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

Abstract

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 A₁(LO) phonon mode while 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 A₁(2LO)/A₁(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.