Institut des
NanoSciences de Paris
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Thesis Defense - Jingfeng Li - Thursday 20 october 2016 at 2 pm

INSP - UPMC - 4 place Jussieu - 75005 Paris - Barre 22-23, 3e étage, salle 317

Jingfeng Li, PhD Student - Team Low-dimensional oxides

Origin, location and transport of excess electrons in titanium dioxide

Abstract

The many applications of titanium dioxide TiO2, both existing and potential, have long attracted an intense research activity. In particular, TiO2 is nowadays foreseen to play a key role in environmental issues and alternative energy sources. Chemically inactive when stoichiometric, its applications mostly rely on reduced forms TiO2-x which points to properties of oxygen vacancies (Ovac) and titanium interstitials (Tiint) through which stoichiometry changes occur. The reduction of the oxide, that formally transforms Ti4+ in Ti3+, generates excess electrons that populate Ti 3d band-gap states (BGS). Central in chemistry, excess charges are also at the origin of the n-type electron conductivity of the oxide. The present work has been dealing with three central questions that are still pending, the respective role of Ovac and Tiint in the formation of BGS, the apparent contradiction between the n-type conductivity of the oxide and the deep donor character of the BGS and the nature of the electronic transport in the oxide. The origin, location and transport of excess charges were explored on rutile TiO2(110)-(1×1) by High Resolution Electron Energy Loss Spectroscopy (HREELS) measurements combined with dielectric simulations. Specific sample treatments and experimental approaches were used, including : (i) surface annealing in the 400-900 K range by exposure to a hot filament, while measuring the temperature via the Bose-Einstein loss/gain ratio ; (ii) cooling down to 100 K ; (iii) off-specular measurements ; (iv) electron bombardment which only produces oxygen vacancies ; (v) exposure to either oxygen or water vapours at different temperatures. BGS were directly evidenced in conditions in which either Ovac or Tiint were the unique type of defects of the surface. A schematic profile of the density of excess electrons through the surface region of TiO2(110) was proposed and improved throughout the manuscript by data fitting which suggested their location in the subsurface. Dielectric modelling was used to model the impact of all solid-state excitations on the shaping of HREELS spectra of reduced TiO2. It was proposed to describe BGS by an oscillator and conductivity through a Drude term. While BGS appears as a defined feature in the band gap, the signature of the carriers is less obvious. Instead of, for example zinc oxide, the large static dielectric function of the material and the high effective mass of carriers lead only to a slight broadening of the quasi-elastic peak through the excitation of surface plasmons at moderate carrier density. Enhancing apparent resolution allows glimpsing their existence and their combination modes with surface phonons. Fortunately, the carrier excitations can be tracked not only through the imaginary part of their dielectric function but also through the real part of this function and the resulting screening. Due to its very large oscillator strength, one longitudinal phonon can be used as a reporter ; since its frequency lies between Drude term and BGS, it shifts upwards in energy with the occurrence of plasmon excitation and downwards with the BGS oscillator strength. Next, the energy and angular depth sensitivity of HREELS was employed to gain insights on the density profile of the excess charge and on the surface versus bulk contributions. Firstly, the existence of Drude-like carriers was demonstrated via a combination of evidences including the way in which oxygen exposure affects phonon spectra, the asymmetry of the quasi-elastic peak and its temperature-dependence and the necessary addition of Drude-like carriers to reproduce the phonon line shape. The existence of two kinds of charge carriers was evidenced, one due to bulk defects, the other due to Ovac. Although both have a similar BGS signature, they show quite different transport properties. The estimated mobility of bulk carriers is higher than that of the surface carriers, with activation energies in the range of a few tenths of meV in good agreement with Hall and conductivity measurements. The linear correlation between the BGS oscillator strength and the Drude frequency suggests that BGS behave in a dual way. The fact that the same correlation holds when the temperature is changed from 300 to 100 K strongly supports the idea. Moreover, although the transport property of surface carriers and bulk carriers are different, they both have this dual behaviour character. The main achievement of the research work presented in the manuscript is the description of the reduced titanium dioxide surface region in terms of categories which usually exclude each other, oxygen vacancies and titanium interstitials, surface and bulk defects, free-carriers or bound states. Unlike often developed arguments, it is shown that these categories coexist in a framework that is only outlined. It will have to be described in detail in future to move towards a full understanding of the important family of the reducible oxides.

Jury

Paul Dumas, Synchrotron SOLEIL, Protractor
Jean-Marc Layet, Université de Marseille, Protractor
Abhay Shukla, IMPMC, Examinator
Geoff Thornton, UCL Londres, Examinator
Jacques Jupille, INSP, Thesis director
Rémi Lazzari, INSP, thesis co-director