Preparation And Characterization Of Ge And Si Quantum Dots Doped TiO₂ Nanocomposite Thin Films

As a new generation promising solar energy conversion material, titanium dioxide (TiO₂) has attracted widespread attention due to its non-toxicity, photo-stability and good environmental acceptability. However, its wide band gap (E_g = 3.0-3.2 eV) requires ultraviolet light irradiation activation which accounts for only a small fraction (4%) of the sun's spectrum. Thus, the high photo-induced charge carriers recombination of TiO₂, leading to the decrease in the use ratio of photo-induced charge carriers under visible light. To solve this problem, the main purpose of this paper is to modify TiO₂ film by incorporating semiconductor quantum dots (QDs) like germanium (Ge) and silicon (Si) to narrow the band gap and to make full use of the visible light. And this film will be applied in the new generation of solar cells.In this thesis, Ge and Si single and double QDs doped TiO₂ thin films were synthesized by ion-beam sputtering technology. Si QDs doped TiO₂ films were also prepared by ion implantation method. Optical and structural properties of the films with Ge, Si or Ge and Si QDs doping have been were characterized by SEM, TEM, XRD, XPS, AFM and UV-visible spectroscopy.TEM results show that Ge, Si QDs are doped in the TiO₂ films with a uniform size distribution and high density. The more concentration of Ge leads to the batter crystalline of Ge from XRD spectra. Analysis by XPS indicates that Ge and Si incorporated in the TiO₂ film is in the form of elemental Ge and Si, respectively. The Stranski–Krastanov growth mode is accounted for the growth of Ge QDs in the TiO₂ films by AFM measurements. Optical absorption spectra exhibits that the optical responses of Ge, Si single and double QDs doped TiO₂ nanocomposite films prepared by ion-beam sputtering are shifted from UV to the near infrared region, suggesting a much better optical absorption property than that of pure TiO₂ films, and the band gap of Ge, Si double QDs doped TiO₂ film reduces to 1.31 eV. Si QDs doped TiO₂ films prepared by ion implantation method also show a much better optical absorption property than that of pure TiO₂ films and its band gap reduces to 1.89 eV. This research enriches the study of the doping of TiO₂ and offers potent reference to solar cell research of TiO₂ thin films.