Tuning optical absorption of anatase thin lms across the visible/near-infrared spectral region

The electronic properties of anatase titanium dioxide (TiO2) thin films epitaxially grown on LaAlO3substrates are investigated by synchrotron-x-ray spectroscopy [x-ray absorption spectroscopy (XAS), x-ray photoemission spectroscopy (XPS), and angle-resolved photoemission spectroscopy (ARPES)] andinfrared spectroscopy. The Ti3+fraction in TiO2−xis varied either by changing the oxygen pressure duringdeposition or by postgrowth annealing in ultrahigh vacuum (UHV). Structural investigation of the TiO2thin films provides evidence of highly uniform crystallographic order in both as-grown andin situUHV-annealed samples. The increased amount of Ti3+as a consequence of UHV annealing is calibrated byinsituXPS and XAS analysis. The as-grown TiO2samples, with a low Ti3+concentration, show distinctelectronic properties with respect to the annealed films, namely, absorption in the midinfrared (MIR)region correlated with polaron formation, and another peak in the visible range at 1.6 eV correlated with thepresence of localized defect states (DSs). With the increasing level of Ti3+induced by the postannealingprocess, the MIR peak disappears, while the DS peak is redshifted to the near-infrared region at about 1.0eV. These results indicate the possibility of tailoring the optical absorption of anatase TiO2films from thevisible to the near-infrared region.