Silicon nanocrystals were synthesized at high temperatures and high pressures by the thermolysis of diphenylsilane using a combination of supercritical carbon dioxide and phosphonic acid surfactants. Size and shape evolution from pseudo-spherical silicon nanocrystals to well-faceted tetrahedral-shaped silicon crystals with edge lengths in the range of 30-400 nm were observed with sequentially decreasing surfactant chain lengths. The silicon nanocrystals were characterized by transmission electron microscopy (TEM), energy-dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), photoluminescence (PL), scanning electron microscopy (SEM) and Raman scattering spectroscopy.