Evidence of x-ray absorption-edge shift as a function of luminescence wavelength in porous silicon

X-ray absorption fine structure (XAFS) at the Si K edge in porous silicon has been measured by monitoring the photoluminescence yield (PLY) in different parts of the luminescence band: at increasing luminescence energy, a continuous positive shift of the x-ray absorption edge has been observed. The peculiar selectivity of the partial PLY-XAFS technique to the luminescent sites allows us to perform a size selection of the different nanostructures distributed in a single porous-silicon sample. The recombination of carriers localized in quantum confined nanocrystals is confirmed to be the main cause of the optical emission.