The role played by oxygen vacancies and rare earth (RE) elements in the anatase-to-rutile (A–R) phase transformation of titanium dioxide (TiO2) is still a matter of controversy. Here, we report the A–R transformation of TiO2 thin solid films as obtained by ion beam sputtering a RE-decorated titanium target in an oxygen-rich atmosphere. The samples correspond to undoped, single-doped (Sm, Tm, and Tb), and codoped (Sm:Tb, Sm:Tm, and Sm:Tb:Tm) TiO2 films. In the as-prepared form, the films are amorphous and contain ∼0.5 at. % of each RE. The structural modifications of the TiO2 films due to the RE elements and the annealing treatments in an oxygen atmosphere are described according to the experimental results provided by Raman scattering, X-ray photoelectron spectroscopy, and optical measurements. The A–R transformation depends on both the annealing temperature and the characteristics of the undoped, single-doped, and codoped TiO2 films. As reported in the literature, the A–R transformation can be inhibited or enhanced by the presence of impurities and is mostly related to energetic contributions. The experimental results were analyzed, considering the essential and stabilizing role of the entropy of mixing in the A–R transformation due to the introduction of more and multiple quantum states originated in vacancies and impurities in the anatase phase.

Future advances in the broad fields of photonics, (nano-)electronics or even theranostics rely, in part, on the precise determination and control, with high sensitivity and speed, of the temperature of very well-defined spatial regions. Ideally, these temperature-sensors (T-sensors) should produce minimum (or no) disturbance in the probed regions, as well as to exhibit good resolution and significant dynamic range. Most of these features are consistent with the sharp and distinctive optical transitions of trivalent rare-earth (RE3+) ions that, additionally, are susceptible to their local environment and conditions. Altogether, these aspects form the basis of the present work, in which we propose a new T-sensor involving the light emission of trivalent thulium ions (Tm3+) embedded into crystalline TiO2. The optical characterization of the TiO2:Tm3+ system indicated a Tm3+-related emission at ~676 nm whose main spectral features are: (1) a temperature-induced wavelength shift of −2.2 pm K−1, (2) a rather small line-width increase over the ~85–750 K range, and (3) minimum data deconvolution-processing. The study also included the experimental data of the well-established pressure- and T-sensor ruby (Al2O3:Cr3+) and a comprehensive discussion concerning the identification and the excitation-recombination mechanisms of the Tm3+-related transitions.

This study reports on the properties of nitrogen doped titanium dioxide (TiO 2 ) thin films considering the application as transparent conducting oxide (TCO). Sets of thin films were prepared by sputtering a titanium target under oxygen atmosphere on a quartz substrate at 400 or 500°C. Films were then doped at the same temperature by 150 eV nitrogen ions. The films were prepared in Anatase phase which was maintained after doping. Up to 30at% nitrogen concentration was obtained at the surface, as determined by in situ x-ray photoelectron spectroscopy (XPS). Such high nitrogen concentration at the surface lead to nitrogen diffusion into the bulk which reached about 25 nm. Hall measurements indicate that average carrier density reached over 10 19 cm -3 with mobility in the range of 0.1 to 1 cm 2 V -1 s -1 . Resistivity about 3.10 -1 cm could be obtained with 85% light transmission at 550 nm. These results indicate that low energy implantation is an effective technique for TiO 2 doping that allows an accurate control of the doping process independently from the TiO 2 preparation. Moreover, this doping route seems promising to attain high doping levels without significantly affecting the film structure. Such approach could be relevant for preparation of N:TiO 2 transparent conduction electrodes (TCE). Graphical abstractHighlights  A two-step process for preparation of N:TiO 2 transparent conductor is proposed. Low energy nitrogen ions are used after Anatase thin film deposition. Approach allows excellent control of crystal, optical and electronic properties.  Resistivity as low as 3.10 -1 cm while transparency at 550nm is about 85%.

Er-doped TiO2−xNx films were grown by Ar+ ion-beam sputtering a Ti + Er target under different N2 + O2 high-purity atmospheres. The compositional-structural properties of the samples were investigated after thermal annealing the films up to 1000 °C under a flow of oxygen. Sample characterization included x-ray photoelectron spectroscopy, grazing incidence x-ray diffraction, Raman scattering, and photoluminescence experiments. According to the experimental data, both composition and atomic structure of the samples were very sensitive to the growth conditions and annealing temperature. In the as-deposited form, the N-rich TiO2−xNx films presented TiN crystallites and no photoluminescence. As the thermal treatments proceed, the films were transformed into TiO2 and Er3+-related light emission were observed in the visible and near-infrared ranges at room-temperature. Whereas the development of TiO2 occurred due to the insertion-diffusion of oxygen in the films, light emission originated because of optical bandgap widening and/or structural-chemical variations in the vicinity of the Er3+ ions. Finally, the photoluminescence results in the visible range suggested the potential of the present samples in producing an optically based temperature sensor in the ∼150–500 K range.

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