Optical investigation on cadmium-doped zinc oxide nanoparticles synthesized by using a sonochemical method

Cadmium-doped zinc oxide CdxZn1-xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles were synthesized by using a sonochemical method. The X-ray diffraction (XRD) patterns show that CdxZn1-xO nanoparticles keep a wurtzite structure up to x = 0.40. However, the lattice parameters a, c, and internal parameter u gradually increase with increasing x. The Raman scattering investigation indicates that the Raman mode shows a significant softening in Cd-doped ZnO nanoparticles with increasing x, which suggests an expansion of the Zn-O bond. We also observe a significant red-shift of the band gap for CdxZn1-xO nanoparticles as x increases, i.e., the gap energy E-g similar to 3.21 eV for x = 0 shifts to 2.92 eV for x = 0.40. In addition, the Urbach band tail gradually increases with x-value, which indicates an enhancement of the defect states in samples with Cd incorporation. In CdxZn1-xO nanoparticles, we further find that the near band-edge (NBE) emission band shows a red-shift with increase in x-value. A broad photoluminescence (PL) band is also observed at around 590 nm, which enhances from x = 0 to 0.15. With further increases in the x-value to 0.40, a weak PL band appears around 450 nm while the broad band around 590 nm is suppressed. The variation of PL intensity is ascribed to the change of the Cd-doping induced defect states and the nonradiative energy transfers.