William E. Buhro

Solution-Liquid-Solid Growth of Crystalline III-V Semiconductors: An Analogy to Vapor-Liquid-Solid Growth

Until now, micrometer-scale or larger crystals of the III-V semiconductors have not been grown at low temperatures for lack of suitable crystallization mechanisms for highly covalent nonmolecular solids. A solution-liquid-solid mechanism for the growth of InP, InAs, and GaAs is described that uses simple, low-temperature (≤203°C), solution-phase reactions. The materials are produced as polycrystalline fibers or near-single-crystal whiskers having widths of 10 to 150 nanometers and lengths of up to several micrometers. This mechanism shows that processes analogous to vapor-liquid-solid growth can operate at low temperatures; similar synthesis routes for other covalent solids may be possible.

The Trouble with TOPO; Identification of Adventitious Impurities Beneficial to the Growth of Cadmium Selenide Quantum Dots, Rods, and Wires

Tri-n-octylphosphine oxide (TOPO) is a commonly used solvent for nanocrystal synthesis. Commercial TOPO samples contain varying amounts of phosphorus-containing impurities, some of which significantly influence nanocrystal growth. Consequently, nanocrystal syntheses often give irreproducible results with different batches of TOPO solvent. In this study, we identify TOPO impurities by (31)P NMR, and correlate their presence with the outcomes of CdSe nanocrystal syntheses. We subsequently add the active impurity species, one by one, to purified TOPO to confirm their influence on nanocrystal syntheses. In this manner, di-n-octylphosphine oxide (DOPO) is shown to assist CdSe quantum-dot growth; di-n-octylphosphinic acid (DOPA) and mono-n-octylphosphinic acid (MOPA) are shown to assist CdSe quantum-rod growth, and DOPA is shown to assist CdSe quantum-wire growth. (The TOPO impurity n-octylphosphonic acid, OPA, has been previously shown to assist quantum-rod growth.) The beneficial impurities are prepared on multigram scales and can be added to recrystallized TOPO to provide reproducible synthetic results.

Spectroscopic identification of tri-n-octylphosphine oxide (TOPO) impurities and elucidation of their roles in cadmium selenide quantum-wire growth.

Tri-n-octylphosphine oxide (TOPO) is the most commonly used solvent for the synthesis of colloidal nanocrystals. Here we show that the use of different batches of commercially obtained TOPO solvent introduces significant variability into the outcomes of CdSe quantum-wire syntheses. This irreproducibility is attributed to varying amounts of phosphorus-containing impurities in the different TOPO batches. We employ (31)P NMR to identify 10 of the common TOPO impurities. Their beneficial, harmful, or negligible effects on quantum-wire growth are determined. The impurity di-n-octylphosphinic acid (DOPA) is found to be the important beneficial TOPO impurity for the reproducible growth of high-quality CdSe quantum wires. DOPA is shown to beneficially modify precursor reactivity through ligand substitution. The other significant TOPO impurities are ranked according to their abilities to similarly influence precursor reactivity. The results are likely of general relevance to most nanocrystal syntheses conducted in TOPO.

Lamellar Assembly of Cadmium Selenide Nanoclusters into Quantum Belts

Here, we elucidate a double-lamellar-template pathway for the formation of CdSe quantum belts. The lamellar templates form initially by dissolution of the CdX(2) precursors in the n-octylamine solvent. Exposure of the precursor templates to selenourea at room temperature ultimately affords (CdSe)(13) nanoclusters entrained within the double-lamellar templates. Upon heating, the nanoclusters are transformed to CdSe quantum belts having widths, lengths, and thicknesses that are predetermined by the dimensions within the templates. This template synthesis is responsible for the excellent optical properties exhibited by the quantum belts. We propose that the templated-growth pathway is responsible for the formation of the various flat, colloidal nanocrystals recently discovered, including nanoribbons, nanoplatelets, nanosheets, and nanodisks.

Bismuth, tellurium, and bismuth telluride nanowires

Decomposition of the precursor Bi[N(SiMe3)2]3 in the presence of poly(1-hexadecene)0.67-co-(1-vinylpyrrolidinone)0.33 and a small amount of NaN(SiMe3)2 in 1,3-diisopropylbenzene solution at 203 °C gives Bi nanowires. The single-crystalline wires are several micrometers in length, and possess a mean diameter of 5.9 ± 2.4 nm. Decomposition of TeCl4 in the presence of TOPO in polydecene solution at 250–300 °C gives Te nanowires. These nanowires are also single crystals exhibiting micrometer lengths. The Te nanowire mean diameters are in the range of 30–60 nm, and depend upon the reaction conditions employed. Attempts to grow Bi2Te3 nanowires from reactions of Bi- and Te-containing molecular precursors produce other Bi2Te3 crystallite morphologies. Efforts to convert Bi nanowires into Bi2Te3 nanowires are also unsuccessful. However, reactions of Te nanowires and BiPh3 in polydecene solution at 160 °C do afford continuous Bi2Te3 nanowires, having micrometer lengths and mean diameters larger than those of the precursor Te nanowires. The Bi2Te3 nanowires are found to be encased in amorphous sheath structures, which apparently enforce the one-dimensional wire morphologies.

Origin of High Photoluminescence Efficiencies in CdSe Quantum Belts

CdSe quantum belts (QBs) having lengths of 0.5-1.5 microm and thicknesses of 1.5-2.0 nm exhibit high photoluminescence (PL) efficiencies of approximately 30%. Epifluorescence studies establish the PL spectra to be uniform along single QBs, and nearly the same from QB to QB. Photogenerated excitons are shown to be effectively delocalized over the entire QBs by position-selective excitation. Decoration of the QBs with gold nanoparticles indicates a low density of surface-trap sites, located primarily on the thin belt edges. The high PL efficiencies and effective exciton delocalization are attributed to the minimization of defective {1100} edge surface area or edge-top/bottom (face) line junctions in QBs relative to quantum wires having roughly isotropic cross sections, for which very low PL quantum efficiencies have been reported. The results suggest that trap sites can be minimized in pseudo-one-dimensional nanocrystals, such that the facile transport of energy and charge along their long axes becomes possible.

Synthesis of cadmium telluride quantum wires and the similarity of their effective band gaps to those of equidiameter cadmium telluride quantum dots.

High-quality colloidal CdTe quantum wires having purposefully controlled diameters in the range 5-11 nm are grown by the solution-liquid-solid (SLS) method, using Bi nanoparticle catalysts, cadmium octadecylphosphonate and trioctylphosphine telluride as precursors, and a TOPO solvent. The wires adopt the wurtzite structure and grow along the [002] direction (parallel to the c axis). The size dependence of the effective band gaps in the wires is determined from the absorption spectra and compared to the experimental results for high-quality CdTe quantum dots. In contrast to the predictions of an effective-mass approximation, particle-in-a-box model, and previous experimental results from CdSe and InP dot-wire comparisons, the effective band gaps of CdTe dots and wires of like diameter are found to be experimentally indistinguishable. The present results are analyzed using density functional theory under the local-density approximation by implementing a charge-patching method. The higher-level theoretical analysis finds the general existence of a threshold diameter, above which dot and wire effective band gaps converge. The origin and magnitude of this threshold diameter are discussed.

Isolation of the Magic-Size CdSe Nanoclusters [(CdSe)13(n-octylamine)13] and [(CdSe)13(oleylamine)13]†

The preparation, isolation, stoichiometric characterization, and dissolution of purified (CdSe)13 nanoclusters are described. We[1] and others[2] recently reported that (CdSe)13 nanoclusters were intermediates in the synthesis of CdSe quantum belts (nanoribbons). We now demonstrate that a lamellar intermediate phase[1] collected from the quantum-belt synthesis is [(CdSe)13(n-octylamine)13], the smallest, discrete, magic-size nanocluster of CdSe that has been observed.[3] Kinetic data show that free, soluble [(CdSe)13(oleylamine)13] nanoclusters are released from the insoluble [(CdSe)13(n-octylamine)13] upon ligand exchange.

Preparation and X-ray crystal structures of volatile copper(II) alkoxides

Abstract Volatile amino-alkoxide complexes Cu[OCH2CH2NMe2]2 (1), Cu[OCHMeCH2 NMe2]2 (2) and Cu[OCH2CH2NMeCH2CH2NMe2]2 (3) have been prepared. X-ray crystal structures reveal a mononuclear, square-planar geometry for 2, and a mononuclear, distorted, square-pyramidal geometry for 3. Thermal decomposition (25–300°C) of 1 gives a mixture of copper, CuO and Cu2O whereas thermal decomposition of 2 gives copper.

An Easy Shortcut Synthesis of Size‐Controlled Bismuth Nanoparticles and Their Use in the SLS Growth of High‐Quality Colloidal Cadmium Selenide Quantum Wires

An easy shortcut synthesis of thermally stable, near-monodisperse Bi nanoparticles from BiCl(3) and Na[N(SiMe(3))(2)] is described. The diameters of the Bi nanoparticles are controlled in the range of 4-29 nm by varying the amounts of BiCl(3) and Na[N(SiMe(3))(2)] employed. Standard deviations in their diameter distributions are 5-15% of the mean diameters, consistent with near monodispersity. These Bi nanoparticles are shown to be the best currently available catalysts for the solution-liquid-solid (SLS) growth of high-quality CdSe quantum wires.