Raman spectroscopy of ultranarrow CdS nanostructures

CdS semiconductor nanoparticles, with dimensions above and below the Bohr radius (∼2.5 nm) of bulk material, were prepared in a single-step benchtop procedure. The degree of quantum confinement in these nanoparticles was determined from their optical absorption and photoluminescence spectra. The size-dependent properties of the nanocrystals were studied by resonance Raman spectroscopy as a function of excitation wavelength and temperature. The spectra were composed of the fundamental longitudinal optical (LO) mode around 300 cm^-1, along with the first and second overtones. The shapes and positions of the Raman peaks exhibited only a weak dependence on particle size even for the two extreme cases of bulk and nanostructures. We show that the ratio of the overtone to the fundamental LO frequency was sensitive to the particle diameter and decreased upon reduction of the particle diameter to values below the Bohr radius. Temperature-dependent Raman measurements of ultranarrow nanorods showed a small red shift with decreasing temperature. Very high anti-Stokes intensities were observed for the CdS nanoparticles.