Synthesis and characterization of group IV semiconductor nanocrystals and nanowires

Group IV semiconductor nanostructures, such as Si and Ge nanocrystals and nanowires, have been challenging to synthesize in solution, primarily due to their strong covalent bonding and the resulting need for high temperatures for crystallization. Here, we present solution-based synthetic approaches for solvents including supercritical fluids and coordinating liquid solvents at elevated temperatures.; Si nanowires were successfully synthesized in supercritical fluid alkanes and octanol by thermolysis of an organosilane precursor with gold nanocrystals as growth seeds on a Si substrate. With a flow reactor designed for rapid heatup and constant concentrations of Si precursor, straight Si nanowires were grown with minimal contamination of Si particulates. By decomposing organogermane precursors in a supercritical organic solvent or supercritical CO2 (sc-CO2) in the presence of a hydrocarbon capping ligand, Ge nanocrystals with tunable diameters ranging from 2--70 nm were synthesized. The nanocrystals exhibited high crystallinity and 3--4 nm Ge nanoparticles showed blue-shifted UV-visible and photoluminescence spectra. Compared with similar reactions in supercritical organic fluid, the synthesis in sc-CO2 produced much less organic contamination.; A new synthetic approach was developed to produce Ge nanocrystals and nanowires in a coordinating liquid solvent at atmospheric pressure. A carbon-free and highly reactive Ge precursor, germanium diiodide (GeI2), was employed to synthesize Ge nanocrystals with controllable sizes ranging from 3--10nm and moderate polydispersity. High chemical yield of Ge nanocrystals was achieved due to the high reactivity of the Ge precursor, the moderate reaction temperature, and a reduction in organic by-product with bismuth (Bi) nanocrystals, highly also synthesized by decomposing GeI2 in enabled by the combination of the low decomposition temperature of GeI2, high solubility of GeI2 in the solvent and low eutectic point of Ge and Bi. This is the first example of group IV semiconductor nanowires synthesized in a liquid solvent at atmospheric pressure.