Optical properties of CdS nanoparticles upon annealing

The metastable cubic phase of CdS has been found to be stabilized in the form of nanoparticles. Zinc-blende to Wurtzite structural transformation of CdS nanoparticles, synthesized using chemical precipitation, was investigated using X-ray diffraction (XRD), Raman, photoluminescence (PL) and infrared (IR) absorption spectroscopy. The nanocrystalline powder was annealed in argon atmosphere in the temperature range 473–773 K for 2 h at each temperature. The hexagonal fraction increased monotonically during annealing and the shape of the particle becomes anisotropic. PL spectra exhibited a marginal decrease in peak position for annealing up to 573 K and then an increase. In Raman spectra, the intensity of 1-LO phonon decreases while that of 2-LO phonon increases indicating an increase in electron–phonon interaction with increase in particle size. In addition to the Frohlich surface optical phonon mode and TO phonon mode, a new mode at 195 cm−1 is found in IR spectra, which is attributed to a defect-activated zone-boundary phonon. The changes in the optical properties are attributed to those arising from particle growth and structural transformation.