Raman Scattering from Surface Optic Phonons in Cylindrical and Rectangular Cross-sectional Semiconducting Nanowires†

Raman scattering from surface optic (SO) phonons has been observed and identified in cylindrical GaP and rectangular cross-section ZnS nanowires. We propose that the symmetry breaking mechanism which activates the SO phonon is a periodic modulation of the cross-sectional area along the nanowires. In the case of cylindrical GaP nanowires, Raman scattering from SO phonons in air at room temperature is observed at 394 cm-1, in between the first order longitudinal optic (LO) (401 cm-1) and transverse optic (TO) (367 cm-1), and downshift to 392 cm-1 in dichloromethane (?m=2.0) and 390 cm-1 in aniline (?m=2.56). Raman scattering from the ZnS nanowires in air at room temperature reveals a strong first-order LO mode at 346 cm-1 and two TO modes at 269 and 282 cm-1. The SO Raman band in ZnS is observed at 335 cm-1 in air, and downshifts to 328 cm-1 in dichloromethane and to 326 cm-1 in aniline. The position of the SO band in GaP and ZnS nanowires is consistent with a dielectric continuum model. Theoretical SO phonon dispersion for both cylindrical and rectangular cross-section nanowires is presented and compared to experiment. {\dag}This work was supported by the NSF NIRT program (DMR- 0304178).