Nature of Paramagnetic Species in Nitrogen-Doped SnO2: A Combined Electron Paramagnetic Resonance and Density Functional Theory Study

A combination of electron paramagnetic resonance (EPR) spectra and density functional theory (DFT) calculations is used to characterize the paramagnetic species in rutile N-doped SnO2 samples synthesized by wet chemistry methods. In particular, the nature of paramagnetic N species, substitutional or interstitial, and their effect on the electronic structure are discussed. Complex EPR spectra generated by the interaction of the unpaired electron with N and Sn nuclei have been accurately simulated to obtain the EPR properties (g and A tensors). The results suggest that the N dopants form a rather symmetric structure with three magnetically equivalent or nearly equivalent Sn atoms surrounding the N impurity. After a careful assessment of an all-electron basis set for Sn atoms, realistic models of substitutional and interstitial N-doped SnO2 structures have been designed, and the corresponding hyperfine coupling constants (hpcc) were computed. The comparison between computed and measured hpcc values leads to the assignment of the paramagnetic centers in N-doped SnO2 to substitutional N dopants that take the position of the O atoms in the lattice. The DFT calculations finally suggest the N impurities induce the formation of localized empty states (electron holes) in the intra band gap region.