We have measured Raman spectra of free-standing porous silicon at 10 K. Porous silicon samples were prepared in aqueous and ethanoic electrolytes and the scattering was measured in the range of one- through four-phonon processes. A quantum size effect is clearly observed in the spectral change of the first- and second-order scattering from optical phonons but the scattering of higher orders is much more sensitive to the decrease of nanocrystallite size. The microscopic morphological changes from Si single crystals are discussed.
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Tanino et al. (1996) studied this question.