Germanium nanocrystals were synthesized in supercritical (sc) CO_2 by thermolysis of diphenylgermane (DPG) or tetraethylgermane (TEG) with octanol as a capping ligand at 500 deg C and 27.6 MPa. The Ge nanocrystals were characterized with high resolution transmission electron microscopy (HRTEM), energy-dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD). On the basis of TEM, the mean diameters of the nanocrystals made from DPG and TEG were 10.1 and 5.6 nm, respectively. The synthesis in sc-CO_2 produced much less organic contamination compared with similar reactions in organic supercritical fluids. When the same reaction of DPG with octanol was performed in the gas phase without CO_2 present, bulk Ge crystals were formed instead of nanocrystals. Thus, the solvation of the hydrocarbon ligands by CO_2 was sufficient to provide steric stabilization. The presence of steric stabilization in CO_2 at a reduced temperature of 2.5, with a reduced solvent density of only 0.4, may be attributed to a reduction in the differences between ligand-ligand interactions and ligand-CO_2 interactions relative to thermal energy.
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