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Ligand-Induced Shape Transformation of PbSe NanocrystalsClick to copy article linkArticle link copied!
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† Debye Institute for Nanomaterials Science, University of Utrecht, Utrecht, The Netherlands
‡ EMAT, University of Antwerp, Antwerpen, Belgium
*E-mail: [email protected]
Abstract
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We present a study of the relation between the surface chemistry and nanocrystal shape of PbSe nanocrystals with a variable Pb-to-Se stoichiometry and density of oleate ligands. The oleate ligand density and binding configuration are monitored by nuclear magnetic resonance and Fourier transform infrared absorbance spectroscopy, allowing us to quantify the number of surface-attached ligands per NC and the nature of the surface–Pb–oleate configuration. The three-dimensional shape of the PbSe nanocrystals is obtained from high-angle annular dark field scanning transmission electron microscopy combined with an atom counting method. We show that the enhanced oleate capping results in a stabilization and extension of the {111} facets, and a crystal shape transformation from a truncated nanocube to a truncated octahedron.
Copyright © 2017 American Chemical Society
Introduction
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Nanometer-sized crystals (NCs) of the lead chalcogenide family continue to intrigue researchers for several reasons. First, they show bright exciton luminescence in the near infrared (IR) that can be tuned over a broad energy range by the crystal dimensions. (1) Second, lead chalcogenide nanocrystals (NCs) can be self-assembled into thin-film quantum dot solids, in which the quantum coupling between the NCs can be tuned. Thin-film systems based on NCs separated by short ligand molecules are of interest for IR photodetectors (2) and solar cells. (3-5) Recently, it has been reported that interfacial ordering and epitaxial attachment resulted in NC monolayers with a square or honeycomb nanogeometry. (6, 7)
The energy level structure, photoluminescence, and propensity to form atomically coherent two-dimensional (2D) superstructures depend critically on the size and shape of the lead chalcogenide NCs and on their surface chemistry, as well. (8-12) It has been established that PbSe NCs that are well-capped with oleate have an excess of Pb atoms. (13-15) In addition, it is theoretically shown that the binding strength and configuration of oleate ligands depend on the type of facet. (16, 17)
Lead chalcogenide NCs, with PbSe being of interest here, have a rock salt crystal structure. When we consider the uncoordinated crystal, the {100} facets, containing square ordered Pb and Se, have the lowest surface energy. (17-19) It is thus expected that PbSe NCs have a cubic shape, terminated by six {100} facets. The Wulff reconstruction is presented in section S1 of the Supporting Information. However, in the case of PbSe capped with oleate ligands, different nanocrystal shapes have been reported ranging from a truncated nanocube (or rhombal cuboctahedron) to a truncated octahedron. (6, 8, 20) Hence, besides the {100} facets, {111} and {110} facets are also observed. Moreover, it is not yet clear why different PbSe NC shapes are observed,and how this is related to the surface chemistry.
Here, we present a study of the relation between the surface chemistry and nanocrystal shape of PbSe NCs (5.6 nm) capped with oleate ligands. Pb–oleate binding is monitored by Fourier transform infrared (FTIR) absorbance and nuclear magnetic resonance (NMR) spectroscopy, allowing us to quantify the nature of the surface–Pb–oleate configuration and the number of surface-attached ligands per NC. The shape of the NCs in three dimensions is obtained via high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) combined with an atom counting method that has been established previously. (21, 22) We have studied the relation between the shape and surface chemistry under well-defined chemical conditions. After conventional PbSe synthesis, we have washed the NCs eight times to remove all remaining precursor moieties and reduce the oleate capping to a minimum. Then we gradually added Pb(oleate)2 to the suspension and monitored the attached oleate capping (by NMR) and the NC shape, as well. Our results show that enhanced oleate capping results in a stabilization and extension of the {111} facets and a remarkable transformation of the nanocrystal shape.
Results
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Figure 1A shows an overview of the infrared spectrum of a PbSe nanocrystal suspension in tetrachloroethylene (TCE) that has been washed eight times with methanol. Washing removes unreacted precursor molecules, excess ligand molecules, and possibly ligand molecules that are only weakly bound to the NCs (see section S2 of the Supporting Information). (23, 24) In Figure 1A, three regions can be distinguished. The peaks in region a are due to the carboxylic stretching vibrations and will be detailed below. The peaks in region b are due to the CH2 and CH3 stretching vibrations of the oleate. The IR absorbance in region c, starting with the peak at 0.64 eV and extending to higher energies, reflects the optical transitions in the PbSe NCs. Because of a calibration procedure developed by Moreels et al., (13) we can use this peak to determine the concentration of PbSe NCs in the solution.
Figure 1
Figure 1. Infrared spectrum of well-washed oleate-capped PbSe NCs in tetrachloroethylene. (A) FTIR spectrum of well-washed PbSe NCs (5.6 nm) dissolved in tetrachloroethylene showing three regions of interest: (a) region of C–O vibrations, detailed in Figure 2, (b) a peak due to the CH2 asymmetric stretching vibrations of oleate molecules (attached to the NCs or dissolved), and (c) the IR absorption peak due to the band-edge exciton in the PbSe NCs. From the integration of region c, the concentration of NCs in the solution can be determined. From the integration of peak b, the total concentration of oleate in the suspension (the sum of oleate attached to the NCs and dissolved oleate) can be determined. This is performed by using a calibration curve (panel B), i.e., the integrated absorbance under peak b vs the concentration of Pb(oleate)2 in a solution containing only Pb(oleate)2 (R2 = 0.999).
Using the integrated value under peak b, we can determine the total amount of oleate present in the suspension. We therefore calibrated peak b (see Figure 1B). Subsequently, we used the integrated peak value (Figure 1A) to determine the total number of oleate moieties present in the suspension (attached to the NCs and freely dissolved). Combining the integrated values of peaks b and c, we can obtain the total number of oleate moieties per PbSe nanocrystal present in the suspension.
We remark here that peaks due to H2O and OH are completely absent in the spectra (see Figure S3), excluding the presence of water, hydroxide, and oleic acid as ligands on our PbSe NCs. A comparison of the FTIR spectra of diphenylphosphine (DPP), trioctylphosphine (TOP), and Pb(oleate)2 with the PbSe NC suspension excludes DPP and indicates that TOP can be present in only small quantities, if it is present at all (see Figure S4). These findings contrast with those reported in the literature, where for PbS NCs, the presence of oleic acid and hydroxide on different facets of the surface of NCs has been reported. (17, 25) We remark here that the hydroxide peak found by Grisorio et al. (25) cannot be attributed to oxidation of the dots (see Figure S5). As Grisorio et al. claimed, it can be the result from the use of Pb(oleate)2-hydrate in their advanced synthesis procedure for PbS nanocrystals. (26) We also remark that we did not observe hydroxide and oleic acid signals for the NCs synthesized with the method developed by Campos et al. (27)
Now, we focus on the carboxyl stretch vibrations in the energy region between 1350 and 1750 cm–1. A detailed spectrum of the well-washed PbSe NCs (Figure 2A–C) shows two broad peaks, reflecting the symmetric (ν1) and asymmetric carboxyl stretch (ν2) of the oleate moiety (Figure 2B). Figure 2A also displays the reference spectra of dissolved Pb(oleate)2 (green) and oleic acid (blue), indicating that the Pb(oleate)2 solution contains oleic acid as an impurity. A spectrum that shows only the features of a pure Pb(oleate)2 solution can be obtained by subtraction (see Figure S6). From comparison of the spectra in Figure 2a, it becomes clear that the spectrum of the well-washed PbSe NCs lacks the peak at 0.212 eV, specific for oleic acid, and has a much more pronounced ν1 peak due to the symmetric carboxylic stretch vibration.
Figure 2
Figure 2. Detailed spectra of the carboxylic region. In panel A, the carboxylic regions of washed NCs (red), Pb(oleate)2 (green), and oleic acid (blue) are shown. The carboxylic vibrations are sketched in panel B. Symmetric and asymmetric carboxylic vibrations ν1 and ν2 in PbSe NCs and Pb(oleate)2 are each composed of two peaks. The decomposition and notation are presented in panel C; the meaning is discussed in the text. Panel D shows the carboxylic region for well-washed NCs (red) and titrated NCs (blue). Panel E presents sketches of the bridging configuration and the (a)symmetric chelating bidentate configurations.
Peaks ν1 and ν2 each consist of a doublet (see Figure 2C). For peak ν1, we note the (left) low-energy peak as ν1,b and the one at a higher energy as ν1,s. Notice that for ν2, the lower-energy peak is denoted as ν2,s and the high-energy peak as ν2,b. The peak splitting suggests that we have two types of Pb–oleate binding configurations. Cass et al. showed that the inner couple (ν1,s and ν2,s) can be attributed to an asymmetric chelating bidentate configuration (see Figure 2E) if the splitting is between 100 and 125 cm–1. (28) We find a separation between the inner peaks of 103 cm–1, indicating that the asymmetric Pb–oleate chelating bidentate configuration is one of the dominating configurations. (28, 29) The outer peaks (ν1,b and ν2,b) point to a bridging configuration of the oleate bound to two neighboring Pb atoms. The splitting that we observe, 141 cm–1, is in line with such a bridging configuration. (30, 31) The intensity of the symmetric vibration is much larger for NCs than for dissolved Pb(oleate)2. The relative intensity of a vibrational peak increases with the increasing dipole moment of the vibrating moiety. This would mean that the Pb–oleate bidentate complex attached to a facet has a dipole moment that is stronger than that of the Pb–oleate complexes that form in solution.
Via comparison of the full width at half-maximum (fwhm) of each of the composed peaks, ν1 and ν2, to the corresponding peaks of free Pb(oleate)2, it is clear that there is a strong broadening of the surface-attached Pb–oleate complex (Table 1). The fwhm of the composed symmetric stretch ν1 increases from 30 to 36 cm–1, while the asymmetric stretch ν2 increases from 56 to 68 cm–1. It is tempting to relate this to the roughness of the facets for which we present evidence below.
Table 1. Distances and Full Widths at Half-Maximum of the Carboxylic Peaks
| Δ(ν1b–ν2b) (cm–1) | Δ(ν1s–ν2s) (cm–1) | fwhm ν1 (cm–1) | fwhm ν2 (cm–1) | |
|---|---|---|---|---|
| Pb(oleate)2 | 150 | 112 | 30 | 56 |
| well-washed | 141 | 103 | 36 | 68 |
| titrated NCs | 120 | 108 | 35 | 62 |
Now, we discuss the equilibrium between freely dissolved Pb(oleate)2 and Pb–oleate units at the surface of the PbSe NCs. With the analysis described above (peak b in Figure 1 referred to the NC concentration obtained from peak c in Figure 1) and from NMR results detailed below, we found for the well-washed NC suspension (red curve) an average of 258–276 oleates/NC, corresponding to a ligand density of 2.2–2.3 oleates/nm2 (assuming a spherical shape). To test if this is really the minimum, a small amount of halide salt dissolved in methanol is added to the suspension. The ligand density can in this way be reduced to 2.0 oleates/nm2, because of ligand exchange. We did not observe lower densities for colloidally stable PbSe NCs. Next, we performed a titration experiment starting from a suspension of PbSe NCs, washed eight times, to which we gradually added Pb(oleate)2. In this titration experiment, we added Pb(oleate)2 to the well-washed suspension and followed the evolution of the 1H NMR signal of the two hydrogen atoms bound to the C═C atoms of oleic acid (Figure 3A). It can be seen that because of the large difference in diffusion coefficients, freely dissolved Pb(oleate)2 (black curve) can be distinguished from the oleate attached to the NCs (24, 32) (red curve). Via addition of Pb(oleate)2 (colored curves), the spectra combine the contributions of attached and free oleate. They can be quantified by fitting the curves on the basis of the red (only attached oleate) and black (only free oleate) spectra (see Figure S7). Via a reference molecule in a known amount (an internal standard) and the known concentration of NCs, it is also possible to calculate the number of ligands per NC and the ligand density (Figure S8), as demonstrated previously. (24, 32) The results of the titration experiment are presented in Figure 3B, showing that the number of attached oleate moieties first increases to saturate at a maximum of approximately 368 oleates/NC. This must be the maximal oleate ligand coverage possible per nanocrystal, which corresponds to a ligand density of 3.2 oleates/nm2, in agreement with previous results for as-synthesized NCs. (10, 32)
Figure 3
Figure 3. Titration of a well-washed PbSe NC suspension with Pb(oleate)2. A) Close-up of the 1H NMR spectra for the two hydrogen atoms bound to the C═C atoms in the oleate. The black spectrum of freely dissolved Pb(oleate)2 and the red spectrum of well-washed PbSe NCs are normalized on their methyl integral, so spectra at similar oleate concentrations can be compared. The spectrum of well-washed NCs (red) shows a broad feature, indicating all oleate is bound to the surface. The orange, yellow, etc., spectra were obtained during gradual addition of Pb(oleate)2 to the well-washed NC suspension. It can be seen that first the magnitude of the broad peak increases (maximum at 5.3 ppm) while with an increase in the amount of Pb(oleate)2 added the sharp peak at 5.35 ppm also arises. Deconvolution of both peaks (shown in Figure S7) allowed us to quantify the number of attached and freely dissolved Pb(oleate)2 per NC during the titration experiment. This is shown in panel B, including the red dot determined via FTIR. The derived ligand densities are colored orange (2.4 oleates/nm2), yellow (2.5 oleates/nm2), green (3.2 oleates/nm2), blue (3.1 oleates/nm2), and purple (3.2 oleates/nm2).
We emphasize that our quantification of the chemisorption process in Pb(oleate)2 units does not mean that the chemisorbed entities remain preserved as Pb(oleate)2. In Figure 2 and Table 2, it can be seen that for the titrated NCs the intensity of the outer peaks decreases, indicating relatively less oleate bound in bridging configurations. Moreover, the peak widths are slightly reduced after the titration, indicating that the titration results in facets with less atomic disorder. It will be shown below that Pb(oleate)2 chemisorption is followed by reconfiguration of the Pb–ligand binding and a transformation of the nanocrystal shape.
Table 2. Percentages of Surface Atoms That Have a Specific Coordination Number That Corresponds to Surface Facets
| low ligand density | high ligand density | |
|---|---|---|
| {100} coordination | 12.1% (10.7–13.4%) | 13.6% (12.2–15.0%) |
| {110} coordination | 18.3% (16.7–20.0%) | 14.9% (13.4–16.3%) |
| {111} coordination | 17.3% (15.7–18.9%) | 21.8% (20.1–23.4%) |
| irregular coordination | 52.3% (50.2–54.4%) | 49.8% (47.7–51.8%) |
We have investigated the three-dimensional (3D) atomic structure of PbSe NCs with the minimal and maximal values of oleate capping. Because the NCs in this study are sensitive to the electron beam, high-resolution electron tomography could not be applied to determine their three-dimensional (3D) shape. (33-35) Therefore, the 3D atomic structure is extracted from 2D HAADF-STEM projection images, a statistical atom counting procedure and an energy relaxation procedure, developed in previous work (see Figures S9 and S10). (21, 36, 37)
HAADF-STEM images showing an overview of the particles with minimal and maximal oleate coverage are displayed in panels A and F of Figure 4, respectively. The NCs with a high ligand density assemble into a hexagonal array. HAADF-STEM images acquired using a higher magnification for both samples are presented in panels B and G of Figure 4. Both images show a nanocrystal oriented along a [110] zone axis. These images have been used as input for a statistical atom counting procedure yielding the number of atoms in each projected atomic column (Figure 4C,H). Here, counting results are obtained with 90% single-atom sensitivity, meaning that for 90% of all atom columns the number of atoms is correctly assigned, whereas the probability to miscount by ±1 atom is 10%.
Figure 4
Figure 4. HAADF-STEM study of PbSe NCs with an atom counting technique. The top row (A–E) shows the shape analysis for low-ligand density NCs and the bottom row (F–J) the maximal ligand densities. Panels A and F are HAADF-STEM images of a group of PbSe NCs, whereas panels B and G are magnified images for PbSe NCs that are oriented in such a way that the lead atoms are oriented in columns. In panels C and H, the intensities of the columns are integrated and the numbers of lead atoms in that row are counted and color-coded. Subsequently, 10 NCs of each sample are averaged (see Figure S10 for the procedure) in panels D and I. We propose models E and J for the fitted data shown in panels D and I, respectively. Notice the increased size of the {111} facets for the oleate-saturated NCs.
Starting from these numbers, we can obtain a 3D model for each individual nanocrystal using a Monte Carlo-based energy minimization scheme. (37, 38) We have performed such a procedure for 10 different NCs of both ligand densities, as presented in Figure S9. It can be observed that for the particles with a low ligand density, the crystal facets are rather small and contain several loose atoms. In line with atomic simulations of ligand-free NCs, we attribute this surface roughness to atomic surface reconstructions. (18) The possibility that the electron beam induces some atom sublimation, however, cannot be excluded, although the exposure time is kept as short as possible. The 3D shape determination shows that particles with a high ligand density have {111} facets that are larger than those of low-ligand density particles. To obtain a prototypical shape of the PbSe NCs, an averaging method was applied that uses the 3D models of the individual NCs as a basis (see Figure S10 for a more detailed explanation of the averaging procedure). The result of this procedure is presented in panels D and I of Figure 4; it can be observed that the shape of the well-washed PbSe NCs (low level of oleate capping) is a nanocube in which truncation results in the presence of {110} and {111} facets, besides the {100} facets. In addition, there is a considerable degree of atomic disorder present on each of these facets that we attribute to atomic surface reconstructions of the bare crystal facets.
For the NCs with maximal oleate capping, a remarkable transformation of the shape is observed. The {111} facets become more dominant at the expense of the {110} facets, although the latter do not disappear completely. Thus, the added Pb(oleate)2 ligands stabilize and order the {111} facets. The truncated octahedral shape is similar to that found for PbS crystals (17) and for as-synthesized PbSe NCs. (16, 20) Idealized models are presented in panels E and J of Figure 4. To quantify the increase in the number of {111} facets for the sample with a high ligand density, the coordination number of surface Pb atoms with other Pb atoms can be used. This number can be related to the surface types that are present in the final configurations. The results are presented in Table 2 together with the 68% confidence intervals, indicating that 22% of the surface atoms belong to a {111} facet (coordination number of 9) for the sample with a high ligand density. For the PbSe NCs with a low ligand density, only 17% of the surface atoms belonged to a {111} facet. The number of irregular atoms, not having a {100}, {110}, or {111} coordination number, decreases from 53 to 49% with an increase in ligand density. Edge atoms are also included in the irregular category, because these have a lower coordination number. Thus, the real facets are on average two Pb atoms larger in the two orthogonal directions (see Figure 4) than following from Table 2.
Discussion
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We start with the well-washed PbSe NCs that have a truncated cubic shape with rather rough facets and a minimum of capping ligands. For such a nanocrystal type, the surface energies of the three facets have been calculated by Schapotschnikov et al. (18) for the case in which no capping is present: the {100} facets have an energy much lower than that of the {110} and {111} facets. In fact, naked {111} facets have such a high energy that they tend to be reconstructed in simulations: on Pb-terminated {111} surfaces, for example, Pb atoms are partially pulled inward while Se atoms pop up, resulting in a reconstructed rough surface. The well-washed samples have a relatively small number of oleate ligands [258–276 per nanocrystal (see above)]; it is reasonable to assume that they are preferentially bound to Pb atoms of the {111} surfaces to minimize the energy of the entire nanocrystal. The amount of surface lead atoms on the {111} crystalline facet can be roughly estimated from the averaged model (Figure 4E). This results in 220 Pb atoms; this number is too small to accommodate all oleate ligands (258 or 276 on average), and thus, ∼20% of the oleates is bound the {110} or {100} surfaces. The ideal position of a Pb–oleate moiety would be in the fcc valley of three Se atoms at the {111} facet; the Pb–oleate moieties attached to the {110} or {100} facets together with surface roughness would explain the broadening of the ν1 and ν2 peaks in the carboxylic region (see Figure 2).
What happens to the well-washed PbSe NCs when Pb(oleate)2 is added to the PbSe NC suspension? Initially, the added Pb(oleate)2 can bind to any Se surface atom, as an MX2 or Z-type ligand. (39) It has been reported that the (surface) atoms of PbSe nanocrystals have a high mobility, even at room temperature. (23, 24, 40) This should allow the chemisorbed Pb(oleate)2 to reconfigure such that the energy of the PbSe nanocrystals is minimized. We observed that the {111} facets become more extended at the expense of the {110} facets. This can occur via the following steps. (i) The chemisorbed Pb(oleate)2 is split into a Pb(oleate) moiety that becomes optimally coordinated in the valley of three Se surface atoms. (ii) The remaining oleate binds to a free Pb atom at a valley position. This process is presented in a schematic way in Figure 5. The process of Pb(oleate)2 chemisorption, splitting, and stabilization and extension of Pb–oleate-terminated {111} surfaces is schematically presented in Scheme 1.
Scheme 1
Scheme 1. Chemisorption of Pb(oleate)2 and the Ensuing Reconfiguration
Figure 5
Figure 5. Scheme describing the attachment and integration of a Pb(oleate)2. Attachment of these Z-type ligands on a PbSe NC resulting in stabilization of a Pb–oleate-terminated {111} facet. Panel A shows an unsaturated facet, typical for well-washed PbSe NCs. Panels B and C show the chemisorbtion of Pb(oleate)2 as a Z-type ligand. Panel D shows the stabilization of the Pb–oleate-terminated {111} facets.
This formal notation accounts for charge neutrality and mass balance. The reconfiguration results in a stabilization and extension of the {111} facets, in agreement with the observation. For the well-washed PbSe samples, y is approximately 184, including the edges of the {111} facets. After chemisorption of the added Pb(oleate)2 and the reconfiguration process, the maximal excess of Pb per nanocrystal amounts to y/2 + n. The maximal oleate coverage y + 2n is equal to 368 oleates/NC. This can be compared with the number of available {111} lead surface positions on the averaged PbSe NCs presented in Figure 4I, being 356. This means that PbSe nanocrystals with a maximal oleate coverage have almost all oleate ligands attached to the {111} facets and its edges.
By the addition of Pb(oleate)2 to a suspension of well-washed NCs, the NCs acquire a larger excess of lead atoms, the charge neutrality is preserved, and the ligand stabilization of the {111} surfaces is optimized (see Scheme 1 and Figure 5). This is in line with the reduced IR line widths, indicating reduced levels of atomic disorder on the facets and more (a)symmetric carboxylic configurations (see Table 1 and Figure 2D). Moreover, the stabilization and extension of the {111} facets result in a shape transformation of the nanocrystal from a truncated cube to a cuboctahedron (truncated octahedron) solely with {111} and {100} facets. We believe that the final situation that we reach by addition of Pb(oleate)2 to well-washed samples is close to that of “as-synthesized” PbSe nanocrystals.
In summary, we have used IR absorption spectroscopy and NMR to quantitatively measure the number of Pb(oleate) ligands per PbSe nanocrystal. We have shown that PbSe NCs can be colloidally stable with a wide range of oleate ligand densities. Our washing and titration cycle suggests that the process of removing and adding oleate ligands is reversible. Pb(oleate)2 chemisorption and surface reconfiguration result in a transformation of the nanocrystal shape from a truncated nanocube with rough surface facets to a truncated octahedron with larger and smooth {111} facets.
Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01103.
Experimental methods, explanation of the Wulff reconstruction, several supporting FTIR spectra, separation of oleic acid from Pb(oleate)2, procedure for determining the amount of bound and unbound Pb(oleate)2 via NMR, and supportive HAADF-STEM data, including a detailed explanation of the averaging procedure (Figure S9) (PDF)
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Author Information
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- Daniël Vanmaekelbergh - Debye Institute for Nanomaterials Science, University of Utrecht, Utrecht, The Netherlands;
http://orcid.org/0000-0002-3535-8366;
- Joep L. Peters - Debye Institute for Nanomaterials Science, University of Utrecht, Utrecht, The Netherlands;http://orcid.org/0000-0002-3415-648X
Acknowledgment
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D.V. acknowledges the European Research Council, ERC advanced grant, Project 692691-First Step, for financial support. We also acknowledge the Dutch FOM programme “Designing Dirac carriers in honeycomb semiconductor superlattices” (FOM Program 152) for financial support. The authors gratefully acknowledge funding from the Research Foundation Flanders (G.036915, G.037413, and funding of a Ph.D. research grant to K.H.W.v.d.B. and a postdoctoral grant to B.G.). S.B. acknowledges the European Research Council, ERC Grant 335078-Colouratom.
References
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This article references 40 other publications.
- 1Steckel, J. S.; Coe-Sullivan, S.; Bulović, V.; Bawendi, M. G. 1.3μm to 1.55μm Tunable Electroluminescence from PbSe Quantum Dots Embedded within an Organic Device Adv. Mater. 2003, 15, 1862– 1866 DOI: 10.1002/adma.200305449Google Scholar1https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXpt1yls74%253D&md5=59337ea91bad0841bbe59a64a019fe5e1.3 μm to 1.55 μm tunable electroluminescence from PbSe quantum dots embedded within an organic deviceSteckel, Jonathan S.; Coe-sullivan, Seth; Bulovic, Vladimir; Bawendi, Moungi G.Advanced Materials (Weinheim, Germany) (2003), 15 (21), 1862-1866CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)Large area near-IR (NIR) electroluminescence from quantum dot LEDs are demonstrated. The LED emission is tuned from λ = 1.32-1.56 μm as the PbSe QD diam. is changed dcore = 4-5 nm, without any modification to the device structure. Processing of the colloidally grown semiconductor QDs and the org. small mols. from the same solvent allows for the simple fabrication of double heterojunction devices, where the NIR emissive layer is only 1 monolayer thick. Using CdSe QD-LEDs as a model, significant improvement is expected of PbSe QD-LED performance through both device engineering and chem. optimization of the materials.
- 2McDonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Solution-Processed PbS Quantum Dot Infrared Photodetectors and Photovoltaics Nat. Mater. 2005, 4, 138– 142 DOI: 10.1038/nmat1299Google Scholar2https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXpt1agtg%253D%253D&md5=8451348fe3e450f88e59db69c8fca144Solution-processed PbS quantum dot infrared photodetectors and photovoltaicsMcDonald, Steven A.; Konstantatos, Gerasimos; Zhang, Shiguo; Cyr, Paul W.; Klem, Ethan J. D.; Levina, Larissa; Sargent, Edward H.Nature Materials (2005), 4 (2), 138-142CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)In contrast to traditional semiconductors, conjugated polymers provide ease of processing, low cost, phys. flexibility and large area coverage. These active optoelectronic materials produce and harvest light efficiently in the visible spectrum. The same functions are required in the IR for telecommunications (1,300-1,600 nm), thermal imaging (1,500 nm and beyond), biol. imaging (transparent tissue windows at 800 nm and 1,100 nm), thermal photovoltaics (>1,900 nm), and solar cells (800-2,000 nm). Photoconductive polymer devices have yet to demonstrate sensitivity beyond ∼800 nm. Sensitizing conjugated polymers with IR-active nanocrystal quantum dots provides a spectrally tunable means of accessing the IR while maintaining the advantageous properties of polymers. Here we use such a nanocomposite approach in which PbS nanocrystals tuned by the quantum size effect sensitize the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) into the IR. We achieve, in a soln.-processed device and with sensitivity far beyond 800 nm, harvesting of IR-photogenerated carriers and the demonstration of an IR photovoltaic effect. We also make use of the wavelength tunability afforded by the nanocrystals to show photocurrent spectra tailored to three different regions of the IR spectrum.
- 3Zhang, J.; Gao, J.; Church, C. P.; Miller, E. M.; Luther, J. M.; Klimov, V. I.; Beard, M. C. PbSe Quantum Dot Solar Cells with More than 6% Efficiency Fabricated in Ambient Atmosphere Nano Lett. 2014, 14, 6010– 6015 DOI: 10.1021/nl503085vGoogle Scholar3https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsFSqu7%252FL&md5=34108502bf839de9924d03bd0fc9c996PbSe Quantum Dot Solar Cells with More than 6% Efficiency Fabricated in Ambient AtmosphereZhang, Jianbing; Gao, Jianbo; Church, Carena P.; Miller, Elisa M.; Luther, Joseph M.; Klimov, Victor I.; Beard, Matthew C.Nano Letters (2014), 14 (10), 6010-6015CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Colloidal quantum dots (QDs) are promising candidates for the next generation of photovoltaic (PV) technologies. Much of the progress in QD PVs is based on using PbS QDs, partly because they are stable under ambient conditions. There is considerable interest in extending this work to PbSe QDs, which have shown an enhanced photocurrent due to multiple exciton generation (MEG). One problem complicating such device-based studies is a poor stability of PbSe QDs toward exposure to ambient air. Here a direct cation exchange synthesis is developed to produce PbSe QDs with a large range of sizes and with in situ chloride and cadmium passivation. The synthesized QDs have excellent air stability, maintaining their photoluminescence quantum yield under ambient conditions for > 30 days. Using these QDs, high-performance solar cells are fabricated without any protection and demonstrate a power conversion efficiency exceeding 6%, which is a current record for PbSe QD solar cells.
- 4Marshall, A. R.; Young, M. R.; Nozik, A. J.; Beard, M. C.; Luther, J. M. Exploration of Metal Chloride Uptake for Improved Performance Characteristics of PbSe Quantum Dot Solar Cells J. Phys. Chem. Lett. 2015, 6, 2892– 2899 DOI: 10.1021/acs.jpclett.5b01214Google Scholar4https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtFequ7rO&md5=9e24825b03b6802cbb94d448046296f1Exploration of Metal Chloride Uptake for Improved Performance Characteristics of PbSe Quantum Dot Solar CellsMarshall, Ashley R.; Young, Matthew R.; Nozik, Arthur J.; Beard, Matthew C.; Luther, Joseph M.Journal of Physical Chemistry Letters (2015), 6 (15), 2892-2899CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)We explored the uptake of metal chloride salts with +1 to +3 metals of Na+, K+, Zn2+, Cd2+, Sn2+, Cu2+, and In3+ by PbSe QD solar cells. We also compared CdCl2 to Cd acetate and Cd nitrate treatments. PbSe QD solar cells fabricated with a CdCl2 treatment are stable for more than 270 days stored in air. We studied how temp. and immersion times affect optoelectronic properties and photovoltaic cell performance. Uptake of Cd2+ and Zn2+ increase open circuit voltage, whereas In3+ and K+ increase the photocurrent without influencing the spectral response or first exciton peak position. Using the most beneficial treatments we varied the bandgap of PbSe QD solar cells from 0.78 to 1.3 eV and find the improved VOC is more prevalent for lower bandgap QD solar cells.
- 5Lan, X.; Voznyy, O.; Kiani, A.; García de Arquer, F. P.; Abbas, A. S.; Kim, G.; Liu, M.; Yang, Z.; Walters, G.; Xu, J.; Yuan, M.; Ning, Z.; Fan, F.; Kanjanaboos, P.; Kramer, I.; Zhitomirsky, D.; Lee, P.; Perelgut, A.; Hoogland, S.; Sargent, E. H. Passivation Using Molecular Halides Increases Quantum Dot Solar Cell Performance Adv. Mater. 2016, 28, 299– 304 DOI: 10.1002/adma.201503657Google Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhvVOms7jJ&md5=1c0ae6d5e1f178ea6e8ebf77f4fc38b5Passivation Using Molecular Halides Increases Quantum Dot Solar Cell PerformanceLan, Xinzheng; Voznyy, Oleksandr; Kiani, Amirreza; Garcia de Arquer, F. Pelayo; Abbas, Abdullah Saud; Kim, Gi-Hwan; Liu, Mengxia; Yang, Zhenyu; Walters, Grant; Xu, Jixian; Yuan, Mingjian; Ning, Zhijun; Fan, Fengjia; Kanjanaboos, Pongsakorn; Kramer, Illan; Zhitomirsky, David; Lee, Philip; Perelgut, Alexander; Hoogland, Sjoerd; Sargent, Edward H.Advanced Materials (Weinheim, Germany) (2016), 28 (2), 299-304CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)Passivation using mol. halides increases quantum dot solar cell performance.
- 6Evers, W. H.; Goris, B.; Bals, S.; Casavola, M.; De Graaf, J.; van Roij, R.; Dijkstra, M.; Vanmaekelbergh, D. Low-Dimensional Semiconductor Superlattices Formed by Geometric Control over Nanocrystal Attachment Nano Lett. 2013, 13, 2317– 2323 DOI: 10.1021/nl303322kGoogle Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsVyks77O&md5=3d750a2725ab1f7ac06c4370871bff0dLow-Dimensional Semiconductor Superlattices Formed by Geometric Control over Nanocrystal AttachmentEvers, Wiel H.; Goris, Bart; Bals, Sara; Casavola, Marianna; de Graaf, Joost; Roij, Rene van; Dijkstra, Marjolein; Vanmaekelbergh, DanielNano Letters (2013), 13 (6), 2317-2323CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Oriented attachment, the process in which nanometer-sized crystals fuse by at. bonding of specific crystal facets, is expected to be more difficult to control than nanocrystal self-assembly that is driven by entropic factors or weak van der Waals attractions. Here, we present a study of oriented attachment of PbSe nanocrystals that counteract this tuition. The reaction was studied in a thin film of the suspension casted on an immiscible liq. at a given temp. We report that attachment can be controlled such that it occurs with one type of facets exclusively. By control of the temp. and particle concn. we obtain one- or two-dimensional PbSe single crystals, the latter with a honeycomb or square superimposed periodicity in the nanometer range. We demonstrate the ability to convert these PbSe superstructures into other semiconductor compds. with the preservation of crystallinity and geometry.
- 7Boneschanscher, M. P.; Evers, W. H.; Geuchies, J. J.; Altantzis, T.; Goris, B.; Rabouw, F. T.; van Rossum, S. A. P.; van der Zant, H. S. J.; Siebbeles, L. D. A.; Van Tendeloo, G.; Swart, I.; Hilhorst, J.; Petukhov, A. V.; Bals, S.; Vanmaekelbergh, D. Long-Range Orientation and Atomic Attachment of Nanocrystals in 2D Honeycomb Superlattices Science (Washington, DC, U. S.) 2014, 344, 1377– 1380 DOI: 10.1126/science.1252642Google Scholar7https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXpslygs78%253D&md5=9dccd97b5dd2e26aa7a5b3edfa00df16Long-range orientation and atomic attachment of nanocrystals in 2D honeycomb superlatticesBoneschanscher, M. P.; Evers, W. H.; Geuchies, J. J.; Altantzis, T.; Goris, B.; Rabouw, F. T.; van Rossum, S. A. P.; van der Zant, H. S. J.; Siebbeles, L. D. A.; Van Tendeloo, G.; Swart, I.; Hilhorst, J.; Petukhov, A. V.; Bals, S.; Vanmaekelbergh, D.Science (Washington, DC, United States) (2014), 344 (6190), 1377-1380CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Oriented attachment of synthetic semiconductor nanocrystals is emerging as a route for obtaining new semiconductors that can have Dirac-type electronic bands such as graphene, but also strong spin-orbit coupling. The two-dimensional (2D) assembly geometry will require both at. coherence and long-range periodicity of the superlattices. We show how the interfacial self-assembly and oriented attachment of nanocrystals results in 2D metal chalcogenide semiconductors with a honeycomb superlattice. We present an extensive at. and nanoscale characterization of these systems using direct imaging and wave scattering methods. The honeycomb superlattices are atomically coherent and have an octahedral symmetry that is buckled; the nanocrystals occupy two parallel planes. Considerable necking and large-scale at. motion occurred during the attachment process.
- 8Baumgardner, W. J.; Whitham, K.; Hanrath, T. Confined-but-Connected Quantum Solids via Controlled Ligand Displacement Nano Lett. 2013, 13, 3225– 3231 DOI: 10.1021/nl401298sGoogle Scholar8https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXpsFGmtro%253D&md5=9ad9507c3d9f42e2a56c9837d8784044Confined-but-Connected Quantum Solids via Controlled Ligand DisplacementBaumgardner, William J.; Whitham, Kevin; Hanrath, TobiasNano Letters (2013), 13 (7), 3225-3231CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Confined-but-connected quantum dot solids (QDS) combine the advantages of tunable, quantum-confined energy levels with efficient charge transport through enhanced electronic interdot coupling. The authors report the fabrication of QDS by treating self-assembled films of colloidal PbSe quantum dots with polar nonsolvents. Treatment with DMF balances the rates of self-assembly and ligand displacement to yield confined-but-connected QDS structures with cubic ordering and quasi-epitaxial interdot connections through facets of neighboring dots. The QDS structure was analyzed by a combination of TEM and wide-angle and small-angle x-ray scattering. Excitonic absorption signatures in optical spectroscopy confirm that quantum confinement is preserved. Transport measurements show significantly enhanced cond. in treated films.
- 9Allan, G. Surface Core-Level Shifts and Relaxation of Group-IV A-Element Chalcogenide Semiconductors Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 43, 9594– 9598 DOI: 10.1103/PhysRevB.43.9594Google ScholarThere is no corresponding record for this reference.
- 10Choi, J. J.; Bealing, C. R.; Bian, K.; Hughes, K. J.; Zhang, W.; Smilgies, D. M.; Hennig, R. G.; Engstrom, J. R.; Hanrath, T. Controlling Nanocrystal Superlattice Symmetry and Shape-Anisotropic Interactions through Variable Ligand Surface Coverage J. Am. Chem. Soc. 2011, 133, 3131– 3138 DOI: 10.1021/ja110454bGoogle Scholar10https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhslyhsrY%253D&md5=e2c606e777197de18ab92b57e14e83c7Controlling Nanocrystal Superlattice Symmetry and Shape-Anisotropic Interactions through Variable Ligand Surface CoverageChoi, Joshua J.; Bealing, Clive R.; Bian, Kai-Fu; Hughes, Kevin J.; Zhang, Wen-Yu; Smilgies, Detlef-M.; Hennig, Richard G.; Engstrom, James R.; Hanrath, TobiasJournal of the American Chemical Society (2011), 133 (9), 3131-3138CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)The assembly of colloidal nanocrystals (NCs) into superstructures with long-range translational and orientational order is sensitive to the mol. interactions between ligands bound to the NC surface. The authors illustrate how ligand coverage on colloidal PbS NCs can be exploited as a tunable parameter to direct the self-assembly of superlattices with predefined symmetry. PbS NCs with dense ligand coverage assemble into fcc. superlattices whereas NCs with sparse ligand coverage assemble into bcc. superlattices which also exhibit orientational ordering of NCs in their lattice sites. Surface chem. characterization combined with d. functional theory calcns. suggest that the loss of ligands occurs preferentially on {100} than on reconstructed {111} NC facets. The resulting anisotropic ligand distribution amplifies the role of NC shape in the assembly and gives superlattices with translational and orientational order.
- 11Evers, W. H.; Schins, J. M.; Aerts, M.; Kulkarni, A.; Capiod, P.; Berthe, M.; Grandidier, B.; Delerue, C.; van der Zant, H. S. J.; van Overbeek, C.; Peters, J. L.; Vanmaekelbergh, D.; Siebbeles, L. D. A. High Charge Mobility in Two Dimensional Percolative Networks of PbSe Quantum Dots Connected by Atomic Bonds Nat. Commun. 2015, 6, 8195 DOI: 10.1038/ncomms9195Google Scholar11https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhsFGksbfE&md5=f1f8effce52a0cdf6e0c8650f43195d5High charge mobility in two-dimensional percolative networks of PbSe quantum dots connected by atomic bondsEvers, Wiel H.; Schins, Juleon M.; Aerts, Michiel; Kulkarni, Aditya; Capiod, Pierre; Berthe, Maxime; Grandidier, Bruno; Delerue, Christophe; van der Zant, Herre S. J.; van Overbeek, Carlo; Peters, Joep L.; Vanmaekelbergh, Daniel; Siebbeles, Laurens D. A.Nature Communications (2015), 6 (), 8195CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)Two-dimensional networks of quantum dots connected by at. bonds have an electronic structure that is distinct from that of arrays of quantum dots coupled by ligand mols. We prepd. atomically coherent two-dimensional percolative networks of PbSe quantum dots connected via at. bonds. Here, we show that photoexcitation leads to generation of free charges that eventually decay via trapping. The charge mobility probed with an AC elec. field increases with frequency from 150±15 cm2 V-1 s-1 at 0.2 THz to 260±15 cm2 V-1 s-1 at 0.6 THz. Gated four-probe measurements yield a DC electron mobility of 13±2 cm2 V-1 s-1. The terahertz mobilities are much higher than for arrays of quantum dots coupled via surface ligands and are similar to the highest DC mobilities reported for PbSe nanowires. The terahertz mobility increases only slightly with temp. in the range of 15-290 K. The extent of straight segments in the two-dimensional percolative networks limits the mobility, rather than charge scattering by phonons.
- 12Kang, I.; Wise, F. W. Electronic Structure and Optical Properties of PbS and PbSe Quantum Dots J. Opt. Soc. Am. B 1997, 14, 1632– 1646 DOI: 10.1364/JOSAB.14.001632Google Scholar12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2sXksFSrtLg%253D&md5=d56b1e92c77121e0c86a996d63f22bf5Electronic structure and optical properties of PbS and PbSe quantum dotsKang, Inuk; Wise, Frank W.Journal of the Optical Society of America B: Optical Physics (1997), 14 (7), 1632-1646CODEN: JOBPDE; ISSN:0740-3224. (Optical Society of America)The electronic structure of spherical PbS and PbSe quantum dots is calcd. with a four-band envelope-function formalism. This calcn. accounts for both exciton energies and wave functions with the correct symmetry of the materials. The selection rules and the strength of the dipole transitions of Pb-salt quantum dots are derived accounting for the symmetry of the band-edge Bloch functions of the lead salts. The calcd. energies of the optically allowed exciton states are in good agreement with exptl. data. The effects of many-body perturbations, such as Coulomb interactions and intervalley scattering, are also discussed.
- 13Moreels, I.; Lambert, K.; De Muynck, D.; Vanhaecke, F.; Poelman, D.; Martins, J. C.; Allan, G.; Hens, Z. Composition and Size-Dependent Extinction Coefficient of Colloidal PbSe Quantum Dots Chem. Mater. 2007, 19, 6101– 6106 DOI: 10.1021/cm071410qGoogle Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXhtlSgur3K&md5=676ea9d5ea379efa09cafc1520736b04Composition and Size-Dependent Extinction Coefficient of Colloidal PbSe Quantum DotsMoreels, Iwan; Lambert, Karel; De Muynck, David; Vanhaecke, Frank; Poelman, Dirk; Martins, Jose C.; Allan, Guy; Hens, ZegerChemistry of Materials (2007), 19 (25), 6101-6106CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)Inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV-vis-NIR spectrophotometry and transmission electron microscopy to det. the nanocrystal compn. and molar extinction coeff. ε of colloidal PbSe quantum dot (Q-PbSe) suspensions. The ICP-MS results show a nonstoichiometric Pb/Se ratio, with a systematic excess of lead for all samples studied. The obsd. ratio is consistent with a faceted spherical Q-PbSe model, composed of a quasi stoichiometric Q-PbSe core terminated by a Pb surface shell. At high photon energies, we find that ε scales with the nanocrystal vol., irresp. of the Q-PbSe size. From ε, we calcd. a size-independent absorption coeff. Its value is in good agreement with the theor. value for bulk PbSe. At the band gap, ε is size-dependent. The resulting absorption coeff. increases quadratically with decreasing Q-PbSe size. Calcns. of the oscillator strength of the first optical transition are in good agreement with theor. tight binding calcns., showing that the oscillator strength increases linearly with Q-PbSe size.
- 14Fritzinger, B.; Capek, R. K.; Lambert, K.; Martins, J. C.; Hens, Z. Utilizing Self-Exchange To Address the Binding of Carboxylic Acid Ligands to CdSe Quantum Dots J. Am. Chem. Soc. 2010, 132, 10195– 10201 DOI: 10.1021/ja104351qGoogle Scholar14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXosVGhs7s%253D&md5=abdbd55f677f597415eeff71225c0739Utilizing Self-Exchange To Address the Binding of Carboxylic Acid Ligands to CdSe Quantum DotsFritzinger, Bernd; Capek, Richard K.; Lambert, Karel; Martins, Jose C.; Hens, ZegerJournal of the American Chemical Society (2010), 132 (29), 10195-10201CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)The authors use soln. NMR techniques to analyze the org./inorg. interface of CdSe quantum dots (Q-CdSe) synthesized using oleic acid as a surfactant. The resulting Q-CdSe are stabilized by tightly bound oleic acid species that only exchange upon addn. of free oleic acid. The NMR anal. points toward a two-step exchange mechanism where free ligands are initially physisorbed within the ligand shell to end up as bound, chemisorbed ligands in a 2nd step. Importantly, every ligand is involved in this exchange process. By addn. of oleic acid with a deuterated carboxyl headgroup, the bound ligands are oleate ions and not oleic acid mols. This explains why a dynamic adsorption/desorption equil. only occurs in the presence of excess free oleic acid, which donates the required proton. Comparing the no. of oleate ligands to the excess cadmium per CdSe quantum dot, a ratio of 2:1 were found. This completes the picture of Q-CdSe as org./inorg. entities where the surface excess of Cd2+ is balanced by a double amt. of oleate ligands, yielding overall neutral nanoparticles.
- 15Owen, J. S.; Park, J.; Trudeau, P.-E. E.; Alivisatos, a. P. Reaction Chemistry and Ligand Exchange at Cadmium-Selenide Nanocrystal Surfaces J. Am. Chem. Soc. 2008, 130, 12279– 12281 DOI: 10.1021/ja804414fGoogle Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXhtVert7jI&md5=643d34871585bf85a6075ba99ace8d92Reaction Chemistry and Ligand Exchange at Cadmium-Selenide Nanocrystal SurfacesOwen, Jonathan S.; Park, Jungwon; Trudeau, Paul-Emile; Alivisatos, A. PaulJournal of the American Chemical Society (2008), 130 (37), 12279-12281CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)The surface chem. of cadmium selenide nanocrystals, prepd. from tri-n-octylphosphine selenide and cadmium octadecylphosphonate in tri-n-octylphosphine oxide, was studied with 1H and {1H}31P NMR spectroscopy as well as ESI-MS and XPS. The identity of the surface ligands was inferred from reaction of nanocrystals with Me3Si-X (X = -S-SiMe3, -Se-SiMe3, -Cl and -S-(CH2CH2O)4OCH3) and unambiguous assignment of the org. byproducts, O,O'-bis(trimethylsilyl)octadecylphosphonic acid ester and O,O'-bis(trimethylsilyl)ocatdecylphosphonic acid anhydride ester. Nanocrystals isolated from these reactions have undergone exchange of the octadecylphosphonate ligands for -X as was shown by 1H NMR (X = -S-(CH2CH2O)4OCH3) and XPS (X = -Cl). Addn. of free thiols to as prepd. nanocrystals results in binding of the thiol to the particle surface and quenching of the nanocrystal fluorescence. Isolation of the thiol-ligated nanocrystals shows this chemisorption proceeds without displacement of the octadecylphosphonate ligands, suggesting the presence of unoccupied Lewis-acidic sites on the particle surface. In the presence of added triethylamine, however, the octadecylphosphonate ligands are readily displaced from the particle surface as was shown with 1H and {1H}31P NMR. These results, in conjunction with previous literature reports, indicate that as-prepd. nanocrystal surfaces are terminated by X-type binding of octadecylphosphonate moieties to a layer of excess cadmium ions.
- 16Bealing, C. R.; Baumgardner, W. J.; Choi, J. J.; Hanrath, T.; Hennig, R. G. Predicting Nanocrystal Shape through Consideration of Surface-Ligand Interactions ACS Nano 2012, 6, 2118– 2127 DOI: 10.1021/nn3000466Google Scholar16https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XitFKktbo%253D&md5=8f362f8e23e76d7262897dbf6651ab57Predicting Nanocrystal Shape through Consideration of Surface-Ligand InteractionsBealing, Clive R.; Baumgardner, William J.; Choi, Joshua J.; Hanrath, Tobias; Hennig, Richard G.ACS Nano (2012), 6 (3), 2118-2127CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)D. functional calcns. for the binding energy of oleic acid-based ligands on Pb-rich {100} and {111} facets of PbSe nanocrystals det. the surface energies as a function of ligand coverage. Oleic acid is expected to bind to the nanocrystal surface as lead oleate. The Wulff construction predicts the thermodn. equil. shape of the PbSe nanocrystals. The equil. shape is a function of the ligand surface coverage, which can be controlled by changing the concn. of oleic acid during synthesis. The different binding energy of the ligand on the {100} and {111} facets results in different equil. ligand coverages on the facets, and a transition in the equil. shape from octahedral to cubic is predicted when increasing the ligand concn. during synthesis.
- 17Zherebetskyy, D.; Scheele, M.; Zhang, Y.; Bronstein, N.; Thompson, C.; Britt, D.; Salmeron, M.; Alivisatos, A. P.; Wang, L.-W. Hydroxylation of the Surface of PbS Nanocrystals Passivated with Oleic Acid Science (Washington, DC, U. S.) 2014, 344, 1380– 1384 DOI: 10.1126/science.1252727Google Scholar17https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXpslygs7w%253D&md5=4046b9d228449c83c88672795a527482Hydroxylation of the surface of PbS nanocrystals passivated with oleic acidZherebetskyy, Danylo; Scheele, Marcus; Zhang, Yingjie; Bronstein, Noah; Thompson, Christopher; Britt, David; Salmeron, Miquel; Alivisatos, Paul; Wang, Lin-WangScience (Washington, DC, United States) (2014), 344 (6190), 1380-1384CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Controlling the structure of colloidal nanocrystals (NCs) is key to the generation of their complex functionality. This requires an understanding of the NC surface at the at. level. The structure of colloidal PbS NCs passivated with oleic acid has been studied theor. and exptl. The authors show the existence of surface OH- groups, which play a key role in stabilizing the PbS(111) facets, consistent with XPS as well as other spectroscopic and chem. expts. The role of water in the synthesis process is also revealed. Their model, along with existing observations of NC surface termination and passivation by ligands, helps to explain and predict the properties of NCs and their assemblies.
- 18Schapotschnikow, P.; van Huis, M. A.; Zandbergen, H. W.; Vanmaekelbergh, D.; Vlugt, T. J. H. Morphological Transformations and Fusion of PbSe Nanocrystals Studied Using Atomistic Simulations Nano Lett. 2010, 10, 3966– 3971 DOI: 10.1021/nl101793bGoogle Scholar18https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtFKlu7rI&md5=ea82cab28e64a8d31cb0fdb00cc7b42dMorphological Transformations and Fusion of PbSe Nanocrystals Studied Using Atomistic SimulationsSchapotschnikow, Philipp; van Huis, Marijn A.; Zandbergen, Henny W.; Vanmaekelbergh, Daniel; Vlugt, Thijs J. H.Nano Letters (2010), 10 (10), 3966-3971CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Mol. dynamics simulations are performed on capped and uncapped PbSe nanocrystals, employing newly developed classical interaction potentials. Here, we show that two uncapped nanocrystals fuse efficiently via direct surface attachment, even if they are initially misaligned. In sharp contrast to the general belief, interparticle dipole interactions do not play a significant role in this "oriented attachment" process. Furthermore, it is shown that presumably polar, capped PbSe{111} facets are never fully Pb- or Se-terminated.
- 19Fang, C.; Van Huis, M. a.; Vanmaekelbergh, D.; Zandbergen, H. W. Energetics of Polar and Nonpolar Facets of PbSe Nanocrystals from Theory and Experiment ACS Nano 2010, 4, 211– 218 DOI: 10.1021/nn9013406Google Scholar19https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsV2htbvK&md5=75ff94111588721c09a5c519059bdf78Energetics of Polar and Nonpolar Facets of PbSe Nanocrystals from Theory and ExperimentFang, Changming; van Huis, Marijn A.; Vanmaekelbergh, Daniel; Zandbergen, Henny W.ACS Nano (2010), 4 (1), 211-218CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Surface energies of the distinct facets of nanocrystals are an important factor in the free energy and hence det. the nanocrystal morphol., chem. and phys. properties, and even interparticle dipole interactions. The stability and at. structure of polar and nonpolar PbSe surfaces by combining first-principles calcns. with high-resoln. transmission electron microscopy (TEM) is investigated. For uncapped surfaces, the calcns. predict that the nonpolar {100} surface is the most stable with a surface energy of 0.184 J m-2, while the nonpolar {110} and reconstructed {111}-Pb surfaces have surface energies of 0.318 J m-2 and 0.328 J m-2, resp. Fully polar {111} surfaces are structurally unstable upon relaxation. These findings are in good agreement with TEM observations showing that capped nanocrystals have a nearly spherical, multifaceted morphol., while cubical shapes with predominantly {100} facets are obtained when the capping mols. are removed through heating in vacuum. During this process, multipolar surfaces can temporarily exist just after the removal of the surfactants. These metastable {111} surfaces consist of ribbon-like nanodomains, whereby the ribbons are alternating in polarity. The calcns. confirm that these multipolar surfaces are energetically more favorable than fully polar surfaces. The consequences for capped nanocrystals (a dominant Pb-oleate termination) and nanocrystal fusion (a shorter interaction range of dipole interactions) are discussed.
- 20Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. Designing PbSe Nanowires and Nanorings through Oriented Attachment of Nanoparticles J. Am. Chem. Soc. 2005, 127, 7140– 7147 DOI: 10.1021/ja050107sGoogle Scholar20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXjs1Sisb8%253D&md5=7f42df157281b5b21d0c5e424aabea4dDesigning PbSe Nanowires and Nanorings through Oriented Attachment of NanoparticlesCho, Kyung-Sang; Talapin, Dmitri V.; Gaschler, Wolfgang; Murray, Christopher B.Journal of the American Chemical Society (2005), 127 (19), 7140-7147CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Single-crystal PbSe nanowires were synthesized in soln. through oriented attachment of nanocrystal building blocks. Reaction temps. of 190-250° and multicomponent surfactant mixts. result in a nearly defect-free crystal lattice and high uniformity of nanowire diam. along the entire length. The wires' dimensions are tuned by tailoring reaction conditions in a range from ∼4 to ∼20 nm in diam. with wire lengths up to ∼30 μm. PbSe nanocrystals bind to each other on either {100}, {110}, or {111} faces, depending on the surfactant mols. present in the reaction soln. While PbSe nanocrystals have the centrosym. rocksalt lattice, they can lack central symmetry due to a noncentrosym. arrangement of Pb- and Se-terminated {111} facets and possess dipole driving 1-dimensional oriented attachment of nanocrystals to form nanowires. In addn. to straight nanowires, zigzag, helical, branched, and tapered nanowires as well as single-crystal nanorings can be controllably prepd. in 1-pot reactions by careful adjustment of the reaction conditions.
- 21Van Aert, S.; De Backer, A.; Martinez, G. T.; Goris, B.; Bals, S.; Van Tendeloo, G.; Rosenauer, A. Procedure to Count Atoms with Trustworthy Single-Atom Sensitivity Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 64107 DOI: 10.1103/PhysRevB.87.064107Google ScholarThere is no corresponding record for this reference.
- 22Bals, S.; Casavola, M.; van Huis, M. A.; Van Aert, S.; Batenburg, K. J.; Van Tendeloo, G.; Vanmaekelbergh, D. Three-Dimensional Atomic Imaging of Colloidal Core-Shell Nanocrystals Nano Lett. 2011, 11, 3420– 3424 DOI: 10.1021/nl201826eGoogle Scholar22https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXptlCju70%253D&md5=1b5c32efb69c3c5e9bdb5e007a872bdfThree-Dimensional Atomic Imaging of Colloidal Core-Shell NanocrystalsBals, Sara; Casavola, Marianna; van Huis, Marijn A.; Van Aert, Sandra; Batenburg, K. Joost; Van Tendeloo, Gustaaf; Vanmaekelbergh, DanielNano Letters (2011), 11 (8), 3420-3424CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Colloidal core-shell semiconductor nanocrystals form an important class of optoelectronic materials, in which the exciton wave functions can be tailored by the at. configuration of the core, the interfacial layers, and the shell. Here, the authors provide a trustful 3D characterization at the at. scale of a free-standing PbSe(core)-CdSe(shell) nanocrystal by combining electron microscopy and discrete tomog. The authors results yield unique insights for understanding the process of cation exchange, which is widely employed in the synthesis of core-shell nanocrystals. The study that the authors present is generally applicable to the broad range of colloidal heteronanocrystals that currently emerge as a new class of materials with technol. importance.
- 23Hassinen, A.; Moreels, I.; De Nolf, K.; Smet, P. F.; Martins, J. C.; Hens, Z. Short-Chain Alcohols Strip X-Type Ligands and Quench the Luminescence of PbSe and CdSe Quantum Dots, Acetonitrile Does Not J. Am. Chem. Soc. 2012, 134, 20705– 20712 DOI: 10.1021/ja308861dGoogle Scholar23https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvVWqtLzO&md5=2e0c7f106c59dfa86a3150716f4f6800Short-Chain Alcohols Strip X-Type Ligands and Quench the Luminescence of PbSe and CdSe Quantum Dots, Acetonitrile Does NotHassinen, Antti; Moreels, Iwan; De Nolf, Kim; Smet, Philippe F.; Martins, Jose C.; Hens, ZegerJournal of the American Chemical Society (2012), 134 (51), 20705-20712CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)The effect of short-chain alcs. and MeCN on the ligand shell compn. and the photoluminescence quantum yield of purified PbSe and CdSe quantum dots is analyzed by soln. NMR and photoluminescence spectroscopy. Short-chain alcs. induce the release of X-type carboxylate ligands with a concurrent redn. of the photoluminescence quantum yield, while MeCN does not. The authors interpret this difference in terms of the protic or aprotic character of both nonsolvents, where only the protic alcs. can provide the protons needed to desorb carboxylate ligands. Similar differences between short-chain alcs. and MeCN when used as nonsolvents during the purifn. of crude synthesis products, a result stressing the importance of using aprotic nonsolvents for nanocrystal purifn. or processing were found.
- 24Anderson, N. C.; Hendricks, M. P.; Choi, J. J.; Owen, J. S. Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding J. Am. Chem. Soc. 2013, 135, 18536– 18548 DOI: 10.1021/ja4086758Google Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhslCrsr%252FI&md5=64005629c75e8c287c3b2dfb3047a246Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and BindingAnderson, Nicholas C.; Hendricks, Mark P.; Choi, Joshua J.; Owen, Jonathan S.Journal of the American Chemical Society (2013), 135 (49), 18536-18548CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Metal carboxylate complexes (L-M(O2CR)2, O2CR = oleic, myristic, M = Cd, Pb) are readily displaced from carboxylate-terminated ME nanocrystals (ME = CdSe, CdS, PbSe, PbS) by various Lewis bases (L = tri-n-butylamine, THF, tetradecanol, N,N-dimethyl-n-butylamine, tri-n-butylphosphine, N,N,N',N'-tetramethylbutylene-1,4-diamine, pyridine, N,N,N',N'-tetramethylethylene-1,2-diamine, n-octylamine). The relative displacement potency is measured by 1H NMR spectroscopy and depends most strongly on geometric factors such as sterics and chelation, although also on the hard/soft match with the Cd ion. The results suggest that ligands displace L-M(O2CR)2 by cooperatively complexing the displaced metal ion as well as the nanocrystal. Removal of up to 90% of surface-bound Cd-(O2CR)2 from CdSe and CdS nanocrystals decreases the Cd/Se ratio from 1.1 ± 0.06 to 1.0 ± 0.05, broadens the 1Se-2S3/2h absorption, and decreases the photoluminescence quantum yield (PLQY) from 10% to <1% (CdSe) and from 20% to <1% (CdS). These changes are partially reversed upon rebinding of M-(O2CR)2 at room temp. (∼60%) and fully reversed at elevated temp. A model is proposed in which electron-accepting M-(O2CR)2 complexes (Z-type ligands) reversibly bind to nanocrystals, leading to a range of stoichiometries for a given core size. Nanocrystals lack a single chem. formula, but are instead dynamic structures with concn.-dependent compns. The importance of these findings to the synthesis and purifn. of nanocrystals as well as ligand exchange reactions is discussed.
- 25Grisorio, R.; Debellis, D.; Suranna, G. P.; Gigli, G.; Giansante, C. The Dynamic Organic/Inorganic Interface of Colloidal PbS Quantum Dots Angew. Chem., Int. Ed. 2016, 55, 6628– 6633 DOI: 10.1002/anie.201511174Google Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xlt1yju7Y%253D&md5=5143cb4fac6ba04219f26a091589b33aThe Dynamic Organic/Inorganic Interface of Colloidal PbS Quantum DotsGrisorio, Roberto; Debellis, Doriana; Suranna, Gian Paolo; Gigli, Giuseppe; Giansante, CarloAngewandte Chemie, International Edition (2016), 55 (23), 6628-6633CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)Colloidal quantum dots are composed of nanometer-sized crystallites of inorg. semiconductor materials bearing org. mols. at their surface. The org./inorg. interface markedly affects forms and functions of the quantum dots, therefore its description and control are important for effective application. Herein we demonstrate that archetypal colloidal PbS quantum dots adapt their interface to the surroundings, thus existing in soln. phase as equil. mixts. with their (metal-)org. ligand and inorg. core components. The interfacial equil. are dictated by solvent polarity and concn., show striking size dependence (leading to more stable ligand/core adducts for larger quantum dots), and selectively involve nanocrystal facets. This notion of ligand/core dynamic equil. may open novel synthetic paths and refined nanocrystal surface-chem. strategies.
- 26Nag, A.; Kovalenko, M. V.; Lee, J.; Liu, W.; Spokoyny, B.; Talapin, D. V. Metal-Free Inorganic Ligands for Colloidal Nanocrystals: S2–, HS–, Se2–, HSe–, Te2–, HTe–, TeS32–, OH–, and NH2– as Surface Ligands J. Am. Chem. Soc. 2011, 133, 10612– 10620 DOI: 10.1021/ja2029415Google Scholar26https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXns1emsbY%253D&md5=51f8cbf21be5b525a4abde2e7f3c8acaMetal-free Inorganic Ligands for Colloidal Nanocrystals: S2-, HS-, Se2-, HSe-, Te2-, HTe-, TeS32-, OH-, and NH2- as Surface LigandsNag, Angshuman; Kovalenko, Maksym V.; Lee, Jong-Soo; Liu, Wenyong; Spokoyny, Boris; Talapin, Dmitri V.Journal of the American Chemical Society (2011), 133 (27), 10612-10620CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)All-inorg. colloidal nanocrystals were synthesized by replacing org. capping ligands on chem. synthesized nanocrystals with metal-free inorg. ions such as S2-, HS-, Se2-, HSe-, Te2-, HTe-, TeS32-, OH- and NH2-. These simple ligands adhered to the NC surface and provided colloidal stability in polar solvents. The versatility of such ligand exchange was demonstrated for various semiconductor and metal nanocrystals of different size and shape. The key aspects of Pearson's hard and soft acids and bases (HSAB) principle, originally developed for metal coordination compds., can be applied to the bonding of mol. species to the nanocrystal surface. The use of small inorg. ligands instead of traditional ligands with long hydrocarbon tails facilitated the charge transport between individual nanocrystals and opened up interesting opportunities for device integration of colloidal nanostructures.
- 27Campos, M. P.; Hendricks, M. P.; Beecher, A. N.; Walravens, W.; Swain, R. A.; Cleveland, G. T.; Hens, Z.; Sfeir, M. Y.; Owen, J. S. A Library of Selenourea Precursors to PbSe Nanocrystals with Size Distributions Near the Homogeneous Limit J. Am. Chem. Soc. 2017, 139, 2296– 2305 DOI: 10.1021/jacs.6b11021Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXht1ygtLs%253D&md5=9329da51d16e810de99cbc5628952062A Library of Selenourea Precursors to PbSe Nanocrystals with Size Distributions near the Homogeneous LimitCampos, Michael P.; Hendricks, Mark P.; Beecher, Alexander N.; Walravens, Willem; Swain, Robert A.; Cleveland, Gregory T.; Hens, Zeger; Sfeir, Matthew Y.; Owen, Jonathan S.Journal of the American Chemical Society (2017), 139 (6), 2296-2305CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)We report a tunable library of N,N,N'-trisubstituted selenourea precursors and their reaction with lead oleate at 60-150 °C to form carboxylate-terminated PbSe nanocrystals in quant. yields. Single exponential conversion kinetics can be tailored over 4 orders of magnitude by adjusting the selenourea structure. The wide range of conversion reactivity allows the extent of nucleation ([nanocrystal] = 4.6-56.7 μM) and the size following complete precursor conversion (d = 1.7-6.6 nm) to be controlled. Narrow size distributions (σ = 0.5-2%) are obtained whose spectral line widths are dominated (73-83%) by the intrinsic single particle spectral broadening, as obsd. using spectral hole burning measurements. The intrinsic broadening decreases with increasing size (fwhm = 320-65 meV, d = 1.6-4.4 nm) that derives from exciton fine structure and exciton-phonon coupling rather than broadening caused by the size distribution.
- 28Cass, L. C.; Malicki, M.; Weiss, E. a. The Chemical Environments of Oleate Species within Samples of Oleate-Coated PbS Quantum Dots Anal. Chem. 2013, 85, 6974– 6979 DOI: 10.1021/ac401623aGoogle Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXpslKhtLg%253D&md5=edc8962aa1789352c3e911b8adf10607The Chemical Environments of Oleate Species within Samples of Oleate-Coated PbS Quantum DotsCass, Laura C.; Malicki, Michal; Weiss, Emily A.Analytical Chemistry (Washington, DC, United States) (2013), 85 (14), 6974-6979CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)A combination of FTIR, 1H NMR, nuclear Overhauser effect (NOESY), and diffusion-ordered (DOSY) NMR spectroscopies shows that samples of oleate-coated PbS quantum dots (QDs) with core radii ranging from 1.6 to 2.4 nm, and purified by washing with acetone, contain two species of oleate characterized by the stretching frequencies of their carboxylate groups, the chem. shifts of their protons, and their diffusion coeffs. One of these oleate species exists primarily on the surfaces of the QDs and either chelates a Pb2+ ion or bridges two Pb2+ ions. The ratio of bridging oleates to chelating oleates on the surfaces of the QDs is ∼1:1 for all sizes of the QDs the authors studied. The 2nd oleate species in these samples bridges two Pb2+ ions within clusters or oligomers of lead oleate (with a hydrodynamic radius of ∼1.4 nm), which are byproducts of the QD synthesis. The concn. of these clusters increases with increasing size of the QDs because larger QDs are produced by increasing the concn. of the oleic acid ligand in the reaction mixt. The oleate mols. on the surfaces of the QDs and within the lead oleate clusters are in rapid exchange with each other. Addnl. washes with methanol progressively eliminate the contaminating clusters from the PbS QD samples. This work quant. characterizes the distribution of binding geometries at the inorg./org. interface of the nanocrystals and demonstrates the utility of using org. ligands as probes for the compn. of a colloidal QD sample as a function of the prepn. procedure.
- 29Mesubi, M. A. An Infrared Study of Zinc, Cadmium, and Lead Salts of Some Fatty Acids J. Mol. Struct. 1982, 81, 61– 71 DOI: 10.1016/0022-2860(82)80079-3Google Scholar29https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL38XktFCht78%253D&md5=8657b3308ca95c7795635cff1158dbb2An infrared study of zinc, cadmium, and lead salts of some fatty acidsMesubi, M. AdediranJournal of Molecular Structure (1982), 81 (1-2), 61-71CODEN: JMOSB4; ISSN:0022-2860.The IR spectra between 1800 and 700 cm-1 of Zn, Cd, and Pb carboxylates of even numbered chain length from C6 to C18 inclusive, were measured as a function of temp. from ambient temp. to a few degrees above the m.p. From the COO- stretching frequencies, it was deduced that the coordination of the carboxylate group to the metal ions is unsym. chelating bidentate. The only significant change obsd. prior to the transition point is a decrease in the intensity and a broadening of the progression of the narrow bands between 1350 and 1150 cm-1 and the band at ∼720 cm-1, most of which virtually disappear in the melt. Above the m.p., there is an abrupt change in the shape of the COO- stretching bands. A new band at 1634 cm-1 was obsd. in Zn salts. This band is attributed to the chelating bidentate structure acquiring some monodentate character as the temp. increases. The spectral changes due to thermal effects are reversible.
- 30Deacon, G. B.; Philiips, R. J. Relationships between the Carbon-Oxygen Stretching Frequencies of Carboxylato Complexes and the Type of Carboxylate Coordination Coord. Chem. Rev. 1980, 33, 227– 250 DOI: 10.1016/S0010-8545(00)80455-5Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL3MXjvVOqsw%253D%253D&md5=792c04e5960043e7dda7f4e4d4f3d779Relationships between the carbon-oxygen stretching frequencies of carboxylato complexes and the type of carboxylate coordinationDeacon, G. B.; Phillips, R. J.Coordination Chemistry Reviews (1980), 33 (3), 227-50CODEN: CCHRAM; ISSN:0010-8545.A review with 177 refs.
- 31Ellis, H. A.; White, N. A. S.; Taylor, R. A.; Maragh, P. T. Infrared, X-Ray and Microscopic Studies on the Room Temperature Structure of Anhydrous Lead (II) N-Alkanoates J. Mol. Struct. 2005, 738, 205– 210 DOI: 10.1016/j.molstruc.2004.12.006Google Scholar31https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXhs12ltL4%253D&md5=9915b50933137ec5e4ec9d7044288e88Infrared, X-ray and microscopic studies on the room temperature structure of anhydrous lead (II) n-alkanoatesEllis, Henry A.; White, Nicole A. S.; Taylor, Richard A.; Maragh, Paul T.Journal of Molecular Structure (2005), 738 (1-3), 205-210CODEN: JMOSB4; ISSN:0022-2860. (Elsevier B.V.)The room temp. structures of even-chain length lead (II) n-alkanoates were studied using IR spectroscopy, x-ray diffraction and polarizing light microscopy. . While the bonding is the same for both long and short chain compds., where four carboxylate groups form an unsym. bidentate bond with a Pb atom, with possible carboxylate bridging to adjacent Pb atoms, at least for the short chain length compds., the arrangement of hydrocarbon chains within a lamella is different. Both x-ray and microscopic studies show that for short chain compds., hydrocarbon chains are arranged as bilayers within a lamella and tilted with respect to the lead basal plane. For the long chain compds., probably the chains are arranged as a monolayer within a lamella, in an alternating arrangement, and slightly tilted with respect to the lead basal plane. For all the compds., x-ray data point to a triclinic unit cell contg. four mols. per unit cell with P‾1 symmetry, and chains oriented along the c axis. Crystallog. data are given.
- 32Moreels, I.; Fritzinger, B.; Martins, J. C.; Hens, Z. Surface Chemistry of Colloidal PbSe Nanocrystals J. Am. Chem. Soc. 2008, 130, 15081– 15086 DOI: 10.1021/ja803994mGoogle Scholar32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXht1GnurnJ&md5=6ae8087657d4338ad25df3f157708ac4Surface Chemistry of Colloidal PbSe NanocrystalsMoreels, Iwan; Fritzinger, Bernd; Martins, Jose C.; Hens, ZegerJournal of the American Chemical Society (2008), 130 (45), 15081-15086CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Soln. NMR spectroscopy (NMR) is used to identify and quantify the org. capping of colloidal PbSe nanocrystals (Q-PbSe). The capping consists primarily of tightly bound oleic acid ligands. Only a minor part of the ligand shell (0-5% with respect to the no. of oleic acid ligands) is composed of tri-n-octylphosphine. As a result, tuning of the Q-PbSe size during synthesis is achieved by varying the oleic acid concn. By combining the NMR results with inductively coupled plasma mass spectrometry, a complete Q-PbSe structural model of semiconductor core and org. ligands is constructed. The nanocrystals are nonstoichiometric, with a surface that is composed of lead atoms. The absence of surface selenium atoms is in accordance with an oleic acid ligand shell. NMR results on a Q-PbSe suspension, stored under ambient conditions, suggest that oxidn. leads to the loss of oleic acid ligands and surface Pb atoms, forming dissolved lead oleate.
- 33Van Aert, S.; Batenburg, K. J.; Rossell, M. D.; Erni, R.; Van Tendeloo, G. Three-Dimensional Atomic Imaging of Crystalline Nanoparticles Nature 2011, 470 (7334) 374– 377 DOI: 10.1038/nature09741Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhsVWjt74%253D&md5=aeb5bf85c88b3b272c7c08972ab6e238Three-dimensional atomic imaging of crystalline nanoparticlesVan Aert, Sandra; Batenburg, Kees J.; Rossell, Marta D.; Erni, Rolf; Van Tendeloo, GustaafNature (London, United Kingdom) (2011), 470 (7334), 374-377CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)Detg. the three-dimensional (3D) arrangement of atoms in cryst. nanoparticles is important for nanometer-scale device engineering and also for applications involving nanoparticles, such as optoelectronics or catalysis. A nanoparticle's phys. and chem. properties are controlled by its exact 3D morphol., structure and compn. Electron tomog. enables the recovery of the shape of a nanoparticle from a series of projection images. Although at.-resoln. electron microscopy has been feasible for nearly four decades, neither electron tomog. nor any other exptl. technique has yet demonstrated at. resoln. in three dimensions. Here the authors report the 3D reconstruction of a complex cryst. nanoparticle at at. resoln. To achieve this, we combined aberration-cor. scanning transmission electron microscopy, statistical parameter estn. theory and discrete tomog. Unlike conventional electron tomog., only two images of the target-a silver nanoparticle embedded in an aluminum matrix-are sufficient for the reconstruction when combined with available knowledge about the particle's crystallog. structure. Addnl. projections confirm the reliability of the result. The results we present help close the gap between the at. resoln. achievable in two-dimensional electron micrographs and the coarser resoln. that has hitherto been obtained by conventional electron tomog.
- 34Scott, M. C.; Chen, C.-C.; Mecklenburg, M.; Zhu, C.; Xu, R.; Ercius, P.; Dahmen, U.; Regan, B. C.; Miao, J. Electron Tomography at 2.4-Ångström Resolution Nature 2012, 483, 444– 447 DOI: 10.1038/nature10934Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XksVehu7o%253D&md5=f6a39c25f019f484e63aaa8cc5e7ba93Electron tomography at 2.4-angstrom resolutionScott, M. C.; Chen, Chien-Chun; Mecklenburg, Matthew; Zhu, Chun; Xu, Rui; Ercius, Peter; Dahmen, Ulrich; Regan, B. C.; Miao, JianweiNature (London, United Kingdom) (2012), 483 (7390), 444-447CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)TEM is a powerful imaging tool that found broad application in materials science, nanoscience and biol. With the introduction of aberration-cor. electron lenses, both the spatial resoln. and the image quality in TEM were significantly improved and resoln. <0.5 angstroms was demonstrated. To reveal the 3-dimensional (3D) structure of thin samples, electron tomog. is the method of choice, with cubic-nanometer resoln. currently achievable. Discrete tomog. has recently been used to generate a 3-dimensional at. reconstruction of a Ag nanoparticle 2 to 3 nm in diam., but this statistical method assumes prior knowledge of the particle's lattice structure and requires that the atoms fit rigidly on that lattice. Here the authors report the exptl. demonstration of a general electron tomog. method that achieves at.-scale resoln. without initial assumptions about the sample structure. By combining a novel projection alignment and tomog. reconstruction method with scanning TEM, the authors detd. the 3-dimensional structure of an ∼10-nm Au nanoparticle at 2.4-angstrom resoln. Although the authors cannot definitively locate all of the atoms inside the nanoparticle, individual atoms are obsd. in some regions of the particle and several grains are identified in 3 dimensions. The 3-dimensional surface morphol. and internal lattice structure revealed are consistent with a distorted icosahedral multiply twinned particle. The authors anticipate that this general method can be applied not only to det. the 3-dimensional structure of nanomaterials at at.-scale resoln., but also to improve the spatial resoln. and image quality in other tomog. fields.
- 35Goris, B.; Bals, S.; Van den Broek, W.; Carbó-Argibay, E.; Gómez-Graña, S.; Liz-Marzán, L. M.; Van Tendeloo, G. Atomic-Scale Determination of Surface Facets in Gold Nanorods Nat. Mater. 2012, 11, 930– 935 DOI: 10.1038/nmat3462Google Scholar35https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsFeqsr7K&md5=19395c5bfcb68e426845ff7954d2958aAtomic-scale determination of surface facets in gold nanorodsGoris, Bart; Bals, Sara; Van den Broek, Wouter; Carbo-Argibay, Enrique; Gomez-Grana, Sergio; Liz-Marzan, Luis M.; Van Tendeloo, GustaafNature Materials (2012), 11 (11), 930-935CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)The phys. properties of nanostructures depend on the type of surface facets. For Au nanorods, the surface facets have a major influence on crucial effects such as reactivity and ligand adsorption and there was controversy regarding facet indexing. Aberration-cor. electron microscopy is the ideal technique to study the at. structure of nanomaterials. However, these images correspond to 2D projections of 3D nanoobjects, leading to an incomplete characterization. Recently, much progress was achieved in the field of at.-resoln. electron tomog., but it is still far from being a routinely used technique. Here we propose a methodol. to measure the 3D at. structure of free-standing nanoparticles, which we apply to characterize the surface facets of Au nanorods. This methodol. is applicable to a broad range of nanocrystals, leading to unique insights concerning the connection between the structure and properties of nanostructures.
- 36Bals, S.; Van Aert, S.; Romero, C. P.; Lauwaet, K.; Van Bael, M. J.; Schoeters, B.; Partoens, B.; Yucelen, E.; Lievens, P.; Van Tendeloo, G. Atomic Scale Dynamics of Ultrasmall Germanium Clusters Nat. Commun. 2012, 3, 897 DOI: 10.1038/ncomms1887Google Scholar36https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC38jht1aqsg%253D%253D&md5=b5f532e0ae2e308c684f2a0eaaa45c1eAtomic scale dynamics of ultrasmall germanium clustersBals S; Van Aert S; Romero C P; Lauwaet K; Van Bael M J; Schoeters B; Partoens B; Yucelen E; Lievens P; Van Tendeloo GNature communications (2012), 3 (), 897 ISSN:.Starting from the gas phase, small clusters can be produced and deposited with huge flexibility with regard to composition, materials choice and cluster size. Despite many advances in experimental characterization, a detailed morphology of such clusters is still lacking. Here we present an atomic scale observation as well as the dynamical behaviour of ultrasmall germanium clusters. Using quantitative scanning transmission electron microscopy in combination with ab initio calculations, we are able to characterize the transition between different equilibrium geometries of a germanium cluster consisting of less than 25 atoms. Seven-membered rings, trigonal prisms and some smaller subunits are identified as possible building blocks that stabilize the structure.
- 37Jones, L.; Macarthur, K. E.; Fauske, V. T.; van Helvoort, A. T. J.; Nellist, P. D. Rapid Estimation of Catalyst Nanoparticle Morphology and Atomic- Coordination by High-Resolution Z-Contrast Electron Microscopy Nano Lett. 2014, 14, 6336– 6341 DOI: 10.1021/nl502762mGoogle ScholarThere is no corresponding record for this reference.
- 38Geuchies, J. J.; van Overbeek, C.; Evers, W. H.; Goris, B.; de Backer, A.; Gantapara, A. P.; Rabouw, F. T.; Hilhorst, J.; Peters, J. L.; Konovalov, O.; Petukhov, A. V.; Dijkstra, M.; Siebbeles, L. D. A.; van Aert, S.; Bals, S.; Vanmaekelbergh, D. In Situ Study of the Formation Mechanism of Two-Dimensional Superlattices from PbSe Nanocrystals Nat. Mater. 2016, 15, 1248– 1254 DOI: 10.1038/nmat4746Google Scholar38https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhsVOqtrfL&md5=86f24b5b2667a5d24f39c19f935df7f8In situ study of the formation mechanism of two-dimensional superlattices from PbSe nanocrystalsGeuchies, Jaco J.; van Overbeek, Carlo; Evers, Wiel H.; Goris, Bart; de Backer, Annick; Gantapara, Anjan P.; Rabouw, Freddy T.; Hilhorst, Jan; Peters, Joep L.; Konovalov, Oleg; Petukhov, Andrei V.; Dijkstra, Marjolein; Siebbeles, Laurens D. A.; van Aert, Sandra; Bals, Sara; Vanmaekelbergh, DanielNature Materials (2016), 15 (12), 1248-1254CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)Oriented attachment of PbSe nanocubes can gave 2-dimensional (2D) superstructures with long-range nanoscale and at. order. This questions the applicability of classic models in which the superlattice grows by 1st forming a nucleus, followed by sequential irreversible attachment of nanocrystals, as 1 misaligned attachment would disrupt the 2D order beyond repair. The formation mechanism of 2D PbSe superstructures with square geometry was demonstrated by using in situ grazing-incidence x-ray scattering (small angle and wide angle), ex situ electron microscopy, and Monte Carlo simulations. The authors obsd. nanocrystal adsorption at the liq./gas interface, followed by the formation of a hexagonal nanocrystal monolayer. The hexagonal geometry transforms gradually through a pseudo-hexagonal phase into a phase with square order, driven by attractive interactions between the {100} planes perpendicular to the liq. substrate, which maximize facet-to-facet overlap. The nanocrystals then attach atomically via a necking process, resulting in 2D square superlattices.
- 39Green, M. L. H. A New Approach to the Formal Classification of Covalent Compounds of the Elements J. Organomet. Chem. 1995, 500, 127– 148 DOI: 10.1016/0022-328X(95)00508-NGoogle Scholar39https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXotlektr8%253D&md5=c0a64ae93f1ebb8fb9c77e03007833dfA new approach to the formal classification of covalent compounds of the elementsGreen, M. L. H.Journal of Organometallic Chemistry (1995), 500 (1-2), 127-48CODEN: JORCAI; ISSN:0022-328X. (Elsevier)A new approach to the classification of covalent compds. of the transition metals is presented. The classification introduces the concepts of valency no. and ligand bond no. as alternatives to the concepts of formal oxidn. state and coordination no. Population d. maps (MLX plots) which represent the covalent compds. of an element as a function of all the known ligands are exemplified for chromium; molybdenum and tungsten; and nickel, palladium and platinum. These plots are used to identify the major characteristic chem. properties of the elements including reactivity trends and reaction mechanisms.
- 40Grodzinska, D.; Pietra, F.; van Huis, M. A.; Vanmaekelbergh, D.; de Mello Donega, 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/c0jm04458jGoogle ScholarThere is no corresponding record for this reference.
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Abstract
Figure 1
Figure 1. Infrared spectrum of well-washed oleate-capped PbSe NCs in tetrachloroethylene. (A) FTIR spectrum of well-washed PbSe NCs (5.6 nm) dissolved in tetrachloroethylene showing three regions of interest: (a) region of C–O vibrations, detailed in Figure 2, (b) a peak due to the CH2 asymmetric stretching vibrations of oleate molecules (attached to the NCs or dissolved), and (c) the IR absorption peak due to the band-edge exciton in the PbSe NCs. From the integration of region c, the concentration of NCs in the solution can be determined. From the integration of peak b, the total concentration of oleate in the suspension (the sum of oleate attached to the NCs and dissolved oleate) can be determined. This is performed by using a calibration curve (panel B), i.e., the integrated absorbance under peak b vs the concentration of Pb(oleate)2 in a solution containing only Pb(oleate)2 (R2 = 0.999).
Figure 2
Figure 2. Detailed spectra of the carboxylic region. In panel A, the carboxylic regions of washed NCs (red), Pb(oleate)2 (green), and oleic acid (blue) are shown. The carboxylic vibrations are sketched in panel B. Symmetric and asymmetric carboxylic vibrations ν1 and ν2 in PbSe NCs and Pb(oleate)2 are each composed of two peaks. The decomposition and notation are presented in panel C; the meaning is discussed in the text. Panel D shows the carboxylic region for well-washed NCs (red) and titrated NCs (blue). Panel E presents sketches of the bridging configuration and the (a)symmetric chelating bidentate configurations.
Figure 3
Figure 3. Titration of a well-washed PbSe NC suspension with Pb(oleate)2. A) Close-up of the 1H NMR spectra for the two hydrogen atoms bound to the C═C atoms in the oleate. The black spectrum of freely dissolved Pb(oleate)2 and the red spectrum of well-washed PbSe NCs are normalized on their methyl integral, so spectra at similar oleate concentrations can be compared. The spectrum of well-washed NCs (red) shows a broad feature, indicating all oleate is bound to the surface. The orange, yellow, etc., spectra were obtained during gradual addition of Pb(oleate)2 to the well-washed NC suspension. It can be seen that first the magnitude of the broad peak increases (maximum at 5.3 ppm) while with an increase in the amount of Pb(oleate)2 added the sharp peak at 5.35 ppm also arises. Deconvolution of both peaks (shown in Figure S7) allowed us to quantify the number of attached and freely dissolved Pb(oleate)2 per NC during the titration experiment. This is shown in panel B, including the red dot determined via FTIR. The derived ligand densities are colored orange (2.4 oleates/nm2), yellow (2.5 oleates/nm2), green (3.2 oleates/nm2), blue (3.1 oleates/nm2), and purple (3.2 oleates/nm2).
Figure 4
Figure 4. HAADF-STEM study of PbSe NCs with an atom counting technique. The top row (A–E) shows the shape analysis for low-ligand density NCs and the bottom row (F–J) the maximal ligand densities. Panels A and F are HAADF-STEM images of a group of PbSe NCs, whereas panels B and G are magnified images for PbSe NCs that are oriented in such a way that the lead atoms are oriented in columns. In panels C and H, the intensities of the columns are integrated and the numbers of lead atoms in that row are counted and color-coded. Subsequently, 10 NCs of each sample are averaged (see Figure S10 for the procedure) in panels D and I. We propose models E and J for the fitted data shown in panels D and I, respectively. Notice the increased size of the {111} facets for the oleate-saturated NCs.
Scheme 1
Scheme 1. Chemisorption of Pb(oleate)2 and the Ensuing ReconfigurationFigure 5
Figure 5. Scheme describing the attachment and integration of a Pb(oleate)2. Attachment of these Z-type ligands on a PbSe NC resulting in stabilization of a Pb–oleate-terminated {111} facet. Panel A shows an unsaturated facet, typical for well-washed PbSe NCs. Panels B and C show the chemisorbtion of Pb(oleate)2 as a Z-type ligand. Panel D shows the stabilization of the Pb–oleate-terminated {111} facets.
References
This article references 40 other publications.
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- 2McDonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Solution-Processed PbS Quantum Dot Infrared Photodetectors and Photovoltaics Nat. Mater. 2005, 4, 138– 142 DOI: 10.1038/nmat12992https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXpt1agtg%253D%253D&md5=8451348fe3e450f88e59db69c8fca144Solution-processed PbS quantum dot infrared photodetectors and photovoltaicsMcDonald, Steven A.; Konstantatos, Gerasimos; Zhang, Shiguo; Cyr, Paul W.; Klem, Ethan J. D.; Levina, Larissa; Sargent, Edward H.Nature Materials (2005), 4 (2), 138-142CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)In contrast to traditional semiconductors, conjugated polymers provide ease of processing, low cost, phys. flexibility and large area coverage. These active optoelectronic materials produce and harvest light efficiently in the visible spectrum. The same functions are required in the IR for telecommunications (1,300-1,600 nm), thermal imaging (1,500 nm and beyond), biol. imaging (transparent tissue windows at 800 nm and 1,100 nm), thermal photovoltaics (>1,900 nm), and solar cells (800-2,000 nm). Photoconductive polymer devices have yet to demonstrate sensitivity beyond ∼800 nm. Sensitizing conjugated polymers with IR-active nanocrystal quantum dots provides a spectrally tunable means of accessing the IR while maintaining the advantageous properties of polymers. Here we use such a nanocomposite approach in which PbS nanocrystals tuned by the quantum size effect sensitize the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) into the IR. We achieve, in a soln.-processed device and with sensitivity far beyond 800 nm, harvesting of IR-photogenerated carriers and the demonstration of an IR photovoltaic effect. We also make use of the wavelength tunability afforded by the nanocrystals to show photocurrent spectra tailored to three different regions of the IR spectrum.
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- 4Marshall, A. R.; Young, M. R.; Nozik, A. J.; Beard, M. C.; Luther, J. M. Exploration of Metal Chloride Uptake for Improved Performance Characteristics of PbSe Quantum Dot Solar Cells J. Phys. Chem. Lett. 2015, 6, 2892– 2899 DOI: 10.1021/acs.jpclett.5b012144https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtFequ7rO&md5=9e24825b03b6802cbb94d448046296f1Exploration of Metal Chloride Uptake for Improved Performance Characteristics of PbSe Quantum Dot Solar CellsMarshall, Ashley R.; Young, Matthew R.; Nozik, Arthur J.; Beard, Matthew C.; Luther, Joseph M.Journal of Physical Chemistry Letters (2015), 6 (15), 2892-2899CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)We explored the uptake of metal chloride salts with +1 to +3 metals of Na+, K+, Zn2+, Cd2+, Sn2+, Cu2+, and In3+ by PbSe QD solar cells. We also compared CdCl2 to Cd acetate and Cd nitrate treatments. PbSe QD solar cells fabricated with a CdCl2 treatment are stable for more than 270 days stored in air. We studied how temp. and immersion times affect optoelectronic properties and photovoltaic cell performance. Uptake of Cd2+ and Zn2+ increase open circuit voltage, whereas In3+ and K+ increase the photocurrent without influencing the spectral response or first exciton peak position. Using the most beneficial treatments we varied the bandgap of PbSe QD solar cells from 0.78 to 1.3 eV and find the improved VOC is more prevalent for lower bandgap QD solar cells.
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- 7Boneschanscher, M. P.; Evers, W. H.; Geuchies, J. J.; Altantzis, T.; Goris, B.; Rabouw, F. T.; van Rossum, S. A. P.; van der Zant, H. S. J.; Siebbeles, L. D. A.; Van Tendeloo, G.; Swart, I.; Hilhorst, J.; Petukhov, A. V.; Bals, S.; Vanmaekelbergh, D. Long-Range Orientation and Atomic Attachment of Nanocrystals in 2D Honeycomb Superlattices Science (Washington, DC, U. S.) 2014, 344, 1377– 1380 DOI: 10.1126/science.12526427https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXpslygs78%253D&md5=9dccd97b5dd2e26aa7a5b3edfa00df16Long-range orientation and atomic attachment of nanocrystals in 2D honeycomb superlatticesBoneschanscher, M. P.; Evers, W. H.; Geuchies, J. J.; Altantzis, T.; Goris, B.; Rabouw, F. T.; van Rossum, S. A. P.; van der Zant, H. S. J.; Siebbeles, L. D. A.; Van Tendeloo, G.; Swart, I.; Hilhorst, J.; Petukhov, A. V.; Bals, S.; Vanmaekelbergh, D.Science (Washington, DC, United States) (2014), 344 (6190), 1377-1380CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Oriented attachment of synthetic semiconductor nanocrystals is emerging as a route for obtaining new semiconductors that can have Dirac-type electronic bands such as graphene, but also strong spin-orbit coupling. The two-dimensional (2D) assembly geometry will require both at. coherence and long-range periodicity of the superlattices. We show how the interfacial self-assembly and oriented attachment of nanocrystals results in 2D metal chalcogenide semiconductors with a honeycomb superlattice. We present an extensive at. and nanoscale characterization of these systems using direct imaging and wave scattering methods. The honeycomb superlattices are atomically coherent and have an octahedral symmetry that is buckled; the nanocrystals occupy two parallel planes. Considerable necking and large-scale at. motion occurred during the attachment process.
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- 10Choi, J. J.; Bealing, C. R.; Bian, K.; Hughes, K. J.; Zhang, W.; Smilgies, D. M.; Hennig, R. G.; Engstrom, J. R.; Hanrath, T. Controlling Nanocrystal Superlattice Symmetry and Shape-Anisotropic Interactions through Variable Ligand Surface Coverage J. Am. Chem. Soc. 2011, 133, 3131– 3138 DOI: 10.1021/ja110454b10https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhslyhsrY%253D&md5=e2c606e777197de18ab92b57e14e83c7Controlling Nanocrystal Superlattice Symmetry and Shape-Anisotropic Interactions through Variable Ligand Surface CoverageChoi, Joshua J.; Bealing, Clive R.; Bian, Kai-Fu; Hughes, Kevin J.; Zhang, Wen-Yu; Smilgies, Detlef-M.; Hennig, Richard G.; Engstrom, James R.; Hanrath, TobiasJournal of the American Chemical Society (2011), 133 (9), 3131-3138CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)The assembly of colloidal nanocrystals (NCs) into superstructures with long-range translational and orientational order is sensitive to the mol. interactions between ligands bound to the NC surface. The authors illustrate how ligand coverage on colloidal PbS NCs can be exploited as a tunable parameter to direct the self-assembly of superlattices with predefined symmetry. PbS NCs with dense ligand coverage assemble into fcc. superlattices whereas NCs with sparse ligand coverage assemble into bcc. superlattices which also exhibit orientational ordering of NCs in their lattice sites. Surface chem. characterization combined with d. functional theory calcns. suggest that the loss of ligands occurs preferentially on {100} than on reconstructed {111} NC facets. The resulting anisotropic ligand distribution amplifies the role of NC shape in the assembly and gives superlattices with translational and orientational order.
- 11Evers, W. H.; Schins, J. M.; Aerts, M.; Kulkarni, A.; Capiod, P.; Berthe, M.; Grandidier, B.; Delerue, C.; van der Zant, H. S. J.; van Overbeek, C.; Peters, J. L.; Vanmaekelbergh, D.; Siebbeles, L. D. A. High Charge Mobility in Two Dimensional Percolative Networks of PbSe Quantum Dots Connected by Atomic Bonds Nat. Commun. 2015, 6, 8195 DOI: 10.1038/ncomms919511https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhsFGksbfE&md5=f1f8effce52a0cdf6e0c8650f43195d5High charge mobility in two-dimensional percolative networks of PbSe quantum dots connected by atomic bondsEvers, Wiel H.; Schins, Juleon M.; Aerts, Michiel; Kulkarni, Aditya; Capiod, Pierre; Berthe, Maxime; Grandidier, Bruno; Delerue, Christophe; van der Zant, Herre S. J.; van Overbeek, Carlo; Peters, Joep L.; Vanmaekelbergh, Daniel; Siebbeles, Laurens D. A.Nature Communications (2015), 6 (), 8195CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)Two-dimensional networks of quantum dots connected by at. bonds have an electronic structure that is distinct from that of arrays of quantum dots coupled by ligand mols. We prepd. atomically coherent two-dimensional percolative networks of PbSe quantum dots connected via at. bonds. Here, we show that photoexcitation leads to generation of free charges that eventually decay via trapping. The charge mobility probed with an AC elec. field increases with frequency from 150±15 cm2 V-1 s-1 at 0.2 THz to 260±15 cm2 V-1 s-1 at 0.6 THz. Gated four-probe measurements yield a DC electron mobility of 13±2 cm2 V-1 s-1. The terahertz mobilities are much higher than for arrays of quantum dots coupled via surface ligands and are similar to the highest DC mobilities reported for PbSe nanowires. The terahertz mobility increases only slightly with temp. in the range of 15-290 K. The extent of straight segments in the two-dimensional percolative networks limits the mobility, rather than charge scattering by phonons.
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- 14Fritzinger, B.; Capek, R. K.; Lambert, K.; Martins, J. C.; Hens, Z. Utilizing Self-Exchange To Address the Binding of Carboxylic Acid Ligands to CdSe Quantum Dots J. Am. Chem. Soc. 2010, 132, 10195– 10201 DOI: 10.1021/ja104351q14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXosVGhs7s%253D&md5=abdbd55f677f597415eeff71225c0739Utilizing Self-Exchange To Address the Binding of Carboxylic Acid Ligands to CdSe Quantum DotsFritzinger, Bernd; Capek, Richard K.; Lambert, Karel; Martins, Jose C.; Hens, ZegerJournal of the American Chemical Society (2010), 132 (29), 10195-10201CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)The authors use soln. NMR techniques to analyze the org./inorg. interface of CdSe quantum dots (Q-CdSe) synthesized using oleic acid as a surfactant. The resulting Q-CdSe are stabilized by tightly bound oleic acid species that only exchange upon addn. of free oleic acid. The NMR anal. points toward a two-step exchange mechanism where free ligands are initially physisorbed within the ligand shell to end up as bound, chemisorbed ligands in a 2nd step. Importantly, every ligand is involved in this exchange process. By addn. of oleic acid with a deuterated carboxyl headgroup, the bound ligands are oleate ions and not oleic acid mols. This explains why a dynamic adsorption/desorption equil. only occurs in the presence of excess free oleic acid, which donates the required proton. Comparing the no. of oleate ligands to the excess cadmium per CdSe quantum dot, a ratio of 2:1 were found. This completes the picture of Q-CdSe as org./inorg. entities where the surface excess of Cd2+ is balanced by a double amt. of oleate ligands, yielding overall neutral nanoparticles.
- 15Owen, J. S.; Park, J.; Trudeau, P.-E. E.; Alivisatos, a. P. Reaction Chemistry and Ligand Exchange at Cadmium-Selenide Nanocrystal Surfaces J. Am. Chem. Soc. 2008, 130, 12279– 12281 DOI: 10.1021/ja804414f15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXhtVert7jI&md5=643d34871585bf85a6075ba99ace8d92Reaction Chemistry and Ligand Exchange at Cadmium-Selenide Nanocrystal SurfacesOwen, Jonathan S.; Park, Jungwon; Trudeau, Paul-Emile; Alivisatos, A. PaulJournal of the American Chemical Society (2008), 130 (37), 12279-12281CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)The surface chem. of cadmium selenide nanocrystals, prepd. from tri-n-octylphosphine selenide and cadmium octadecylphosphonate in tri-n-octylphosphine oxide, was studied with 1H and {1H}31P NMR spectroscopy as well as ESI-MS and XPS. The identity of the surface ligands was inferred from reaction of nanocrystals with Me3Si-X (X = -S-SiMe3, -Se-SiMe3, -Cl and -S-(CH2CH2O)4OCH3) and unambiguous assignment of the org. byproducts, O,O'-bis(trimethylsilyl)octadecylphosphonic acid ester and O,O'-bis(trimethylsilyl)ocatdecylphosphonic acid anhydride ester. Nanocrystals isolated from these reactions have undergone exchange of the octadecylphosphonate ligands for -X as was shown by 1H NMR (X = -S-(CH2CH2O)4OCH3) and XPS (X = -Cl). Addn. of free thiols to as prepd. nanocrystals results in binding of the thiol to the particle surface and quenching of the nanocrystal fluorescence. Isolation of the thiol-ligated nanocrystals shows this chemisorption proceeds without displacement of the octadecylphosphonate ligands, suggesting the presence of unoccupied Lewis-acidic sites on the particle surface. In the presence of added triethylamine, however, the octadecylphosphonate ligands are readily displaced from the particle surface as was shown with 1H and {1H}31P NMR. These results, in conjunction with previous literature reports, indicate that as-prepd. nanocrystal surfaces are terminated by X-type binding of octadecylphosphonate moieties to a layer of excess cadmium ions.
- 16Bealing, C. R.; Baumgardner, W. J.; Choi, J. J.; Hanrath, T.; Hennig, R. G. Predicting Nanocrystal Shape through Consideration of Surface-Ligand Interactions ACS Nano 2012, 6, 2118– 2127 DOI: 10.1021/nn300046616https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XitFKktbo%253D&md5=8f362f8e23e76d7262897dbf6651ab57Predicting Nanocrystal Shape through Consideration of Surface-Ligand InteractionsBealing, Clive R.; Baumgardner, William J.; Choi, Joshua J.; Hanrath, Tobias; Hennig, Richard G.ACS Nano (2012), 6 (3), 2118-2127CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)D. functional calcns. for the binding energy of oleic acid-based ligands on Pb-rich {100} and {111} facets of PbSe nanocrystals det. the surface energies as a function of ligand coverage. Oleic acid is expected to bind to the nanocrystal surface as lead oleate. The Wulff construction predicts the thermodn. equil. shape of the PbSe nanocrystals. The equil. shape is a function of the ligand surface coverage, which can be controlled by changing the concn. of oleic acid during synthesis. The different binding energy of the ligand on the {100} and {111} facets results in different equil. ligand coverages on the facets, and a transition in the equil. shape from octahedral to cubic is predicted when increasing the ligand concn. during synthesis.
- 17Zherebetskyy, D.; Scheele, M.; Zhang, Y.; Bronstein, N.; Thompson, C.; Britt, D.; Salmeron, M.; Alivisatos, A. P.; Wang, L.-W. Hydroxylation of the Surface of PbS Nanocrystals Passivated with Oleic Acid Science (Washington, DC, U. S.) 2014, 344, 1380– 1384 DOI: 10.1126/science.125272717https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXpslygs7w%253D&md5=4046b9d228449c83c88672795a527482Hydroxylation of the surface of PbS nanocrystals passivated with oleic acidZherebetskyy, Danylo; Scheele, Marcus; Zhang, Yingjie; Bronstein, Noah; Thompson, Christopher; Britt, David; Salmeron, Miquel; Alivisatos, Paul; Wang, Lin-WangScience (Washington, DC, United States) (2014), 344 (6190), 1380-1384CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Controlling the structure of colloidal nanocrystals (NCs) is key to the generation of their complex functionality. This requires an understanding of the NC surface at the at. level. The structure of colloidal PbS NCs passivated with oleic acid has been studied theor. and exptl. The authors show the existence of surface OH- groups, which play a key role in stabilizing the PbS(111) facets, consistent with XPS as well as other spectroscopic and chem. expts. The role of water in the synthesis process is also revealed. Their model, along with existing observations of NC surface termination and passivation by ligands, helps to explain and predict the properties of NCs and their assemblies.
- 18Schapotschnikow, P.; van Huis, M. A.; Zandbergen, H. W.; Vanmaekelbergh, D.; Vlugt, T. J. H. Morphological Transformations and Fusion of PbSe Nanocrystals Studied Using Atomistic Simulations Nano Lett. 2010, 10, 3966– 3971 DOI: 10.1021/nl101793b18https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtFKlu7rI&md5=ea82cab28e64a8d31cb0fdb00cc7b42dMorphological Transformations and Fusion of PbSe Nanocrystals Studied Using Atomistic SimulationsSchapotschnikow, Philipp; van Huis, Marijn A.; Zandbergen, Henny W.; Vanmaekelbergh, Daniel; Vlugt, Thijs J. H.Nano Letters (2010), 10 (10), 3966-3971CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Mol. dynamics simulations are performed on capped and uncapped PbSe nanocrystals, employing newly developed classical interaction potentials. Here, we show that two uncapped nanocrystals fuse efficiently via direct surface attachment, even if they are initially misaligned. In sharp contrast to the general belief, interparticle dipole interactions do not play a significant role in this "oriented attachment" process. Furthermore, it is shown that presumably polar, capped PbSe{111} facets are never fully Pb- or Se-terminated.
- 19Fang, C.; Van Huis, M. a.; Vanmaekelbergh, D.; Zandbergen, H. W. Energetics of Polar and Nonpolar Facets of PbSe Nanocrystals from Theory and Experiment ACS Nano 2010, 4, 211– 218 DOI: 10.1021/nn901340619https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsV2htbvK&md5=75ff94111588721c09a5c519059bdf78Energetics of Polar and Nonpolar Facets of PbSe Nanocrystals from Theory and ExperimentFang, Changming; van Huis, Marijn A.; Vanmaekelbergh, Daniel; Zandbergen, Henny W.ACS Nano (2010), 4 (1), 211-218CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Surface energies of the distinct facets of nanocrystals are an important factor in the free energy and hence det. the nanocrystal morphol., chem. and phys. properties, and even interparticle dipole interactions. The stability and at. structure of polar and nonpolar PbSe surfaces by combining first-principles calcns. with high-resoln. transmission electron microscopy (TEM) is investigated. For uncapped surfaces, the calcns. predict that the nonpolar {100} surface is the most stable with a surface energy of 0.184 J m-2, while the nonpolar {110} and reconstructed {111}-Pb surfaces have surface energies of 0.318 J m-2 and 0.328 J m-2, resp. Fully polar {111} surfaces are structurally unstable upon relaxation. These findings are in good agreement with TEM observations showing that capped nanocrystals have a nearly spherical, multifaceted morphol., while cubical shapes with predominantly {100} facets are obtained when the capping mols. are removed through heating in vacuum. During this process, multipolar surfaces can temporarily exist just after the removal of the surfactants. These metastable {111} surfaces consist of ribbon-like nanodomains, whereby the ribbons are alternating in polarity. The calcns. confirm that these multipolar surfaces are energetically more favorable than fully polar surfaces. The consequences for capped nanocrystals (a dominant Pb-oleate termination) and nanocrystal fusion (a shorter interaction range of dipole interactions) are discussed.
- 20Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. Designing PbSe Nanowires and Nanorings through Oriented Attachment of Nanoparticles J. Am. Chem. Soc. 2005, 127, 7140– 7147 DOI: 10.1021/ja050107s20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXjs1Sisb8%253D&md5=7f42df157281b5b21d0c5e424aabea4dDesigning PbSe Nanowires and Nanorings through Oriented Attachment of NanoparticlesCho, Kyung-Sang; Talapin, Dmitri V.; Gaschler, Wolfgang; Murray, Christopher B.Journal of the American Chemical Society (2005), 127 (19), 7140-7147CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Single-crystal PbSe nanowires were synthesized in soln. through oriented attachment of nanocrystal building blocks. Reaction temps. of 190-250° and multicomponent surfactant mixts. result in a nearly defect-free crystal lattice and high uniformity of nanowire diam. along the entire length. The wires' dimensions are tuned by tailoring reaction conditions in a range from ∼4 to ∼20 nm in diam. with wire lengths up to ∼30 μm. PbSe nanocrystals bind to each other on either {100}, {110}, or {111} faces, depending on the surfactant mols. present in the reaction soln. While PbSe nanocrystals have the centrosym. rocksalt lattice, they can lack central symmetry due to a noncentrosym. arrangement of Pb- and Se-terminated {111} facets and possess dipole driving 1-dimensional oriented attachment of nanocrystals to form nanowires. In addn. to straight nanowires, zigzag, helical, branched, and tapered nanowires as well as single-crystal nanorings can be controllably prepd. in 1-pot reactions by careful adjustment of the reaction conditions.
- 21Van Aert, S.; De Backer, A.; Martinez, G. T.; Goris, B.; Bals, S.; Van Tendeloo, G.; Rosenauer, A. Procedure to Count Atoms with Trustworthy Single-Atom Sensitivity Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 64107 DOI: 10.1103/PhysRevB.87.064107There is no corresponding record for this reference.
- 22Bals, S.; Casavola, M.; van Huis, M. A.; Van Aert, S.; Batenburg, K. J.; Van Tendeloo, G.; Vanmaekelbergh, D. Three-Dimensional Atomic Imaging of Colloidal Core-Shell Nanocrystals Nano Lett. 2011, 11, 3420– 3424 DOI: 10.1021/nl201826e22https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXptlCju70%253D&md5=1b5c32efb69c3c5e9bdb5e007a872bdfThree-Dimensional Atomic Imaging of Colloidal Core-Shell NanocrystalsBals, Sara; Casavola, Marianna; van Huis, Marijn A.; Van Aert, Sandra; Batenburg, K. Joost; Van Tendeloo, Gustaaf; Vanmaekelbergh, DanielNano Letters (2011), 11 (8), 3420-3424CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Colloidal core-shell semiconductor nanocrystals form an important class of optoelectronic materials, in which the exciton wave functions can be tailored by the at. configuration of the core, the interfacial layers, and the shell. Here, the authors provide a trustful 3D characterization at the at. scale of a free-standing PbSe(core)-CdSe(shell) nanocrystal by combining electron microscopy and discrete tomog. The authors results yield unique insights for understanding the process of cation exchange, which is widely employed in the synthesis of core-shell nanocrystals. The study that the authors present is generally applicable to the broad range of colloidal heteronanocrystals that currently emerge as a new class of materials with technol. importance.
- 23Hassinen, A.; Moreels, I.; De Nolf, K.; Smet, P. F.; Martins, J. C.; Hens, Z. Short-Chain Alcohols Strip X-Type Ligands and Quench the Luminescence of PbSe and CdSe Quantum Dots, Acetonitrile Does Not J. Am. Chem. Soc. 2012, 134, 20705– 20712 DOI: 10.1021/ja308861d23https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvVWqtLzO&md5=2e0c7f106c59dfa86a3150716f4f6800Short-Chain Alcohols Strip X-Type Ligands and Quench the Luminescence of PbSe and CdSe Quantum Dots, Acetonitrile Does NotHassinen, Antti; Moreels, Iwan; De Nolf, Kim; Smet, Philippe F.; Martins, Jose C.; Hens, ZegerJournal of the American Chemical Society (2012), 134 (51), 20705-20712CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)The effect of short-chain alcs. and MeCN on the ligand shell compn. and the photoluminescence quantum yield of purified PbSe and CdSe quantum dots is analyzed by soln. NMR and photoluminescence spectroscopy. Short-chain alcs. induce the release of X-type carboxylate ligands with a concurrent redn. of the photoluminescence quantum yield, while MeCN does not. The authors interpret this difference in terms of the protic or aprotic character of both nonsolvents, where only the protic alcs. can provide the protons needed to desorb carboxylate ligands. Similar differences between short-chain alcs. and MeCN when used as nonsolvents during the purifn. of crude synthesis products, a result stressing the importance of using aprotic nonsolvents for nanocrystal purifn. or processing were found.
- 24Anderson, N. C.; Hendricks, M. P.; Choi, J. J.; Owen, J. S. Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding J. Am. Chem. Soc. 2013, 135, 18536– 18548 DOI: 10.1021/ja408675824https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhslCrsr%252FI&md5=64005629c75e8c287c3b2dfb3047a246Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and BindingAnderson, Nicholas C.; Hendricks, Mark P.; Choi, Joshua J.; Owen, Jonathan S.Journal of the American Chemical Society (2013), 135 (49), 18536-18548CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Metal carboxylate complexes (L-M(O2CR)2, O2CR = oleic, myristic, M = Cd, Pb) are readily displaced from carboxylate-terminated ME nanocrystals (ME = CdSe, CdS, PbSe, PbS) by various Lewis bases (L = tri-n-butylamine, THF, tetradecanol, N,N-dimethyl-n-butylamine, tri-n-butylphosphine, N,N,N',N'-tetramethylbutylene-1,4-diamine, pyridine, N,N,N',N'-tetramethylethylene-1,2-diamine, n-octylamine). The relative displacement potency is measured by 1H NMR spectroscopy and depends most strongly on geometric factors such as sterics and chelation, although also on the hard/soft match with the Cd ion. The results suggest that ligands displace L-M(O2CR)2 by cooperatively complexing the displaced metal ion as well as the nanocrystal. Removal of up to 90% of surface-bound Cd-(O2CR)2 from CdSe and CdS nanocrystals decreases the Cd/Se ratio from 1.1 ± 0.06 to 1.0 ± 0.05, broadens the 1Se-2S3/2h absorption, and decreases the photoluminescence quantum yield (PLQY) from 10% to <1% (CdSe) and from 20% to <1% (CdS). These changes are partially reversed upon rebinding of M-(O2CR)2 at room temp. (∼60%) and fully reversed at elevated temp. A model is proposed in which electron-accepting M-(O2CR)2 complexes (Z-type ligands) reversibly bind to nanocrystals, leading to a range of stoichiometries for a given core size. Nanocrystals lack a single chem. formula, but are instead dynamic structures with concn.-dependent compns. The importance of these findings to the synthesis and purifn. of nanocrystals as well as ligand exchange reactions is discussed.
- 25Grisorio, R.; Debellis, D.; Suranna, G. P.; Gigli, G.; Giansante, C. The Dynamic Organic/Inorganic Interface of Colloidal PbS Quantum Dots Angew. Chem., Int. Ed. 2016, 55, 6628– 6633 DOI: 10.1002/anie.20151117425https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xlt1yju7Y%253D&md5=5143cb4fac6ba04219f26a091589b33aThe Dynamic Organic/Inorganic Interface of Colloidal PbS Quantum DotsGrisorio, Roberto; Debellis, Doriana; Suranna, Gian Paolo; Gigli, Giuseppe; Giansante, CarloAngewandte Chemie, International Edition (2016), 55 (23), 6628-6633CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)Colloidal quantum dots are composed of nanometer-sized crystallites of inorg. semiconductor materials bearing org. mols. at their surface. The org./inorg. interface markedly affects forms and functions of the quantum dots, therefore its description and control are important for effective application. Herein we demonstrate that archetypal colloidal PbS quantum dots adapt their interface to the surroundings, thus existing in soln. phase as equil. mixts. with their (metal-)org. ligand and inorg. core components. The interfacial equil. are dictated by solvent polarity and concn., show striking size dependence (leading to more stable ligand/core adducts for larger quantum dots), and selectively involve nanocrystal facets. This notion of ligand/core dynamic equil. may open novel synthetic paths and refined nanocrystal surface-chem. strategies.
- 26Nag, A.; Kovalenko, M. V.; Lee, J.; Liu, W.; Spokoyny, B.; Talapin, D. V. Metal-Free Inorganic Ligands for Colloidal Nanocrystals: S2–, HS–, Se2–, HSe–, Te2–, HTe–, TeS32–, OH–, and NH2– as Surface Ligands J. Am. Chem. Soc. 2011, 133, 10612– 10620 DOI: 10.1021/ja202941526https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXns1emsbY%253D&md5=51f8cbf21be5b525a4abde2e7f3c8acaMetal-free Inorganic Ligands for Colloidal Nanocrystals: S2-, HS-, Se2-, HSe-, Te2-, HTe-, TeS32-, OH-, and NH2- as Surface LigandsNag, Angshuman; Kovalenko, Maksym V.; Lee, Jong-Soo; Liu, Wenyong; Spokoyny, Boris; Talapin, Dmitri V.Journal of the American Chemical Society (2011), 133 (27), 10612-10620CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)All-inorg. colloidal nanocrystals were synthesized by replacing org. capping ligands on chem. synthesized nanocrystals with metal-free inorg. ions such as S2-, HS-, Se2-, HSe-, Te2-, HTe-, TeS32-, OH- and NH2-. These simple ligands adhered to the NC surface and provided colloidal stability in polar solvents. The versatility of such ligand exchange was demonstrated for various semiconductor and metal nanocrystals of different size and shape. The key aspects of Pearson's hard and soft acids and bases (HSAB) principle, originally developed for metal coordination compds., can be applied to the bonding of mol. species to the nanocrystal surface. The use of small inorg. ligands instead of traditional ligands with long hydrocarbon tails facilitated the charge transport between individual nanocrystals and opened up interesting opportunities for device integration of colloidal nanostructures.
- 27Campos, M. P.; Hendricks, M. P.; Beecher, A. N.; Walravens, W.; Swain, R. A.; Cleveland, G. T.; Hens, Z.; Sfeir, M. Y.; Owen, J. S. A Library of Selenourea Precursors to PbSe Nanocrystals with Size Distributions Near the Homogeneous Limit J. Am. Chem. Soc. 2017, 139, 2296– 2305 DOI: 10.1021/jacs.6b1102127https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXht1ygtLs%253D&md5=9329da51d16e810de99cbc5628952062A Library of Selenourea Precursors to PbSe Nanocrystals with Size Distributions near the Homogeneous LimitCampos, Michael P.; Hendricks, Mark P.; Beecher, Alexander N.; Walravens, Willem; Swain, Robert A.; Cleveland, Gregory T.; Hens, Zeger; Sfeir, Matthew Y.; Owen, Jonathan S.Journal of the American Chemical Society (2017), 139 (6), 2296-2305CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)We report a tunable library of N,N,N'-trisubstituted selenourea precursors and their reaction with lead oleate at 60-150 °C to form carboxylate-terminated PbSe nanocrystals in quant. yields. Single exponential conversion kinetics can be tailored over 4 orders of magnitude by adjusting the selenourea structure. The wide range of conversion reactivity allows the extent of nucleation ([nanocrystal] = 4.6-56.7 μM) and the size following complete precursor conversion (d = 1.7-6.6 nm) to be controlled. Narrow size distributions (σ = 0.5-2%) are obtained whose spectral line widths are dominated (73-83%) by the intrinsic single particle spectral broadening, as obsd. using spectral hole burning measurements. The intrinsic broadening decreases with increasing size (fwhm = 320-65 meV, d = 1.6-4.4 nm) that derives from exciton fine structure and exciton-phonon coupling rather than broadening caused by the size distribution.
- 28Cass, L. C.; Malicki, M.; Weiss, E. a. The Chemical Environments of Oleate Species within Samples of Oleate-Coated PbS Quantum Dots Anal. Chem. 2013, 85, 6974– 6979 DOI: 10.1021/ac401623a28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXpslKhtLg%253D&md5=edc8962aa1789352c3e911b8adf10607The Chemical Environments of Oleate Species within Samples of Oleate-Coated PbS Quantum DotsCass, Laura C.; Malicki, Michal; Weiss, Emily A.Analytical Chemistry (Washington, DC, United States) (2013), 85 (14), 6974-6979CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)A combination of FTIR, 1H NMR, nuclear Overhauser effect (NOESY), and diffusion-ordered (DOSY) NMR spectroscopies shows that samples of oleate-coated PbS quantum dots (QDs) with core radii ranging from 1.6 to 2.4 nm, and purified by washing with acetone, contain two species of oleate characterized by the stretching frequencies of their carboxylate groups, the chem. shifts of their protons, and their diffusion coeffs. One of these oleate species exists primarily on the surfaces of the QDs and either chelates a Pb2+ ion or bridges two Pb2+ ions. The ratio of bridging oleates to chelating oleates on the surfaces of the QDs is ∼1:1 for all sizes of the QDs the authors studied. The 2nd oleate species in these samples bridges two Pb2+ ions within clusters or oligomers of lead oleate (with a hydrodynamic radius of ∼1.4 nm), which are byproducts of the QD synthesis. The concn. of these clusters increases with increasing size of the QDs because larger QDs are produced by increasing the concn. of the oleic acid ligand in the reaction mixt. The oleate mols. on the surfaces of the QDs and within the lead oleate clusters are in rapid exchange with each other. Addnl. washes with methanol progressively eliminate the contaminating clusters from the PbS QD samples. This work quant. characterizes the distribution of binding geometries at the inorg./org. interface of the nanocrystals and demonstrates the utility of using org. ligands as probes for the compn. of a colloidal QD sample as a function of the prepn. procedure.
- 29Mesubi, M. A. An Infrared Study of Zinc, Cadmium, and Lead Salts of Some Fatty Acids J. Mol. Struct. 1982, 81, 61– 71 DOI: 10.1016/0022-2860(82)80079-329https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL38XktFCht78%253D&md5=8657b3308ca95c7795635cff1158dbb2An infrared study of zinc, cadmium, and lead salts of some fatty acidsMesubi, M. AdediranJournal of Molecular Structure (1982), 81 (1-2), 61-71CODEN: JMOSB4; ISSN:0022-2860.The IR spectra between 1800 and 700 cm-1 of Zn, Cd, and Pb carboxylates of even numbered chain length from C6 to C18 inclusive, were measured as a function of temp. from ambient temp. to a few degrees above the m.p. From the COO- stretching frequencies, it was deduced that the coordination of the carboxylate group to the metal ions is unsym. chelating bidentate. The only significant change obsd. prior to the transition point is a decrease in the intensity and a broadening of the progression of the narrow bands between 1350 and 1150 cm-1 and the band at ∼720 cm-1, most of which virtually disappear in the melt. Above the m.p., there is an abrupt change in the shape of the COO- stretching bands. A new band at 1634 cm-1 was obsd. in Zn salts. This band is attributed to the chelating bidentate structure acquiring some monodentate character as the temp. increases. The spectral changes due to thermal effects are reversible.
- 30Deacon, G. B.; Philiips, R. J. Relationships between the Carbon-Oxygen Stretching Frequencies of Carboxylato Complexes and the Type of Carboxylate Coordination Coord. Chem. Rev. 1980, 33, 227– 250 DOI: 10.1016/S0010-8545(00)80455-530https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL3MXjvVOqsw%253D%253D&md5=792c04e5960043e7dda7f4e4d4f3d779Relationships between the carbon-oxygen stretching frequencies of carboxylato complexes and the type of carboxylate coordinationDeacon, G. B.; Phillips, R. J.Coordination Chemistry Reviews (1980), 33 (3), 227-50CODEN: CCHRAM; ISSN:0010-8545.A review with 177 refs.
- 31Ellis, H. A.; White, N. A. S.; Taylor, R. A.; Maragh, P. T. Infrared, X-Ray and Microscopic Studies on the Room Temperature Structure of Anhydrous Lead (II) N-Alkanoates J. Mol. Struct. 2005, 738, 205– 210 DOI: 10.1016/j.molstruc.2004.12.00631https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXhs12ltL4%253D&md5=9915b50933137ec5e4ec9d7044288e88Infrared, X-ray and microscopic studies on the room temperature structure of anhydrous lead (II) n-alkanoatesEllis, Henry A.; White, Nicole A. S.; Taylor, Richard A.; Maragh, Paul T.Journal of Molecular Structure (2005), 738 (1-3), 205-210CODEN: JMOSB4; ISSN:0022-2860. (Elsevier B.V.)The room temp. structures of even-chain length lead (II) n-alkanoates were studied using IR spectroscopy, x-ray diffraction and polarizing light microscopy. . While the bonding is the same for both long and short chain compds., where four carboxylate groups form an unsym. bidentate bond with a Pb atom, with possible carboxylate bridging to adjacent Pb atoms, at least for the short chain length compds., the arrangement of hydrocarbon chains within a lamella is different. Both x-ray and microscopic studies show that for short chain compds., hydrocarbon chains are arranged as bilayers within a lamella and tilted with respect to the lead basal plane. For the long chain compds., probably the chains are arranged as a monolayer within a lamella, in an alternating arrangement, and slightly tilted with respect to the lead basal plane. For all the compds., x-ray data point to a triclinic unit cell contg. four mols. per unit cell with P‾1 symmetry, and chains oriented along the c axis. Crystallog. data are given.
- 32Moreels, I.; Fritzinger, B.; Martins, J. C.; Hens, Z. Surface Chemistry of Colloidal PbSe Nanocrystals J. Am. Chem. Soc. 2008, 130, 15081– 15086 DOI: 10.1021/ja803994m32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXht1GnurnJ&md5=6ae8087657d4338ad25df3f157708ac4Surface Chemistry of Colloidal PbSe NanocrystalsMoreels, Iwan; Fritzinger, Bernd; Martins, Jose C.; Hens, ZegerJournal of the American Chemical Society (2008), 130 (45), 15081-15086CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Soln. NMR spectroscopy (NMR) is used to identify and quantify the org. capping of colloidal PbSe nanocrystals (Q-PbSe). The capping consists primarily of tightly bound oleic acid ligands. Only a minor part of the ligand shell (0-5% with respect to the no. of oleic acid ligands) is composed of tri-n-octylphosphine. As a result, tuning of the Q-PbSe size during synthesis is achieved by varying the oleic acid concn. By combining the NMR results with inductively coupled plasma mass spectrometry, a complete Q-PbSe structural model of semiconductor core and org. ligands is constructed. The nanocrystals are nonstoichiometric, with a surface that is composed of lead atoms. The absence of surface selenium atoms is in accordance with an oleic acid ligand shell. NMR results on a Q-PbSe suspension, stored under ambient conditions, suggest that oxidn. leads to the loss of oleic acid ligands and surface Pb atoms, forming dissolved lead oleate.
- 33Van Aert, S.; Batenburg, K. J.; Rossell, M. D.; Erni, R.; Van Tendeloo, G. Three-Dimensional Atomic Imaging of Crystalline Nanoparticles Nature 2011, 470 (7334) 374– 377 DOI: 10.1038/nature0974133https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhsVWjt74%253D&md5=aeb5bf85c88b3b272c7c08972ab6e238Three-dimensional atomic imaging of crystalline nanoparticlesVan Aert, Sandra; Batenburg, Kees J.; Rossell, Marta D.; Erni, Rolf; Van Tendeloo, GustaafNature (London, United Kingdom) (2011), 470 (7334), 374-377CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)Detg. the three-dimensional (3D) arrangement of atoms in cryst. nanoparticles is important for nanometer-scale device engineering and also for applications involving nanoparticles, such as optoelectronics or catalysis. A nanoparticle's phys. and chem. properties are controlled by its exact 3D morphol., structure and compn. Electron tomog. enables the recovery of the shape of a nanoparticle from a series of projection images. Although at.-resoln. electron microscopy has been feasible for nearly four decades, neither electron tomog. nor any other exptl. technique has yet demonstrated at. resoln. in three dimensions. Here the authors report the 3D reconstruction of a complex cryst. nanoparticle at at. resoln. To achieve this, we combined aberration-cor. scanning transmission electron microscopy, statistical parameter estn. theory and discrete tomog. Unlike conventional electron tomog., only two images of the target-a silver nanoparticle embedded in an aluminum matrix-are sufficient for the reconstruction when combined with available knowledge about the particle's crystallog. structure. Addnl. projections confirm the reliability of the result. The results we present help close the gap between the at. resoln. achievable in two-dimensional electron micrographs and the coarser resoln. that has hitherto been obtained by conventional electron tomog.
- 34Scott, M. C.; Chen, C.-C.; Mecklenburg, M.; Zhu, C.; Xu, R.; Ercius, P.; Dahmen, U.; Regan, B. C.; Miao, J. Electron Tomography at 2.4-Ångström Resolution Nature 2012, 483, 444– 447 DOI: 10.1038/nature1093434https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XksVehu7o%253D&md5=f6a39c25f019f484e63aaa8cc5e7ba93Electron tomography at 2.4-angstrom resolutionScott, M. C.; Chen, Chien-Chun; Mecklenburg, Matthew; Zhu, Chun; Xu, Rui; Ercius, Peter; Dahmen, Ulrich; Regan, B. C.; Miao, JianweiNature (London, United Kingdom) (2012), 483 (7390), 444-447CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)TEM is a powerful imaging tool that found broad application in materials science, nanoscience and biol. With the introduction of aberration-cor. electron lenses, both the spatial resoln. and the image quality in TEM were significantly improved and resoln. <0.5 angstroms was demonstrated. To reveal the 3-dimensional (3D) structure of thin samples, electron tomog. is the method of choice, with cubic-nanometer resoln. currently achievable. Discrete tomog. has recently been used to generate a 3-dimensional at. reconstruction of a Ag nanoparticle 2 to 3 nm in diam., but this statistical method assumes prior knowledge of the particle's lattice structure and requires that the atoms fit rigidly on that lattice. Here the authors report the exptl. demonstration of a general electron tomog. method that achieves at.-scale resoln. without initial assumptions about the sample structure. By combining a novel projection alignment and tomog. reconstruction method with scanning TEM, the authors detd. the 3-dimensional structure of an ∼10-nm Au nanoparticle at 2.4-angstrom resoln. Although the authors cannot definitively locate all of the atoms inside the nanoparticle, individual atoms are obsd. in some regions of the particle and several grains are identified in 3 dimensions. The 3-dimensional surface morphol. and internal lattice structure revealed are consistent with a distorted icosahedral multiply twinned particle. The authors anticipate that this general method can be applied not only to det. the 3-dimensional structure of nanomaterials at at.-scale resoln., but also to improve the spatial resoln. and image quality in other tomog. fields.
- 35Goris, B.; Bals, S.; Van den Broek, W.; Carbó-Argibay, E.; Gómez-Graña, S.; Liz-Marzán, L. M.; Van Tendeloo, G. Atomic-Scale Determination of Surface Facets in Gold Nanorods Nat. Mater. 2012, 11, 930– 935 DOI: 10.1038/nmat346235https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsFeqsr7K&md5=19395c5bfcb68e426845ff7954d2958aAtomic-scale determination of surface facets in gold nanorodsGoris, Bart; Bals, Sara; Van den Broek, Wouter; Carbo-Argibay, Enrique; Gomez-Grana, Sergio; Liz-Marzan, Luis M.; Van Tendeloo, GustaafNature Materials (2012), 11 (11), 930-935CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)The phys. properties of nanostructures depend on the type of surface facets. For Au nanorods, the surface facets have a major influence on crucial effects such as reactivity and ligand adsorption and there was controversy regarding facet indexing. Aberration-cor. electron microscopy is the ideal technique to study the at. structure of nanomaterials. However, these images correspond to 2D projections of 3D nanoobjects, leading to an incomplete characterization. Recently, much progress was achieved in the field of at.-resoln. electron tomog., but it is still far from being a routinely used technique. Here we propose a methodol. to measure the 3D at. structure of free-standing nanoparticles, which we apply to characterize the surface facets of Au nanorods. This methodol. is applicable to a broad range of nanocrystals, leading to unique insights concerning the connection between the structure and properties of nanostructures.
- 36Bals, S.; Van Aert, S.; Romero, C. P.; Lauwaet, K.; Van Bael, M. J.; Schoeters, B.; Partoens, B.; Yucelen, E.; Lievens, P.; Van Tendeloo, G. Atomic Scale Dynamics of Ultrasmall Germanium Clusters Nat. Commun. 2012, 3, 897 DOI: 10.1038/ncomms188736https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC38jht1aqsg%253D%253D&md5=b5f532e0ae2e308c684f2a0eaaa45c1eAtomic scale dynamics of ultrasmall germanium clustersBals S; Van Aert S; Romero C P; Lauwaet K; Van Bael M J; Schoeters B; Partoens B; Yucelen E; Lievens P; Van Tendeloo GNature communications (2012), 3 (), 897 ISSN:.Starting from the gas phase, small clusters can be produced and deposited with huge flexibility with regard to composition, materials choice and cluster size. Despite many advances in experimental characterization, a detailed morphology of such clusters is still lacking. Here we present an atomic scale observation as well as the dynamical behaviour of ultrasmall germanium clusters. Using quantitative scanning transmission electron microscopy in combination with ab initio calculations, we are able to characterize the transition between different equilibrium geometries of a germanium cluster consisting of less than 25 atoms. Seven-membered rings, trigonal prisms and some smaller subunits are identified as possible building blocks that stabilize the structure.
- 37Jones, L.; Macarthur, K. E.; Fauske, V. T.; van Helvoort, A. T. J.; Nellist, P. D. Rapid Estimation of Catalyst Nanoparticle Morphology and Atomic- Coordination by High-Resolution Z-Contrast Electron Microscopy Nano Lett. 2014, 14, 6336– 6341 DOI: 10.1021/nl502762mThere is no corresponding record for this reference.
- 38Geuchies, J. J.; van Overbeek, C.; Evers, W. H.; Goris, B.; de Backer, A.; Gantapara, A. P.; Rabouw, F. T.; Hilhorst, J.; Peters, J. L.; Konovalov, O.; Petukhov, A. V.; Dijkstra, M.; Siebbeles, L. D. A.; van Aert, S.; Bals, S.; Vanmaekelbergh, D. In Situ Study of the Formation Mechanism of Two-Dimensional Superlattices from PbSe Nanocrystals Nat. Mater. 2016, 15, 1248– 1254 DOI: 10.1038/nmat474638https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhsVOqtrfL&md5=86f24b5b2667a5d24f39c19f935df7f8In situ study of the formation mechanism of two-dimensional superlattices from PbSe nanocrystalsGeuchies, Jaco J.; van Overbeek, Carlo; Evers, Wiel H.; Goris, Bart; de Backer, Annick; Gantapara, Anjan P.; Rabouw, Freddy T.; Hilhorst, Jan; Peters, Joep L.; Konovalov, Oleg; Petukhov, Andrei V.; Dijkstra, Marjolein; Siebbeles, Laurens D. A.; van Aert, Sandra; Bals, Sara; Vanmaekelbergh, DanielNature Materials (2016), 15 (12), 1248-1254CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)Oriented attachment of PbSe nanocubes can gave 2-dimensional (2D) superstructures with long-range nanoscale and at. order. This questions the applicability of classic models in which the superlattice grows by 1st forming a nucleus, followed by sequential irreversible attachment of nanocrystals, as 1 misaligned attachment would disrupt the 2D order beyond repair. The formation mechanism of 2D PbSe superstructures with square geometry was demonstrated by using in situ grazing-incidence x-ray scattering (small angle and wide angle), ex situ electron microscopy, and Monte Carlo simulations. The authors obsd. nanocrystal adsorption at the liq./gas interface, followed by the formation of a hexagonal nanocrystal monolayer. The hexagonal geometry transforms gradually through a pseudo-hexagonal phase into a phase with square order, driven by attractive interactions between the {100} planes perpendicular to the liq. substrate, which maximize facet-to-facet overlap. The nanocrystals then attach atomically via a necking process, resulting in 2D square superlattices.
- 39Green, M. L. H. A New Approach to the Formal Classification of Covalent Compounds of the Elements J. Organomet. Chem. 1995, 500, 127– 148 DOI: 10.1016/0022-328X(95)00508-N39https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXotlektr8%253D&md5=c0a64ae93f1ebb8fb9c77e03007833dfA new approach to the formal classification of covalent compounds of the elementsGreen, M. L. H.Journal of Organometallic Chemistry (1995), 500 (1-2), 127-48CODEN: JORCAI; ISSN:0022-328X. (Elsevier)A new approach to the classification of covalent compds. of the transition metals is presented. The classification introduces the concepts of valency no. and ligand bond no. as alternatives to the concepts of formal oxidn. state and coordination no. Population d. maps (MLX plots) which represent the covalent compds. of an element as a function of all the known ligands are exemplified for chromium; molybdenum and tungsten; and nickel, palladium and platinum. These plots are used to identify the major characteristic chem. properties of the elements including reactivity trends and reaction mechanisms.
- 40Grodzinska, D.; Pietra, F.; van Huis, M. A.; Vanmaekelbergh, D.; de Mello Donega, 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/c0jm04458jThere is no corresponding record for this reference.
Supporting Information
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01103.
Experimental methods, explanation of the Wulff reconstruction, several supporting FTIR spectra, separation of oleic acid from Pb(oleate)2, procedure for determining the amount of bound and unbound Pb(oleate)2 via NMR, and supportive HAADF-STEM data, including a detailed explanation of the averaging procedure (Figure S9) (PDF)
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