Structural and near-infra red luminescence properties of Nd-doped TiO2 films deposited by RF sputtering

We report the effect of neodymium (Nd) doping of TiO2 thin films produced on quartz, soda lime and silicon substrates by RF co-sputtering of two targets of TiO2 and metallic Nd. The Nd/Ti ratio was varied in the range of 0.9-100 at.%. A study based on a joint use of Auger electron spectroscopy and X-ray fluorescence analysis allowed us to define the chemical composition of all the deposited films. A piece of direct evidence of the structural changes (phase transformation from anatase to rutile for Nd/Ti atomic ratio up to 43 at.%) and amorphization for an Nd/Ti ratio equal to 100 at.%, a lattice distortion due to Nd incorporation and also the rare-earth ion location site was obtained by X-ray diffraction studies, in correlation with the optical properties analysis. NIR photoluminescence properties of the doped films in the wavelength range of 800-1200 nm were investigated by direct (green, blue-green and blue lines at 514.5, 496 and 476 nm respectively) and indirect excitations of Nd (using the UV line at 355 nm), in the latter case excitation through the titania host. In both cases, an intense NIR photoluminescence was observed at 907 and 1093 nm pertaining to the transitions between the excited F-4(3/2) to I-4(9/2) and I-4(11/2) levels of the Nd3+ ions for the lowest ion concentration (Nd/Ti ratio equal to 0.9 at.%) and a correlation between the phase structure of the matrix and Nd concentration and the emission properties was found.