High-resolution core-and valence-level photoemission spectra of Nb-doped TiO 2 ceramics (Ti 1Ϫx Nb x O 2 with 0.01ϽxϽ0.8) have been measured using monochromatic x-ray excitation. Nb doping produces a welldefined photoemission peak in the bulk band gap of rutile, whose intensity increases with increasing doping level. Core-level spectroscopy shows that the Nb is incorporated within the rutile lattice at low doping levels mainly as Nb͑V͒ and that the gap state is associated with Ti͑III͒ ions. This conclusion is reinforced by variable energy photoemission measurements on Ti 0.9 Nb 0.1 O 2 in the vicinity of the Ti 3p and Nb 4p core thresholds. The photoemission resonance profile for the gap states reaches half maximum intensity at the same energy as found for oxygen-deficient TiO 2Ϫx but is shifted from the resonance profile for the Nb 4d states of NbO 2 . STM images on Nb-doped TiO 2 ͑110͒ are considered in relation to the spectroscopic measurements. Nb dopant atoms are imaged as ''bright spot'' clusters, implying delocalization of charge from Nb onto neighboring Ti ions. The experimental x-ray photoelectron spectroscopy data are compared with density-of-states profiles derived from local-density approximation calculations on pure and Nb-doped TiO 2 clusters. These calculations show that Nb doping of TiO 2 introduces new states of mixed Nb 4d -Ti 3d character above the O 2p valence band of the host material. In addition, there is increased x-ray photoemission intensity across the O 2p valence band owing to strong Nb 4d/O 2p hybridization and a cross section for ionization of Nb 4d states that is an order of magnitude larger than that for O 2p or Ti 3d states.
High resolution valence-and core-level photoemission spectra of undoped and 3% Sb-doped SnO 2 are presented. Conduction-band occupation due to Sb doping in SnO 2 leads to a shift of valence-band features to high binding energy. However, the shift is less than the width of the occupied part of the conduction band. This is attributed to a shrinkage of the bulk band gap with doping, arising from an attractive dopant electron interaction and screening of the Coulomb repulsion between valence and conduction electrons. Core-level spectra provide evidence for strong screening by the conduction electron gas in 3% Sb-doped SnO 2 , giving rise to ''screened'' and ''unscreened'' final-state peaks in photoemission. The dominant screening response involves excitation of conduction electron plasmons.
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