  Gallium arsenide nanowires were synthesized by gallium-assisted molecular
beam epitaxy. By varying the growth time, nanowires with diameters ranging from
30 to 160 nm were obtained. Raman spectra of the nanowires ensembles were
measured. The small line width of the optical phonon modes agree with an
excellent crystalline quality. A surface phonon mode was also revealed, as a
shoulder at lower frequencies of the longitudinal optical mode. In agreement
with the theory, the surface mode shifts to lower wave numbers when the
diameter of the nanowires is decreased or the environment dielectric constant
increased.
