Chemical preparation of crystalline, nonmolecular solids, including solution-liquid-solid (SLS) growth of semiconductor fibers and varied routes to nanocrystalline molybdenum disilicide.

摘要

New methods for the preparation of crystalline, nonmolecular solids under milder conditions and/or with control of crystallite size or morphology were developed in two separate projects. In one project, polycrystalline 13-15 semiconductor fibers (dimensions 10-100 nm x 50-1000 nm) were grown by solution-based chemical methods. Crystal precursor species of the general formula (R{dollar}rmsb{lcub}x{rcub}InEHsb{lcub}x{rcub}rbracksb{lcub}n{rcub},{dollar} where E is a pnictide and R is an alkyl group, were prepared by the phosphinolysis or arsinolysis of alkylindanes in aromatic solvents. Thermal decomposition of these precursors in solution, which was catalyzed by various protic reagents (MeOH, PhSH, Et{dollar}sb2{dollar}NH, or PhCO{dollar}sb2{dollar}H), resulted in crystalline InE when In metal was present in the form of submicron droplets dispersed in the solvent. Crystallization was determined to occur (at the lowest temperatures reported for 13-15 semiconductors, {dollar}<{dollar}164{dollar}spcirc{dollar}C) by a previously unreported solution-liquid-solid (SLS) mechanism reminiscent of vapor-liquid-solid (VLS) growth of single-crystal whiskers.; Some analogous reactions were investigated in which t-Bu{dollar}sb3{dollar}Ga was mixed with the alkylindane in order to prepare ternary alloys {dollar}rm(Insb{lcub}x{rcub}Gasb{lcub}1-x{rcub}As).{dollar} Product crystallinity and composition was dependent on, though not exclusively determined by, the indane/gallane ratio. Crystals with composition within the miscibility gap for this alloy system were grown.; The focus of the other project was the preparation of nanocrystalline (crystallite dimensions {dollar}<{dollar}100 nm) MoSi{dollar}sb2.{dollar} Two successful methods were developed. The first method consisted of coreducing molybdenum and silicon halides by NaK alloy in an ultrasonically agitated hydrocarbon solvent followed by thermal processing (900{dollar}spcirc{dollar}C) under vacuum to eliminate byproduct salts. MoSi{dollar}sb2{dollar} crystallites averaging 20-50 nm were obtained. Solvent degradation during this process resulted in the incorporation of substantial carbonaceous impurity (believed to be SiC) in these products. To eliminate the carbon, similar solventless reductions (without ultrasound) were conducted in molten magnesium, but average particle sizes have not been refined into the nanometer regime (currently 100-200 nm). The second method was the reaction of MoCl{dollar}sb3{dollar} and Si in the solid state. These reactants underwent an ignition at approximately 500{dollar}spcirc{dollar}C that resulted in the evolution of SiCl{dollar}sb4{dollar} and the formation of MoSi{dollar}sb2{dollar} crystallites. Crystallite size was dependent on reaction scale and was only nanocrystalline for very small scales. Addition of an inert salt to the reaction mixture, however, moderated the exothermic process and allowed for the preparation of nanocrystalline product ({dollar}sim{dollar}50 nm).; Some of the nanocrystalline MoSi{dollar}sb2{dollar} powders were consolidated and tested for mechanical properties. Enhanced strength and hardness compared to conventional MoSi{dollar}sb2{dollar} were established.