Theory of the phonon properties of pure and ion-doped ZnO nanoparticles

Abstract

Based on the s-d model including electron–phonon and spin–phonon interaction, and using a Green’s function technique we have studied surface, size, and ion doping effects on the phonon properties of ZnO nanoparticles (NPs). We have shown that the electron–phonon and anharmonic phonon–phonon interactions play an important role in pure ZnO NPs, whereas in the transition metal (TM) and rare earth (RE) doped ZnO NPs the spin–phonon interaction must be taken into account in order to explain the experimental data. Due to surface and size effects, the phonon frequency decreases and the phonon damping increases with decreasing of particle size. By TM ion doping we obtain hardening, whereas by doping with Mn or RE ions-softening of the phonon energy with increasing the dopant concentration. This is due to the different radii of the doping ions compared to the host Zn ion radius and to the induced different lattice strain and spin–phonon interactions. The phonon damping is always enhanced compared to the undoped case. The observed results are in qualitative agreement with the experimental data. Our model and theory can be applied to all pure and doped diluted magnetic semiconductor NPs.