Oxygen induced strained ZnO nanoparticles: an investigation of Raman scattering and visible photoluminescence

We report the influence of the nanosized effect on the optical properties of non-centrosymmetric ZnO nanoparticles. In this study confocal Raman scattering was employed to investigate the strain effect of the softening A(1)(LO) phonon mode white controlling particle sizes from 55 +/- 1 to 32 +/- 1 nm. The observations reveal a positive Poisson ratio between the compressive-and tensile-strain. The intensity ratio of the A(1)(2LO)/A(1)(LO) modes exhibits strong size dependence. As the particle sizes decrease further, the ratio decreases rapidly, signaling the short-range electron-phonon coupling effect which confines the electrons and holes within a smaller volume. An energy red-shift in the photoluminescence peak was observed, because of a lowering in the strain of local symmetry at O2- sites caused by excess oxygen, and by strong coupling between the electron and phonon vibration. The variation of the bandgap is very sensitive to the electron-phonon coupling and the distinct size effect of strained ZnO nanoparticles.