TiO2 films obtained by the sol–gel process and doped with Yb3+ and Er3+ ions

The upconversion mechanism based on lanthanide ions has been studied in several systems and has been applied in various fields, such as sensors, biological markers, and photovoltaic cells. This work investigates the properties of the lanthanides Er3+ and Yb3+ doped into titanium oxide at different molar concentrations. The sol–gel methodology and the spin-coating technique were used to prepare the thin films. To promote the upward energy conversion mechanism, the materials were submitted to thermal treatment at 500 °C after the depositions. Before and after thermal treatment, the films presented 80% and 83% optical transparency above 350 nm, respectively. The X-ray diffractograms of all the films attested to the beginning of TiO2 crystallization, as evidenced by the characteristic peak of the titania phase (JCPDS # 23-1446). The emission spectra of the films excited at 980 nm exhibited the characteristic Er3+ emission bands in the green (525 and 555 nm) and red (660 nm) regions, which corresponded to the 4H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions, respectively. On the basis of the emission intensity as a function of laser power, two photons were involved in the process. The Fluorescence Intensity Ratio (FIR) indicated a rise in the local temperature, which was induced by excitation light, or the laser power.