| AVS 63rd International Symposium & Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS+AS-TuA |
| Session: | Structure and Characterization of Oxides |
| Presenter: | Remi Lazzari, Institut des NanoSciences de Paris, France |
| Authors: | R. Lazzari, Institut des NanoSciences de Paris, France J. Li, Institut des NanoSciences de Paris, France J. Jupille, Institut des NanoSciences de Paris, France |
| Correspondent: | Click to Email |
Stoichiometry defects play a tremendous role in the surface chemistry of titanium oxide [1,2]. Reduced rutile is indubitably a n-type semiconductor in terms of electrical transport but electron-based spectroscopies and scanning tunnelling microscopy show the existence of a defect-related gap state lying 0.8-1eV below the Fermi level [1,2]. Its nature i.e. surface oxygen vacancies [2,3] versus sub-surface interstitial titaniums [4] is still highly debated in the literature as well the actual (de)localisation of the associated excess electrons [4,5,6].
In our work, electron energy loss spectroscopy in low and high resolution modes was used to probe band gap state and phonon excitations in TiO2(110) as a function of oxygen exposure at 100 and 300K. By comparing surfaces, from reduced to fully oxidized obtained by various means including electron bombardment, and by using EELS depth sensitivity in out-of-specular detection, a contribution from sub-surface defects is clearly evidenced. A method to prepare defect-free surfaces (as observed by EELS) is even proposed. Using dielectric modelling of spectra including phonons, carriers, gap state and interband transitions and multiple excitations, it was shown that “free-like” carriers characterized by their plasmon excitation coexists with band gap states. While the latter give rise to an obvious peak in the band gap, the former induce a temperature dependent broadening of the quasi-elastic peak and a sizeable screening and upward frequency shift of phonons compared to stoichiometric samples. Through data fitting, both surface and bulk carrier densities and dampings could be quantified as well their profile. A very different dynamics of the healing of the associated signals upon O2 exposure was also observed. The implication of such findings in terms polaronic nature of excess electron will be discussed.
[1] U. Diebold, Surf. Sci. Rep. 48 (2003) 43; C. Pang et al., Chem. Rev. 113 (2013) 3887
[2] C. Yim et al., Phys. Rev. Lett. 104 (2010) 036806
[3] P. Kruger et al., Phys. Rev. Lett 100 (2008) 055501
[4] S Wendt et al., Science 320 (2008) 1755
[5] M. Setvin et al., Phys. Rev. Lett. 113 (2014) 086402
[6] A. Janotti et al., Phys. Stat. Solidi RRL 7 (2013) 199