Raman spectroscopy of PbS nanocrystalline semiconductors

We have studied the effect of crystalline size and surface roughness on Raman intensity for PbS nanocrystalline semiconductors of different crystalline sizes prepared by an electrochemical route. Rutherford back scattering spectroscopy provides the estimation of surface roughness, thickness, and composition of the PbS samples. The Raman spectrum shows a low-frequency wing at $\sim 205\hspace{0.5em}{\mathrm{cm}}^{-1}$ besides the characteristic first order longitudinal optical phonon mode at $\sim 210\hspace{0.5em}{\mathrm{cm}}^{-1}$ when excited with a laser of wavelength 514.5 nm. The observed variation of the Raman shifts, widths, and intensities of these two peaks have been discussed as a function of crystalline size. The low frequency wing is identified as a surface phonon mode. In addition to these two peaks, a peak at $271\hspace{0.5em}{\mathrm{cm}}^{-1}$ and another at $415\hspace{0.5em}{\mathrm{cm}}^{-1}$ are observed in good agreement with earlier reported results.