Shining Light on Indium Phosphide Quantum Dots: Understanding the

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Shining Light on Indium Phosphide Quantum Dots: Understanding the Interplay among Precursor Conversion, Nucleation, and Growth† Brandi M. Cossairt* Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States ABSTRACT: InP quantum dots have emerged as an exciting class of phosphors for displays and energy-efficient solid state lighting. Unfortunately, the synthesis of these materials has lagged behind that of related IIVI and IV-VI materials. It is becoming increasingly apparent that this may be due, in many cases, to the inability to control quantum dot nucleation and crystallization using precursor conversion kinetics. In this perspective, recent work on understanding the nucleation and growth of InP from the perspective of nonclassical nucleation models is discussed. In particular, the recent discovery that kinetically persistent magic-size nanoclusters build up during the high temperature synthesis of this material will be highlighted. Isolation and complete structural characterization of one such InP nanocluster has offered unprecedented insight into the structure and surface chemistry of InP and its deviation from how we think about quantum dot structure and composition based on models built for II-VI materials. Moreover, this cluster offers an exciting playground to test hypotheses related to ligand and cation exchange as well as serving as a stepping stone to develop a more complete understanding of the properties that govern nanomaterial nucleation.

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INTRODUCTION Colloidal semiconductor nanocrystals (quantum dots, QDs) have attracted a great deal of interest over the past 3 decades, as evident from the many thousands of journal articles and patents on the subject, as well as the emergence of a number of commercial products containing these materials. Quantum dots exhibiting size-dependent optical properties were first prepared in glass matrices by Alexey Ekimov in 1981,1 and the first colloidal syntheses of quantum confined semiconductor nanoparticles was reported by Louis Brus in 1983.2 Moungi Bawendi and co-workers elaborated on this work using a solution-phase hot-injection technique to prepare nearly monodisperse quantum dots of CdSe with diameters between 2 and 11 nm.3 Since these landmark discoveries, it has been shown that it is possible to tailor the band structure as well as the surface chemistry of semiconductor nanocrystals to meet a wide range of demands such as narrow, tunable absorption and emission, high carrier mobility, solution processability in a range of organic and aqueous media, and size and shape control. The versatility of nanocrystals holds great promise for a wide range of applications, including photovoltaics,4−7 biological imaging,8−14 and light emitting devices.15−18 A colloidal approach is particularly attractive for industrial applications as it is inexpensive, scalable, and compatible with solution processing techniques, such as roll-to-roll printing.19 The rapid progress in reproducible and scalable batch synthesis of uniform QDs has led to the emergence of commercial display products containing quantum dot downconverters.20 By exploiting, the narrow, tunable emission profiles of nanocrystals, these displays are able to provide a

wider color gamut than those that incorporate the traditional cerium-doped yttrium aluminum garnet phosphor.20−22 For a quantum dot sample to emit light with a high degree of color purity, the sample must have minimal dispersion in particle size and shape. The historical focus of the community on the synthesis of II-VI quantum dots, particularly of CdSe, led to the development of scalable, size-tunable syntheses of monodisperse nanocrystals of CdSe making it the state-of-the-art material for these down-conversion applications in terms of fullwidth at half-maximum (fwhm) and photoluminescence quantum yield (PLQY).20,23,24 Given the inherent toxicity of cadmium,25−28 however, there has been a push by the display industry and regulatory agencies to limit the amount of cadmium in consumer electronics.29 InP QDs have therefore emerged as an attractive alternative to CdSe QDs because of their ability to emit over the same range of visible wavelengths. Although empirical optimization has led to displays containing InP as a replacement, these devices underperform their CdSe counterparts in both color gamut and energy efficiency due to the relatively poor size distributions obtained from traditional InP QD syntheses.21,24,30,31 This shortcoming provides compelling motivation to understand the mechanisms by which InP grows in solution and to determine how these mechanisms differ from those of other colloidal semiconductors. Received: August 15, 2016 Revised: September 26, 2016 Published: September 26, 2016

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This Perspective is part of the Up-and-Coming series. © 2016 American Chemical Society

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DOI: 10.1021/acs.chemmater.6b03408 Chem. Mater. 2016, 28, 7181−7189

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Chemistry of Materials

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MODELS GOVERNING QUANTUM DOT NUCLEATION AND GROWTH To develop a synthesis of monodisperse QDs, researchers have sought to understand the fundamental steps involved in the nucleation and growth of colloidal particles. The most widely discussed model in this regard is that proposed by LaMer and Dinegar in 1950.32 In the LaMer model, a colloidal synthesis from molecular precursors proceeds through three main phases (Figure 1). During the first phase of the reaction, the chemical

solution of one or more reactants to a separate solution of the remaining reactants at an elevated temperature. In this way, there is an initial spike in the number of monomers in the solution due to rapid precursor conversion kinetics at the initial temperature and precursor concentrations. Shortly after injection, the temperature has equilibrated and a significant fraction of potential monomers has been consumed via nucleation, effectively retarding precursor conversion kinetics in order to maintain a steady monomer supply for the subsequent size-focusing growth stage. Developing precursor-conversion limited reaction schemes where nucleation and growth of the resultant semiconductor nanocrystals adheres to the main features of the LaMer model has been shown to be effective for preparing a range of II-VI and IV-VI semiconductors.3,38,39 These successes have led researchers in the field to begin with this classical nucleation model as their starting hypothesis when attempting to design syntheses of other classes of semiconductor nanomaterials. The LaMer model can be considered as a classical homogeneous nucleation model wherein nanocrystals evolve in size by interacting solely with monomers.40−42 Classical nucleation theory also assumes that small clusters that form during the course of crystallization have the same structure as the final crystalline phase obtained at the end of the reaction.41 It is becoming increasingly apparent that for many nanoscale systems the formation of ordered phases involves nonclassical nucleation and growth processes. Extensive experimental and computational work has been reported that reveals the occurrence of metastable phases, multistep nucleation, and intermediate-mediated self-assembly, all of which impact the product outcomes of these reactions.41,43,44 Although a range of nonclassical nucleation models have been developed to explain these observations that differ in their precise mechanism, these models are united by the fact that during the course of the reaction, intermediates containing multiple monomer units build up in concentration and their subsequent reactivity impacts the later course of the crystallization process (Figure 2).40,41,43 As will be discussed below, InP shows signatures of nonclassical nucleation necessitating new approaches to the synthesis of this material in colloidal solutions.

Figure 1. Kinetics of solute supersaturation according to LaMer. Three phases of the LaMer model include (I) supersaturation, (II) nucleation, and (III) growth. Precursor conversion reactions that limit the crystallization determine the temporal evolution of monomer concentration as well as the steady state supersaturation during the growth phase. Figure reproduced with permission from ref 37.

potential of a soluble form of the desired colloid (called the monomer) is rapidly increased. In a typical quantum dot synthesis, this is achieved through the in situ generation of the monomer species from the conversion of one or more molecular precursors. In the second phase of the reaction, the chemical potential of the monomer reaches a critical limit and nuclei form. Ideally, this critical limit achieved by the monomer should be well above the supersaturation level of the solution so that nucleation occurs nearly instantaneously and brings the solution below supersaturation. This criterion is crucial as temporal separation of nucleation and growth is necessary to avoid differences in particle growth history that will invariably broaden the particle size distribution. The final phase of the LaMer model is growth of the newly formed particles either from the remaining monomer species left over in the solution or from newly generated monomers from continued precursor conversion. During this stage of the reaction, a size-focusing kinetic regime can be obtained.33,34 At a sufficiently high monomer concentration, particle growth will be favored over dissolution back into monomers. Remaining monomers will preferentially adsorb onto smaller particles due to their higher surface energies relative to larger particles. These growth kinetics result in a decrease in size distributions with time. If monomer concentrations are too low during the growth stage, then particle dissolution back to monomers will be favored over monomer adsorption, and growth will proceed via an Ostwald ripening mechanism with an overall increase in size distribution. With this model in mind, an ideal synthesis of nanocrystals should target temporally separated nucleation and growth stages while maintaining sufficiently high monomer concentrations during particle growth. In line with these criteria, a typical technique employed to obtain a monodisperse sample of nanocrystals is the hot-injection method.33,35,36 The hotinjection method rapidly introduces a room temperature

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APPROACHES TO INDIUM PHOSPHIDE QD SYNTHESIS A recent review by Peter Reiss and co-workers describes the breadth of methods that have been developed to prepare InP QDs.45 Briefly, the predominant approaches can be separated into three classes: (i) InX3 + P(SiMe3)3 (X = halide or carboxylate), (ii) In(0) + P(0), and (iii) InX3 + P(NMe2)3 (X = halide). One of the first successful and widely implemented routes to InP was Wells’ dehalosilylation reaction involving the use of indium trihalide and P(SiMe3)3.46−48 One hallmark of this procedure, and its later variants involving the use of alternative In(III) precursors or alternative silylphosphines, is the fact that it starts with ionic precursors with equal and opposite charge (In3+ and P3−), and therefore no redox chemistry is needed for the formation of InP. In the second reaction class, wherein particles of In(0) are combined with a zerovalent source of P, such as P4, the presumed mechanism proceeds via deposition of P at the surface of the In(0) particles and intercalation via a surface-mediated mechanism where the reaction begins at the particle surface and diffusion to the interior follows.49,50 This approach often yields amorphous phases and/or hollow 7182


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explored a variety of sterically bulky and electronically tunable para-substituted triarylsilylphosphines, P(Si(C6H4-X)3)3 (X = H, F, NMe2, and OMe), as reaction partners with In(O2CR)3.58 It should be noted that steric control of precursor conversion kinetics was first explored by Jeong and co-workers using P(SiMe2(tBu))3 as a P3− source, and has recently been expanded on by Bawendi and co-workers using P(SiMe2iPr)3.59,60 The use of P(SiPh3)3, for example, allowed us to enter into a situation where precursor consumption was slow and coincided with the appearance of InP over the course of hours at 270 °C. In this case, however, nucleation occurred throughout the entire course of the reaction, leading to a disparity in particle growth histories and polydisperse populations of InP QDs. Given the extremely sluggish reactivity of P(SiPh3)3, we then chose to use it as a monomer reservoir in a reaction where particles were nucleated rapidly from P(SiMe3)3. In this way, we were able to observe particle growth over a wide range of sizes that could be attributed to precursor conversion rather than Ostwald ripening (Figure 3). Moreover, we could tune the final particle size by changing the relative ratio of P(SiMe3)3 to P(SiPh3)3, allowing us to confirm that nucleation was largely occurring from P(SiMe3)3 and growth was predominantly arising from P(SiPh3)3. Despite this

Figure 2. Plots of free energy change as a function of particle size and schematic representations of crystallization for classical (A) and nonclassical (B) nucleation models. In the classical model, formation of the critical nucleus is rate determining and subsequent growth occurs in a continuous manner from monomers in solution. In nonclassical models, cluster intermediates form (often prenucleation) and then must transform (in a series of steps that can be slow) prior to crystal formation.

particles via the Kirkendall effect.50 Furthermore, the final particle size distribution is largely dictated by the starting In(0) population. In the most recently developed class of reactions for InP synthesis, InX3 is combined with phosphorus trisdimethylamide, often in the presence of a Zn2+ source and a primary amine.51,52 In this reaction, an In3+ source is being combined with a P3+ source and thus redox chemistry is needed to generate InP (similar to reactions beginning with InX3 and alkylphosphines). Recent mechanistic studies have shown this likely occurs by involvement of the amine and a P(III/V) redox couple.53,54 Until the past few years with the emergence of the P(NMe2)3-based chemistry, the most widely explored and modified procedure for preparing InP was based on a synthesis from Xiaogang Peng and co-workers that is a modification of Wells’ dehalosilylation procedure.55 In the first version of this chemistry, In(O2CR)3, where R is a long chain fatty acid like myristate or oleate, is combined with P(SiMe3)3 at elevated temperature. A major limitation of this approach is the extreme reactivity of P(SiMe3)3 under these conditions: it is typically consumed within the first few seconds of a hot-injection reaction precluding monomer reservoirs for subsequent growth. This has led many researchers to explore alternative P3− sources for this chemistry as a way to modulate the precursor conversion kinetics. One notable study in this regard involved the development of P(GeMe3)3 as a precursor.56 In this study, it was found that although P(GeMe3)3 was far less reactive than the corresponding silyl variant, the resulting particle size distributions were still poor in comparison to those seen in related II-VI and IV-VI hot injection syntheses. Similarly, amine has been explored in combination with P(SiMe3)3 as a strategy to slow down precursor conversion kinetics by competing with indium for phosphine coordination.57 In our group, we initially

Figure 3. (A) UV−vis evolution of InP prepared from a 1:1 mixture of P(SiMe3)3 and P(SiPh3)3 with In(O2CR)3 at 270 °C and TEM image (B) of the final QD sample. Modified figure reproduced with permission from ref 58. 7183


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Chemistry of Materials ability to toggle artificially nucleation and growth in the absence of Ostwald ripening, we were still unable to obtain monodisperse size distributions, leading us to question whether there was some other unaccounted for chemistry that was causing a departure from a classical nucleation model.

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CLUSTER INTERMEDIATES EN ROUTE TO INDIUM PHOSPHIDE QDS As discussed above, tuning precursor conversion kinetics appears insufficient to allow preparation of monodisperse samples at high yield over a range of sizes. This result prompted our group to re-evaluate the assumption that in this chemistry precursor conversion is the rate limiting step. If it is not, then precursor conversion kinetics alone should not determine the nucleation profile of InP QDs. We have hypothesized that kinetically persistent magic-size cluster (MSC) intermediates build up during InP QD synthesis creating a low energy bottleneck in the reaction coordinate diagram (Figure 4).61

Figure 4. Two-step nucleation model proposed for InP showing the formation of kinetically persistent magic-size cluster intermediates that build up en route to InP QDs. Figure reproduced with permission from ref 61.

Prior to our work on InP, a variety of binary semiconductor MSCs have been investigated including lead selenide (PbSe),62 zinc selenide (ZnSe),63 zinc telluride (ZnTe),64,65 cadmium sulfide (CdS),66 cadmium selenide (CdSe),67−72 cadmium telluride (CdTe),70 and cadmium phosphide (Cd3P2).73 This class of nanoclusters has shown utility for applications ranging from white LEDs,66,67 blue LEDs, room-temperature nucleants for nanoplatelets,74 to in vivo biological imaging.62 CdSe clusters in particular have proven to be a versatile starting material for a variety of nanostructures such as rods, rice, tadpoles,75 ribbons,76 nanosheets,77 and quantum belts.78 The major distinction between many of these cluster intermediates and those proposed to exist for InP is their stability. Owen and co-workers, for example, have shown that Cd35Se20 clusters spontaneously grow in solution at 25 °C and are not present at appreciable concentrations during high temperature synthesis.79 For MSCs to perturb QD nucleation, they must form and persist under the conditions used for QD synthesis. The extreme persistence of such clusters during high-temperature InP synthesis has recently been demonstrated using MALDI mass spectrometry.80 For InP prepared from In(O2CR)3 and silylphosphines, we identified and isolated an MSC and demonstrated its significance in the synthesis of InP QDs (Figure 5).61 This cluster, with a lowest energy maximum at 386 nm, appears and persists as the only InP-containing product when the reaction temperature is maintained below 120 °C. It has also been observed previously under similar synthetic conditions.81 Indeed, the as-synthesized cluster is a robust single-source precursor for the synthesis of InP QDs, as would be expected based on the proposed two-step nucleation model. Although

Figure 5. (A) Growth of InP QDs from isolated InP MSCs in squalane at 400 °C. (B) Trend in final product optical spectra as a function of InP MSC concentration. (C) TEM image showing final particles obtained from the reaction shown in panel A. Figure reproduced with permission from ref 61.

the precise mechanism of the MSC to QD transformation remains an open question, we have shown that it is likely not to proceed via an aggregative growth mechanism and that some extent of dissolution and renucleation is likely necessary for subsequent crystallization.

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IDENTIFICATION OF IN37P20(O2CR)51 In our initial publication on the intermediacy of InP386, we hypothesized that the MSC may be an equilibrium mixture of multiple species due to the asymmetry of the optical spectrum and the complexity of the corresponding 31P NMR spectrum. Since that time, we have prepared analogous MSCs with a variety of carboxylate ligands, all of which were characterized by nearly identical UV−vis spectral signatures and similar solutionphase NMR spectra, suggesting that perhaps these spectral signatures are indeed attributable to a single species in solution. Using phenylacetate as our ligand on indium, we were able to 7184


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characterized II-VI semiconductor clusters, which can qualitatively be described as being based on the bulk zinc blende structure type, this lower symmetry structure was a surprise. Interestingly, recent calculations on gas-phase InP clusters suggest that low symmetry may be an intrinsic property of this material.86 The intermediacy of this cluster in InP QD synthesis from In(O2CR)3 and silylphosphines and its deviation from the bulk structure type further reinforces the notion that nucleation of InP in solution proceeds through a nonclassical mechanism. Although we have demonstrated that phenylacetate-ligated clusters can serve as single-source precursors to InP QDs, the question now arises: is the In37P20 structure conserved when more traditional fatty carboxylate ligands are used? Using a suite of spectroscopic tools we can now confidently say that the answer is yes. Overlay of the UV−vis spectra and pair distribution function analysis of PXRD data of clusters prepared with phenylacetate and myristate ligand sets show the close correspondence between the two species (Figure 7). As shown

obtain gram quantities of this MSC and to grow X-ray quality single crystals from concentrated solutions of ethyl acetate.82 A solution to the single crystal X-ray diffraction data was obtained to a resolution of 0.83 Å with an R1 value of 0.1107, remarkable quality for a structure of this type. The data showed that InP386 contained an inorganic core of In37P20 with C2 symmetry and a highly interconnected ligand shell of 51 bidentate and mostly bridging phenylacetate anions (Figure 6).

Figure 7. Overlay of UV−vis spectra and PDF data (inset) for InP MSCs prepared using myristate (green) and phenylacetate (red). The computed PDF model from the single crystal X-ray structure of In37P20(O2CCH2C6H5)51 is shown in blue.

using TDDFT calculations carried out by our collaborators Alessio Petrone and Xiaosong Li, we now understand the asymmetric peak shape of the UV−vis data to be a hallmark of the low symmetry of the inorganic cluster core. The PDF data collected and analyzed by our collaborators Maxwell Terban and Simon Billinge show agreement between the computed PDF obtained from the single crystal solution and the powder data collected for bulk quantities of the clusters with the two ligand types. Furthermore, the solution-phase 31P{1H} NMR data is also consistent with the C2 symmetric In37P20 structure. The solution-phase 31P{1H} NMR data for oleate capped clusters is characterized by a 9-line pattern (Figure 8). Inversion recovery experiments were conducted in order to choose appropriate delay times to obtain an integratable spectrum that clearly shows two peaks that integrate to 1, which agrees well with the two P atoms that are coincident with the C2 axis of the cluster. The spectrum we postulate is largely interpretable based on shielding arguments with the most core like P atoms (4,4′) coming the most upfield and the most surface like P atom (1) appearing the most downfield. Further information on analysis of solution phase NMR data and their use in understanding cluster reactivity will appear in a forthcoming paper.

Figure 6. (A) In 3 7 P 2 0 core and (B) full structure of In37P20(O2CCH2Ph)51 obtained from structure refinement based on single crystal X-ray diffraction data. H atoms omitted for clarity. Figure reproduced with permission from ref 82.

Several notable features emerged from this structure. First is the composition of the inorganic core itself; there are 16 surface indium atoms that each coordinate to a single phosphorus atom to complete the tetrahedral coordination sphere of the phosphorus. Examining the fused core aside from these 16 surface In atoms, reveals a stoichiometry of In21P20: that is, the fused core itself is nonstoichiometric and charged. Next, examining the ligand coordination shell reveals a dense and interconnected network of indium carboxylates with many of the 16 surface indium atoms connected to core indium atoms via bridging carboxylates. This dense and interconnected ligand shell likely is critical for the stability and structure of this cluster and presents a very different picture of the quantum dot surface from the model we had previously, based on thiolate capped nanocluster structures where ligands could be easily classified as X-type and L-type independent of one another.83−85 The final point to highlight about this structure is the low C2 symmetry adopted by the inorganic core. Given the number of previously 7185


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Figure 8. Solution-phase 31P{1H} NMR spectrum of In37P20(O2C(CH2)7(CHCH)(CH2)7CH3)51 showing the characteristic 11-line pattern for the C2 symmetric structure as well as assignments of the most upfield and most downfield resonances based on shielding arguments.

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OUTLOOK Understanding the full implications of cluster chemistry on the nucleation and growth of QDs will require us to develop an understanding of the parameters that dictate cluster structure and stability. Three features that we have considered to be important in this regard include surface chemistry, cation charge, and lattice covalency. One thing we know to be true given our preliminary studies with coordinating Lewis bases is that the structure and reactivity of cluster intermediates is highly dependent on surface chemistry. We have observed, for example, that addition of primary amine destabilizes the cluster by perturbation of the interconnected indium carboxylate ligand network, allowing for growth of InP at lower temperatures without persistence of In37P20(O2CR)51.61 Further, we have determined that In37P20(O2CR)51 can be converted into a new, more symmetric, and more thermally stable cluster of similar size upon complete exchange of indium carboxylate for indium phosphonate.61 We are excited about the prospects of exploring the evolution of cluster structure caused by ligand exchange and studying the impact of such transformations on the cluster to QD evolution. Key differences are observed in the cluster chemistries of InP compared to II-VI materials, and these differences likely affect the reactivities of these clusters in important ways that impact the eventual QD products. First, the ligand density at metal-rich InP MSC surfaces is significantly greater than for related II-VI clusters, attributable to the higher charge on indium versus cadmium (Figure 9). This difference potentially renders the InP MSC surface more inert toward further reaction, thereby increasing the kinetic stability of these III-V clusters over II-VI clusters of similar size. The second major difference is the greater covalency of the InP lattice. Covalency has been widely invoked for rationalizing the differences in reactivity and chemistry of III-V vs II-VI semiconductor nanocrystals.88 Comparing the fractional ionic characters (computed from electronegativities) of InP (0.42) and CdSe (0.70) and the computed density of states for the two materials,89 we see that although InP has a greater degree of covalent character, it is still

Figure 9. Space-filling model of (A) Cd32Se14(SePh)36(PPh3)487 and (B) In37P20(O2CCH2Ph)51 demonstrating the difference in ligand coverage in similarly sized II-VI versus III-V nanoclusters with cationrich surfaces.

significantly ionic in terms of its bonding. We are eager to explore the parameters (covalency, cation charge, etc.) controlling nucleation and growth mechanisms in greater detail. It will be exciting as a community to work toward a universal model for QD syntheses that can equally well take into account complex reaction mechanisms, the evolution of stable intermediates, and rate-limiting precursor conversion (Figure 10). With such a model we could develop a set of

Figure 10. Generic potential energy diagram for QD synthesis. For example, rate-limiting precursor conversion would be represented by EA1 ≫ EA2 and syntheses where clusters build up and persist would have EA2 > EA1.

predictive design principles that allow for the deductive formulation of new syntheses based on fundamental material properties rather than empirical optimization.

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AUTHOR INFORMATION

Corresponding Author

*B. M. Cossairt. E-mail: [email protected]. 7186


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Chemistry of Materials Notes

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The author declares no competing financial interest. Biography Asst. Prof. Brandi Cossairt was born and raised in Miami, Florida and began working in the laboratory of Professor Anthony J. Hynes at the University of Miami Rosenstiel School of Marine and Atmospheric Science as a high school student. She is a first-generation college graduate, having obtained her B.S. in Chemistry from the California Institute of Technology in 2006 where she carried out undergraduate research with Professor Jonas C. Peters. Brandi went on to pursue graduate studies at the Massachusetts Institute of Technology under the guidance of Professor Christopher C. Cummins and was awarded her Ph.D. in inorganic chemistry in 2010. She then continued her academic career as an NIH NRSA Postdoctoral Fellow at Columbia University between 2010 and 2012. Brandi joined the Department of Chemistry at the University of Washington as an Assistant Professor in July of 2012. She has received a number of awards for her research including a Sloan Research Fellowship, a 3M Non-Tenured Faculty Award, a David and Lucile Packard Foundation Fellowship, an NSF CAREER Award, and the Seattle AWIS Award for Early Career Achievement. In addition to leading her research group, Brandi is the cofounder of the Chemistry Women Mentorship Network and serves as the faculty sponsor for the UW Department of Chemistry’s Women in Chemical Sciences group.

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ACKNOWLEDGMENTS I thank the University of Washington (startup funds, Royalty Research Fund, and Innovation Award), the 3M Non-Tenured Faculty Award program, the Alfred P. Sloan Foundation, and the David and Lucile Packard Foundation for generous support of the research presented in this perspective article. I also thank my students Dylan Gary, Benjamin Glassy, and Sarah Flowers and my collaborators Xiaosong Li, Alessio Petrone, Simon Billinge, and Maxwell Terban for their contributions to this research.

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REFERENCES

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Shining Light on Indium Phosphide Quantum Dots: Understanding the

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