Structure of epitaxial (Fe,N) codoped rutile TiO2thin films by x-ray absorption

Abstract

Homoepitaxial thin films of Fe:TiO 2 and (Fe,N):TiO 2 were deposited on rutile(110) by molecular beam epitaxy. X-ray absorption near edge spectroscopy (XANES) spectra were collected at the Ti L-edge, Fe L-edge, Ti K-edge, O K-edge, and N K-edge. No evidence of structural disorder associated with a high concentration of oxygen vacancies is observed. Substitution of Fe for Ti could be inferred, and secondary phases such as Fe 2 O 3 , Fe 3 O 4 , and FeTiO 3 can be ruled out. The similarity of the N K-edge spectra to O, and the presence of a strong x-ray linear dichroism signal for the N K-edge, indicates that N is substitutional for O in the rutile lattice and is not present as a secondary phase such as TiN. Simulations of the XANES spectra confirm substitution, although N appears to be present in more than one local environment. Neither Fe:TiO 2 nor (Fe,N):TiO 2 exhibit intrinsic room temperature ferromagnetism, despite the presence of mixed valent Fe(II)/Fe(III) in the reduced (Fe,N):TiO 2 film.

Nb‐Dopant‐Induced Tuning of Optical and Electrical Property of Anatase TiO2 Nanocrystals

“…The formation of dopant induced impurity states depends on the type of orbitals and energetic position of atomic orbitals of dopant element. Electrical and optical properties of TiO 2 have been modified by doping with different aliovalent 3d and 4d cations like V, Mn, Cr, Fe, Mo, Ta and W incorporating impurity levels . High (> 90%) ionization efficiency due to low ionization energy (4‐30 meV) of niobium (Nb) atom compared to other elements suggests that Nb dopants are very good electron donor to wide band gap semiconductors.…”

Nanoscale Structural Modulation and Low-temperature Magnetic Response in Mixed-layer Aurivillius-type Oxides

“…Normally, because of Fe: 2p spin-orbit coupling, the overall spectral shapes are similar for all iron species with an intense peak at around 708∼710 eV (L 3 ) and a less intense peak at 721∼723 eV (L 2 ). While the L 3 and L 2 peak positions do not differ significantly for Fe 2+ and Fe 3+ , the fine structure is sensitive to the local environment of the absorber 18 . Our Fe L-edge XAS spectra exhibit fine structure at both the L 3 and L 2 edges, in contrast to the L-edge spectra for metallic Fe, where any fine structure is absent 19 .…”

“…3(b). Because titanium is dominated by the dipole 2p-3d transition, spin-orbit splitting of the 2p orbitals into 2p 3/2 (L 3 ) and 2p 1/2 (L 2 ), and crystal field splitting of the 3d orbitals into e g and t 2g , result in a manifold of four absorption peaks for the Ti L-edge 18 . If the tetragonal distortion from perfect octahedral symmetry exists, the e g states will be further split, e.g .…”

“…anatase and rutile, where the intensity ratio of the split e g peaks is opposite for the two crystal structures 20 . Compared the observed Ti L-edge XAS spectra with that of TiO 2 18 , Ti should be a ‘+4’ valence and octahedrally coordinated by oxygen in BFCT- x .

Figure 3XAS spectra of BFCT- x . ( a ) Fe L-edge; ( b ) Ti L-edge; ( c ) Co L-edge; ( d ) O K-edge.

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“…Distinctly different XANES spectral behaviours of the Co-doped and un-doped samples demonstrate that the local electronic structures of BFCT-0.0 are modified by Co-substitution. To the pre-peaks in the O K-edge, titanium oxides usually consist of two strong pre-edge peaks arising from transitions from O 1  s to hybridized Ti 3d - O 2p orbitals and crystal field splitting of the Ti 3d orbitals into e g and t 2g components 18 , yet iron (cobalt) oxides have only one pre-edge peak originating from transitions from O 1  s to hybridized Fe (Co) 3d - O 2p orbitals 19,23 . It is reasonable to understand that the change of the concentration ratio between Ti and Fe (Co) in the unit of BFCT- x gives rise to this different O K-edge XANES spectral behaviours, corresponding to the aforementioned structural transformation from the dominant 5-layer slabs (Ti/Fe ∼ 3/2) to the homogeneous 4-layer slabs (Ti/Fe ∼ 3/1).…”