Photoelectrochemical and Raman characterization of nanocrystalline CdS grown on ZnO by successive ionic layer adsorption and reaction method

Abstract

Properties of CdS nanoparticles (NPs) grown by successive ionic layer adsorption and reaction (SILAR) method on the surface of electrodeposited ZnO films were studied by Raman, photocurrent and UV–Vis absorption spectroscopies. The CdS nanoparticles deposited at a SILAR cycle number (N) from 5 to 10 exhibit a broadening of the band gap (Eg) by 0.17–0.31eV as compared with that of the CdS particles grown at N=30. The size quantization of the interband transition energy in CdS nanoparticles is in accordance with the Raman spectroscopic data demonstrating a considerable increase in the LO peak intensity with increasing the N from 5 to 10 as a result of transition to resonant light scattering. The spectral width of the LO peak decreases from 50 to 15cm−1 as the N increases from 5 to 30 reflecting a less pronounced effect of the nanoparticle surface on the phonon scattering. A large spectral width of the Raman peaks is assumed to originate from a complex structure of the CdS nanoparticles comprising crystallinity domains that can affect the phonon confinement. The photocurrent spectroscopy of ZnO/CdS heterostructures showed that the band gap of CdS NPs deposited at N>20 is smaller by ~0.08eV than that of bulk cadmium sulfide. It was concluded that this effect is not associated with photoexcitation of structural defects but rather reflects intrinsic electronic properties of SILAR-deposited CdS nanoparticles.