Raman Scattering from Surface Phonons in Rectangular Cross-sectional w-ZnS Nanowires

Wurtzite (w-) ZnS nanowires with nearly square cross sections have been grown by pulsed laser vaporization of a (ZnS) 0.9 Au 0.1 target in a flow of Ar 5%H 2 . Growth proceeds by the vapor−liquid−solid mechanism. Raman scattering from the ZnS nanowires in air at room temperature reveals a strong first-order longitudinal optic (LO) phonon (346 cm -1 ) and two transverse optic (TO) phonons (269 and 282 cm -1 ), as well as several second-order features. Peak assignments based on bulk ZnS can be made for all the first- and second-order features, except for one band located between the LO and TO bands. This additional Raman band is observed at 335 cm -1 in air, and downshifts to 328 cm -1 in dichloromethane ( ε m = 2.0) and to 326 cm -1 in aniline ( ε m = 2.56). This band is therefore assigned to surface optic (SO) phonons. The position of the SO band is consistent with a dielectric continuum model for rectangular cross section wires. A symmetry breaking mechanism which may activate the SO mode is also discussed.