Figure 1.

Thermal oxidation decreases the size of the Si skeletal core and increases the thickness of the SiO₂ shell. (a) Top: Photographs of thermally treated porous silicon microparticles as a function of thermal oxidation time. The brownish color observed derives from the crystalline Si core, which is completely oxidized when the material is treated at a temperature of 800 °C for 1 h (“Full Oxidized”). Middle: The particles illuminated with UV (365 nm) light display differing degrees of luminescence intensity. Only the partially oxidized particles displayed detectable photoluminescence. Values of d_Si indicate average size of the luminescent Si features, estimated from eq 1. Bottom: A schematic depicting the proposed nanostructural changes is shown below the relevant images; the crystalline Si core decreases in diameter and the SiO₂ shell increases in thickness with increasing extent of oxidation. (b) Relative photoluminescence spectra of the pictured samples (λ_ex = 365 nm, 500 nm long-pass filter). The photoluminescence spectra exhibit a blue shift as the Si core oxidizes, corresponding to a reduction in average size of the crystalline Si domains in the sample.