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Controlled Synthesis of Perovskite Nanocrystals at Room Temperature by Liquid Crystalline Templates
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  • Jun-Hyung Im
    Jun-Hyung Im
    Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
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    Orcidhttps://orcid.org/0009-0002-1504-8544
  • Myeonggeun Han
    Myeonggeun Han
    Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
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  • Jisu Hong
    Jisu Hong
    Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
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  • Hyein Kim
    Hyein Kim
    Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
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  • Kwang-Suk Oh
    Kwang-Suk Oh
    Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
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  • Taesu Choi
    Taesu Choi
    Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
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  • Abd Rashid bin Mohd Yusoff
    Abd Rashid bin Mohd Yusoff
    Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
    Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru 81310, Malaysia
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  • Maria Vasilopoulou
    Maria Vasilopoulou
    Institute of Nanoscience and Nanotechnology, National Centre for Scientific Research Demokritos, Attica 15341, Greece
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    Orcidhttps://orcid.org/0000-0001-8893-1691
  • Eunsook Lee
    Eunsook Lee
    Pohang Accelerator Laboratory, Pohang 37673, Republic of Korea
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  • Chan-Cuk Hwang
    Chan-Cuk Hwang
    Pohang Accelerator Laboratory, Pohang 37673, Republic of Korea
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  • Yong-Young Noh*
    Yong-Young Noh
    Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0001-7222-2401
  • Young-Ki Kim*
    Young-Ki Kim
    Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-4420-3489
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ACS Nano

Cite this: ACS Nano 2025, 19, 1, 1177–1189
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https://doi.org/10.1021/acsnano.4c13217
Published January 2, 2025

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Abstract

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Perovskite nanocrystals (PNCs) are promising active materials because of their outstanding optoelectronic properties, which are finely tunable via size and shape. However, previous synthetic methods such as hot-injection and ligand-assisted reprecipitation require a high synthesis temperature or provide limited access to homogeneous PNCs, leading to the present lack of commercial value and real-world applications of PNCs. Here, we report a room-temperature approach to synthesize PNCs within a liquid crystalline antisolvent, enabling access to PNCs with a precisely defined size and shape and with reduced surface defects. We demonstrate that elastic strains and long-range molecular ordering of the liquid crystals play a key role in not only regulating the growth of PNCs but also promoting high surface passivation of PNCs with ligands. The approach is a simple, rapid, and room-temperature process, yet it enables access to highly homogeneous PNCs on a mass scale with substantially reduced surface defect states leading to significantly enhanced optoelectronic features. Our results provide a versatile and generalizable strategy to be broadly compatible with a range of nanomaterials and other synthetic methods such as ligand exchange and microfluidic processes.

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Introduction

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Metal halide perovskites generally have a stoichiometry of ABX3, where A represents an organic cation (e.g., methylammonium) or a monovalent alkali metal (Cs+), B represents a divalent cation (e.g., Pb2+, Sn2+), and X represents anionic halides. Perovskite nanocrystals (PNCs), in particular, have been identified as one of the most notable materials in both fundamental sciences and practical applications including solar cells, photodiodes, and light-emitting diodes (LEDs) due to their outstanding optoelectronic properties compared to conventional nanomaterials. (1,2) Because the properties of PNCs are closely associated with their size, shape, crystallinity, and surface defects, it is critical to develop simple and scalable synthesis methods permitting high levels of control over them. Accordingly, a variety of preparation techniques have been proposed based on bottom-up and top-down approaches, (2,3) and significant advances have been made in the controlled synthesis of PNCs mainly via two representative methods, hot-injection (4,5) and ligand-assisted reprecipitation (LARP). (6,7) However, such approaches require a high synthesis temperature (>140 °C) or still offer limited access to monodispersed PNCs leading to additional size purification processes and thus a low production yield, which restricts mass production of PNCs and their broader applications.
To address this challenge, we design a novel synthetic method by leveraging the accumulating knowledge on (i) the room-temperature LARP processes for the synthesis of nanocrystals (2,3,8) and (ii) the liquid crystal (LC) template to mediate molecular/colloidal self-assembly resulting in nanostructures (e.g., nanofibers, colloidal crystals) with a precisely defined size and shape. (9−15) LCs are an intermediate phase of matter that possess both fluidity of liquid and long-range molecular ordering of the crystal. In LC phases, molecules assume a preferred orientation (defined by the director n), which underlies intriguing characteristics of LCs such as optical birefringence and elasticity. (16,17) In particular, the elasticity permits certain strain modes of LCs in an LC bulk represented by splay, twist, and bend director configurations. (17) When inclusions (e.g., colloidal particles, ligands, surfactants) are present in LC media, their behaviors are dictated by the energies associated with the elastic deformation of LCs (elastic bulk free energy, ∼KL) and orientation-dependent interactions of LCs with the surface of inclusions (surface anchoring energy, ∼WL2); L is the size of inclusion, K is the Frank elastic constant of the LC, and W is the surface anchoring energy density. (13,14) Recent studies have demonstrated that LCs can regulate the growth of polymeric inclusions, thus permitting the synthesis of nanofiber arrays with a well-defined size and organization by utilizing the so-called extrapolation length of LCs, defined as ξ = K/W. (14,15) Specifically, when the polymeric inclusions are smaller than ξ (i.e., WL2 < KL), the molecular ordering of LCs is not perturbed by the presence of inclusions due to the higher energetic cost for the elastic deformation of LCs. By contrast, if the inclusions become larger than ξ (i.e., WL2 > KL), to minimize WL2, the LC molecules in the vicinity of inclusions are reoriented along the direction imposed by the surface of inclusions. Due to the long-range molecular ordering of LCs, the local reorientation leads to macroscopic LC reordering, resulting in considerable elastic strains around the inclusions. (13,14,18,19) Moreover, the inclusions of L > ξ in LC media must be accompanied by topological defects, which are the nanoscopic regions where LC ordering cannot be defined and the free energy density is extremely high. (10−12,20) Therefore, the presence of both elastic strains and topological defects profoundly impacts the growth of inclusions in an LC medium.
By leveraging the previous studies, in this work, we find that simple substitution of isotropic antisolvents (e.g., toluene) by anisotropic LC antisolvents in the existing LARP method enables the synthesis of PNCs at room temperature with a precisely defined size and shape and reduces surface defects of PNCs, leading to a significantly enhanced photoluminescence quantum yield (PLQY). The proposed synthetic method is also demonstrated to provide broad compatibility with other synthetic techniques, such as shape transformation of PNCs, microfluidics, and ligand exchange processes.

Results and Discussion

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Controlled Synthesis of PNCs in LC Antisolvents

The LARP method is implemented by injecting a “good” solvent such as dimethylformamide (DMF), where perovskite precursors and ligands are dissolved (Figure 1a), into an antisolvent (e.g., toluene) that is under agitation and promotes the nucleation of PNCs. The organic ligands (n-octylamine and oleic acid, Figure 1a) serve as capping agents to passivate surface defects of PNCs and also to facilitate termination of the growth of PNCs in a nanometer size range. It is a simple and room-temperature process but offers limited access to homogeneous PNCs in size and shape, requiring additional purification processes to achieve the PNCs with the desired size and shape. (6,21,22) Unlike in the conventional antisolvent, inclusions (i.e., PNCs) in an LC antisolvent (Figure 1b) experience the competition between KL and WL2. (13,14) For instance, when the PNCs are smaller than ξ (i.e., WL2 < KL), the molecular ordering of LCs remains unperturbed (Figure 1c), allowing the PNCs to grow in the LC antisolvent. However, once the PNCs exceed ξ (i.e., WL2 > KL, Figure 1d), the LC molecules in the vicinity of PNCs reorient to minimize WL2, resulting in considerable elastic strains around the PNCs (Figure 1d). (13,14,18,19) In analogy to the polymeric nanofibers synthesized in LC, (14,15) therefore, the induced elastic strains are also expected to contribute to suppressing the further growth of PNCs over ξ. Therefore, we predict that a simple replacement of the isotropic antisolvent of toluene with an LC antisolvent in the LARP technique (we call it the LC-LARP method; Figure 1a–d) provide an additional level of control over the growth of PNCs in company with the contribution of ligand passivation.

Figure 1

Figure 1. Controlled synthesis of perovskite nanocrystals in a liquid crystalline antisolvent. Chemical structures of (a) perovskite precursors (CH3NH3Br and PbBr2) and organic ligands (n-octylamine and oleic acid) and (b) the nematic liquid crystal (LC), E7. Schematic illustrations for reconstructed profiles of LC molecules (purple ellipsoids) around growing perovskite nanocrystals (PNCs) when their size L (blue double arrows) is (c) smaller and (d) larger than the extrapolation length ξ (red double arrows and circles) of the LCs. n indicates the director of LC. (e) Ultraviolet–visible absorbance (purple and blue lines) and photoluminescence (PL) spectra (red and black lines) measured from the LC-PNC (top) and T-PNC (bottom) solutions before (dashed lines) and after (solid lines) size purification processes. The PNCs are synthesized for 720 min at T = 25 °C. The insets show photographs of the as-synthesized PNC solutions under ambient light. a.u.: arbitrary units. (f) Size distribution of LC-PNCs (top) and T-PNCs (bottom) measured from electron micrographs. n ≥ 1000 measurements. L = 8 ± 2 nm (mean ± SD) for LC-PNCs. Micrographs of (g, h) LC-PNCs and (i, j) T-PNCs obtained by (g, i) scanning and (h, j) transmission electron microscopy.

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To explore the feasibility of the LC-LARP method, therefore, we synthesize the representative inorganic–organic hybrid metal halide perovskite (CH3NH3PbBr3) through the typical recipe of the LARP technique at the temperature (T) of 25 °C, (6) yet with the use of nematic LC, E7, as an antisolvent (Figure 1a,b). In brief, a precursor solution prepared by dissolving CH3NH3Br, PbBr2, oleic acid, and n-octylamine in DMF is added to vigorously stirred E7. After the synthesis, E7 and unreacted reagents are removed and the resulting precipitates are dispersed into toluene for systematic comparison with the PNCs synthesized from conventional LARP using the antisolvent of toluene. PNCs synthesized via LC-LARP and conventional LARP are henceforth denoted as LC-PNCs and T-PNCs, respectively. We note that in order to verify the role of the LC antisolvent for the controlled synthesis of PNCs, we use LC-PNC solutions without any size purification processes in the entire experiments, unless we specify otherwise (see the Experimental Section).
From the ultraviolet–visible absorbance (the purple line shown in Figure 1e), we confirm CH3NH3PbBr3 to be successfully synthesized via LC-LARP. (6,23) Surprisingly, we find that even before any size purification processes, the resulting LC-PNC solution shows a single photoluminescence (PL) peak at 517 nm with a narrow full width at half-maximum (FWHM) of 28 nm (the red dashed line shown in Figure 1e), reflecting excellent homogeneity of resulting LC-PNCs. After size purification processes using centrifugation (see the Experimental Section), the PL spectrum remains almost unaltered (the red solid line shown in Figure 1e), indicating the high monodispersity of LC-PNCs. On the contrary, the T-PNC solution exhibits multiple PL peaks at 450, 467, 514, and 533 nm before size purification processes, indicative of the polydispersity of T-PNCs (the black dashed line shown in Figure 1e). Therefore, additional size purification processes, which lead to a low production yield, are essential for their practical uses (the black solid line shown in Figure 1e). As shown in Figure 1f–j, electron microscopy further demonstrates LC-LARP to provide highly homogeneous cubic-shaped PNCs in the size of L = 8 ± 2 nm even without any size purification processes, while T-PNCs exhibit a much broader distribution in size (including even micrometer-sized perovskite crystals) and shape from cubes to rods (one-dimensional structure, 1D) to sheets (two-dimensional structure, 2D).

Underlying Mechanisms of the LC-LARP Method

In order to verify our proposition that the growth of PNCs is regulated by the elastic strains of LCs, we determine ξ of the LC antisolvent and compare it with L of LC-PNCs (Figure 2a and see the Experimental Section). First, because the injection of the isotropic “good” solvent (DMF) into E7 changes the phase behavior of E7, we measure a phase diagram of the LC solution (i.e., E7 including DMF) in our synthetic condition (see the Experimental Section). The LC solution shows a nematic phase (possessing only orientational ordering of LCs) at room temperature, a nematic–isotropic coexistence phase from 40 °C (= TN–N+I) to 45 °C, and an isotropic phase above 45 °C (= Tc), where N and I indicate nematic LC and isotropic phases, respectively (Figure S1a, Supporting Information). Any crystallization of the LC solution is not observed until 0 °C. Therefore, to omit the contribution of other phases rather than LC, the following measurements and synthesis are performed in a deep N phase at the temperature range of 25 °C ≤ T ≤ 35 °C sufficiently away from the phase transitions to I (TN–N+I = 40 °C) and the crystal. Based on the measured clearing temperature (Tc = 45 °C), we theoretically estimate K = Ko(1 – T/Tc)2β of the LC solution (Figure S1b, Supporting Information), where Ko is the elastic constant of LC at 0 K and β = 0.2–0.3 is a material constant. (14,24) Subsequently, we measure W from the LC solution placed onto a substrate coated with a film of LC-PNCs (Figure S2, Supporting Information) and determine ξ with respect to T (Figure 2a). Notably, we find L of LC-PNCs measured from electron microscopy to be closely correlated with ξ of the LC solution. When the LC solution is in an I phase where ξ is not defined, we achieve polydisperse PNCs analogous to those from conventional LARP (Figure 1f–j). The LC solution within the deep N phase, however, produces the PNCs with well-defined L, which is shown to scale with the temperature-dependent value of ξ (Figure 2a). Accordingly, through changes in synthesis T (and thus ξ and L), we demonstrate the optical property of LC-PNCs to be finely tunable (Figure 2b,c). For example, the variation of synthesis T from 25 to 35 °C results in increases of ξ from 49 to 483 nm, L from 8 to 9.5 nm (Figure 2a), and peak wavelength (λpeak) of PL spectra from 517 to 521 nm (Figure 2b,c). We stress that LC-LARP at different T (and thus ξ) still produces well-defined nanosized PNCs even without the size purification processes (Figure S3, Supporting Information). This result is in good agreement with our proposed mechanism, which suggests that the elastic strain of LCs plays a significant role in regulating the growth of PNCs. In addition, because a variety of LC mixtures exist with a wide nematic temperature range, LC-LARP is also capable of modulating the size of LC-PNCs in a wide range. Therefore, the proposed approach can provide a promising control knob for tuning the size of PNCs with high homogeneity.

Figure 2

Figure 2. Synthesis mechanism and optical properties of LC-PNCs. (a) Mean values of ξ (red circles, n ≥ 5) and L (blue squares, n ≥ 500) with respect to synthesis T. (b) PL spectra of LC-PNCs synthesized at T = 25 °C (black squares), 30 °C (red triangles), and 35 °C (blue circles) and (c) the corresponding λpeak with respect to synthesis T. PL spectra from the PNC solutions that are synthesized at T = 25 °C for (d) ts = 10 min and (e) 720 min. PL spectra of T-PNC solutions (black) are normalized by the maximum peak intensity of LC-PNC solutions (red). (f) λpeak for the LC-PNC (red stars) and T-PNC (black squares) solutions with respect to ts. PL spectra are measured with the PNC solutions (e) with and (d, f) without size purification processes.

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Despite the correlative coupling between ξ and L, however, there exists a difference between the amplitude of these values (Figure 2a). We attribute this discrepancy to the three following factors: (i) Due to the absence of experimental techniques, one cannot measure the actual surface anchoring energy density (W0) directly from the surface of the nanoparticle, but can instead measure W indirectly from the large flat surface comprising nanoparticles as we carried out (Figure S2, Supporting Information). Previous works demonstrated W0 to be much greater than W from evident elastic interactions between nanoparticles of L ≪ ξ. (18) We make consistent observations with LC-PNCs, signifying that the actual extrapolation length ξ0 (= K/W0, W0 ≫ W) should be substantially smaller than ξ. More details will be discussed in the next section (Figures 3 and S4, Supporting Information). (ii) The growing PNC of the cubic shape in the LC media must involve not only the reorientation of surrounding LCs (Figure 1c,d) but also the formation of topological line defects (so-called disclinations) along the edges of and around the PNC. (19) Therefore, the substantial energetic cost associated with the increase in the total length of the line defects should also contribute to terminate the further growth of LC-PNCs. For simply evaluating the contribution, we assume that the size of PNCs grows from L = 8 nm (regulated L by LC at 25 °C, see Figure 1f) to 9 nm. The net change in elastic and defect energies exhibits a positive value (ΔE ∼ 11 kBT where kB is the Boltzmann constant, see the Experimental Section), with the defect energy change also being positive (∼19 kBT). This implies that the growth of PNCs beyond ξ is energetically unfavorable and influenced not only by the elastic strain but also by the topological defects of LCs. (iii) Given the high surface coverage of ligands onto LC-PNCs (Figures 4 and 5, see below for a detalied analysis), it is reasonable to postulate that the actual size (L0) of LC-PNCs is greater than L measured from electron microscopy that does not visualize the ligands; the length of single ligands used in this work is in the order of a few nanometers. (25,26)

Figure 3

Figure 3. Elastic interactions of PNCs in LC media. Schematic illustrations describing the elastic interaction associated with the (a, b) attraction and (c) repulsion of PNCs in LC media. In situ fluorescence confocal microscopy (FCM) images of elastic interactions of PNCs in LC media, where LC molecules are aligned along the x-axis. FCM images are measured at (d) 0, (e) 166, and (f) 258 s. n0 indicates the far-field director of LCs.

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Figure 4

Figure 4. Optical characteristics of PNCs and underlying mechanisms. (a) Time-resolved PL spectra of LC-PNCs (red) and T-PNCs (black). X-ray photoelectron spectroscopy spectra of LC-PNCs (red circles, top) and T-PNCs (black squares, bottom) for (b) N 1s, (c) Br 3d, and (d) Pb 4f. The blue and purple solid lines are fitting curves, dashed lines guide the position of peaks, and arrows indicate shoulder peaks resulting from metallic Pb. For systematic comparison, the measurements in panels (a–d) are carried out with LC-PNCs and T-PNCs achieved after the same size purification processes, albeit the purification is not necessary for LC-PNCs (see the Experimental Section).

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Figure 5

Figure 5. Orientational coupling between LC molecules and ligands. Schematic image describing (a) LC-LARP and (b, c) molecular ordering of LCs on LC-PNCs. Schematic image describing (d) conventional LARP and (e, f) molecular ordering of LCs on T-PNCs. (b, e) Side-view illustration and (c, f) the corresponding optical micrographs of LC films on the substrates coated with a film of (b, c) LC-PNCs and (e,f) T-PNCs. For homogeneous films of PNCs, LC-PNCs and T-PNCs are used after the size purification processes (see the Experimental Section). The inset in panel (c) shows the conoscopic Maltese cross. The optical micrographs are observed between crossed-polarizers. “A” and “P” represent the analyzer and polarizer, respectively.

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Characteristics of LC-PNCs

Regarding the results above, we make three additional key observations. First, we observe LC-PNCs synthesized at a higher T to be of relatively broader size distributions (e.g., L = 8 ± 2 nm at T = 25 °C and L = 9.5 ± 3.5 nm at T = 35 °C, Figure S3). It can be ascribed to the increase in fluctuations on the molecular ordering of LCs at high T, which influences the value of ξ because K and W are correlatively associated with LC ordering. (14,24,27) Therefore, this observation provides additional support for the regulated growth of LC-PNCs by the elastic strain of LCs. We note that the ordering fluctuation of LC molecules is also likely responsible for the different change trends of ξ and L at high T (Figure 2a).
Second, we find LC-LARP to offer a significantly higher production yield of PNCs with a desired size than conventional LARP, which is supported by the considerable difference in PL intensities measured from LC-PNC and T-PNC solutions (Figure 2d,e). Specifically, at the synthesis time (ts) of 10 and 720 min, we collect the PNC solutions resulting from LC-LARP and conventional LARP and measure the PL spectra from PNCs with the desired size (L ∼ 8 nm). To this end, the LC-PNC and T-PNC solutions at ts = 10 min (Figure 2d) are used without a size purification process because both solutions contain only the desired PNCs so far, reflected by a single PL peak at λpeak ∼ 520 nm. For both solutions collected at ts = 720 min (Figure 2e), however, we conduct the same size purification processes in order to directly compare the PL signals only from the desired PNCs because the resulting T-PNCs at ts = 720 min are of very wide size distributions (the black dashed line shown in Figure 1e and in the bottom of Figure 1f). As shown in Figure 2d,e, the amplitude of the PL peak measured from the LC-PNC solution (red lines) is much higher than that from the T-PNC solution (black lines), indicating a high production yield of PNCs with the desired size by LC-LARP. From the total mass of achieved PNCs, we estimate the production yield of desired PNCs to be ∼80% for LC-LARP and less than 1% for conventional LARP at ts = 720 min and T = 25 °C (see the Experimental Section).
Another noticeable feature is that upon conventional LARP, the PL intensity from the desired PNCs gets weaker as ts increases (black lines shown in Figure 2d,e), indicative of a decrease in the population of T-PNCs with the desired size. Additional experiments show that the main λpeak of PL spectra measured from the T-PNC solutions (without size purification processes) is shifted to a larger value as ts increases (the black line shown in Figure 2f), while the main λpeak of the LC-PNC solutions remains nearly unchanged with respect to ts (the red line shown in Figure 2f). The results indicate that under the LC-LARP process, the size of the main population of PNCs is well maintained regardless of ts, but the size becomes larger beyond the desired size (L = 8 nm) as the synthesis continues via conventional LARP. In conventional LARP, ts is an important synthesis factor because persistent occurrence of aggregations and coalescences of T-PNCs during the synthesis gives rise to size increases, a broader size distribution, and thus a low production yield of the desired size and properties of PNCs. (28) By contrast, such aggregation and coalescence are strictly restricted in the LC antisolvents due to the involvement of large elastic strains and topological defects around PNCs (L > ξ). (13,18,19) Previous studies have demonstrated that these two factors promote the so-called elastic interactions between the colloids (L > ξ) in analogy to the interactions between electric dipoles or quadrupoles. (18,29,30) As a result, the colloids in an LC medium attract each other due to topological charges (Figure 3a), but cannot make surface-to-surface contact (i.e., no aggregation and coalescence of colloids) because the elastic strains become more severe as they approach (Figure 3b), which generate considerable elastic repulsion between colloids (Figure 3c). We make consistent observations using fluorescence confocal microscopy that two resulting LC-PNCs in the LC antisolvent initially attract (Figure 3d,e) and then repel each other (Figure 3f), while aggregation of LC-PNCs is observed in isotropic media (Figure S4e–g). The existence of elastic repulsion contributes to preventing further growth of LC-PNCs by aggregation and coalescence. Therefore, LC-LARP can be performed until all precursors are consumed for the synthesis of LC-PNCs with a specific size being kept constant regardless of ts, resulting in a high production yield of homogeneous PNCs (Figure 2d–f). The observed findings also support our proposition that ξ0 is smaller than the measured value because such elastic behaviors of LC-PNCs occur only when L > ξ. (18,29,30) In addition, the presence of abundant topological defects in the bulk of the LC antisolvent, especially that is under agitation, would serve as nucleation points for PNCs because the high energy density of the topological defect drives absorption of inclusions (e.g., precursors, ligands) into the core of the defect, thus promoting their self-assemblies. (10−12,20) Consequently, LC-LARP is capable of rapidly terminating the reaction by consuming reactants through a multitude of topological defects without allowing further growth of PNCs, leading to the high production yield of PNCs with the desired size and shape (i.e., desired optoelectronic properties). This is not possible in conventional LARP that undergoes continuous growth of PNCs during synthesis (Figures 2f and S4e–g, Supporting Information).
Third, we observe the PLQY of LC-PNCs to be higher than that of T-PNCs; average PLQY = 80% for LC-PNCs and 68% for T-PNCs. We note that the PLQY of T-PNCs with the same compositions and processes used in this work have been reported to be from 50 to 70%. (6) The enhanced PLQY of LC-PNCs is also attributed to the intrinsic properties of LCs because LC-LARP uses the exact same recipe (including size purification processes) as conventional LARP, yet simply replaces the antisolvent from toluene to LC. To gain insight into the underlying mechanism of this improvement, time-resolved PL is measured from the LC-PNC and T-PNC solutions (Figure 4a). From the analysis of the PL decay curves using the triexponential model, we find an average recombination lifetime (τave) of LC-PNCs (18.1 ns) to be longer than τave of T-PNCs (11.5 ns). This is resulting from an increased contribution of radiative recombination but a reduced contribution from trap-assisted recombination (Table S1, Supporting Information). (6,31,32) Because the recombinations are closely correlated with crystallinity and surface defects of PNCs, (1,3) we carry out a range of structural analyses. The wide-angle X-ray scattering and selected area electron diffraction, however, indicate no sign of improvement in the crystallinity of LC-PNCs compared to T-PNCs (Figure S5, Supporting Information).
Therefore, we turn our focus to detailed investigations on the surface defects of PNCs through X-ray photoelectron spectroscopy (XPS) at the Pohang Accelerator Laboratory. Surprisingly, we observe that the XPS spectra of N 1s, Br 3d, and Pb 4f of LC-PNCs are significantly distinct from those of T-PNCs (Figure 4b–d), despite the use of the same synthesis recipe (i.e., same composition, synthesis T, ts, and size purification process). Specifically, in the N 1s spectrum (Figure 4b), a peak at 399.5 eV corresponding to n-octylamine (ligand) bound to the PNCs’ surface is evident only from LC-PNCs; a peak at ∼402 eV observed in both LC-PNCs and T-PNCs indicates methylammonium (CH3NH3+). (6) This offers solid evidence that the LC-LARP approach results in high surface passivation of the LC-PNCs with ligands, leading to a significant reduction in surface defect states and thus the improvement of the PLQY. In the Br 3d spectra (Figure 4c), both LC-PNCs and T-PNCs show two clear peaks, yet with an obvious difference in the ratio of the first (left peaks shown in Figure 4c) to second peaks (right peaks shown in Figure 4c); the ratio of 0.93 for LC-PNCs and 0.55 for T-PNCs. Since the first and second peaks correspond to surface and inner Br ions, respectively, (6) it indicates LC-PNCs to be enriched with the surface Br ions that play a role in preventing electron trapping on the surface defects. (7) In the Pb 4f spectra (Figure 4d), we find two shoulder peaks, which are indicative of metallic Pb and evident in T-PNCs at 142.0 and 137.2 eV, to disappear in LC-PNCs; two main peaks at 143.9 and 139.0 eV are assigned to Pb 4f5/2 and Pb 4f7/2, respectively. (23) The depletion in LC-PNCs contributes to an increase in the PLQY because metallic Pb acts as an electron trap site. (33) In addition, the shift of two peaks both in the Br 3d and Pb 4f spectra of LC-PNCs to a lower binding energy, as compared to those of T-PNCs (dashed lines shown in Figure 4c,d), reflects weakened interactions of Br– with Pb2+. It can be also indicative of the high surface passivation of LC-PNCs with ligands because the binding of oleic acid onto Pb2+ defects reduces cationic charges on Pb2+, thereby disturbing their interactions with Br–. (34−36)
We ascribe the high surface passivation of LC-PNCs with ligands to interactions between the LC molecules and ligands. It has been well-established that there exists a significant orientational coupling of LCs with aliphatic tails of amphiphile molecules (e.g., ligands, surfactants). (17) When ligands possessing long aliphatic tails are dispersed in an LC medium, the tails interdigitate between LC molecules (see Figure 5a,b), which drives the tails to be stretched linearly and thus decreases the effective volume of ligands. Consequently, as compared to isotropic media, the LC media permit ligands to be densely packed at LC interfaces (i.e., at the surface of LC-PNCs here). (37) In addition, the topological defects generated at the surface of colloids in LC media energetically drive the ligands to be self-assembled into the core of topological defects. (10−12,20) Because the LC-PNCs are synthesized within LC media and are of topological defects evidenced by their elastic interactions (Figure 3d–f), therefore, we expect the LC-PNCs’ surface to be highly passivated by the ligands (oleic acid and n-octylamine), corroborated by the XPS measurements (Figure 4).
The high surface passivation of LC-PNCs with ligands is further proven by our observations regarding the orientation of LCs at the surface of the PNCs (Figure 5). Owing to the optical birefringence of LCs, micro-thick LC films exhibit distinct optical textures depending on their internal molecular ordering. (16,17) As shown in Figure 5c, we observe the micro-thick film of E7 (thickness d = 18 μm) hosted onto the substrates coated with a film of LC-PNCs to show a dark optical texture between crossed-polarizers corresponding to a vertical alignment of LCs within the film (Figure 5b). Because E7 was shown to assume a vertical anchoring at the air interface, (16,17) the result implies that the surface of LC-PNCs also promotes the vertical anchoring of LCs (Figure 5b). By contrast, the LC film onto the substrate coated with T-PNCs shows a birefringent texture (Figure 5f), indicative of tilted or planar LC anchoring at the surface of T-PNCs (Figure 5e). Because the orientational coupling of LCs with aliphatic tails of ligands was demonstrated to drive the vertical anchoring of LCs, these results reflect the higher surface passivation of LC-PNCs with ligands than that of T-PNCs.

Compatibility with Other Synthetic Techniques

In order to demonstrate the versatility of the LC-LARP method, we validate the compatibility of LC-LARP with a number of synthetic techniques for PNCs. For instance, due to the binding competition between precursors (e.g., CH3NH3+, Cs+) and ligands for the lattice sites of PNCs, variations in the concentration of primary aliphatic amines in the LARP method were shown to manipulate the shape of PNCs that is one of the crucial factors to determine the optoelectronic properties of PNCs. (38,39) Since the LC antisolvent is shown to drive the enhanced surface passivation of PNCs with ligands (Figures 4b–d and 5a–c), we anticipate that LCs can further promote the role of ligands in transforming the shape of PNCs, along with the excellent homogeneity of the PNC size. For example, upon an increase in the concentration of n-octylamine (COAm), we demonstrate that LC-LARP can also convert the shape of the resulting LC-PNCs from cubes (Figure 6a) to sheets (Figure 6d) with intermediate transition states where cubes, rods, and sheets coexist (Figure 6b,c). The striking feature is that LC-LARP also opens access to high homogeneity in the shape of resulting PNCs without any purification processes in contrast to previous studies, (38,39) demonstrated by the single peak and narrow FWHM of PL spectra for the cubes (the red line shown in Figure 6e) and sheets (the purple line shown in Figure 6e). We note that the challenge for the synthesis of homogeneous PNC rods lies in the control of their length and aspect ratio (Figure 6b,c and yellow and blue lines shown in Figure 6e). In light of our results in combination with the previous LC-templated synthesis, (14,15) we expect that LC-LARP can also provide important guidance in synthesizing the PNCs with a well-defined 1D structure. From the preliminary experiment, we selectively synthesized the PNC rods using the LC film within which LC molecules are unidirectionally aligned (Figure S6, Supporting Information). The underlying mechanism regarding the anisotropic growth of PNCs is not fully understood yet, but we suppose that the uniform alignment of LC molecules provides elastic anisotropy around the growing PNCs and thus governs the growth direction of PNCs. Furthermore, the simple, rapid, and room-temperature operation enables the successful integration of LC-LARP with a microfluidic platform, (40) permitting the continuous production of LC-PNCs (Figure 6f,g) and in situ modification on their size and shape (and thus their optical properties) by controlling synthetic conditions such as the flow rate (Figure 6h, and Figure S7a,b, Supporting Information) and COAm (Figures 6i, and S7c,d, Supporting Information). We also demonstrate that LC-PNCs can be applied to previous ligand exchange processes and enable access to improving the performance of PNC-based optoelectronic devices such as LEDs (Figure S8, Supporting Information). When used in conjunction with recent demonstrations for the extraordinary performance of perovskite devices such as LEDs (EQE ∼ 37% for green) (41) and solar cells (power conversion efficiency > 18%), (42) therefore, LC-LARP would offer an additional route to further improving their performances.

Figure 6

Figure 6. Compatibility of LC-LARP with other synthetic techniques. Controlled shape transformation of LC-PNCs using different concentrations of n-octylamine (COAm): (a–d) transmission electron micrographs and (e) PL spectra from the LC-PNCs synthesized with COAm = 0.12 (a, red line in e), 0.13 (b, yellow line in e), 0.15 (c, blue line in e), and 0.24 mmol (d, purple line in e). (f) Schematic illustration and (g) the corresponding fluorescence micrograph for LC-LARP applied into the microfluidic platform. The fluorescence signal (green in panel (g)) indicates the synthesis of LC-PNCs. In situ modification on PL spectra of the resulting LC-PNCs by precisely controlling their size via (h) the flow rate of the precursor solution (h and Figure S7a,b) or (i) their shape via COAm in the precursor solution. The inset of panel (i) is the photograph of the LC-PNC solutions excited by 365 nm light. All measurements are done with LC-PNCs without any size purification processes.

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Overall, our results demonstrate that LC-LARP is a versatile and generalizable process that is compatible with other synthetic techniques. Beyond these discoveries, LC-LARP can also be extended to synthesize all-inorganic PNCs such as CsPbBr3 at room temperature with a precisely defined size and shape and with reduced surface defects (Figures S9–S12, Supporting Information), consistent with the results of the inorganic–organic hybrid PNCs (CH3NH3PbBr3).

Conclusions

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In summary, we report that a simple substitution of isotropic antisolvents by anisotropic LC antisolvents in the existing LARP methods offers a versatile and generalizable strategy to synthesize highly monodisperse PNCs at room temperature. By leveraging the elasticity and topological defects of LCs, we achieve a precisely defined size and shape of PNCs with a high production yield. Furthermore, the orientational coupling of LCs with ligands promotes dense surface passivation of PNCs with ligands, reducing surface defect states and enhancing optical properties of PNCs. We also find that LC-LARP can be expanded to various perovskite materials and combined with established techniques, such as shape transformation of PNCs, microfluidics, ligand exchange, and optoelectronic devices. We anticipate that our results will trigger much future research because the distinct features of LC-LARP reported in this paper would not only improve the applicability of PNCs but also allow detailed studies in nanoscopic phenomena of nanocrystals under controlled conditions, such as quantum confinement effects and superfluorescence. (43,44) Given the excellent compatibility and the rapid room-temperature process, we also expect that LC-LARP can be extended to other optoelectronic materials beyond perovskites and can enable a scalable batch process, restricted in the hot-injection method that is currently used for commercial production of nanocrystals (e.g., quantum dot displays). (4,5,45,46)

Experimental Section

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Materials

LCs, E7 and HPC860100–100, were purchased from Jiangsu Hecheng Display Technology Co., Ltd. To synthesize PNCs, lead(II) bromide (PbBr2, 99.999%, Sigma-Aldrich), methylammonium bromide (CH3NH3Br, 99.999%, Sigma-Aldrich), cesium bromide (CsBr, 99.999%, Sigma-Aldrich), n-octylamine (99.5%, Sigma-Aldrich), oleic acid (>99.0%, TCI), toluene (anhydrous, 99.8%, Sigma-Aldrich), N,N-dimethylformamide (DMF, anhydrous, 99.8%, Sigma-Aldrich), and butyl acetate (anhydrous, ≥99%, Sigma-Aldrich) were used as received without further purification. To prepare the microfluidic platform, polydimethylsiloxane (PDMS) components (Sylgard 184 silicone elastomer kit) were purchased from Dow Corning and the SU-8 50 photoresist was purchased from Microchem.

Synthesis of PNCs

We used conventional LARP method based on the past literature (6) and modified it for our LC-LARP method. Specifically, the precursor solution was prepared by dissolving 0.16 mmol of CH3NH3Br (CsBr as well), 0.2 mmol of PbBr2, 0.5 mL of oleic acid, and 20 μL of n-octylamine in 5 mL of DMF. 200 μL and 1 mL of the precursor solutions were injected dropwise into vigorously stirred 5 mL of E7 (LC-PNCs) and toluene (T-PNCs), respectively, in ambient conditions. We referred to this as-synthesized solution as the “crude PNC solution”. To remove the antisolvent (i.e., E7 and toluene) and unreacted precursors and ligands, the crude PNC solution was centrifuged (1st centrifugation). Subsequently, the supernatant was discarded, and only PNC precipitates were obtained. The precipitates were redispersed to toluene for obtaining PNC solutions. We regarded this PNC solution as the solution before the size purification processes. To corroborate various characteristics of PNCs (e.g., growing mechanisms, enhanced optical properties, and structural analysis), we modified the synthesis of T, ts, and the first centrifugation procedure (details are given below).
For Figures 1e–j, 2e, 3d–f, S4, and S9 and the production yield of PNCs, both LC-PNCs and T-PNCs were synthesized at T = 25 °C for ts = 720 min to reach a termination of the reaction (Figures 2f and S11a). For Figure 2d, LC-PNCs were synthesized at T = 25 °C for ts = 10 min. For Figure 2f, LC-PNCs were synthesized with varying ts at T = 25 °C. To thoroughly remove the LC antisolvent, 2 mL of butyl acetate was added to the crude PNC solution, (47) followed by centrifuging it at 7580 rpm at 10 °C for 10 min. We also implemented the same centrifugation procedure for the T-PNCs.
For Figures 2a (for L),b, S3, and S10d-h(for L), LC-PNCs were synthesized at T = 25, 27.5, 30, 32.5, and 35 °C for ts = 720 min. We added 2 mL of butyl acetate to the crude PNC solution to strictly remove the LC antisolvent. (47) Subsequently, the solution was centrifuged at 7580 rpm at 10 °C for 10 min.
For Figures 2a (for ξ), S2, and S10a–d (for ξ), LC-PNCs were synthesized at T = 25, 27.5, 30, 32.5, and 35 °C for ts = 720 min. For Figure 5, LC-PNCs were synthesized at T = 25 °C for ts = 720 min. To minimize additional effects of butyl acetate on the ligand passivation of PNCs, (47) we centrifuged the crude PNC solution without adding butyl acetate at 8000 rpm at 10 °C for 1 h.
For Figure 4a and PLQY measurement, both LC-PNCs and T-PNCs were synthesized for short ts = 120 min to avoid the decrease of optical properties of PNCs in ambient conditions. (2) Furthermore, to omit additional effects of butyl acetate on the optical properties of PNCs, (47) we centrifuged the crude PNC solution without using butyl acetate at 7580 rpm at 10 °C for 10 min.
For Figures 4b–d and S12f–h, we altered ts (30 min for CH3NH3PbBr3 and 180 min for CsPbBr3) due to the low stability of PNCs in ambient conditions. (2) The first centrifugation of crude PNC solutions was performed at 7580 rpm at 10 °C for 10 min without using butyl acetate to minimize the effects of butyl acetate on the ligand passivation of PNCs. (47)

Size Purification Processes for PNCs

PNC solutions with the first centrifugation were centrifuged again at 7580 rpm at 10 °C for 10 min (2nd centrifugation), and then only the supernatant was obtained. We designated this second centrifugation as “size purification processes”.
For Figures 1e, 2e, 4, 5, S2, S9a, S11c, and S12f–h, the production yield of PNCs, and PLQY measurement, the solutions of both LC-PNCs and T-PNCs were performed with size purification processes.

Production Yield of PNCs

PNC solutions after size purification processes were dried under vacuum to thoroughly remove the solvent and only obtain the PNCs with the desired size (L ∼ 8 nm). The production yield of PNCs was calculated using the formula mass of PNCsmass of precursors+mass of ligands×100 (%).

Preparation of Substrates Coated with PNCs

For Figures 2a (for ξ) and S2, the LC-PNC solutions of CH3NH3PbBr3 (without the size purification processes) were filtered using a syringe filter (hydrophobic polytetrafluoroethylene with a 0.20 μm pore size, Hyundai Micro). For Figure S10a–d (for ξ), the LC-PNC solutions of CsPbBr3 (without the size purification processes) were directly used. The LC-PNC solutions of CH3NH3PbBr3 and CsPbBr3 were dried under vacuum, and then we obtained the powder of PNCs. The powder of PNCs was redispersed in toluene with a concentration of 200 mg/mL. 2 μL of this PNC solution was drop-cast on a glass substrate, followed by being spin-coated at 2000 rpm for 60 s. During spin coating, we dropped butyl acetate to eliminate any remaining E7 in PNCs. Subsequently, this film of PNCs was flattened by the slit-coating method with a slide glass. Finally, the flattened film of PNCs was spin-coated again at 2000 rpm for 60 s by dropping butyl acetate to totally rinse off any remaining E7. The final film of PNCs was dense and thick (>200 nm, measured by a profilometer, NanoMap-PS), thus removing the surface effects of the glass substrate.
For Figures 4b–d and S12f–h, the PNC solution with the size purification processes was dried in vacuum, and then it was redispersed in toluene. This colloidal solution was drop-cast on a p-type silicon wafer, followed by being spin-coated at 1000 rpm for 5 s. A few drops of butyl acetate were dropped onto the substrates coated with PNCs during spin coating to thoroughly remove the remaining antisolvents and unreacted reagents.
For Figure 5, the PNC solution with the size purification processes was dried in vacuum, and then it was redispersed in toluene with a concentration of 140 mg/mL. This colloidal solution was drop-cast on a bare glass substrate, followed by being spin-coated at 2000 rpm for 60 s. A few drops of butyl acetate (as a washing solvent) were dropped onto substrates coated with PNCs during spin coating.

Measurement of L (Figures 1f, 2a, S3, S9b, and S10)

To measure the L of PNCs, more than 10 transmission electron microscopy (TEM) images were analyzed by using ImageJ. The L of PNCs was determined by measuring the longitudinal length of PNCs without size purification processes. For PNCs of CH3NH3PbBr3 (Figures 1f, 2a, and S3), L = 8 ± 2, 9 ± 3.5, 10 ± 3.5, 10 ± 4, and 9.5 ± 3.5 nm at T = 25, 27.5, 30, 32.5, and 35 °C, respectively. For PNCs of CsPbBr3 (Figures S9b and S10), L = 8 ± 2.5, 12 ± 6, 11.5 ± 5, 16 ± 5, and 13 ± 6 nm at T = 25, 27.5, 30, 32.5, and 35 °C, respectively. Data are mean ± SD; L and its standard deviation of PNCs were obtained from the Gaussian fit of the histogram with a bin size = 1 nm.

Analysis for the Phase Diagram of the LC Solution

Due to the addition of 200 μL of the precursor solution, which is composed of ∼90 vol % of DMF, in 5 mL of E7, we considered 3.6 vol % of DMF in E7 to alter the physical properties of E7. In addition, as shown in Figure S1a, DMF mainly changed the Tc of E7, as compared to two ligands (oleic acid and n-octylamine). Therefore, we only considered DMF to simplify the measurement of the transition temperature of LC in our experiments. We referred to E7 including 3.6 vol % of DMF as the “LC solution”. The transition temperature of the LC solution was measured using a temperature controller T96 equipped with a hot stage PE120 (Linkam Scientific Instruments) at cooling rates of 1 °C/min. LC solutions were filled into 4.5 μm sandwich cells that were prepared by assembling two glasses coated with polyimide (Nissan Chemical Korea). The polyimide was coated on the glasses at 1000 rpm for 10 s followed by 2500 rpm for 30 s. Subsequently, the polyimide-coated glass was annealed at 110 °C for 100 s, followed by 230 °C for 60 min. The gaps between glasses were set by glass spacers mixed with UV glue (NOA 65). To minimize evaporation of DMF during measurement, we blocked the inlet and outlet of LC-filled sandwich cells with vacuum grease (Dow corning).

Measurement of W and ξ (Figures 2a, S2b, and S10c,d)

To verify the change of ξ = K/W of LC with varying T, we experimentally measured W and theoretically estimated K, where K = (K11 + K33)/2 (one constant approximation of the Frank elastic constant) and K11 and K33 are splay and bend elastic constants of LC, respectively. (13,14) Because PNCs of CH3NH3PbBr3 and CsPbBr3 induce the vertical anchoring of E7 (Figures 5 and S10b), we considered LC molecules to reorient toward PNCs along the polar angle when L becomes larger than ξ. Therefore, we calculated the polar surface anchoring energy density (Wp) at the vertical anchoring surface. The Wp can be obtained by measuring the polar angle θ of LC at the surfaces that confine LCs. (48)
WP=2K11I(θ0,θd)d·1+κsin2⁡θdsin(2θd)
where I(θ0,θd)=∫θ0θd1+κsin2⁡θdθ, κ=K33−K11K11 and d is the thickness of the LC film. θd and θ0 are the polar angles of the LC molecules at vertical and planar anchoring surfaces, respectively. We determined the polar angles via measuring the optical retardation of the LC film with a hybrid alignment (Rh) at various wavelengths (λ) of normal incident light. (48)
Rh(λ)=2πnodλI(θ0,θd)∫θ0θd(11−νcos2⁡θ−1)×1+κsin2⁡θdθ
where ν=ne2−no2ne2, no and ne are the ordinary and extraordinary refractive indices of LC, respectively.
We measured the Wp of E7 including DMF (i.e., LC solution), instead of pure E7, because the “good” solvent DMF changed the physical properties (K11, K33, no, and ne) of LC. Based on the order parameter S = (1 – T/Tc)β (T is the temperature of LC and β is a material constant), (24) we could modify K11, K33, no, and ne of E7 with Tc = 45 °C.
First, K11 and K33 can be theoretically estimated as Kii = (Kii)o(1 – T/Tc)2β. (24) To obtain modified K11 and K33 of E7, we extracted (Kii)o and β of pure E7 from the literature; (49) (K11)o = 14.5 with β = 0.23, and (K33)o = 25.6 with β = 0.29. By using the aforementioned (Kii)o, β, and Tc (= 45 °C), we could plot K11 and K33 of the LC solution with respect to T (Figure S1b). K of the LC solution is 12.9, 12.0, 11.0, 10.0, and 8.9 pN at 25, 27.5, 30, 32.5, and 35 °C, respectively.
Second, no and ne can be rewritten as no=⟨n⟩−13Δn and ne=⟨n⟩+23Δn, (50) where ⟨n⟩=(ne2+2no2)/3 is the average reactive index (51) and Δn is the birefringence. To obtain modified no and ne with respect to T, we measured Δn of the LC solution at 25, 30, 35, and 40 °C by measuring the optical retardation R = Δnd of LCs with a planar alignment via a Berek compensator (U-CTB, Olympus) (Figure S1c). We injected the LC solution into a sandwich cell that was prepared by assembling two glasses coated with polyimide (Nissan Chemical Korea). The polyimide was spin-coated on the glass at 1000 rpm for 10 s followed by 2500 rpm for 30 s. Subsequently, the polyimide-coated glass was annealed at 110 °C for 100 s followed by 230 °C for 60 min. To induce unidirectional orientation of the LC molecules (i.e., uniform R), we gently rubbed both glass substrates 10 times using a velvet cloth and assembled them in an antiparallel fashion. Additionally, we applied a strong in-plane electric field (12–13 V/μm) to the sandwich cell by using electrodes made from aluminum foils in order to remove pretilted LC molecules on the substrates. The gaps between substrates were determined by the thickness of the aluminum foils (∼23 μm). Therefore, based on R = Δnd, we could calculate Δn at 25, 30, 35, and 40 °C. Because R is the function of the wavelength, Δn was measured with 488, 550, and 656 nm wavelengths of light, respectively. With the experimentally measured Δn (at 25, 30, 35, and 40 °C) and Tc (= 45 °C) of the LC solution, we plotted Δn(T) by using an equation Δn = (Δn)o(1 – T/Tc)β (Figure S1c). (24) We assumed ⟨n⟩ of the LC solution with that of pure E7. (50) Based on obtained Δn(T) at various wavelengths and ⟨n⟩, we theoretically calculated no and ne of the LC solution with respect to T at 488, 550, and 656 nm.
Third, Rh was measured by using a Berek compensator (U-CTB, Olympus) with three wavelengths of 488, 550, and 656 nm according to synthesis T (Figure S2a). The LC film was prepared by filling the sandwich cell with the LC solution at 25 °C (nematic phase) to inhibit the evaporation of DMF. The sandwich cell was prepared by assembling two glass substrates coated with polyimide (SE7492K, Nissan Chemical) and a thick film of LC-PNCs (Figure S2a). The substrate coated with polyimide (bottom) induced planar anchoring (formation of θ0) of the LC molecules. On the other hand, the substrate coated with LC-PNCs (top) resulted in vertical anchoring (formation of θd) of LC molecules. The polyimide-coated substrate was rubbed 10 times with a velvet cloth to induce a unidirectional alignment of E7. The temperature was controlled by a temperature controller T96 with a hot stage PE120.
Lastly, with the modified K11, K33, no, and ne of E7, numerical solutions of Wp and ξ were obtained by fitting the measured values of Rh to the theoretical equation. (7) To acquire high accuracy of Wp, we used thin LC films (<2 μm). (48) d of the LC film was determined by subtracting the thickness of the PNC film from the gap of the sandwich cell that was set by the glass spacer. Additionally, we set boundary conditions for θ0, θd, and d. We determined the pretilted angle of θ0 > ∼4° based on planar polyimide SE7492K. The possible range of θd in the thin hybrid alignment of the LC cell (<2 μm) was decided from the literature. (48) Furthermore, we manipulated the range of θd with respect to synthesis T in order to fit the measured Rh to the equation (details are given below). The boundary condition of d was set in accuracy of tens of the nanometer scale. (48) We determined numerical solutions of Wp (Figures S2b and S10c) and ξ (Figures 2a and S10d) by considering the possible error of the Rh measurement with root-mean-square (RMS) deviation. Because the Berek compensator is able to measure with an accuracy of 0.0001 rad, numerical solutions that deviated from measured values of Rh within the range of 0.0001 rad in RMS were selected. For PNCs of CH3NH3PbBr3 (Figure 2a), ξ = 49 ± 23, 104 ± 24, 207 ± 48, 477 ± 190, and 483 ± 230 nm were obtained with the boundary conditions of θd = 80–90, 80–90, 70–90, 60–90, and 40–90°, respectively. For PNCs of CsPbBr3 (Figure S10d), ξ = 92 ± 32, 217 ± 84, 211 ± 41, 394 ± 86, and 565 ± 114 nm were obtained with the boundary conditions of θd = 80–90, 70–90, 70–90, 60–90, and 40–90°, respectively. The values of ξ and the range of boundary conditions of θd corresponded to synthesis conditions T = 25, 27.5, 30, 32.5, and 35 °C, respectively.

Calculation of the Elastic and Defect Energies

When the cubic-shaped PNC grows larger than ξ in the LC media, it must involve the reorientation of the surrounding LCs and the formation of topological line defects along the edges of and around the PNCs. The relevant elastic free energy (Ec) and elastic-defect free energy for a line defect (Eh) with topological strengths of m = −1/2 and a surface line defect (Eq) of m = +1/4 are estimated as (19)
Ec≈−AπK(Sc−L2)
(1)
Eh≈14πKLln⁡Scrh+πrh2Lεc
(2)
Eq≈116πKLln⁡Scrq+πrq2Lεc
(3)
where A is a numerical constant, Sc is a characteristic length of elastic deformation, rh and rq are, respectively, the radius of the defect core of m = −1/2 and +1/4, and εc is the energy density of the defect core.
We calculated the difference in the elastic and defect energies (ΔE) when a PNC grows from L = 8 nm (resulting from LC-LARP at 25 °C) to 9 nm, which depicts the situation where the PNC becomes larger than the actual ξ0. We estimated the elastic-defect energies of states with a PNC of 8 nm (E8 nm) and a PNC of 9 nm (E9 nm) and ΔE as follows (19)
E8nm=Ec,8nm+(22+2)Eh,8nm+12Eq,8nm≈−AπK(Sc,8nm−L8nm2)+(22+2)[14πKL8nmln⁡Sc,8nmrh+πrh2L8nmεc]+12(116πKL8nmln⁡Sc,8nmrq+πrq2L8nmεc)
(4)
E9nm=Ec,9nm+(22+2)Eh,9nm+12Eq,9nm≈−AπK(Sc,9nm−L9nm2)+(22+2)[14πKL9nmln⁡Sc,9nmrh+πrh2L9nmεc]+12(116πKL9nmln⁡Sc,9nmrq+πrq2L9nmεc)
(5)
ΔE=E9nm−E8nm≈[−AπK(Sc,9nm−L9nm2−Sc,8nm+L8nm2)+(22+2)πK4(L9nmlnS9nmrh−L8nmlnS8nmrh)+3πK4(L9nmlnS9nmrq−L8nmlnS8nmrq)]elastic+[(22+2)πrh2εc(L9nm−L8nm)+12πrq2εc(L9nm−L8nm)]defect
(6)
To calculate ΔE, we used the following parameters: (19) A = 6, K = 12.9 pN (at 25 °C, see the Experimental Section), rc = 20 nm, rh = 10 nm, rq = 5 nm, εc(≈K/rc2) = 3.2 × 104 J/m2, L8 nm = 8 nm, Sc,8 nm = 8 nm, L9 nm = 9 nm, and Sc,9 nm = 9 nm. Due to kBT = 4.1 × 10–21 J at T = 25 °C, the difference in the elastic and defect energies ΔE = 10.9 kBT. The defect energy (the second term in ΔE) is 19.2 kBT.

Observation of the PNC Behavior in LC and Isotropic Media (Figure 3)

LC-PNCs of CH3NH3PbBr3 were dispersed in E7. This mixture was injected into a 13 μm gap of sandwich cells prepared by the assembly of two glass substrates coated with polyimide (SE7492K). Specifically, the polyimide was spin-coated on the glass at 1000 rpm for 10 s and 2500 rpm for 30 s under ambient conditions. Subsequently, the polyimide-coated glass was annealed at 100 °C for 100 s, followed by 210 °C for 60 min. To induce unidirectional orientation of the LC molecules, we gently rubbed the polyimide-coated glass substrates 10 times with a velvet cloth and assembled them in an antiparallel fashion. The gap was set by glass spacers mixed with UV glue (NOA 65). The samples were observed with a fluorescence confocal microscope using a 405 nm excitation laser in the emission range of 500–530 nm. Isolated PNCs in optical images were decided by comparing the PNCs’ size with the size of 25 and 100 nm of silica nanoparticles (HiQ-Nano) that emit 525 nm under 488 nm excitation laser.

Preparation of LC Films on the Substrates Coated with PNCs (Figure 5)

To observe LC films with an optical microscope, a copper grid for TEM (18 μm thickness, Electron Microscopy Sciences) was hosted on the glass substrate coated with PNCs (synthesized at T = 25 °C), providing microwells. Subsequently, we filled the microwells with E7 including 3.6 vol % DMF at the temperature above Tc to remove the memory effect.

Shape Transformation of LC-PNCs (Figure 6a–e)

We synthesized LC-PNCs of CH3NH3PbBr3 by injecting 200 μL of the precursor solution with different amounts of n-octylamine as 20, 22, 25, and 40 μL (equal to COAm = 0.12, 0.13, 0.15, and 0.24 mmol, respectively) in vigorously agitated 5 mL of E7 in ambient conditions. LC-PNCs were synthesized at T = 25 °C for ts = 720 min, followed by being performed with the first centrifugation by using 2 mL of butyl acetate. We analyzed the PNC solutions without the size purification processes.

Synthesis of LC-PNCs with a Microfluidic Platform (Figure 6f–i)

We prepared a microfluidic platform using PDMS. The microchannels with 1000 and 80 μm width and 45 μm height were prepared by replication of SU-8 50 masters, which were obtained by a photolithography process on silicon wafers according to a previously reported method. (52) To synthesize PNCs, the precursor solution and HPC860100–100 (nematic LC, Tc = 141 °C) were pumped together using syringe pumps and flowed into a T-junction through a 10 cm polytetrafluoroethylene tubing (1.5 mm ID) in ambient conditions at room temperature, as shown in Figure 6f. The resulting mixture was collected in a container, followed by being vigorously stirred at room temperature for 30 min. The final product was centrifuged at 7580 rpm for 10 min and the supernatant containing any unreacted precursors was discarded. The remaining precipitates were redispersed in toluene. We measured optical properties of the PNC solutions without the size purification processes. Detailed conditions of the precursor solution and the flow rate are described below.
For Figures 6h and S7a,b, we prepared the precursor solution of PNCs of CH3NH3PbBr3 by dissolving 0.16 mmol of CH3NH3Br, 0.2 mmol of PbBr2, 0.5 mL of oleic acid, and 20 μL of n-octylamine in 5 mL of DMF. The flow rates of the precursor solution varied at 200, 230, 260, 400, and 500 μL/h, while the flow rate of LC was set to 2000 μL/h.
For Figure 6i, we prepared the precursor solution of PNCs of CH3NH3PbBr3 by dissolving 0.08 mmol of CH3NH3Br, 0.08 mmol of PbBr2, 2.5 mL of oleic acid, and various concentrations of n-octylamine (67, 200, 333, and 468 μL, which are equal to COAm = 0.4, 1.2, 2.0, and 2.8 mmol, respectively) in 5 mL of DMF. The flow rates of the precursor solution and LC were set to 230 and 2000 μL/h, respectively.
For Figure S7c,d, we prepared the precursor solution of PNCs of CH3NH3PbBr3 by dissolving 0.16 mmol of CH3NH3Br, 0.2 mmol of PbBr2, 0.5 mL of oleic acid, and various concentrations of n-octylamine (20, 30, 40, 50, and 60 μL, which are equal to COAm = 0.12, 0.18, 0.24, 0.30, and 0.36 mmol, respectively) in 5 mL of DMF. The flow rates of the precursor solution and LC were set to 230 and 2000 μL/h, respectively.

PNC Characterization

Scanning electron microscopy images were obtained by using a field-emission scanning electron microscope (Hitachi S4800). TEM was performed by JEM-2100F and JEM-2200FS instruments with an excitation voltage at 200 kV. Ultraviolet–visible absorption spectra of the PNC solutions were obtained by using JASCO V-770. PL of the PNC solutions was measured by FluoroMax (Horiba) with 365 nm excitation. The PLQY was analyzed with the PNC solutions using a FluoroMax (Horiba) equipped with an integrated sphere under 365 nm excitation. Time-resolved PL was characterized with the PNC solutions using a time-correlated single photon counting system (HAMAMATSU/C11367–31) with a laser source wavelength of 372 nm. The wide-angle X-ray scattering analysis was performed at the 9A beamline of the Pohang Accelerator Laboratory (PAL), Korea. XPS measurements were made at the 10D beamline of the PAL equipped with Scienta DA30 and PHOIBOS 150 analyzers. XPS spectra were obtained using a monochromatic Al Kα source (1486.7 eV) to probe deeper layers after monitoring core-level spectra with several photon energies from synchrotron radiation. The spectra were collected using a pass energy of 20 eV with a resolution of 0.1 eV. We confirmed no sample charging at the beginning of the measurements. All of the data were collected at room temperature. To analyze the performance of PNC-based LEDs, the curves of the current density and luminance and curves of the external quantum efficiency (EQE) and current density were obtained using a Keithley 2400 with a Photo Research PR-670 under room temperature and ambient conditions.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.4c13217.

  • Physical properties of the LC antisolvent; characterization of W of CH3NH3PbBr3; size distributions of LC-PNCs of CH3NH3PbBr3; behaviors of PNCs in LC and isotropic media; structural analysis of PNCs of CH3NH3PbBr3; anisotropic growth of PNCs via LC films; detailed optical and structural analysis of LC-PNCs of CH3NH3PbBr3 in a microfluidic platform; detailed characterization of PNC-based LEDs; controlled synthesis of LC-PNCs of CsPbBr3; growth mechanism of LC-PNCs of CsPbBr3; detailed PL spectra of LC-PNCs of CsPbBr3 depending on ts; structural analysis of PNCs of CsPbBr3; and detailed PL lifetime of PNCs of CH3NH3PbBr3 (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Jun-Hyung Im - Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea;  Orcidhttps://orcid.org/0009-0002-1504-8544
    • Myeonggeun Han - Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
    • Jisu Hong - Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
    • Hyein Kim - Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
    • Kwang-Suk Oh - Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
    • Taesu Choi - Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
    • Abd Rashid bin Mohd Yusoff - Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea;  Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru 81310, Malaysia
    • Maria Vasilopoulou - Institute of Nanoscience and Nanotechnology, National Centre for Scientific Research Demokritos, Attica 15341, Greece;  Orcidhttps://orcid.org/0000-0001-8893-1691
    • Eunsook Lee - Pohang Accelerator Laboratory, Pohang 37673, Republic of Korea
    • Chan-Cuk Hwang - Pohang Accelerator Laboratory, Pohang 37673, Republic of Korea
  • Author Contributions

    J.-H.I., M.H., and J.H. contributed equally to this work. The main idea and experimental strategy were proposed initially by Y.-Y.N. and Y.-K.K. and developed by all authors. Experiments were mainly performed by J.-H.I., M.H., and J.H. with assistance from H.K., K.-S.O., T.C., A.R.B.M.Y., M.V., E.L., and C.-C.H. The experiments of X-ray photoelectron spectroscopy were performed in the Pohang Accelerator Laboratory by E.L. and C.-C.H. with assistance from J.-H.I. All experimental and theoretical data were analyzed primarily by J.-H.I., Y.-Y.N., and Y.-K.K. with assistance from all other authors. The initial manuscript was prepared by J.-H.I., M.H., and J.H. and revised by J.-H.I., Y.-Y.N., and Y.-K.K.

  • Notes
    The authors declare the following competing financial interest(s): The Pohang University of Science and Technology has registered a patent (KR 10-2601025) and filed patent applications (US 17/420,769, US 17/373,964, EU 20908460.7, EU 21184803.1, KR 10-2021-0088480) on the work described in this manuscript. The inventors listed on the patents are J.H., H.K., Y.-Y.N., and Y.-K.K.

Acknowledgments

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This work was primarily funded by the National Research Foundation of Korea (through grants RS-2023-00302586, RS-2023-00212739, and RS-2023-00260608). We thank T. Park, J. K. Kim, X. Wang, S. Zhou, and D. P. Singh for fruitful discussions regarding the results in this manuscript.

References

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    Nature (London, United Kingdom) (2018), 557 (7706), 539-544CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Liq. crystals (LCs) are anisotropic fluids that combine the long-range order of crystals with the mobility of liqs.1,2. This combination of properties has been widely used to create reconfigurable materials that optically report information about their environment, such as changes in elec. fields (smart-phone displays)3, temp. (thermometers)4 or mech. shear5, and the arrival of chem. and biol. stimuli (sensors)6,7. An unmet need exists, however, for responsive materials that not only report their environment but also transform it through self-regulated chem. interactions. A range of stimuli can trigger pulsatile (transient) or continuous release of microcargo (aq. microdroplets or solid microparticles and their chem. contents) that is trapped initially within LCs. The resulting LC materials self-report and self-regulate their chem. response to targeted phys., chem. and biol. events in ways that can be preprogrammed through an interplay of elastic, elec. double-layer, buoyant and shear forces in diverse geometries (such as wells, films and emulsion droplets). These LC materials can carry out complex functions that go beyond the capabilities of conventional materials used for controlled microcargo release, such as optically reporting a stimulus (for example, mech. shear stresses generated by motile bacteria) and then responding in a self-regulated manner via a feedback loop (for example, to release the min. amt. of biocidal agent required to cause bacterial cell death).
  14. 14
    Cheng, K. C. K.; Bedolla-Pantoja, M. A.; Kim, Y.-K.; Gregory, J. V.; Xie, F.; de France, A.; Hussal, C.; Sun, K.; Abbott, N. L.; Lahann, J. Templated Nanofiber Synthesis via Chemical Vapor Polymerization into Liquid Crystalline Films. Science 2018, 362, 804– 808,  DOI: 10.1126/science.aar8449
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    Templated nanofiber synthesis via chemical vapor polymerization into liquid crystalline films
    Cheng, Kenneth C. K.; Bedolla-Pantoja, Marco A.; Kim, Young-Ki; Gregory, Jason V.; Xie, Fan; de France, Alexander; Hussal, Christoph; Sun, Kai; Abbott, Nicholas L.; Lahann, Joerg
    Science (Washington, DC, United States) (2018), 362 (6416), 804-808CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Extrusion, electrospinning, and microdrawing are widely used to create fibrous polymer mats, but these approaches offer limited access to oriented arrays of nanometer-scale fibers with controlled size, shape, and lateral organization. We show that chem. vapor polymn. can be performed on surfaces coated with thin films of liq. crystals to synthesize organized assemblies of end-attached polymer nanofibers. The process uses low concns. of radical monomers formed initially in the vapor phase and then diffused into the liq.-crystal template. This minimizes monomer-induced changes to the liq.-crystal phase and enables access to nanofiber arrays with complex yet precisely defined structures and compns. The nanofiber arrays permit tailoring of a wide range of functional properties, including adhesion that depends on nanofiber chirality.
  15. 15
    Roh, S.; Kim, J.; Varadharajan, D.; Lahann, J.; Abbott, N. L. Sharing of Strain Between Nanofiber Forests and Liquid Crystals Leads to Programmable Responses to Electric Fields. Adv. Funct. Mater. 2022, 32, 2200830  DOI: 10.1002/adfm.202200830
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    15
    Sharing of Strain Between Nanofiber Forests and Liquid Crystals Leads to Programmable Responses to Electric Fields
    Roh, Sangchul; Kim, John; Varadharajan, Divya; Lahann, Joerg; Abbott, Nicholas L.
    Advanced Functional Materials (2022), 32 (27), 2200830CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Fibers embedded in soft matrixes are widely encountered in biol. systems, with the fibers providing mech. reinforcement or encoding of instructions for shape changes. Here, the mech. coupling of end-attached polymeric nanofiber forests and liq. crystals (LCs) is explored, where the nanofibers are templated into prescribed shapes by the chem. vapor polymn. of paracyclophane-based monomers in supported films of the LCs. It is shown that the elastic energies of the nanofibers and LCs are comparable in magnitude, leading to reversible straining of nanofibers via the application of an elec. field to the LC. This coupling is shown to encode complex electrooptical responses in the LC (e.g., optical vortices), thus illustrating how LC-templated nanofiber forests offer the basis of fresh approaches for programming configurational changes in soft materials.
  16. 16
    Kim, W.-S.; Im, J.-H.; Kim, H.; Choi, J.-K.; Choi, Y.; Kim, Y.-K. Liquid Crystalline Systems from Nature and Interaction of Living Organisms with Liquid Crystals. Adv. Mater. 2023, 35, 2204275  DOI: 10.1002/adma.202204275
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    Liquid Crystalline Systems from Nature and Interaction of Living Organisms with Liquid Crystals
    Kim, Won-Sik; Im, Jun-Hyung; Kim, Hyein; Choi, Jin-Kang; Choi, Yena; Kim, Young-Ki
    Advanced Materials (Weinheim, Germany) (2023), 35 (4), 2204275CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Biomaterials, which are substances interacting with biol. systems, have been extensively explored to understand living organisms and obtain scientific inspiration (such as biomimetics). However, many aspects of biomaterials have yet to be fully understood. Because liq. cryst. phases are ubiquitously found in biomaterials (e.g., cholesterol, amphiphile, DNA, cellulose, bacteria), therefore, a wide range of research has made attempts to approach unresolved issues with the concept of liq. crystals (LCs). This review presents these studies that address the interactive correlation between biomaterials and LCs. Specifically, intrinsic LC behavior of various biomaterials such as DNA, cellulose nanocrystals, and bacteriaare first introduced. Second, the dynamics of bacteria in LC media are addressed, with focus on how bacteria interact with LCs, and how dynamics of bacteria can be controlled by exploiting the characteristics of LCs. Lastly, how the strong correlation between LCs and biomaterials has been leveraged to design a new class of biosensors with addnl. functionalities (e.g., self-regulated drug release) that are not available in previous systems is reviewed. Examples addressed in this review convey the message that the intersection between biomaterials and LCs offers deep insights into fundamental understanding of biomaterials, and provides resources for development of transformative technologies.
  17. 17
    Choi, Y.; Choi, D.; Choi, J.-K.; Oh, K.-S.; Cho, E.; Im, J.-H.; Singh, D. P.; Kim, Y.-K. Stimuli-Responsive Materials from Liquid Crystals. ACS Appl. Opt. Mater. 2023, 1, 1879– 1897,  DOI: 10.1021/acsaom.3c00282
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    Ryzhkova, A. V.; Muševič, I. Particle Size Effects on Nanocolloidal Interactions in Nematic Liquid Crystals. Phys. Rev. E 2013, 87, 032501  DOI: 10.1103/PhysRevE.87.032501
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    18
    Particle size effects on nanocolloidal interactions in nematic liquid crystals
    Ryzhkova, A. V.; Musevic, I.
    Physical Review E: Statistical, Nonlinear, and Soft Matter Physics (2013), 87 (3-B), 032501/1-032501/12CODEN: PRESCM; ISSN:1539-3755. (American Physical Society)
    We study the interactions of submicrometer diam. silica particles, surface functionalized with DMOAP (N,N-dimethyl-n-octadecyl-3-aminopropyl-trimethoxysilyl chloride), in the nematic liq. crystal 5CB (pentylcyanobiphenil). Using the methods of video-tracking dark-field microscopy, we have measured the pair-binding energy of 35- to 450-nm-diam. silica particles, which is in the range between 100 and 1000 kBT. It is therefore high enough for the formation of thermally stable nanocolloidal pairs of 35 nm diam. We find that smaller colloids with the diam. around 22 nm do not form thermally stable pairs, which seems to be currently the lower limit for nanocolloidal assembly in the nematic liq. crystals. We also study the particle interactions with point and Saturn-ring defects and discuss the possibility of hierarchical structures generated by particles of different sizes assembled by topol. defects.
  19. 19
    Shields, C. W.; Kim, Y.-K.; Han, K.; Murphy, A. C.; Scott, A. J.; Abbott, N. L.; Velev, O. D. Control of the Folding Dynamics of Self-Reconfiguring Magnetic Microbots Using Liquid Crystallinity. Adv. Intell. Syst. 2020, 2, 1900114  DOI: 10.1002/aisy.201900114
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    Lin, I.-H.; Miller, D. S.; Bertics, P. J.; Murphy, C. J.; de Pablo, J. J.; Abbott, N. L. Endotoxin-Induced Structural Transformations in Liquid Crystalline Droplets. Science 2011, 332, 1297– 1300,  DOI: 10.1126/science.1195639
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    Endotoxin-Induced Structural Transformations in Liquid Crystalline Droplets
    Lin, I.-Hsin; Miller, Daniel S.; Bertics, Paul J.; Murphy, Christopher J.; de Pablo, Juan J.; Abbott, Nicholas L.
    Science (Washington, DC, United States) (2011), 332 (6035), 1297-1300CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The ordering of liq. crystals (LCs) is known to be influenced by surfaces and contaminants. Here, we report that picogram per mL concns. of endotoxin in water trigger ordering transitions in micrometer-size LC droplets. The ordering transitions, which occur at surface concns. of endotoxin that are less than 10-5 Langmuir, are not due to adsorbate-induced changes in the interfacial energy of the LC. The sensitivity of the LC to endotoxin was measured to change by six orders of magnitude with the geometry of the LC (droplet vs. slab), supporting the hypothesis that interactions of endotoxin with topol. defects in the LC mediate the response of the droplets. The LC ordering transitions depend strongly on glycophospholipid structure and provide new designs for responsive soft matter.
  21. 21
    Hassan, Y.; Park, J. H.; Crawford, M. L.; Sadhanala, A.; Lee, J.; Sadighian, J. C.; Mosconi, E.; Shivanna, R.; Radicchi, E.; Jeong, M.; Yang, C.; Choi, H.; Park, S. H.; Song, M. H.; De Angelis, F.; Wong, C. Y.; Friend, R. H.; Lee, B. R.; Snaith, H. J. Ligand-Engineered Bandgap Stability in Mixed-Halide Perovskite LEDs. Nature 2021, 591, 72– 77,  DOI: 10.1038/s41586-021-03217-8
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    Ligand-engineered bandgap stability in mixed-halide perovskite LEDs
    Hassan, Yasser; Park, Jong Hyun; Crawford, Michael L.; Sadhanala, Aditya; Lee, Jeongjae; Sadighian, James C.; Mosconi, Edoardo; Shivanna, Ravichandran; Radicchi, Eros; Jeong, Mingyu; Yang, Changduk; Choi, Hyosung; Park, Sung Heum; Song, Myoung Hoon; De Angelis, Filippo; Wong, Cathy Y.; Friend, Richard H.; Lee, Bo Ram; Snaith, Henry J.
    Nature (London, United Kingdom) (2021), 591 (7848), 72-77CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Abstr.: Lead halide perovskites are promising semiconductors for light-emitting applications because they exhibit bright, bandgap-tunable luminescence with high color purity1,2. Photoluminescence quantum yields close to unity have been achieved for perovskite nanocrystals across a broad range of emission colors, and light-emitting diodes with external quantum efficiencies exceeding 20 per cent-approaching those of com. org. light-emitting diodes-have been demonstrated in both the IR and the green emission channels1,3,4. However, owing to the formation of lower-bandgap iodide-rich domains, efficient and color-stable red electroluminescence from mixed-halide perovskites has not yet been realized5,6. Here we report the treatment of mixed-halide perovskite nanocrystals with multidentate ligands to suppress halide segregation under electroluminescent operation. We demonstrate color-stable, red emission centered at 620 nm, with an electroluminescence external quantum efficiency of 20.3 per cent. We show that a key function of the ligand treatment is to 'clean' the nanocrystal surface through the removal of lead atoms. D. functional theory calcns. reveal that the binding between the ligands and the nanocrystal surface suppresses the formation of iodine Frenkel defects, which in turn inhibits halide segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)cryst. surface and presents a route through which to control the formation and migration of surface defects. This is crit. to achieve bandgap stability for light emission and could also have a broader impact on other optoelectronic applications-such as photovoltaics-for which bandgap stability is required.
  22. 22
    Kim, Y.-H.; Park, J.; Kim, S.; Kim, J. S.; Xu, H.; Jeong, S.-H.; Hu, B.; Lee, T.-W. Exploiting the Full Advantages of Colloidal Perovskite Nanocrystals for Large-Area Efficient Light-Emitting Diodes. Nat. Nanotechnol. 2022, 17, 590– 597,  DOI: 10.1038/s41565-022-01113-4
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    22
    Exploiting the full advantages of colloidal perovskite nanocrystals for large-area efficient light-emitting diodes
    Kim, Young-Hoon; Park, Jinwoo; Kim, Sungjin; Kim, Joo Sung; Xu, Hengxing; Jeong, Su-Hun; Hu, Bin; Lee, Tae-Woo
    Nature Nanotechnology (2022), 17 (6), 590-597CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
    Cost-effective, high-throughput industrial applications of metal halide perovskites in large-area displays are hampered by the fundamental difficulty of controlling the process of polycryst. film formation from precursors, which results in the random growth of crystals, leading to non-uniform large grains and thus low electroluminescence efficiency in large-area perovskite light-emitting diodes (PeLEDs). Here we report that highly efficient large-area PeLEDs with high uniformity can be realized through the use of colloidal perovskite nanocrystals (PNCs), decoupling the crystn. of perovskites from film formation. PNCs were precrystd. and surrounded by org. ligands, and thus they were not affected by the film formation process, in which a simple modified bar-coating method facilitated the evapn. of residual solvent to provide uniform large-area films. PeLEDs incorporating the uniform bar-coated PNC films achieved an external quantum efficiency (EQE) of 23.26% for a pixel size of 4 mm2 and an EQE of 22.5% for a large pixel area of 102 mm2 with high reproducibility. This method provides a promising approach towards the development of large-scale industrial displays and solid-state lighting using perovskite emitters.
  23. 23
    Huang, H.; Raith, J.; Kershaw, S. V.; Kalytchuk, S.; Tomanec, O.; Jing, L.; Susha, A. S.; Zboril, R.; Rogach, A. L. Growth Mechanism of Strongly Emitting CH3NH3PbBr3 Perovskite Nanocrystals with a Tunable Bandgap. Nat. Commun. 2017, 8, 996  DOI: 10.1038/s41467-017-00929-2
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    Growth mechanism of strongly emitting CH3NH3PbBr3 perovskite nanocrystals with a tunable bandgap
    Huang He; Raith Johannes; Kershaw Stephen V; Susha Andrei S; Rogach Andrey L; Kalytchuk Sergii; Tomanec Ondrej; Zboril Radek; Jing Lihong
    Nature communications (2017), 8 (1), 996 ISSN:.
    Metal halide perovskite nanocrystals are promising materials for a diverse range of applications, such as light-emitting devices and photodetectors. We demonstrate the bandgap tunability of strongly emitting CH3NH3PbBr3 nanocrystals synthesized at both room and elevated (60 °C) temperature through the variation of the precursor and ligand concentrations. We discuss in detail the role of two ligands, oleylamine and oleic acid, in terms of the coordination of the lead precursors and the nanocrystal surface. The growth mechanism of nanocrystals is elucidated by combining the experimental results with the principles of nucleation/growth models. The proposed formation mechanism of perovskite nanocrystals will be helpful for further studies in this field and can be used as a guide to improve the synthetic methods in the future.The development of perovskite nanocrystals is limited by poor mechanistic understanding of their growth. Here, the authors systematically study the ligand-assisted reprecipitation synthesis of CH3NH3PbBr3 nanocrystals, revealing the effect of precursor and ligand concentrations on bandgap tunability.
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    Wang, H.; Wu, T. X.; Gauza, S.; Wu, J. R.; Wu, S.-T. A Method to Estimate the Leslie Coefficients of Liquid Crystals Based on MBBA Data. Liq. Cryst. 2006, 33, 91– 98,  DOI: 10.1080/02678290500446111
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    Wang, T.; Zhuang, J.; Lynch, J.; Chen, O.; Wang, Z.; Wang, X.; LaMontagne, D.; Wu, H.; Wang, Z.; Cao, Y. C. Self-Assembled Colloidal Superparticles from Nanorods. Science 2012, 338, 358– 363,  DOI: 10.1126/science.1224221
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    Self-Assembled Colloidal Superparticles from Nanorods
    Wang, Tie; Zhuang, Jiaqi; Lynch, Jared; Chen, Ou; Wang, Zhongliang; Wang, Xirui; LaMontagne, Derek; Wu, Huimeng; Wang, Zhongwu; Cao, Y. Charles
    Science (Washington, DC, United States) (2012), 338 (6105), 358-363CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Colloidal superparticles are nanoparticle assemblies in the form of colloidal particles. The assembly of nanoscopic objects into mesoscopic or macroscopic complex architectures allows bottom-up fabrication of functional materials. We report that the self-assembly of cadmium selenide-cadmium sulfide (CdSe-CdS) core-shell semiconductor nanorods, mediated by shape and structural anisotropy, produces mesoscopic colloidal superparticles having multiple well-defined supercryst. domains. Moreover, functionality-based anisotropic interactions between these CdSe-CdS nanorods can be kinetically introduced during the self-assembly and, in turn, yield single-domain, needle-like superparticles with parallel alignment of constituent nanorods. Unidirectional patterning of these mesoscopic needle-like superparticles gives rise to the lateral alignment of CdSe-CdS nanorods into macroscopic, uniform, freestanding polymer films that exhibit strong photoluminescence with a striking anisotropy, enabling their use as downconversion phosphors to create polarized light-emitting diodes.
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    Giuntini, D.; Zhao, S.; Krekeler, T.; Li, M.; Blankenburg, M.; Bor, B.; Schaan, G.; Domènech, B.; Müller, M.; Scheider, I.; Ritter, M.; Schneider, G. A. Defects and Plasticity in Ultrastrong Supercrystalline Nanocomposites. Sci. Adv. 2021, 7, eabb6063  DOI: 10.1126/sciadv.abb6063
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    Faetti, S.; Gatti, M.; Palleschi, V.; Sluckin, T. J. Almost Critical Behavior of the Anchoring Energy at the Interface between a Nematic Liquid Crystal and a SiO Substrate. Phys. Rev. Lett. 1985, 55, 1681– 1684,  DOI: 10.1103/PhysRevLett.55.1681
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    Almost critical behavior of the anchoring energy at the interface between a nematic liquid crystal and a silicon monoxide substrate
    Faetti, Sandro; Gatti, Marta; Palleschi, Vincenzo; Sluckin, Timothy J.
    Physical Review Letters (1985), 55 (16), 1681-4CODEN: PRLTAO; ISSN:0031-9007.
    The anchoring energy at the interface between the nematic liq. crystal 4-pentyl-4'-cyanobiphenyl (5CB) and a glass plate treated by oblique evapn. of SiO was studied by measuring the torque exerted on the surface by the nematic subject to an orienting magnetic field. The anchoring energy is sharply reduced as the clearing temp. is approached. The expt. provides the 1st direct evidence of a strong redn. of the surface order parameter for this system in the anisotropic phase.
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    Seth, S.; Samanta, A. A Facile Methodology for Engineering the Morphology of CsPbX3 Perovskite Nanocrystals under Ambient Condition. Sci. Rep. 2016, 6, 37693  DOI: 10.1038/srep37693
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    A Facile Methodology for Engineering the Morphology of CsPbX3 Perovskite Nanocrystals under Ambient Condition
    Seth, Sudipta; Samanta, Anunay
    Scientific Reports (2016), 6 (), 37693CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    A facile and highly reproducible room temp., open atm. synthesis of cesium lead halide perovskite nanocrystals of six different morphologies is reported just by varying the solvent, ligand and reaction time. Sequential evolution of the quantum dots, nanoplates and nanobars in one medium and nanocubes, nanorods and nanowires in another medium is demonstrated. These perovskite nanoparticles are shown to be of excellent cryst. quality with high fluorescence quantum yield. A mechanism of the formation of nanoparticles of different shapes and sizes is proposed. Considering the key role of morphol. in nanotechnol., this simple method of fabrication of a wide range of high quality nanocrystals of different shapes and sizes of all-inorg. lead halide perovskites, whose potential is already demonstrated in light emitting and photovoltaic applications, is likely to help widening the scope and utility of these materials in optoelectronic devices.
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    Lapointe, C. P.; Mason, T. G.; Smalyukh, I. I. Shape-Controlled Colloidal Interactions in Nematic Liquid Crystals. Science 2009, 326, 1083– 1086,  DOI: 10.1126/science.1176587
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    Shape-Controlled Colloidal Interactions in Nematic Liquid Crystals
    Lapointe, Clayton P.; Mason, Thomas G.; Smalyukh, Ivan I.
    Science (Washington, DC, United States) (2009), 326 (5956), 1083-1086CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Robust control over the positions, orientations, and assembly of nonspherical colloids may aid in the creation of new types of structured composite materials that are important from both technol. and fundamental standpoints. With the use of lithog. fabricated equilateral polygonal platelets, we demonstrate that colloidal interactions and self-assembly in anisotropic nematic fluids can be effectively tailored via control over the particles' shapes. The particles disturb the uniform alignment of the surrounding nematic host, resulting in both a distinct equil. alignment and highly directional pair interactions. Interparticle forces between polygonal platelets exhibit either dipolar or quadrupolar symmetries, depending on whether their no. of sides is odd or even, and drive the assembly of a no. of ensuing self-assembled colloidal structures.
  30. 30
    Senyuk, B.; Liu, Q.; Nystrom, P. D.; Smalyukh, I. I. Repulsion–Attraction Switching of Nematic Colloids Formed by Liquid Crystal Dispersions of Polygonal Prisms. Soft Matter 2017, 13, 7398– 7405,  DOI: 10.1039/C7SM01186E
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    Repulsion-attraction switching of nematic colloids formed by liquid crystal dispersions of polygonal prisms
    Senyuk, B.; Liu, Q.; Nystrom, P. D.; Smalyukh, I. I.
    Soft Matter (2017), 13 (40), 7398-7405CODEN: SMOABF; ISSN:1744-6848. (Royal Society of Chemistry)
    Self-assembly of colloidal particles due to elastic interactions in nematic liq. crystals promises tunable composite materials and can be guided by exploiting surface functionalization, geometric shape and topol., though these means of controlling self-assembly remain limited. Here, we realize low-symmetry achiral and chiral elastic colloids in the nematic liq. crystals using colloidal polygonal concave and convex prisms. We show that the controlled pinning of disclinations at the prism edges alters the symmetry of director distortions around the prisms and their orientation with respect to the far-field director. The controlled localization of the disclinations at the prism's edges significantly influences the anisotropy of the diffusion properties of prisms dispersed in liq. crystals and allows one to modify their self-assembly. We show that elastic interactions between polygonal prisms can be switched between repulsive and attractive just by controlled re-pinning the disclinations at different edges using laser tweezers. Our findings demonstrate that elastic interactions between colloidal particles dispersed in nematic liq. crystals are sensitive to the topol. equiv. but geometrically rich controlled configurations of the particle-induced defects.
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    Zhang, M.; Yu, H.; Lyu, M.; Wang, Q.; Yun, J.-H.; Wang, L. Composition-Dependent Photoluminescence Intensity and Prolonged Recombination Lifetime of Perovskite CH3NH3PbBr3–xClx Films. Chem. Commun. 2014, 50, 11727– 11730,  DOI: 10.1039/C4CC04973J
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    Composition-dependent photoluminescence intensity and prolonged recombination lifetime of perovskite CH3NH3PbBr3-xClx films
    Zhang, Meng; Yu, Hua; Lyu, Miaoqiang; Wang, Qiong; Yun, Jung-Ho; Wang, Lianzhou
    Chemical Communications (Cambridge, United Kingdom) (2014), 50 (79), 11727-11730CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Mixed halide perovskites CH3NH3PbBr3-xClx (x = 0.6-1.2) with different compns. of halogens exhibit drastically changed optical properties. In particular, the thin films prepd. with these perovskites demonstrate extraordinary photoluminescence emission intensities and prolonged recombination lifetimes up to 446 ns, which are desirable for light emitting and photovoltaic applications.
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    Cho, H.; Jeong, S.-H.; Park, M.-H.; Kim, Y.-H.; Wolf, C.; Lee, C.-L.; Heo, J. H.; Sadhanala, A.; Myoung, N.; Yoo, S.; Im, S. H.; Friend, R. H.; Lee, T.-W. Overcoming the Electroluminescence Efficiency Limitations of Perovskite Light-Emitting Diodes. Science 2015, 350, 1222– 1225,  DOI: 10.1126/science.aad1818
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    Overcoming the electroluminescence efficiency limitations of perovskite light-emitting diodes
    Cho, Himchan; Jeong, Su-Hun; Park, Min-Ho; Kim, Young-Hoon; Wolf, Christoph; Lee, Chang-Lyoul; Heo, Jin Hyuck; Sadhanala, Aditya; Myoung, NoSoung; Yoo, Seunghyup; Im, Sang Hyuk; Friend, Richard H.; Lee, Tae-Woo
    Science (Washington, DC, United States) (2015), 350 (6265), 1222-1225CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Org.-inorg. hybrid perovskites are emerging low-cost emitters with very high color purity, but their low luminescent efficiency is a crit. drawback. We boosted the current efficiency (CE) of perovskite light-emitting diodes with a simple bilayer structure to 42.9 candela per A, similar to the CE of phosphorescent org. light-emitting diodes OLED , with two modifications: We prevented the formation of metallic lead (Pb) atoms that cause strong exciton quenching through a small increase in methylammonium bromide (MABr) molar proportion, and we spatially confined the exciton in uniform MAPbBr3 nanograins (av. diam. = 99.7 nm) formed by a nanocrystal pinning process and concomitant redn. of exciton diffusion length to 67 nm. These changes caused substantial increases in steady-state photoluminescence intensity and efficiency of MAPbBr3 nanograin layers.
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    Jang, H. M.; Kim, J.-S.; Heo, J.-M.; Lee, T.-W. Enhancing Photoluminescence Quantum Efficiency of Metal Halide Perovskites by Examining Luminescence-Limiting Factors. APL Mater. 2020, 8, 020904  DOI: 10.1063/1.5136308
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    Enhancing photoluminescence quantum efficiency of metal halide perovskites by examining luminescence-limiting factors
    Jang, Hyun Myung; Kim, Joo-Sung; Heo, Jung-Min; Lee, Tae-Woo
    APL Materials (2020), 8 (2), 020904CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    A review. Metal halide perovskites (MHPs) show superior optoelectronic properties, which give them the great potential for use in next generation light-emitting diodes (LEDs). In particular, their narrow emission linewidths can achieve ultrahigh color purity. However, the reported luminescence efficiency (LE) values are not high enough to be commercialized in displays and solid-state lightings. Moreover, the operational stability of LEDs assocd. with the overshooting of luminance and the high relative std. deviation of reported external quantum efficiencies are still problematic. In this perspective, we review photophys. factors that limit the photoluminescence quantum efficiency of perovskite-based LEDs. These factors are categorized into (i) weak exciton binding, (ii) nonradiative recombinations, (iii) slow cooling of long-lived hot carriers, (iv) deep-level defects, and (v) interband transition rates. We then present various physicochem. methods to effectively overcome these luminescence-limiting factors. We finally suggest some useful research directions to further improve the LE of MHP emitters as core components in displays and solid-state lightings. (c) 2020 American Institute of Physics.
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    Smock, S. R.; Williams, T. J.; Brutchey, R. L. Quantifying the Thermodynamics of Ligand Binding to CsPbBr3 Quantum Dots. Angew. Chem., Int. Ed. 2018, 57, 11711– 11715,  DOI: 10.1002/anie.201806916
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    Quantifying the Thermodynamics of Ligand Binding to CsPbBr3 Quantum Dots
    Smock, Sara R.; Williams, Travis J.; Brutchey, Richard L.
    Angewandte Chemie, International Edition (2018), 57 (36), 11711-11715CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Cesium lead halide perovskites are an emerging class of quantum dots (QDs) that have shown promise in a variety of applications; however, their properties are highly dependent on their surface chem. To this point, the thermodn. of ligand binding remain unstudied. Herein, 1H NMR methods were used to quantify the thermodn. of ligand exchange on CsPbBr3 QDs. Both oleic acid and oleylamine native ligands dynamically interact with the CsPbBr3 QD surface, having individual surface densities of 1.2-1.7 nm-2. 10-Undecenoic acid undergoes an exergonic exchange equil. with bound oleate (Keq = 1.97) at 25 °C while 10-undecenylphosphonic acid undergoes irreversible ligand exchange. Undec-10-en-1-amine exergonically exchanges with oleylamine (Keq = 2.52) at 25 °C. Exchange occurs with carboxylic acids, phosphonic acids, and amines on CsPbBr3 QDs without etching of the nanocrystal surface; increases in the steady-state PL intensities correlate with more strongly bound conjugate base ligands.
  35. 35
    Almeida, G.; Infante, I.; Manna, L. Resurfacing Halide Perovskite Nanocrystals. Science 2019, 364, 833– 834,  DOI: 10.1126/science.aax5825
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    Resurfacing halide perovskite nanocrystals
    Almeida, Guilherme; Infante, Ivan; Manna, Liberato
    Science (Washington, DC, United States) (2019), 364 (6443), 833-834CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Metal halide perovskite semiconductors are ionic compds. with the formula AMX3 (A and M are cations, and X can be Cl-, Br-, I-, or binary mixts. of these anions). In the form of colloidal nanocrystals, these materials have extraordinary potential as light emitters. Not only do they exhibit high photoluminescence quantum yields (PLQYs), but the emission color can be finely tuned across the entire visible spectrum by changing the proportions of mixed halide anions (). However, the surface chem. of these nanocrystals makes them susceptible to degrdn. and long-term instability, and the surface can introduce surface centers (midgap states) that promote nonradiative recombination of charge carriers that lower PLQYs. Thus, the characterization of the interface between the perovskite nanocrystals and the org. ligands is fundamental to developing strategies to control surface defects, tuning the opto-electronic properties, and improving device performance and stability.
  36. 36
    Kong, L.; Zhang, X.; Li, Y.; Wang, H.; Jiang, Y.; Wang, S.; You, M.; Zhang, C.; Zhang, T.; Kershaw, S. V.; Zheng, W.; Yang, Y.; Lin, Q.; Yuan, M.; Rogach, A. L.; Yang, X. Smoothing the Energy Transfer Pathway in Quasi-2D Perovskite Films Using Methanesulfonate Leads to Highly Efficient Light-Emitting Devices. Nat. Commun. 2021, 12, 1246  DOI: 10.1038/s41467-021-21522-8
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    Smoothing the energy transfer pathway in quasi-2D perovskite films using methanesulfonate leads to highly efficient light-emitting devices
    Kong, Lingmei; Zhang, Xiaoyu; Li, Yunguo; Wang, Haoran; Jiang, Yuanzhi; Wang, Sheng; You, Mengqing; Zhang, Chengxi; Zhang, Ting; Kershaw, Stephen V.; Zheng, Weitao; Yang, Yingguo; Lin, Qianqian; Yuan, Mingjian; Rogach, Andrey L.; Yang, Xuyong
    Nature Communications (2021), 12 (1), 1246CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Quasi-two-dimensional (quasi-2D) Ruddlesden-Popper (RP) perovskites such as BA2Csn-1PbnBr3n+1 (BA = butylammonium, n > 1) are promising emitters, but their electroluminescence performance is limited by a severe non-radiative recombination during the energy transfer process. Here, we make use of methanesulfonate (MeS) that can interact with the spacer BA cations via strong hydrogen bonding interaction to reconstruct the quasi-2D perovskite structure, which increases the energy acceptor-to-donor ratio and enhances the energy transfer in perovskite films, thus improving the light emission efficiency. MeS additives also lower the defect d. in RP perovskites, which is due to the elimination of uncoordinated Pb2+ by the electron-rich Lewis base MeS and the weakened adsorbate blocking effect. As a result, green light-emitting diodes fabricated using these quasi-2D RP perovskite films reach current efficiency of 63 cd A-1 and 20.5% external quantum efficiency, which are the best reported performance for devices based on quasi-2D perovskites so far.
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    Brake, J. M.; Mezera, A. D.; Abbott, N. L. Effect of Surfactant Structure on the Orientation of Liquid Crystals at Aqueous–Liquid Crystal Interfaces. Langmuir 2003, 19, 6436– 6442,  DOI: 10.1021/la034132s
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    Effect of Surfactant Structure on the Orientation of Liquid Crystals at Aqueous-Liquid Crystal Interfaces
    Brake, Jeffrey M.; Mezera, Andrew D.; Abbott, Nicholas L.
    Langmuir (2003), 19 (16), 6436-6442CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    It is known that the orientations assumed by thermotropic liq. crystals (LCs) in contact with water are sensitive to the types and concns. of surfactants and/or polymers present in the aq. phase. This work expands upon these past observations by developing criteria for surfactants that give rise to a particular orientation of a contacting nematic LC formed formed from 4'-pentyl-4-cyanobiphenyl (5CB). We observe surfactants that have a bolaform structure ((11-hydroxyundecyl)trimethylammonium bromide (HTAB), dodecyl-1,12-bis(trimethylammonium bromide) (DBTAB), 11-(ferrocenylundecyl)trimethylammonium bromide (FTMA)) and which adopt looped configurations at air-water/oil-water interfaces cause planar anchoring of 5CB. In contrast, classical surfactants (alkyltrimethylammonium halides (CnTABs, n > 8), sodium dodecyl sulfate (SDS), and N,N-dimethylferrocenylalkylammonium bromides (FCnABs, n > 12)) that assume tilted orientations at air-water/oil-water interfaces can give rise to a homeotropic orientation of 5CB. By comparing SDS, dodecyl trimethylammonium halide (DTAB), and tetra(ethylene glycol) monododecyl ether (C12E4), we conclude that the nature of these headgroups does not measurably influence the orientation of the LC. However, the orientation of the LC is found to depend on the aliph. chain length and the areal d. of the adsorbed surfactant. When using surfactants with short alkyl chain lengths (n = 8 for CnTAB and n = 7 and 12 for FCnAB), we observe the orientation of 5CB to remain parallel to the interface up to concns. at which the 5CB begins to be solubilized by the surfactant. These results, when combined, lead us to conclude that interactions between the aliph. chains of the surfactant and 5CB, which are influenced by the conformation of the surfactant, largely dictate the orientation of the 5CB.
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    Sichert, J. A.; Tong, Y.; Mutz, N.; Vollmer, M.; Fischer, S.; Milowska, K. Z.; García Cortadella, R.; Nickel, B.; Cardenas-Daw, C.; Stolarczyk, J. K.; Urban, A. S.; Feldmann, J. Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets. Nano Lett. 2015, 15, 6521– 6527,  DOI: 10.1021/acs.nanolett.5b02985
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    Quantum size effect in organometal halide perovskite nanoplatelets
    Sichert, Jasmina A.; Tong, Yu; Mutz, Niklas; Vollmer, Mathias; Fischer, Stefan; Milowska, Karolina Z.; Garcia-Cortadella, Ramon; Nickel, Bert; Cardenas-Daw, Carlos; Stolarczyk, Jacek K.; Urban, Alexander S.; Feldmann, Jochen
    Nano Letters (2015), 15 (10), 6521-6527CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Organometal halide perovskites have recently emerged displaying a huge potential for not only photovoltaic, but also light-emitting applications. Exploiting the optical properties of specifically tailored perovskite nanocrystals could greatly enhance the efficiency and functionality of applications based on this material. In this study, we investigate the quantum size effect in colloidal organometal halide perovskite nanoplatelets. By tuning the ratio of the org. cations used, we can control the thickness and consequently the photoluminescence emission of the platelets. Quantum mech. calcns. match well with the exptl. values. We find that not only do the properties of the perovskite, but also those of the org. ligands play an important role. Stacking of nanoplatelets leads to the formation of minibands, further shifting the bandgap energies. In addn., we find a large exciton binding energy of up to several hundreds of meV for nanoplatelets thinner than three unit cells, partially counteracting the blueshift induced by quantum confinement. Understanding of the quantum size effects in perovskite nanoplatelets and the ability to tune them provide an addnl. method with which to manipulate the optical properties of organometal halide perovskites.
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    Almeida, G.; Goldoni, L.; Akkerman, Q.; Dang, Z.; Khan, A. H.; Marras, S.; Moreels, I.; Manna, L. Role of Acid–Base Equilibria in the Size, Shape, and Phase Control of Cesium Lead Bromide Nanocrystals. ACS Nano 2018, 12, 1704– 1711,  DOI: 10.1021/acsnano.7b08357
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    Role of Acid-Base Equilibria in the Size, Shape, and Phase Control of Cesium Lead Bromide Nanocrystals
    Almeida, Guilherme; Goldoni, Luca; Akkerman, Quinten; Dang, Zhiya; Khan, Ali Hossain; Marras, Sergio; Moreels, Iwan; Manna, Liberato
    ACS Nano (2018), 12 (2), 1704-1711CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    A binary ligand system composed of aliph. carboxylic acids and primary amines of various chain lengths is commonly employed in diverse synthesis methods for CsPbBr3 nanocrystals (NCs). The authors have carried out a systematic study examg. how the concn. of ligands (oleylamine and oleic acid) and the resulting acidity (or basicity) affects the hot-injection synthesis of CsPbBr3 NCs. The authors devise a general synthesis scheme for cesium lead bromide NCs which allows control over size, size distribution, shape, and phase (CsPbBr3 or Cs4PbBr6) by combining key insights on the acid-base interactions that rule this ligand system. Also, the authors' findings shed light upon the soly. of PbBr2 in this binary ligand system, and plausible mechanisms are suggested to understand the ligand-mediated phase control and structural stability of CsPbBr3 NCs.
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    Lignos, I.; Stavrakis, S.; Nedelcu, G.; Protesescu, L.; deMello, A. J.; Kovalenko, M. V. Synthesis of Cesium Lead Halide Perovskite Nanocrystals in a Droplet-Based Microfluidic Platform: Fast Parametric Space Mapping. Nano Lett. 2016, 16, 1869– 1877,  DOI: 10.1021/acs.nanolett.5b04981
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    Synthesis of Cesium Lead Halide Perovskite Nanocrystals in a Droplet-Based Microfluidic Platform: Fast Parametric Space Mapping
    Lignos, Ioannis; Stavrakis, Stavros; Nedelcu, Georgian; Protesescu, Loredana; de Mello, Andrew J.; Kovalenko, Maksym V.
    Nano Letters (2016), 16 (3), 1869-1877CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Prior to this work, fully inorg. nanocrystals of cesium lead halide perovskite (CsPbX3, X = Br, I, Cl and Cl/Br and Br/I mixed halide systems), exhibiting bright and tunable photoluminescence, have been synthesized using conventional batch (flask-based) reactions. Unfortunately, our understanding of the parameters governing the formation of these nanocrystals is still very limited due to extremely fast reaction kinetics and multiple variables involved in ion-metathesis-based synthesis of such multinary halide systems. Herein, we report the use of a droplet-based microfluidic platform for the synthesis of CsPbX3 nanocrystals. The combination of online photoluminescence and absorption measurements and the fast mixing of reagents within such a platform allows the rigorous and rapid mapping of the reaction parameters, including molar ratios of Cs, Pb, and halide precursors, reaction temps., and reaction times. This translates into enormous savings in reagent usage and screening times when compared to analogous batch synthetic approaches. The early-stage insight into the mechanism of nucleation of metal halide nanocrystals suggests similarities with multinary metal chalcogenide systems, albeit with much faster reaction kinetics in the case of halides. Furthermore, we show that microfluidics-optimized synthesis parameters are also directly transferrable to the conventional flask-based reaction.
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    Lee, H.-D.; Woo, S.-J.; Kim, S.; Kim, J.; Zhou, H.; Han, S. J.; Jang, K. Y.; Kim, D.-H.; Park, J.; Yoo, S.; Lee, T.-W. Valley-Centre Tandem Perovskite Light-Emitting Diodes. Nat. Nanotechnol. 2024, 19, 624– 631,  DOI: 10.1038/s41565-023-01581-2
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    Aqoma, H.; Lee, S.-H.; Imran, I. F.; Hwang, J.-H.; Lee, S.-H.; Jang, S.-Y. Alkyl Ammonium Iodide-Based Ligand Exchange Strategy for High-Efficiency Organic-Cation Perovskite Quantum Dot Solar Cells. Nat. Energy 2024, 9, 324– 332,  DOI: 10.1038/s41560-024-01450-9
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    Rainò, G.; Becker, M. A.; Bodnarchuk, M. I.; Mahrt, R. F.; Kovalenko, M. V.; Stöferle, T. Superfluorescence from Lead Halide Perovskite Quantum Dot Superlattices. Nature 2018, 563, 671– 675,  DOI: 10.1038/s41586-018-0683-0
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    Superfluorescence from lead halide perovskite quantum dot superlattices
    Raino, Gabriele; Becker, Michael A.; Bodnarchuk, Maryna I.; Mahrt, Rainer F.; Kovalenko, Maksym V.; Stoferle, Thilo
    Nature (London, United Kingdom) (2018), 563 (7733), 671-675CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    An ensemble of emitters can behave very differently from its individual constituents when they interact coherently via a common light field. After excitation of such an ensemble, collective coupling can give rise to a many-body quantum phenomenon that results in short, intense bursts of light-so-called superfluorescence1. Because this phenomenon requires a fine balance of interactions between the emitters and their decoupling from the environment, together with close identity of the individual emitters, superfluorescence has thus far been obsd. only in a limited no. of systems, such as certain at. and mol. gases and a few solid-state systems2-7. The generation of superfluorescent light in colloidal nanocrystals (which are bright photonic sources practically suited for optoelectronics8,9) has been precluded by inhomogeneous emission broadening, low oscillator strength, and fast exciton dephasing. Here we show that cesium lead halide (CsPbX3, X = Cl, Br) perovskite nanocrystals10-13 that are self-organized into highly ordered three-dimensional superlattices exhibit key signatures of superfluorescence. These are dynamically red-shifted emission with more than 20-fold accelerated radiative decay, extension of the first-order coherence time by more than a factor of four, photon bunching, and delayed emission pulses with Burnham-Chiao ringing behavior14 at high excitation d. These mesoscopically extended coherent states could be used to boost the performance of optoelectronic devices15 and enable entangled multi-photon quantum light sources16,17.
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    Hou, J.; Chen, P.; Shukla, A.; Krajnc, A.; Wang, T.; Li, X.; Doasa, R.; Tizei, L. H. G.; Chan, B.; Johnstone, D. N.; Lin, R.; Schülli, T. U.; Martens, I.; Appadoo, D.; Ari, M. S.; Wang, Z.; Wei, T.; Lo, S.-C.; Lu, M.; Li, S. Liquid-Phase Sintering of Lead Halide Perovskites and Metal-Organic Framework Glasses. Science 2021, 374, 621– 625,  DOI: 10.1126/science.abf4460
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    Liquid-phase sintering of lead halide perovskites and metal-organic framework glasses
    Hou, Jingwei; Chen, Peng; Shukla, Atul; Krajnc, Andraz; Wang, Tiesheng; Li, Xuemei; Doasa, Rana; Tizei, Luiz H. G.; Chan, Bun; Johnstone, Duncan N.; Lin, Rijia; Schulli, Tobias U.; Martens, Isaac; Appadoo, Dominique; S'Ari, Mark; Wang, Zhiliang; Wei, Tong; Lo, Shih-Chun; Lu, Mingyuan; Li, Shichun; Namdas, Ebinazar B.; Mali, Gregor; Cheetham, Anthony K.; Collins, Sean M.; Chen, Vicki; Wang, Lianzhou; Bennett, Thomas D.
    Science (Washington, DC, United States) (2021), 374 (6567), 621-625CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    Lead halide perovskite (LHP) semiconductors show exceptional optoelectronic properties. Barriers for their applications, however, lie in their polymorphism, instability to polar solvents, phase segregation, and susceptibility to the leaching of lead ions. We report a family of scalable composites fabricated through liq.-phase sintering of LHPs and metal-org. framework glasses. The glass acts as a matrix for LHPs, effectively stabilizing nonequil. perovskite phases through interfacial interactions. These interactions also passivate LHP surface defects and impart bright, narrow-band photoluminescence with a wide gamut for creating white light-emitting diodes (LEDs). The processable composites show high stability against immersion in water and org. solvents as well as exposure to heat, light, air, and ambient humidity. These properties, together with their lead self-sequestration capability, can enable breakthrough applications for LHPs.
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    Kim, J. I.; Zeng, Q.; Park, S.; Lee, H.; Park, J.; Kim, T.; Lee, T.-W. Strategies to Extend the Lifetime of Perovskite Downconversion Films for Display Applications. Adv. Mater. 2023, 35, 2209784  DOI: 10.1002/adma.202209784
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    Proppe, A. H.; Berkinsky, D. B.; Zhu, H.; Šverko, T.; Kaplan, A. E. K.; Horowitz, J. R.; Kim, T.; Chung, H.; Jun, S.; Bawendi, M. G. Highly Stable and Pure Single-Photon Emission with 250 ps Optical Coherence Times in InP Colloidal Quantum Dots. Nat. Nanotechnol. 2023, 18, 993– 999,  DOI: 10.1038/s41565-023-01432-0
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    Highly stable and pure single-photon emission with 250 ps optical coherence times in InP colloidal quantum dots
    Proppe, Andrew H.; Berkinsky, David B.; Zhu, Hua; Sverko, Tara; Kaplan, Alexander E. K.; Horowitz, Jonah R.; Kim, Taehyung; Chung, Heejae; Jun, Shinae; Bawendi, Moungi G.
    Nature Nanotechnology (2023), 18 (9), 993-999CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
    Quantum photonic technologies such as quantum communication, sensing or computation require efficient, stable and pure single-photon sources. Epitaxial quantum dots (QDs) have been made capable of on-demand photon generation with high purity, indistinguishability and brightness, although they require precise fabrication and face challenges in scalability. By contrast, colloidal QDs are batch synthesized in soln. but typically have broader linewidths, low single-photon purities and unstable emission. Here we demonstrate spectrally stable, pure and narrow-linewidth single-photon emission from InP/ZnSe/ZnS colloidal QDs. Using photon correlation Fourier spectroscopy, we observe single-dot linewidths as narrow as ∼5μeV at 4 K, giving a lower-bounded optical coherence time, T2, of ∼250 ps. These dots exhibit minimal spectral diffusion on timescales of microseconds to minutes, and narrow linewidths are maintained on timescales up to 50 ms, orders of magnitude longer than other colloidal systems. Moreover, these InP/ZnSe/ZnS dots have single-photon purities g(2) (τ = 0) of 0.077-0.086 in the absence of spectral filtering. This work demonstrates the potential of heavy-metal-free InP-based QDs as spectrally stable sources of single photons.
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    Chiba, T.; Hoshi, K.; Pu, Y.-J.; Takeda, Y.; Hayashi, Y.; Ohisa, S.; Kawata, S.; Kido, J. High-Efficiency Perovskite Quantum-Dot Light-Emitting Devices by Effective Washing Process and Interfacial Energy Level Alignment. ACS Appl. Mater. Interfaces 2017, 9, 18054– 18060,  DOI: 10.1021/acsami.7b03382
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    High-Efficiency Perovskite Quantum-Dot Light-Emitting Devices by Effective Washing Process and Interfacial Energy Level Alignment
    Chiba, Takayuki; Hoshi, Keigo; Pu, Yong-Jin; Takeda, Yuya; Hayashi, Yukihiro; Ohisa, Satoru; Kawata, So; Kido, Junji
    ACS Applied Materials & Interfaces (2017), 9 (21), 18054-18060CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    All inorg. perovskites quantum dots (PeQDs) have attracted much attention for used in thin film display applications and solid-state lighting applications, owing to their narrow band emission with high luminescence quantum yields (PLQYs), color tunability, and soln. processability. Fabricated low-driving-voltage and high-efficiency CsPbBr3 PeQDs light-emitting devices (PeQD-LEDs) using a PeQDs washing process with an ester solvent contg. BuOAc to remove excess ligands from the PeQDs. The CsPbBr3 PeQDs film washed with AcOBu exhibited a PLQY of 42%, and a narrow PL emission with a full width at half-max. of 19 nm. Energy level alignment of the PeQD-LED to achieve effective hole injection into PeQDs from the adjacent hole injection layer was demonstrated. The PeQD-LED with AcOBu-washed PeQDs exhibited a max. power efficiency of 31.7 lm W-1 and EQE of 8.73%. Control of the interfacial PeQDs through ligand removal and energy level alignment in the device structure are promising methods for obtaining high PLQYs in film state and high device efficiency.
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    Microfabrication uses integrated-circuit manufg. technol. supplemented by its own processes to create objects with dimensions in the range of micrometers to millimeters. These objects can have miniature moving parts, stationary structures, or both. Microfabrication has been used for many applications in biol. and medicine. These applications fall into four domains: tools for mol. biol. and biochem., tools for cell biol., medical devices, and biosensors. Microfabricated device structures may provide significantly enhanced function with respect to a conventional device. Sometimes microfabrication can enable devices with novel capabilities. These enhancing and enabling qualities are conferred when microfabrication is used appropriately to address the right types of problems. Herein, we describe microfabrication technol. and its application to biol. and medicine. We detail several classes of advantages conferred by microfabrication and how these advantages have been used to date.

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  2. Pei-Xi Wang, Tingting Zhou, Penghao Guo, Xuelian Jiang, Hongbo Zhao, Qing Zhang. Semiconducting Liquid Crystalline Dispersions with Precisely Adjustable Band Gaps and Polarized Photoluminescence. Materials Horizons 2025, https://doi.org/10.1039/D4MH01876A
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  • Abstract

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    Figure 1

    Figure 1. Controlled synthesis of perovskite nanocrystals in a liquid crystalline antisolvent. Chemical structures of (a) perovskite precursors (CH3NH3Br and PbBr2) and organic ligands (n-octylamine and oleic acid) and (b) the nematic liquid crystal (LC), E7. Schematic illustrations for reconstructed profiles of LC molecules (purple ellipsoids) around growing perovskite nanocrystals (PNCs) when their size L (blue double arrows) is (c) smaller and (d) larger than the extrapolation length ξ (red double arrows and circles) of the LCs. n indicates the director of LC. (e) Ultraviolet–visible absorbance (purple and blue lines) and photoluminescence (PL) spectra (red and black lines) measured from the LC-PNC (top) and T-PNC (bottom) solutions before (dashed lines) and after (solid lines) size purification processes. The PNCs are synthesized for 720 min at T = 25 °C. The insets show photographs of the as-synthesized PNC solutions under ambient light. a.u.: arbitrary units. (f) Size distribution of LC-PNCs (top) and T-PNCs (bottom) measured from electron micrographs. n ≥ 1000 measurements. L = 8 ± 2 nm (mean ± SD) for LC-PNCs. Micrographs of (g, h) LC-PNCs and (i, j) T-PNCs obtained by (g, i) scanning and (h, j) transmission electron microscopy.

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    Figure 2

    Figure 2. Synthesis mechanism and optical properties of LC-PNCs. (a) Mean values of ξ (red circles, n ≥ 5) and L (blue squares, n ≥ 500) with respect to synthesis T. (b) PL spectra of LC-PNCs synthesized at T = 25 °C (black squares), 30 °C (red triangles), and 35 °C (blue circles) and (c) the corresponding λpeak with respect to synthesis T. PL spectra from the PNC solutions that are synthesized at T = 25 °C for (d) ts = 10 min and (e) 720 min. PL spectra of T-PNC solutions (black) are normalized by the maximum peak intensity of LC-PNC solutions (red). (f) λpeak for the LC-PNC (red stars) and T-PNC (black squares) solutions with respect to ts. PL spectra are measured with the PNC solutions (e) with and (d, f) without size purification processes.

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    Figure 3

    Figure 3. Elastic interactions of PNCs in LC media. Schematic illustrations describing the elastic interaction associated with the (a, b) attraction and (c) repulsion of PNCs in LC media. In situ fluorescence confocal microscopy (FCM) images of elastic interactions of PNCs in LC media, where LC molecules are aligned along the x-axis. FCM images are measured at (d) 0, (e) 166, and (f) 258 s. n0 indicates the far-field director of LCs.

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    Figure 4

    Figure 4. Optical characteristics of PNCs and underlying mechanisms. (a) Time-resolved PL spectra of LC-PNCs (red) and T-PNCs (black). X-ray photoelectron spectroscopy spectra of LC-PNCs (red circles, top) and T-PNCs (black squares, bottom) for (b) N 1s, (c) Br 3d, and (d) Pb 4f. The blue and purple solid lines are fitting curves, dashed lines guide the position of peaks, and arrows indicate shoulder peaks resulting from metallic Pb. For systematic comparison, the measurements in panels (a–d) are carried out with LC-PNCs and T-PNCs achieved after the same size purification processes, albeit the purification is not necessary for LC-PNCs (see the Experimental Section).

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    Figure 5

    Figure 5. Orientational coupling between LC molecules and ligands. Schematic image describing (a) LC-LARP and (b, c) molecular ordering of LCs on LC-PNCs. Schematic image describing (d) conventional LARP and (e, f) molecular ordering of LCs on T-PNCs. (b, e) Side-view illustration and (c, f) the corresponding optical micrographs of LC films on the substrates coated with a film of (b, c) LC-PNCs and (e,f) T-PNCs. For homogeneous films of PNCs, LC-PNCs and T-PNCs are used after the size purification processes (see the Experimental Section). The inset in panel (c) shows the conoscopic Maltese cross. The optical micrographs are observed between crossed-polarizers. “A” and “P” represent the analyzer and polarizer, respectively.

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    Figure 6

    Figure 6. Compatibility of LC-LARP with other synthetic techniques. Controlled shape transformation of LC-PNCs using different concentrations of n-octylamine (COAm): (a–d) transmission electron micrographs and (e) PL spectra from the LC-PNCs synthesized with COAm = 0.12 (a, red line in e), 0.13 (b, yellow line in e), 0.15 (c, blue line in e), and 0.24 mmol (d, purple line in e). (f) Schematic illustration and (g) the corresponding fluorescence micrograph for LC-LARP applied into the microfluidic platform. The fluorescence signal (green in panel (g)) indicates the synthesis of LC-PNCs. In situ modification on PL spectra of the resulting LC-PNCs by precisely controlling their size via (h) the flow rate of the precursor solution (h and Figure S7a,b) or (i) their shape via COAm in the precursor solution. The inset of panel (i) is the photograph of the LC-PNC solutions excited by 365 nm light. All measurements are done with LC-PNCs without any size purification processes.

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      Metal halides perovskites, such as hybrid org.-inorg. MeNH3PbI3, are newcomer optoelectronic materials that have attracted enormous attention as soln.-deposited absorbing layers in solar cells with power conversion efficiencies reaching 20%. A new avenue for halide perovskites was demonstrated by designing highly luminescent perovskite-based colloidal quantum dot materials. Monodisperse colloidal nanocubes (4-15 nm edge lengths) of fully inorg. perovskites (CsPbX3, X = Cl, Br, and I or mixed halide systems Cl/Br and Br/I) were synthesized using inexpensive com. precursors. Through compositional modulations and quantum size-effects, the bandgap energies and emission spectra are readily tunable over the entire visible spectral region of 410-700 nm. The luminescence of CsPbX3 nanocrystals is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color std., high quantum yields of ≤90%, and radiative lifetimes at 1-29 ns. The compelling combination of enhanced optical properties and chem. robustness makes CsPbX3 nanocrystals appealing for optoelectronic applications, particularly for blue and green spectral regions (410-530 nm), where typical metal chalcogenide-based quantum dots suffer from photodegrdn.
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      Zhang, F.; Zhong, H.; Chen, C.; Wu, X.-g.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y. Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano 2015, 9, 4533– 4542,  DOI: 10.1021/acsnano.5b01154
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      A ligand-assisted repptn. strategy to fabricate brightly luminescent and color-tunable colloidal CH3NH3PbX3 (X = Br, I, Cl) quantum dots with abs. quantum yield up to 70% at room temp. and low excitation fluencies is reported. To illustrate the photoluminescence enhancements in these quantum dots, comprehensive compn. and surface characterizations and time- and temp.-dependent photoluminescence spectra were studied. Comparisons between small-sized CH3NH3PbBr3 quantum dots (av. diam. 3.3 nm) and corresponding micrometer-sized bulk particles (2-8 μm) suggested that the intense increased photoluminescence quantum yield originates from the increase of exciton binding energy due to size redn. as well as proper chem. passivations of the Br-rich surface. Wide-color gamut white-light-emitting diodes using green emissive CH3NH3PbBr3 quantum dots and red emissive K2SiF6:Mn4+ as color converters were fabricated, providing enhanced color quality for display technol. The colloidal CH3NH3PbX3 quantum dots are expected to exhibit interesting nanoscale excitonic properties and also have other potential applications in lasers, electroluminescence devices, and optical sensors.
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      Li, X.; Wu, Y.; Zhang, S.; Cai, B.; Gu, Y.; Song, J.; Zeng, H. CsPbX3 Quantum Dots for Lighting and Displays: Room-Temperature Synthesis, Photoluminescence Superiorities, Underlying Origins and White Light-Emitting Diodes. Adv. Funct. Mater. 2016, 26, 2435– 2445,  DOI: 10.1002/adfm.201600109
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      Recently, Kovalenko and Li developed CsPbX3 (X = Cl, Br, I) inorg. perovskite quantum dots (IPQDs), which exhibited ultrahigh photoluminescence (PL) quantum yields (QYs), low-threshold lasing, and multicolor electroluminescence. However, the usual synthesis needs high temp., inert gas protection, and localized injection operation, which are severely against applications. Moreover, the so unexpectedly high QYs are very confusing. Here, for the first time, the IPQDs' room-temp. (RT) synthesis, superior PL, underlying origins and potentials in lighting and displays are reported. The synthesis is designed according to supersatd. recrystn. (SR), which is operated at RT, within few seconds, free from inert gas and injection operation. Although formed at RT, IPQDs' PLs have QYs of 80%, 95%, 70%, and FWHMs of 35, 20, and 18 nm for red, green, and blue emissions. As to the origins, the obsd. 40 meV exciton binding energy, halogen self-passivation effect, and CsPbX3@X quantum-well band alignment are proposed to guarantee the excitons generation and high-rate radiative recombination at RT. Moreover, such superior optical merits endow them with promising potentials in lighting and displays, which are primarily demonstrated by the white light-emitting diodes with tunable color temp. and wide color gamut.
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      A review. In the past decade, colloidal solns. were assumed to be very efficient templates for controlling particle size and shape. A large no. of groups used reverse micelles to control the size of spherical nanoparticles. This makes it possible to det. the various parameters involved in such processes, and demonstrates that nanoparticles can be considered to be efficient nanoreactors. However, some discrepancies arise. There are few reports concerning the control of particle shape, and it is still rather difficult to det. the key parameters, such as the adsorption of salts and other mols., and the synthesis procedure. Here, the authors discuss these controls of the size and shape of inorg. nanomaterials.
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      Small water droplets dispersed in a nematic liq. crystal exhibit a novel class of colloidal interactions, arising from the orientational elastic energy of the anisotropic host fluid. These interactions include a short-range repulsion and a long-range dipolar attraction, and they lead to the formation of anisotropic chainlike structures by the colloidal particles. The repulsive interaction can lead to novel mechanisms for colloid stabilization.
    10. 10
      Castles, F.; Day, F. V.; Morris, S. M.; Ko, D.-H.; Gardiner, D. J.; Qasim, M. M.; Nosheen, S.; Hands, P. J. W.; Choi, S. S.; Friend, R. H.; Coles, H. J. Blue-Phase Templated Fabrication of Three-Dimensional Nanostructures for Photonic Applications. Nat. Mater. 2012, 11, 599– 603,  DOI: 10.1038/nmat3330
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      Blue-phase templated fabrication of three-dimensional nanostructures for photonic applications
      Castles, F.; Day, F. V.; Morris, S. M.; Ko, D.-H.; Gardiner, D. J.; Qasim, M. M.; Nosheen, S.; Hands, P. J. W.; Choi, S. S.; Friend, R. H.; Coles, H. J.
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      A promising approach to the fabrication of materials with nanoscale features is the transfer of liq.-cryst. structure to polymers. However, this has not been achieved in systems with full three-dimensional periodicity. Here we demonstrate the fabrication of self-assembled three-dimensional nanostructures by polymer templating blue phase I, a chiral liq. crystal with cubic symmetry. Blue phase I was photopolymd. and the remaining liq. crystal removed to create a porous free-standing cast metals, which retains the chiral three-dimensional structure of the blue phase, yet contains no chiral additive mols. The cast metals may in turn be used as a hard template for the fabrication of new materials. By refilling the cast metals with an achiral nematic liq. crystal, we created templated blue phases that have unprecedented thermal stability in the range -125 to 125 °C, and that act as both mirrorless lasers and switchable electro-optic devices. Blue-phase templated materials will facilitate advances in device architectures for photonics applications in particular.
    11. 11
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      Wang, Xiaoguang; Miller, Daniel S.; Bukusoglu, Emre; de Pablo, Juan J.; Abbott, Nicholas L.
      Nature Materials (2016), 15 (1), 106-112CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Topol. defects in liq. crystals (LCs) have been widely used to organize colloidal dispersions and template polymn., leading to a range of assemblies, elastomers and gels. However, little is understood about mol.-level assembly processes within defects. Here, the authors report that nanoscopic environments defined by LC topol. defects can selectively trigger processes of mol. self-assembly. By using fluorescence microscopy, cryogenic TEM and super-resoln. optical microscopy, the authors obsd. signatures of mol. self-assembly of amphiphilic mols. in topol. defects, including cooperativity, reversibility and controlled growth. Also nanoscopic o-rings synthesized from Saturn-ring disclinations and other mol. assemblies templated by defects can be preserved by using photocrosslinkable amphiphiles. In analogy to other classes of macromol. templates such as polymer-surfactant complexes, topol. defects in LCs are a versatile class of 3-dimensional, dynamic and reconfigurable templates that can direct processes of mol. self-assembly.
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      Wang, X.; Kim, Y.-K.; Bukusoglu, E.; Zhang, B.; Miller, D. S.; Abbott, N. L. Experimental Insights into the Nanostructure of the Cores of Topological Defects in Liquid Crystals. Phys. Rev. Lett. 2016, 116, 147801  DOI: 10.1103/PhysRevLett.116.147801
      12
      Experimental insights into the nanostructure of the cores of topological defects in liquid crystals
      Wang, Xiaoguang; Kim, Young-Ki; Bukusoglu, Emre; Zhang, Bo; Miller, Daniel S.; Abbott, Nicholas L.
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      The nanoscopic structure of the cores of topol. defects in anisotropic condensed matter is an unresolved issue, although a no. of theor. predictions have been reported. In the exptl. study reported in this Letter, we template the assembly of amphiphilic mols. from the cores of defects in liq. crystals and thereby provide the first exptl. evidence that the cores of singular defects that appear optically to be points (with strength m = +1) are nanometer-sized closed-loop, disclination lines. We also analyze this result in the context of a model that describes the influence of amphiphilic assemblies on the free energy and stability of the defects. Overall, our exptl. results and theor. predictions reveal that the cores of defects with opposite strengths (e.g., m = +1 vs m = -1) differ in ways that profoundly influence processes of mol. self-assembly.
    13. 13
      Kim, Y.-K.; Wang, X.; Mondkar, P.; Bukusoglu, E.; Abbott, N. L. Self-Reporting and Self-Regulating Liquid Crystals. Nature 2018, 557, 539– 544,  DOI: 10.1038/s41586-018-0098-y
      13
      Self-reporting and self-regulating liquid crystals
      Kim, Young-Ki; Wang, Xiaoguang; Mondkar, Pranati; Bukusoglu, Emre; Abbott, Nicholas L.
      Nature (London, United Kingdom) (2018), 557 (7706), 539-544CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Liq. crystals (LCs) are anisotropic fluids that combine the long-range order of crystals with the mobility of liqs.1,2. This combination of properties has been widely used to create reconfigurable materials that optically report information about their environment, such as changes in elec. fields (smart-phone displays)3, temp. (thermometers)4 or mech. shear5, and the arrival of chem. and biol. stimuli (sensors)6,7. An unmet need exists, however, for responsive materials that not only report their environment but also transform it through self-regulated chem. interactions. A range of stimuli can trigger pulsatile (transient) or continuous release of microcargo (aq. microdroplets or solid microparticles and their chem. contents) that is trapped initially within LCs. The resulting LC materials self-report and self-regulate their chem. response to targeted phys., chem. and biol. events in ways that can be preprogrammed through an interplay of elastic, elec. double-layer, buoyant and shear forces in diverse geometries (such as wells, films and emulsion droplets). These LC materials can carry out complex functions that go beyond the capabilities of conventional materials used for controlled microcargo release, such as optically reporting a stimulus (for example, mech. shear stresses generated by motile bacteria) and then responding in a self-regulated manner via a feedback loop (for example, to release the min. amt. of biocidal agent required to cause bacterial cell death).
    14. 14
      Cheng, K. C. K.; Bedolla-Pantoja, M. A.; Kim, Y.-K.; Gregory, J. V.; Xie, F.; de France, A.; Hussal, C.; Sun, K.; Abbott, N. L.; Lahann, J. Templated Nanofiber Synthesis via Chemical Vapor Polymerization into Liquid Crystalline Films. Science 2018, 362, 804– 808,  DOI: 10.1126/science.aar8449
      14
      Templated nanofiber synthesis via chemical vapor polymerization into liquid crystalline films
      Cheng, Kenneth C. K.; Bedolla-Pantoja, Marco A.; Kim, Young-Ki; Gregory, Jason V.; Xie, Fan; de France, Alexander; Hussal, Christoph; Sun, Kai; Abbott, Nicholas L.; Lahann, Joerg
      Science (Washington, DC, United States) (2018), 362 (6416), 804-808CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Extrusion, electrospinning, and microdrawing are widely used to create fibrous polymer mats, but these approaches offer limited access to oriented arrays of nanometer-scale fibers with controlled size, shape, and lateral organization. We show that chem. vapor polymn. can be performed on surfaces coated with thin films of liq. crystals to synthesize organized assemblies of end-attached polymer nanofibers. The process uses low concns. of radical monomers formed initially in the vapor phase and then diffused into the liq.-crystal template. This minimizes monomer-induced changes to the liq.-crystal phase and enables access to nanofiber arrays with complex yet precisely defined structures and compns. The nanofiber arrays permit tailoring of a wide range of functional properties, including adhesion that depends on nanofiber chirality.
    15. 15
      Roh, S.; Kim, J.; Varadharajan, D.; Lahann, J.; Abbott, N. L. Sharing of Strain Between Nanofiber Forests and Liquid Crystals Leads to Programmable Responses to Electric Fields. Adv. Funct. Mater. 2022, 32, 2200830  DOI: 10.1002/adfm.202200830
      15
      Sharing of Strain Between Nanofiber Forests and Liquid Crystals Leads to Programmable Responses to Electric Fields
      Roh, Sangchul; Kim, John; Varadharajan, Divya; Lahann, Joerg; Abbott, Nicholas L.
      Advanced Functional Materials (2022), 32 (27), 2200830CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Fibers embedded in soft matrixes are widely encountered in biol. systems, with the fibers providing mech. reinforcement or encoding of instructions for shape changes. Here, the mech. coupling of end-attached polymeric nanofiber forests and liq. crystals (LCs) is explored, where the nanofibers are templated into prescribed shapes by the chem. vapor polymn. of paracyclophane-based monomers in supported films of the LCs. It is shown that the elastic energies of the nanofibers and LCs are comparable in magnitude, leading to reversible straining of nanofibers via the application of an elec. field to the LC. This coupling is shown to encode complex electrooptical responses in the LC (e.g., optical vortices), thus illustrating how LC-templated nanofiber forests offer the basis of fresh approaches for programming configurational changes in soft materials.
    16. 16
      Kim, W.-S.; Im, J.-H.; Kim, H.; Choi, J.-K.; Choi, Y.; Kim, Y.-K. Liquid Crystalline Systems from Nature and Interaction of Living Organisms with Liquid Crystals. Adv. Mater. 2023, 35, 2204275  DOI: 10.1002/adma.202204275
      16
      Liquid Crystalline Systems from Nature and Interaction of Living Organisms with Liquid Crystals
      Kim, Won-Sik; Im, Jun-Hyung; Kim, Hyein; Choi, Jin-Kang; Choi, Yena; Kim, Young-Ki
      Advanced Materials (Weinheim, Germany) (2023), 35 (4), 2204275CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Biomaterials, which are substances interacting with biol. systems, have been extensively explored to understand living organisms and obtain scientific inspiration (such as biomimetics). However, many aspects of biomaterials have yet to be fully understood. Because liq. cryst. phases are ubiquitously found in biomaterials (e.g., cholesterol, amphiphile, DNA, cellulose, bacteria), therefore, a wide range of research has made attempts to approach unresolved issues with the concept of liq. crystals (LCs). This review presents these studies that address the interactive correlation between biomaterials and LCs. Specifically, intrinsic LC behavior of various biomaterials such as DNA, cellulose nanocrystals, and bacteriaare first introduced. Second, the dynamics of bacteria in LC media are addressed, with focus on how bacteria interact with LCs, and how dynamics of bacteria can be controlled by exploiting the characteristics of LCs. Lastly, how the strong correlation between LCs and biomaterials has been leveraged to design a new class of biosensors with addnl. functionalities (e.g., self-regulated drug release) that are not available in previous systems is reviewed. Examples addressed in this review convey the message that the intersection between biomaterials and LCs offers deep insights into fundamental understanding of biomaterials, and provides resources for development of transformative technologies.
    17. 17
      Choi, Y.; Choi, D.; Choi, J.-K.; Oh, K.-S.; Cho, E.; Im, J.-H.; Singh, D. P.; Kim, Y.-K. Stimuli-Responsive Materials from Liquid Crystals. ACS Appl. Opt. Mater. 2023, 1, 1879– 1897,  DOI: 10.1021/acsaom.3c00282
      There is no corresponding record for this reference.
    18. 18
      Ryzhkova, A. V.; Muševič, I. Particle Size Effects on Nanocolloidal Interactions in Nematic Liquid Crystals. Phys. Rev. E 2013, 87, 032501  DOI: 10.1103/PhysRevE.87.032501
      18
      Particle size effects on nanocolloidal interactions in nematic liquid crystals
      Ryzhkova, A. V.; Musevic, I.
      Physical Review E: Statistical, Nonlinear, and Soft Matter Physics (2013), 87 (3-B), 032501/1-032501/12CODEN: PRESCM; ISSN:1539-3755. (American Physical Society)
      We study the interactions of submicrometer diam. silica particles, surface functionalized with DMOAP (N,N-dimethyl-n-octadecyl-3-aminopropyl-trimethoxysilyl chloride), in the nematic liq. crystal 5CB (pentylcyanobiphenil). Using the methods of video-tracking dark-field microscopy, we have measured the pair-binding energy of 35- to 450-nm-diam. silica particles, which is in the range between 100 and 1000 kBT. It is therefore high enough for the formation of thermally stable nanocolloidal pairs of 35 nm diam. We find that smaller colloids with the diam. around 22 nm do not form thermally stable pairs, which seems to be currently the lower limit for nanocolloidal assembly in the nematic liq. crystals. We also study the particle interactions with point and Saturn-ring defects and discuss the possibility of hierarchical structures generated by particles of different sizes assembled by topol. defects.
    19. 19
      Shields, C. W.; Kim, Y.-K.; Han, K.; Murphy, A. C.; Scott, A. J.; Abbott, N. L.; Velev, O. D. Control of the Folding Dynamics of Self-Reconfiguring Magnetic Microbots Using Liquid Crystallinity. Adv. Intell. Syst. 2020, 2, 1900114  DOI: 10.1002/aisy.201900114
      There is no corresponding record for this reference.
    20. 20
      Lin, I.-H.; Miller, D. S.; Bertics, P. J.; Murphy, C. J.; de Pablo, J. J.; Abbott, N. L. Endotoxin-Induced Structural Transformations in Liquid Crystalline Droplets. Science 2011, 332, 1297– 1300,  DOI: 10.1126/science.1195639
      20
      Endotoxin-Induced Structural Transformations in Liquid Crystalline Droplets
      Lin, I.-Hsin; Miller, Daniel S.; Bertics, Paul J.; Murphy, Christopher J.; de Pablo, Juan J.; Abbott, Nicholas L.
      Science (Washington, DC, United States) (2011), 332 (6035), 1297-1300CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The ordering of liq. crystals (LCs) is known to be influenced by surfaces and contaminants. Here, we report that picogram per mL concns. of endotoxin in water trigger ordering transitions in micrometer-size LC droplets. The ordering transitions, which occur at surface concns. of endotoxin that are less than 10-5 Langmuir, are not due to adsorbate-induced changes in the interfacial energy of the LC. The sensitivity of the LC to endotoxin was measured to change by six orders of magnitude with the geometry of the LC (droplet vs. slab), supporting the hypothesis that interactions of endotoxin with topol. defects in the LC mediate the response of the droplets. The LC ordering transitions depend strongly on glycophospholipid structure and provide new designs for responsive soft matter.
    21. 21
      Hassan, Y.; Park, J. H.; Crawford, M. L.; Sadhanala, A.; Lee, J.; Sadighian, J. C.; Mosconi, E.; Shivanna, R.; Radicchi, E.; Jeong, M.; Yang, C.; Choi, H.; Park, S. H.; Song, M. H.; De Angelis, F.; Wong, C. Y.; Friend, R. H.; Lee, B. R.; Snaith, H. J. Ligand-Engineered Bandgap Stability in Mixed-Halide Perovskite LEDs. Nature 2021, 591, 72– 77,  DOI: 10.1038/s41586-021-03217-8
      21
      Ligand-engineered bandgap stability in mixed-halide perovskite LEDs
      Hassan, Yasser; Park, Jong Hyun; Crawford, Michael L.; Sadhanala, Aditya; Lee, Jeongjae; Sadighian, James C.; Mosconi, Edoardo; Shivanna, Ravichandran; Radicchi, Eros; Jeong, Mingyu; Yang, Changduk; Choi, Hyosung; Park, Sung Heum; Song, Myoung Hoon; De Angelis, Filippo; Wong, Cathy Y.; Friend, Richard H.; Lee, Bo Ram; Snaith, Henry J.
      Nature (London, United Kingdom) (2021), 591 (7848), 72-77CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Abstr.: Lead halide perovskites are promising semiconductors for light-emitting applications because they exhibit bright, bandgap-tunable luminescence with high color purity1,2. Photoluminescence quantum yields close to unity have been achieved for perovskite nanocrystals across a broad range of emission colors, and light-emitting diodes with external quantum efficiencies exceeding 20 per cent-approaching those of com. org. light-emitting diodes-have been demonstrated in both the IR and the green emission channels1,3,4. However, owing to the formation of lower-bandgap iodide-rich domains, efficient and color-stable red electroluminescence from mixed-halide perovskites has not yet been realized5,6. Here we report the treatment of mixed-halide perovskite nanocrystals with multidentate ligands to suppress halide segregation under electroluminescent operation. We demonstrate color-stable, red emission centered at 620 nm, with an electroluminescence external quantum efficiency of 20.3 per cent. We show that a key function of the ligand treatment is to 'clean' the nanocrystal surface through the removal of lead atoms. D. functional theory calcns. reveal that the binding between the ligands and the nanocrystal surface suppresses the formation of iodine Frenkel defects, which in turn inhibits halide segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)cryst. surface and presents a route through which to control the formation and migration of surface defects. This is crit. to achieve bandgap stability for light emission and could also have a broader impact on other optoelectronic applications-such as photovoltaics-for which bandgap stability is required.
    22. 22
      Kim, Y.-H.; Park, J.; Kim, S.; Kim, J. S.; Xu, H.; Jeong, S.-H.; Hu, B.; Lee, T.-W. Exploiting the Full Advantages of Colloidal Perovskite Nanocrystals for Large-Area Efficient Light-Emitting Diodes. Nat. Nanotechnol. 2022, 17, 590– 597,  DOI: 10.1038/s41565-022-01113-4
      22
      Exploiting the full advantages of colloidal perovskite nanocrystals for large-area efficient light-emitting diodes
      Kim, Young-Hoon; Park, Jinwoo; Kim, Sungjin; Kim, Joo Sung; Xu, Hengxing; Jeong, Su-Hun; Hu, Bin; Lee, Tae-Woo
      Nature Nanotechnology (2022), 17 (6), 590-597CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
      Cost-effective, high-throughput industrial applications of metal halide perovskites in large-area displays are hampered by the fundamental difficulty of controlling the process of polycryst. film formation from precursors, which results in the random growth of crystals, leading to non-uniform large grains and thus low electroluminescence efficiency in large-area perovskite light-emitting diodes (PeLEDs). Here we report that highly efficient large-area PeLEDs with high uniformity can be realized through the use of colloidal perovskite nanocrystals (PNCs), decoupling the crystn. of perovskites from film formation. PNCs were precrystd. and surrounded by org. ligands, and thus they were not affected by the film formation process, in which a simple modified bar-coating method facilitated the evapn. of residual solvent to provide uniform large-area films. PeLEDs incorporating the uniform bar-coated PNC films achieved an external quantum efficiency (EQE) of 23.26% for a pixel size of 4 mm2 and an EQE of 22.5% for a large pixel area of 102 mm2 with high reproducibility. This method provides a promising approach towards the development of large-scale industrial displays and solid-state lighting using perovskite emitters.
    23. 23
      Huang, H.; Raith, J.; Kershaw, S. V.; Kalytchuk, S.; Tomanec, O.; Jing, L.; Susha, A. S.; Zboril, R.; Rogach, A. L. Growth Mechanism of Strongly Emitting CH3NH3PbBr3 Perovskite Nanocrystals with a Tunable Bandgap. Nat. Commun. 2017, 8, 996  DOI: 10.1038/s41467-017-00929-2
      23
      Growth mechanism of strongly emitting CH3NH3PbBr3 perovskite nanocrystals with a tunable bandgap
      Huang He; Raith Johannes; Kershaw Stephen V; Susha Andrei S; Rogach Andrey L; Kalytchuk Sergii; Tomanec Ondrej; Zboril Radek; Jing Lihong
      Nature communications (2017), 8 (1), 996 ISSN:.
      Metal halide perovskite nanocrystals are promising materials for a diverse range of applications, such as light-emitting devices and photodetectors. We demonstrate the bandgap tunability of strongly emitting CH3NH3PbBr3 nanocrystals synthesized at both room and elevated (60 °C) temperature through the variation of the precursor and ligand concentrations. We discuss in detail the role of two ligands, oleylamine and oleic acid, in terms of the coordination of the lead precursors and the nanocrystal surface. The growth mechanism of nanocrystals is elucidated by combining the experimental results with the principles of nucleation/growth models. The proposed formation mechanism of perovskite nanocrystals will be helpful for further studies in this field and can be used as a guide to improve the synthetic methods in the future.The development of perovskite nanocrystals is limited by poor mechanistic understanding of their growth. Here, the authors systematically study the ligand-assisted reprecipitation synthesis of CH3NH3PbBr3 nanocrystals, revealing the effect of precursor and ligand concentrations on bandgap tunability.
    24. 24
      Wang, H.; Wu, T. X.; Gauza, S.; Wu, J. R.; Wu, S.-T. A Method to Estimate the Leslie Coefficients of Liquid Crystals Based on MBBA Data. Liq. Cryst. 2006, 33, 91– 98,  DOI: 10.1080/02678290500446111
      There is no corresponding record for this reference.
    25. 25
      Wang, T.; Zhuang, J.; Lynch, J.; Chen, O.; Wang, Z.; Wang, X.; LaMontagne, D.; Wu, H.; Wang, Z.; Cao, Y. C. Self-Assembled Colloidal Superparticles from Nanorods. Science 2012, 338, 358– 363,  DOI: 10.1126/science.1224221
      25
      Self-Assembled Colloidal Superparticles from Nanorods
      Wang, Tie; Zhuang, Jiaqi; Lynch, Jared; Chen, Ou; Wang, Zhongliang; Wang, Xirui; LaMontagne, Derek; Wu, Huimeng; Wang, Zhongwu; Cao, Y. Charles
      Science (Washington, DC, United States) (2012), 338 (6105), 358-363CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Colloidal superparticles are nanoparticle assemblies in the form of colloidal particles. The assembly of nanoscopic objects into mesoscopic or macroscopic complex architectures allows bottom-up fabrication of functional materials. We report that the self-assembly of cadmium selenide-cadmium sulfide (CdSe-CdS) core-shell semiconductor nanorods, mediated by shape and structural anisotropy, produces mesoscopic colloidal superparticles having multiple well-defined supercryst. domains. Moreover, functionality-based anisotropic interactions between these CdSe-CdS nanorods can be kinetically introduced during the self-assembly and, in turn, yield single-domain, needle-like superparticles with parallel alignment of constituent nanorods. Unidirectional patterning of these mesoscopic needle-like superparticles gives rise to the lateral alignment of CdSe-CdS nanorods into macroscopic, uniform, freestanding polymer films that exhibit strong photoluminescence with a striking anisotropy, enabling their use as downconversion phosphors to create polarized light-emitting diodes.
    26. 26
      Giuntini, D.; Zhao, S.; Krekeler, T.; Li, M.; Blankenburg, M.; Bor, B.; Schaan, G.; Domènech, B.; Müller, M.; Scheider, I.; Ritter, M.; Schneider, G. A. Defects and Plasticity in Ultrastrong Supercrystalline Nanocomposites. Sci. Adv. 2021, 7, eabb6063  DOI: 10.1126/sciadv.abb6063
      There is no corresponding record for this reference.
    27. 27
      Faetti, S.; Gatti, M.; Palleschi, V.; Sluckin, T. J. Almost Critical Behavior of the Anchoring Energy at the Interface between a Nematic Liquid Crystal and a SiO Substrate. Phys. Rev. Lett. 1985, 55, 1681– 1684,  DOI: 10.1103/PhysRevLett.55.1681
      27
      Almost critical behavior of the anchoring energy at the interface between a nematic liquid crystal and a silicon monoxide substrate
      Faetti, Sandro; Gatti, Marta; Palleschi, Vincenzo; Sluckin, Timothy J.
      Physical Review Letters (1985), 55 (16), 1681-4CODEN: PRLTAO; ISSN:0031-9007.
      The anchoring energy at the interface between the nematic liq. crystal 4-pentyl-4'-cyanobiphenyl (5CB) and a glass plate treated by oblique evapn. of SiO was studied by measuring the torque exerted on the surface by the nematic subject to an orienting magnetic field. The anchoring energy is sharply reduced as the clearing temp. is approached. The expt. provides the 1st direct evidence of a strong redn. of the surface order parameter for this system in the anisotropic phase.
    28. 28
      Seth, S.; Samanta, A. A Facile Methodology for Engineering the Morphology of CsPbX3 Perovskite Nanocrystals under Ambient Condition. Sci. Rep. 2016, 6, 37693  DOI: 10.1038/srep37693
      28
      A Facile Methodology for Engineering the Morphology of CsPbX3 Perovskite Nanocrystals under Ambient Condition
      Seth, Sudipta; Samanta, Anunay
      Scientific Reports (2016), 6 (), 37693CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      A facile and highly reproducible room temp., open atm. synthesis of cesium lead halide perovskite nanocrystals of six different morphologies is reported just by varying the solvent, ligand and reaction time. Sequential evolution of the quantum dots, nanoplates and nanobars in one medium and nanocubes, nanorods and nanowires in another medium is demonstrated. These perovskite nanoparticles are shown to be of excellent cryst. quality with high fluorescence quantum yield. A mechanism of the formation of nanoparticles of different shapes and sizes is proposed. Considering the key role of morphol. in nanotechnol., this simple method of fabrication of a wide range of high quality nanocrystals of different shapes and sizes of all-inorg. lead halide perovskites, whose potential is already demonstrated in light emitting and photovoltaic applications, is likely to help widening the scope and utility of these materials in optoelectronic devices.
    29. 29
      Lapointe, C. P.; Mason, T. G.; Smalyukh, I. I. Shape-Controlled Colloidal Interactions in Nematic Liquid Crystals. Science 2009, 326, 1083– 1086,  DOI: 10.1126/science.1176587
      29
      Shape-Controlled Colloidal Interactions in Nematic Liquid Crystals
      Lapointe, Clayton P.; Mason, Thomas G.; Smalyukh, Ivan I.
      Science (Washington, DC, United States) (2009), 326 (5956), 1083-1086CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Robust control over the positions, orientations, and assembly of nonspherical colloids may aid in the creation of new types of structured composite materials that are important from both technol. and fundamental standpoints. With the use of lithog. fabricated equilateral polygonal platelets, we demonstrate that colloidal interactions and self-assembly in anisotropic nematic fluids can be effectively tailored via control over the particles' shapes. The particles disturb the uniform alignment of the surrounding nematic host, resulting in both a distinct equil. alignment and highly directional pair interactions. Interparticle forces between polygonal platelets exhibit either dipolar or quadrupolar symmetries, depending on whether their no. of sides is odd or even, and drive the assembly of a no. of ensuing self-assembled colloidal structures.
    30. 30
      Senyuk, B.; Liu, Q.; Nystrom, P. D.; Smalyukh, I. I. Repulsion–Attraction Switching of Nematic Colloids Formed by Liquid Crystal Dispersions of Polygonal Prisms. Soft Matter 2017, 13, 7398– 7405,  DOI: 10.1039/C7SM01186E
      30
      Repulsion-attraction switching of nematic colloids formed by liquid crystal dispersions of polygonal prisms
      Senyuk, B.; Liu, Q.; Nystrom, P. D.; Smalyukh, I. I.
      Soft Matter (2017), 13 (40), 7398-7405CODEN: SMOABF; ISSN:1744-6848. (Royal Society of Chemistry)
      Self-assembly of colloidal particles due to elastic interactions in nematic liq. crystals promises tunable composite materials and can be guided by exploiting surface functionalization, geometric shape and topol., though these means of controlling self-assembly remain limited. Here, we realize low-symmetry achiral and chiral elastic colloids in the nematic liq. crystals using colloidal polygonal concave and convex prisms. We show that the controlled pinning of disclinations at the prism edges alters the symmetry of director distortions around the prisms and their orientation with respect to the far-field director. The controlled localization of the disclinations at the prism's edges significantly influences the anisotropy of the diffusion properties of prisms dispersed in liq. crystals and allows one to modify their self-assembly. We show that elastic interactions between polygonal prisms can be switched between repulsive and attractive just by controlled re-pinning the disclinations at different edges using laser tweezers. Our findings demonstrate that elastic interactions between colloidal particles dispersed in nematic liq. crystals are sensitive to the topol. equiv. but geometrically rich controlled configurations of the particle-induced defects.
    31. 31
      Zhang, M.; Yu, H.; Lyu, M.; Wang, Q.; Yun, J.-H.; Wang, L. Composition-Dependent Photoluminescence Intensity and Prolonged Recombination Lifetime of Perovskite CH3NH3PbBr3–xClx Films. Chem. Commun. 2014, 50, 11727– 11730,  DOI: 10.1039/C4CC04973J
      31
      Composition-dependent photoluminescence intensity and prolonged recombination lifetime of perovskite CH3NH3PbBr3-xClx films
      Zhang, Meng; Yu, Hua; Lyu, Miaoqiang; Wang, Qiong; Yun, Jung-Ho; Wang, Lianzhou
      Chemical Communications (Cambridge, United Kingdom) (2014), 50 (79), 11727-11730CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Mixed halide perovskites CH3NH3PbBr3-xClx (x = 0.6-1.2) with different compns. of halogens exhibit drastically changed optical properties. In particular, the thin films prepd. with these perovskites demonstrate extraordinary photoluminescence emission intensities and prolonged recombination lifetimes up to 446 ns, which are desirable for light emitting and photovoltaic applications.
    32. 32
      Cho, H.; Jeong, S.-H.; Park, M.-H.; Kim, Y.-H.; Wolf, C.; Lee, C.-L.; Heo, J. H.; Sadhanala, A.; Myoung, N.; Yoo, S.; Im, S. H.; Friend, R. H.; Lee, T.-W. Overcoming the Electroluminescence Efficiency Limitations of Perovskite Light-Emitting Diodes. Science 2015, 350, 1222– 1225,  DOI: 10.1126/science.aad1818
      32
      Overcoming the electroluminescence efficiency limitations of perovskite light-emitting diodes
      Cho, Himchan; Jeong, Su-Hun; Park, Min-Ho; Kim, Young-Hoon; Wolf, Christoph; Lee, Chang-Lyoul; Heo, Jin Hyuck; Sadhanala, Aditya; Myoung, NoSoung; Yoo, Seunghyup; Im, Sang Hyuk; Friend, Richard H.; Lee, Tae-Woo
      Science (Washington, DC, United States) (2015), 350 (6265), 1222-1225CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Org.-inorg. hybrid perovskites are emerging low-cost emitters with very high color purity, but their low luminescent efficiency is a crit. drawback. We boosted the current efficiency (CE) of perovskite light-emitting diodes with a simple bilayer structure to 42.9 candela per A, similar to the CE of phosphorescent org. light-emitting diodes OLED , with two modifications: We prevented the formation of metallic lead (Pb) atoms that cause strong exciton quenching through a small increase in methylammonium bromide (MABr) molar proportion, and we spatially confined the exciton in uniform MAPbBr3 nanograins (av. diam. = 99.7 nm) formed by a nanocrystal pinning process and concomitant redn. of exciton diffusion length to 67 nm. These changes caused substantial increases in steady-state photoluminescence intensity and efficiency of MAPbBr3 nanograin layers.
    33. 33
      Jang, H. M.; Kim, J.-S.; Heo, J.-M.; Lee, T.-W. Enhancing Photoluminescence Quantum Efficiency of Metal Halide Perovskites by Examining Luminescence-Limiting Factors. APL Mater. 2020, 8, 020904  DOI: 10.1063/1.5136308
      33
      Enhancing photoluminescence quantum efficiency of metal halide perovskites by examining luminescence-limiting factors
      Jang, Hyun Myung; Kim, Joo-Sung; Heo, Jung-Min; Lee, Tae-Woo
      APL Materials (2020), 8 (2), 020904CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      A review. Metal halide perovskites (MHPs) show superior optoelectronic properties, which give them the great potential for use in next generation light-emitting diodes (LEDs). In particular, their narrow emission linewidths can achieve ultrahigh color purity. However, the reported luminescence efficiency (LE) values are not high enough to be commercialized in displays and solid-state lightings. Moreover, the operational stability of LEDs assocd. with the overshooting of luminance and the high relative std. deviation of reported external quantum efficiencies are still problematic. In this perspective, we review photophys. factors that limit the photoluminescence quantum efficiency of perovskite-based LEDs. These factors are categorized into (i) weak exciton binding, (ii) nonradiative recombinations, (iii) slow cooling of long-lived hot carriers, (iv) deep-level defects, and (v) interband transition rates. We then present various physicochem. methods to effectively overcome these luminescence-limiting factors. We finally suggest some useful research directions to further improve the LE of MHP emitters as core components in displays and solid-state lightings. (c) 2020 American Institute of Physics.
    34. 34
      Smock, S. R.; Williams, T. J.; Brutchey, R. L. Quantifying the Thermodynamics of Ligand Binding to CsPbBr3 Quantum Dots. Angew. Chem., Int. Ed. 2018, 57, 11711– 11715,  DOI: 10.1002/anie.201806916
      34
      Quantifying the Thermodynamics of Ligand Binding to CsPbBr3 Quantum Dots
      Smock, Sara R.; Williams, Travis J.; Brutchey, Richard L.
      Angewandte Chemie, International Edition (2018), 57 (36), 11711-11715CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Cesium lead halide perovskites are an emerging class of quantum dots (QDs) that have shown promise in a variety of applications; however, their properties are highly dependent on their surface chem. To this point, the thermodn. of ligand binding remain unstudied. Herein, 1H NMR methods were used to quantify the thermodn. of ligand exchange on CsPbBr3 QDs. Both oleic acid and oleylamine native ligands dynamically interact with the CsPbBr3 QD surface, having individual surface densities of 1.2-1.7 nm-2. 10-Undecenoic acid undergoes an exergonic exchange equil. with bound oleate (Keq = 1.97) at 25 °C while 10-undecenylphosphonic acid undergoes irreversible ligand exchange. Undec-10-en-1-amine exergonically exchanges with oleylamine (Keq = 2.52) at 25 °C. Exchange occurs with carboxylic acids, phosphonic acids, and amines on CsPbBr3 QDs without etching of the nanocrystal surface; increases in the steady-state PL intensities correlate with more strongly bound conjugate base ligands.
    35. 35
      Almeida, G.; Infante, I.; Manna, L. Resurfacing Halide Perovskite Nanocrystals. Science 2019, 364, 833– 834,  DOI: 10.1126/science.aax5825
      35
      Resurfacing halide perovskite nanocrystals
      Almeida, Guilherme; Infante, Ivan; Manna, Liberato
      Science (Washington, DC, United States) (2019), 364 (6443), 833-834CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Metal halide perovskite semiconductors are ionic compds. with the formula AMX3 (A and M are cations, and X can be Cl-, Br-, I-, or binary mixts. of these anions). In the form of colloidal nanocrystals, these materials have extraordinary potential as light emitters. Not only do they exhibit high photoluminescence quantum yields (PLQYs), but the emission color can be finely tuned across the entire visible spectrum by changing the proportions of mixed halide anions (). However, the surface chem. of these nanocrystals makes them susceptible to degrdn. and long-term instability, and the surface can introduce surface centers (midgap states) that promote nonradiative recombination of charge carriers that lower PLQYs. Thus, the characterization of the interface between the perovskite nanocrystals and the org. ligands is fundamental to developing strategies to control surface defects, tuning the opto-electronic properties, and improving device performance and stability.
    36. 36
      Kong, L.; Zhang, X.; Li, Y.; Wang, H.; Jiang, Y.; Wang, S.; You, M.; Zhang, C.; Zhang, T.; Kershaw, S. V.; Zheng, W.; Yang, Y.; Lin, Q.; Yuan, M.; Rogach, A. L.; Yang, X. Smoothing the Energy Transfer Pathway in Quasi-2D Perovskite Films Using Methanesulfonate Leads to Highly Efficient Light-Emitting Devices. Nat. Commun. 2021, 12, 1246  DOI: 10.1038/s41467-021-21522-8
      36
      Smoothing the energy transfer pathway in quasi-2D perovskite films using methanesulfonate leads to highly efficient light-emitting devices
      Kong, Lingmei; Zhang, Xiaoyu; Li, Yunguo; Wang, Haoran; Jiang, Yuanzhi; Wang, Sheng; You, Mengqing; Zhang, Chengxi; Zhang, Ting; Kershaw, Stephen V.; Zheng, Weitao; Yang, Yingguo; Lin, Qianqian; Yuan, Mingjian; Rogach, Andrey L.; Yang, Xuyong
      Nature Communications (2021), 12 (1), 1246CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Quasi-two-dimensional (quasi-2D) Ruddlesden-Popper (RP) perovskites such as BA2Csn-1PbnBr3n+1 (BA = butylammonium, n > 1) are promising emitters, but their electroluminescence performance is limited by a severe non-radiative recombination during the energy transfer process. Here, we make use of methanesulfonate (MeS) that can interact with the spacer BA cations via strong hydrogen bonding interaction to reconstruct the quasi-2D perovskite structure, which increases the energy acceptor-to-donor ratio and enhances the energy transfer in perovskite films, thus improving the light emission efficiency. MeS additives also lower the defect d. in RP perovskites, which is due to the elimination of uncoordinated Pb2+ by the electron-rich Lewis base MeS and the weakened adsorbate blocking effect. As a result, green light-emitting diodes fabricated using these quasi-2D RP perovskite films reach current efficiency of 63 cd A-1 and 20.5% external quantum efficiency, which are the best reported performance for devices based on quasi-2D perovskites so far.
    37. 37
      Brake, J. M.; Mezera, A. D.; Abbott, N. L. Effect of Surfactant Structure on the Orientation of Liquid Crystals at Aqueous–Liquid Crystal Interfaces. Langmuir 2003, 19, 6436– 6442,  DOI: 10.1021/la034132s
      37
      Effect of Surfactant Structure on the Orientation of Liquid Crystals at Aqueous-Liquid Crystal Interfaces
      Brake, Jeffrey M.; Mezera, Andrew D.; Abbott, Nicholas L.
      Langmuir (2003), 19 (16), 6436-6442CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      It is known that the orientations assumed by thermotropic liq. crystals (LCs) in contact with water are sensitive to the types and concns. of surfactants and/or polymers present in the aq. phase. This work expands upon these past observations by developing criteria for surfactants that give rise to a particular orientation of a contacting nematic LC formed formed from 4'-pentyl-4-cyanobiphenyl (5CB). We observe surfactants that have a bolaform structure ((11-hydroxyundecyl)trimethylammonium bromide (HTAB), dodecyl-1,12-bis(trimethylammonium bromide) (DBTAB), 11-(ferrocenylundecyl)trimethylammonium bromide (FTMA)) and which adopt looped configurations at air-water/oil-water interfaces cause planar anchoring of 5CB. In contrast, classical surfactants (alkyltrimethylammonium halides (CnTABs, n > 8), sodium dodecyl sulfate (SDS), and N,N-dimethylferrocenylalkylammonium bromides (FCnABs, n > 12)) that assume tilted orientations at air-water/oil-water interfaces can give rise to a homeotropic orientation of 5CB. By comparing SDS, dodecyl trimethylammonium halide (DTAB), and tetra(ethylene glycol) monododecyl ether (C12E4), we conclude that the nature of these headgroups does not measurably influence the orientation of the LC. However, the orientation of the LC is found to depend on the aliph. chain length and the areal d. of the adsorbed surfactant. When using surfactants with short alkyl chain lengths (n = 8 for CnTAB and n = 7 and 12 for FCnAB), we observe the orientation of 5CB to remain parallel to the interface up to concns. at which the 5CB begins to be solubilized by the surfactant. These results, when combined, lead us to conclude that interactions between the aliph. chains of the surfactant and 5CB, which are influenced by the conformation of the surfactant, largely dictate the orientation of the 5CB.
    38. 38
      Sichert, J. A.; Tong, Y.; Mutz, N.; Vollmer, M.; Fischer, S.; Milowska, K. Z.; García Cortadella, R.; Nickel, B.; Cardenas-Daw, C.; Stolarczyk, J. K.; Urban, A. S.; Feldmann, J. Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets. Nano Lett. 2015, 15, 6521– 6527,  DOI: 10.1021/acs.nanolett.5b02985
      38
      Quantum size effect in organometal halide perovskite nanoplatelets
      Sichert, Jasmina A.; Tong, Yu; Mutz, Niklas; Vollmer, Mathias; Fischer, Stefan; Milowska, Karolina Z.; Garcia-Cortadella, Ramon; Nickel, Bert; Cardenas-Daw, Carlos; Stolarczyk, Jacek K.; Urban, Alexander S.; Feldmann, Jochen
      Nano Letters (2015), 15 (10), 6521-6527CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Organometal halide perovskites have recently emerged displaying a huge potential for not only photovoltaic, but also light-emitting applications. Exploiting the optical properties of specifically tailored perovskite nanocrystals could greatly enhance the efficiency and functionality of applications based on this material. In this study, we investigate the quantum size effect in colloidal organometal halide perovskite nanoplatelets. By tuning the ratio of the org. cations used, we can control the thickness and consequently the photoluminescence emission of the platelets. Quantum mech. calcns. match well with the exptl. values. We find that not only do the properties of the perovskite, but also those of the org. ligands play an important role. Stacking of nanoplatelets leads to the formation of minibands, further shifting the bandgap energies. In addn., we find a large exciton binding energy of up to several hundreds of meV for nanoplatelets thinner than three unit cells, partially counteracting the blueshift induced by quantum confinement. Understanding of the quantum size effects in perovskite nanoplatelets and the ability to tune them provide an addnl. method with which to manipulate the optical properties of organometal halide perovskites.
    39. 39
      Almeida, G.; Goldoni, L.; Akkerman, Q.; Dang, Z.; Khan, A. H.; Marras, S.; Moreels, I.; Manna, L. Role of Acid–Base Equilibria in the Size, Shape, and Phase Control of Cesium Lead Bromide Nanocrystals. ACS Nano 2018, 12, 1704– 1711,  DOI: 10.1021/acsnano.7b08357
      39
      Role of Acid-Base Equilibria in the Size, Shape, and Phase Control of Cesium Lead Bromide Nanocrystals
      Almeida, Guilherme; Goldoni, Luca; Akkerman, Quinten; Dang, Zhiya; Khan, Ali Hossain; Marras, Sergio; Moreels, Iwan; Manna, Liberato
      ACS Nano (2018), 12 (2), 1704-1711CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A binary ligand system composed of aliph. carboxylic acids and primary amines of various chain lengths is commonly employed in diverse synthesis methods for CsPbBr3 nanocrystals (NCs). The authors have carried out a systematic study examg. how the concn. of ligands (oleylamine and oleic acid) and the resulting acidity (or basicity) affects the hot-injection synthesis of CsPbBr3 NCs. The authors devise a general synthesis scheme for cesium lead bromide NCs which allows control over size, size distribution, shape, and phase (CsPbBr3 or Cs4PbBr6) by combining key insights on the acid-base interactions that rule this ligand system. Also, the authors' findings shed light upon the soly. of PbBr2 in this binary ligand system, and plausible mechanisms are suggested to understand the ligand-mediated phase control and structural stability of CsPbBr3 NCs.
    40. 40
      Lignos, I.; Stavrakis, S.; Nedelcu, G.; Protesescu, L.; deMello, A. J.; Kovalenko, M. V. Synthesis of Cesium Lead Halide Perovskite Nanocrystals in a Droplet-Based Microfluidic Platform: Fast Parametric Space Mapping. Nano Lett. 2016, 16, 1869– 1877,  DOI: 10.1021/acs.nanolett.5b04981
      40
      Synthesis of Cesium Lead Halide Perovskite Nanocrystals in a Droplet-Based Microfluidic Platform: Fast Parametric Space Mapping
      Lignos, Ioannis; Stavrakis, Stavros; Nedelcu, Georgian; Protesescu, Loredana; de Mello, Andrew J.; Kovalenko, Maksym V.
      Nano Letters (2016), 16 (3), 1869-1877CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Prior to this work, fully inorg. nanocrystals of cesium lead halide perovskite (CsPbX3, X = Br, I, Cl and Cl/Br and Br/I mixed halide systems), exhibiting bright and tunable photoluminescence, have been synthesized using conventional batch (flask-based) reactions. Unfortunately, our understanding of the parameters governing the formation of these nanocrystals is still very limited due to extremely fast reaction kinetics and multiple variables involved in ion-metathesis-based synthesis of such multinary halide systems. Herein, we report the use of a droplet-based microfluidic platform for the synthesis of CsPbX3 nanocrystals. The combination of online photoluminescence and absorption measurements and the fast mixing of reagents within such a platform allows the rigorous and rapid mapping of the reaction parameters, including molar ratios of Cs, Pb, and halide precursors, reaction temps., and reaction times. This translates into enormous savings in reagent usage and screening times when compared to analogous batch synthetic approaches. The early-stage insight into the mechanism of nucleation of metal halide nanocrystals suggests similarities with multinary metal chalcogenide systems, albeit with much faster reaction kinetics in the case of halides. Furthermore, we show that microfluidics-optimized synthesis parameters are also directly transferrable to the conventional flask-based reaction.
    41. 41
      Lee, H.-D.; Woo, S.-J.; Kim, S.; Kim, J.; Zhou, H.; Han, S. J.; Jang, K. Y.; Kim, D.-H.; Park, J.; Yoo, S.; Lee, T.-W. Valley-Centre Tandem Perovskite Light-Emitting Diodes. Nat. Nanotechnol. 2024, 19, 624– 631,  DOI: 10.1038/s41565-023-01581-2
      There is no corresponding record for this reference.
    42. 42
      Aqoma, H.; Lee, S.-H.; Imran, I. F.; Hwang, J.-H.; Lee, S.-H.; Jang, S.-Y. Alkyl Ammonium Iodide-Based Ligand Exchange Strategy for High-Efficiency Organic-Cation Perovskite Quantum Dot Solar Cells. Nat. Energy 2024, 9, 324– 332,  DOI: 10.1038/s41560-024-01450-9
      There is no corresponding record for this reference.
    43. 43
      Rainò, G.; Becker, M. A.; Bodnarchuk, M. I.; Mahrt, R. F.; Kovalenko, M. V.; Stöferle, T. Superfluorescence from Lead Halide Perovskite Quantum Dot Superlattices. Nature 2018, 563, 671– 675,  DOI: 10.1038/s41586-018-0683-0
      43
      Superfluorescence from lead halide perovskite quantum dot superlattices
      Raino, Gabriele; Becker, Michael A.; Bodnarchuk, Maryna I.; Mahrt, Rainer F.; Kovalenko, Maksym V.; Stoferle, Thilo
      Nature (London, United Kingdom) (2018), 563 (7733), 671-675CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      An ensemble of emitters can behave very differently from its individual constituents when they interact coherently via a common light field. After excitation of such an ensemble, collective coupling can give rise to a many-body quantum phenomenon that results in short, intense bursts of light-so-called superfluorescence1. Because this phenomenon requires a fine balance of interactions between the emitters and their decoupling from the environment, together with close identity of the individual emitters, superfluorescence has thus far been obsd. only in a limited no. of systems, such as certain at. and mol. gases and a few solid-state systems2-7. The generation of superfluorescent light in colloidal nanocrystals (which are bright photonic sources practically suited for optoelectronics8,9) has been precluded by inhomogeneous emission broadening, low oscillator strength, and fast exciton dephasing. Here we show that cesium lead halide (CsPbX3, X = Cl, Br) perovskite nanocrystals10-13 that are self-organized into highly ordered three-dimensional superlattices exhibit key signatures of superfluorescence. These are dynamically red-shifted emission with more than 20-fold accelerated radiative decay, extension of the first-order coherence time by more than a factor of four, photon bunching, and delayed emission pulses with Burnham-Chiao ringing behavior14 at high excitation d. These mesoscopically extended coherent states could be used to boost the performance of optoelectronic devices15 and enable entangled multi-photon quantum light sources16,17.
    44. 44
      Hou, J.; Chen, P.; Shukla, A.; Krajnc, A.; Wang, T.; Li, X.; Doasa, R.; Tizei, L. H. G.; Chan, B.; Johnstone, D. N.; Lin, R.; Schülli, T. U.; Martens, I.; Appadoo, D.; Ari, M. S.; Wang, Z.; Wei, T.; Lo, S.-C.; Lu, M.; Li, S. Liquid-Phase Sintering of Lead Halide Perovskites and Metal-Organic Framework Glasses. Science 2021, 374, 621– 625,  DOI: 10.1126/science.abf4460
      44
      Liquid-phase sintering of lead halide perovskites and metal-organic framework glasses
      Hou, Jingwei; Chen, Peng; Shukla, Atul; Krajnc, Andraz; Wang, Tiesheng; Li, Xuemei; Doasa, Rana; Tizei, Luiz H. G.; Chan, Bun; Johnstone, Duncan N.; Lin, Rijia; Schulli, Tobias U.; Martens, Isaac; Appadoo, Dominique; S'Ari, Mark; Wang, Zhiliang; Wei, Tong; Lo, Shih-Chun; Lu, Mingyuan; Li, Shichun; Namdas, Ebinazar B.; Mali, Gregor; Cheetham, Anthony K.; Collins, Sean M.; Chen, Vicki; Wang, Lianzhou; Bennett, Thomas D.
      Science (Washington, DC, United States) (2021), 374 (6567), 621-625CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
      Lead halide perovskite (LHP) semiconductors show exceptional optoelectronic properties. Barriers for their applications, however, lie in their polymorphism, instability to polar solvents, phase segregation, and susceptibility to the leaching of lead ions. We report a family of scalable composites fabricated through liq.-phase sintering of LHPs and metal-org. framework glasses. The glass acts as a matrix for LHPs, effectively stabilizing nonequil. perovskite phases through interfacial interactions. These interactions also passivate LHP surface defects and impart bright, narrow-band photoluminescence with a wide gamut for creating white light-emitting diodes (LEDs). The processable composites show high stability against immersion in water and org. solvents as well as exposure to heat, light, air, and ambient humidity. These properties, together with their lead self-sequestration capability, can enable breakthrough applications for LHPs.
    45. 45
      Kim, J. I.; Zeng, Q.; Park, S.; Lee, H.; Park, J.; Kim, T.; Lee, T.-W. Strategies to Extend the Lifetime of Perovskite Downconversion Films for Display Applications. Adv. Mater. 2023, 35, 2209784  DOI: 10.1002/adma.202209784
      There is no corresponding record for this reference.
    46. 46
      Proppe, A. H.; Berkinsky, D. B.; Zhu, H.; Šverko, T.; Kaplan, A. E. K.; Horowitz, J. R.; Kim, T.; Chung, H.; Jun, S.; Bawendi, M. G. Highly Stable and Pure Single-Photon Emission with 250 ps Optical Coherence Times in InP Colloidal Quantum Dots. Nat. Nanotechnol. 2023, 18, 993– 999,  DOI: 10.1038/s41565-023-01432-0
      46
      Highly stable and pure single-photon emission with 250 ps optical coherence times in InP colloidal quantum dots
      Proppe, Andrew H.; Berkinsky, David B.; Zhu, Hua; Sverko, Tara; Kaplan, Alexander E. K.; Horowitz, Jonah R.; Kim, Taehyung; Chung, Heejae; Jun, Shinae; Bawendi, Moungi G.
      Nature Nanotechnology (2023), 18 (9), 993-999CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
      Quantum photonic technologies such as quantum communication, sensing or computation require efficient, stable and pure single-photon sources. Epitaxial quantum dots (QDs) have been made capable of on-demand photon generation with high purity, indistinguishability and brightness, although they require precise fabrication and face challenges in scalability. By contrast, colloidal QDs are batch synthesized in soln. but typically have broader linewidths, low single-photon purities and unstable emission. Here we demonstrate spectrally stable, pure and narrow-linewidth single-photon emission from InP/ZnSe/ZnS colloidal QDs. Using photon correlation Fourier spectroscopy, we observe single-dot linewidths as narrow as ∼5μeV at 4 K, giving a lower-bounded optical coherence time, T2, of ∼250 ps. These dots exhibit minimal spectral diffusion on timescales of microseconds to minutes, and narrow linewidths are maintained on timescales up to 50 ms, orders of magnitude longer than other colloidal systems. Moreover, these InP/ZnSe/ZnS dots have single-photon purities g(2) (τ = 0) of 0.077-0.086 in the absence of spectral filtering. This work demonstrates the potential of heavy-metal-free InP-based QDs as spectrally stable sources of single photons.
    47. 47
      Chiba, T.; Hoshi, K.; Pu, Y.-J.; Takeda, Y.; Hayashi, Y.; Ohisa, S.; Kawata, S.; Kido, J. High-Efficiency Perovskite Quantum-Dot Light-Emitting Devices by Effective Washing Process and Interfacial Energy Level Alignment. ACS Appl. Mater. Interfaces 2017, 9, 18054– 18060,  DOI: 10.1021/acsami.7b03382
      47
      High-Efficiency Perovskite Quantum-Dot Light-Emitting Devices by Effective Washing Process and Interfacial Energy Level Alignment
      Chiba, Takayuki; Hoshi, Keigo; Pu, Yong-Jin; Takeda, Yuya; Hayashi, Yukihiro; Ohisa, Satoru; Kawata, So; Kido, Junji
      ACS Applied Materials & Interfaces (2017), 9 (21), 18054-18060CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      All inorg. perovskites quantum dots (PeQDs) have attracted much attention for used in thin film display applications and solid-state lighting applications, owing to their narrow band emission with high luminescence quantum yields (PLQYs), color tunability, and soln. processability. Fabricated low-driving-voltage and high-efficiency CsPbBr3 PeQDs light-emitting devices (PeQD-LEDs) using a PeQDs washing process with an ester solvent contg. BuOAc to remove excess ligands from the PeQDs. The CsPbBr3 PeQDs film washed with AcOBu exhibited a PLQY of 42%, and a narrow PL emission with a full width at half-max. of 19 nm. Energy level alignment of the PeQD-LED to achieve effective hole injection into PeQDs from the adjacent hole injection layer was demonstrated. The PeQD-LED with AcOBu-washed PeQDs exhibited a max. power efficiency of 31.7 lm W-1 and EQE of 8.73%. Control of the interfacial PeQDs through ligand removal and energy level alignment in the device structure are promising methods for obtaining high PLQYs in film state and high device efficiency.
    48. 48
      Hung, L. T.; Oka, S.; Kimura, M.; Akahane, T. Determination of Polar Anchoring Strength at Vertical Alignment Nematic Liquid Crystal-Wall Interface Using Thin Hybrid Alignment Nematic Cell. Jpn. J. Appl. Phys. 2004, 43, L649– L651,  DOI: 10.1143/JJAP.43.L649
      There is no corresponding record for this reference.
    49. 49
      Raynes, E. P.; Tough, R. J. A.; Davies, K. A. Voltage Dependence of the Capacitance of a Twisted Nematic Liquid Crystal Layer. Mol. Cryst. Liq. Cryst. 1979, 56, 63– 68,  DOI: 10.1080/01406567908071968
      49
      Voltage dependence of the capacitance of a twisted nematic liquid crystal layer
      Raynes, E. P.; Tough, R. J. A.; Davies, K. A.
      Molecular Crystals and Liquid Crystals (1979), 56 (2), 63-8CODEN: MCLCA5; ISSN:0026-8941.
      The voltage dependence of the capacitance of the nematic mixt. E7 was measured just above the Freedericksz transition voltage. The results agree with theor. predictions.
    50. 50
      Li, J.; Wen, C.-H.; Gauza, S.; Lu, R.; Wu, S.-T. Refractive Indices of Liquid Crystals for Display Applications. J. Display Technol. 2005, 1, 51
      50
      Refractive indices of liquid crystals for display applications
      Li, Jun; Wen, Chien-Hui; Gauza, Sebastian; Lu, Ruibo; Wu, Shin-Tson
      Journal of Display Technology (2005), 1 (1), 51-61CODEN: IJDTAL; ISSN:1551-319X. (Institute of Electrical and Electronics Engineers)
      This paper reviews the extended Cauchy model and the four-parameter model for describing the wavelength and temp. effects of liq. crystal (LC) refractive indexes. The refractive indexes of nine com. LCs, MLC-9200-000, MLC-9200-100, MLC-6608, MLC-6241-000, 5PCH, 5CB, TL-216, E7, and E44 are measured by the Multi-wavelength Abbe Refractometer. These exptl. data are used to validate the theor. models. Excellent agreement between expt. and theory is obtained.
    51. 51
      Senyuk, B.; Kim, Y.-K.; Tortora, L.; Shin, S.-T.; Shiyanovskii, S. V.; Lavrentovich, O. D. Surface Alignment, Anchoring Transitions, Optical Properties and Topological Defects in Nematic Bent-Core Materials C7 and C12. Mol. Cryst. Liq. Cryst. 2011, 540, 20– 41,  DOI: 10.1080/15421406.2011.568324
      There is no corresponding record for this reference.
    52. 52
      Voldman, J.; Gray, M. L.; Schmidt, M. A. Microfabrication in Biology and Medicine. Annu. Rev. Biomed. Eng. 1999, 1, 401– 425,  DOI: 10.1146/annurev.bioeng.1.1.401
      52
      Microfabrication in biology and medicine
      Voldman, Joel; Gray, Martha L.; Schmidt, Martin A.
      Annual Review of Biomedical Engineering (1999), 1 (), 401-425CODEN: ARBEF7; ISSN:1523-9829. (Annual Reviews Inc.)
      Microfabrication uses integrated-circuit manufg. technol. supplemented by its own processes to create objects with dimensions in the range of micrometers to millimeters. These objects can have miniature moving parts, stationary structures, or both. Microfabrication has been used for many applications in biol. and medicine. These applications fall into four domains: tools for mol. biol. and biochem., tools for cell biol., medical devices, and biosensors. Microfabricated device structures may provide significantly enhanced function with respect to a conventional device. Sometimes microfabrication can enable devices with novel capabilities. These enhancing and enabling qualities are conferred when microfabrication is used appropriately to address the right types of problems. Herein, we describe microfabrication technol. and its application to biol. and medicine. We detail several classes of advantages conferred by microfabrication and how these advantages have been used to date.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.4c13217.

    • Physical properties of the LC antisolvent; characterization of W of CH3NH3PbBr3; size distributions of LC-PNCs of CH3NH3PbBr3; behaviors of PNCs in LC and isotropic media; structural analysis of PNCs of CH3NH3PbBr3; anisotropic growth of PNCs via LC films; detailed optical and structural analysis of LC-PNCs of CH3NH3PbBr3 in a microfluidic platform; detailed characterization of PNC-based LEDs; controlled synthesis of LC-PNCs of CsPbBr3; growth mechanism of LC-PNCs of CsPbBr3; detailed PL spectra of LC-PNCs of CsPbBr3 depending on ts; structural analysis of PNCs of CsPbBr3; and detailed PL lifetime of PNCs of CH3NH3PbBr3 (PDF)


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