Investigation of additional Raman modes in ZnO and Eu0.01Zn0.99O nanoparticles synthesized by the solution combustion method

In this study, we report on the origin of Raman additional modes (AM) in ZnO and Eu0.01Zn0.99O nanoparticles (NPs) synthesized by the solution combustion method (SCM). The as-produced samples were thermally annealed at different temperatures and under different conditions (air, N2-flow and He-flow). The as-produced NPs exhibited the characteristic color (salmon) of N-doped ZnO. Typical patterns of ZnO wurtzite were found by X-ray diffraction data and no evidences of additional phases were detected. Raman spectra of as-produced ZnO and Eu0.01Zn0.99O NPs showed AM features usually found in N-doped ZnO. These features disappeared after thermal annealing in air, but they do not disappeared after thermal annealing in N2- and He-flow. Effects of Eu-doping on AM modes were observed, indicating that co-doping with Europium can be used to increase the Nitrogen solubility in ZnO. Using the Raman data the maximum N-content in Eu0.01Zn0.99O NPs was estimated in 1018 cm−3. Our results concerning thermal annealing at different conditions indicated that it is unlikely that Zni-Oi complexes are the origin of the ∼510 cm−1 AM, as it is currently proposed in the literature. Instead, the Raman AM features observed in this study were attributed to disorder activated Raman modes favored by N-doping.