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 (epsilon(m) = 2.0) and to 326 cm(-1) in aniline (epsilon(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.