N-2(+) Ion Bombardment Effect on the Band Gap of Anatase TiO2 Ultrathin Films

We report a study of the effect of nitrogen ion bombardment on the optical properties of anatase TiO2 ultrathin films, particularly the band gap energy. The TiO2 films were prepared by a sol-gel method and dip-coating process. The as-prepared TiO2 films were then exposed to a N-2(+) low-energy ion beam from a microwave electron cyclotron resonance (ESR) ion source. Raman and spectroscopic ellipsometry (SE) analysis were performed on TiO2 films prepared at different N-2(+) exposure times. The Raman measurements reveal the conservation of the anatase TiO2 crystalline structure after the ion beam exposure. From a detailed ellipsometric study, the thickness of layers, the dielectric function, the band gap and the Urbach energies were determined. The obtained results show an increase of the TiO2 band gap with the decrease of thickness of films during NI exposure time. The band gap energy was blue shifted from 20 meV to 140 meV as the exposure time was increased from 5 min to 20 min when the thickness was decreased from 30 nm to 21 nm. This increasing of band gap energy could be explained by the thickness effect. From the band tail, the Urbach energy was also affected by N-2(+) ion beam. These results are in good agreement with the observed broadending of the Raman band the O-Ti-O bending vibration mode, as the exposure time increases.