Weak Quantum Confinement and Polaritons in ZnO and ZnO Cu Nanocrystals Prepared by Electrochemical Method

Photoluminescence (PL), X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Raman scattering have been used for the optical and structural characterization of ZnO nanocrystals (NCs) of various sizes. The samples prepared by an electrochemical method have a size of NCs from the range 60 to 600 nm after a heat treatment for 2 hours at 400 ^oC in ambient air. The Raman scattering technique presents the several active modes including the surface phonon mode. The Raman intensity increases with decreasing NC size and the Raman spectrum shifts to low energy. Simultaneously the X-ray diffraction diagrams present a small shift of XRD peaks that testifies on a change in the lattice parameters in ZnO NCs with decreasing NC size.

Photoluminescence spectra show a free exciton and defect-related emission. The intensity stimulation of exciton-related PL bands with NC size decreasing up to 60 nm is attributed to the realization of the week confinement and the exciton-light coupling with the formation of polariton in small size of ZnO NCs. The numerical calculation of recombination times in the frame of this model has confirmed the presented explanation.