Shell-wise oxidation of nanocrystalline silicon observed by in situ x-ray diffraction

The oxidation of nanocrystalline silicon powder (average crystallite size of 87 Ó), prepared by laser-driven pyrolysis of SiH4 , has been studied by X-ray di+raction and transmission electron microscopy. In a previous study the Si particles were shown to form perfect isolated near-spherical "single crystalsÏ that are stable up to 1073 K under high vacuum conditions. We have measured the size reduction of the Si nanoparticles in a Ñow of oxygen as a function of the degree of oxidation. The results are in agreement with model calculations for a shell-wise oxidation of silicon surface layers to amorphous silicon oxide. An oxidation to 50 vol.% implies a size reduction of 20% and tenfold improved visible photoluminescence intensity. Preliminary kinetic studies at 656 K in oxygen indicate a self-limiting type of oxidation, which is reached for an oxide layer thickness of B6.8 Ó