Solution phase synthesis and characterization of III-V, II-VI and CdSe.₀₈Te.₉₂ semiconductor nanowires
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There are many advantages to the solution phase synthesis of semiconductor nanowires, the most notable of which are the ease of scalability and the production of nanowires in higher yields than those typically obtained in chemical vapor deposition (CVD) based processes. The solution phase synthesis of high quality, high aspect ratio (>100) narrow diameter semiconductor nanowires depends sensitively on three parameters: the diameter of the nanocrystals utilized to promote (seed) nanowire growth, molecular precursor decomposition kinetics and the choice of solvent in which the nanowires are grown. Bismuth is a low melting point (270 °C) semimetal and thus an ideal candidate for the solution-liquid-solid (SLS) growth of nanowires. A bismuth nanocrystal synthesis was developed that affords nanocrystals with average diameters from 4 - 20 nm. The nanocrystal diameter is controlled by varying the capping ligand (TOPO) to bismuth molar ratio. The synthesis of Au2Bi nanocrystals was also studied as it also affords small diameter (~ 2 nm) nanocrystals that are suitable for SLS nanowire growth. Molecular precursor decomposition kinetics can have a significant impact on nanowire yield and quality. Precursors that decompose too quickly can produce relatively large diameter nanowires, while precursors that decompose too slowly can produce nanowires with a highly tortuous morphology as a result of a high density of crystallographic defects. The choice of molecular precursor for the synthesis of III-V and II-VI nanowires was investigated. The solvent in which nanowires are grown can also have a significant effect on nanowire yield, quality and morphology. Coordinating solvents such as alkylphosphine oxides and alkylamines can interact with the atoms, or atomic complexes, that constitute nanowires and thus mediate the nanowire growth rate. In some instances, for example InAs nanowires grown in TOPO, this interaction can completely quench nanowire growth. This solvent effect has been investigated for the growth of III-V and II-VI nanowires. Solvents can also affect nanowire morphology. Branched ZnSe nanowires, i.e. hybrid nanostructures in which ZnSe nanorods grow epitaxially from the surface of ZnSe nanowires, are synthesized in trioctylamine whereas TOPO suppresses this branched growth. Finally, a mechanism which allows for the synthesis of narrow diameter nanowires seeded by much larger diameter nanocrystals is investigated. Bismuth nanocrystals with an average diameter of ~ 20 nm are utilized to promote the growth of narrow diameter (~ 6 nm) CdSe.₀₈Te.₉₂ nanowires.