Morphology control of cadmium selenide nanocrystals: insights into the roles of di-n-octylphosphine oxide (DOPO) and ucid (DOPA)

The Huge Role of Tiny Impurities in Nanoscale Synthesis

Nanotechnology is vital to many current industries, including electronics, energy, textiles, agriculture, and theranostics. Understanding the chemical mechanisms of nanomaterial synthesis has contributed to the tunability of their unique properties, although studies frequently overlook the potential impact of impurities. Impurities can show adverse effects, clouding the interpretation of results or limiting the practical utility of the nanomaterial. On the other hand, as successful doping has demonstrated, the intentional introduction of impurities can be a powerful tool for enhancing the properties of a nanomaterial. This Review examines the complex role of impurities, unintentionally or intentionally added, during nanoscale synthesis and their effects on the performance and usefulness of the most common classes of nanomaterials: nanocarbons, noble metal and metal oxide nanoparticles, semiconductor quantum dots, thermoelectrics, and perovskites.

Covalent inorganic complexes enabled zinc blende to wurtzite phase changes in CdSe nanoplatelets

Phase changes in colloidal semiconductor nanocrystals (NCs) are essential in material design and device applications. However, the transition pathways have yet to be sufficiently studied, and a better understanding of the underlying mechanisms is needed. In this work, a complete ligand-assisted phase transition from zinc blende (ZB) to wurtzite (WZ) is observed in CdSe nanoplatelets (NPLs). By monitoring with in situ absorption spectra along with electrospray ionization mass spectrometry (ESI-MS), we demonstrated that the transition process is a ligand-assisted covalent inorganic complex (CIC)-mediated phase transition pathway, which involves three steps, ligand exchange on ZB CdSe NPLs (Step 1), dissolution of NPLs to form CICs (Step 2), and conversion of CdSe-CIC assemblies to WZ CdSe NPLs (Step 3). In particular, CICs can be directly anisotropically grown to WZ CdSe NPL without other intermediates, following pseudo-first-order kinetics (kobs = 9.17 × 10-5 s-1). Furthermore, we demonstrated that CICs are also present and play an essential role in the phase transition of ZnS NPLs from WZ to ZB structure. This study proposes a new crystal transformation pathway and elucidates a general phase-transition mechanism, facilitating precise functional nanomaterial design.

Monoalkyl Phosphinic Acids as Ligands in Nanocrystal Synthesis

Ligands play a crucial role in the synthesis of colloidal nanocrystals. Nevertheless, only a handful molecules are currently used, oleic acid being the most typical example. Here, we show that monoalkyl phosphinic acids are another interesting ligand class, forming metal complexes with a reactivity that is intermediate between the traditional carboxylates and phosphonates. We first present the synthesis of n-hexyl, 2-ethylhexyl, n-tetradecyl, n-octadecyl, and oleylphosphinic acid. These compounds are suitable ligands for high-temperature nanocrystal synthesis (240-300 °C) since, in contrast to phosphonic acids, they do not form anhydride oligomers. Consequently, CdSe quantum dots synthesized with octadecylphosphinic acid are conveniently purified, and their UV-vis spectrum is free from background scattering. The CdSe nanocrystals have a low polydispersity and a photoluminescence quantum yield up to 18% (without shell). Furthermore, we could synthesize CdSe and CdS nanorods using phosphinic acid ligands with high shape purity. We conclude that the reactivity toward TOP-S and TOP-Se precursors decreases in the following series: cadmium carboxylate > cadmium phosphinate > cadmium phosphonate. By introducing a third and intermediate class of surfactants, we enhance the versatility of surfactant-assisted syntheses.

Synthetic Mechanisms in the Formation of SnTe Nanocrystals

Infrared active colloidal semiconducting nanocrystals (NCs) are important for applications including photodetectors and photovoltaics. While much research has been conducted on nanocrystalline materials such as the Pb and Hg chalcogenides, less toxic alternatives such as SnTe have been far less explored. Previous synthetic work on SnTe NCs have characterized photophysical properties of the nanoparticles. This study focuses on understanding the fundamental chemical mechanisms involved in SnTe NC formation, with the aim to improve synthetic outcomes. The solvent oleylamine, common to all SnTe syntheses, is found to form a highly reactive, heteroleptic Sn-oleylamine precursor that is the primary molecular Sn species initiating NC formation and growth. Further, the capping ligand oleic acid (OA) reacts with this amine to produce tin oxide (SnO_x), facilitating the formation of an NC SnO_x shell. Therefore, the use of OA during synthesis is counterproductive to the formation of stoichiometric SnTe nanoparticles. The knowledge of chemical reaction mechanisms creates a foundation for the production of high-quality, unoxidized, and stoichiometric SnTe NCs.

Cow-to-cow variation in nanocrystal synthesis: learning from technical-grade oleylamine

Many technical-grade reagents, including oleylamine, are broadly used as ligands in nanocrystal synthesis, allowing for cost-effective, and more environmentally friendly, preparation of materials in useful quantities. Impurities can represent 30% or more of these reagent blends, and have frequently emerged as substantial drivers of nanocrystal morphology, assembly, or other physical properties, making it important to understand their composition. Some functional alkyl reagents are derived from natural sources (e.g. often beef tallow, in the case of oleylamine), introducing alkyl chain structures very different than those that might be expected as side products of synthesis from pure feedstocks. Additionally, impurities can exhibit variations based on biological factors (e.g. species, diet, season). In biology, blends of alkyl chains allow for surprisingly sophisticated function of amphiphiles in the cell membrane, pointing to the possibility of similar control in synthetic materials if reagent composition were either better controlled or better understood. Here, we provide brief context on the breadth of roles technical-grade impurities have played in nanocrystal materials, followed by a perspective on oleylamine impurities, their physical properties, and their potential contributions to nanomaterial function.

'Beneficial impurities' in colloidal synthesis of surfactant coated inorganic nanoparticles

Colloidal synthesis of nanoparticles (NP) has advanced tremendously over the past 25 years, with an increasing number of research papers introducing nanomaterials with a variety of compositions, shapes, sizes, and phases. Although much progress has been achieved, commonly used synthetic procedures often fail to reproduce results, and the fine details of the syntheses are often disregarded. Reproducibility issues in synthesis can be ascribed to the effects of impurities, trace amounts of chemical moieties which significantly affect the reaction products. Impurities in NP synthesis are rarely reported or regularly studied, despite their impact, deleterious, or beneficial. This topical review discusses several case studies of colloidal NP synthesis where the sources and the chemistry of impurities are highlighted, and their role is examined.

Exploiting Functional Impurities for Fast and Efficient Incorporation of Manganese into Quantum Dots

The incorporation of manganese (Mn) ions into Cd(Zn)-chalcogenide QDs activates strong spin-exchange interactions between the magnetic ions and intrinsic QD excitons that have been exploited for color conversion, sunlight harvesting, electron photoemission, and advanced imaging and sensing. The ability to take full advantage of novel functionalities enabled by Mn dopants requires accurate control of doping levels over a wide range of Mn contents. This, however, still represents a considerable challenge. Specific problems include the difficulty in obtaining high Mn contents, considerable broadening of QD size dispersion during the doping procedure, and large batch-to-batch variations in the amount of incorporated Mn. Here, we show that these problems originate from the presence of unreacted cadmium (Cd) complexes whose abundance is linked to uncontrolled impurities participating in the QD synthesis. After identifying these impurities as secondary phosphines, we modify the synthesis by introducing controlled amounts of "functional" secondary phosphine species. This allows us to realize a regime of nearly ideal QD doping when incorporation of magnetic ions occurs solely via addition of Mn-Se units without uncontrolled deposition of Cd-Se species. Using this method, we achieve very high per-dot Mn contents (>30% of all cations) and thereby realize exceptionally strong exciton-Mn exchange coupling with g-factors of ∼600.

In situ SAXS probing the evolution of the precursors and onset of nucleation of ZnSe colloidal semiconductor quantum dots

The effect of diphenyl phosphine (HPPh₂) on precursors conversion reaction and nucleation/growth of quantum dots (QDs) were in situ investigated by the combination of SAXS and UV-vis.

Quantum scale biomimicry of low dimensional growth: An unusual complex amorphous precursor route to TiO2 band confinement by shape adaptive biopolymer-like flexibility for energy applications

Crystallization via an amorphous pathway is often preferred by biologically driven processes enabling living species to better regulate activation energies to crystal formation that are intrinsically linked to shape and size of dynamically evolving morphologies. Templated ordering of 3-dimensional space around amorphous embedded non-equilibrium phases at heterogeneous polymer─metal interfaces signify important routes for the genesis of low-dimensional materials under stress-induced polymer confinement. We report the surface induced catalytic loss of P=O ligands to bond activated aromatization of C−C C=C and Ti=N resulting in confinement of porphyrin-TiO₂ within polymer nanocages via particle attachment. Restricted growth nucleation of TiO₂ to the quantum scale (≤2 nm) is synthetically assisted by nitrogen, phosphine and hydrocarbon polymer chemistry via self-assembly. Here, the amorphous arrest phase of TiO₂ is reminiscent of biogenic amorphous crystal growth patterns and polymer coordination has both a chemical and biomimetic significance arising from quantum scale confinement which is atomically challenging. The relative ease in adaptability of non-equilibrium phases renders host structures more shape compliant to congruent guests increasing the possibility of geometrical confinement. Here, we provide evidence for synthetic biomimicry akin to bio-polymerization mechanisms to steer disorder-to-order transitions via solvent plasticization-like behaviour. This challenges the rationale of quantum driven confinement processes by conventional processes. Further, we show the change in optoelectronic properties under quantum confinement is intrinsically related to size that affects their optical absorption band energy range in DSSC.

Coordination chemistry and catalysis with secondary phosphine oxides

Review on synthesis, coordination chemistry and catalysis with secondary phosphine oxides.

Regular patterns of the effects of hydrogen-containing additives on the formation of CdSe monomer

It is unclear at the molecular level why HY (HY = RSH, or ROH, or RNH2) with HPPh2 additives kinetically affects the reaction pathway to the formation of different monomers (Ph2P-SeCd-Y or Ph2P-SeCdSe-Y) in the systhesis of semiconductor nanocrystals. In the present work, it was found that in a [Cd(OA)2 + Se[double bond, length as m-dash]P(C8H17)3 + HPPh2 + HY] mixture, HY behaves as a mediator for the formation of the initial kind of monomer, besides as a hydrogen/proton donor in the release of oleic acid and as an accelerant in the Se-P bond cleavage, which follows the mechanism of hydrogen-shift/nucleophilic-attack. The capability of the HY additive to provide a H-source decreases in the order SePPh2H > RSH > HPPh2 > ROH > RNH2, while the performance of HY to accelerate Se-P bond cleavage decreases in the order HPPh2 > RSH > RNH2 > ROH. The capacity of HY to promote the formation of the Ph2P-SeCd-Y monomer decreases in the order RSH > HPPh2 > ROH > RNH2, while the effect of HY to drive the formation of the Ph2P-SeCdSe-Y monomer decreases in the order HPPh2 > RSH > RNH2 > ROH. The activation strain energy plays a key role in both the Se-P and H-Y bond cleavage, which correlates negatively to the size of the coordinated atom radius. When only HPPh2 is present without other HY species (HY = RNH2, or RSH, or ROH), Ph2P-SeCdSe-PPh2 is preferentially formed. Alternatively, when both HY (HY = RNH2, or RSH, or ROH) and HPPh2 are present, Ph2P-SeCd-Y is favorably formed. For the formation of Ph2P-SeCd-Y (Y = -PPh2, -SR, -OR, and -NHR), SePPh2H embodies the catalytic performance, while HPPh2 serves as the catalyst for the formation of Ph2P-SeCdSe-Y (Y = -NHR or -OR). Our study brings a molecular-level insight into the relationship between the CdSe monomer and the phosphorous-containing side-product, which may advance the rational design and synthesis of quantum dots.

Role of Precursor-Conversion Chemistry in the Crystal-Phase Control of Catalytically Grown Colloidal Semiconductor Quantum Wires

Crystal-phase control is one of the most challenging problems in nanowire growth. We demonstrate that, in the solution-phase catalyzed growth of colloidal cadmium telluride (CdTe) quantum wires (QWs), the crystal phase can be controlled by manipulating the reaction chemistry of the Cd precursors and tri-n-octylphosphine telluride (TOPTe) to favor the production of either a CdTe solute or Te, which consequently determines the composition and (liquid or solid) state of the Bi_xCd_yTe_z catalyst nanoparticles. Growth of single-phase (e.g., wurtzite) QWs is achieved only from solid catalysts (y ≪ z) that enable the solution-solid-solid growth of the QWs, whereas the liquid catalysts (y ≈ z) fulfill the solution-liquid-solid growth of the polytypic QWs. Factors that affect the precursor-conversion chemistry are systematically accounted for, which are correlated with a kinetic study of the composition and state of the catalyst nanoparticles to understand the mechanism. This work reveals the role of the precursor-reaction chemistry in the crystal-phase control of catalytically grown colloidal QWs, opening the possibility of growing phase-pure QWs of other compositions.

Spectroscopic Properties of Phase-Pure and Polytypic Colloidal Semiconductor Quantum Wires

We report ensemble extinction and photoluminesence spectra for colloidal CdTe quantum wires (QWs) with nearly phase-pure, defect-free wurtzite (WZ) structure, having spectral line widths comparable to the best ensemble or single quantum-dot values, to the single polytypic (having WZ and zinc blende (ZB) alternations) QW values, and to those of two-dimensional quantum belts or platelets. The electronic structures determined from the multifeatured extinction spectra are in excellent agreement with the theoretical results of WZ QWs having the same crystallographic orientation. Optical properties of polytypic QWs of like diameter and diameter distribution are provided for comparison, which exhibit smaller bandgaps and broader spectral line widths. The nonperiodic WZ-ZB alternations are found to generate non-negligible shifts of the bandgap to intermediate energies between the quantum-confined WZ and ZB energies. The alternations and variations in the domain sizes result in inhomogeneous spectral line width broadening that may be more significant than that arising from the 12-13% diameter distributions within the QW ensembles.

General low-temperature reaction pathway from precursors to monomers before nucleation of compound semiconductor nanocrystals

Little is known about the molecular pathway to monomers of semiconductor nanocrystals. Here we report a general reaction pathway, which is based on hydrogen-mediated ligand loss for the precursor conversion to 'monomers' at low temperature before nucleation. We apply (31)P nuclear magnetic resonance spectroscopy to monitor the key phosphorous-containing products that evolve from MXn+E=PPh2H+HY mixtures, where MXn, E=PPh2H, and HY are metal precursors, chalcogenide precursors, and additives, respectively. Surprisingly, the phosphorous-containing products detected can be categorized into two groups, Ph2P-Y and Ph2P(E)-Y. On the basis of our experimental and theoretical results, we propose two competing pathways to the formation of M2En monomers, each of which is accompanied by one of the two products. Our study unravels the pathway of precursor evolution into M2En monomers, the stoichiometry of which directly correlates with the atomic composition of the final compound nanocrystals.

Quantum Dot Surface Engineering: Toward Inert Fluorophores with Compact Size and Bright, Stable Emission

The surfaces of colloidal nanocrystals are complex interfaces between solid crystals, coordinating ligands, and liquid solutions. For fluorescent quantum dots, the properties of the surface vastly influence the efficiency of light emission, stability, and physical interactions, and thus determine their sensitivity and specificity when they are used to detect and image biological molecules. But after more than 30 years of study, the surfaces of quantum dots remain poorly understood and continue to be an important subject of both experimental and theoretical research. In this article, we review the physics and chemistry of quantum dot surfaces and describe approaches to engineer optimal fluorescent probes for applications in biomolecular imaging and sensing. We describe the structure and electronic properties of crystalline facets, the chemistry of ligand coordination, and the impact of ligands on optical properties. We further describe recent advances in compact coatings that have significantly improved their properties by providing small hydrodynamic size, high stability and fluorescence efficiency, and minimal nonspecific interactions with cells and biological molecules. While major progress has been made in both basic and applied research, many questions remain in the chemistry and physics of quantum dot surfaces that have hindered key breakthroughs to fully optimize their properties.

Two-dimensional semiconductor nanocrystals: properties, templated formation, and magic-size nanocluster intermediates

CONSPECTUS: Semiconductor nanocrystals having an extended length dimension and capable of efficiently transporting energy and charge would have useful applications in solar-energy conversion and other emerging technologies. Pseudocylindrical semiconductor nanowires and quantum wires are available that could potentially serve in this role. Sadly, however, their defective surfaces contain significant populations of surface trap sites that preclude efficient transport. The very large surface area of long wires is at least part of the problem. As electrons, holes, and excitons migrate along a nanowire or quantum wire, they are exposed to an extensive surface and to potentially large numbers of trap sites. A solution to this dilemma might be found by identifying "long" semiconductor nanocrystals of other morphologies that are better passivated. In this Account, we discuss a newly emerging family of flat semiconductor nanocrystals that have surprising characteristics. These thin, flat nanocrystals have up to micrometer-scale (orthogonal) lateral dimensions and thus very large surface areas. Even so, their typical photoluminescence efficiencies of 30% are astonishingly high and are 2 orders of magnitude higher than those typical of semiconductor quantum wires. The very sharp emission spectra of the pseudo-two-dimensional nanocrystals reflect a remarkable uniformity in their discrete thicknesses. Evidence that excitons are effectively delocalized and hence transported over the full dimensions of these nanocrystals has been obtained. The excellent optical properties of the flat semiconductor nanocrystals confirm that they are exceptionally well passivated. This Account summarizes the two synthetic methods that have been developed for the preparation of pseudo-two-dimensional semiconductor nanocrystals. A discussion of their structural features accounts for their discrete, uniform thicknesses and details the crystal-lattice expansions and contractions they exhibit. An analysis of their optical properties justifies the sharp photoluminescence spectra and high photoluminescence efficiencies. Finally, a bilayer mesophase template pathway is elucidated for the formation of the nanocrystals, explaining their flat morphologies. Magic-size nanocluster intermediates are found to be potent nanocrystal nucleants, allowing the synthesis temperatures to be decreased to as low as room temperature. The potential of these flat semiconductor nanocrystals in the form of nanoribbons or nanosheets for long-range energy and charge transport appears to be high.

Selective epitaxial growth of zinc blende-derivative on wurtzite-derivative: the case of polytypic Cu2CdSn(S(1-x)Se(x))4 nanocrystals

Polytypic nanocrystals with zinc blende (ZB) cores and wurtzite (WZ) arms, such as tetrapod and octopod nanocrystals, have been widely reported. However, polytypic nanocrystals with WZ cores and ZB arms or ends have been rarely reported. Here, we report a facile, solution-based approach to the synthesis of polytypic Cu2CdSn(S1-xSex)4 (CCTSSe) nanocrystals with ZB-derivative selectively engineered on (000±2)WZ facets of WZ-derived cores. Accordingly, two typical morphologies, i.e., bullet-like nanocrystals with a WZ-derivative core and one ZB-derivative end, and rugby ball-like nanocrystals with a WZ-derivative core and two ZB-derivative ends, can be selectively prepared. The epitaxial growth mechanism is confirmed by the time-dependent experiments. The ratio of rugby ball-like and bullet-like polytypic CCTSSe nanocrystals can be tuned through changing the amount of Cd precursor to adjust the reactivity difference between (0002)WZ and (000-2)WZ facets. These unique polytypic CCTSSe nanocrystals may find applications in energetic semiconducting materials for energy conversion in the future.

Mechanistic insights into the role of alkylamine in the synthesis of CdSe nanocrystals

This paper reports a detailed mechanistic study of the effect of alkylamine on the synthesis of CdSe nanocrystals. Alkylamines are one of the most important additives for the synthesis of colloidal semiconductor nanocrystals. However, their effect on the monomer production as well as nanocrystal nucleation and growth are not well understood, as indicted by inconsistent and contradictory conclusions in the literature. We found that alkylamines slow down the reaction between cadmium oleate and trialkyl phosphine selenide by binding to cadmium and preventing the activation of trialkyl phosphine selenide. A linear correlation was observed between the observed reaction rate constant and the (31)P NMR chemical shift or (1)J(P-Se) of phosphine selenide. In the presence of alkylamine, an alkylaminophosphonium intermediate was observed. Mechanistic study suggests that the cleavage of P═Se bond is through nucleophilic attack by carboxylate instead of alkylamine. Interestingly, although alkylamines decrease the rate of monomer production, it increases the rate of CdSe nanocrystal growth. Although seemingly contradictory, this is due to a drastic decrease in the nanocrystal nucleation events in the presence of alkylamines. As a result, each nucleus is fed with more monomers and grows faster in the presence of alkylamine than in its absence.

Shape-programmed nanofabrication: understanding the reactivity of dichalcogenide precursors

Dialkyl and diaryl dichalcogenides are highly versatile and modular precursors for the synthesis of colloidal chalcogenide nanocrystals. We have used a series of commercially available dichalcogenide precursors to unveil the molecular basis for the outcome of nanocrystal preparations, more specifically, how precursor molecular structure and reactivity affect the final shape and size of II-VI semiconductor nanocrystals. Dichalcogenide precursors used were diallyl, dibenzyl, di-tert-butyl, diisopropyl, diethyl, dimethyl, and diphenyl disulfides and diethyl, dimethyl, and diphenyl diselenides. We find that the presence of two distinctively reactive C-E and E-E bonds makes the chemistry of these precursors much richer and interesting than that of other conventional precursors such as the more common phosphine chalcogenides. Computational studies (DFT) reveal that the dissociation energy of carbon-chalcogen (C-E) bonds in dichalcogenide precursors (R-E-E-R, E=S or Se) increases in the order (R): diallyl<dibenzyl<di-tert-butyl<diisopropyl<diethyl<dimethyl<diphenyl. The dissociation energy of chalcogen-chalcogen (E-E) bonds remains relatively constant across the series. The only exceptions are diphenyl dichalcogenides, which have a much lower E-E bond dissociation energy. An increase in C-E bond dissociation energy results in a decrease in R-E-E-R precursor reactivity, leading to progressively slower nucleation and higher selectivity for anisotropic growth, all the way from dots to pods to tetrapods. Under identical experimental conditions, we obtain CdS and CdSe nanocrystals with spherical, elongated, or tetrapodal morphology by simply varying the identity and reactivity of the dichalcogenide precursor. Interestingly, we find that precursors with strong C-E and weak E-E bond dissociation energies such as Ph-S-S-Ph serve as a ready source of thiol radicals that appear to stabilize small CdE nuclei, facilitating anisotropic growth. These CdS and CdSe nanocrystals have been characterized using structural and spectroscopic methods. An intimate understanding of how molecular structure affects the chemical reactivity of molecular precursors enables highly predictable and reproducible synthesis of colloidal nanocrystals with specific sizes, shapes, and optoelectronic properties for customized applications.

Molecular Chemistry to the Fore: New Insights into the Fascinating World of Photoactive Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals possess unique properties that are unmatched by other chromophores such as organic dyes or transition-metal complexes. These versatile building blocks have generated much scientific interest and found applications in bioimaging, tracking, lighting, lasing, photovoltaics, photocatalysis, thermoelectrics, and spintronics. Despite these advances, important challenges remain, notably how to produce semiconductor nanostructures with predetermined architecture, how to produce metastable semiconductor nanostructures that are hard to isolate by conventional syntheses, and how to control the degree of surface loading or valence per nanocrystal. Molecular chemists are very familiar with these issues and can use their expertise to help solve these challenges. In this Perspective, we present our group's recent work on bottom-up molecular control of nanoscale composition and morphology, low-temperature photochemical routes to semiconductor heterostructures and metastable phases, solar-to-chemical energy conversion with semiconductor-based photocatalysts, and controlled surface modification of colloidal semiconductors that bypasses ligand exchange.

Recent advances in the synthesis of colloidal nanowires

We review critically the advances in the synthesis of colloidal nanowires that have occurred over the past three years, with a focus on those that produced very thin (or “ultrathin”) nanowires (∼2–3 nm in diameter or less). We discuss the importance of these ultrathin nanowires, especially in light of the emerging evidence of their topological properties and their potential similarities with polymers.

The importance of nanocrystal precursor conversion kinetics: mechanism of the reaction between cadmium carboxylate and cadmium bis(diphenyldithiophosphinate)

We describe the synthesis of cadmium bis(diphenyldithiophosphinate) (Cd(S(2)PPh(2))(2)) from secondary phosphine sulfides and its conversion to cadmium sulfide nanocrystals. Heating Cd(S(2)PPh(2))(2) and cadmium tetradecanoate (≥4 equiv) to 240 °C results in complete conversion of Cd(S(2)PPh(2))(2) to cadmium sulfide nanocrystals with tetradecanoate surface termination. The nanocrystals have a narrow size distribution (d = 3.8-4.1 nm, σ < 10%) that is evident from the line width of the lowest energy absorption feature (λ = 412-422 nm, fwhm = 0.17 eV) and display bright photoluminescence (PLQY(band edge+trap) = 36%). Interestingly, the final diameter is insensitive to the reaction conditions, including the total concentration of precursors and initial cadmium to sulfur ratio. Monitoring the reaction with (31)P NMR, UV-visible, and infrared absorption spectroscopies shows that the production of cadmium diphenylphosphinate (Cd(O(2)PPh(2))(2)) and tetradecanoic anhydride co-products is coupled with the formation of cadmium sulfide. From these measurements we propose a balanced chemical equation for the conversion reaction and use it to optimize a synthesis that affords CdS nanocrystals in quantitative yield. In light of these results we discuss the importance of well-defined precursor reactivity to reproducible conversion kinetics and the synthesis of nanocrystals with unambiguous chemical composition.

Ultraviolet ZnSe₁-xSx gradient-alloyed nanocrystals via a noninjection approach

Highly emissive ultraviolet ZnSeS nanocrystals (NCs), with a core-shell-like structure, were designed and synthesized via a one-step noninjection approach in 1-octadecene (ODE). These ultraviolet ZnSeS NCs exhibit bright bandgap emission with high color purity and little trap emission. With full width at half-maximum (fwhm) of ∼21 nm only, photoluminescent (PL) quantum yield (QY) of ∼60% was estimated for one ensemble dispersed in toluene exhibiting bandgap absorption peaking at ∼380 nm and bandgap emission at ∼389 nm. These alloyed ZnSeS NCs present a cubic crystal structure consisting of a Se-rich core and a S-rich shell. Such a gradiently alloyed structure was suggested by our investigation on the temporal evolution of optical properties of the growing ZnSeS NCs monitored from 80 to 300 °C, together with structural and compositional characterization performed with XRD, XPS, EDX, and TEM. This newly developed one-step noninjection approach was achieved with zinc oleate (Zn(OA)(2)), diphenylphosphine selenide (SeDPP), and diphenylphosphine sulfide (SDPP) as Zn, Se, and S precursors, respectively. ZnSe monomers mainly participated in nucleation at ∼120 °C, while both ZnSe and ZnS monomers contributed to NC formation in later growth stages (∼160 °C and higher). (31)P NMR study demonstrates that SeDPP is more reactive than SDPP toward Zn(OA)(2), and also supports such a model proposed on the combination of ZnSe and ZnS monomers leading to nucleation/growth of ZnSeS alloyed NCs. The present study offers conceptual methodology to various highly photoluminescent alloyed NCs with high quality, high particle yield, and high synthetic reproducibility.