Tuning the surface morphology and local atomic structure of Mn-TiO2 thin films using rapid thermal annealing

Abstract

In this study, Ti0.95Mn0.05O2-delta nanostructured thin films were fabricated by pulsed laser deposition technique followed by rapid thermal annealing (RTA) in pure O-2 and N-2 atmospheres. The RTA process induced a substantial change in the surfaces morphology and local atomic structure around Ti4+ cation that have been studied by means of atomic force microscopy, Raman scattering and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Raman spectra of the films were resembled to that of TiO2 rutile phase, and the change in the width of E-g (434 cm(-1)) Raman active modes has been attributed to oxygen non-stoichiometry. NEXAFS spectra were carried out in synchrotron facility at Ti/Mn L (3,2) edges and O-K edge. The ligand-field splitting, estimated from the energy difference between t(2g) and e(g) features in O K-edge spectra were similar to 2.81 eV for pristine and annealed film, which is a characteristic of the TiO2 rutile structure, and the asymmetry of t(2g) and e(g) bands at the O-K edge has been ascribed to oxygen vacancy (Vo(2+)). The annealing of film in O-2 gas optimized the surface structure and healed the Vo(2+) bridging, while the RTA in N-2 gas introduced Vo and reduced the valence state of Ti4+ (TiO2) into Ti3+ (Ti2O3) that have been probed by comparing the NEXAFS spectra of N-2 annealed film with the reference spectra of Ti2O3. Experimental and atomic multiplet calculations revealed that the Mn ions exist in 2+ valence state.