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Table of contents
1 Introduction
1.1 The origin of the bang gap states
1.2 The location of the energy levels of the band gap states
1.3 The nature and transport property of excess electrons
1.4 Outline
2 Electron Energy Loss Spectroscopy: experimental set-up and methods
2.1 The ultra-high vacuum system
2.2 The TiO2(110) surface and its preparation
2.2.1 Bulk rutile titanium dioxide
2.2.2 The TiO2(110) surface
2.2.3 The surface preparation
2.3 (High Resolution) Electron Energy Loss Spectroscopy
2.3.1 Introduction: basics of EELS
2.3.2 The spectrometer
2.3.3 Electron-surface interaction modes and their cross-section
2.3.3.1 Dipole scattering
2.3.3.1.1 Classical single loss cross section in the dielectric approach
2.3.3.1.2 Semi-classical treatment of multiple losses
2.3.3.2 Impact scattering
3 Dielectric modelling of Electron Energy Loss from TiO2
3.1 Introduction
3.2 Dielectric modelling of EELS from TiO2 : the interplay between carrier excitations, phonons, band gap states and interband transitions
3.2.1 Theoretical reminder about dielectric theory of EELS
3.2.1.1 The single loss probability
3.2.1.2 The sensitivity function and the slit integration
3.2.1.3 Losses from an anisotropic material
3.2.1.4 Losses from a stratified medium
3.2.1.5 Mulitple losses
3.2.2 The dielectric function of TiO2 from far-infrared to ultraviolet
3.2.2.1 Phonons
3.2.2.2 Interband transitions
3.2.2.3 The static dielectric function: electron-phonon coupling and polaronic distorsion
3.2.2.4 Defects induced band gap states and optical absorption
3.2.2.5 Excitation due to itinerant motion of carriers: the Drude model
3.2.3 Numerical implementation: the HREELS program
3.2.4 The interplay and screening between reduced TiO2 excitations
3.2.4.1 Effect of dielectric anisotropy
3.2.4.2 Quasi-elastic peak broadening due to carriers
3.2.4.3 Screening of phonons by carrier excitations
3.2.4.4 Screening of phonons by band gap states
3.2.5 Surface versus bulk excitations: the question of depth sensitivity
3.2.5.1 Probing depth in EELS ?
3.2.5.2 A few examples of effects of dielectric function profile
3.3 Resolution enhancement in EELS based on iterative semi-blind Lucy-Richardson algorithm
3.4 Conclusion
4 Surface versus bulk contribution to the band gap states in TiO2
4.1 Introduction
4.2 Exposure to oxygen of reduced rutile samples
4.3 Surface annealing
4.3.1 When the hot filament only allows surface annealing
4.3.2 Surface temperature measurements
4.3.3 BGS due to Ti interstitial diffusion for a surface temperature of 420 K
4.3.4 BGS due to a combination of Ti interstitials and oxygen vacancies at various surface temperatures
4.3.5 Toward a defect-free TiO2(110) surface
4.4 How water adsorption can heal BGS associated with surface vacancies ?
4.5 Creation of oxygen vacancies by electron bombardment
4.5.1 Creation of only oxygen vacancies
4.5.2 The limited efficiency of electron bombardment
4.6 Out-of-specular EELS spectra and the profile of excess electrons
4.6.1 BGS recorded from different probing depths
4.6.2 Qualitative description of the profile of excess charges
4.7 Conclusion
5 Excess electrons in reducible TiO2 rutile: dual behaviour or coexistence of trapped and free states ?
5.1 Position of the question
5.2 (HR)EELS from reduced TiO2(110) surface
5.2.1 On the existence of carrier excitations
5.2.1.1 Effect of oxygen exposure
5.2.1.2 Quasi-elastic peak: shape and temperature dependence
5.2.1.3 Phonon line shape
5.2.2 The profile of dielectric function for fits
5.3 Bulk and surface excess electrons: dual behavior or coexistence of trapped and free states ?
5.3.1 Bulk excess electrons
5.3.2 Surface excess electrons
5.4 Conclusion
6 Conclusion
7 Annex: Published paper in Rev. Sci. Inst. 86 (2015) 013906
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