Raman spectroscopy of CdS nanocrystalline semiconductors

Raman scattering measurements were performed on nanostructured II-VI semiconductor CdS prepared by a chemical route. The Raman spectrum shows a low-frequency wing at 295 cm(-1) besides the characteristic first-order longitudinal optical phonon (1LO) mode at 305 cm(-1) when excited with a laser of wavelength 457.9 nm. The observed variation of the Raman shifts, widths and intensities of these two lines with the size of the nanoparticles is consistent with the interpretation that the low-frequency peak is a surface phonon (SP) mode. Increasing the wavelength of the exciting laser lowers the intensity of the LO mode, while shifting the lower-frequency SP mode to the higher-frequency side and simultaneously increases its width. This anomalous behavior is attributed to the possible electron hole excitation by the SP due to the presence of a continuum of localized and acceptor states within CdS band gap. The effect of temperature, on these modes, is also studied and discussed. (C) 1999 Elsevier Science B.V. All rights reserved.