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

Abstract

In this study, Ti0.95Mn0.05O2−δ nanostructured thin films were fabricated by pulsed laser deposition technique followed by rapid thermal annealing (RTA) in pure O2 and N2 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 Eg (434 cm−1) Raman active modes has been attributed to oxygen non-stoichiometry. NEXAFS spectra were carried out in synchrotron facility at Ti/Mn L3,2 edges and O-K edge. The ligand-field splitting, estimated from the energy difference between t2g and eg features in O K-edge spectra were ~ 2.81 eV for pristine and annealed film, which is a characteristic of the TiO2 rutile structure, and the asymmetry of t2g and eg bands at the O-K edge has been ascribed to oxygen vacancy (Vo2+). The annealing of film in O2 gas optimized the surface structure and healed the Vo2+ bridging, while the RTA in N2 gas introduced Vo and reduced the valence state of Ti4+ (TiO2) into Ti3+ (Ti2O3) that have been probed by comparing the NEXAFS spectra of N2 annealed film with the reference spectra of Ti2O3. Experimental and atomic multiplet calculations revealed that the Mn ions exist in 2+ valence state.