Raman spectroscopy of free-standing Ge nanocrystals

Ge nanocrystals, with an average diameter of 5 nm, are grown in a silica matrix. Free-standing nanocrystals are obtained by selectively etching the oxide. Embedded nanocrystals experience considerable compressive stress relative to the bulk. The Raman line position of free-standing nanocrystals is redshifted by $\sim $6 cm$^{-1}$ relative to that of embedded nanocrystals, indicating relief of the compressive stress. Mixed surface/bulk vibrational modes between 125 and 250 cm$^{-1}$ and surface modes below 125 cm$^{-1}$ are observed by Raman spectroscopy on free-standing nanocrystals. These modes are not observed for the case of embedded nanocrystals. Exposed nanocrystals are stable under ambient atmospheric conditions after the formation of a thin, self-limiting native oxide layer. The effect of oxide layer thickness on the vibrational spectra of free-standing nanocrystals will be discussed. This work is supported in part by U.S. NSF Grant Nos. DMR-0109844 {\&} EEC-0085569 and in part by U.S. DOE under Contract No. DE-AC03-76F00098.