P. C. Gibbons

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.

CONSTRAINTS ON STELLAR GRAIN FORMATION FROM PRESOLAR GRAPHITE IN THE MURCHISON METEORITE

We report the results of isotopic, chemical, structural, and crystallographic microanalyses of graphitic spherules (0.3E9 km) extracted from the Murchison meteorite. The spherules have 12C/13C ratios ranging over 3 orders of magnitude (from 0.02 to 80 times solar), clearly establishing their presolar origin as stellar condensates. These and other isotopic constraints point to a variety of stellar types as sources of the carbon, including low-mass asymptotic giant branch (AGB) stars and supernovae. Transmission elec- tron microscopy (TEM) of ultrathin sections of the spherules revealed that many have a composite struc- ture consisting of a core of nanocrystalline carbon surrounded by a mantle of well-graphitized carbon. The nanocrystalline cores are compact masses consisting of randomly oriented graphene sheets, from PAH-sized units up to sheets 3E4 nm in diameter, with little graphitic layering order. These sheets prob- ably condensed as isolated particles that subsequently coalesced to form the cores, after which the sur- rounding graphitic mantles were added by vapor deposition. We also detected internal crystals of metal carbides in one-third of the spherules. These crystals (5E200 nm) have compositions ranging from nearly pure TiC to nearly pure Zr-Mo carbide. Some of these car- bides occur at the centers of the spherules and are surrounded by well-graphitized carbon, having evi- dently served as heterogeneous nucleation centers for condensation of carbon. Others were entrained by carbon as the spherules grew. The chemical and textural evidence indicates that these carbides formed prior to carbon condensation, which indicates that the C/O ratios in the stellar sources were very close to unity. Only one of the 67 spherules studied in the TEM contained SiC, from which we infer that carbon condensation nearly always preceded SiC formation. This observation places stringent limits on the possible delay of graphite formation and is consistent with the predictions of equilibrium thermody- namics in the inferred range of pressure and C/O ratios. We model the formation of the observed refractory carbides under equilibrium conditions, both with and without s-process enrichment of Zr and Mo, and show that the chemical variation among internal crystals is consistent with the predicted equilibrium condensation sequence. The compositions of most of the Zr-Mo-Ti carbides require an s-process enrichment of both Zr and Mo to at least 30 times their solar abundances relative to Ti. However, to account for crystals in which Mo is also enriched relative to Zr, it is necessary to suppose that Zr is removed by separation of the earliest formed ZrC crystals from their parent gas. We also explore the formation constraints imposed by kinetics, equilibrium thermodynamics, and the observation of clusters of carbide crystals in some spherules, and conclude that relatively high formation pressures dynes cm~2), and/or condensable carbon number densities cm~3) are required. (Z0.1 (Z108 The graphite spherules with 12C/13C ratios less than the solar value may have originated in AGB stellar winds. However, in the spherically symmetric AGB atmospheres customarily assumed in models of stellar grain formation, pressures are much too low (by factors of to produce carbide crystals or Z102) graphite spherules of the sizes observed within plausible timescales. If some of the graphite spherules formed in the winds from such stars, it thus appears necessary to assume that the regions of grain forma- tion are density concentrations with length scales less than a stellar radius. Some of the spherules with both 12C/13C ratios greater than the solar value and 28Si excesses probably grew in the ejecta of super- novae. The isotopic compositions and growth constraints imply that they must have formed at high den- sities (e.g., with g cm~3) from mixtures of inner-shell material with material from the C-rich

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.

STABLE TI-BASED QUASICRYSTAL OFFERS PROSPECT FOR IMPROVED HYDROGEN STORAGE

The desorption of hydrogen from a novel material, a Ti45Zr38Ni17‐H quasicrystal, was observed using high‐temperature powder x‐ray diffraction, demonstrating the potential utility of Ti‐based quasicrystals in place of crystalline or amorphous hydrides for hydrogen storage applications. The maximum observed change in hydrogen concentration was from 61 at. %, corresponding to a hydrogen‐to‐metal ratio (H/M) of 1.54, at 91 °C to less than 2.5 at. % (H/M=0.025) at 620 °C. The onset temperature of desorption is below 350 °C. Surface oxidation was found to promote the formation of crystalline hydride phases. Highly oxidized samples transformed to a mixture of the C14 Laves and C15 Laves crystalline hydrides, and the Ti2Ni phase. When the oxidation was less severe, a reversible transformation between the quasicrystal and crystalline hydride phases was clearly observed, demonstrating the stability of the Ti45Zr38Ni17 quasicrystal at very low hydrogen concentrations, and temperatures as high as 661 °C. This is the ...

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.

Hydrogen Storage in Quasicrystals

Quasicrystals may have important applications as new technological materials. In particular, work in our laboratory has shown that some quasicrystals may be useful as hydrogen-storage materials. Some transition metals have a capacity to store hydrogen to a density exceeding that of liquid hydrogen. Such systems allow for basic investigations of solid-state phenomena such as phase transitions, atomic diffusion, and electronic structure. They may also be critical materials for the future energy economy. The depletion of the worlds petroleum reserves and the increased environmental impact of conventional combustion-engine powered automobiles are leading to renewed interest in hydrogen. TiFe hydrides have already been used as storage tanks for stationary nonpolluting hydrogen internal-combustion engines. Nickel metal-hydride batteries are commonly used in a wide range of applications, most notably as power sources for portable electronic devices—particularly computers. The light weight and low cost of titanium-transition-metal alloys offer significant advantages for such applications. Unfortunately they tend to form stable hydrides, which prevents the ready desorption of the stored hydrogen for the intended use. Some titanium/zirconium quasicrystals have a larger capacity for reversible hydrogen storage than do competing crystalline materials. Hydrogen can be loaded from the gas phase at temperatures as low as room temperature and from an electrolytic solution. The hydrogen goes into solution in the quasicrystal structure, often avoiding completely the formation of undesirable crystalline hydride phases. The proven ability to reversibly store variable quantities of hydrogen in a quasicrystal not only points to important areas of application but also opens the door to previously inaccessible information about the structure and dynamics of this novel phase. Selected results illustrating these points appear briefly here.

Hydrogenation of Pd-coated samples of the Ti–Zr-based icosahedral phase and related crystalline phases

Abstract Although Ti-based quasicrystals and related crystalline phases are candidate materials for hydrogen storage applications, their absorption, desorption and cycling have been inhibited by the presence of a surface barrier due to an oxide coating. To circumvent this, melt-spun ribbons and annealed ingots of Ti 45 Zr 38 Ni 17 , containing the thermodynamically stable icosahedral phase (i-phase), were plasma-etched and a thin Pd coating was applied by vapor deposition. This dramatically reduced the loading time, enabled absorption at room temperature, and allowed reversible desorption of the stored hydrogen. This procedure also allowed an investigation of hydrogen absorption in the C14 phase in these alloys and in the i-phases and related crystal phases in Ti–(Cr, Mn)–Si–O alloys. The relative loading capacities of these phases are presented and discussed.

Electron energy loss spectrometry of interstellar diamonds

The results are reported of electron energy loss spectra (EELS) measurements on diamond residues from carbonaceous meteorites designed to elucidate the structure and composition of interstellar diamonds. Dynamic effective medium theory is used to model the dielectric properties of the diamonds and in particular to synthesize the observed spectra as mixtures of diamond and various pi-bonded carbons. The results are shown to be quantitatively consistent with the idea that diamonds and their surfaces are the only contributors to the electron energy loss spectra of the diamond residues and that these peculiar spectra are the result of the exceptionally small grain size and large specific surface area of the interstellar diamonds. 35 refs.

Cluster structure and hydrogen in Ti–Zr–Ni quasicrystals and approximants

Abstract Elastic neutron diffraction data from icosahedral Ti 45 Zr 38 Ni 17 are presented and analyzed using information from the 1/1 approximant Ti 50 Zr 35 Ni 15 . These data indicate that similar clusters exist in the approximant and the i-phase. This is shown to be consistent with simulated diffraction from an icosahedral glass model of the quasicrystal, placing a Bergman cluster on the glass sites. An electrochemical method was used to hydrogenate Ti-based quasicrystals and their crystal approximants. This technique gives a consistently high hydrogen to metal atom ratio of 1.9, without crystal hydride formation in the quasicrystal.