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ArticleOctober 16, 2019

Tailoring Cu⁺ for Ga³⁺ Cation Exchange in Cu_2–xS and CuInS₂ Nanocrystals by Controlling the Ga Precursor Chemistry
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Stijn O. M. Hinterding
Stijn O. M. Hinterding
Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
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http://orcid.org/0000-0002-3940-1253
Anne C. Berends
Anne C. Berends
Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
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Mert Kurttepeli
Mert Kurttepeli
Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
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Marc-Etienne Moret
Marc-Etienne Moret
Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
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Johannes D. Meeldijk
Johannes D. Meeldijk
Electron Microscopy Utrecht, Debye Institute for Nanomaterials Science, Utrecht University, 3584 CH Utrecht, The Netherlands
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Sara Bals
Sara Bals
Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
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http://orcid.org/0000-0002-4249-8017
Ward van der Stam
Ward van der Stam
Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
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http://orcid.org/0000-0001-8155-5400
Celso de Mello Donega*
Celso de Mello Donega
Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
*E-mail: [email protected]
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http://orcid.org/0000-0002-4403-3627

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ACS Nano

Cite this: ACS Nano 2019, 13, 11, 12880–12893

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https://doi.org/10.1021/acsnano.9b05337

Published October 16, 2019

Abstract

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Nanoscale cation exchange (CE) has resulted in colloidal nanomaterials that are unattainable by direct synthesis methods. Aliovalent CE is complex and synthetically challenging because the exchange of an unequal number of host and guest cations is required to maintain charge balance. An approach to control aliovalent CE reactions is the use of a single reactant to both supply the guest cation and extract the host cation. Here, we study the application of GaCl₃–L complexes [L = trioctylphosphine (TOP), triphenylphosphite (TPP), diphenylphosphine (DPP)] as reactants in the exchange of Cu⁺ for Ga³⁺ in Cu_2–xS nanocrystals. We find that noncomplexed GaCl₃ etches the nanocrystals by S^2– extraction, whereas GaCl₃–TOP is unreactive. Successful exchange of Cu⁺ for Ga³⁺ is only possible when GaCl₃ is complexed with either TPP or DPP. This is attributed to the pivotal role of the Cu_2–xS–GaCl₃–L activated complex that forms at the surface of the nanocrystal at the onset of the CE reaction, which must be such that simultaneous Ga³⁺ insertion and Cu⁺ extraction can occur. This requisite is only met if GaCl₃ is bound to a phosphine ligand, with a moderate bond strength, to allow facile dissociation of the complex at the nanocrystal surface. The general validity of this mechanism is demonstrated by using GaCl₃–DPP to convert CuInS₂ into (Cu,Ga,In)S₂ nanocrystals, which increases the photoluminescence quantum yield 10-fold, while blue-shifting the photoluminescence into the NIR biological window. This highlights the general applicability of the mechanistic insights provided by our work.

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Compound copper chalcogenide nanocrystals (NCs) offer a promising alternative to the widely studied CdX and PbX NCs (X = S, Se, Te), as they consist of less toxic elements and display exciting optoelectronic properties, such as photoluminescence (PL) tunability in the visible to the near-infrared (NIR) spectral window and localized surface plasmon resonances (LSPR) in the NIR. (1−4) Another advantage of these nanomaterials is the wide range of compositions that can be obtained, ranging from binary to multinary compounds with up to five different elements. (1−3) This makes them a versatile class of materials, offering ample opportunities for property engineering. (1−3,5) However, some morphologies and compositions are not easily accessible by direct approaches due to synthetic limitations, such as the lack of suitable precursors for multinary compounds. (1) Nanoscale cation exchange (CE) has emerged as a promising postsynthetic tool to circumvent these limitations, providing pathways to NCs and hetero-NCs with compositions, morphologies, crystal structures, and heteroarchitectures that would otherwise remain unavailable. (5−8) The extent of the CE (ranging from doping, e.g., Ag:CdSe NCs, (9) Zn:CsPbBr₃ NCs, (10) Mn:ZnTe magic-size clusters, (11) to full replacement) (5−8) and the elemental distribution profile of the product NCs (i.e., homogeneous or gradient alloys, e.g., (Cd,Zn)Se NCs, (12) or heterostructured NCs, e.g., ZnSe/CdSe, (12) PbSe/CdSe (13) core/shell quantum dots) depends on a number of physicochemical properties, such as the miscibility of the parent and product phases and the diffusion rates of the incoming and outgoing cations within the NCs and can be tailored by adjusting reaction variables, such as reaction temperature and concentration of guest cations in solution. (5−13)

In isovalent CE, such as Zn²⁺ for Cd²⁺ exchange in ZnSe NCs (12) and Pb²⁺ for Cd²⁺ in PbX NCs, (13−15) a direct place exchange reaction occurs at the surface of the template NCs, through which the outgoing host cation and the incoming guest cation swap places (i.e., the guest cation is incorporated in the NC, while the host cation forms a complex with the ligand molecule that was carrying the guest cation). (12) This is in contrast with aliovalent CE, such as Cu⁺ for In³⁺ in Cu_2–xS NCs (16) or Cd²⁺ for Ag⁺ in CdSe, (5,9,17) which typically occurs through two separate chemical pathways. (5−8,16) This makes balancing the in- and outgoing diffusion rates challenging. An approach to better control aliovalent CE reactions is the use of a single precursor, which both provides the guest cation and extracts the host cation. In the case of the Cu⁺ for In³⁺ exchange, a stoichiometric InCl₃–trioctylphosphine complex has been shown to be a suitable precursor, even allowing for topotactic sequential CE (viz., Cd²⁺ for Cu⁺, followed by partial Cu⁺ for In³⁺) in large and complex anisotropic NCs such as CdSe/CdS dot-in-rod nanorods, with preservation of the size, shape, and heterostructure of the template NCs in the product CuInSe₂/CuInS₂ dot-in-rod nanorods. (18) The strict control over the CE rates provided by this approach has been recently exploited to fabricate air-stable CuInSe₂ NC transistors by postdeposition sequential CE. (19) Here, we extend the single precursor approach to the exchange of Cu⁺ for Ga³⁺ in Cu_2–xS and CuInS₂ NCs by using stoichiometric GaCl₃–phosphine complexes to simultaneously deliver Ga³⁺ and extract Cu⁺. Our choice is motivated by the fact that CuGaS₂ (CGS) and Cu(In,Ga)S₂ (CIGS) exhibit interesting optoelectronic properties, which make them promising materials for applications in photocatalysis, (20,21) light-emitting devices, (22,23) and photovoltaics. (24−27) Moreover, studies of Cu⁺ for Ga³⁺ and In³⁺ for Ga³⁺ cation exchange are scarce. (28−30) Although Ga³⁺ and In³⁺ are both trivalent cations of group IIIA metals, they have significantly different ionic radii (r_Ga³⁺ = 47 pm, r_In³⁺ = 62 pm) (31) and Lewis acidity (η_Ga³⁺ = 17 eV, η_In³⁺ = 13 eV) (32) and are thus expected to behave differently in cation exchange reactions. Our study thus offers an ideal testbed to investigate the relationship between the chemistry of the metal chloride–phosphine complexes and the outcome of cation exchange reactions in Cu chalcogenide NCs.

We investigated different GaCl₃–L Lewis acid–base adduct complexes [L = trioctylphosphine (TOP), triphenylphosphite (TPP), and diphenylphosphine DPP)] as reagents for the exchange of Cu⁺ for Ga³⁺ and found that the choice of ligand has a profound impact on the reaction outcome. Noncomplexed GaCl₃ is a strong Lewis acid that yields hollow NCs through etching by sulfide extraction, whereas the complex GaCl₃–trioctylphosphine leads to neither etching nor cation exchange. In contrast, both GaCl₃–triphenylphosphite and GaCl₃–diphenylphosphine successfully converted Cu_2–xS NCs in CuGaS₂ NCs through Cu⁺ for Ga³⁺ exchange. These different reactivities are attributed to the stability and geometry of the activated Cu_2–xS–GaCl₃–L complex that must be formed at the surface of the NCs prior to the cation exchange. Density functional theory (DFT) calculations show that, in these complexes, the Ga–P bond strength decreases with increasing electron-withdrawing nature of the side groups on the phosphine ligands (i.e., the Ga–P bond strength ε follows the trend ε_{GaCl₃–TOP} > ε_{GaCl₃–DPP} > ε_{GaCl₃–TPP}). Complexing GaCl₃ with DPP or TPP lowers the activation energy barrier for the CE reaction, while kinetically favoring the Cu⁺ for Ga³⁺ place exchange over the sulfide extraction. These insights were used to successfully convert CuInS₂ NCs into Cu-poor (Cu,In,Ga)S₂ NCs through Cu⁺ for Ga³⁺ exchange, yielding NCs with vastly improved photoluminescence quantum yields with respect to the parent NCs. The findings presented in this work are thus generally applicable, resulting in nanomaterials with combinations of size, shape, composition, and crystal structure that are not directly accessible by conventional synthesis routes.

Results and Discussion

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Stoichiometric InCl₃–TOP and GaCl₃–TOP as Reactants for Cation Exchange in Cu_2–xS NCs

Throughout this work, low-chalcocite 12 nm Cu_2–xS (x < 0.04) bifrustum NCs are used as parent NCs to investigate the Cu⁺ for Ga³⁺ cation exchange reaction. For comparison, these NCs (Figure 1a,d) are converted into wurtzite CuInS₂ (CIS) bifrustum NCs (Figure 1b,e) by partial, self-limited Cu⁺ for In³⁺ CE at 100 °C, using a stoichiometric InCl₃–trioctylphosphine (InCl₃–TOP) Lewis acid–base adduct as precursor, as previously reported by our group. (18) The transformation from low chalcocite to wurtzite is evidenced by the electron diffraction (ED) patterns (Figure 1a,b,d,e). The powder ED (PED) pattern obtained after azimuthal integration of the 2D ED pattern of the product NCs (Figure 1e) corresponds well to the reference pattern of wurtzite CIS. The shape and size of the NCs are preserved after the reaction (Supporting Information, Figure S1), confirming the topotactic nature of the Cu⁺ for In³⁺ CE reaction. Energy-dispersive X-ray spectroscopy (EDS) measurements further confirm the conversion of Cu_2–xS NCs into ternary CIS NCs (Cu/In = 1.00:0.79; Supporting Information, Figure S2). Compared to the Cu_2–xS parent NCs, the product CIS NCs no longer exhibit a LSPR absorption band in the NIR (Supporting Information, Figure S3a,b). The absorption spectrum of the CIS NCs agrees well with spectra reported in the literature. (33) In contrast, the use of the stoichiometric GaCl₃–TOP Lewis acid–base adduct as a precursor does not lead to any significant change in the composition (Cu/Ga = 1.00:0.02, Supporting Information, Figure S4), crystal structure, size, and shape of the NCs (Figure 1c,f). The absorption spectrum of NCs obtained after reaction with GaCl₃–TOP exhibits little to no LSPR band (Supporting Information, Figure S3c), which indicates the absence of excess free holes in these NCs, due to (limited) reduction at the NC surface.

Figure 1. TEM images of (a) parent Cu_2–xS bifrustum NCs, (b) product Cu_1.03In_0.81S₂ (CIS) bifrustum NCs, after reaction with InCl₃–TOP at 100 °C, and (c) Cu_1.99Ga_0.04S nanocrystals, after reaction with GaCl₃–TOP at 100 °C. Top-right insets display the corresponding ED pattern. Bottom panels show azimuthally integrated ED (PED) patterns of (d) parent low-chalcocite Cu_2–xS nanocrystals, (e) product wurtzite CIS nanocrystals, and (f) product low-chalcocite Cu_1.99Ga_0.04S nanocrystals. Reference bars in (d,f) are calculated patterns based on the low-chalcocite Cu_2–xS crystal structure described in ref (34). Reference bars in (e) were obtained from JCPDS PDF-card 01-077-9459 for wurtzite CIS.

Reaction of Noncomplexed GaCl₃ with Cu_2–xS NCs

Clearly, the Cu⁺ for Ga³⁺ CE does not significantly occur when GaCl₃–TOP is used as the reactant under the same conditions that successfully lead to Cu⁺ for In³⁺ CE using InCl₃–TOP. Therefore, other Ga precursors were examined, starting with the simplest possible case: GaCl₃ dissolved in neat toluene. The reaction with GaCl₃ at the same temperature used with GaCl₃–TOP (100 °C) was found to severely deteriorate the NCs (Supporting Information, Figure S5). To minimize the degradation, the temporal evolution of this reaction was studied at a lower temperature (30 °C) (Figure 2b–e). The TEM images of the product NCs show that, despite the lower reaction temperature, a low-contrast spot appears in the NCs already after 15 min of reaction (Figure 2b, red arrows) and increases in size over time (Figure 2c–e). The PED pattern shows that the crystal structure of the NCs remains low-chalcocite Cu_2–xS, even after 120 min of reaction (Figure 2f), despite the dramatic morphological changes (Figure 2e). After 120 min of reaction, these NCs no longer exhibit an LSPR absorption band (Supporting Information, Figure S3f), presumably due to their poorly defined shape.

Figure 2. TEM images of (a) parent Cu_2–xS NCs and (b–e) product NCs obtained after reacting the NCs shown in (a) with GaCl₃ at 30 °C for (b) 15 min, (c) 30 min, (d) 60 min, and (e) 120 min. Red arrows indicate regions of low contrast within the nanocrystals (cartoons in the lower-left corner give an impression of the observed contrast in the NCs). Top-right inset in (e) depicts an electron diffraction pattern of the corresponding sample. Panel (f) is the azimuthally integrated ED pattern depicted in (e). Reference bars in (f) are calculated patterns based on the low-chalcocite Cu_2–xS crystal structure described in ref (34). The blue curve in (f) corresponds to the PED pattern of the parent Cu_2–xS nanocrystals. Dotted lines serve as guides for the eye.

To determine the nature of the low contrast region and its position within the particles, high-angle annular dark-field scanning transmission microscopy (HAADF-STEM) tomography measurements were performed. Visualizations of the 3D reconstructed volumes of NCs after different reaction times are shown in Figure 3. Animated versions of the tomograms are also provided in the web-enhanced movies 1, 2, and 3. The tomography measurements show that, after 15 min of reaction with GaCl₃, a small cavity is present at the surface of some Cu_2–xS NCs (Figure 3a) and increases in size as the reaction progresses (Figure 3b,c). Acquired orthoslices, which were used to reconstruct the 3D morphologies, are displayed in Figure 3d–f. These measurements indicate that the brighter regions observed in TEM images coincide with cavities at the surface of the NCs. The cavities appear to form at the surface of the NCs at early reaction stages and subsequently grow inward over time. A small portion of the NCs (indicated by red arrows in Figure 3) appears to be hollow, without an obvious opening at the surface.

Figure 3. 3D characterization of tomographic visualization of the Cu_2–xS bifrustum NCs after reaction with GaCl₃ at 30 °C for (a) 15 min, (b) 30 min, and (c) 60 min. After 15 min, only a small dimple is present at the surface of the NCs. After longer reaction times, these cavities are found to be larger. Red arrows indicate hollow NCs without an apparent opening at the surface. (d–f) Orthoslices used to reconstruct the 3D morphologies presented in (a–c). Animated (rotating) versions of the models depicted in panels (a–c) are provided.

These observations bear similarities with previous reports on hollow nanoparticles, whose formation has been ascribed to the nanoscale Kirkendall effect, (35,36) which involves an imbalance between outward and inward diffusion fluxes during the chemical conversion of a NC into a different material through reaction with a reactant present in solution. The surface of the parent NC is first converted into a shell of the product material by reaction with a species present in solution, after which further conversion can only take place through solid-state diffusion of the reactant species through the NC. (35) In case the outward diffusion rate is faster than the inward diffusion rate, there is a net flow of material going out of the NC, leaving vacancies behind. (35) Condensation of these vacancies gives rise to voids that eventually collect in a central void in the NCs. (35) The nanoscale Kirkendall process has also been recently proposed as an explanation for the conversion of Cu_2–xS nanoplatelets into CuInS₂ nanorings through partial Cu⁺ for In³⁺ CE. (37) Our observations, however, are not consistent with the nanoscale Kirkendall effect, as the cavities form at the surface of the NCs and grow inward, in striking contrast with the outward growing inner voids that characterize the Kirkendall effect.

Surface erosion of the NCs and preservation of a low-chalcocite Cu_2–xS shell after the reaction with GaCl₃ indicate that a reaction other than cation exchange is taking place. In the successful Cu⁺ for In³⁺ cation exchange reaction using the InCl₃–TOP Lewis acid–base adduct complex, TOP (a soft Lewis base) acts as Cu⁺ extractor, (5,18) likely forming CuCl–TOP Lewis acid–base adduct complexes. In the present reaction using noncomplexed GaCl₃, there are no species present to directly extract Cu⁺. However, GaCl₃ is a strong Lewis acid (38,39) and therefore can extract S^2– from the NC as S^2– is a Lewis base. This extraction destabilizes the crystal lattice, as the S^2– anions form the framework of Cu_2–xS NCs, resulting in etching of the NCs and formation of cavities at the surface. The collapse of the NC framework will release Cu⁺ ions, which are likely stabilized in solution by binding to the chloride ions released from GaCl₃ after its reaction with the NC sulfides.

Although the NCs react with a strong Lewis acid, they do not dissolve completely, even after 300 min (Supporting Information, Figure S6). Interestingly, in micrographs obtained using HAADF-STEM, most particles exhibit a uniform contrast after reacting for 300 min (Supporting Information, Figure S6e). This uniform contrast indicates that the previously formed cavities are filled with new material. EDS elemental maps show that this material consists mostly of gallium. These observations suggest that Ga-containing monomers, formed upon extraction of S^2– by GaCl₃, precipitate inside the NC cavities after longer reaction times. This refilling of the cavities likely blocks further reactions on the NC surface, preventing further dissolution.

Tailoring the Ga Precursor Chemistry To Boost the Cu⁺ for Ga³⁺ Cation Exchange

We have discussed above two extreme cases of reactivity imbalance: whereas GaCl₃–TOP is too inert, hardly leading to any change in the parent Cu_2–xS NCs, noncomplexed GaCl₃ is too reactive toward the sulfide anions, thereby etching the parent NCs without any significant cation exchange. In the following, we will tailor the chemistry of the Ga precursor complex to allow successful Cu⁺ for Ga³⁺ cation exchange.

The observation that the reactivity of the GaCl₃–TOP Lewis acid–base adduct complex as a cation exchange precursor is much lower than that of the InCl₃–TOP complex is surprising, considering that the metal–phosphorus bond in GaCl₃–TOP should be weaker than that in InCl₃–TOP as Ga³⁺ is a harder Lewis acid than In³⁺ (η = 17 and 13 eV, respectively), (32) whereas TOP is a soft Lewis base (η ≈ 6 eV). (5,40) This implies that other variables are more relevant in determining the Cu⁺ for Ga³⁺ cation exchange rates and prompted us to explore the influence of the nature of the phosphine ligand on the cation exchange reaction by using triphenylphosphite and diphenylphosphine as ligands for GaCl₃ (Figure 4b,c). The coordination behavior of a phosphine ligand with respect to a metal center is determined by its steric and electronic properties. (41,42) The most frequently used parameter to quantify the steric properties of phosphines is the Tolman cone angle, defined as the apex angle of a cylindrical cone with origin 2.28 Å from the center of the phosphorus atom, whose sides just touch the Van der Waals surfaces of the outermost atoms of the organic substituents. (41) The three phosphine ligands used in the present work have significantly different electronic properties and total molecular volume, but their Tolman cone angles are essentially the same: 128° for both DPP and TPP and 132° for TOP. (41) The value for TOP is taken to be the same as that reported by Tolman for other tertiary alkylphosphines, such as tri-n-butylphosphine, although a recent work has estimated it to be 128°. (40) Steric effects are thus not expected to play a significant role in the cation exchange reactions investigated in our work. The electronic properties of phosphines are determined by their σ-donor ability and π-acceptor strength as they bind to metals by a combination of σ-donation from the lone pair of the P atom and π-back bonding from the d orbitals of the metal into their empty P–R σ* molecular orbital (R = organic substituent). (41,42) More electronegative (i.e., electron-withdrawing) substituents on the phosphorus atom decrease the basicity and σ-donating ability of the phosphine, while increasing its π-acceptor strength. (40−43) This effect is due to the decrease of the electronic density on the phosphorus atom and increase of the s-character of the lone electron pair. (41,42) An increase in electronegativity of the substituents on the phosphorus atom should lead to weaker bonding to the metal, as should be the case for GaCl₃–TPP and GaCl₃–DPP compared to GaCl₃–TOP.

Figure 4. (Top row) Ground-state geometries of three GaCl₃–phosphine complexes obtained from DFT calculations (see below): (a) GaCl₃ complexed with triethylphosphine, used as model for GaCl₃ complexed with trioctylphosphine (GaCl₃–TOP), (b) GaCl₃ complexed with triphenylphosphite (GaCl₃–TPP), and (c) GaCl₃ complexed with diphenylphosphine (GaCl₃–DPP). (Middle row) TEM images of product NCs obtained after reaction of Cu_2–xS NCs with (d) GaCl₃–TOP at 100 °C, (e) GaCl₃–TPP at 100 °C, and (f) GaCl₃–DPP at 50 °C. Black arrows indicate intraparticle contrast in the NCs reacted with GaCl₃–TPP. Top-right insets show the corresponding ED patterns. Bottom row (g–i) shows the corresponding azimuthally integrated ED patterns. Reference bars in (h,i) are calculated patterns based on the wurtzite CuGaS₂ crystals structure, (44) and reference bars in (g) are based on the low-chalcocite Cu_2–xS crystal structure. (34)

Indeed, the use of both GaCl₃–TPP and GaCl₃–DPP as reactants results in successful CE reactions even after reaction times much shorter than those used for GaCl₃–TOP (1 h instead of 12 h, at the same temperature), as clearly demonstrated by EDS analysis of the product NCs obtained after reaction with GaCl₃–TPP and GaCl₃–DPP at 100 °C, which reveals Cu/Ga ratios of 1.00:1.38 and 1.00:1.48, respectively (Supporting Information, Figures S7 and S8). The NCs that reacted with GaCl₃–TPP are partially deformed and exhibit intraparticle contrast in TEM images (Figure 4e, indicated by black arrows), similar to the NCs that reacted with GaCl₃ (see Figures 2 and 3). The NCs that reacted with GaCl₃–DPP form agglomerates (Supporting Information, Figure S9), which is likely due to displacement of the original organic capping ligands (dodecanethiol) with DPP during the CE reaction. DPP is a relatively small molecule and thus poorly suitable as a steric stabilizer. In comparison, the NCs cluster less after reaction with GaCl₃–TPP as triphenylphosphite is bulky enough to provide steric stabilization. To increase the colloidal stability of the NCs reacted with GaCl₃–DPP, oleylamine (OLAM) was added to the reaction mixture and a lower reaction temperature (50 °C) was employed, which indeed inhibited NC agglomeration (Figure 4f). However, under these milder reaction conditions, the Cu⁺ for Ga³⁺ exchange proceeds to a lesser extent, as shown by EDS analysis (Cu/Ga ratio of 1.00:0.62, Supporting Information, Figure S10). The PED pattern (Figure 4e,h) of the NCs reacted with GaCl₃–TPP corresponds to the wurtzite crystal structure, confirming that Cu⁺ for Ga³⁺ CE has indeed taken place. Interestingly, the peaks in the ED pattern are shifted to lower q values compared to those of the reference wurtzite CGS, indicating that the product CGS NCs have a larger lattice constant than the reference. This observation will be discussed in more detail below. The PED pattern (Figure 4f,i) of the NCs reacted with GaCl₃–DPP exhibits broad, poorly defined peaks, indicating that these particles have partially lost their crystallinity during the CE procedure, which precludes a definitive assignment of the crystal structure. The NCs reacted with GaCl₃–TPP and GaCl₃–DPP are both smaller than the parent Cu_2–xS NCs (diameters = 10.8 ± 1.0, 11.1 ± 1.1, and 12.3 ± 1.0 nm, respectively, Supporting Information, Figure S1). The NCs reacted with GaCl₃–TPP no longer exhibit an LSPR absorption band, whereas those reacted with GaCl₃–DPP exhibit an LSPR absorption band with a reduced intensity (Supporting Information, Figure S3d,e). In the latter NCs, Cu⁺ was exchanged for Ga³⁺ to a lesser extent; the presence of an LSPR absorption band thus indicates that these NCs remain cation-deficient, requiring excess holes for charge neutrality.

The intraparticle contrast observed in the NCs after reaction with GaCl₃–TPP was studied with HAADF-STEM and EDS elemental mapping. In micrographs obtained using HAADF-STEM, the intraparticle contrast appears similar to that observed in TEM images (compare Figure 5a to Figure 4e). In the elemental maps, the regions of low contrast coincide with regions of low signal for both cations (Figure 5, indicated by orange arrows). As the Cu and Ga distributions correlate well (Figure 5b,c), the intraparticle contrast is clearly not due to an inhomogeneous composition. Instead, it appears that the low contrast regions in HAADF-STEM and TEM images simply contain less material. Likely, surface erosion by GaCl₃ is not completely suppressed when GaCl₃–TPP is used as reactant, resulting in the formation of small cavities at the NC surface.

Figure 5. (a) HAADF-STEM images and EDS elemental maps of (b, blue) Cu and (c, red) Ga of product NCs obtained by reacting Cu_2–xS NCs with GaCl₃–TPP at 100 °C. Orange arrows indicate regions of low contrast (HAADF-STEM) coinciding with regions of low signal (EDS maps).

Similar to the previously reported conversion of low-chalcocite Cu_2–xS into wurtzite CIS NCs by Cu⁺ for In³⁺ CE, (16,18,33) the exchange of Cu⁺ for Ga³⁺ in Cu_2–xS NCs using GaCl₃–DPP or GaCl₃–TPP as precursors is also partial and self-limited as CGS, rather than Ga₂S₃, NCs are produced. In the case of the Cu⁺ for In³⁺ CE in Cu_2–xS NCs, the topotactic, partial, and self-limited character of the exchange was attributed to the high energy barrier associated with the conversion of the hexagonal close-packed anionic sublattice of the parent low-chalcocite Cu_2–xS NCs into the face-centered cubic sublattice of the spinel structure of In₂S₃, which requires a substantial reorganization of the sulfide anions (by 58% of a S–S interatomic distance). (16) In contrast, the hexagonal wurtzite structure of CuInS₂ requires only minor rearrangements of the sulfide sublattice with respect to that of the template NCs (1.4 and 6.4% contraction within and between the hexagonal S^2– layers, respectively). (16) The topotactic nature of the cation exchange in Cu_2–xS NCs has been recently exploited to achieve structure-selective conversion of Cu_2–xS NC templates into ZnS, CdS, and CuInS₂, with hexagonal roxbyite Cu_1.78S and cubic digenite Cu_1.8S yielding product NCs with the wurtzite and the zinc blende structures, respectively. (45)

In the present case, however, the structural differences between the possible products and the template NCs are less pronounced because low-chalcocite Cu_2–xS, wurtzite CuGaS₂, and the stable form of bulk Ga₂S₃, monoclinic α′-Ga₂S₃, all have a hexagonal close-packed sulfide sublattice, (34,44,46) with hexagonally structured layers stacked in an ...ABAB... fashion (Figure 6a,b). However, we observe that, in all three dimensions, the wurtzite CuGaS₂ structure exhibits a significant lattice contraction compared to that of low-chalcocite Cu_2–xS (Figure 6c,d). These contractions are 5.6, 5.7, and 9.5% in x, y, and z, respectively. On the other hand, the anion positions of wurtzite CuGaS₂ and α′-Ga₂S₃ are very similar: transforming the former into the latter, the lattice has to contract by only 0.6, 0.5, and 2.5% in x, y, and z, respectively (the definition of x, y, and z directions in terms of crystallographic directions are provided in the caption of Figure 6). It is thus clear that, based on the bulk crystal structure parameters, the sulfide sublattice of the template low-chalcocite Cu_2–xS would have to undergo similar degrees of reconstruction, regardless of whether the cation exchange would be partial, yielding wurtzite CuGaS₂, or complete, yielding α′-Ga₂S₃. However, we note that the product wurtzite CGS NCs (with Cu/Ga ratios ranging from 1:1.48 to 1:0.62) obtained in our work exhibit a larger lattice constant than the wurtzite CuGaS₂ reference (Figure 4h,i), indicating that the degree of contraction was smaller than expected based on the bulk lattice parameters, and therefore, that the product CGS NCs are under tensile strain. This suggests that the mild reaction conditions used in our work do not allow the anion sublattice to undergo full reconstruction in order to relax the strain induced by the CE.

Figure 6. (a,b) Hexagonal close-packed structure viewed from two different orientations: (a) characteristic arrangement into hexagonally constructed layers, defined here as the x,y-plane, and (b) characteristic ...ABAB... stacking of these hexagonal layers, defined here as the x,z-plane. (c,d) Anion positions of (black) low-chalcocite Cu_2–xS, (cyan) wurtzite CuGaS₂, and (purple) α′-Ga₂S₃ overlaid onto each other. (c) Hexagonal ordering in the x,y-plane and (d) ...ABAB... stacking in the x,z-plane. Δx, Δy, and Δz, denoted in (a,b), correspond to the lattice distances used to determine the lattice contractions mentioned in the main text. The x,y,z-directions correspond to the following crystallographic directions: (low-chalcocite Cu_2–xS) [201],[010],[001]; (wurtzite CuGaS₂) [11̅0],[110],[001]; (monoclinic α′-Ga₂S₃) [010],[100],[102]. Anion coordinates were obtained from the crystal structures of low-chalcocite Cu_2–xS, (34) wurtzite CuGaS₂, (44) and monoclinic α′-Ga₂S₃. (46)

Recently, lattice strain was shown to play an important role in the conversion of djurleite Cu_2–xS nanorods (NRs) into Cu_2–xS/CGS hetero-NRs by partial Cu⁺ for Ga³⁺ CE. (30) Shim and co-workers showed that lattice strain prevented the homogeneous conversion of Cu_2–xS NRs (∼170 nm long, 20 nm diameter) into CGS NRs, resulting instead in hetero-NRs containing one or multiple wurtzite CGS domains. (30) The authors argued that the strain induced by the lattice mismatch between the parent djurleite Cu_2–xS phase and the product wurtzite CGS phase would make Ga³⁺ diffusion into Cu_2–xS unfavorable, thereby limiting the extent of the CE reaction and leading to wurtzite–djurleite superlattice formation as a way to minimize the resulting strain. However, we note that in the present work Cu_2–xS NCs were successfully converted into homogeneous CGS NCs, despite the tensile strain resulting from insufficient contraction to the lattice parameters of fully relaxed bulk CuGaS₂. This difference can be attributed to the fact that the NCs used here are much smaller than those used by Shim and co-workers (viz. 12 nm diameter vs ∼170 nm long, 20 nm diameter rods). The smaller volume of the template NCs used in our work favors conversion to homogeneous CGS NCs because smaller NCs can more easily accommodate lattice strain. This also offers a possible explanation for the self-limiting character of the Cu⁺ for Ga³⁺ exchange in Cu_2–xS NC templates: the α′-Ga₂S₃ structure is probably unable to accommodate as much tensile strain as wurtzite CGS, preventing full CE.

Density Functional Theory Calculations

To corroborate our assumption of weaker Ga–P bond strengths in GaCl₃–DPP and GaCl₃–TPP with respect to GaCl₃–TOP, we conducted DFT calculations at the B3LYP/6-311+G(d,p);Ga:SDD level to estimate both the ground-state geometry of the complexes and the Ga–P bond enthalpy. In the DFT calculations, we modeled TOP as triethylphosphine to avoid an extensive conformational search due to the many degrees of freedom of the long alkyl chains. We expect that only the first few CH₂ groups of TOP significantly influence the Ga–P bond, making PEt₃ an acceptable surrogate for TOP. To verify whether the outcome of these calculations is physically reasonable, we compare calculated geometries to experimental structures obtained from X-ray crystallography. We include an additional complex (GaCl₃–PPh₃), as, to the best of our knowledge, the structures of GaCl₃–DPP and GaCl₃–TPP have not been resolved experimentally. The results of the DFT calculations (Ga–P bond lengths and dissociation enthalpies) and the experimentally determined Ga–P bond lengths are shown in Table 1. The Ga–P bond lengths determined by DFT deviate from the experimental ones by only 4–5%. Moreover, the DFT results reproduce the experimental trend in Ga–P bond length from GaCl₃–PEt₃ to GaCl₃–PPh₃. Thus, although the absolute values obtained from the DFT calculations are 4–5% longer than the experimental ones, the trends in the DFT results are reliable. We observe that, with increasing electron-withdrawing nature of the side groups on the phosphine ligand, the Ga–P bond length L increases (viz., L_{GaCl₃–TOP} < L_{GaCl₃–DPP} < L_{GaCl₃–TPP}). The dissociation enthalpy follows the reverse trend. These results show that, indeed, the Ga–P bonds in GaCl₃–DPP and GaCl₃–TPP are weaker than those in GaCl₃–TOP.

Table 1. DFT-Calculated (B3LYP/6-311+G(d,p);Ga:SDD) and Experimental Ga–P Bond Lengths and DFT-Calculated Dissociation Enthalpies of GaCl₃–PEt₃, GaCl₃–PPh₃, GaCl₃–DPP, and GaCl₃–TPP

complex	Ga–P bond length (Å), calculated	Ga–P bond length (Å), experimental	dissociation enthalpy (kcal/mol), calculated
GaCl₃–PEt₃	2.453	2.353 (47)	31.2
GaCl₃–PPh₃	2.487	2.372 (48)	27.2
GaCl₃–DPP	2.498	not available	23.0
GaCl₃–TPP	2.540	not available	14.7

Mechanism for the Cu⁺ for Ga³⁺ Exchange in Cu_2–xS NCs

Nanoscale CE reactions consist of a number of linked elementary kinetic steps: (5,6,8,12) (i) extraction of the host cations from the NC surface, (ii) incorporation of guest cations at the NC surface, (iii) solid-state diffusion of the guest cations into the NC, and (iv) solid-state diffusion of the native cations to the NC surface. The thermodynamic driving force for the reaction is determined by the energy balance of the overall reaction. (5,6,8) However, the energy input or output involved in each step is important as all steps are kinetically linked and must proceed in a concerted manner. Therefore, the outcome of nanoscale CE reactions (i.e., extent of the exchange, homogeneous or heterostructured nanocrystals, etc.) is dictated by the kinetic balance between a number of competing processes taking place in solution, at the NC surface and within the NC.

Aliovalent CE reactions are inherently more complex than isovalent ones as preservation of charge balance is in this case kinetically more challenging because a different number of host and guest cations must be exchanged to keep charge neutrality (e.g., three Cu⁺ cations must be extracted to allow the incorporation of one In³⁺ or Ga³⁺ cation). Aliovalent CE involving Cu⁺ has been extensively investigated because the small charge and high solid-state diffusion rates of Cu⁺ ions make both their incorporation into and extraction from NCs facile and fast. This makes copper chalcogenide NCs attractive not only as templates for other binary, multinary, or hetero-NCs that cannot be easily obtained by direct routes (e.g., CuInS₂, (16,28,30,33,37,45) CuInSe₂, (8,49,50) Cu₄SnS₄, (19,51) Cu_2–xSe/ZnSe, (52) ZnS/Cu_1.8S, (28,53)etc.) but also as intermediates in sequential CE reactions, making it possible to synthesize NCs and hetero-NCs that would otherwise remain inaccessible (e.g., PbS NRs from CdS NRs by Cd²⁺ for Cu⁺ followed by Cu⁺ for Pb²⁺ CE, (5) wurtzite ZnSe/ZnS and CuInSe₂/CuInS₂ core/shell NRs from CdSe/CdS core/shell NRs (18,54)). As a result of this extensive research, the thermodynamics and kinetics of aliovalent CE reactions involving Cu⁺ are reasonably well-understood and known to depend on many variables: presence of Cu-extracting ligands in solution, (5,6,8) leaving ability of the ligands bound to the incoming guest Mⁿ⁺ cation, (37) concentration of Cu⁺ vacancies, (52,55) valence and diffusion rate of the incoming guest Mⁿ⁺ cation, (8,28,30) crystal structure of the template Cu_2–xS NCs, (8,45) and miscibility of the parent and product phases. (8) As we will discuss below, the results presented above unravel one more crucial parameter governing the kinetics of CE reactions: the activated complex that must form at the surface of the template NC when a direct place exchange reaction is used (i.e., when the Cu-extracting ligand is bound to the guest heterocation).

As previously demonstrated by Buhro and co-workers, (37) the aliovalent Cu⁺ for In³⁺ exchange in Cu_2–xS NCs is in the reaction-limited regime. The incorporation of In³⁺ is preceded by the adsorption of the InX₃ precursor on the NC surface. The displacement of the X^– ligand is proposed to occur by nucleophilic attack of lattice S^2– ions on the adsorbed InX₃ precursor. (37) The height of the energy barrier for this reaction is given by the leaving-group ability of the X^– ligand. (37) We add that when a direct place exchange reaction is promoted by using a TOP–InCl₃ complex, the energy barrier is lowered by the formation of an incipient bond between the P donor group and a Cu⁺ ion in the vicinity of the S^2– ion responsible for the nucleophilic attack on the In–Cl bond. This makes the reaction rates faster and allows for a direct place exchange between the incoming In³⁺ guest cation and one outgoing Cu⁺ cation, while facilitating extraction of two additional Cu⁺ ions as neutral CuCl units. However, the formation of such an activated complex imposes constraints on the orientation of the binding TOP–InCl₃ with respect to the surface Cu⁺ and S^2–. Nevertheless, in the case of Cu⁺ for In³⁺ CE, these constraints are not very strict because the activated complex can follow only two possible reaction pathways: CE or desorption without reaction, in which case it may bind again in the proper orientation later.

In the case of Cu⁺ for Ga³⁺ CE, the constraints imposed on the activated complex are more severe because GaCl₃ is a much stronger Lewis acid than InCl₃, therefore being capable of extracting lattice S^2– from the template Cu_2–xS NCs (Ga–S = 393 kJ/mol, In–S = 313–316 kJ/mol), (56) leading to etching rather than CE (Figure 7a), as demonstrated above (Figures 2 and 3). Complexation of GaCl₃ with TOP suppresses the NC etching but does not lead to CE. This demonstrates that the activated complex formed upon adsorption of GaCl₃–TOP on the NC surface effectively weakens the interaction between Ga and the lattice S^2– and also prevents interaction between the P donor head and the Cu⁺ ions (Figure 7b). In contrast, both GaCl₃–TPP and GaCl₃–DPP result in successful Cu⁺ for Ga³⁺ CE. All three ligands (TOP, TPP, DPP) are bound to GaCl₃ through the lone pair on the phosphorus atom and an empty sp³ orbital of gallium. (48,57,58) To extract Cu⁺, the phosphine acts as Lewis base and binds to Cu⁺ through the same lone pair. In order for the phosphine to bind to Cu⁺, while still preventing etching by attack of the Ga³⁺ to lattice S^2– ions, the activated complex at the NC surface (Figure 7c) must be such that the Cu–P bond formation and the Ga–P bond breaking occur simultaneously (Figure 7c, steps 1–3) and are followed by the immediate place exchange between Cu⁺ and Ga³⁺. We attribute the higher CE reactivities of the GaCl₃–TPP and GaCl₃–DPP to a more favorable geometry of the activated complex formed upon their adsorption on the surface of the template Cu_2–xS NCs, as well as a weaker Ga–P bond compared to GaCl₃–TOP. In GaCl₃–TPP and GaCl₃–DPP, the Ga–P bond is weaker than that in GaCl₃–TOP (see Table 1), fulfilling the conditions for a fast place exchange between Ga³⁺ and Cu⁺, while still making the competing reaction between GaCl₃ and the lattice S^2– ions less likely (Figure 7c, steps 1–3). We note that the Cu⁺ for Ga³⁺ place exchange is thermodynamically favored as it results in the formation of two new bonds that are stronger than the initial ones (viz., the Cu–P bond in the Cu–phosphine Lewis acid–base adduct, which is favored because Cu⁺ is a much softer Lewis acid than Ga³⁺, η = 6.28 and 17 eV, respectively, (32) and the Ga–S bond, which has a bond strength of 393 kJ/mol while the enthalpy of formation of Cu₂S is only −79.5 kJ/mol (59)). Nevertheless, some degree of etching still occurs as the NCs reacted with GaCl₃–TPP and GaCl₃–DPP are smaller than the template NCs (viz., 10.8 ± 1.0, 11.1 ± 1.1, and 12.3 ± 1.0 nm, respectively, Supporting Information, Figure S1). The observed shrinkage (12.2 and 9.76%, respectively) is too large to be attributed to a decrease in unit cell volume alone (see discussion on crystal lattice contraction above). The difference in reactivity between InCl₃–TOP and GaCl₃–TOP as CE reactants may be due to the fact that In³⁺ exhibits three-, four-, five-, and six-coordination, whereas Ga³⁺ is limited to three- and four-coordination. (57) InCl₃–TOP has thus more degrees of freedom to rearrange itself at the nanocrystal surface to form the Cu_2–xS–InCl₃–TOP activated complex, whereas GaCl₃–TOP does not, making the formation of the Cu_2–xS–GaCl₃–TOP activated complex subjected to more stringent orientational constraints and therefore less likely.

Figure 7. Schematic depiction of the Cu⁺ for Ga³⁺ cation exchange reaction in Cu_2–xS nanocrystals. (a) Free GaCl₃ reacts as a Lewis acid, extracting S^2–, leading to etching of the nanocrystals. (b) Extraction of S^2– is inhibited when GaCl₃ is complexed with a phosphine ligand. (c) Successful cation exchange involves four steps: (i) cleavage of the Ga–P bond at the Cu_2–xS nanocrystal surface, at which point Ga–S and P–Cu bonds are made simultaneously; (ii) extraction of Cu⁺ by the phosphine; (iii) subsequent incorporation of Ga³⁺ into the nanocrystal; and (iv) reaction of remaining Cl^– with Cu⁺, extracting two additional Cu⁺ ions, completing the exchange.

As mentioned above, the steric demands of all three phosphines used in the present work are essentially the same (viz., 128° for DPP and TPP, 132° for TOP), (41) and therefore, steric effects are not expected to play a significant role in the outcome of the cation exchange reactions. However, Tolman cone angles are just a first approximation as they do not take into account the effect of different M–P bond lengths, different metal centers, and the overall interaction with other ligands coordinated to the same metal center. (43,60,61) Therefore, actual cone angles obtained from X-ray structural data (the so-called solid cone angles) typically deviate from the Tolman cone angles, varying over a wide range depending on the nature of the metal center and bond length (shorter bonds result in larger cone angles). (60) For example, the solid cone angles determined for triphenylphosphine complexes range from 129 to 168°, (61) whereas the Tolman cone angle of triphenylphosphine is 145°. (41,43) In the case of Ni complexes, a slightly larger solid cone angle has been reported for TPP with respect to DPP complexes (135 and 124°, respectively), (43) consistent with the bulkier nature of the former (containing three phenyl groups) with respect to the latter (which contains only two phenyl groups). If the impact of steric effects on the outcome of the cation exchange reactions were significant, one would expect a difference between the reactivity of the complexes using TPP and DPP as ligands, in contrast with our experimental data, which show that GaCl₃–TPP and GaCl₃–DPP have similar reactivities. This implies that the kinetics of the cation exchange reactions studied in our work are primarily dictated by the electronic properties of the phosphine ligands used, which determine the Ga–P bond strength of the resulting Lewis acid–base adduct complexes with GaCl₃ and the geometry of the activated Cu_2–xS–GaCl₃–L complex that forms at the surface of the Cu_2–xS NC at the onset of the cation exchange reaction. After the place exchange between the outgoing Cu⁺ and the incoming Ga³⁺ cations has taken place, the two chlorides still bound to the incorporated Ga³⁺ cation will react with two neighboring Cu⁺ cations, extracting them from the NC surface as neutral CuCl units, which are highly soluble in the apolar reaction medium used in our experiments (Figure 7c, step 4). This likely increases the thermodynamic driving force of the overall CE reaction both entropically and enthalpically as two more chemical species are released in solution and solvated. (5,6,8) The two Cu⁺ vacancies left will further promote the CE reaction by allowing more Cu⁺ cations to diffuse to the surface, while allowing the incorporated Ga³⁺ guest cation to diffuse inward. As previously shown for CE reactions in copper chalcogenide NCs, the CE rates increase with increasing concentration of Cu⁺ vacancies. (52,62)

Cu⁺ for Ga³⁺ Exchange in Luminescent CuInS₂ NCs

To illustrate the general applicability of the findings presented above, we used GaCl₃–DPP to convert luminescent CuInS₂ (CIS) NCs into (Cu,In,Ga)S₂ (CIGS) NCs with much brighter photoluminescence (see Methods for details). The parent CIS NCs have an indium-rich composition with a Cu/In ratio of 0.68 ± 0.008:1. EDS analysis of the product NCs gives a Cu/Ga/In ratio of 0.20 ± 0.001:0.16 ± 0.001:1, indicating that Cu⁺ ions are exchanged for Ga³⁺. These cation ratios imply that the charge balance is preserved through the CE reaction, as the relative concentration of Ga ions multiplied by their charge of +3 exactly corresponds to the decrease in Cu concentration (with charge +1; 0.16 × 3 = 0.48; 0.68–0.20 = 0.48). Interestingly, the size (∼5.5 nm) and pyramidal shape of the CIS NCs are preserved in the product CIGS NCs (see Figure 8a,d), despite the removal of a significant amount of cations from the NC. Fast Fourier transform (FFT) analysis of the high-resolution TEM images allows determination of the lattice spacings, being 0.18, 0.21, and 0.34 nm before and after the reaction (i.e., they are not significantly changed by the CE reaction). However, analysis of the ED patterns shows a shift to higher q values (Figure 8e), indicating a small contraction of the unit cell. This contraction is consistent with the exchange of the relatively large Cu⁺ for the relatively small Ga³⁺ (r_Cu⁺ = 60 pm, r_Ga³⁺ = 47 pm). (31)

Figure 8. Cu⁺ for Ga³⁺ cation exchange in luminescent CIS NCs. (a) High-resolution TEM image of the parent CIS NCs. The inset shows the FFT of the image. (b) High-resolution TEM image of the product CIGS NCs after reaction; the inset shows the FFT of the image. (c,d) Enlarged TEM image of single NCs before (c) and after (d) the Ga³⁺ for Cu⁺ CE reaction, with the corresponding FFTs. Both scale bars correspond to 5 nm. (e) Azimuthally integrated ED patterns of the parent CIS NCs and product CIGS NCs, and bulk roquesite CIS X-ray diffraction reference pattern (JPCDS PDF-card 00-047-1372). (f) Absorption and (g) PL spectra of the parent CIS NCs and product CIGS NCs. The inset in (f) shows the normalized absorption spectra. The PL quantum yield of the product CIGS NCs is at least 10 times higher than that of the parent CIS NCs. The dip in the PL spectrum around 1150 nm is caused by absorption of emitted light by toluene, which was used as solvent.

The optical properties of the template CIS NCs and the product Ga-containing (CIGS) NCs are shown in Figure 8f,g. Both the absorption (Figure 8f) and photoluminescence (PL, Figure 8g) spectra show a blue shift after the CE reaction, indicating a widening of the band gap. This can be attributed to either an increase in quantum confinement due to a smaller effective CIS core size, (2,33) in case a CIS/CIGS core/shell NC is formed, or by the intrinsic increase in band gap, (63,64) in case a homogeneous CIGS alloy is formed, as the band gap of bulk CuGaS₂ is larger than that of CuInS₂ (2.50 and 1.55 eV, respectively). (1−3) Our results, however, do not allow a distinction between the two cases to be made. The PL quantum yield (QY) of the product CIGS NCs increases by 1 order of magnitude in comparison to that of the parent CIS NCs (Figure 8g). Such an increase in PLQY could originate from a strong enhancement of the radiative recombination rates, from elimination of nonradiative recombination pathways, or from a combination of both. Considering that similar PLQY enhancements have been previously observed upon shelling of CIS NCs with both ZnS and CdS, and attributed to passivation of nonradiative recombination centers at the surface, (1,65−67) it is likely that the higher PLQY of the CIGS NCs obtained by cation exchange is also due to improved surface passivation. It is interesting to note that PL of CIGS NCs is generally not reported, (26,68) suggesting that the exciton recombination in these materials tends to be dominated by nonradiative decay pathways, in striking contrast to the (Cu-poor) CIGS NCs prepared in our work by postsynthetic Cu⁺ for Ga³⁺ cation exchange in CIS NCs. The scarce literature on the PL of CIGS NCs makes it difficult to unambiguously identify the radiative recombination mechanism in these materials. However, their large global Stokes shift and broad PL peak suggest that, similar to CIS and other I–III–VI₂ NCs, the PL in these nanomaterials most likely originates from the radiative recombination of a delocalized conduction band electron with a hole localized at a Cu(I) ion. (69) Time-resolved spectroscopic measurements could provide additional insights into the origin of the radiative recombination in these nanomaterials but are beyond the scope of the present work. It should be noted that the PL of these Pb- and Cd-free NCs is centered in the NIR biological window. (65) This, in combination with their high PLQY, makes them promising candidates for bioimaging applications. Most importantly, the results discussed above show that the CE mechanism proposed in the present work is widely applicable.

Conclusions

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Our work shows the importance of reactant chemistry in aliovalent cation exchange reactions in Cu_2–xS nanocrystals. Noncomplexed GaCl₃ reacts as Lewis acid, extracting S^2– from the nanocrystals, thereby etching them. Etching is inhibited when GaCl₃ is bound to a phosphine ligand in a GaCl₃–L complex (L = trioctylphosphine, triphenylphosphite, and diphenylphosphine). GaCl₃–TOP was found to be unreactive, whereas GaCl₃–TPP and GaCl₃–DPP lead to successful exchange of Cu⁺ for Ga³⁺, converting the parent Cu_2–xS nanocrystals into CuGaS₂ nanocrystals. This is rationalized by considering that the activated Cu_2–xS–GaCl₃–L complex that forms at the surface of the nanocrystal upon adsorption of the GaCl₃–L complex must allow for simultaneous Ga³⁺ insertion and Cu⁺ extraction, while preventing S^2– extraction. This requirement is only fulfilled by activated complexes in which the Ga–P bond is sufficiently weak. DFT calculations show that the Ga–P bond strength decreases with increasing electron-withdrawing nature of the side groups of the phosphine ligand. Our findings highlight several factors (viz., Lewis acidity, influence of ligand electronic properties on reactivity) which need be taken into account when designing cation exchange protocols. Finally, we showed that the mechanism proposed here is generally applicable by using GaCl₃–DPP to convert CuInS₂ nanocrystals into Cu-poor (Cu,Ga,In)S₂ nanocrystals with an increased photoluminescence quantum yield in the NIR spectral window.

Methods

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Materials

Copper(II) sulfate pentahydrate (CuSO₄·5H₂O, 99.995%), oleic acid (OA, tech., 90%), 1-dodecanethiol (DDT, ≥98%), oleylamine (tech., 70%), gallium(III) chloride (GaCl₃, anhydrous, beads), trioctylphosphine (90%), diphenylphosphine (98%), triphenylphosphite (97%), indium(III) acetate (In(Ac)₃, 99.99%), copper(I) iodide (CuI, 98%), anhydrous methanol, anhydrous butanol, and anhydrous toluene were all purchased from Sigma-Aldrich and used as supplied, except for OA and OLAM, which were degassed prior to use.

Synthesis of Cu_2–xS Bifrustum Nanocrystals

Cu_2–xS bifrustum nanocrystals were prepared according to the method described in the literature. (16) Typically, 0.203 g of CuSO₄·5H₂O, 6 mL of OA, and 7.5 mL of DDT were brought into a three-neck round-bottom flask, connected to a Schlenk line. Under constant stirring and nitrogen atmosphere, the mixture was heated to 200 °C at a rate of about 30 °C/min. After reaction for 2 h at 200 °C, the mixture was allowed to cool to approximately 110 °C, when it was quenched by injection of 5 mL of toluene. The product was precipitated by addition of a mixture of methanol and butanol as nonsolvent and isolated by centrifugation at 2500 rpm for 10 min. The supernatant was removed, and the residue was redispersed in toluene. The mass of product was estimated by weighing the empty sample vials prior to sample retrieval and after removal of the supernatant. The amount of toluene used to redisperse the particles was chosen such that a concentration of approximately 20 g/L was obtained.

Preparation of Cation Exchange Precursors

Ga– and In–phosphine precursors were prepared in a way similar to our previously reported method. (18)

InCl₃–TOP was prepared in a N₂-filled glovebox, by addition of 1.250 mL (2.8 mmol) of TOP to an equimolar amount (0.620 g) of InCl₃, followed by heating to 50 °C under constant stirring. The resulting turbid, white, viscous liquid was diluted by addition of 1.250 mL of toluene.

GaCl₃–TOP was prepared in a N₂-filled glovebox, by addition of 0.500 mL (1.12 mmol) TOP to an equimolar amount (0.197 g) of GaCl₃, followed by heating to 50 °C under constant stirring, yielding a transparent, orange, viscous liquid.

GaCl₃–DPP was prepared in a N₂-filled glovebox, by addition of 0.815 mL (4.683 mmol) DPP to an equimolar amount (0.8246 g) of GaCl₃, followed by heating to 50 °C under constant stirring. After a few minutes, an opaque, white, viscous liquid formed. Over the course of several hours, the liquid changed into an opaque, white, crystalline solid, which was dissolved in 10 mL of toluene at 100 °C.

GaCl₃–TPP was prepared in a N₂-filled glovebox, by addition of 1.125 mL (4.281 mmol) TPP to an equimolar amount (0.7538 g) of GaCl₃, followed by heating to 50 °C under constant stirring. After a few minutes, an opaque, white, crystalline solid formed, which was dissolved in 10 mL of toluene at 100 °C.

GaCl₃ cation exchange precursor was prepared in a N₂-filled glovebox, by making a 1 mol/L (176.08 g/L) solution of GaCl₃ in toluene, resulting in a transparent, bright green liquid. It was found that the solution degraded over time, even when stored under inert atmosphere. Degradation was indicated by a change of color from bright green to dark red. In all reactions with GaCl₃ as precursor, a fresh batch of precursor was used.

Cation Exchange in Cu_2–xS Bifrustum Nanocrystals

The cation exchange reactions were performed following the method reported in ref (18) inside a N₂-filled glovebox. The equivalent of 0.2 mmol cation exchange precursor (0.200 mL of InCl₃–TOP, 0.100 mL of GaCl₃–TOP, 0.425 mL of GaCl₃–DPP, 0.465 mL of GaCl₃–TPP, 0.200 mL of GaCl₃ solution) was placed in a vial. Toluene was added to make the total reaction volume after addition of the NC dispersion 2.700 mL (added volume: with InCl₃–TOP, 2.000 mL; with GaCl₃–TOP, 2.100 mL; with GaCl₃–DPP, 1.775 mL; with GaCl₃–TPP, 1.735 mL; with GaCl₃, 2.000 mL). The mixture of toluene and cation exchange precursor was heated to the reaction temperature (100 °C with InCl₃–TOP, GaCl₃–TOP, GaCl₃–DPP, GaCl₃–TPP and 30 °C with GaCl₃) under constant stirring at 400 rpm. Once the desired temperature was reached, 0.500 mL of parent NC dispersion (equivalent to approximately 10 mg of parent particles) was swiftly injected. The reaction was stopped (reaction time: with InCl₃–TOP, overnight; with GaCl₃–TOP, overnight; with GaCl₃–TPP, 60 min; with GaCl₃–DPP, 60 min; with GaCl₃, 15, 30, 60, 120, and 300 min) by removing the vial from the heater and quenched by the addition of methanol and butanol as nonsolvents. All samples were washed at least twice prior to analysis, by addition of methanol and butanol as nonsolvent, centrifugation and removal of the supernatant. Finally, the particles were redispersed in toluene.

Cation Exchange under Milder Reaction Conditions with GaCl₃–DPP

To prevent particle agglomeration, the reaction of Cu_2–xS nanocrystals with GaCl₃–DPP was also performed under milder reaction conditions. This reaction was performed at half the scale of the reactions discussed above, inside a N₂-filled glovebox. To this end, 0.1 mmol (0.210 mL) GaCl₃–DPP, 0.860 mL of toluene, and 0.025 mL of OLAM were placed in a vial. The mixture was heated to 50 °C, under constant stirring at 400 rpm. Once the desired temperature was reached, 0.250 mL of parent NC dispersion (equivalent to approximately 5 mg of parent particles) was swiftly injected. The reaction was stopped by removing the vial from the heater and quenched by the addition of methanol and butanol as nonsolvents. The product was washed twice prior to analysis, by addition of methanol and butanol as nonsolvent, centrifugation, and removal of the supernatant. Finally, the particles were redispersed in toluene.

Synthesis of CuInS₂ NCs

For the synthesis of luminescent CuInS₂ (CIS) NCs a protocol reported earlier by Li et al. (66) was adapted. Briefly, 297 mg of In(Ac)₃, 191 mg of CuI, and 5 mL of DDT were mixed and degassed under vacuum for 1 h at 80 °C. The reaction mixture was then heated under N₂ flow to 230 °C and allowed to react for 100 min. The reaction was stopped by removing the heating mantle. The NCs were precipitated with a methanol/butanol 1/1 mixture and, after centrifugation and decanting the supernatant, dispersed in 10 mL of toluene.

Cation Exchange in CuInS₂ NCs

CIS NCs (0.250 mL) in toluene were diluted in a vial with 1 mL of toluene and mixed with 0.1 mL of DDT (extra ligand) and 0.1 mL of GaCl₃–DPP (prepared as described above, however, not dissolved in toluene), which was melted at a heating plate with a plate temperature of 100 °C. The vial containing this mixture was placed on the heating plate with T_plate = 100 °C for 30 min, after which the vial was removed and the NCs were precipitated with a methanol/butanol 1/1 mixture and, after centrifugation and decanting the supernatant, dispersed in 0.2 mL of toluene.

Optical Spectroscopy

Samples were prepared by placing a diluted dispersion of nanocrystals in toluene in a quartz cuvette with 10 mm path length. Absorption spectra were measured using a PerkinElmer Lambda 16 UV–vis–NIR spectrometer. Photoluminescence spectra were measured on an Edinburgh Instruments FLS920 spectrofluorimeter equipped with a Hamamatsu R5509-72 PMT with a monochromator grated at 1200 nm. A 900 W Xe lamp was used as the excitation source.

Transmission Electron Microscopy

TEM images were recorded on Thermo Fisher Scientific (formerly Philips/FEI) Tecnai 10, Tecnai 12, and Tecnai 20 electron microscopes. TEM samples were prepared by drop-casting a dispersion of particles in toluene onto a copper TEM grid, with a carbon-coated polymer support film.

Energy-Dispersive X-ray Spectroscopy

EDS measurements were performed on Thermo Fisher Scientific Tecnai 20F and Talos F200X electron microscopes using Tecnai Imaging and Analysis software. Sample preparation was similar to the preparation of TEM samples. In all cases, aluminum TEM grids were used. A low-background sample holder was used. Measurements were performed on areas with >500 nanoparticles. Discussed elemental compositions are normalized to the stoichiometric sulfur contents.

Elemental Mapping

Elemental mapping was performed on a Talos F200X electron microscope equipped with a 200 kV XFEG. X-ray detection was done using four symmetrically placed SuperX-EDX detectors. Imaging was performed in high-angle annular dark-field scanning transmission electron microscopy mode, with a current of 700 pA using a Fischione Instruments HAADF detector. Obtained data were analyzed using Bruker Esprit software.

HAADF-STEM Tomography

Tilt series for HAADF-STEM tomography were acquired with a FEI Tecnai Osiris operated at 200 kV in combination with an advanced tomography holder from Fischione Instruments and the FEI XPlore3D acquisition software. Tilt series consisting of 31 HAADF-STEM images were acquired with tilt increments of 5° over a range of ±75°. Alignment of the data was carried out using the FEI Inspect3D software package. The reconstruction was performed using the simultaneous iterative reconstruction technique, with 25 iterations, implemented in Inspect3D. Amira (Visage Imaging GmbH) was used for the visualization of the reconstructed volume.

Electron Diffraction

Due to the large amount of sample required for X-ray diffraction (XRD) measurements and low signal-to-noise ratio in XRD measurements for the samples presented in this work, ED was used instead to determine the crystal structure and crystallinity of the samples. ED patterns were measured on a Tecnai 12 electron microscope with an acceleration voltage of 120 kV at a camera length of 1 m.

Data processing of ED patterns measured on Cu_2–xS NCs and the particles obtained after reaction of these parent NCs with cation exchange precursors was performed as follows: measured two-dimensional patterns were reduced to one dimension by azimuthal integration of the patterns around the center, which was manually defined for every pattern. The measured ED patterns were calibrated to a gold reference sample (Agar S106). The calibration procedure is described in detail in the Supporting Information.

Data processing of ED patterns measured on CIS NCs and the particles obtained after reaction of these parent NCs with cation exchange precursors, was performed using the Crystallographic Tool Box (CrysTBox) software, (70) which automatically performed the required steps of center localization, azimuthal integration, and calibration to a gold reference sample.

Calculated Diffraction Patterns

Diffraction patterns for the low-chalcocite Cu_2–xS and wurtzite CuGaS₂ crystal structures were calculated from the relevant crystal structures (for low-chalcocite Cu_2–xS, the crystal structure reported in ref (34); for wurtzite CGS, the crystal structure reported in ref (44)) using the Visualization for Electronic and Structural Analysis (VESTA) software program (version 3.3.9), (71) using the default settings. The calculated lattice distances in real space, d, were transformed into distances in reciprocal space, q, using the equation q = 2π/d.

Comparison of the Anion Sublattices

Using the VESTA software program, (71) all cations were removed from the crystal structures of low-chalcocite Cu_2–xS, (34) wurtzite CGS, (44) and monoclinic α′-Ga₂S₃, (46) and a supercell (consisting of multiple unit cells) was constructed. Data corresponding to a single hexagonal layer and a single ...ABAB... layer was exported for each crystal structure. The anion coordinates were translated and rotated to orient the three data sets in the same manner. In this common orientation, the x, y, and z directions correspond to the following crystallographic directions: low-chalcocite Cu_2–xS: [201],[010],[001]; wurtzite CuGaS₂: [11̅0],[110],[001]; monoclinic α′-Ga₂S₃: [010],[100],[102]. Lattice distances Δx, Δy, and Δz were measured over 5, 8, and 5 anion–anion distances in x, y, and z, respectively. Relative lattice contraction values, corresponding to a transformation of lattice A into B, are defined as

(and similarly for the y and z directions).

DFT Calculations

All DFT calculations were performed using the Gaussian 09, revision D.01, software package. (72) Geometry optimizations were performed using the B3LYP (Becke, three-parameter, Lee–Yang–Parr) functional, the relativistic Stuttgart–Dresden (SDD) pseudopotential and basis set on Ga and In and the 6-311+G(d,p) basis set on all other atoms. The structures were optimized without any symmetry restraints. Frequency analyses at the same level were performed on all calculations to verify that the obtained stationary points are in fact energy minima. Thermochemical analysis was performed using a temperature of 298.15 K. Depicted visualizations of the geometry-optimized structures were prepared using the VESTA software program. (71)

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b05337.

NC size histograms, EDS of NCs before and after CE reaction, absorption spectra of NCs before and after CE, TEM images of NCs after reaction with GaCl₃ and GaCl₃–DPP at 100 °C, HAADF-STEM images and EDS elemental maps of Cu_2–xS NCs after reaction with GaCl₃, detailed description of PED calibration procedure (PDF)
File showing the results of the DFT calculations (ground-state geometries and bond enthalpies for the phosphine complexes used in our work (XYZ)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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Corresponding Author
- Celso de Mello Donega - Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands; http://orcid.org/0000-0002-4403-3627; Email: [email protected]
Authors
- Stijn O. M. Hinterding - Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands; Present Address: S.O.M.H.: Soft Condensed Matter/Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands; http://orcid.org/0000-0002-3940-1253
- Anne C. Berends - Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands; Present Address: A.C.B.: Seaborough Research B.V., Science Park 106, 1098XG Amsterdam, The Netherlands
- Mert Kurttepeli - Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium; Present Address: M.K.: Foseco Nederland B.V., Pantheon 30, 7521PR Enschede, The Netherlands
- Marc-Etienne Moret - Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
- Johannes D. Meeldijk - Electron Microscopy Utrecht, Debye Institute for Nanomaterials Science, Utrecht University, 3584 CH Utrecht, The Netherlands
- Sara Bals - Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium; http://orcid.org/0000-0002-4249-8017
- Ward van der Stam - Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands; Present Address: W.v.d.S.: Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, Universiteitsweg 99, 3584CG Utrecht, The Netherlands; http://orcid.org/0000-0001-8155-5400
Notes
The authors declare no competing financial interest.

Acknowledgments

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S.O.M.H., W.v.d.S., A.C.B., and C.d.M.D. acknowledge financial support from the division of Chemical Sciences (CW) of The Netherlands Organization for Scientific Research (NWO) under Grant Nos. ECHO.712.012.0001 and ECHO.712.014.001. S.B. acknowledges financial support from the European Research Council (ERC Consolidator Grant No. 815128-REALNANO). S.O.M.H. is supported by The Netherlands Center for Multiscale Catalytic Energy Conversion (MCEC), an NWO Gravitation Programme funded by the Ministry of Education, Culture and Science of the government of The Netherlands. DFT calculations were carried out on the Dutch national e-infrastructure with the support of SURF Cooperative. This work was sponsored by NWO Physical Sciences for the use of supercomputer facilities. The authors thank Jessi van der Hoeven for EDS and TEM measurements.

References

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This article references 72 other publications.

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A review. Cation exchange is an age-old technique for the chem. conversion of liqs. or extended solids by place-exchanging the cations in an ionic material with a different set of cations. The technique is undergoing a major revival with the advent of high-quality nanocrystals: researchers are now able to overcome the limitations in bulk systems and fully exploit cation exchange for materials synthesis and discovery via rapid, low-temp. transformations in the solid state. In this tutorial review, we discuss cation exchange as a promising materials synthesis and discovery tool. Exchange on the nanoscale exhibits some unique attributes: rapid kinetics at room temp. (orders of magnitude faster than in the bulk) and the tuning of reactivity via control of nanocrystal size, shape, and surface faceting. These features make cation exchange a convenient tool for accessing nanocrystal compns. and morphologies for which conventional synthesis may not be established. A simple exchange reaction allows extension of nanochem. to a larger part of the periodic table, beyond the typical gamut of II-VI, IV-VI, and III-V materials. Cation exchange transformations in nanocrystals can be topotactic and size- and shape-conserving, allowing nanocrystals synthesized by conventional methods to be used as templates for prodn. of compositionally novel, multicomponent, or doped nanocrystals. Since phases and compns. resulting from an exchange reaction can be kinetically controlled, rather than governed by the phase diagram, nanocrystals of metastable and hitherto inaccessible compns. are attainable. Outside of materials synthesis, applications for cation exchange exist in water purifn., chem. staining, and sensing. Since nanoscale cation exchange occurs rapidly at room temp., it can be integrated with sensitive environments such as those in biol. systems. Cation exchange is already allowing access to a variety of new materials and processes. With better mechanistic understanding and control, researchers may be able to advance the field to a stage where a custom nanostructure of arbitrary complexity would be achievable by simple cation exchange chem. and a basic understanding of the periodic table.
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De Trizio, Luca; Manna, Liberato
Chemical Reviews (Washington, DC, United States) (2016), 116 (18), 10852-10887CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. Among the various postsynthesis treatments of colloidal nanocrystals that have been developed to date, transformations by cation exchange have recently emerged as an extremely versatile tool that has given access to a wide variety of materials and nanostructures. One notable example in this direction is represented by partial cation exchange, by which preformed nanocrystals can be either transformed to alloy nanocrystals or to various types of nanoheterostructures possessing core/shell, segmented, or striped architectures. This review provides an up to date overview of the complex colloidal nanostructures that could be prepd. so far by cation exchange. At the same time, the review gives an account of the fundamental thermodn. and kinetic parameters governing these types of reactions, as they are currently understood, and outlines the main open issues and possible future developments in the field.
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Sahu, A.; Kang, M. S.; Kompch, A.; Notthoff, C.; Wills, A. W.; Deng, D.; Winterer, M.; Frisbie, C. D.; Norris, D. J. Electronic Impurity Doping in CdSe Nanocrystals. Nano Lett. 2012, 12, 2587– 2594, DOI: 10.1021/nl300880g
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Electronic Impurity Doping in CdSe Nanocrystals
Sahu, Ayaskanta; Kang, Moon Sung; Kompch, Alexander; Notthoff, Christian; Wills, Andrew W.; Deng, Donna; Winterer, Markus; Frisbie, C. Daniel; Norris, David J.
Nano Letters (2012), 12 (5), 2587-2594CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
We dope CdSe nanocrystals with Ag impurities and investigate their optical and elec. properties. Doping leads not only to dramatic changes but surprising complexity. The addn. of just a few Ag atoms per nanocrystal causes a large enhancement in the fluorescence, reaching efficiencies comparable to core-shell nanocrystals. While Ag was expected to be a substitutional acceptor, nonmonotonic trends in the fluorescence and Fermi level suggest that Ag changes from an interstitial (n-type) to a substitutional (p-type) impurity with increased doping.
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Van der Stam, W.; Geuchies, J. J.; Altantzis, T.; Van den Bos, K. H. W.; Meeldijk, J. D.; Van Aert, S.; Bals, S.; Vanmaekelbergh, D.; de Mello Donegá, C. Highly Emissive Divalent Ion Doped Colloidal CsPb_1–xM_xBr₃ Perovskite Nanocrystals through Cation Exchange. J. Am. Chem. Soc. 2017, 139, 4087– 4097, DOI: 10.1021/jacs.6b13079
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Highly Emissive Divalent-Ion-Doped Colloidal CsPb1-xMxBr3 Perovskite Nanocrystals through Cation Exchange
van der Stam, Ward; Geuchies, Jaco J.; Altantzis, Thomas; van den Bos, Karel H. W.; Meeldijk, Johannes D.; Van Aert, Sandra; Bals, Sara; Vanmaekelbergh, Daniel; de Mello Donega, Celso
Journal of the American Chemical Society (2017), 139 (11), 4087-4097CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
A method is presented that allows partial cation exchange in colloidal CsPbBr3 nanocrystals (NCs), whereby Pb2+ is exchanged for several isovalent cations, resulting in doped CsPb1-xMxBr3 NCs (M = Sn2+, Cd2+, and Zn2+; 0 < x ≤ 0.1), with preservation of the original NC shape. The size of the parent NCs is also preserved in the product NCs, apart from a small (few %) contraction of the unit cells upon incorporation of the guest cations. The partial Pb2+ for M2+ exchange leads to a blue-shift of the optical spectra, while maintaining the high luminescence quantum yields (>50%), sharp absorption features, and narrow emission of the parent CsPbBr3 NCs. The blue-shift in the optical spectra is attributed to the lattice contraction that accompanies the Pb2+ for M2+ cation exchange and is obsd. to scale linearly with the lattice contraction. This work opens up new possibilities to engineer the properties of halide perovskite NCs, which to date are the only known system where cation and anion exchange reactions can be sequentially combined while preserving the original NC shape, resulting in compositionally diverse perovskite NCs.
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Eilers, J.; Groeneveld, E.; de Mello Donegá, C.; Meijerink, A. Optical Properties of Mn-Doped ZnTe Magic Size Nanocrystals. J. Phys. Chem. Lett. 2012, 3, 1663– 1667, DOI: 10.1021/jz300300g
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Optical Properties of Mn-Doped ZnTe Magic Size Nanocrystals
Eilers, Joren; Groeneveld, Esther; de Mello Donega, Celso; Meijerink, Andries
Journal of Physical Chemistry Letters (2012), 3 (12), 1663-1667CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
The authors report successful doping of ZnTe magic size nanocrystals (MSNCs) with Mn2+. Colloidal ZnTe MSNCs are prepd. via a hot-injection method and doped with Mn2+ via cation exchange. The doped MSNCs show an emission band centered at 620 nm with a radiative decay time of 45 μs, characteristic of Mn2+ in ZnTe. The excitation spectrum of the Mn2+ emission shows narrow absorption bands corresponding to different sizes of ZnTe MSNCs providing further evidence that the 620 nm emission originates from Mn2+ incorporated in the ZnTe host, rather than Mn2+ bound to the surface. The Mn2+-doped ZnTe clusters may serve as nuclei for the growth of larger ZnTe quantum dots doped with a single Mn2+ ion.
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Groeneveld, E.; Witteman, L.; Lefferts, M.; Ke, X.; Bals, S.; Van Tendeloo, G.; de Mello Donegá, C. Tailoring ZnSe-CdSe Colloidal Quantum Dots via Cation Exchange: From Core/Shell to Alloy Nanocrystals. ACS Nano 2013, 7, 7913– 7930, DOI: 10.1021/nn402931y
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Tailoring ZnSe-CdSe Colloidal Quantum Dots via Cation Exchange: From Core/Shell to Alloy Nanocrystals
Groeneveld, Esther; Witteman, Leon; Lefferts, Merel; Ke, Xiaoxing; Bals, Sara; Van Tendeloo, Gustaaf; de Mello Donega, Celso
ACS Nano (2013), 7 (9), 7913-7930CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
We report a study of Zn2+ by Cd2+ cation exchange (CE) in colloidal ZnSe nanocrystals (NCs). Our results reveal that CE in ZnSe NCs is a thermally activated isotropic process. The CE efficiency (i.e., fraction of Cd2+ ions originally in soln., Cdsol, that is incorporated in the ZnSe NC) increases with temp. and depends also on the Cdsol/ZnSe ratio. Interestingly, the reaction temp. can be used as a sensitive parameter to tailor both the compn. and the elemental distribution profile of the product (Zn,Cd)Se NCs. At 150° C ZnSe/CdSe core/shell hetero-NCs (HNCs) are obtained, while higher temps. (200 and 220° C) produce (Zn1-xCdx)Se gradient alloy NCs, with increasingly smoother gradients as the temp. increases, until homogeneous alloy NCs are obtained at T ≥ 240° C. Remarkably, sequential heating (150° C followed by 220° C) leads to ZnSe/CdSe core/shell HNCs with thicker shells, rather than (Zn1-xCdx)Se gradient alloy NCs. Thermal treatment at 250° C converts the ZnSe/CdSe core/shell HNCs into (Zn1-xCdx)Se homogeneous alloy NCs, while preserving the NC shape. A mechanism for the cation exchange in ZnSe NCs is proposed, in which fast CE takes place at the NC surface, and is followed by relatively slower thermally activated solid-state cation diffusion, which is mediated by Frenkel defects. The findings presented here demonstrate that cation exchange in colloidal ZnSe NCs provides a very sensitive tool to tailor the nature and localization regime of the electron and hole wave functions and the optoelectronic properties of colloidal ZnSe-CdSe NCs.
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Grodzińska, D.; Pietra, F.; Van Huis, M. A.; Vanmaekelbergh, D.; de Mello Donegá, C. Thermally Induced Atomic Reconstruction of PbSe/CdSe Core/Shell Quantum Dots into PbSe/CdSe Bi-Hemisphere Hetero-Nanocrystals. J. Mater. Chem. 2011, 21, 11556– 11565, DOI: 10.1039/c0jm04458j
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Thermally induced atomic reconstruction of PbSe/CdSe core/shell quantum dots into PbSe/CdSe bi-hemisphere hetero-nanocrystals
Grodzinska, Dominika; Pietra, Francesca; van Huis, Marijn A.; Vanmaekelbergh, Daniel; Donega, Celso de Mello
Journal of Materials Chemistry (2011), 21 (31), 11556-11565CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
The properties of hetero-nanocrystals (HNCs) depend strongly on the mutual arrangement of the nanoscale components. The structural and morphol. evolution of colloidal PbSe/CdSe core/shell quantum dots upon annealing under vacuum was studied. Prior to annealing the PbSe core has an approx. octahedral morphol. with 8 {111} facets, and the CdSe shell has Zn-blende crystal structure. Thermal annealing under vacuum at 150-200° induces a structural and morphol. reconstruction of the HNCs whereby the PbSe core and the CdSe shell are reorganized into 2 hemispheres joined by a common {111} Se plane. This thermally induced reconstruction leads to considerable changes in the optical properties of the colloidal PbSe/CdSe HNCs.
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Fan, Z.; Lin, L.-C.; Buijs, W.; Vlugt, T. J. H.; Van Huis, M. A. Atomistic Understanding of Cation Exchange in PbS Nanocrystals Using Simulations with Pseudoligands. Nat. Commun. 2016, 7, 11503, DOI: 10.1038/ncomms11503
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Atomistic understanding of cation exchange in PbS nanocrystals using simulations with pseudoligands
Fan, Zhaochuan; Lin, Li-Chiang; Buijs, Wim; Vlugt, Thijs J. H.; van Huis, Marijn A.
Nature Communications (2016), 7 (), 11503CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
Cation exchange is a powerful tool for the synthesis of nanostructures such as core-shell nanocrystals, however, the underlying mechanism is poorly understood. Interactions of cations with ligands and solvent mols. are systematically ignored in simulations. Here, we introduce the concept of pseudoligands to incorporate cation-ligand-solvent interactions in mol. dynamics. This leads to excellent agreement with exptl. data on cation exchange of PbS nanocrystals, whereby Pb ions are partially replaced by Cd ions from soln. The temp. and the ligand-type control the exchange rate and equil. compn. of cations in the nanocrystal. Our simulations reveal that Pb ions are kicked out by exchanged Cd interstitials and migrate through interstitial sites, aided by local relaxations at core-shell interfaces and point defects. We also predict that high-pressure conditions facilitate strongly enhanced cation exchange reactions at elevated temps. Our approach is easily extendable to other semiconductor compds. and to other families of nanocrystals.
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Hewavitharana, I. K.; Brock, S. L. When Ligand Exchange Leads to Ion Exchange: Nanocrystal Facets Dictate the Outcome. ACS Nano 2017, 11, 11217– 11224, DOI: 10.1021/acsnano.7b05534
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When Ligand Exchange Leads to Ion Exchange: Nanocrystal Facets Dictate the Outcome
Hewavitharana, Indika K.; Brock, Stephanie L.
ACS Nano (2017), 11 (11), 11217-11224CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
This study demonstrates that ligand exchange of nanocrystals (NCs) is not always an innocuous process, but can lead to facile (room temp.) ion exchange, depending on the surface crystal faceting. Rock salt PbTe NCs prepd. as cubes with neutral facets undergo room-temp. ligand exchange with sulfide ions, whereas cuboctahedron-shaped particles with neutral {100} and polar {111} facets are transformed to PbS, driven by ion exchange along the 〈111〉 direction. Likewise, cation exchange (with Ag+) occurs rapidly for cuboctahedra, whereas cubes remain inert. This dramatic difference is attributed to the relative surface area of {111} facets that promote rapid ion exchange and shows how facet engineering is a powerful knob for the control of reaction pathways in nanoparticles.
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Van der Stam, W.; Berends, A. C.; Rabouw, F. T.; Willhammar, T.; Ke, X.; Meeldijk, J. D.; Bals, S.; de Mello Donegá, C. Luminescent CuInS₂ Quantum Dots by Partial Cation Exchange in Cu_2-xS Nanocrystals. Chem. Mater. 2015, 27, 621– 628, DOI: 10.1021/cm504340h
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Luminescent CuInS2 Quantum Dots by Partial Cation Exchange in Cu2-xS Nanocrystals
van der Stam, Ward; Berends, Anne C.; Rabouw, Freddy T.; Willhammar, Tom; Ke, Xiaoxing; Meeldijk, Johannes D.; Bals, Sara; de Mello Donega, Celso
Chemistry of Materials (2015), 27 (2), 621-628CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
Partial cation exchange reactions in Cu2-xS nanocrystals (NCs) yield luminescent CuInS2 (CIS) NCs. The approach of mild reaction conditions ensures slow Cu extn. rates, which results in a balance with the slow In incorporation rate. With this method, the authors obtain CIS NCs with luminescence (PL) far in the near-IR (NIR), which cannot be directly synthesized by currently available synthesis protocols. The factors that favor partial, self-limited cation exchange from Cu2-xS to CIS NCs, rather than complete cation exchange to In2S3 are discussed. The product CIS NCs have the wurtzite crystal structure, which is understood in terms of conservation of the hexagonal close packing of the anionic sublattice of the parent NCs into the product NCs. These results are an important step toward the design of CIS NCs with sizes and shapes that are not attainable by direct synthesis protocols and may thus impact a no. of potential applications.
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Jharimune, S.; Sathe, A. A.; Rioux, R. M. Thermochemical Measurements of Cation Exchange in CdSe Nanocrystals Using Isothermal Titration Calorimetry. Nano Lett. 2018, 18, 6795– 6803, DOI: 10.1021/acs.nanolett.8b02661
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Thermochemical Measurements of Cation Exchange in CdSe Nanocrystals Using Isothermal Titration Calorimetry
Jharimune, Suprita; Sathe, Ajay A.; Rioux, Robert M.
Nano Letters (2018), 18 (11), 6795-6803CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
Among the various reported post synthetic modifications of colloidal nanocrystals, cation exchange (CE) is one of the most promising and versatile approaches for the synthesis of nanostructures that cannot be directly synthesized from their constitutive precursors. Numerous studies have reported on the qual. anal. of these reactions, but rigorous quant. study of the thermodn. of CE in colloidal nanoparticles is still lacking. We demonstrate using isothermal titrn. calorimetry (ITC), the thermodn. of the CE between cadmium selenide (CdSe) nanocrystals and silver in soln. can be quantified. We survey the influence of CdSe nanocrystal diam., capping ligands and temp. on the thermodn. of the exchange reaction. Results obtained from ITC provide a detailed description of overall thermodn. parameters-equil. const. (Keq), enthalpy (ΔH), entropy (ΔS) and stoichiometry (n)-of the exchange reaction. We compared the free energy change of reaction (ΔG) between CdSe and Ag+ obtained directly from ITC for both CdSe bulk and nanoparticles with values calcd. from previously reported methods. While the calcd. value is closer to the exptl. obtained ΔGrxn for bulk particles, nanocrystals show an addnl. Gibbs free energy stabilization of ∼-14 kJ/mol Se. We discuss a thermochem. cycle elucidating the steps involved in the overall cation exchange process. This work demonstrates the application of ITC to probe the thermochem. of nanoscale transformations under relevant soln. conditions.
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Van der Stam, W.; Bladt, E.; Rabouw, F. T.; Bals, S.; de Mello Donegá, C. Near-Infrared Emitting CuInSe₂/CuInS₂ Dot Core/Rod Shell Heteronanorods by Sequential Cation Exchange. ACS Nano 2015, 9, 11430– 11438, DOI: 10.1021/acsnano.5b05496
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Near-Infrared Emitting CuInSe2/CuInS2 Dot Core/Rod Shell Heteronanorods by Sequential Cation Exchange
van der Stam, Ward; Bladt, Eva; Rabouw, Freddy T.; Bals, Sara; Donega, Celso de Mello
ACS Nano (2015), 9 (11), 11430-11438CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
The direct synthesis of heteronanocrystals (HNCs) combining different ternary semiconductors is challenging and has not yet been successful. A sequential topotactic cation exchange (CE) pathway that yields CuInSe2/CuInS2 dot core/rod shell nanorods with near-IR luminescence is reported. The Cu+ extn. rate is coupled to the In3+ incorporation rate using a stoichiometric trioctylphosphine-InCl3 complex, which fulfills the roles of both In-source and Cu-extg. agent. In this way, Cu+ ions can be extd. by trioctylphosphine ligands only when the In-P bond is broken. This results in readily available In3+ ions at the same surface site from which the Cu+ is extd., making the process a direct place exchange reaction and shifting the overall energy balance in favor of the CE. Controlled cation exchange can occur even in large and anisotropic heterostructured nanocrystals with preservation of the size, shape, and heterostructuring of the template NCs into the product NCs. The cation exchange is self-limited, stopping when the ternary core/shell CuInSe2/CuInS2 compn. is reached. The method is very versatile, yielding a variety of luminescent CuInX2 (X = S, Se, and Te) quantum dots, nanorods, and HNCs, by using Cd-chalcogenide NCs and HNCs as templates. The approach reported here thus opens up routes toward materials with unprecedented properties, which would otherwise remain inaccessible.
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Wang, H.; Butler, D. J.; Straus, D. B.; Oh, N.; Wu, F.; Guo, J.; Xue, K.; Lee, J. D.; Murray, C. B.; Kagan, C. R. Air-Stable CuInSe₂ Nanocrystal Transistors and Circuits via Post-Deposition Cation Exchange. ACS Nano 2019, 13, 2324– 2333, DOI: 10.1021/acsnano.8b09055
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Air-Stable CuInSe2 Nanocrystal Transistors and Circuits via Post-Deposition Cation Exchange
Wang, Han; Butler, Derrick J.; Straus, Daniel B.; Oh, Nuri; Wu, Fengkai; Guo, Jiacen; Xue, Kun; Lee, Jennifer D.; Murray, Christopher B.; Kagan, Cherie R.
ACS Nano (2019), 13 (2), 2324-2333CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
Colloidal semiconductor nanocrystals (NCs) are a promising materials class for soln.-processable, next-generation electronic devices. However, most high-performance devices and circuits have been achieved using NCs contg. toxic elements, which may limit their further device development. We fabricate high mobility CuInSe2 NC field-effect transistors (FETs) using a soln.-based, post-deposition, sequential cation exchange process that starts with electronically coupled, thiocyanate (SCN)-capped CdSe NC thin films. First Cu+ is substituted for Cd2+ transforming CdSe NCs to Cu-rich Cu2Se NC films. Next, Cu2Se NC films are dipped into a Na2Se soln. to Se-enrich the NCs, thus compensating the Cu-rich surface, promoting fusion of the Cu2Se NCs, and providing sites for subsequent In-dopants. The liq.-coordination-complex trioctylphosphine-indium chloride (TOP-InCl3) is used as a source of In3+ to partially exchange and n-dope CuInSe2 NC films. We demonstrate Al2O3-encapsulated, air-stable CuInSe2 NC FETs with linear (satn.) electron mobilities of 8.2 ± 1.8 cm2/(V s) (10.5 ± 2.4 cm2/(V s)) and with current modulation of 105, comparable to that for high-performance Cd-, Pb-, and As-based NC FETs. The CuInSe2 NC FETs are used as building blocks of integrated inverters to demonstrate their promise for low-cost, low-toxicity NC circuits.
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Regulacio, M. D.; Ye, C.; Lim, S. H.; Zheng, Y.; Xu, Q.-H.; Han, M.-Y. Facile Noninjection Synthesis and Photocatalytic Properties of Wurtzite-Phase CuGaS₂ Nanocrystals with Elongated Morphologies. CrystEngComm 2013, 15, 5214– 5217, DOI: 10.1039/c3ce40352a
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Facile noninjection synthesis and photocatalytic properties of wurtzite-phase CuGaS2 nanocrystals with elongated morphologies
Regulacio, Michelle D.; Ye, Chen; Lim, Suo Hon; Zheng, Yuangang; Xu, Qing-Hua; Han, Ming-Yong
CrystEngComm (2013), 15 (26), 5214-5217CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
The authors present the colloidal prepn. of ternary CuGaS2 (CGS) nanocrystals that exhibit elongated morphologies and possess the metastable wurtzite crystal structure. A facile noninjection-based synthetic strategy was used wherein dithiocarbamate complexes of Cu and Ga were thermally decompd. in the presence of dodecanethiol. The anisotropic CGS nanocrystals were found to display promising photocatalytic behavior under visible-light illumination.
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Guijarro, N.; Prévot, M. S.; Yu, X.; Jeanbourquin, X. A.; Bornoz, P.; Bourée, W.; Johnson, M.; Le Formal, F.; Sivula, K. A Bottom-Up Approach toward All-Solution-Processed High-Efficiency Cu(In,Ga)S₂ Photocathodes for Solar Water Splitting. Adv. Energy Mater. 2016, 6, 1501949, DOI: 10.1002/aenm.201501949
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Lee, K.-H.; Kim, J.-H.; Jang, H. S.; Do, Y. R.; Yang, H. Quantum-Dot-Based White Lighting Planar Source through Downconversion by Blue Electroluminescence. Opt. Lett. 2014, 39, 1208– 1211, DOI: 10.1364/OL.39.001208
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Quantum-dot-based white lighting planar source through downconversion by blue electroluminescence
Lee, Ki-Heon; Kim, Jong-Hoon; Jang, Ho Seong; Do, Young Rag; Yang, Heesun
Optics Letters (2014), 39 (5), 1208-1211CODEN: OPLEDP; ISSN:0146-9592. (Optical Society of America)
We report the unprecedented fabrication of a planar white lighting quantum dot light-emitting diode (QD-LED) through integrating a CdZnS QD-based blue electroluminescence (EL) device with a free-standing polymethyl methacrylate (PMMA) composite film embedded with orange-emitting Cu-In-S (CIS) green-greenish yellow-emitting Cu-In-Ga-S (CIGS) QDs. The hybrid device successfully generates bicolored white emission that comprises blue EL and downconverted QD photoluminescence. The hybrid QD-LEDs loaded with the composite film embedded with one type of QDs exhibit a limited white spectral coverage, consequently producing low values (<65) in color rendering index (CRI). Thus, the QD-PMMA film consisting of a blend of green CIGS and orange CIS QD down-converters is applied for obtaining a higher-CRI white light through the spectral extension, resulting in a much improved CRI of 75-77. Various EL performances of the hybrid planar white device vs. the ref. blue QD-LED are also characterized in details.
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Kim, J.-H.; Lee, K.-H.; Jo, D.-Y.; Lee, Y.; Hwang, J. Y.; Yang, H. Cu-In-Ga-S Quantum Dot Composition-Dependent Device Performance of Electrically Driven Light-Emitting Diodes. Appl. Phys. Lett. 2014, 105, 133104, DOI: 10.1063/1.4896911
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Cu-In-Ga-S quantum dot composition-dependent device performance of electrically driven light-emitting diodes
Kim, Jong-Hoon; Lee, Ki-Heon; Jo, Dae-Yeon; Lee, Yangjin; Hwang, Jun Yeon; Yang, Heesun
Applied Physics Letters (2014), 105 (13), 133104/1-133104/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
Colloidal synthesis of ternary and quaternary quantum dots (QDs) of In/Ga ratio-varied CuIn1-xGaxS2 (CIGS) with nominal x = 0, 0.5, 0.7, and 1 and their application for the fabrication of quantum dot LEDs (QLEDs) are reported. Four QLEDs having CIGS QDs with different compns. are all soln.-processed in the framework of multilayered structure, where QD emitting layer is sandwiched by hybrid charge transport layers of poly(9-vinylcarbazole) and ZnO nanoparticles. The device performance such as luminance and efficiency is strongly dependent on the compn. of CIGS QDs, and well interpreted by the device energy level diagram proposed through the detn. of QD valence band min. by photoelectron emission spectroscopic measurement. (c) 2014 American Institute of Physics.
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Moon, S. H.; Park, S. J.; Kim, S. H.; Lee, M. W.; Han, J.; Kim, J. Y.; Kim, H.; Hwang, Y. J.; Lee, D.-K.; Min, B. K. Monolithic DSSC/CIGS Tandem Solar Cell Fabricated by a Solution Process. Sci. Rep. 2015, 5, 8970, DOI: 10.1038/srep08970
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Monolithic DSSC/CIGS tandem solar cell fabricated by a solution process
Moon, Sung Hwan; Park, Se Jin; Kim, Sang Hoon; Lee, Min Woo; Han, Jisu; Kim, Jin Young; Kim, Honggon; Hwang, Yun Jeong; Lee, Doh-Kwon; Min, Byoung Koun
Scientific Reports (2015), 5 (), 8970CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
Tandem architecture between org. (dye-sensitized solar cell, DSSC) and inorg. (CuInGaSe2 thin film solar cell, CIGS) single-junction solar cells was constructed particularly based on a soln. process. Arc-plasma deposition was employed for the Pt interfacial layer to minimize the damage to the layers of the CIGS bottom cell. Solar cell efficiency of 13% was achieved, which is significant progress from individual single-junction solar cells (e.g., 7.25 and 6.2% for DSSC and CIGS, resp.).
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Yang, Y.; Chen, Q.; Hsieh, Y.-T.; Song, T.-B.; De Marco, N.; Zhou, H.; Yang, Y. Multilayer Transparent Top Electrode for Solution Processed Perovskite/Cu(In,Ga)(Se,S)₂ Four Terminal Tandem Solar Cells. ACS Nano 2015, 9, 7714– 7721, DOI: 10.1021/acsnano.5b03189
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Multilayer Transparent Top Electrode for Solution Processed Perovskite/Cu(In,Ga)(Se,S)2 Four Terminal Tandem Solar Cells
Yang, Yang; Chen, Qi; Hsieh, Yao-Tsung; Song, Tze-Bin; Marco, Nicholas De; Zhou, Huanping; Yang, Yang
ACS Nano (2015), 9 (7), 7714-7721CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
Halide perovskites (PVSK) have attracted much attention in recent years due to their high potential as a next generation solar cell material. To further improve perovskites progress toward a state-of-the-art technol., it is desirable to create a tandem structure in which perovskite may be stacked with a current prevailing solar cell such as Si or Cu(In,Ga)(Se,S)2 (CIGS). The transparent top electrode is one of the key components as well as challenges to realize such tandem structure. Herein, the authors develop a multilayer transparent top electrode for perovskite photovoltaic devices delivering an 11.5% efficiency in top illumination mode. The transparent electrode is based on a dielec./metal/dielec. structure, featuring an ultrathin Au seeded Ag layer. A 4 terminal tandem solar cell employing soln. processed CIGS and perovskite cells is also demonstrated with over 15% efficiency.
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Zhao, J.; Zhang, J.; Wang, W.; Wang, P.; Li, F.; Ren, D.; Si, H.; Sun, X.; Ji, F.; Hao, Y. Facile Synthesis of CuInGaS₂ Quantum Dot Nanoparticles for Bilayer-Sensitized Solar Cells. Dalton Trans 2014, 43, 16588– 16592, DOI: 10.1039/C4DT02150A
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Facile synthesis of CuInGaS2 quantum dot nanoparticles for bilayer-sensitized solar cells
Zhao, Jinjin; Zhang, Jiangbin; Wang, Wenna; Wang, Peng; Li, Feng; Ren, Deliang; Si, Huanyan; Sun, Xiuguo; Ji, Fengqiu; Hao, Yanzhong
Dalton Transactions (2014), 43 (44), 16588-16592CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
CuIn0.7Ga0.3S2 quantum dots (QDs) with particle size of 2-5 nm were directly synthesized by a vacuum one-pot-nanocasting process and homogeneously anchored on TiO2 nanocrystals (<50 nm) for the first time. We further present CuIn0.7Ga0.3S2 quantum dots and dye bilayer-sensitized solar cells with a power conversion efficiency 36.3% higher than mono-dye sensitized solar cells.
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Dilena, E.; Xie, Y.; Brescia, R.; Prato, M.; Maserati, L.; Krahne, R.; Paolella, A.; Bertoni, G.; Povia, M.; Moreels, I.; Manna, L. CuIn_xGa_1-xS₂ Nanocrystals with Tunable Composition and Band Gap Synthesized via a Phosphine-Free and Scalable Procedure. Chem. Mater. 2013, 25, 3180– 3187, DOI: 10.1021/cm401563u
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CuInxGa1-xS2 Nanocrystals with Tunable Composition and Band Gap Synthesized via a Phosphine-Free and Scalable Procedure
Dilena, Enrico; Xie, Yi; Brescia, Rosaria; Prato, Mirko; Maserati, Lorenzo; Krahne, Roman; Paolella, Andrea; Bertoni, Giovanni; Povia, Mauro; Moreels, Iwan; Manna, Liberato
Chemistry of Materials (2013), 25 (15), 3180-3187CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
We report a phosphine-free colloidal synthesis of CuInxGa1-xS2 (CIGS) nanocrystals (NCs) by heating a mixt. of metal salts, elemental sulfur, octadecene, and oleylamine. In contrast with the more commonly used hot injection, this procedure is highly suitable for large-scale NC prodn., which we tested by performing a gram-scale synthesis. The compn. of the CIGS NCs could be tuned by varying the In and Ga precursor ratios, and the samples showed a compn.-dependent band gap energy. The av. particle size was scaled from 13 to 19 nm by increasing the reaction temp. from 230 to 270 °C. Two concomitant growth mechanisms took place: in one, covellite (CuS) NCs nucleated already at room temp. and then incorporated increasing amts. of In and Ga until they evolved into chalcopyrite CIGS NCs. In the second mechanism, CIGS NCs directly nucleated at intermediate temps. They were smaller than the NCs formed by the first mechanism, but richer in In and Ga. In the final sample, obtained by prolonged heating at 230-270 °C, all NCs were homogeneous in size and compn. Attempts to replace the native ligands on the surface of the NCs with sulfur ions (following literature procedures) resulted in only around 50% exchange. Films prepd. using the partially ligand-exchanged NCs exhibited good homogeneity and an ohmic dark cond. and photocond. with a resistivity of about 50 Ω·cm.
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Liu, Y.; Yin, D.; Swihart, M. T. Valence Selectivity of Cation Incorporation into Covellite CuS Nanoplatelets. Chem. Mater. 2018, 30, 1399– 1407, DOI: 10.1021/acs.chemmater.7b05198
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Valence selectivity of cation incorporation into covellite CuS nanoplatelets
Liu, Yang; Yin, Deqiang; Swihart, Mark T.
Chemistry of Materials (2018), 30 (4), 1399-1407CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
Synthesis of copper sulfide-based nanomaterials by cation incorporation into copper deficient copper sulfide (Cu2-xS) is of interest as a powerful means to obtain nanostructures with otherwise inaccessible combinations of size, shape, compn., and crystal phase. Incorporation of a heterocation (M) may produce heterogeneous Cu2-xS-MS nanocrystals (NCs) or homogeneous Cu-M-S alloys. However, the factors detg. whether heterogeneous NCs or homogeneous alloy NCs are produced have not been fully elucidated. In this report, we incorporate diverse cations into covellite CuS nanoplatelet (NPl) templates in the presence of dodecanethiol (DDT). These cations are categorized by their valencies. We demonstrate that trivalent and tetravalent cations can be incorporated into reduced CuS NPls to produce homogeneous ternary alloy NPls, while the divalent cations cannot coexist with Cu+ ions in the Cu2-xS phase. In turn, the incorporation of divalent cations leads to formation of heterogeneous NPls and finally produces copper-free metal sulfide NPls. The cation valence selectivity arises from conflicts between charge balance and coordination between Cu+ and divalent cations. This study not only provides better understanding of the relationship among the compn., morphol., and crystal structure of copper sulfide-based nanomaterials but also provides a pathway to controllable synthesis of complex nanostructures.
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Song, J.; Zhang, Y.; Dai, Y.; Hu, J.; Zhu, L.; Xu, X.; Yu, Y.; Li, H.; Yao, B.; Zhou, H. Polyelectrolyte-Mediated Nontoxic AgGa_xIn_1-xS₂ QDs/Low-Density Lipoprotein Nanoprobe for Selective 3D Fluorescence Imaging of Cancer Stem Cells. ACS Appl. Mater. Interfaces 2019, 11, 9884– 9892, DOI: 10.1021/acsami.9b00121
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Polyelectrolyte-Mediated Nontoxic AgGaxIn1-xS2 QDs/Low-Density Lipoprotein Nanoprobe for Selective 3D Fluorescence Imaging of Cancer Stem Cells
Song, Jiangluqi; Zhang, Yan; Dai, Yiwen; Hu, Jinhang; Zhu, Lixin; Xu, Xiaoliang; Yu, Yue; Li, Huan; Yao, Bo; Zhou, Huixin
ACS Applied Materials & Interfaces (2019), 11 (10), 9884-9892CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
Cancer stem cells, which are a population of cancer cells sharing common properties with normal stem cells, have strong self-renewal ability and multi-lineage differentiation potential to trigger tumor proliferation, metastases, and recurrence. From this, targeted therapy for cancer stem cells may be one of the most promising strategies for comprehensive treatment of tumors in the future. We design a facile approach to establish the colon cancer stem cells-selective fluorescent probe based on the low-d. lipoprotein (LDL) and the novel AgGaxIn(1-x)S2 quantum dots (AGIS QDs). The AGIS QDs with a high crystallinity are obtained for the first time via cation-exchange protocol of Ga3+ to In3+ starting from parent AgInS2 QDs. Photoluminescence peak of AGIS QDs can be turned from 502 to 719 nm by regulating the reaction conditions, with the highest quantum yield up to 37%. Subsequently, AGIS QDs-conjugated LDL nanocomposites (NCs) are fabricated, in which a cationic polyelectrolyte was used as a coupling reagent to guarantee the electrostatic self-assembly. The structural integrity and physicochem. properties of the LDL-QDs NCs are found to be maintained in vitro, and the NCs exhibit remarkable biocompatibility. The LDL-QDs can be selectively delivered into cancer stem cells that overexpress LDL receptor, and three-dimensional imaging of cancer stem cells is realized. The results of this study not only demonstrate the versatility of nature-derived lipoprotein nanoparticles, but also confirm the feasibility of electrostatic conjugation using cationic polyelectrolyte, allowing researchers to design nanoarchitectures for targeted diagnosis and treatment of cancer.
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Zhai, Y.; Flanagan, J. C.; Shim, M. Lattice Strain and Ligand Effects on the Formation of Cu_2–xS/I-III-VI₂ Nanorod Heterostructures through Partial Cation Exchange. Chem. Mater. 2017, 29, 6161– 6167, DOI: 10.1021/acs.chemmater.7b02392
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Lattice Strain and Ligand Effects on the Formation of Cu2-xS/I-III-VI2 Nanorod Heterostructures through Partial Cation Exchange
Zhai, You; Flanagan, Joseph C.; Shim, Moonsub
Chemistry of Materials (2017), 29 (14), 6161-6167CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
The authors report on the effects of lattice strain and the choice of ligands on the formation of Cu2-xS/ I-III-VI2 colloidal nanorod heterostructures through partial cation exchange starting from Cu2-xS nanorods. Lattice strain can induce alternating Cu2-xS/CuGaS2 segments along a colloidal nanorod if CuGaS2 can nucleate easily from the sides of the nanorods. The choice in coordinating ligands can alter this preference to favor tip nucleation, in which case the resulting heterostructure has CuGaS2/Cu2-xS/CuGaS2 rod/ rod/ rod geometry. In the less strained CuInS2 case, superlattice-like alternating segmentation does not occur but the ligand induced difference in the preference of where nucleation initiates can still lead to distinct heterostructure morphologies. These results demonstrate how surface accessibility varied by the choice of ligands can be exploited synergistically with the driving force that creates interfaces to provide synthetic control over nanoscale heterostructure formation.
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Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751– 767, DOI: 10.1107/S0567739476001551
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Pearson, R. G. Absolute Electronegativity and Hardness: Application to Inorganic Chemistry. Inorg. Chem. 1988, 27, 734– 740, DOI: 10.1021/ic00277a030
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Absolute electronegativity and hardness: application to inorganic chemistry
Pearson, Ralph G.
Inorganic Chemistry (1988), 27 (4), 734-40CODEN: INOCAJ; ISSN:0020-1669.
The recent concepts of abs. electronegativity, χ, and abs. hardness, η, are discussed. The operational definition, χ = (I + A)/2 and η = (I - A)/2, are used to calc. exptl. values for a large no. of cations, atoms, radicals, and mols. The resulting values are in good agreement with the chem. behavior both as to acid-base character and as to chem. hardness or softness. Anions are modeled by their corresponding radicals, and the importance of local softness for delocalized anions is pointed out. Applications of the use of tabulated values of η, both for rank ordering and in numerical calcn., are given.
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Xia, C.; Wu, W.; Yu, T.; Xie, X.; Van Oversteeg, C.; Gerritsen, H. C.; de Mello Donegá, C. Size-Dependent Band-Gap and Molar Absorption Coefficients of Colloidal CuInS₂ Quantum Dots. ACS Nano 2018, 12, 8350– 8361, DOI: 10.1021/acsnano.8b03641
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Size-Dependent Band-Gap and Molar Absorption Coefficients of Colloidal CuInS2 Quantum Dots
Xia, Chenghui; Wu, Weiwei; Yu, Ting; Xie, Xiaobin; van Oversteeg, Christina; Gerritsen, Hans C.; de Mello Donega, Celso
ACS Nano (2018), 12 (8), 8350-8361CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
The knowledge of the quantum dot (QD) concn. in a colloidal suspension and the quant. understanding of the size-dependence of the band gap of QDs are of crucial importance from both applied and fundamental viewpoints. In this work, we investigate the size-dependence of the optical properties of nearly spherical wurtzite (wz) CuInS2 (CIS) QDs in the 2.7 to 6.1 nm diam. range (polydispersity ≤10%). The QDs are synthesized by partial Cu+ for In3+ cation exchange in template Cu2-xS nanocrystals, which yields CIS QDs with very small compn. variations (In/Cu = 0.91 ± 0.11), regardless of their sizes. These well-defined QDs are used to investigate the size-dependence of the band gap of wz CIS QDs. A sizing curve is also constructed for chalcopyrite CIS QDs by collecting and reanalyzing literature data. We observe that both sizing curves follow primarily a 1/d dependence. Moreover, the molar absorption coeffs. and the absorption cross-section per CIS formula unit, both at 3.1 eV and at the band gap, are analyzed. The results demonstrate that the molar absorption coeffs. of CIS QDs follow a power law at the first exciton transition energy (εE1 = 5208d2.45) and scale with the QD vol. at 3.1 eV. This latter observation implies that the absorption cross-section per unit cell at 3.1 eV is size-independent and therefore can be estd. from bulk optical consts. These results also demonstrate that the molar absorption coeffs. at 3.1 eV are more reliable for anal. purposes, since they are less sensitive to size and shape dispersion.
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Evans, H. T. Crystal Structure of Low Chalcocite. Nature, Phys. Sci. 1971, 232, 69– 70, DOI: 10.1038/physci232069a0
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Crystal structure of low chalcocite
Evans, Howard T., Jr.
Nature (London), Physical Science (1971), 232 (29), 69-70CODEN: NPSCA6; ISSN:0300-8746.
By x-ray structural anal., low chalcocite, Cu2S, is monoclinic, space group P21/c, with a 15.246 ± 0.004, b 11.884 ± 0.002, c 13.494 ± 0.003 Å, and β 116.35° ± 0.01°; Z = 48. All Cu atoms are in a triangular coordination with S; 1 Cu atom is displaced strongly from the triangular plane toward tetrahedral site. The Cu atoms (1/3) lie in the hexagonal S layers normal to the c axis and the remaining atoms are situated in triangular CuS3 groups lying tilted between the S layers.
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Wang, W.; Dahl, M.; Yin, Y. Hollow Nanocrystals through the Nanoscale Kirkendall Effect. Chem. Mater. 2013, 25, 1179– 1189, DOI: 10.1021/cm3030928
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Hollow Nanocrystals through the Nanoscale Kirkendall Effect
Wang, Wenshou; Dahl, Michael; Yin, Yadong
Chemistry of Materials (2013), 25 (8), 1179-1189CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
A review. Colloidal hollow nanocrystals with controlled hollow interior and shell thickness represent a class of important nanostructured materials, because of their promising applications for nanoreactors, drug delivery, and catalysis. Since the first report in 2004 on the synthesis of CoS and CoO hollow nanocrystals by sulfidation and oxidn. of Co nanocrystals, several different kinds of hollow nanocrystals have been prepd. by a similar approach that involves the nanoscale Kirkendall effect. The application of this well-known classical phenomenon in metallurgy in the synthesis of hollow nanocrystals is discussed. The authors start with a brief introduction to the synthesis of hollow nanocrystals, then discuss the concepts and applications of nanoscale Kirkendall effect for the synthesis of hollow nanocrystals, and finally touch on the extension of the process to the formation of nanotubes. A summary and perspectives on the directions in which future work on this field might be focused is presented.
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Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Formation of Hollow Nanocrystals through the Nanoscale Kirkendall Effect. Science 2004, 304, 711– 714, DOI: 10.1126/science.1096566
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Formation of Hollow Nanocrystals Through the Nanoscale Kirkendall Effect
Yin, Yadong; Rioux, Robert M.; Erdonmez, Can K.; Hughes, Steven; Somorjai, Gabor A.; Alivisatos, A. Paul
Science (Washington, DC, United States) (2004), 304 (5671), 711-714CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
A review is presented on the synthesis of hollow nanocrystals by a mechanism analogous to the Kirkendall Effect, in which pores form because of the difference in diffusion rates between two components in a diffusion couple. Starting with cobalt nanocrystals, their reaction in soln. with oxygen and either sulfur or selenium gives hollow nanocrystals of the resulting oxide and chalcogenides. This process provides a general route to the synthesis of hollow nanostructures of a large no. of compds. A simple extension of the process yielded platinum-cobalt oxide yolk-shell nanostructures, which may serve as nanoscale reactors in catalytic applications.
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Mu, L.; Wang, F.; Sadtler, B.; Loomis, R. A.; Buhro, W. E. Influence of the Nanoscale Kirkendall Effect on the Morphology of Copper Indium Disulfide Nanoplatelets Synthesized by Ion Exchange. ACS Nano 2015, 9, 7419– 7428, DOI: 10.1021/acsnano.5b02427
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Influence of the Nanoscale Kirkendall Effect on the Morphology of Copper Indium Disulfide Nanoplatelets Synthesized by Ion Exchange
Mu, Linjia; Wang, Fudong; Sadtler, Bryce; Loomis, Richard A.; Buhro, William E.
ACS Nano (2015), 9 (7), 7419-7428CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
CuInS2 nanocrystals were prepd. by ion exchange with template Cu2-xS nanoplatelets and InX3 [X = chloride, iodide, acetate (OAc), or acetylacetonate (acac)]. The morphologies of the resultant nanocrystals depend on the InX3 precursor and the reaction temp. Exchange with InCl3 at 150°C produces CuInS2 nanoplatelets having central holes and thickness variations, whereas the exchange at 200°C produces intact CuInS2 nanoplatelets in which the initial morphol. is preserved. Exchange with InI3 at 150°C produces CuInS2 nanoplatelets in which the central hollowing is more extreme, whereas exchange with In(OAc)3 or In(acac)3 at 150°C produces intact CuInS2 nanoplatelets. The results establish that the ion exchange occurs through the thin nanoplatelet edge facets. The hollowing and hole formation are due to a nanoscale Kirkendall Effect operating in the reaction-limited regime for displacement of X- at the edges, to allow insertion of In3+ into the template nanoplatelets.
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Ogawa, A.; Fujimoto, H. Lewis Acidity of Gallium Halides. Inorg. Chem. 2002, 41, 4888– 4894, DOI: 10.1021/ic020268m
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Lewis Acidity of Gallium Halides
Ogawa, Atsushi; Fujimoto, Hiroshi
Inorganic Chemistry (2002), 41 (19), 4888-4894CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
The Lewis acidity of GaF3, GaF2Cl, GaFCl2, and GaCl3 in acid-base interactions has been studied by taking ammonia as their electron-donating counterpart. We have derived an unoccupied reactive orbital that shows the max. localization on the Ga at. center for each species. The orbital is located lower in energy compared to those in the corresponding boron and aluminum halides. In contrast to boron halides, the unoccupied reactive orbital of the acid site tends to be delocalized considerably on the halogens as the fluorines are substituted by chlorines in gallium halides. The trend obsd. in the effects of fluorine and chlorine on the acidity of the gallium halides is opposite to those found in the boron halides. This cannot be interpreted solely in terms of the electron-accepting strength of the gallium center, but can be understood by including electrostatic interactions and closed-shell repulsion with ammonia in the adducts. The origin of the difference in Lewis acidity of BCl3, AlCl3, and GaCl3 has been clarified.
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Hogg, J. M.; Coleman, F.; Ferrer-Ugalde, A.; Atkins, M. P.; Swadźba-Kwaśny, M. Liquid Coordination Complexes: A New Class of Lewis Acids as Safer Alternatives to BF₃ in Synthesis of Polyalphaolefins. Green Chem. 2015, 17, 1831– 1841, DOI: 10.1039/C4GC02080D
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Liquid coordination complexes: a new class of Lewis acids as safer alternatives to BF3 in synthesis of polyalphaolefins
Hogg, James M.; Coleman, Fergal; Ferrer-Ugalde, Albert; Atkins, Martin P.; Swadzba-Kwasny, Malgorzata
Green Chemistry (2015), 17 (3), 1831-1841CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
Liq. coordination complexes (LCCs) are a new class of liq. Lewis acids, prepd. by combining an excess of a metal halide (e.g. GaCl3) with a basic donor mol. (e.g. amides, amines or phosphines). LCCs were used to catalyze oligomerization of 1-decene to polyalphaolefins (PAOs). Mol. wt. distribution and phys. properties of the produced oils were compliant with those required for low viscosity synthetic (Group IV) lubricant base oils. Kinematic viscosities at 100 °C of ca. 4 or 6 cSt were obtained, along with viscosity indexes above 120 and pour points below -57 °C. In industry, to achieve similar properties, BF3 gas is used as a catalyst. LCCs are proposed as a safer and economically attractive alternative to BF3 gas for the prodn. of polyalphaolefins.
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Ruberu, T. P. A.; Albright, H. R.; Callis, B.; Ward, B.; Cisneros, J.; Fan, H.-J.; Vela, J. Molecular Control of the Nanoscale: Effect of Phosphine–Chalcogenide Reactivity on CdS–CdSe Nanocrystal Composition and Morphology. ACS Nano 2012, 6, 5348– 5359, DOI: 10.1021/nn301182h
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Molecular Control of the Nanoscale: Effect of Phosphine-Chalcogenide Reactivity on CdS-CdSe Nanocrystal Composition and Morphology
Ruberu, T. Purnima A.; Albright, Haley R.; Callis, Brandon; Ward, Brittney; Cisneros, Joana; Fan, Hua-Jun; Vela, Javier
ACS Nano (2012), 6 (6), 5348-5359CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
The authors demonstrate mol. control of nanoscale compn., alloying, and morphol. (aspect ratio) in CdS-CdSe nanocrystal dots and rods by modulating the chem. reactivity of phosphine-chalcogenide precursors. Specific mol. precursors studied were sulfides and selenides of triphenylphosphite (TPP), diphenylpropylphosphine (DPP), tributylphosphine (TBP), trioctylphosphine (TOP), and hexaethylphosphorustriamide (HPT). Computational (DFT), NMR (31P and 77Se), and high-temp. crossover studies unambiguously confirm a chem. bonding interaction between P and chalcogen atoms in all precursors. Phosphine-chalcogenide precursor reactivity increases in the order: HPTE < TOPE < TBPE < DPPE < TPPE (E = S, Se). For a given phosphine, the selenide is always more reactive than the sulfide. CdS1-xSex quantum dots were synthesized via single injection of a R3PS-R3PSe mixt. to Cd oleate at 250°. XRD, TEM, and UV/visible and PL optical spectroscopy reveal that relative R3PS and R3PSe reactivity dictates CdS1-xSex dot chalcogen content and the extent of radial alloying (alloys vs. core/shells). CdS, CdSe, and CdS1-xSex quantum rods were synthesized by injection of a single R3PE (E = S or Se) precursor or a R3PS-R3PSe mixt. to Cd-phosphonate at 320 or 250°. XRD and TEM reveal that the length-to-diam. aspect ratio of CdS and CdSe nanorods is inversely proportional to R3PE precursor reactivity. Purposely matching or mismatching R3PS-R3PSe precursor reactivity leads to CdS1-xSex nanorods without or with axial compn. gradients, resp. The authors expect these observations will lead to scalable and highly predictable bottom-up programmed syntheses of finely heterostructured nanomaterials with well-defined architectures and properties that are tailored for precise applications.
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Tolman, C. A. Steric Effects of Phosphorus Ligands in Organometallic Chemistry and Homogeneous Catalysis. Chem. Rev. 1977, 77, 313– 348, DOI: 10.1021/cr60307a002
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Steric effects of phosphorus ligands in organometallic chemistry and homogeneous catalysis
Tolman, Chadwick A.
Chemical Reviews (Washington, DC, United States) (1977), 77 (3), 313-48CODEN: CHREAY; ISSN:0009-2665.
A review, with 298 refs.
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Kühl, O. Predicting the Net Donating Ability of Phosphines - Do We Need Sophisticated Theoretical Methods?. Coord. Chem. Rev. 2005, 249, 693– 704, DOI: 10.1016/j.ccr.2004.08.021
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Predicting the net donating ability of phosphines-do we need sophisticated theoretical methods?
Kuehl, Olaf
Coordination Chemistry Reviews (2005), 249 (5-6), 693-704CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
A review; several approaches to quantify and classify the electronic properties of phosphines and similar ligands exptl. are reviewed and compared to recent attempts to calc. these properties with theor. methods.
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Brown, T. L.; Lee, K. J. Ligand Steric Properties. Coord. Chem. Rev. 1993, 128, 89– 116, DOI: 10.1016/0010-8545(93)80025-Z
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Ligand steric properties
Brown, Theodore L.; Lee, Kevin J.
Coordination Chemistry Reviews (1993), 128 (1-2), 89-116CODEN: CCHRAM; ISSN:0010-8545.
A review, with >50 refs., is given on methods of estg. the steric requirements of ligands. The most widely employed measure is the cone angle, θ, 1st proposed by C. A. Tolman (1970, 1974, 1977). Elaborations on the cone angle concept include math. methods for its estn., ests. based on x-ray structural data, and solid cone angle measures. The ligand repulsive parameter, ER, is based upon mol. mechanics calcns. of the structures of Cr(CO)5 complexes of the various ligands. The θ and ER parameters correlate reasonably well, but significant disparities are found among a large group of P, As and N ligands for which both θ and ER values are available. Consideration of the various approaches to estg. ligand steric requirements indicates that each ligand has a range of steric requirements relative to other ligands, depending on the details of the particular complex or reaction involved. Applications of ligand steric requirements include quant. linear free energy relations. Given the relative imprecision with which the steric (and probably also the electronic) parameters can be detd., the use of addnl. parameters to account for the relative importances of σ and π bonding, or the existence of a steric threshold may not be justified by the no. of and variety of data available. Nevertheless, despite their lack of high precision, linear free energy relations can provide important information regarding the electronic and steric demands of the transition state relative to the ground state in chem. reactions.
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Xiao, N.; Zhu, L.; Wang, K.; Dai, Q.; Wang, Y.; Li, S.; Sui, Y.; Ma, Y.; Liu, J.; Liu, B.; Zou, G.; Zou, B. Synthesis and High-Pressure Transformation of Metastable Wurtzite-Structured CuGaS₂ Nanocrystals. Nanoscale 2012, 4, 7443– 7447, DOI: 10.1039/c2nr31629c
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Synthesis and high-pressure transformation of metastable wurtzite-structured CuGaS2 nanocrystals
Xiao, Ningru; Zhu, Li; Wang, Kai; Dai, Quanqin; Wang, Yingnan; Li, Shourui; Sui, Yongming; Ma, Yanming; Liu, Jing; Liu, Bingbing; Zou, Guangtian; Zou, Bo
Nanoscale (2012), 4 (23), 7443-7447CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
The metastable wurtzite nanocrystals of CuGaS2 have been synthesized through a one-pot solvothermal approach. Through the Rietveld refinement on exptl. X-ray diffraction patterns, the structural parameters and the disordered nature of the wurtzite phase. were detd. The metastability of wurtzite structure with respect to the stable chalcopyrite structure was testified by a precise theor. total energy calcn. Subsequent high-pressure expts. were performed to establish the isothermal phase stability of this wurtzite phase in the pressure range of 0-15.9 GPa, above which another disordered rock salt phase crystd. and remained stable up to 30.3 GPa, the highest pressure studied. Upon release of pressure, the sample was irreversibly converted into the energetically more favorable and ordered chalcopyrite structure as revealed by the synchrotron X-ray diffraction and the high-resoln. transmission electron microscopic measurements. The obsd. phase transitions were rationalized by first-principles calcns. The current research establishes a novel phase transition sequence of disorder, disorder, order, where pressure has played a significant role in effectively tuning stabilities of these different phases.
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Fenton, J. L.; Steimle, B. C.; Schaak, R. E. Structure-Selective Synthesis of Wurtzite and Zincblende ZnS, CdS, and CuInS₂ Using Nanoparticle Cation Exchange Reactions. Inorg. Chem. 2019, 58, 672– 678, DOI: 10.1021/acs.inorgchem.8b02880
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Structure-Selective Synthesis of Wurtzite and Zincblende ZnS, CdS, and CuInS2 Using Nanoparticle Cation Exchange Reactions
Fenton, Julie L.; Steimle, Benjamin C.; Schaak, Raymond E.
Inorganic Chemistry (2019), 58 (1), 672-678CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
For polymorphic solid-state systems contg. multiple distinct crystal structures of the same compn., identifying rational pathways to selectively target one particular structure is an important synthetic capability. Cation exchange reactions can transform a growing library of metal chalcogenide nanocrystals into different phases by replacing the cation sublattice, often while retaining morphol. and crystal structure. However, only a few examples have been demonstrated where multiple distinct phases in a polymorphic system could be selectively accessed using nanocrystal cation exchange reactions. Here, we show that roxbyite (hexagonal) and digenite (cubic) Cu2-xS nanoparticles transform upon cation exchange with Cd2+, Zn2+, and In3+ to wurtzite (hexagonal) and zincblende (cubic) CdS, ZnS, and CuInS2, resp. These products retain the anion and cation sublattice features programmed into the copper sulfide template, and each phase forms to the exclusion of other known crystal structures. These results significantly expand the scope of structure-selective cation exchange reactions in polymorphic systems.
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Pardo, M. P.; Guittard, M.; Chilouet, A.; Tomas, A. Diagramme de Phases Gallium-Soufre et Études Structurales Des Phases Solides. J. Solid State Chem. 1993, 102, 423– 433, DOI: 10.1006/jssc.1993.1054
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Phase diagram of the gallium-sulfur system and structural study of the solid phases
Pardo, M. P.; Guittard, M.; Chilouet, A.; Tomas, A.
Journal of Solid State Chemistry (1993), 102 (2), 423-33CODEN: JSSCBI; ISSN:0022-4596.
A description of the phase diagram for Ga-S and structural studies of all the solid phases are given. Two types of compds. are found. The Ga2S3 occurs in 4 forms. The monoclinic form α' is exactly stoichiometric and stable from room temp. to melting. The hexagonal form α and the wurtzite-type form β exist only at high temp.; their formation requires a very small defect of S. Passage from the hexagonal form α to the wurtzite form β is progressive. The γ-blende-type form is substoichiometric and only exists in a small domain of temp. Atoms of Ga are in tetrahedra for all the crystallog. forms. Two varieties of GaS are found: 2H and 3R. A structural study is made using powder data for GaS 3R; this form is metastable for all temps. of the solid state. Pairs of Ga are inside antiprisms of S atoms for the 3R form and inside triangular prisms for the 2H form. Two liq.-liq. immiscible phases are obsd. between Ga and GaS and between Ga2S3 and S.
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Nogai, S.; Schmidbaur, H. Dichlorogallane (HGaCl₂)₂: Its Molecular Structure and Synthetic Potential. Inorg. Chem. 2002, 41, 4770– 4774, DOI: 10.1021/ic0203015
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Dichlorogallane (HGaCl2)2: Its Molecular Structure and Synthetic Potential
Nogai, Stefan; Schmidbaur, Hubert
Inorganic Chemistry (2002), 41 (18), 4770-4774CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
Dichlorogallane (HGaCl2)2 is readily prepd. from GaCl3 and triethylsilane in quant. yield. Its crystal structure was detd. by single crystal x-ray diffraction. In the Cl-bridged dimers of crystallog. imposed C2h symmetry, the terminal H atoms are in trans positions. In the reaction of dichlorogallane with 2 equiv of triethylphosphine, mononuclear (Et3P)GaHCl2 is formed. Thermal decompn. of (HGaCl2)2 affords H2 gas and quant. yields of GaCl2 as mixed-valent Ga[GaCl4]. Treatment of this product with triethylphosphine gives the sym., Ga-Ga-bonded Ga(II) complex [GaCl2(PEt3)]2 with an ethane-type structure and with the phosphine ligands in a single-trans conformation. The corresponding [GaBr2(PEt3)]2 complex was prepd. from Ga[GaBr4] and has an analogous structure. (Et3P)GaCl3 was synthesized and structurally characterized as a ref. compd.
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Cheng, F.; Codgbrook, H. L.; Hector, A. L.; Levason, W.; Reid, G.; Webster, M.; Zhang, W. Gallium(III) Halide Complexes with Phosphines, Arsines and Phosphine Oxides – a Comparative Study. Polyhedron 2007, 26, 4147– 4155, DOI: 10.1016/j.poly.2007.05.008
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Gallium(III) halide complexes with phosphines, arsines and phosphine oxides - a comparative study
Cheng, Fei; Codgbrook, Hannah L.; Hector, Andrew L.; Levason, William; Reid, Gillian; Webster, Michael; Zhang, Wenjian
Polyhedron (2007), 26 (15), 4147-4155CODEN: PLYHDE; ISSN:0277-5387. (Elsevier B.V.)
The phosphine oxide complexes [GaX3(Me3PO)] and [(GaX3)2{μ-o-C6H4(CH2P(O)Ph2)2}] were prepd. and characterized by microanal., IR and multinuclear NMR (1H, 13C{1H}, 31P{1H} and 71Ga) spectroscopy. The structures of [GaCl3(Me3PO)], [(GaBr3)2{μ-o-C6H4(CH2P(O)Ph2)2}] and of the ionic product [GaI2(Me3PO)2][GaI4] were detd. and show that the Lewis acidity of the Ga halides towards phosphinoyl ligands diminishes as the halogen becomes heavier. The [GaX3(Ph3E)] (X = Cl, Br or I; E = P or As) and [(GaX3)2{μ-o-C6H4(CH2PPh2)2}] (X = Br or I) were prepd. and their structural and spectroscopic properties compared with those of the phosphinoyl complexes. The results, and competitive soln. NMR studies, show that Ga(III) binds the hard R3PO in preference to the softer phosphine or arsine ligands. Hydrolysis of Ga(III) phosphines is shown to lead to [R3PH][GaX4], but in contrast to some other p-block halides, GaX3 do not promote air-oxidn. of R3P to R3PO.
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Yarema, O.; Yarema, M.; Wood, V. Tuning the Composition of Multicomponent Semiconductor Nanocrystals: The Case of I-III-VI Materials. Chem. Mater. 2018, 30, 1446– 1461, DOI: 10.1021/acs.chemmater.7b04710
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Tuning the Composition of Multicomponent Semiconductor Nanocrystals: The Case of I-III-VI Materials
Yarema, Olesya; Yarema, Maksym; Wood, Vanessa
Chemistry of Materials (2018), 30 (5), 1446-1461CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
A review. Among the advantages of multicomponent nanocrystals is the possibility to adjust their electronic and optical properties with compn. as well as size. However, the synthesis of multicomponent nanocrystals is challenging due to the presence of several metal precursors in the reaction mixt. This review takes I-III-VI semiconductor materials as an example class of multicomponent nanocrystals to highlight the underestimated importance of compn., which can affect the electronic and optical properties of nanocrystals as much as size. We discuss synthetic strategies, which enable the compn. control, and show that the ability to sep. choose nanocrystal size and nanocrystal compn. can be beneficial for many optoelectronic and biomedical applications.
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Berends, A. C.; Van der Stam, W.; Akkerman, Q. A.; Meeldijk, J. D.; Van der Lit, J.; de Mello Donegá, C. Anisotropic 2D Cu_2-xSe Nanocrystals from Dodecaneselenol and Their Conversion to CdSe and CuInSe₂ Nanoparticles. Chem. Mater. 2018, 30, 3836– 3846, DOI: 10.1021/acs.chemmater.8b01143
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Anisotropic 2D Cu2-xSe nanocrystals from dodecaneselenol and conversion to CdSe and CuInSe2 nanoparticles
Berends, Anne C.; van der Stam, Ward; Akkerman, Quinten A.; Meeldijk, Johannes D.; van der Lit, Joost; de Mello Donega, Celso
Chemistry of Materials (2018), 30 (11), 3836-3846CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
We present the synthesis of colloidal anisotropic Cu2-xSe nanocrystals (NCs) with excellent size and shape control, using the unexplored phosphine-free selenium precursor 1-dodecaneselenol (DDSe). This precursor forms lamellar complexes with Cu(I) that enable tailoring the NC morphol. from 0D polyhedral to highly anisotropic 2D shapes. The Cu2-xSe NCs are subsequently used as templates in postsynthetic cation exchange reactions, through which they are successfully converted to CdSe and CuInSe2 quantum dots, nanoplatelets, and ultrathin nanosheets. The shape of the template hexagonal nanoplatelets is preserved during the cation exchange reaction, despite a substantial reorganization of the anionic sublattice, which leads to conversion of the tetragonal umangite crystal structure of the parent Cu2-xSe NCs into hexagonal wurtzite CdSe and CuInSe2, accompanied by a change of both the thickness and the lateral dimensions of the nanoplatelets. The crystallog. transformation and reconstruction of the product NCs are attributed to a combination of the unit cell dimensionalities of the parent and product crystal phases and an internal ripening process. This work provides novel tools for the rational design of shape-controlled colloidal anisotropic Cu2-xSe NCs, which, besides their promising optoelectronic properties, also constitute a new family of cation exchange templates for the synthesis of shape-controlled NCs of wurtzite CdSe, CuInSe2, and other metal selenides that cannot be attained through direct synthesis approaches. Moreover, the insights provided here are likely applicable also to the direct synthesis of shape-controlled NCs of other metal selenides, since DDSe may be able to form lamellar complexes with several other metals.
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Tan, J. M. R.; Scott, M. C.; Hao, W.; Baikie, T.; Nelson, C. T.; Pedireddy, S.; Tao, R.; Ling, X.; Magdassi, S.; White, T.; Li, S.; Minor, A. M.; Zheng, H.; Wong, L. H. Revealing Cation-Exchange-Induced Phase Transformations in Multielemental Chalcogenide Nanoparticles. Chem. Mater. 2017, 29, 9192– 9199, DOI: 10.1021/acs.chemmater.7b03029
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Revealing cation-exchange-induced phase transformations in multielemental chalcogenide nanoparticles
Tan, Joel M. R.; Scott, Mary C.; Hao, Wei; Baikie, Tom; Nelson, Christopher T.; Pedireddy, Srikanth; Tao, Runzhe; Ling, Xingyi; Magdassi, Shlomo; White, Timothy; Li, Shuzhou; Minor, Andrew M.; Zheng, Haimei; Wong, Lydia H.
Chemistry of Materials (2017), 29 (21), 9192-9199CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
To control the process of cation exchange (CE) in a multielemental system, a detailed understanding of structural changes at the microscopic level is imperative. However, the synthesis of a multielemental system has so far relied on the CE phenomenon of a binary system, which does not necessarily extend to the higher-order systems. Here, direct exptl. evidence supported by theor. calcns. reveals a growth model of binary Cu-S to ternary Cu-Sn-S to quaternary Cu-Zn-Sn-S, which shows that cations preferentially diffuse along a specific lattice plane with the preservation of sulfuric anionic framework. In addn., we also discover that, unlike the commonly accepted structure (P63mc), the metastable crystal structure of Cu-Zn-Sn-S phase possesses fixed Sn occupancy sites. By revealing the preferential nature of cations diffusion and growth mechanism, our work provides insight into controlling the stoichiometry and phase purity of novel multielemental materials.
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Lesnyak, V.; Brescia, R.; Messina, G. C.; Manna, L. Cu Vacancies Boost Cation Exchange Reactions in Copper Selenide Nanocrystals. J. Am. Chem. Soc. 2015, 137, 9315– 9323, DOI: 10.1021/jacs.5b03868
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Cu Vacancies Boost Cation Exchange Reactions in Copper Selenide Nanocrystals
Lesnyak, Vladimir; Brescia, Rosaria; Messina, Gabriele C.; Manna, Liberato
Journal of the American Chemical Society (2015), 137 (29), 9315-9323CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The cation exchange reactions in copper selenide nanocrystals have been studied using two divalent ions as guest cations (Zn2+ and Cd2+) and comparing the reactivity of close to stoichiometric (i.e., Cu2Se) nanocrystals with that of nonstoichiometric (Cu2-xSe) nanocrystals, to gain insights into the mechanism of cation exchange at the nanoscale. The presence of a large d. of copper vacancies significantly accelerated the exchange process at room temp. and corroborated vacancy diffusion as one of the main drivers in these reactions. Partially exchanged samples exhibited Janus-like heterostructures made of immiscible domains sharing epitaxial interfaces. No alloy or core-shell structures were obsd. The role of phosphines, like tri-n-octylphosphine, in these reactions, is multifaceted: besides acting as selective solvating ligands for Cu+ ions exiting the nanoparticles during exchange, they also enable anion diffusion, by extg. an appreciable amt. of selenium to the soln. phase, which may further promote the exchange process. In reactions run at a higher temp. (150 °C), copper vacancies were quickly eliminated from the nanocrystals and major differences in Cu stoichiometries, as well as in reactivities, between the initial Cu2Se and Cu2-xSe samples were rapidly smoothed out. These expts. indicate that cation exchange, under the specific conditions of this work, is more efficient at room temp. than at higher temp.
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Fenton, J. L.; Steimle, B. C.; Schaak, R. E. Tunable Intraparticle Frameworks for Creating Complex Heterostructured Nanoparticle Libraries. Science 2018, 360, 513– 517, DOI: 10.1126/science.aar5597
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Tunable intraparticle frameworks for creating complex heterostructured nanoparticle libraries
Fenton, Julie L.; Steimle, Benjamin C.; Schaak, Raymond E.
Science (Washington, DC, United States) (2018), 360 (6388), 513-517CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
Complex heterostructured nanoparticles with precisely defined materials and interfaces are important for many applications. However, rationally incorporating such features into nanoparticles with rigorous morphol. control remains a synthetic bottleneck. We define a modular divergent synthesis strategy that progressively transforms simple nanoparticle synthons into increasingly sophisticated products. We introduce a series of tunable interfaces into zero-, one-, and two-dimensional copper sulfide nanoparticles using cation exchange reactions. Subsequent manipulation of these intraparticle frameworks yielded a library of 47 distinct heterostructured metal sulfide derivs., including particles that contain asym., patchy, porous, and sculpted nanoarchitectures. This generalizable mix-and-match strategy provides predictable retrosynthetic pathways to complex nanoparticle features that are otherwise inaccessible.
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Li, H.; Brescia, R.; Krahne, R.; Bertoni, G.; Alcocer, M. J. P.; D’Andrea, C.; Scotognella, F.; Tassone, F.; Zanella, M.; De Giorgi, M.; Manna, L. Blue-UV-Emitting ZnSe(Dot)/ZnS(Rod) Core/Shell Nanocrystals Prepared from CdSe/CdS Nanocrystals by Sequential Cation Exchange. ACS Nano 2012, 6, 1637– 1647, DOI: 10.1021/nn204601n
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Blue-UV-Emitting ZnSe(Dot)/ZnS(Rod) Core/Shell Nanocrystals Prepared from CdSe/CdS Nanocrystals by Sequential Cation Exchange
Li, Hongbo; Brescia, Rosaria; Krahne, Roman; Bertoni, Giovanni; Alcocer, Marcelo J. P.; D'Andrea, Cosimo; Scotognella, Francesco; Tassone, Francesco; Zanella, Marco; De Giorgi, Milena; Manna, Liberato
ACS Nano (2012), 6 (2), 1637-1647CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
Great control over size, shape and optical properties is now possible in colloidal Cd-based nanocrystals, which has paved the way for many fundamental studies and applications. One popular example of such class of nanocrystals is represented by CdSe(spherical core)/CdS(rod shell) nanorods. These can be nearly monodisperse in size and shape and have strong and stable photoluminescence that is tunable in the visible range (mainly by varying the size of the CdSe core). The corresponding Zn-based core/shell nanorods would be good candidates for tunable emission in the blue-UV region. However, while the synthesis of ZnS nanocrystals with elongated shapes was demonstrated based on the oriented-attachment mechanism, elongated ZnS shells are difficult to fabricate because the more common cubic phase of ZnS has a highly sym. crystal structure. The authors report here a procedure based on a sequence of 2 cation exchange reactions, namely, Cd2+ Cu+ and then Cu+ Zn2+, by which the authors transform colloidal CdSe(core)/CdS(shell) nanorods 1st into Cu2Se/Cu2S nanorods, which are then converted into blue-UV fluorescent ZnSe(core)/ZnS(shell) nanorods. The procedure transfers the morphol. and structural information of the initial Cd-based nanorods to the Zn-based nanorods. Therefore, the final nanoparticles are made by a ZnSe dot embedded in a rod-shaped shell of wurtzite ZnS. Since in the starting Cd-based nanorods the size of the CdSe core and the length of the CdS shell can be well controlled, the same holds for the final Zn-based rods. In the 2nd step of the exchange reaction (Cu+ Zn2+), a large excess of Zn2+ ions added over the Cu+ ions present in the Cu2Se/Cu2S nanorods is the key requisite to obtain bright, band-edge emission (with quantum yields approaching 15%) with narrow line widths (approaching 75 meV). In these ZnSe/ZnS nanorods, photogenerated carriers appear to be more confined in the core region compared to their parent CdSe/CdS nanorods.
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Chakraborty, P.; Jin, Y.; Barrows, C. J.; Dunham, S. T.; Gamelin, D. R. Kinetics of Isovalent (Cd²⁺) and Aliovalent (In³⁺) Cation Exchange in Cd_1-xMn_xSe Nanocrystals. J. Am. Chem. Soc. 2016, 138, 12885– 12893, DOI: 10.1021/jacs.6b05949
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Kinetics of Isovalent (Cd2+) and Aliovalent (In3+) Cation Exchange in Cd1-xMnxSe Nanocrystals
Chakraborty, Pradip; Jin, Yu; Barrows, Charles J.; Dunham, Scott T.; Gamelin, Daniel R.
Journal of the American Chemical Society (2016), 138 (39), 12885-12893CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Ion exchange, in which an in-diffusing ion replaces a lattice ion, has been widely exploited as a synthetic tool for semiconductor doping and solid-to-solid chem. transformations, both in bulk and at the nanoscale. Here, we present a systematic investigation of cation-exchange reactions that involve the displacement of Mn2+ from CdSe nanocrystals by Cd2+ or In3+. For both incoming cations, Mn2+ displacement is spontaneous but thermally activated, following Arrhenius behavior over a broad exptl. temp. range. At any given temp., cation exchange by In3+ is approx. 2 orders of magnitude faster than that by Cd2+, illustrating a crit. dependence on the incoming cation. Quant. anal. of the kinetics data within a Fick's-law diffusion model yields diffusion barriers (ED) and limiting diffusivities (D0) for both incoming ions. Despite their very different kinetics, indistinguishable diffusion barriers of ED ≈ 1.1 eV are found for both reactions (In3+ and Cd2+). A dramatically enhanced diffusivity is found for Mn2+ cation exchange by In3+. Overall, these findings provide unique exptl. insights into cation diffusion within colloidal semiconductor nanocrystals, contributing to our fundamental understanding of this rich and important area of nanoscience.
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Zuckerman, J. J.; Hagen, A. P. Inorganic Reactions and Methods: Formation of Bonds to O, S, Se, Te, Po (Part 1), 1st ed.; VCH Publishers, Inc.: New York, 1992; Vol. 5, p 217.
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Burt, J.; Levason, W.; Reid, G. Coordination Chemistry of the Main Group Elements with Phosphine, Arsine and Stibine Ligands. Coord. Chem. Rev. 2014, 260, 65– 115, DOI: 10.1016/j.ccr.2013.09.020
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Coordination chemistry of the main group elements with phosphine, arsine and stibine ligands
Burt, Jennifer; Levason, William; Reid, Gillian
Coordination Chemistry Reviews (2014), 260 (), 65-115CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
A review. Complexes of Group 2, 12, 13, 14, 15 and 16 elements with mono-, bi-, and polydentate phosphine and arsine ligands (and including the very few examples of stibine and bismuthine donor ligands) are described. Polydentate ligand complexes contg. neutral or charged N, O, C, or S donor groups in addn. to phosphino or arsino donor groups are included, but charged P or As (phosphides, arsenides, phosphinomethanides etc.) ligands are excluded. Emphasis is placed upon the X-ray structures, multinuclear NMR data and reactions. The major differences of this class of complexes compared to the familiar d-block phosphine/arsine complexes are discussed and rationalized in terms of the E-M bonding models. Literature coverage is focused on the last 20 years, although key older work is also included where necessary for comparison purposes, and the article includes work published up to early 2013.
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Chen, F.; Ma, G.; Bernard, G. M.; Wasylishen, R. E.; Cavell, R. G.; McDonald, R.; Ferguson, M. J. An Investigation of 1:1 Adducts of Gallium Trihalides with Triarylphosphines by Solid-State ^69/71Ga and ³¹P NMR Spectroscopy. Chem. - Eur. J. 2013, 19, 2826– 2838, DOI: 10.1002/chem.201202954
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An Investigation of 1:1 Adducts of Gallium Trihalides with Triarylphosphines by Solid-State 69/71Ga and 31P NMR Spectroscopy
Chen, Fu; Ma, Guibin; Bernard, Guy M.; Wasylishen, Roderick E.; Cavell, Ronald G.; McDonald, Robert; Ferguson, Michael J.
Chemistry - A European Journal (2013), 19 (8), 2826-2838CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
Several 1:1 adducts of Ga trihalides with triarylphosphines, X3Ga(PR3) (X=Cl, Br, and I; PR3 = triarylphosphine ligand), were studied by using solid-state 69/71Ga and 31P NMR spectroscopy at different magnetic-field strengths. The 69/71Ga nuclear quadrupolar coupling parameters, as well as the Ga and P magnetic shielding tensors, were detd. The magnitude of the 71Ga quadrupolar coupling consts. (CQ(71Ga)) range from ∼0.9 to 11.0 MHz. The spans of the Ga magnetic shielding tensors for these complexes, δ11-δ33, range from ∼30 to 380 ppm; those detd. for P range from 10 to 40 ppm. For any given phosphine ligand, the Ga nuclei are most shielded for X = I and least shielded for X=Cl, a trend previously obsd. for InIII-phosphine complexes. This exptl. trend, attributed to spin-orbit effects of the halogen ligands, is reproduced by DFT calcns. The signs of CQ(69/71Ga) for some of the adducts were detd. from the anal. of the 31P NMR spectra acquired with magic angle spinning (MAS). The 1J(69/71Ga,31P) and ΔJ(69/71Ga, 31P) values, as well as their signs, were also detd.; values of 1J(71Ga,31P) range from ∼380 to 1590 Hz. Values of 1J(69/71Ga,31P) and ΔJ(69/71Ga,31P) calcd. by using DFT have comparable magnitudes and generally reproduce exptl. trends. Both the Fermi-contact and spin-dipolar Fermi-contact mechanisms make important contributions to the 1J(69/71Ga,31P) tensors. The 31P NMR spectra of several adducts in soln., obtained as a function of temp., are contrasted with those obtained in the solid state. Finally, to complement the anal. of NMR spectra for these adducts, single-crystal x-ray diffraction data for Br3Ga[P(p-Anis)3] and I3Ga[P(p-Anis)3] were obtained.
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Haynes, W. M. CRC Handbook of Chemistry and Physics, 95th ed.; CRC Press: Boca Raton, FL, 2014; section 5–11.
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Clavier, H.; Nolan, S. P. Percent Buried Volume for Phosphine and N-Heterocyclic Carbene Ligands: Steric Properties in Organometallic Chemistry. Chem. Commun. (Cambridge, U. K.) 2010, 46, 841– 861, DOI: 10.1039/b922984a
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Percent buried volume for phosphine and N-heterocyclic carbene ligands: steric properties in organometallic chemistry
Clavier, Herve; Nolan, Steven P.
Chemical Communications (Cambridge, United Kingdom) (2010), 46 (6), 841-861CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
A review. Electronic and steric ligand effects both play major roles in organometallic chem. and consequently in metal-mediated catalysis. Quantifying such parameters is of interest to better understand not only the parameters governing catalyst performance but also reaction mechanisms. Nowadays, ligand mol. architectures are becoming significantly more elaborate and existing models describing ligand sterics prove lacking. This review presents the development of a more general method to det. the steric parameter of organometallic ligands. Two case studies are presented: the tertiary phosphines and the N-heterocyclic carbenes.
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Snelders, D. J. M.; Van Koten, G.; Klein Gebbink, R. J. M. Steric, Electronic, and Secondary Effects on the Coordination Chemistry of Ionic Phosphine Ligands and the Catalytic Behavior of Their Metal Complexes. Chem. - Eur. J. 2011, 17, 42– 57, DOI: 10.1002/chem.201002508
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Steric, Electronic, and Secondary Effects on the Coordination Chemistry of Ionic Phosphine Ligands and the Catalytic Behavior of Their Metal Complexes
Snelders, Dennis J. M.; van Koten, Gerard; Klein Gebbink, Robertus J. M.
Chemistry - A European Journal (2011), 17 (1), 42-57CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. The effects of introducing ionic functionalities in phosphine ligands on the coordination chem. of these ligands and the catalytic behavior of the corresponding metal complexes are reviewed. The steric and electronic consequences of such functionalizations are discussed. Apart from these steric and electronic effects, the presence of charged groups often leads to addnl., supramol. interactions that occur in the 2nd coordination sphere of the metal complex, such as intramol., interligand H bonding and Coulombic repulsion. These interactions can significantly alter the behavior of the phosphine ligand in question. Such effects were obsd. in phosphine-metal assocn./dissocn. equil., ligand substitution reactions, and stereoisomerism in phosphine-metal complexes. By drawing general conclusions, this review offers an insight into the coordination and catalytic behavior of phosphine ligands contg. ionic functionalities and their corresponding metal complexes.
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Chakraborty, P.; Jin, Y.; Barrows, C. J.; Dunham, S. T.; Gamelin, D. R. Kinetics of Isovalent (Cd²⁺) and Aliovalent (In³⁺) Cation Exchange in Cd_1-xMn_xSe Nanocrystals. J. Am. Chem. Soc. 2016, 138, 12885– 12893, DOI: 10.1021/jacs.6b05949
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Kinetics of Isovalent (Cd2+) and Aliovalent (In3+) Cation Exchange in Cd1-xMnxSe Nanocrystals
Chakraborty, Pradip; Jin, Yu; Barrows, Charles J.; Dunham, Scott T.; Gamelin, Daniel R.
Journal of the American Chemical Society (2016), 138 (39), 12885-12893CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Ion exchange, in which an in-diffusing ion replaces a lattice ion, has been widely exploited as a synthetic tool for semiconductor doping and solid-to-solid chem. transformations, both in bulk and at the nanoscale. Here, we present a systematic investigation of cation-exchange reactions that involve the displacement of Mn2+ from CdSe nanocrystals by Cd2+ or In3+. For both incoming cations, Mn2+ displacement is spontaneous but thermally activated, following Arrhenius behavior over a broad exptl. temp. range. At any given temp., cation exchange by In3+ is approx. 2 orders of magnitude faster than that by Cd2+, illustrating a crit. dependence on the incoming cation. Quant. anal. of the kinetics data within a Fick's-law diffusion model yields diffusion barriers (ED) and limiting diffusivities (D0) for both incoming ions. Despite their very different kinetics, indistinguishable diffusion barriers of ED ≈ 1.1 eV are found for both reactions (In3+ and Cd2+). A dramatically enhanced diffusivity is found for Mn2+ cation exchange by In3+. Overall, these findings provide unique exptl. insights into cation diffusion within colloidal semiconductor nanocrystals, contributing to our fundamental understanding of this rich and important area of nanoscience.
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Choi, Y.-M.; Lee, Y.-I.; Kim, S.; Choa, Y.-H. Metallic Alloy Nanoparticle-Based Fabrication and Optical Properties of a Cu(In_1-xGa_x)S₂ Absorber Layer for Solar Cells. J. Alloys Compd. 2014, 615, 496– 500, DOI: 10.1016/j.jallcom.2014.06.174
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Metallic alloy nanoparticle-based fabrication and optical properties of a Cu(In1-xGax)S2 absorber layer for solar cells
Choi, Yo-Min; Lee, Young-In; Kim, Seil; Choa, Yong-Ho
Journal of Alloys and Compounds (2014), 615 (), 496-500CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
A highly-densified Cu(In1-xGax)S2 (CIGS2) absorber layer was fabricated using Cu, In and Ga (CIG) metallic alloy nanoparticles synthesized by salt-assisted ultrasonic spray pyrolysis (SAUSP) followed by direct thermal redn. The redn. process in salt matrix minimized aggregation of CIG metallic alloy nanoparticles that have the potential to lead to higher film densification during sulfurization. To optimize the amt. of salt, various NaCl/precursor ratios were used for SAUSP and Cu-In metallic alloy nanoparticles with av. particle size of 79 nm were obtained. The CIGS2 obtained in the present study exhibited a variable band-gap ranging from 1.46 to 2.4 eV depending on the Ga/(In + Ga) ratio, which corresponded to the resp. bulk materials.
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Chang, S.-H.; Chiang, M.-Y.; Chiang, C.-C.; Yuan, F.-W.; Chen, C.-Y.; Chiu, B.-C.; Kao, T.-L.; Lai, C.-H.; Tuan, H.-Y. Facile Colloidal Synthesis of Quinary CuIn_1-xGa_x(S_ySe_1-y) (CIGSSe) Nanocrystal Inks with Tunable Band Gaps for Use in Low-Cost Photovoltaics. Energy Environ. Sci. 2011, 4, 4929– 4932, DOI: 10.1039/c1ee02341a
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Facile colloidal synthesis of quinary CuIn1-xGax(SySe1-y)2 (CIGSSe) nanocrystal inks with tunable band gaps for use in low-cost photovoltaics
Chang, Shu-Hao; Chiang, Ming-Yi; Chiang, Chien-Chih; Yuan, Fang-Wei; Chen, Chia-Yu; Chiu, Bo-Cheng; Kao, Tzu-Lun; Lai, Chi-Huang; Tuan, Hsing-Yu
Energy & Environmental Science (2011), 4 (12), 4929-4932CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
We report, for the first time, colloidal synthesis of quinary CuIn1-xGax(SySe1-y)2 (CIGSSe) nanocrystals across the entire compn. range (x,y) = 0 to 1 with band gaps tunable in the range of 0.98 to 2.40 eV by facile chem. synthesis. As a proof-of-concept, thin-film solar cells made by using the CIGSSe nanocrystal inks as an absorber layer precursor exhibited an efficiency over 1% under AM 1.5 illumination.
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Xia, C.; Meeldijk, J. D.; Gerritsen, H. C.; de Mello Donegá, C. Highly Luminescent Water-Dispersible NIR-Emitting Wurtzite CuInS₂/ZnS Core/Shell Colloidal Quantum Dots. Chem. Mater. 2017, 29, 4940– 4951, DOI: 10.1021/acs.chemmater.7b01258
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Highly Luminescent Water-Dispersible NIR-Emitting Wurtzite CuInS2/ZnS Core/Shell Colloidal Quantum Dots
Xia, Chenghui; Meeldijk, Johannes D.; Gerritsen, Hans C.; de Mello Donega, Celso
Chemistry of Materials (2017), 29 (11), 4940-4951CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
Copper indium sulfide (CIS) quantum dots (QDs) are attractive as labels for biomedical imaging, since they have large absorption coeffs. across a broad spectral range, size- and compn.-tunable photoluminescence from the visible to the near-IR, and low toxicity. However, the application of NIR-emitting CIS QDs is still hindered by large size and shape dispersions and low photoluminescence quantum yields (PLQYs). In this work, we develop an efficient pathway to synthesize highly luminescent NIR-emitting wurtzite CIS/ZnS QDs, starting from template Cu2-xS nanocrystals (NCs), which are converted by topotactic partial Cu+ for In3+ exchange into CIS NCs. These NCs are subsequently used as cores for the overgrowth of ZnS shells (≤1 nm thick). The CIS/ZnS core/shell QDs exhibit PL tunability from the first to the second NIR window (750-1100 nm), with PLQYs ranging from 75% (at 820 nm) to 25% (at 1050 nm), and can be readily transferred to water upon exchange of the native ligands for mercaptoundecanoic acid. The resulting water-dispersible CIS/ZnS QDs possess good colloidal stability over at least 6 mo and PLQYs ranging from 39% (at 820 nm) to 6% (at 1050 nm). These PLQYs are superior to those of commonly available water-sol. NIR-fluorophores (dyes and QDs), making the hydrophilic CIS/ZnS QDs developed in this work promising candidates for further application as NIR emitters in bioimaging. The hydrophobic CIS/ZnS QDs obtained immediately after the ZnS shelling are also attractive as fluorophores in luminescent solar concentrators.
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Li, L.; Pandey, A.; Werder, D. J.; Khanal, B. P.; Pietryga, J. M.; Klimov, V. I. Efficient Synthesis of Highly Luminescent Copper Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission. J. Am. Chem. Soc. 2011, 133, 1176– 1179, DOI: 10.1021/ja108261h
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Efficient synthesis of highly luminescent copper indium sulfide-based core/shell nanocrystals with surprisingly long-lived emission
Li, Liang; Pandey, Anshu; Werder, Donald J.; Khanal, Bishnu P.; Pietryga, Jeffrey M.; Klimov, Victor I.
Journal of the American Chemical Society (2011), 133 (5), 1176-1179CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
We report an efficient synthesis of copper indium sulfide nanocrystals with strong photoluminescence in the visible to near-IR. This method can produce gram quantities of material with a chem. yield in excess of 90% with minimal solvent waste. The overgrowth of as-prepd. nanocrystals with a few monolayers of CdS or ZnS increases the photoluminescence quantum efficiency to > 80%. On the basis of time-resolved spectroscopic studies of core/shell particles, we conclude that the emission is due to an optical transition that couples a quantized electron state to a localized hole state, which is most likely assocd. with an internal defect.
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Berends, A. C.; Rabouw, F. T.; Spoor, F. C. M.; Bladt, E.; Grozema, F. C.; Houtepen, A. J.; Siebbeles, L. D. A.; De Mello Donegá, C. Radiative and Nonradiative Recombination in CuInS₂ Nanocrystals and CuInS₂-Based Core/Shell Nanocrystals. J. Phys. Chem. Lett. 2016, 7, 3503– 3509, DOI: 10.1021/acs.jpclett.6b01668
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Radiative and Nonradiative Recombination in CuInS2 Nanocrystals and CuInS2-Based Core/Shell Nanocrystals
Berends, Anne C.; Rabouw, Freddy T.; Spoor, Frank C. M.; Bladt, Eva; Grozema, Ferdinand C.; Houtepen, Arjan J.; Siebbeles, Laurens D. A.; Donega, Celso de Mello
Journal of Physical Chemistry Letters (2016), 7 (17), 3503-3509CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Luminescent CuInS2 (CIS) nanocrystals are a potential soln. to the toxicity issues assocd. with Cd- and Pb-based nanocrystals. The development of high-quality CIS nanocrystals was complicated by insufficient knowledge of the electronic structure and the factors that lead to luminescence quenching. The exciton decay pathways in CIS nanocrystals were studied using time-resolved luminescence and transient absorption spectroscopy. Core-only CIS nanocrystals with low quantum yield are compared to core/shell nanocrystals (CIS/ZnS and CIS/CdS) with higher quantum yield. The measurements support the model of luminescence by radiative recombination of a conduction band electron with a localized hole. Luminescence quenching in low-quantum-yield nanocrystals involves initially uncoupled decay pathways for the electron and hole. The electron decay pathway dets. whether the exciton recombines radiatively or nonradiatively. The development of high-quality CIS nanocrystals should therefore focus on the elimination of electron traps.
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Bai, X.; Purcell-Milton, F.; Gun’ko, Y. Optical Properties, Synthesis, and Potential Applications of Cu-Based Ternary or Quaternary Anisotropic Quantum Dots, Polytypic Nanocrystals, and Core/Shell Heterostructures. Nanomaterials 2019, 9, 85, DOI: 10.3390/nano9010085
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Optical properties, synthesis, and potential applications of Cu-based ternary or quaternary anisotropic quantum dots, polytypic nanocrystals, and core/shell heterostructures
Bai, Xue; Milton, Finn Purcell; Gun'ko, Yuri K.
Nanomaterials (2019), 9 (1), 85/1-85/36CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
This review summaries the optical properties, recent progress in synthesis, and a range of applications of luminescent Cu-based ternary or quaternary quantum dots (QDs). We first present the unique optical properties of the Cu-based multicomponent QDs, regarding their emission mechanism, high photoluminescent quantum yields (PLQYs), size-dependent bandgap, compn.-dependent bandgap, broad emission range, large Stokes' shift, and long photoluminescent (PL) lifetimes. Huge progress has taken place in this area over the past years, via detailed experimenting and modeling, giving a much more complete understanding of these nanomaterials and enabling the means to control and therefore take full advantage of their important properties. We then fully explore the techniques to prep. the various types of Cu-based ternary or quaternary QDs (including anisotropic nanocrystals (NCs), polytypic NCs, and spherical, nanorod and tetrapod core/shell heterostructures) are introduced in subsequent sections. To date, various strategies have been employed to understand and control the QDs distinct and new morphologies, with the recent development of Cu-based nanorod and tetrapod structure synthesis highlighted. Next, we summarize a series of applications of these luminescent Cu-based anisotropic and core/shell heterostructures, covering luminescent solar concentrators (LSCs), bioimaging and light emitting diodes (LEDs). Finally, we provide perspectives on the overall current status, challenges, and future directions in this field. The confluence of advances in the synthesis, properties, and applications of these Cu-based QDs presents an important opportunity to a wide-range of fields and this piece gives the reader the knowledge to grasp these exciting developments.
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Berends, A. C.; Mangnus, M. J. J.; Xia, C.; Rabouw, F. T.; De Mello Donegá, C. Optoelectronic Properties of Ternary I-III-VI₂ Semiconductor Nanocrystals: Bright Prospects with Elusive Origins. J. Phys. Chem. Lett. 2019, 10, 1600– 1616, DOI: 10.1021/acs.jpclett.8b03653
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Optoelectronic Properties of Ternary I-III-VI2 Semiconductor Nanocrystals: Bright Prospects with Elusive Origins
Berends, Anne C.; Mangnus, Mark J. J.; Xia, Chenghui; Rabouw, Freddy T.; de Mello Donega, Celso
Journal of Physical Chemistry Letters (2019), 10 (7), 1600-1616CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Colloidal nanocrystals of ternary I-III-VI2 semiconductors are emerging as promising alternatives to Cd- and Pb-chalcogenide nanocrystals because of their inherently lower toxicity, while still offering widely tunable photoluminescence. These properties make them promising materials for a variety of applications. However, the realization of their full potential was hindered by both their underdeveloped synthesis and the poor understanding of their optoelectronic properties, whose origins are still under intense debate. In this Perspective, the authors provide novel insights on the latter aspect by critically discussing the accumulated body of knowledge on I-III-VI2 nanocrystals. From the authors' anal., the luminescence in these nanomaterials most likely originates from the radiative recombination of a delocalized conduction band electron with a hole localized at the group-I cation, which results in broad bandwidths, large Stokes shifts, and long exciton lifetimes. Finally, the authors highlight the remaining open questions and propose expts. to address them.
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Klinger, M.; Jäger, A. Crystallographic Tool Box (CrysTBox): Automated Tools for Transmission Electron Microscopists and Crystallographers. J. Appl. Crystallogr. 2015, 48, 2012– 2018, DOI: 10.1107/S1600576715017252
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Crystallographic Tool Box (CrysTBox): automated tools for transmission electron microscopists and crystallographers
Klinger, Miloslav; Jager, Ales
Journal of Applied Crystallography (2015), 48 (6), 2012-2018CODEN: JACGAR; ISSN:1600-5767. (International Union of Crystallography)
Three tools for an automated anal. of electron diffraction pattern and crystallog. visualization are presented. Firstly, diffractGUI dets. the zone axis from selected area diffraction, convergent beam diffraction or nanodiffraction patterns and allows for indexing of individual reflections. Secondly, ringGUI identifies crystallog. planes corresponding to the depicted rings in the ring diffraction pattern and can select the sample material from a list of candidates. Both diffractGUI and ringGUI employ methods of computer vision for a fast, robust and accurate anal. Thirdly, cellViewer is an intuitive visualization tool which is also helpful for crystallog. calcns. or educational purposes. diffractGUI and cellViewer can be used together during a transmission electron microscopy session to det. the sample holder tilts required to reach a desired zone axis. All the tools offer a graphical user interface. The toolbox is distributed as a standalone application, so it can be installed on the microscope computer and launched directly from DigitalMicrograph (Gatan Inc.).
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Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272– 1276, DOI: 10.1107/S0021889811038970
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VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data
Momma, Koichi; Izumi, Fujio
Journal of Applied Crystallography (2011), 44 (6), 1272-1276CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
VESTA is a 3D visualization system for crystallog. studies and electronic state calcns. It was upgraded to the latest version, VESTA 3, implementing new features including drawing the external morphpol. of crysals; superimposing multiple structural models, volumetric data and crystal faces; calcn. of electron and nuclear densities from structure parameters; calcn. of Patterson functions from the structure parameters or volumetric data; integration of electron and nuclear densities by Voronoi tessellation; visualization of isosurfaces with multiple levels, detn. of the best plane for selected atoms; an extended bond-search algorithm to enable more sophisticated searches in complex mols. and cage-like structures; undo and redo is graphical user interface operations; and significant performance improvements in rendering isosurfaces and calcg. slices.
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ACS Nano

Cite this: ACS Nano 2019, 13, 11, 12880–12893

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https://doi.org/10.1021/acsnano.9b05337

Published October 16, 2019

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Abstract
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Figure 1
Figure 1. TEM images of (a) parent Cu_2–xS bifrustum NCs, (b) product Cu_1.03In_0.81S₂ (CIS) bifrustum NCs, after reaction with InCl₃–TOP at 100 °C, and (c) Cu_1.99Ga_0.04S nanocrystals, after reaction with GaCl₃–TOP at 100 °C. Top-right insets display the corresponding ED pattern. Bottom panels show azimuthally integrated ED (PED) patterns of (d) parent low-chalcocite Cu_2–xS nanocrystals, (e) product wurtzite CIS nanocrystals, and (f) product low-chalcocite Cu_1.99Ga_0.04S nanocrystals. Reference bars in (d,f) are calculated patterns based on the low-chalcocite Cu_2–xS crystal structure described in ref (34). Reference bars in (e) were obtained from JCPDS PDF-card 01-077-9459 for wurtzite CIS.
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Figure 2
Figure 2. TEM images of (a) parent Cu_2–xS NCs and (b–e) product NCs obtained after reacting the NCs shown in (a) with GaCl₃ at 30 °C for (b) 15 min, (c) 30 min, (d) 60 min, and (e) 120 min. Red arrows indicate regions of low contrast within the nanocrystals (cartoons in the lower-left corner give an impression of the observed contrast in the NCs). Top-right inset in (e) depicts an electron diffraction pattern of the corresponding sample. Panel (f) is the azimuthally integrated ED pattern depicted in (e). Reference bars in (f) are calculated patterns based on the low-chalcocite Cu_2–xS crystal structure described in ref (34). The blue curve in (f) corresponds to the PED pattern of the parent Cu_2–xS nanocrystals. Dotted lines serve as guides for the eye.
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Figure 3
Figure 3. 3D characterization of tomographic visualization of the Cu_2–xS bifrustum NCs after reaction with GaCl₃ at 30 °C for (a) 15 min, (b) 30 min, and (c) 60 min. After 15 min, only a small dimple is present at the surface of the NCs. After longer reaction times, these cavities are found to be larger. Red arrows indicate hollow NCs without an apparent opening at the surface. (d–f) Orthoslices used to reconstruct the 3D morphologies presented in (a–c). Animated (rotating) versions of the models depicted in panels (a–c) are provided.
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Figure 4
Figure 4. (Top row) Ground-state geometries of three GaCl₃–phosphine complexes obtained from DFT calculations (see below): (a) GaCl₃ complexed with triethylphosphine, used as model for GaCl₃ complexed with trioctylphosphine (GaCl₃–TOP), (b) GaCl₃ complexed with triphenylphosphite (GaCl₃–TPP), and (c) GaCl₃ complexed with diphenylphosphine (GaCl₃–DPP). (Middle row) TEM images of product NCs obtained after reaction of Cu_2–xS NCs with (d) GaCl₃–TOP at 100 °C, (e) GaCl₃–TPP at 100 °C, and (f) GaCl₃–DPP at 50 °C. Black arrows indicate intraparticle contrast in the NCs reacted with GaCl₃–TPP. Top-right insets show the corresponding ED patterns. Bottom row (g–i) shows the corresponding azimuthally integrated ED patterns. Reference bars in (h,i) are calculated patterns based on the wurtzite CuGaS₂ crystals structure, (44) and reference bars in (g) are based on the low-chalcocite Cu_2–xS crystal structure. (34)
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Figure 5
Figure 5. (a) HAADF-STEM images and EDS elemental maps of (b, blue) Cu and (c, red) Ga of product NCs obtained by reacting Cu_2–xS NCs with GaCl₃–TPP at 100 °C. Orange arrows indicate regions of low contrast (HAADF-STEM) coinciding with regions of low signal (EDS maps).
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Figure 6
Figure 6. (a,b) Hexagonal close-packed structure viewed from two different orientations: (a) characteristic arrangement into hexagonally constructed layers, defined here as the x,y-plane, and (b) characteristic ...ABAB... stacking of these hexagonal layers, defined here as the x,z-plane. (c,d) Anion positions of (black) low-chalcocite Cu_2–xS, (cyan) wurtzite CuGaS₂, and (purple) α′-Ga₂S₃ overlaid onto each other. (c) Hexagonal ordering in the x,y-plane and (d) ...ABAB... stacking in the x,z-plane. Δx, Δy, and Δz, denoted in (a,b), correspond to the lattice distances used to determine the lattice contractions mentioned in the main text. The x,y,z-directions correspond to the following crystallographic directions: (low-chalcocite Cu_2–xS) [201],[010],[001]; (wurtzite CuGaS₂) [11̅0],[110],[001]; (monoclinic α′-Ga₂S₃) [010],[100],[102]. Anion coordinates were obtained from the crystal structures of low-chalcocite Cu_2–xS, (34) wurtzite CuGaS₂, (44) and monoclinic α′-Ga₂S₃. (46)
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Figure 7
Figure 7. Schematic depiction of the Cu⁺ for Ga³⁺ cation exchange reaction in Cu_2–xS nanocrystals. (a) Free GaCl₃ reacts as a Lewis acid, extracting S^2–, leading to etching of the nanocrystals. (b) Extraction of S^2– is inhibited when GaCl₃ is complexed with a phosphine ligand. (c) Successful cation exchange involves four steps: (i) cleavage of the Ga–P bond at the Cu_2–xS nanocrystal surface, at which point Ga–S and P–Cu bonds are made simultaneously; (ii) extraction of Cu⁺ by the phosphine; (iii) subsequent incorporation of Ga³⁺ into the nanocrystal; and (iv) reaction of remaining Cl^– with Cu⁺, extracting two additional Cu⁺ ions, completing the exchange.
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Figure 8
Figure 8. Cu⁺ for Ga³⁺ cation exchange in luminescent CIS NCs. (a) High-resolution TEM image of the parent CIS NCs. The inset shows the FFT of the image. (b) High-resolution TEM image of the product CIGS NCs after reaction; the inset shows the FFT of the image. (c,d) Enlarged TEM image of single NCs before (c) and after (d) the Ga³⁺ for Cu⁺ CE reaction, with the corresponding FFTs. Both scale bars correspond to 5 nm. (e) Azimuthally integrated ED patterns of the parent CIS NCs and product CIGS NCs, and bulk roquesite CIS X-ray diffraction reference pattern (JPCDS PDF-card 00-047-1372). (f) Absorption and (g) PL spectra of the parent CIS NCs and product CIGS NCs. The inset in (f) shows the normalized absorption spectra. The PL quantum yield of the product CIGS NCs is at least 10 times higher than that of the parent CIS NCs. The dip in the PL spectrum around 1150 nm is caused by absorption of emitted light by toluene, which was used as solvent.
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References
This article references 72 other publications.
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  Control over the doping d. in copper sulfide nanocrystals is of great importance and dets. its use in optoelectronic applications such as NIR optical switches and photovoltaic devices. Here, we demonstrate that we can reversibly control the hole carrier d. (varying from >1022 cm-3 to intrinsic) in copper sulfide nanocrystals by electrochem. methods. We can control the type of charge injection, i.e., capacitive charging or ion intercalation, via the choice of the charge compensating cation (e.g., ammonium salts vs. Li+). Further, the type of intercalating ion dets. whether the charge injection is fully reversible (for Li+) or leads to permanent changes in doping d. (for Cu+). Using fully reversible lithium intercalation allows us to switch between thin films of covellite CuS NCs (Eg = 2.0 eV, hole d. 1022 cm-3, strong localized surface plasmon resonance) and low-chalcocite CuLiS NCs (Eg = 1.2 eV, intrinsic, no localized surface plasmon resonance), and back. Electrochem. Cu+ ion intercalation leads to a permanent phase transition to intrinsic low-chalcocite Cu2S nanocrystals that display air stable fluorescence, centered around 1050 nm (fwhm ∼145 meV, PLQY ca. 1.8%), which is the first observation of narrow near-IR fluorescence for copper sulfide nanocrystals. The dynamic control over the hole doping d. and fluorescence of copper sulfide nanocrystals presented in this work and the ability to switch between plasmonic and fluorescent semiconductor nanocrystals might lead to their successful implementation into photovoltaic devices, NIR optical switches and smart windows.
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  A review. The development of nanomaterials for next generation photonic, optoelectronic, and catalytic applications requires a robust synthetic toolkit for systematically tuning compn., phase, and morphol. at nanometer length scales. While de novo synthetic methods for prepg. nanomaterials from mol. precursors have advanced considerably in recent years, post-synthetic modifications of these preformed nanostructures have enabled the stepwise construction of complex nanomaterials. Among these post-synthetic transformations, cation exchange reactions, in which the cations ligated within a nanocrystal host lattice are substituted with those in soln., have emerged as particularly powerful tools for fine-grained control over nanocrystal compn. and phase. The authors review the fundamental thermodn. and kinetic basis for cation exchange reactions in colloidal semiconductor nanocrystals and highlight its synthetic versatility for accessing nanomaterials intractable by direct synthetic methods from mol. precursors. Unlike analogous ion substitution reactions in extended solids, cation exchange reactions at the nanoscale benefit from rapid reaction rates and facile modulation of reaction thermodn. via selective ion coordination in soln. The preservation of the morphol. of the initial nanocrystal template upon exchange, coupled with stoichiometric control over the extent of reaction, enables the formation of nanocrystals with compns., morphologies, and crystal phases that are not readily accessible by conventional synthetic methods.
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6. 6
  Rivest, J. B.; Jain, P. K. Cation Exchange on the Nanoscale: An Emerging Technique for New Material Synthesis, Device Fabrication, and Chemical Sensing. Chem. Soc. Rev. 2013, 42, 89– 96, DOI: 10.1039/C2CS35241A
  6
  Cation exchange on the nanoscale: an emerging technique for new material synthesis, device fabrication, and chemical sensing
  Rivest, Jessy B.; Jain, Prashant K.
  Chemical Society Reviews (2013), 42 (1), 89-96CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
  A review. Cation exchange is an age-old technique for the chem. conversion of liqs. or extended solids by place-exchanging the cations in an ionic material with a different set of cations. The technique is undergoing a major revival with the advent of high-quality nanocrystals: researchers are now able to overcome the limitations in bulk systems and fully exploit cation exchange for materials synthesis and discovery via rapid, low-temp. transformations in the solid state. In this tutorial review, we discuss cation exchange as a promising materials synthesis and discovery tool. Exchange on the nanoscale exhibits some unique attributes: rapid kinetics at room temp. (orders of magnitude faster than in the bulk) and the tuning of reactivity via control of nanocrystal size, shape, and surface faceting. These features make cation exchange a convenient tool for accessing nanocrystal compns. and morphologies for which conventional synthesis may not be established. A simple exchange reaction allows extension of nanochem. to a larger part of the periodic table, beyond the typical gamut of II-VI, IV-VI, and III-V materials. Cation exchange transformations in nanocrystals can be topotactic and size- and shape-conserving, allowing nanocrystals synthesized by conventional methods to be used as templates for prodn. of compositionally novel, multicomponent, or doped nanocrystals. Since phases and compns. resulting from an exchange reaction can be kinetically controlled, rather than governed by the phase diagram, nanocrystals of metastable and hitherto inaccessible compns. are attainable. Outside of materials synthesis, applications for cation exchange exist in water purifn., chem. staining, and sensing. Since nanoscale cation exchange occurs rapidly at room temp., it can be integrated with sensitive environments such as those in biol. systems. Cation exchange is already allowing access to a variety of new materials and processes. With better mechanistic understanding and control, researchers may be able to advance the field to a stage where a custom nanostructure of arbitrary complexity would be achievable by simple cation exchange chem. and a basic understanding of the periodic table.
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7. 7
  Gupta, S.; Kershaw, S. V.; Rogach, A. L. 25th Anniversary Article: Ion Exchange in Colloidal Nanocrystals. Adv. Mater. 2013, 25, 6923– 6944, DOI: 10.1002/adma.201302400
  7
  25th Anniversary Article: Ion Exchange in Colloidal Nanocrystals
  Gupta, Shuchi; Kershaw, Stephen V.; Rogach, Andrey L.
  Advanced Materials (Weinheim, Germany) (2013), 25 (48), 6923-6944CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review; we review the progress in ion exchange in a variety of nanocrystal structures from the earliest accounts dating back over two decades ago to the present day. In recent years the no. of groups using this method to form otherwise difficult or inaccessible nanoparticle shapes and morphologies has increased considerably and the field has experienced a resurgence of interest. While most of the early work on cation exchange centered on II-VI materials, the methodol. has been expanded to cover a far broader range of semiconductor nanocrystals including low toxicity I-III-VI materials and the much less facile III-V materials. The extent of exchange can be controlled leading to lightly doped nanoparticles, alloys, core-shells, segmented rods and dots-in-rods. Progress has been driven by a better understanding of the underlying principles of the exchange process - from thermodn. factors (differences in cation solubilities); the interactions between ions and transfer agents (solvents, ligands, anions, co-dopants); ionic in-diffusion mechanisms and kinetics. More recent availability of very detailed electron microscopy coupled with image reconstruction techniques has been a valuable tool to investigate the resulting heterostructures and internal interfaces. We start by surveying the range of synthetic approaches most often used to carry out ion exchange, mainly focusing on cation replacement strategies, and then describe the rich variety of nanostructures these techniques can bring forth. We also describe some of the principles that are used to establish the relative ease of exchange and to systematically improve the process where the basic energetics are less favorable. To help further the understanding of the underlying fundamentals we have gathered together useful data from the literature on solubilities, cation and anion hardness, ligand and solvent Lewis acid or base strengths for a wide range of chem. species generally used. We offer a perspective on the outlook for the field in terms of the emerging applications and the ion exchange derived materials that will enable them.
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8. 8
  De Trizio, L.; Manna, L. Forging Colloidal Nanostructures via Cation Exchange Reactions. Chem. Rev. 2016, 116, 10852– 10887, DOI: 10.1021/acs.chemrev.5b00739
  8
  Forging Colloidal Nanostructures via Cation Exchange Reactions
  De Trizio, Luca; Manna, Liberato
  Chemical Reviews (Washington, DC, United States) (2016), 116 (18), 10852-10887CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. Among the various postsynthesis treatments of colloidal nanocrystals that have been developed to date, transformations by cation exchange have recently emerged as an extremely versatile tool that has given access to a wide variety of materials and nanostructures. One notable example in this direction is represented by partial cation exchange, by which preformed nanocrystals can be either transformed to alloy nanocrystals or to various types of nanoheterostructures possessing core/shell, segmented, or striped architectures. This review provides an up to date overview of the complex colloidal nanostructures that could be prepd. so far by cation exchange. At the same time, the review gives an account of the fundamental thermodn. and kinetic parameters governing these types of reactions, as they are currently understood, and outlines the main open issues and possible future developments in the field.
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9. 9
  Sahu, A.; Kang, M. S.; Kompch, A.; Notthoff, C.; Wills, A. W.; Deng, D.; Winterer, M.; Frisbie, C. D.; Norris, D. J. Electronic Impurity Doping in CdSe Nanocrystals. Nano Lett. 2012, 12, 2587– 2594, DOI: 10.1021/nl300880g
  9
  Electronic Impurity Doping in CdSe Nanocrystals
  Sahu, Ayaskanta; Kang, Moon Sung; Kompch, Alexander; Notthoff, Christian; Wills, Andrew W.; Deng, Donna; Winterer, Markus; Frisbie, C. Daniel; Norris, David J.
  Nano Letters (2012), 12 (5), 2587-2594CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  We dope CdSe nanocrystals with Ag impurities and investigate their optical and elec. properties. Doping leads not only to dramatic changes but surprising complexity. The addn. of just a few Ag atoms per nanocrystal causes a large enhancement in the fluorescence, reaching efficiencies comparable to core-shell nanocrystals. While Ag was expected to be a substitutional acceptor, nonmonotonic trends in the fluorescence and Fermi level suggest that Ag changes from an interstitial (n-type) to a substitutional (p-type) impurity with increased doping.
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10. 10
  Van der Stam, W.; Geuchies, J. J.; Altantzis, T.; Van den Bos, K. H. W.; Meeldijk, J. D.; Van Aert, S.; Bals, S.; Vanmaekelbergh, D.; de Mello Donegá, C. Highly Emissive Divalent Ion Doped Colloidal CsPb_1–xM_xBr₃ Perovskite Nanocrystals through Cation Exchange. J. Am. Chem. Soc. 2017, 139, 4087– 4097, DOI: 10.1021/jacs.6b13079
  10
  Highly Emissive Divalent-Ion-Doped Colloidal CsPb1-xMxBr3 Perovskite Nanocrystals through Cation Exchange
  van der Stam, Ward; Geuchies, Jaco J.; Altantzis, Thomas; van den Bos, Karel H. W.; Meeldijk, Johannes D.; Van Aert, Sandra; Bals, Sara; Vanmaekelbergh, Daniel; de Mello Donega, Celso
  Journal of the American Chemical Society (2017), 139 (11), 4087-4097CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  A method is presented that allows partial cation exchange in colloidal CsPbBr3 nanocrystals (NCs), whereby Pb2+ is exchanged for several isovalent cations, resulting in doped CsPb1-xMxBr3 NCs (M = Sn2+, Cd2+, and Zn2+; 0 < x ≤ 0.1), with preservation of the original NC shape. The size of the parent NCs is also preserved in the product NCs, apart from a small (few %) contraction of the unit cells upon incorporation of the guest cations. The partial Pb2+ for M2+ exchange leads to a blue-shift of the optical spectra, while maintaining the high luminescence quantum yields (>50%), sharp absorption features, and narrow emission of the parent CsPbBr3 NCs. The blue-shift in the optical spectra is attributed to the lattice contraction that accompanies the Pb2+ for M2+ cation exchange and is obsd. to scale linearly with the lattice contraction. This work opens up new possibilities to engineer the properties of halide perovskite NCs, which to date are the only known system where cation and anion exchange reactions can be sequentially combined while preserving the original NC shape, resulting in compositionally diverse perovskite NCs.
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11. 11
  Eilers, J.; Groeneveld, E.; de Mello Donegá, C.; Meijerink, A. Optical Properties of Mn-Doped ZnTe Magic Size Nanocrystals. J. Phys. Chem. Lett. 2012, 3, 1663– 1667, DOI: 10.1021/jz300300g
  11
  Optical Properties of Mn-Doped ZnTe Magic Size Nanocrystals
  Eilers, Joren; Groeneveld, Esther; de Mello Donega, Celso; Meijerink, Andries
  Journal of Physical Chemistry Letters (2012), 3 (12), 1663-1667CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  The authors report successful doping of ZnTe magic size nanocrystals (MSNCs) with Mn2+. Colloidal ZnTe MSNCs are prepd. via a hot-injection method and doped with Mn2+ via cation exchange. The doped MSNCs show an emission band centered at 620 nm with a radiative decay time of 45 μs, characteristic of Mn2+ in ZnTe. The excitation spectrum of the Mn2+ emission shows narrow absorption bands corresponding to different sizes of ZnTe MSNCs providing further evidence that the 620 nm emission originates from Mn2+ incorporated in the ZnTe host, rather than Mn2+ bound to the surface. The Mn2+-doped ZnTe clusters may serve as nuclei for the growth of larger ZnTe quantum dots doped with a single Mn2+ ion.
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12. 12
  Groeneveld, E.; Witteman, L.; Lefferts, M.; Ke, X.; Bals, S.; Van Tendeloo, G.; de Mello Donegá, C. Tailoring ZnSe-CdSe Colloidal Quantum Dots via Cation Exchange: From Core/Shell to Alloy Nanocrystals. ACS Nano 2013, 7, 7913– 7930, DOI: 10.1021/nn402931y
  12
  Tailoring ZnSe-CdSe Colloidal Quantum Dots via Cation Exchange: From Core/Shell to Alloy Nanocrystals
  Groeneveld, Esther; Witteman, Leon; Lefferts, Merel; Ke, Xiaoxing; Bals, Sara; Van Tendeloo, Gustaaf; de Mello Donega, Celso
  ACS Nano (2013), 7 (9), 7913-7930CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  We report a study of Zn2+ by Cd2+ cation exchange (CE) in colloidal ZnSe nanocrystals (NCs). Our results reveal that CE in ZnSe NCs is a thermally activated isotropic process. The CE efficiency (i.e., fraction of Cd2+ ions originally in soln., Cdsol, that is incorporated in the ZnSe NC) increases with temp. and depends also on the Cdsol/ZnSe ratio. Interestingly, the reaction temp. can be used as a sensitive parameter to tailor both the compn. and the elemental distribution profile of the product (Zn,Cd)Se NCs. At 150° C ZnSe/CdSe core/shell hetero-NCs (HNCs) are obtained, while higher temps. (200 and 220° C) produce (Zn1-xCdx)Se gradient alloy NCs, with increasingly smoother gradients as the temp. increases, until homogeneous alloy NCs are obtained at T ≥ 240° C. Remarkably, sequential heating (150° C followed by 220° C) leads to ZnSe/CdSe core/shell HNCs with thicker shells, rather than (Zn1-xCdx)Se gradient alloy NCs. Thermal treatment at 250° C converts the ZnSe/CdSe core/shell HNCs into (Zn1-xCdx)Se homogeneous alloy NCs, while preserving the NC shape. A mechanism for the cation exchange in ZnSe NCs is proposed, in which fast CE takes place at the NC surface, and is followed by relatively slower thermally activated solid-state cation diffusion, which is mediated by Frenkel defects. The findings presented here demonstrate that cation exchange in colloidal ZnSe NCs provides a very sensitive tool to tailor the nature and localization regime of the electron and hole wave functions and the optoelectronic properties of colloidal ZnSe-CdSe NCs.
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13. 13
  Grodzińska, D.; Pietra, F.; Van Huis, M. A.; Vanmaekelbergh, D.; de Mello Donegá, C. Thermally Induced Atomic Reconstruction of PbSe/CdSe Core/Shell Quantum Dots into PbSe/CdSe Bi-Hemisphere Hetero-Nanocrystals. J. Mater. Chem. 2011, 21, 11556– 11565, DOI: 10.1039/c0jm04458j
  13
  Thermally induced atomic reconstruction of PbSe/CdSe core/shell quantum dots into PbSe/CdSe bi-hemisphere hetero-nanocrystals
  Grodzinska, Dominika; Pietra, Francesca; van Huis, Marijn A.; Vanmaekelbergh, Daniel; Donega, Celso de Mello
  Journal of Materials Chemistry (2011), 21 (31), 11556-11565CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
  The properties of hetero-nanocrystals (HNCs) depend strongly on the mutual arrangement of the nanoscale components. The structural and morphol. evolution of colloidal PbSe/CdSe core/shell quantum dots upon annealing under vacuum was studied. Prior to annealing the PbSe core has an approx. octahedral morphol. with 8 {111} facets, and the CdSe shell has Zn-blende crystal structure. Thermal annealing under vacuum at 150-200° induces a structural and morphol. reconstruction of the HNCs whereby the PbSe core and the CdSe shell are reorganized into 2 hemispheres joined by a common {111} Se plane. This thermally induced reconstruction leads to considerable changes in the optical properties of the colloidal PbSe/CdSe HNCs.
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14. 14
  Fan, Z.; Lin, L.-C.; Buijs, W.; Vlugt, T. J. H.; Van Huis, M. A. Atomistic Understanding of Cation Exchange in PbS Nanocrystals Using Simulations with Pseudoligands. Nat. Commun. 2016, 7, 11503, DOI: 10.1038/ncomms11503
  14
  Atomistic understanding of cation exchange in PbS nanocrystals using simulations with pseudoligands
  Fan, Zhaochuan; Lin, Li-Chiang; Buijs, Wim; Vlugt, Thijs J. H.; van Huis, Marijn A.
  Nature Communications (2016), 7 (), 11503CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
  Cation exchange is a powerful tool for the synthesis of nanostructures such as core-shell nanocrystals, however, the underlying mechanism is poorly understood. Interactions of cations with ligands and solvent mols. are systematically ignored in simulations. Here, we introduce the concept of pseudoligands to incorporate cation-ligand-solvent interactions in mol. dynamics. This leads to excellent agreement with exptl. data on cation exchange of PbS nanocrystals, whereby Pb ions are partially replaced by Cd ions from soln. The temp. and the ligand-type control the exchange rate and equil. compn. of cations in the nanocrystal. Our simulations reveal that Pb ions are kicked out by exchanged Cd interstitials and migrate through interstitial sites, aided by local relaxations at core-shell interfaces and point defects. We also predict that high-pressure conditions facilitate strongly enhanced cation exchange reactions at elevated temps. Our approach is easily extendable to other semiconductor compds. and to other families of nanocrystals.
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15. 15
  Hewavitharana, I. K.; Brock, S. L. When Ligand Exchange Leads to Ion Exchange: Nanocrystal Facets Dictate the Outcome. ACS Nano 2017, 11, 11217– 11224, DOI: 10.1021/acsnano.7b05534
  15
  When Ligand Exchange Leads to Ion Exchange: Nanocrystal Facets Dictate the Outcome
  Hewavitharana, Indika K.; Brock, Stephanie L.
  ACS Nano (2017), 11 (11), 11217-11224CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  This study demonstrates that ligand exchange of nanocrystals (NCs) is not always an innocuous process, but can lead to facile (room temp.) ion exchange, depending on the surface crystal faceting. Rock salt PbTe NCs prepd. as cubes with neutral facets undergo room-temp. ligand exchange with sulfide ions, whereas cuboctahedron-shaped particles with neutral {100} and polar {111} facets are transformed to PbS, driven by ion exchange along the 〈111〉 direction. Likewise, cation exchange (with Ag+) occurs rapidly for cuboctahedra, whereas cubes remain inert. This dramatic difference is attributed to the relative surface area of {111} facets that promote rapid ion exchange and shows how facet engineering is a powerful knob for the control of reaction pathways in nanoparticles.
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16. 16
  Van der Stam, W.; Berends, A. C.; Rabouw, F. T.; Willhammar, T.; Ke, X.; Meeldijk, J. D.; Bals, S.; de Mello Donegá, C. Luminescent CuInS₂ Quantum Dots by Partial Cation Exchange in Cu_2-xS Nanocrystals. Chem. Mater. 2015, 27, 621– 628, DOI: 10.1021/cm504340h
  16
  Luminescent CuInS2 Quantum Dots by Partial Cation Exchange in Cu2-xS Nanocrystals
  van der Stam, Ward; Berends, Anne C.; Rabouw, Freddy T.; Willhammar, Tom; Ke, Xiaoxing; Meeldijk, Johannes D.; Bals, Sara; de Mello Donega, Celso
  Chemistry of Materials (2015), 27 (2), 621-628CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  Partial cation exchange reactions in Cu2-xS nanocrystals (NCs) yield luminescent CuInS2 (CIS) NCs. The approach of mild reaction conditions ensures slow Cu extn. rates, which results in a balance with the slow In incorporation rate. With this method, the authors obtain CIS NCs with luminescence (PL) far in the near-IR (NIR), which cannot be directly synthesized by currently available synthesis protocols. The factors that favor partial, self-limited cation exchange from Cu2-xS to CIS NCs, rather than complete cation exchange to In2S3 are discussed. The product CIS NCs have the wurtzite crystal structure, which is understood in terms of conservation of the hexagonal close packing of the anionic sublattice of the parent NCs into the product NCs. These results are an important step toward the design of CIS NCs with sizes and shapes that are not attainable by direct synthesis protocols and may thus impact a no. of potential applications.
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17. 17
  Jharimune, S.; Sathe, A. A.; Rioux, R. M. Thermochemical Measurements of Cation Exchange in CdSe Nanocrystals Using Isothermal Titration Calorimetry. Nano Lett. 2018, 18, 6795– 6803, DOI: 10.1021/acs.nanolett.8b02661
  17
  Thermochemical Measurements of Cation Exchange in CdSe Nanocrystals Using Isothermal Titration Calorimetry
  Jharimune, Suprita; Sathe, Ajay A.; Rioux, Robert M.
  Nano Letters (2018), 18 (11), 6795-6803CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  Among the various reported post synthetic modifications of colloidal nanocrystals, cation exchange (CE) is one of the most promising and versatile approaches for the synthesis of nanostructures that cannot be directly synthesized from their constitutive precursors. Numerous studies have reported on the qual. anal. of these reactions, but rigorous quant. study of the thermodn. of CE in colloidal nanoparticles is still lacking. We demonstrate using isothermal titrn. calorimetry (ITC), the thermodn. of the CE between cadmium selenide (CdSe) nanocrystals and silver in soln. can be quantified. We survey the influence of CdSe nanocrystal diam., capping ligands and temp. on the thermodn. of the exchange reaction. Results obtained from ITC provide a detailed description of overall thermodn. parameters-equil. const. (Keq), enthalpy (ΔH), entropy (ΔS) and stoichiometry (n)-of the exchange reaction. We compared the free energy change of reaction (ΔG) between CdSe and Ag+ obtained directly from ITC for both CdSe bulk and nanoparticles with values calcd. from previously reported methods. While the calcd. value is closer to the exptl. obtained ΔGrxn for bulk particles, nanocrystals show an addnl. Gibbs free energy stabilization of ∼-14 kJ/mol Se. We discuss a thermochem. cycle elucidating the steps involved in the overall cation exchange process. This work demonstrates the application of ITC to probe the thermochem. of nanoscale transformations under relevant soln. conditions.
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18. 18
  Van der Stam, W.; Bladt, E.; Rabouw, F. T.; Bals, S.; de Mello Donegá, C. Near-Infrared Emitting CuInSe₂/CuInS₂ Dot Core/Rod Shell Heteronanorods by Sequential Cation Exchange. ACS Nano 2015, 9, 11430– 11438, DOI: 10.1021/acsnano.5b05496
  18
  Near-Infrared Emitting CuInSe2/CuInS2 Dot Core/Rod Shell Heteronanorods by Sequential Cation Exchange
  van der Stam, Ward; Bladt, Eva; Rabouw, Freddy T.; Bals, Sara; Donega, Celso de Mello
  ACS Nano (2015), 9 (11), 11430-11438CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  The direct synthesis of heteronanocrystals (HNCs) combining different ternary semiconductors is challenging and has not yet been successful. A sequential topotactic cation exchange (CE) pathway that yields CuInSe2/CuInS2 dot core/rod shell nanorods with near-IR luminescence is reported. The Cu+ extn. rate is coupled to the In3+ incorporation rate using a stoichiometric trioctylphosphine-InCl3 complex, which fulfills the roles of both In-source and Cu-extg. agent. In this way, Cu+ ions can be extd. by trioctylphosphine ligands only when the In-P bond is broken. This results in readily available In3+ ions at the same surface site from which the Cu+ is extd., making the process a direct place exchange reaction and shifting the overall energy balance in favor of the CE. Controlled cation exchange can occur even in large and anisotropic heterostructured nanocrystals with preservation of the size, shape, and heterostructuring of the template NCs into the product NCs. The cation exchange is self-limited, stopping when the ternary core/shell CuInSe2/CuInS2 compn. is reached. The method is very versatile, yielding a variety of luminescent CuInX2 (X = S, Se, and Te) quantum dots, nanorods, and HNCs, by using Cd-chalcogenide NCs and HNCs as templates. The approach reported here thus opens up routes toward materials with unprecedented properties, which would otherwise remain inaccessible.
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19. 19
  Wang, H.; Butler, D. J.; Straus, D. B.; Oh, N.; Wu, F.; Guo, J.; Xue, K.; Lee, J. D.; Murray, C. B.; Kagan, C. R. Air-Stable CuInSe₂ Nanocrystal Transistors and Circuits via Post-Deposition Cation Exchange. ACS Nano 2019, 13, 2324– 2333, DOI: 10.1021/acsnano.8b09055
  19
  Air-Stable CuInSe2 Nanocrystal Transistors and Circuits via Post-Deposition Cation Exchange
  Wang, Han; Butler, Derrick J.; Straus, Daniel B.; Oh, Nuri; Wu, Fengkai; Guo, Jiacen; Xue, Kun; Lee, Jennifer D.; Murray, Christopher B.; Kagan, Cherie R.
  ACS Nano (2019), 13 (2), 2324-2333CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  Colloidal semiconductor nanocrystals (NCs) are a promising materials class for soln.-processable, next-generation electronic devices. However, most high-performance devices and circuits have been achieved using NCs contg. toxic elements, which may limit their further device development. We fabricate high mobility CuInSe2 NC field-effect transistors (FETs) using a soln.-based, post-deposition, sequential cation exchange process that starts with electronically coupled, thiocyanate (SCN)-capped CdSe NC thin films. First Cu+ is substituted for Cd2+ transforming CdSe NCs to Cu-rich Cu2Se NC films. Next, Cu2Se NC films are dipped into a Na2Se soln. to Se-enrich the NCs, thus compensating the Cu-rich surface, promoting fusion of the Cu2Se NCs, and providing sites for subsequent In-dopants. The liq.-coordination-complex trioctylphosphine-indium chloride (TOP-InCl3) is used as a source of In3+ to partially exchange and n-dope CuInSe2 NC films. We demonstrate Al2O3-encapsulated, air-stable CuInSe2 NC FETs with linear (satn.) electron mobilities of 8.2 ± 1.8 cm2/(V s) (10.5 ± 2.4 cm2/(V s)) and with current modulation of 105, comparable to that for high-performance Cd-, Pb-, and As-based NC FETs. The CuInSe2 NC FETs are used as building blocks of integrated inverters to demonstrate their promise for low-cost, low-toxicity NC circuits.
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  https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXitlKnsLw%253D&md5=e8a9f11d81ba5a2475b4d090aa720e4d
20. 20
  Regulacio, M. D.; Ye, C.; Lim, S. H.; Zheng, Y.; Xu, Q.-H.; Han, M.-Y. Facile Noninjection Synthesis and Photocatalytic Properties of Wurtzite-Phase CuGaS₂ Nanocrystals with Elongated Morphologies. CrystEngComm 2013, 15, 5214– 5217, DOI: 10.1039/c3ce40352a
  20
  Facile noninjection synthesis and photocatalytic properties of wurtzite-phase CuGaS2 nanocrystals with elongated morphologies
  Regulacio, Michelle D.; Ye, Chen; Lim, Suo Hon; Zheng, Yuangang; Xu, Qing-Hua; Han, Ming-Yong
  CrystEngComm (2013), 15 (26), 5214-5217CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
  The authors present the colloidal prepn. of ternary CuGaS2 (CGS) nanocrystals that exhibit elongated morphologies and possess the metastable wurtzite crystal structure. A facile noninjection-based synthetic strategy was used wherein dithiocarbamate complexes of Cu and Ga were thermally decompd. in the presence of dodecanethiol. The anisotropic CGS nanocrystals were found to display promising photocatalytic behavior under visible-light illumination.
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21. 21
  Guijarro, N.; Prévot, M. S.; Yu, X.; Jeanbourquin, X. A.; Bornoz, P.; Bourée, W.; Johnson, M.; Le Formal, F.; Sivula, K. A Bottom-Up Approach toward All-Solution-Processed High-Efficiency Cu(In,Ga)S₂ Photocathodes for Solar Water Splitting. Adv. Energy Mater. 2016, 6, 1501949, DOI: 10.1002/aenm.201501949
  There is no corresponding record for this reference.
22. 22
  Lee, K.-H.; Kim, J.-H.; Jang, H. S.; Do, Y. R.; Yang, H. Quantum-Dot-Based White Lighting Planar Source through Downconversion by Blue Electroluminescence. Opt. Lett. 2014, 39, 1208– 1211, DOI: 10.1364/OL.39.001208
  22
  Quantum-dot-based white lighting planar source through downconversion by blue electroluminescence
  Lee, Ki-Heon; Kim, Jong-Hoon; Jang, Ho Seong; Do, Young Rag; Yang, Heesun
  Optics Letters (2014), 39 (5), 1208-1211CODEN: OPLEDP; ISSN:0146-9592. (Optical Society of America)
  We report the unprecedented fabrication of a planar white lighting quantum dot light-emitting diode (QD-LED) through integrating a CdZnS QD-based blue electroluminescence (EL) device with a free-standing polymethyl methacrylate (PMMA) composite film embedded with orange-emitting Cu-In-S (CIS) green-greenish yellow-emitting Cu-In-Ga-S (CIGS) QDs. The hybrid device successfully generates bicolored white emission that comprises blue EL and downconverted QD photoluminescence. The hybrid QD-LEDs loaded with the composite film embedded with one type of QDs exhibit a limited white spectral coverage, consequently producing low values (<65) in color rendering index (CRI). Thus, the QD-PMMA film consisting of a blend of green CIGS and orange CIS QD down-converters is applied for obtaining a higher-CRI white light through the spectral extension, resulting in a much improved CRI of 75-77. Various EL performances of the hybrid planar white device vs. the ref. blue QD-LED are also characterized in details.
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23. 23
  Kim, J.-H.; Lee, K.-H.; Jo, D.-Y.; Lee, Y.; Hwang, J. Y.; Yang, H. Cu-In-Ga-S Quantum Dot Composition-Dependent Device Performance of Electrically Driven Light-Emitting Diodes. Appl. Phys. Lett. 2014, 105, 133104, DOI: 10.1063/1.4896911
  23
  Cu-In-Ga-S quantum dot composition-dependent device performance of electrically driven light-emitting diodes
  Kim, Jong-Hoon; Lee, Ki-Heon; Jo, Dae-Yeon; Lee, Yangjin; Hwang, Jun Yeon; Yang, Heesun
  Applied Physics Letters (2014), 105 (13), 133104/1-133104/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
  Colloidal synthesis of ternary and quaternary quantum dots (QDs) of In/Ga ratio-varied CuIn1-xGaxS2 (CIGS) with nominal x = 0, 0.5, 0.7, and 1 and their application for the fabrication of quantum dot LEDs (QLEDs) are reported. Four QLEDs having CIGS QDs with different compns. are all soln.-processed in the framework of multilayered structure, where QD emitting layer is sandwiched by hybrid charge transport layers of poly(9-vinylcarbazole) and ZnO nanoparticles. The device performance such as luminance and efficiency is strongly dependent on the compn. of CIGS QDs, and well interpreted by the device energy level diagram proposed through the detn. of QD valence band min. by photoelectron emission spectroscopic measurement. (c) 2014 American Institute of Physics.
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24. 24
  Moon, S. H.; Park, S. J.; Kim, S. H.; Lee, M. W.; Han, J.; Kim, J. Y.; Kim, H.; Hwang, Y. J.; Lee, D.-K.; Min, B. K. Monolithic DSSC/CIGS Tandem Solar Cell Fabricated by a Solution Process. Sci. Rep. 2015, 5, 8970, DOI: 10.1038/srep08970
  24
  Monolithic DSSC/CIGS tandem solar cell fabricated by a solution process
  Moon, Sung Hwan; Park, Se Jin; Kim, Sang Hoon; Lee, Min Woo; Han, Jisu; Kim, Jin Young; Kim, Honggon; Hwang, Yun Jeong; Lee, Doh-Kwon; Min, Byoung Koun
  Scientific Reports (2015), 5 (), 8970CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
  Tandem architecture between org. (dye-sensitized solar cell, DSSC) and inorg. (CuInGaSe2 thin film solar cell, CIGS) single-junction solar cells was constructed particularly based on a soln. process. Arc-plasma deposition was employed for the Pt interfacial layer to minimize the damage to the layers of the CIGS bottom cell. Solar cell efficiency of 13% was achieved, which is significant progress from individual single-junction solar cells (e.g., 7.25 and 6.2% for DSSC and CIGS, resp.).
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25. 25
  Yang, Y.; Chen, Q.; Hsieh, Y.-T.; Song, T.-B.; De Marco, N.; Zhou, H.; Yang, Y. Multilayer Transparent Top Electrode for Solution Processed Perovskite/Cu(In,Ga)(Se,S)₂ Four Terminal Tandem Solar Cells. ACS Nano 2015, 9, 7714– 7721, DOI: 10.1021/acsnano.5b03189
  25
  Multilayer Transparent Top Electrode for Solution Processed Perovskite/Cu(In,Ga)(Se,S)2 Four Terminal Tandem Solar Cells
  Yang, Yang; Chen, Qi; Hsieh, Yao-Tsung; Song, Tze-Bin; Marco, Nicholas De; Zhou, Huanping; Yang, Yang
  ACS Nano (2015), 9 (7), 7714-7721CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  Halide perovskites (PVSK) have attracted much attention in recent years due to their high potential as a next generation solar cell material. To further improve perovskites progress toward a state-of-the-art technol., it is desirable to create a tandem structure in which perovskite may be stacked with a current prevailing solar cell such as Si or Cu(In,Ga)(Se,S)2 (CIGS). The transparent top electrode is one of the key components as well as challenges to realize such tandem structure. Herein, the authors develop a multilayer transparent top electrode for perovskite photovoltaic devices delivering an 11.5% efficiency in top illumination mode. The transparent electrode is based on a dielec./metal/dielec. structure, featuring an ultrathin Au seeded Ag layer. A 4 terminal tandem solar cell employing soln. processed CIGS and perovskite cells is also demonstrated with over 15% efficiency.
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26. 26
  Zhao, J.; Zhang, J.; Wang, W.; Wang, P.; Li, F.; Ren, D.; Si, H.; Sun, X.; Ji, F.; Hao, Y. Facile Synthesis of CuInGaS₂ Quantum Dot Nanoparticles for Bilayer-Sensitized Solar Cells. Dalton Trans 2014, 43, 16588– 16592, DOI: 10.1039/C4DT02150A
  26
  Facile synthesis of CuInGaS2 quantum dot nanoparticles for bilayer-sensitized solar cells
  Zhao, Jinjin; Zhang, Jiangbin; Wang, Wenna; Wang, Peng; Li, Feng; Ren, Deliang; Si, Huanyan; Sun, Xiuguo; Ji, Fengqiu; Hao, Yanzhong
  Dalton Transactions (2014), 43 (44), 16588-16592CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
  CuIn0.7Ga0.3S2 quantum dots (QDs) with particle size of 2-5 nm were directly synthesized by a vacuum one-pot-nanocasting process and homogeneously anchored on TiO2 nanocrystals (<50 nm) for the first time. We further present CuIn0.7Ga0.3S2 quantum dots and dye bilayer-sensitized solar cells with a power conversion efficiency 36.3% higher than mono-dye sensitized solar cells.
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27. 27
  Dilena, E.; Xie, Y.; Brescia, R.; Prato, M.; Maserati, L.; Krahne, R.; Paolella, A.; Bertoni, G.; Povia, M.; Moreels, I.; Manna, L. CuIn_xGa_1-xS₂ Nanocrystals with Tunable Composition and Band Gap Synthesized via a Phosphine-Free and Scalable Procedure. Chem. Mater. 2013, 25, 3180– 3187, DOI: 10.1021/cm401563u
  27
  CuInxGa1-xS2 Nanocrystals with Tunable Composition and Band Gap Synthesized via a Phosphine-Free and Scalable Procedure
  Dilena, Enrico; Xie, Yi; Brescia, Rosaria; Prato, Mirko; Maserati, Lorenzo; Krahne, Roman; Paolella, Andrea; Bertoni, Giovanni; Povia, Mauro; Moreels, Iwan; Manna, Liberato
  Chemistry of Materials (2013), 25 (15), 3180-3187CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  We report a phosphine-free colloidal synthesis of CuInxGa1-xS2 (CIGS) nanocrystals (NCs) by heating a mixt. of metal salts, elemental sulfur, octadecene, and oleylamine. In contrast with the more commonly used hot injection, this procedure is highly suitable for large-scale NC prodn., which we tested by performing a gram-scale synthesis. The compn. of the CIGS NCs could be tuned by varying the In and Ga precursor ratios, and the samples showed a compn.-dependent band gap energy. The av. particle size was scaled from 13 to 19 nm by increasing the reaction temp. from 230 to 270 °C. Two concomitant growth mechanisms took place: in one, covellite (CuS) NCs nucleated already at room temp. and then incorporated increasing amts. of In and Ga until they evolved into chalcopyrite CIGS NCs. In the second mechanism, CIGS NCs directly nucleated at intermediate temps. They were smaller than the NCs formed by the first mechanism, but richer in In and Ga. In the final sample, obtained by prolonged heating at 230-270 °C, all NCs were homogeneous in size and compn. Attempts to replace the native ligands on the surface of the NCs with sulfur ions (following literature procedures) resulted in only around 50% exchange. Films prepd. using the partially ligand-exchanged NCs exhibited good homogeneity and an ohmic dark cond. and photocond. with a resistivity of about 50 Ω·cm.
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28. 28
  Liu, Y.; Yin, D.; Swihart, M. T. Valence Selectivity of Cation Incorporation into Covellite CuS Nanoplatelets. Chem. Mater. 2018, 30, 1399– 1407, DOI: 10.1021/acs.chemmater.7b05198
  28
  Valence selectivity of cation incorporation into covellite CuS nanoplatelets
  Liu, Yang; Yin, Deqiang; Swihart, Mark T.
  Chemistry of Materials (2018), 30 (4), 1399-1407CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  Synthesis of copper sulfide-based nanomaterials by cation incorporation into copper deficient copper sulfide (Cu2-xS) is of interest as a powerful means to obtain nanostructures with otherwise inaccessible combinations of size, shape, compn., and crystal phase. Incorporation of a heterocation (M) may produce heterogeneous Cu2-xS-MS nanocrystals (NCs) or homogeneous Cu-M-S alloys. However, the factors detg. whether heterogeneous NCs or homogeneous alloy NCs are produced have not been fully elucidated. In this report, we incorporate diverse cations into covellite CuS nanoplatelet (NPl) templates in the presence of dodecanethiol (DDT). These cations are categorized by their valencies. We demonstrate that trivalent and tetravalent cations can be incorporated into reduced CuS NPls to produce homogeneous ternary alloy NPls, while the divalent cations cannot coexist with Cu+ ions in the Cu2-xS phase. In turn, the incorporation of divalent cations leads to formation of heterogeneous NPls and finally produces copper-free metal sulfide NPls. The cation valence selectivity arises from conflicts between charge balance and coordination between Cu+ and divalent cations. This study not only provides better understanding of the relationship among the compn., morphol., and crystal structure of copper sulfide-based nanomaterials but also provides a pathway to controllable synthesis of complex nanostructures.
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29. 29
  Song, J.; Zhang, Y.; Dai, Y.; Hu, J.; Zhu, L.; Xu, X.; Yu, Y.; Li, H.; Yao, B.; Zhou, H. Polyelectrolyte-Mediated Nontoxic AgGa_xIn_1-xS₂ QDs/Low-Density Lipoprotein Nanoprobe for Selective 3D Fluorescence Imaging of Cancer Stem Cells. ACS Appl. Mater. Interfaces 2019, 11, 9884– 9892, DOI: 10.1021/acsami.9b00121
  29
  Polyelectrolyte-Mediated Nontoxic AgGaxIn1-xS2 QDs/Low-Density Lipoprotein Nanoprobe for Selective 3D Fluorescence Imaging of Cancer Stem Cells
  Song, Jiangluqi; Zhang, Yan; Dai, Yiwen; Hu, Jinhang; Zhu, Lixin; Xu, Xiaoliang; Yu, Yue; Li, Huan; Yao, Bo; Zhou, Huixin
  ACS Applied Materials & Interfaces (2019), 11 (10), 9884-9892CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
  Cancer stem cells, which are a population of cancer cells sharing common properties with normal stem cells, have strong self-renewal ability and multi-lineage differentiation potential to trigger tumor proliferation, metastases, and recurrence. From this, targeted therapy for cancer stem cells may be one of the most promising strategies for comprehensive treatment of tumors in the future. We design a facile approach to establish the colon cancer stem cells-selective fluorescent probe based on the low-d. lipoprotein (LDL) and the novel AgGaxIn(1-x)S2 quantum dots (AGIS QDs). The AGIS QDs with a high crystallinity are obtained for the first time via cation-exchange protocol of Ga3+ to In3+ starting from parent AgInS2 QDs. Photoluminescence peak of AGIS QDs can be turned from 502 to 719 nm by regulating the reaction conditions, with the highest quantum yield up to 37%. Subsequently, AGIS QDs-conjugated LDL nanocomposites (NCs) are fabricated, in which a cationic polyelectrolyte was used as a coupling reagent to guarantee the electrostatic self-assembly. The structural integrity and physicochem. properties of the LDL-QDs NCs are found to be maintained in vitro, and the NCs exhibit remarkable biocompatibility. The LDL-QDs can be selectively delivered into cancer stem cells that overexpress LDL receptor, and three-dimensional imaging of cancer stem cells is realized. The results of this study not only demonstrate the versatility of nature-derived lipoprotein nanoparticles, but also confirm the feasibility of electrostatic conjugation using cationic polyelectrolyte, allowing researchers to design nanoarchitectures for targeted diagnosis and treatment of cancer.
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30. 30
  Zhai, Y.; Flanagan, J. C.; Shim, M. Lattice Strain and Ligand Effects on the Formation of Cu_2–xS/I-III-VI₂ Nanorod Heterostructures through Partial Cation Exchange. Chem. Mater. 2017, 29, 6161– 6167, DOI: 10.1021/acs.chemmater.7b02392
  30
  Lattice Strain and Ligand Effects on the Formation of Cu2-xS/I-III-VI2 Nanorod Heterostructures through Partial Cation Exchange
  Zhai, You; Flanagan, Joseph C.; Shim, Moonsub
  Chemistry of Materials (2017), 29 (14), 6161-6167CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  The authors report on the effects of lattice strain and the choice of ligands on the formation of Cu2-xS/ I-III-VI2 colloidal nanorod heterostructures through partial cation exchange starting from Cu2-xS nanorods. Lattice strain can induce alternating Cu2-xS/CuGaS2 segments along a colloidal nanorod if CuGaS2 can nucleate easily from the sides of the nanorods. The choice in coordinating ligands can alter this preference to favor tip nucleation, in which case the resulting heterostructure has CuGaS2/Cu2-xS/CuGaS2 rod/ rod/ rod geometry. In the less strained CuInS2 case, superlattice-like alternating segmentation does not occur but the ligand induced difference in the preference of where nucleation initiates can still lead to distinct heterostructure morphologies. These results demonstrate how surface accessibility varied by the choice of ligands can be exploited synergistically with the driving force that creates interfaces to provide synthetic control over nanoscale heterostructure formation.
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31. 31
  Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751– 767, DOI: 10.1107/S0567739476001551
  There is no corresponding record for this reference.
32. 32
  Pearson, R. G. Absolute Electronegativity and Hardness: Application to Inorganic Chemistry. Inorg. Chem. 1988, 27, 734– 740, DOI: 10.1021/ic00277a030
  32
  Absolute electronegativity and hardness: application to inorganic chemistry
  Pearson, Ralph G.
  Inorganic Chemistry (1988), 27 (4), 734-40CODEN: INOCAJ; ISSN:0020-1669.
  The recent concepts of abs. electronegativity, χ, and abs. hardness, η, are discussed. The operational definition, χ = (I + A)/2 and η = (I - A)/2, are used to calc. exptl. values for a large no. of cations, atoms, radicals, and mols. The resulting values are in good agreement with the chem. behavior both as to acid-base character and as to chem. hardness or softness. Anions are modeled by their corresponding radicals, and the importance of local softness for delocalized anions is pointed out. Applications of the use of tabulated values of η, both for rank ordering and in numerical calcn., are given.
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33. 33
  Xia, C.; Wu, W.; Yu, T.; Xie, X.; Van Oversteeg, C.; Gerritsen, H. C.; de Mello Donegá, C. Size-Dependent Band-Gap and Molar Absorption Coefficients of Colloidal CuInS₂ Quantum Dots. ACS Nano 2018, 12, 8350– 8361, DOI: 10.1021/acsnano.8b03641
  33
  Size-Dependent Band-Gap and Molar Absorption Coefficients of Colloidal CuInS2 Quantum Dots
  Xia, Chenghui; Wu, Weiwei; Yu, Ting; Xie, Xiaobin; van Oversteeg, Christina; Gerritsen, Hans C.; de Mello Donega, Celso
  ACS Nano (2018), 12 (8), 8350-8361CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  The knowledge of the quantum dot (QD) concn. in a colloidal suspension and the quant. understanding of the size-dependence of the band gap of QDs are of crucial importance from both applied and fundamental viewpoints. In this work, we investigate the size-dependence of the optical properties of nearly spherical wurtzite (wz) CuInS2 (CIS) QDs in the 2.7 to 6.1 nm diam. range (polydispersity ≤10%). The QDs are synthesized by partial Cu+ for In3+ cation exchange in template Cu2-xS nanocrystals, which yields CIS QDs with very small compn. variations (In/Cu = 0.91 ± 0.11), regardless of their sizes. These well-defined QDs are used to investigate the size-dependence of the band gap of wz CIS QDs. A sizing curve is also constructed for chalcopyrite CIS QDs by collecting and reanalyzing literature data. We observe that both sizing curves follow primarily a 1/d dependence. Moreover, the molar absorption coeffs. and the absorption cross-section per CIS formula unit, both at 3.1 eV and at the band gap, are analyzed. The results demonstrate that the molar absorption coeffs. of CIS QDs follow a power law at the first exciton transition energy (εE1 = 5208d2.45) and scale with the QD vol. at 3.1 eV. This latter observation implies that the absorption cross-section per unit cell at 3.1 eV is size-independent and therefore can be estd. from bulk optical consts. These results also demonstrate that the molar absorption coeffs. at 3.1 eV are more reliable for anal. purposes, since they are less sensitive to size and shape dispersion.
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34. 34
  Evans, H. T. Crystal Structure of Low Chalcocite. Nature, Phys. Sci. 1971, 232, 69– 70, DOI: 10.1038/physci232069a0
  34
  Crystal structure of low chalcocite
  Evans, Howard T., Jr.
  Nature (London), Physical Science (1971), 232 (29), 69-70CODEN: NPSCA6; ISSN:0300-8746.
  By x-ray structural anal., low chalcocite, Cu2S, is monoclinic, space group P21/c, with a 15.246 ± 0.004, b 11.884 ± 0.002, c 13.494 ± 0.003 Å, and β 116.35° ± 0.01°; Z = 48. All Cu atoms are in a triangular coordination with S; 1 Cu atom is displaced strongly from the triangular plane toward tetrahedral site. The Cu atoms (1/3) lie in the hexagonal S layers normal to the c axis and the remaining atoms are situated in triangular CuS3 groups lying tilted between the S layers.
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35. 35
  Wang, W.; Dahl, M.; Yin, Y. Hollow Nanocrystals through the Nanoscale Kirkendall Effect. Chem. Mater. 2013, 25, 1179– 1189, DOI: 10.1021/cm3030928
  35
  Hollow Nanocrystals through the Nanoscale Kirkendall Effect
  Wang, Wenshou; Dahl, Michael; Yin, Yadong
  Chemistry of Materials (2013), 25 (8), 1179-1189CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  A review. Colloidal hollow nanocrystals with controlled hollow interior and shell thickness represent a class of important nanostructured materials, because of their promising applications for nanoreactors, drug delivery, and catalysis. Since the first report in 2004 on the synthesis of CoS and CoO hollow nanocrystals by sulfidation and oxidn. of Co nanocrystals, several different kinds of hollow nanocrystals have been prepd. by a similar approach that involves the nanoscale Kirkendall effect. The application of this well-known classical phenomenon in metallurgy in the synthesis of hollow nanocrystals is discussed. The authors start with a brief introduction to the synthesis of hollow nanocrystals, then discuss the concepts and applications of nanoscale Kirkendall effect for the synthesis of hollow nanocrystals, and finally touch on the extension of the process to the formation of nanotubes. A summary and perspectives on the directions in which future work on this field might be focused is presented.
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36. 36
  Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Formation of Hollow Nanocrystals through the Nanoscale Kirkendall Effect. Science 2004, 304, 711– 714, DOI: 10.1126/science.1096566
  36
  Formation of Hollow Nanocrystals Through the Nanoscale Kirkendall Effect
  Yin, Yadong; Rioux, Robert M.; Erdonmez, Can K.; Hughes, Steven; Somorjai, Gabor A.; Alivisatos, A. Paul
  Science (Washington, DC, United States) (2004), 304 (5671), 711-714CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  A review is presented on the synthesis of hollow nanocrystals by a mechanism analogous to the Kirkendall Effect, in which pores form because of the difference in diffusion rates between two components in a diffusion couple. Starting with cobalt nanocrystals, their reaction in soln. with oxygen and either sulfur or selenium gives hollow nanocrystals of the resulting oxide and chalcogenides. This process provides a general route to the synthesis of hollow nanostructures of a large no. of compds. A simple extension of the process yielded platinum-cobalt oxide yolk-shell nanostructures, which may serve as nanoscale reactors in catalytic applications.
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37. 37
  Mu, L.; Wang, F.; Sadtler, B.; Loomis, R. A.; Buhro, W. E. Influence of the Nanoscale Kirkendall Effect on the Morphology of Copper Indium Disulfide Nanoplatelets Synthesized by Ion Exchange. ACS Nano 2015, 9, 7419– 7428, DOI: 10.1021/acsnano.5b02427
  37
  Influence of the Nanoscale Kirkendall Effect on the Morphology of Copper Indium Disulfide Nanoplatelets Synthesized by Ion Exchange
  Mu, Linjia; Wang, Fudong; Sadtler, Bryce; Loomis, Richard A.; Buhro, William E.
  ACS Nano (2015), 9 (7), 7419-7428CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  CuInS2 nanocrystals were prepd. by ion exchange with template Cu2-xS nanoplatelets and InX3 [X = chloride, iodide, acetate (OAc), or acetylacetonate (acac)]. The morphologies of the resultant nanocrystals depend on the InX3 precursor and the reaction temp. Exchange with InCl3 at 150°C produces CuInS2 nanoplatelets having central holes and thickness variations, whereas the exchange at 200°C produces intact CuInS2 nanoplatelets in which the initial morphol. is preserved. Exchange with InI3 at 150°C produces CuInS2 nanoplatelets in which the central hollowing is more extreme, whereas exchange with In(OAc)3 or In(acac)3 at 150°C produces intact CuInS2 nanoplatelets. The results establish that the ion exchange occurs through the thin nanoplatelet edge facets. The hollowing and hole formation are due to a nanoscale Kirkendall Effect operating in the reaction-limited regime for displacement of X- at the edges, to allow insertion of In3+ into the template nanoplatelets.
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38. 38
  Ogawa, A.; Fujimoto, H. Lewis Acidity of Gallium Halides. Inorg. Chem. 2002, 41, 4888– 4894, DOI: 10.1021/ic020268m
  38
  Lewis Acidity of Gallium Halides
  Ogawa, Atsushi; Fujimoto, Hiroshi
  Inorganic Chemistry (2002), 41 (19), 4888-4894CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  The Lewis acidity of GaF3, GaF2Cl, GaFCl2, and GaCl3 in acid-base interactions has been studied by taking ammonia as their electron-donating counterpart. We have derived an unoccupied reactive orbital that shows the max. localization on the Ga at. center for each species. The orbital is located lower in energy compared to those in the corresponding boron and aluminum halides. In contrast to boron halides, the unoccupied reactive orbital of the acid site tends to be delocalized considerably on the halogens as the fluorines are substituted by chlorines in gallium halides. The trend obsd. in the effects of fluorine and chlorine on the acidity of the gallium halides is opposite to those found in the boron halides. This cannot be interpreted solely in terms of the electron-accepting strength of the gallium center, but can be understood by including electrostatic interactions and closed-shell repulsion with ammonia in the adducts. The origin of the difference in Lewis acidity of BCl3, AlCl3, and GaCl3 has been clarified.
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39. 39
  Hogg, J. M.; Coleman, F.; Ferrer-Ugalde, A.; Atkins, M. P.; Swadźba-Kwaśny, M. Liquid Coordination Complexes: A New Class of Lewis Acids as Safer Alternatives to BF₃ in Synthesis of Polyalphaolefins. Green Chem. 2015, 17, 1831– 1841, DOI: 10.1039/C4GC02080D
  39
  Liquid coordination complexes: a new class of Lewis acids as safer alternatives to BF3 in synthesis of polyalphaolefins
  Hogg, James M.; Coleman, Fergal; Ferrer-Ugalde, Albert; Atkins, Martin P.; Swadzba-Kwasny, Malgorzata
  Green Chemistry (2015), 17 (3), 1831-1841CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
  Liq. coordination complexes (LCCs) are a new class of liq. Lewis acids, prepd. by combining an excess of a metal halide (e.g. GaCl3) with a basic donor mol. (e.g. amides, amines or phosphines). LCCs were used to catalyze oligomerization of 1-decene to polyalphaolefins (PAOs). Mol. wt. distribution and phys. properties of the produced oils were compliant with those required for low viscosity synthetic (Group IV) lubricant base oils. Kinematic viscosities at 100 °C of ca. 4 or 6 cSt were obtained, along with viscosity indexes above 120 and pour points below -57 °C. In industry, to achieve similar properties, BF3 gas is used as a catalyst. LCCs are proposed as a safer and economically attractive alternative to BF3 gas for the prodn. of polyalphaolefins.
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40. 40
  Ruberu, T. P. A.; Albright, H. R.; Callis, B.; Ward, B.; Cisneros, J.; Fan, H.-J.; Vela, J. Molecular Control of the Nanoscale: Effect of Phosphine–Chalcogenide Reactivity on CdS–CdSe Nanocrystal Composition and Morphology. ACS Nano 2012, 6, 5348– 5359, DOI: 10.1021/nn301182h
  40
  Molecular Control of the Nanoscale: Effect of Phosphine-Chalcogenide Reactivity on CdS-CdSe Nanocrystal Composition and Morphology
  Ruberu, T. Purnima A.; Albright, Haley R.; Callis, Brandon; Ward, Brittney; Cisneros, Joana; Fan, Hua-Jun; Vela, Javier
  ACS Nano (2012), 6 (6), 5348-5359CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  The authors demonstrate mol. control of nanoscale compn., alloying, and morphol. (aspect ratio) in CdS-CdSe nanocrystal dots and rods by modulating the chem. reactivity of phosphine-chalcogenide precursors. Specific mol. precursors studied were sulfides and selenides of triphenylphosphite (TPP), diphenylpropylphosphine (DPP), tributylphosphine (TBP), trioctylphosphine (TOP), and hexaethylphosphorustriamide (HPT). Computational (DFT), NMR (31P and 77Se), and high-temp. crossover studies unambiguously confirm a chem. bonding interaction between P and chalcogen atoms in all precursors. Phosphine-chalcogenide precursor reactivity increases in the order: HPTE < TOPE < TBPE < DPPE < TPPE (E = S, Se). For a given phosphine, the selenide is always more reactive than the sulfide. CdS1-xSex quantum dots were synthesized via single injection of a R3PS-R3PSe mixt. to Cd oleate at 250°. XRD, TEM, and UV/visible and PL optical spectroscopy reveal that relative R3PS and R3PSe reactivity dictates CdS1-xSex dot chalcogen content and the extent of radial alloying (alloys vs. core/shells). CdS, CdSe, and CdS1-xSex quantum rods were synthesized by injection of a single R3PE (E = S or Se) precursor or a R3PS-R3PSe mixt. to Cd-phosphonate at 320 or 250°. XRD and TEM reveal that the length-to-diam. aspect ratio of CdS and CdSe nanorods is inversely proportional to R3PE precursor reactivity. Purposely matching or mismatching R3PS-R3PSe precursor reactivity leads to CdS1-xSex nanorods without or with axial compn. gradients, resp. The authors expect these observations will lead to scalable and highly predictable bottom-up programmed syntheses of finely heterostructured nanomaterials with well-defined architectures and properties that are tailored for precise applications.
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41. 41
  Tolman, C. A. Steric Effects of Phosphorus Ligands in Organometallic Chemistry and Homogeneous Catalysis. Chem. Rev. 1977, 77, 313– 348, DOI: 10.1021/cr60307a002
  41
  Steric effects of phosphorus ligands in organometallic chemistry and homogeneous catalysis
  Tolman, Chadwick A.
  Chemical Reviews (Washington, DC, United States) (1977), 77 (3), 313-48CODEN: CHREAY; ISSN:0009-2665.
  A review, with 298 refs.
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42. 42
  Kühl, O. Predicting the Net Donating Ability of Phosphines - Do We Need Sophisticated Theoretical Methods?. Coord. Chem. Rev. 2005, 249, 693– 704, DOI: 10.1016/j.ccr.2004.08.021
  42
  Predicting the net donating ability of phosphines-do we need sophisticated theoretical methods?
  Kuehl, Olaf
  Coordination Chemistry Reviews (2005), 249 (5-6), 693-704CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
  A review; several approaches to quantify and classify the electronic properties of phosphines and similar ligands exptl. are reviewed and compared to recent attempts to calc. these properties with theor. methods.
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43. 43
  Brown, T. L.; Lee, K. J. Ligand Steric Properties. Coord. Chem. Rev. 1993, 128, 89– 116, DOI: 10.1016/0010-8545(93)80025-Z
  43
  Ligand steric properties
  Brown, Theodore L.; Lee, Kevin J.
  Coordination Chemistry Reviews (1993), 128 (1-2), 89-116CODEN: CCHRAM; ISSN:0010-8545.
  A review, with >50 refs., is given on methods of estg. the steric requirements of ligands. The most widely employed measure is the cone angle, θ, 1st proposed by C. A. Tolman (1970, 1974, 1977). Elaborations on the cone angle concept include math. methods for its estn., ests. based on x-ray structural data, and solid cone angle measures. The ligand repulsive parameter, ER, is based upon mol. mechanics calcns. of the structures of Cr(CO)5 complexes of the various ligands. The θ and ER parameters correlate reasonably well, but significant disparities are found among a large group of P, As and N ligands for which both θ and ER values are available. Consideration of the various approaches to estg. ligand steric requirements indicates that each ligand has a range of steric requirements relative to other ligands, depending on the details of the particular complex or reaction involved. Applications of ligand steric requirements include quant. linear free energy relations. Given the relative imprecision with which the steric (and probably also the electronic) parameters can be detd., the use of addnl. parameters to account for the relative importances of σ and π bonding, or the existence of a steric threshold may not be justified by the no. of and variety of data available. Nevertheless, despite their lack of high precision, linear free energy relations can provide important information regarding the electronic and steric demands of the transition state relative to the ground state in chem. reactions.
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44. 44
  Xiao, N.; Zhu, L.; Wang, K.; Dai, Q.; Wang, Y.; Li, S.; Sui, Y.; Ma, Y.; Liu, J.; Liu, B.; Zou, G.; Zou, B. Synthesis and High-Pressure Transformation of Metastable Wurtzite-Structured CuGaS₂ Nanocrystals. Nanoscale 2012, 4, 7443– 7447, DOI: 10.1039/c2nr31629c
  44
  Synthesis and high-pressure transformation of metastable wurtzite-structured CuGaS2 nanocrystals
  Xiao, Ningru; Zhu, Li; Wang, Kai; Dai, Quanqin; Wang, Yingnan; Li, Shourui; Sui, Yongming; Ma, Yanming; Liu, Jing; Liu, Bingbing; Zou, Guangtian; Zou, Bo
  Nanoscale (2012), 4 (23), 7443-7447CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
  The metastable wurtzite nanocrystals of CuGaS2 have been synthesized through a one-pot solvothermal approach. Through the Rietveld refinement on exptl. X-ray diffraction patterns, the structural parameters and the disordered nature of the wurtzite phase. were detd. The metastability of wurtzite structure with respect to the stable chalcopyrite structure was testified by a precise theor. total energy calcn. Subsequent high-pressure expts. were performed to establish the isothermal phase stability of this wurtzite phase in the pressure range of 0-15.9 GPa, above which another disordered rock salt phase crystd. and remained stable up to 30.3 GPa, the highest pressure studied. Upon release of pressure, the sample was irreversibly converted into the energetically more favorable and ordered chalcopyrite structure as revealed by the synchrotron X-ray diffraction and the high-resoln. transmission electron microscopic measurements. The obsd. phase transitions were rationalized by first-principles calcns. The current research establishes a novel phase transition sequence of disorder, disorder, order, where pressure has played a significant role in effectively tuning stabilities of these different phases.
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45. 45
  Fenton, J. L.; Steimle, B. C.; Schaak, R. E. Structure-Selective Synthesis of Wurtzite and Zincblende ZnS, CdS, and CuInS₂ Using Nanoparticle Cation Exchange Reactions. Inorg. Chem. 2019, 58, 672– 678, DOI: 10.1021/acs.inorgchem.8b02880
  45
  Structure-Selective Synthesis of Wurtzite and Zincblende ZnS, CdS, and CuInS2 Using Nanoparticle Cation Exchange Reactions
  Fenton, Julie L.; Steimle, Benjamin C.; Schaak, Raymond E.
  Inorganic Chemistry (2019), 58 (1), 672-678CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  For polymorphic solid-state systems contg. multiple distinct crystal structures of the same compn., identifying rational pathways to selectively target one particular structure is an important synthetic capability. Cation exchange reactions can transform a growing library of metal chalcogenide nanocrystals into different phases by replacing the cation sublattice, often while retaining morphol. and crystal structure. However, only a few examples have been demonstrated where multiple distinct phases in a polymorphic system could be selectively accessed using nanocrystal cation exchange reactions. Here, we show that roxbyite (hexagonal) and digenite (cubic) Cu2-xS nanoparticles transform upon cation exchange with Cd2+, Zn2+, and In3+ to wurtzite (hexagonal) and zincblende (cubic) CdS, ZnS, and CuInS2, resp. These products retain the anion and cation sublattice features programmed into the copper sulfide template, and each phase forms to the exclusion of other known crystal structures. These results significantly expand the scope of structure-selective cation exchange reactions in polymorphic systems.
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46. 46
  Pardo, M. P.; Guittard, M.; Chilouet, A.; Tomas, A. Diagramme de Phases Gallium-Soufre et Études Structurales Des Phases Solides. J. Solid State Chem. 1993, 102, 423– 433, DOI: 10.1006/jssc.1993.1054
  46
  Phase diagram of the gallium-sulfur system and structural study of the solid phases
  Pardo, M. P.; Guittard, M.; Chilouet, A.; Tomas, A.
  Journal of Solid State Chemistry (1993), 102 (2), 423-33CODEN: JSSCBI; ISSN:0022-4596.
  A description of the phase diagram for Ga-S and structural studies of all the solid phases are given. Two types of compds. are found. The Ga2S3 occurs in 4 forms. The monoclinic form α' is exactly stoichiometric and stable from room temp. to melting. The hexagonal form α and the wurtzite-type form β exist only at high temp.; their formation requires a very small defect of S. Passage from the hexagonal form α to the wurtzite form β is progressive. The γ-blende-type form is substoichiometric and only exists in a small domain of temp. Atoms of Ga are in tetrahedra for all the crystallog. forms. Two varieties of GaS are found: 2H and 3R. A structural study is made using powder data for GaS 3R; this form is metastable for all temps. of the solid state. Pairs of Ga are inside antiprisms of S atoms for the 3R form and inside triangular prisms for the 2H form. Two liq.-liq. immiscible phases are obsd. between Ga and GaS and between Ga2S3 and S.
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47. 47
  Nogai, S.; Schmidbaur, H. Dichlorogallane (HGaCl₂)₂: Its Molecular Structure and Synthetic Potential. Inorg. Chem. 2002, 41, 4770– 4774, DOI: 10.1021/ic0203015
  47
  Dichlorogallane (HGaCl2)2: Its Molecular Structure and Synthetic Potential
  Nogai, Stefan; Schmidbaur, Hubert
  Inorganic Chemistry (2002), 41 (18), 4770-4774CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
  Dichlorogallane (HGaCl2)2 is readily prepd. from GaCl3 and triethylsilane in quant. yield. Its crystal structure was detd. by single crystal x-ray diffraction. In the Cl-bridged dimers of crystallog. imposed C2h symmetry, the terminal H atoms are in trans positions. In the reaction of dichlorogallane with 2 equiv of triethylphosphine, mononuclear (Et3P)GaHCl2 is formed. Thermal decompn. of (HGaCl2)2 affords H2 gas and quant. yields of GaCl2 as mixed-valent Ga[GaCl4]. Treatment of this product with triethylphosphine gives the sym., Ga-Ga-bonded Ga(II) complex [GaCl2(PEt3)]2 with an ethane-type structure and with the phosphine ligands in a single-trans conformation. The corresponding [GaBr2(PEt3)]2 complex was prepd. from Ga[GaBr4] and has an analogous structure. (Et3P)GaCl3 was synthesized and structurally characterized as a ref. compd.
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48. 48
  Cheng, F.; Codgbrook, H. L.; Hector, A. L.; Levason, W.; Reid, G.; Webster, M.; Zhang, W. Gallium(III) Halide Complexes with Phosphines, Arsines and Phosphine Oxides – a Comparative Study. Polyhedron 2007, 26, 4147– 4155, DOI: 10.1016/j.poly.2007.05.008
  48
  Gallium(III) halide complexes with phosphines, arsines and phosphine oxides - a comparative study
  Cheng, Fei; Codgbrook, Hannah L.; Hector, Andrew L.; Levason, William; Reid, Gillian; Webster, Michael; Zhang, Wenjian
  Polyhedron (2007), 26 (15), 4147-4155CODEN: PLYHDE; ISSN:0277-5387. (Elsevier B.V.)
  The phosphine oxide complexes [GaX3(Me3PO)] and [(GaX3)2{μ-o-C6H4(CH2P(O)Ph2)2}] were prepd. and characterized by microanal., IR and multinuclear NMR (1H, 13C{1H}, 31P{1H} and 71Ga) spectroscopy. The structures of [GaCl3(Me3PO)], [(GaBr3)2{μ-o-C6H4(CH2P(O)Ph2)2}] and of the ionic product [GaI2(Me3PO)2][GaI4] were detd. and show that the Lewis acidity of the Ga halides towards phosphinoyl ligands diminishes as the halogen becomes heavier. The [GaX3(Ph3E)] (X = Cl, Br or I; E = P or As) and [(GaX3)2{μ-o-C6H4(CH2PPh2)2}] (X = Br or I) were prepd. and their structural and spectroscopic properties compared with those of the phosphinoyl complexes. The results, and competitive soln. NMR studies, show that Ga(III) binds the hard R3PO in preference to the softer phosphine or arsine ligands. Hydrolysis of Ga(III) phosphines is shown to lead to [R3PH][GaX4], but in contrast to some other p-block halides, GaX3 do not promote air-oxidn. of R3P to R3PO.
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49. 49
  Yarema, O.; Yarema, M.; Wood, V. Tuning the Composition of Multicomponent Semiconductor Nanocrystals: The Case of I-III-VI Materials. Chem. Mater. 2018, 30, 1446– 1461, DOI: 10.1021/acs.chemmater.7b04710
  49
  Tuning the Composition of Multicomponent Semiconductor Nanocrystals: The Case of I-III-VI Materials
  Yarema, Olesya; Yarema, Maksym; Wood, Vanessa
  Chemistry of Materials (2018), 30 (5), 1446-1461CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  A review. Among the advantages of multicomponent nanocrystals is the possibility to adjust their electronic and optical properties with compn. as well as size. However, the synthesis of multicomponent nanocrystals is challenging due to the presence of several metal precursors in the reaction mixt. This review takes I-III-VI semiconductor materials as an example class of multicomponent nanocrystals to highlight the underestimated importance of compn., which can affect the electronic and optical properties of nanocrystals as much as size. We discuss synthetic strategies, which enable the compn. control, and show that the ability to sep. choose nanocrystal size and nanocrystal compn. can be beneficial for many optoelectronic and biomedical applications.
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50. 50
  Berends, A. C.; Van der Stam, W.; Akkerman, Q. A.; Meeldijk, J. D.; Van der Lit, J.; de Mello Donegá, C. Anisotropic 2D Cu_2-xSe Nanocrystals from Dodecaneselenol and Their Conversion to CdSe and CuInSe₂ Nanoparticles. Chem. Mater. 2018, 30, 3836– 3846, DOI: 10.1021/acs.chemmater.8b01143
  50
  Anisotropic 2D Cu2-xSe nanocrystals from dodecaneselenol and conversion to CdSe and CuInSe2 nanoparticles
  Berends, Anne C.; van der Stam, Ward; Akkerman, Quinten A.; Meeldijk, Johannes D.; van der Lit, Joost; de Mello Donega, Celso
  Chemistry of Materials (2018), 30 (11), 3836-3846CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  We present the synthesis of colloidal anisotropic Cu2-xSe nanocrystals (NCs) with excellent size and shape control, using the unexplored phosphine-free selenium precursor 1-dodecaneselenol (DDSe). This precursor forms lamellar complexes with Cu(I) that enable tailoring the NC morphol. from 0D polyhedral to highly anisotropic 2D shapes. The Cu2-xSe NCs are subsequently used as templates in postsynthetic cation exchange reactions, through which they are successfully converted to CdSe and CuInSe2 quantum dots, nanoplatelets, and ultrathin nanosheets. The shape of the template hexagonal nanoplatelets is preserved during the cation exchange reaction, despite a substantial reorganization of the anionic sublattice, which leads to conversion of the tetragonal umangite crystal structure of the parent Cu2-xSe NCs into hexagonal wurtzite CdSe and CuInSe2, accompanied by a change of both the thickness and the lateral dimensions of the nanoplatelets. The crystallog. transformation and reconstruction of the product NCs are attributed to a combination of the unit cell dimensionalities of the parent and product crystal phases and an internal ripening process. This work provides novel tools for the rational design of shape-controlled colloidal anisotropic Cu2-xSe NCs, which, besides their promising optoelectronic properties, also constitute a new family of cation exchange templates for the synthesis of shape-controlled NCs of wurtzite CdSe, CuInSe2, and other metal selenides that cannot be attained through direct synthesis approaches. Moreover, the insights provided here are likely applicable also to the direct synthesis of shape-controlled NCs of other metal selenides, since DDSe may be able to form lamellar complexes with several other metals.
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51. 51
  Tan, J. M. R.; Scott, M. C.; Hao, W.; Baikie, T.; Nelson, C. T.; Pedireddy, S.; Tao, R.; Ling, X.; Magdassi, S.; White, T.; Li, S.; Minor, A. M.; Zheng, H.; Wong, L. H. Revealing Cation-Exchange-Induced Phase Transformations in Multielemental Chalcogenide Nanoparticles. Chem. Mater. 2017, 29, 9192– 9199, DOI: 10.1021/acs.chemmater.7b03029
  51
  Revealing cation-exchange-induced phase transformations in multielemental chalcogenide nanoparticles
  Tan, Joel M. R.; Scott, Mary C.; Hao, Wei; Baikie, Tom; Nelson, Christopher T.; Pedireddy, Srikanth; Tao, Runzhe; Ling, Xingyi; Magdassi, Shlomo; White, Timothy; Li, Shuzhou; Minor, Andrew M.; Zheng, Haimei; Wong, Lydia H.
  Chemistry of Materials (2017), 29 (21), 9192-9199CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  To control the process of cation exchange (CE) in a multielemental system, a detailed understanding of structural changes at the microscopic level is imperative. However, the synthesis of a multielemental system has so far relied on the CE phenomenon of a binary system, which does not necessarily extend to the higher-order systems. Here, direct exptl. evidence supported by theor. calcns. reveals a growth model of binary Cu-S to ternary Cu-Sn-S to quaternary Cu-Zn-Sn-S, which shows that cations preferentially diffuse along a specific lattice plane with the preservation of sulfuric anionic framework. In addn., we also discover that, unlike the commonly accepted structure (P63mc), the metastable crystal structure of Cu-Zn-Sn-S phase possesses fixed Sn occupancy sites. By revealing the preferential nature of cations diffusion and growth mechanism, our work provides insight into controlling the stoichiometry and phase purity of novel multielemental materials.
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52. 52
  Lesnyak, V.; Brescia, R.; Messina, G. C.; Manna, L. Cu Vacancies Boost Cation Exchange Reactions in Copper Selenide Nanocrystals. J. Am. Chem. Soc. 2015, 137, 9315– 9323, DOI: 10.1021/jacs.5b03868
  52
  Cu Vacancies Boost Cation Exchange Reactions in Copper Selenide Nanocrystals
  Lesnyak, Vladimir; Brescia, Rosaria; Messina, Gabriele C.; Manna, Liberato
  Journal of the American Chemical Society (2015), 137 (29), 9315-9323CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The cation exchange reactions in copper selenide nanocrystals have been studied using two divalent ions as guest cations (Zn2+ and Cd2+) and comparing the reactivity of close to stoichiometric (i.e., Cu2Se) nanocrystals with that of nonstoichiometric (Cu2-xSe) nanocrystals, to gain insights into the mechanism of cation exchange at the nanoscale. The presence of a large d. of copper vacancies significantly accelerated the exchange process at room temp. and corroborated vacancy diffusion as one of the main drivers in these reactions. Partially exchanged samples exhibited Janus-like heterostructures made of immiscible domains sharing epitaxial interfaces. No alloy or core-shell structures were obsd. The role of phosphines, like tri-n-octylphosphine, in these reactions, is multifaceted: besides acting as selective solvating ligands for Cu+ ions exiting the nanoparticles during exchange, they also enable anion diffusion, by extg. an appreciable amt. of selenium to the soln. phase, which may further promote the exchange process. In reactions run at a higher temp. (150 °C), copper vacancies were quickly eliminated from the nanocrystals and major differences in Cu stoichiometries, as well as in reactivities, between the initial Cu2Se and Cu2-xSe samples were rapidly smoothed out. These expts. indicate that cation exchange, under the specific conditions of this work, is more efficient at room temp. than at higher temp.
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53. 53
  Fenton, J. L.; Steimle, B. C.; Schaak, R. E. Tunable Intraparticle Frameworks for Creating Complex Heterostructured Nanoparticle Libraries. Science 2018, 360, 513– 517, DOI: 10.1126/science.aar5597
  53
  Tunable intraparticle frameworks for creating complex heterostructured nanoparticle libraries
  Fenton, Julie L.; Steimle, Benjamin C.; Schaak, Raymond E.
  Science (Washington, DC, United States) (2018), 360 (6388), 513-517CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  Complex heterostructured nanoparticles with precisely defined materials and interfaces are important for many applications. However, rationally incorporating such features into nanoparticles with rigorous morphol. control remains a synthetic bottleneck. We define a modular divergent synthesis strategy that progressively transforms simple nanoparticle synthons into increasingly sophisticated products. We introduce a series of tunable interfaces into zero-, one-, and two-dimensional copper sulfide nanoparticles using cation exchange reactions. Subsequent manipulation of these intraparticle frameworks yielded a library of 47 distinct heterostructured metal sulfide derivs., including particles that contain asym., patchy, porous, and sculpted nanoarchitectures. This generalizable mix-and-match strategy provides predictable retrosynthetic pathways to complex nanoparticle features that are otherwise inaccessible.
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54. 54
  Li, H.; Brescia, R.; Krahne, R.; Bertoni, G.; Alcocer, M. J. P.; D’Andrea, C.; Scotognella, F.; Tassone, F.; Zanella, M.; De Giorgi, M.; Manna, L. Blue-UV-Emitting ZnSe(Dot)/ZnS(Rod) Core/Shell Nanocrystals Prepared from CdSe/CdS Nanocrystals by Sequential Cation Exchange. ACS Nano 2012, 6, 1637– 1647, DOI: 10.1021/nn204601n
  54
  Blue-UV-Emitting ZnSe(Dot)/ZnS(Rod) Core/Shell Nanocrystals Prepared from CdSe/CdS Nanocrystals by Sequential Cation Exchange
  Li, Hongbo; Brescia, Rosaria; Krahne, Roman; Bertoni, Giovanni; Alcocer, Marcelo J. P.; D'Andrea, Cosimo; Scotognella, Francesco; Tassone, Francesco; Zanella, Marco; De Giorgi, Milena; Manna, Liberato
  ACS Nano (2012), 6 (2), 1637-1647CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  Great control over size, shape and optical properties is now possible in colloidal Cd-based nanocrystals, which has paved the way for many fundamental studies and applications. One popular example of such class of nanocrystals is represented by CdSe(spherical core)/CdS(rod shell) nanorods. These can be nearly monodisperse in size and shape and have strong and stable photoluminescence that is tunable in the visible range (mainly by varying the size of the CdSe core). The corresponding Zn-based core/shell nanorods would be good candidates for tunable emission in the blue-UV region. However, while the synthesis of ZnS nanocrystals with elongated shapes was demonstrated based on the oriented-attachment mechanism, elongated ZnS shells are difficult to fabricate because the more common cubic phase of ZnS has a highly sym. crystal structure. The authors report here a procedure based on a sequence of 2 cation exchange reactions, namely, Cd2+ Cu+ and then Cu+ Zn2+, by which the authors transform colloidal CdSe(core)/CdS(shell) nanorods 1st into Cu2Se/Cu2S nanorods, which are then converted into blue-UV fluorescent ZnSe(core)/ZnS(shell) nanorods. The procedure transfers the morphol. and structural information of the initial Cd-based nanorods to the Zn-based nanorods. Therefore, the final nanoparticles are made by a ZnSe dot embedded in a rod-shaped shell of wurtzite ZnS. Since in the starting Cd-based nanorods the size of the CdSe core and the length of the CdS shell can be well controlled, the same holds for the final Zn-based rods. In the 2nd step of the exchange reaction (Cu+ Zn2+), a large excess of Zn2+ ions added over the Cu+ ions present in the Cu2Se/Cu2S nanorods is the key requisite to obtain bright, band-edge emission (with quantum yields approaching 15%) with narrow line widths (approaching 75 meV). In these ZnSe/ZnS nanorods, photogenerated carriers appear to be more confined in the core region compared to their parent CdSe/CdS nanorods.
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55. 55
  Chakraborty, P.; Jin, Y.; Barrows, C. J.; Dunham, S. T.; Gamelin, D. R. Kinetics of Isovalent (Cd²⁺) and Aliovalent (In³⁺) Cation Exchange in Cd_1-xMn_xSe Nanocrystals. J. Am. Chem. Soc. 2016, 138, 12885– 12893, DOI: 10.1021/jacs.6b05949
  55
  Kinetics of Isovalent (Cd2+) and Aliovalent (In3+) Cation Exchange in Cd1-xMnxSe Nanocrystals
  Chakraborty, Pradip; Jin, Yu; Barrows, Charles J.; Dunham, Scott T.; Gamelin, Daniel R.
  Journal of the American Chemical Society (2016), 138 (39), 12885-12893CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Ion exchange, in which an in-diffusing ion replaces a lattice ion, has been widely exploited as a synthetic tool for semiconductor doping and solid-to-solid chem. transformations, both in bulk and at the nanoscale. Here, we present a systematic investigation of cation-exchange reactions that involve the displacement of Mn2+ from CdSe nanocrystals by Cd2+ or In3+. For both incoming cations, Mn2+ displacement is spontaneous but thermally activated, following Arrhenius behavior over a broad exptl. temp. range. At any given temp., cation exchange by In3+ is approx. 2 orders of magnitude faster than that by Cd2+, illustrating a crit. dependence on the incoming cation. Quant. anal. of the kinetics data within a Fick's-law diffusion model yields diffusion barriers (ED) and limiting diffusivities (D0) for both incoming ions. Despite their very different kinetics, indistinguishable diffusion barriers of ED ≈ 1.1 eV are found for both reactions (In3+ and Cd2+). A dramatically enhanced diffusivity is found for Mn2+ cation exchange by In3+. Overall, these findings provide unique exptl. insights into cation diffusion within colloidal semiconductor nanocrystals, contributing to our fundamental understanding of this rich and important area of nanoscience.
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56. 56
  Zuckerman, J. J.; Hagen, A. P. Inorganic Reactions and Methods: Formation of Bonds to O, S, Se, Te, Po (Part 1), 1st ed.; VCH Publishers, Inc.: New York, 1992; Vol. 5, p 217.
  There is no corresponding record for this reference.
57. 57
  Burt, J.; Levason, W.; Reid, G. Coordination Chemistry of the Main Group Elements with Phosphine, Arsine and Stibine Ligands. Coord. Chem. Rev. 2014, 260, 65– 115, DOI: 10.1016/j.ccr.2013.09.020
  57
  Coordination chemistry of the main group elements with phosphine, arsine and stibine ligands
  Burt, Jennifer; Levason, William; Reid, Gillian
  Coordination Chemistry Reviews (2014), 260 (), 65-115CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
  A review. Complexes of Group 2, 12, 13, 14, 15 and 16 elements with mono-, bi-, and polydentate phosphine and arsine ligands (and including the very few examples of stibine and bismuthine donor ligands) are described. Polydentate ligand complexes contg. neutral or charged N, O, C, or S donor groups in addn. to phosphino or arsino donor groups are included, but charged P or As (phosphides, arsenides, phosphinomethanides etc.) ligands are excluded. Emphasis is placed upon the X-ray structures, multinuclear NMR data and reactions. The major differences of this class of complexes compared to the familiar d-block phosphine/arsine complexes are discussed and rationalized in terms of the E-M bonding models. Literature coverage is focused on the last 20 years, although key older work is also included where necessary for comparison purposes, and the article includes work published up to early 2013.
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58. 58
  Chen, F.; Ma, G.; Bernard, G. M.; Wasylishen, R. E.; Cavell, R. G.; McDonald, R.; Ferguson, M. J. An Investigation of 1:1 Adducts of Gallium Trihalides with Triarylphosphines by Solid-State ^69/71Ga and ³¹P NMR Spectroscopy. Chem. - Eur. J. 2013, 19, 2826– 2838, DOI: 10.1002/chem.201202954
  58
  An Investigation of 1:1 Adducts of Gallium Trihalides with Triarylphosphines by Solid-State 69/71Ga and 31P NMR Spectroscopy
  Chen, Fu; Ma, Guibin; Bernard, Guy M.; Wasylishen, Roderick E.; Cavell, Ronald G.; McDonald, Robert; Ferguson, Michael J.
  Chemistry - A European Journal (2013), 19 (8), 2826-2838CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Several 1:1 adducts of Ga trihalides with triarylphosphines, X3Ga(PR3) (X=Cl, Br, and I; PR3 = triarylphosphine ligand), were studied by using solid-state 69/71Ga and 31P NMR spectroscopy at different magnetic-field strengths. The 69/71Ga nuclear quadrupolar coupling parameters, as well as the Ga and P magnetic shielding tensors, were detd. The magnitude of the 71Ga quadrupolar coupling consts. (CQ(71Ga)) range from ∼0.9 to 11.0 MHz. The spans of the Ga magnetic shielding tensors for these complexes, δ11-δ33, range from ∼30 to 380 ppm; those detd. for P range from 10 to 40 ppm. For any given phosphine ligand, the Ga nuclei are most shielded for X = I and least shielded for X=Cl, a trend previously obsd. for InIII-phosphine complexes. This exptl. trend, attributed to spin-orbit effects of the halogen ligands, is reproduced by DFT calcns. The signs of CQ(69/71Ga) for some of the adducts were detd. from the anal. of the 31P NMR spectra acquired with magic angle spinning (MAS). The 1J(69/71Ga,31P) and ΔJ(69/71Ga, 31P) values, as well as their signs, were also detd.; values of 1J(71Ga,31P) range from ∼380 to 1590 Hz. Values of 1J(69/71Ga,31P) and ΔJ(69/71Ga,31P) calcd. by using DFT have comparable magnitudes and generally reproduce exptl. trends. Both the Fermi-contact and spin-dipolar Fermi-contact mechanisms make important contributions to the 1J(69/71Ga,31P) tensors. The 31P NMR spectra of several adducts in soln., obtained as a function of temp., are contrasted with those obtained in the solid state. Finally, to complement the anal. of NMR spectra for these adducts, single-crystal x-ray diffraction data for Br3Ga[P(p-Anis)3] and I3Ga[P(p-Anis)3] were obtained.
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59. 59
  Haynes, W. M. CRC Handbook of Chemistry and Physics, 95th ed.; CRC Press: Boca Raton, FL, 2014; section 5–11.
  There is no corresponding record for this reference.
60. 60
  Clavier, H.; Nolan, S. P. Percent Buried Volume for Phosphine and N-Heterocyclic Carbene Ligands: Steric Properties in Organometallic Chemistry. Chem. Commun. (Cambridge, U. K.) 2010, 46, 841– 861, DOI: 10.1039/b922984a
  60
  Percent buried volume for phosphine and N-heterocyclic carbene ligands: steric properties in organometallic chemistry
  Clavier, Herve; Nolan, Steven P.
  Chemical Communications (Cambridge, United Kingdom) (2010), 46 (6), 841-861CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  A review. Electronic and steric ligand effects both play major roles in organometallic chem. and consequently in metal-mediated catalysis. Quantifying such parameters is of interest to better understand not only the parameters governing catalyst performance but also reaction mechanisms. Nowadays, ligand mol. architectures are becoming significantly more elaborate and existing models describing ligand sterics prove lacking. This review presents the development of a more general method to det. the steric parameter of organometallic ligands. Two case studies are presented: the tertiary phosphines and the N-heterocyclic carbenes.
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61. 61
  Snelders, D. J. M.; Van Koten, G.; Klein Gebbink, R. J. M. Steric, Electronic, and Secondary Effects on the Coordination Chemistry of Ionic Phosphine Ligands and the Catalytic Behavior of Their Metal Complexes. Chem. - Eur. J. 2011, 17, 42– 57, DOI: 10.1002/chem.201002508
  61
  Steric, Electronic, and Secondary Effects on the Coordination Chemistry of Ionic Phosphine Ligands and the Catalytic Behavior of Their Metal Complexes
  Snelders, Dennis J. M.; van Koten, Gerard; Klein Gebbink, Robertus J. M.
  Chemistry - A European Journal (2011), 17 (1), 42-57CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. The effects of introducing ionic functionalities in phosphine ligands on the coordination chem. of these ligands and the catalytic behavior of the corresponding metal complexes are reviewed. The steric and electronic consequences of such functionalizations are discussed. Apart from these steric and electronic effects, the presence of charged groups often leads to addnl., supramol. interactions that occur in the 2nd coordination sphere of the metal complex, such as intramol., interligand H bonding and Coulombic repulsion. These interactions can significantly alter the behavior of the phosphine ligand in question. Such effects were obsd. in phosphine-metal assocn./dissocn. equil., ligand substitution reactions, and stereoisomerism in phosphine-metal complexes. By drawing general conclusions, this review offers an insight into the coordination and catalytic behavior of phosphine ligands contg. ionic functionalities and their corresponding metal complexes.
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62. 62
  Chakraborty, P.; Jin, Y.; Barrows, C. J.; Dunham, S. T.; Gamelin, D. R. Kinetics of Isovalent (Cd²⁺) and Aliovalent (In³⁺) Cation Exchange in Cd_1-xMn_xSe Nanocrystals. J. Am. Chem. Soc. 2016, 138, 12885– 12893, DOI: 10.1021/jacs.6b05949
  62
  Kinetics of Isovalent (Cd2+) and Aliovalent (In3+) Cation Exchange in Cd1-xMnxSe Nanocrystals
  Chakraborty, Pradip; Jin, Yu; Barrows, Charles J.; Dunham, Scott T.; Gamelin, Daniel R.
  Journal of the American Chemical Society (2016), 138 (39), 12885-12893CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Ion exchange, in which an in-diffusing ion replaces a lattice ion, has been widely exploited as a synthetic tool for semiconductor doping and solid-to-solid chem. transformations, both in bulk and at the nanoscale. Here, we present a systematic investigation of cation-exchange reactions that involve the displacement of Mn2+ from CdSe nanocrystals by Cd2+ or In3+. For both incoming cations, Mn2+ displacement is spontaneous but thermally activated, following Arrhenius behavior over a broad exptl. temp. range. At any given temp., cation exchange by In3+ is approx. 2 orders of magnitude faster than that by Cd2+, illustrating a crit. dependence on the incoming cation. Quant. anal. of the kinetics data within a Fick's-law diffusion model yields diffusion barriers (ED) and limiting diffusivities (D0) for both incoming ions. Despite their very different kinetics, indistinguishable diffusion barriers of ED ≈ 1.1 eV are found for both reactions (In3+ and Cd2+). A dramatically enhanced diffusivity is found for Mn2+ cation exchange by In3+. Overall, these findings provide unique exptl. insights into cation diffusion within colloidal semiconductor nanocrystals, contributing to our fundamental understanding of this rich and important area of nanoscience.
  >> More from SciFinder ^®
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63. 63
  Choi, Y.-M.; Lee, Y.-I.; Kim, S.; Choa, Y.-H. Metallic Alloy Nanoparticle-Based Fabrication and Optical Properties of a Cu(In_1-xGa_x)S₂ Absorber Layer for Solar Cells. J. Alloys Compd. 2014, 615, 496– 500, DOI: 10.1016/j.jallcom.2014.06.174
  63
  Metallic alloy nanoparticle-based fabrication and optical properties of a Cu(In1-xGax)S2 absorber layer for solar cells
  Choi, Yo-Min; Lee, Young-In; Kim, Seil; Choa, Yong-Ho
  Journal of Alloys and Compounds (2014), 615 (), 496-500CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
  A highly-densified Cu(In1-xGax)S2 (CIGS2) absorber layer was fabricated using Cu, In and Ga (CIG) metallic alloy nanoparticles synthesized by salt-assisted ultrasonic spray pyrolysis (SAUSP) followed by direct thermal redn. The redn. process in salt matrix minimized aggregation of CIG metallic alloy nanoparticles that have the potential to lead to higher film densification during sulfurization. To optimize the amt. of salt, various NaCl/precursor ratios were used for SAUSP and Cu-In metallic alloy nanoparticles with av. particle size of 79 nm were obtained. The CIGS2 obtained in the present study exhibited a variable band-gap ranging from 1.46 to 2.4 eV depending on the Ga/(In + Ga) ratio, which corresponded to the resp. bulk materials.
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64. 64
  Chang, S.-H.; Chiang, M.-Y.; Chiang, C.-C.; Yuan, F.-W.; Chen, C.-Y.; Chiu, B.-C.; Kao, T.-L.; Lai, C.-H.; Tuan, H.-Y. Facile Colloidal Synthesis of Quinary CuIn_1-xGa_x(S_ySe_1-y) (CIGSSe) Nanocrystal Inks with Tunable Band Gaps for Use in Low-Cost Photovoltaics. Energy Environ. Sci. 2011, 4, 4929– 4932, DOI: 10.1039/c1ee02341a
  64
  Facile colloidal synthesis of quinary CuIn1-xGax(SySe1-y)2 (CIGSSe) nanocrystal inks with tunable band gaps for use in low-cost photovoltaics
  Chang, Shu-Hao; Chiang, Ming-Yi; Chiang, Chien-Chih; Yuan, Fang-Wei; Chen, Chia-Yu; Chiu, Bo-Cheng; Kao, Tzu-Lun; Lai, Chi-Huang; Tuan, Hsing-Yu
  Energy & Environmental Science (2011), 4 (12), 4929-4932CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
  We report, for the first time, colloidal synthesis of quinary CuIn1-xGax(SySe1-y)2 (CIGSSe) nanocrystals across the entire compn. range (x,y) = 0 to 1 with band gaps tunable in the range of 0.98 to 2.40 eV by facile chem. synthesis. As a proof-of-concept, thin-film solar cells made by using the CIGSSe nanocrystal inks as an absorber layer precursor exhibited an efficiency over 1% under AM 1.5 illumination.
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65. 65
  Xia, C.; Meeldijk, J. D.; Gerritsen, H. C.; de Mello Donegá, C. Highly Luminescent Water-Dispersible NIR-Emitting Wurtzite CuInS₂/ZnS Core/Shell Colloidal Quantum Dots. Chem. Mater. 2017, 29, 4940– 4951, DOI: 10.1021/acs.chemmater.7b01258
  65
  Highly Luminescent Water-Dispersible NIR-Emitting Wurtzite CuInS2/ZnS Core/Shell Colloidal Quantum Dots
  Xia, Chenghui; Meeldijk, Johannes D.; Gerritsen, Hans C.; de Mello Donega, Celso
  Chemistry of Materials (2017), 29 (11), 4940-4951CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  Copper indium sulfide (CIS) quantum dots (QDs) are attractive as labels for biomedical imaging, since they have large absorption coeffs. across a broad spectral range, size- and compn.-tunable photoluminescence from the visible to the near-IR, and low toxicity. However, the application of NIR-emitting CIS QDs is still hindered by large size and shape dispersions and low photoluminescence quantum yields (PLQYs). In this work, we develop an efficient pathway to synthesize highly luminescent NIR-emitting wurtzite CIS/ZnS QDs, starting from template Cu2-xS nanocrystals (NCs), which are converted by topotactic partial Cu+ for In3+ exchange into CIS NCs. These NCs are subsequently used as cores for the overgrowth of ZnS shells (≤1 nm thick). The CIS/ZnS core/shell QDs exhibit PL tunability from the first to the second NIR window (750-1100 nm), with PLQYs ranging from 75% (at 820 nm) to 25% (at 1050 nm), and can be readily transferred to water upon exchange of the native ligands for mercaptoundecanoic acid. The resulting water-dispersible CIS/ZnS QDs possess good colloidal stability over at least 6 mo and PLQYs ranging from 39% (at 820 nm) to 6% (at 1050 nm). These PLQYs are superior to those of commonly available water-sol. NIR-fluorophores (dyes and QDs), making the hydrophilic CIS/ZnS QDs developed in this work promising candidates for further application as NIR emitters in bioimaging. The hydrophobic CIS/ZnS QDs obtained immediately after the ZnS shelling are also attractive as fluorophores in luminescent solar concentrators.
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66. 66
  Li, L.; Pandey, A.; Werder, D. J.; Khanal, B. P.; Pietryga, J. M.; Klimov, V. I. Efficient Synthesis of Highly Luminescent Copper Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission. J. Am. Chem. Soc. 2011, 133, 1176– 1179, DOI: 10.1021/ja108261h
  66
  Efficient synthesis of highly luminescent copper indium sulfide-based core/shell nanocrystals with surprisingly long-lived emission
  Li, Liang; Pandey, Anshu; Werder, Donald J.; Khanal, Bishnu P.; Pietryga, Jeffrey M.; Klimov, Victor I.
  Journal of the American Chemical Society (2011), 133 (5), 1176-1179CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  We report an efficient synthesis of copper indium sulfide nanocrystals with strong photoluminescence in the visible to near-IR. This method can produce gram quantities of material with a chem. yield in excess of 90% with minimal solvent waste. The overgrowth of as-prepd. nanocrystals with a few monolayers of CdS or ZnS increases the photoluminescence quantum efficiency to > 80%. On the basis of time-resolved spectroscopic studies of core/shell particles, we conclude that the emission is due to an optical transition that couples a quantized electron state to a localized hole state, which is most likely assocd. with an internal defect.
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67. 67
  Berends, A. C.; Rabouw, F. T.; Spoor, F. C. M.; Bladt, E.; Grozema, F. C.; Houtepen, A. J.; Siebbeles, L. D. A.; De Mello Donegá, C. Radiative and Nonradiative Recombination in CuInS₂ Nanocrystals and CuInS₂-Based Core/Shell Nanocrystals. J. Phys. Chem. Lett. 2016, 7, 3503– 3509, DOI: 10.1021/acs.jpclett.6b01668
  67
  Radiative and Nonradiative Recombination in CuInS2 Nanocrystals and CuInS2-Based Core/Shell Nanocrystals
  Berends, Anne C.; Rabouw, Freddy T.; Spoor, Frank C. M.; Bladt, Eva; Grozema, Ferdinand C.; Houtepen, Arjan J.; Siebbeles, Laurens D. A.; Donega, Celso de Mello
  Journal of Physical Chemistry Letters (2016), 7 (17), 3503-3509CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Luminescent CuInS2 (CIS) nanocrystals are a potential soln. to the toxicity issues assocd. with Cd- and Pb-based nanocrystals. The development of high-quality CIS nanocrystals was complicated by insufficient knowledge of the electronic structure and the factors that lead to luminescence quenching. The exciton decay pathways in CIS nanocrystals were studied using time-resolved luminescence and transient absorption spectroscopy. Core-only CIS nanocrystals with low quantum yield are compared to core/shell nanocrystals (CIS/ZnS and CIS/CdS) with higher quantum yield. The measurements support the model of luminescence by radiative recombination of a conduction band electron with a localized hole. Luminescence quenching in low-quantum-yield nanocrystals involves initially uncoupled decay pathways for the electron and hole. The electron decay pathway dets. whether the exciton recombines radiatively or nonradiatively. The development of high-quality CIS nanocrystals should therefore focus on the elimination of electron traps.
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68. 68
  Bai, X.; Purcell-Milton, F.; Gun’ko, Y. Optical Properties, Synthesis, and Potential Applications of Cu-Based Ternary or Quaternary Anisotropic Quantum Dots, Polytypic Nanocrystals, and Core/Shell Heterostructures. Nanomaterials 2019, 9, 85, DOI: 10.3390/nano9010085
  68
  Optical properties, synthesis, and potential applications of Cu-based ternary or quaternary anisotropic quantum dots, polytypic nanocrystals, and core/shell heterostructures
  Bai, Xue; Milton, Finn Purcell; Gun'ko, Yuri K.
  Nanomaterials (2019), 9 (1), 85/1-85/36CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
  This review summaries the optical properties, recent progress in synthesis, and a range of applications of luminescent Cu-based ternary or quaternary quantum dots (QDs). We first present the unique optical properties of the Cu-based multicomponent QDs, regarding their emission mechanism, high photoluminescent quantum yields (PLQYs), size-dependent bandgap, compn.-dependent bandgap, broad emission range, large Stokes' shift, and long photoluminescent (PL) lifetimes. Huge progress has taken place in this area over the past years, via detailed experimenting and modeling, giving a much more complete understanding of these nanomaterials and enabling the means to control and therefore take full advantage of their important properties. We then fully explore the techniques to prep. the various types of Cu-based ternary or quaternary QDs (including anisotropic nanocrystals (NCs), polytypic NCs, and spherical, nanorod and tetrapod core/shell heterostructures) are introduced in subsequent sections. To date, various strategies have been employed to understand and control the QDs distinct and new morphologies, with the recent development of Cu-based nanorod and tetrapod structure synthesis highlighted. Next, we summarize a series of applications of these luminescent Cu-based anisotropic and core/shell heterostructures, covering luminescent solar concentrators (LSCs), bioimaging and light emitting diodes (LEDs). Finally, we provide perspectives on the overall current status, challenges, and future directions in this field. The confluence of advances in the synthesis, properties, and applications of these Cu-based QDs presents an important opportunity to a wide-range of fields and this piece gives the reader the knowledge to grasp these exciting developments.
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  Berends, Anne C.; Mangnus, Mark J. J.; Xia, Chenghui; Rabouw, Freddy T.; de Mello Donega, Celso
  Journal of Physical Chemistry Letters (2019), 10 (7), 1600-1616CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Colloidal nanocrystals of ternary I-III-VI2 semiconductors are emerging as promising alternatives to Cd- and Pb-chalcogenide nanocrystals because of their inherently lower toxicity, while still offering widely tunable photoluminescence. These properties make them promising materials for a variety of applications. However, the realization of their full potential was hindered by both their underdeveloped synthesis and the poor understanding of their optoelectronic properties, whose origins are still under intense debate. In this Perspective, the authors provide novel insights on the latter aspect by critically discussing the accumulated body of knowledge on I-III-VI2 nanocrystals. From the authors' anal., the luminescence in these nanomaterials most likely originates from the radiative recombination of a delocalized conduction band electron with a hole localized at the group-I cation, which results in broad bandwidths, large Stokes shifts, and long exciton lifetimes. Finally, the authors highlight the remaining open questions and propose expts. to address them.
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  Three tools for an automated anal. of electron diffraction pattern and crystallog. visualization are presented. Firstly, diffractGUI dets. the zone axis from selected area diffraction, convergent beam diffraction or nanodiffraction patterns and allows for indexing of individual reflections. Secondly, ringGUI identifies crystallog. planes corresponding to the depicted rings in the ring diffraction pattern and can select the sample material from a list of candidates. Both diffractGUI and ringGUI employ methods of computer vision for a fast, robust and accurate anal. Thirdly, cellViewer is an intuitive visualization tool which is also helpful for crystallog. calcns. or educational purposes. diffractGUI and cellViewer can be used together during a transmission electron microscopy session to det. the sample holder tilts required to reach a desired zone axis. All the tools offer a graphical user interface. The toolbox is distributed as a standalone application, so it can be installed on the microscope computer and launched directly from DigitalMicrograph (Gatan Inc.).
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  VESTA is a 3D visualization system for crystallog. studies and electronic state calcns. It was upgraded to the latest version, VESTA 3, implementing new features including drawing the external morphpol. of crysals; superimposing multiple structural models, volumetric data and crystal faces; calcn. of electron and nuclear densities from structure parameters; calcn. of Patterson functions from the structure parameters or volumetric data; integration of electron and nuclear densities by Voronoi tessellation; visualization of isosurfaces with multiple levels, detn. of the best plane for selected atoms; an extended bond-search algorithm to enable more sophisticated searches in complex mols. and cage-like structures; undo and redo is graphical user interface operations; and significant performance improvements in rendering isosurfaces and calcg. slices.
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Supporting Information
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b05337.
- NC size histograms, EDS of NCs before and after CE reaction, absorption spectra of NCs before and after CE, TEM images of NCs after reaction with GaCl₃ and GaCl₃–DPP at 100 °C, HAADF-STEM images and EDS elemental maps of Cu_2–xS NCs after reaction with GaCl₃, detailed description of PED calibration procedure (PDF)
- File showing the results of the DFT calculations (ground-state geometries and bond enthalpies for the phosphine complexes used in our work (XYZ)
- nn9b05337_si_001.pdf (1.02 MB)
- nn9b05337_si_005.xyz (13.96 kb)
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Tailoring Cu+ for Ga3+ Cation Exchange in Cu2–xS and CuInS2 Nanocrystals by Controlling the Ga Precursor ChemistryClick to copy article linkArticle link copied!

ACS Nano

Abstract

Note

Results and Discussion

Stoichiometric InCl3–TOP and GaCl3–TOP as Reactants for Cation Exchange in Cu2–xS NCs

Figure 1

Reaction of Noncomplexed GaCl3 with Cu2–xS NCs

Figure 2

Figure 3

Tailoring the Ga Precursor Chemistry To Boost the Cu+ for Ga3+ Cation Exchange

Figure 4

Figure 5

Figure 6

Density Functional Theory Calculations

Mechanism for the Cu+ for Ga3+ Exchange in Cu2–xS NCs

Figure 7

Cu+ for Ga3+ Exchange in Luminescent CuInS2 NCs

Figure 8

Conclusions

Methods

Materials

Synthesis of Cu2–xS Bifrustum Nanocrystals

Preparation of Cation Exchange Precursors

Cation Exchange in Cu2–xS Bifrustum Nanocrystals

Cation Exchange under Milder Reaction Conditions with GaCl3–DPP

Synthesis of CuInS2 NCs

Cation Exchange in CuInS2 NCs

Optical Spectroscopy

Transmission Electron Microscopy

Energy-Dispersive X-ray Spectroscopy

Elemental Mapping

HAADF-STEM Tomography

Electron Diffraction

Calculated Diffraction Patterns

Comparison of the Anion Sublattices

DFT Calculations

Supporting Information

Terms & Conditions

Author Information

Acknowledgments

References

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Abstract

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

References

Supporting Information

Supporting Information

Terms & Conditions

Tailoring Cu⁺ for Ga³⁺ Cation Exchange in Cu_2–xS and CuInS₂ Nanocrystals by Controlling the Ga Precursor Chemistry
Click to copy article linkArticle link copied!

Stoichiometric InCl₃–TOP and GaCl₃–TOP as Reactants for Cation Exchange in Cu_2–xS NCs

Reaction of Noncomplexed GaCl₃ with Cu_2–xS NCs

Tailoring the Ga Precursor Chemistry To Boost the Cu⁺ for Ga³⁺ Cation Exchange

Mechanism for the Cu⁺ for Ga³⁺ Exchange in Cu_2–xS NCs

Cu⁺ for Ga³⁺ Exchange in Luminescent CuInS₂ NCs

Synthesis of Cu_2–xS Bifrustum Nanocrystals

Cation Exchange in Cu_2–xS Bifrustum Nanocrystals

Cation Exchange under Milder Reaction Conditions with GaCl₃–DPP

Synthesis of CuInS₂ NCs

Cation Exchange in CuInS₂ NCs