Ti 3d Band Research Articles

Observation of Electronic Structure in Conduction Band of CaTiO<sub>3</sub>

Electronic structure in the conduction band of CaTiO3 was examined by the bremsstrahlung isochromat spectroscopy (BIS), X-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS). Two broad bands were observed at 2 and 7eV above the Fermi energy in the BIS spectrum. The features of the XAS spectra for Ca L2, 3, Ti L2, 3, and O K edges were almost the same as those of the EELS spectra, and the XAS spectrum for each element was in reasonable agreement with its partial density of states estimated by an energy band calculation. A trial spectrum was synthesized by the sum of the XAS spectra, and it was found that the feature of the synthesized spectrum is similar to that of the BIS spectrum. From the comparison between XAS and BIS spectra, the two broad bands at 2 and 7eV in the BIS spectrum were assigned to Ti 3d bands and Ca 4s bands, respectively.

Electronic structure of Pt overlayers on (1 × 3) reconstructed TiO 2(100) surfaces

The effect of Pt atoms on the Ti 3d emission of Ti 3+ states at (1 × 3) reconstructed TiO 2(100) surfaces was studied by means of valence band photoemission spectroscopy using synchrotron radiation with excitation energies between 30 and 120 eV. A clean (1 × 3) reconstructed TiO 2(100) surface was prepared by Ar + sputtering and annealing at T = 870 K as checked with low energy electron diffraction. Subsequently, Pt was evaporated in situ using a resistively heated Pt filament. Evaporation was performed stepwise up to a nominal coverage of 1.8 monolayers (ML) as checked with Auger electron spectroscopy (AES). The Pt 4f core levels are shifted to higher binding energies with respect to Pt metal, indicating the presence of small Pt clusters. Difference spectra show a dublet between the TiO 2 valence band maximum and the Fermi energy which is attributed to Pt 5d states. We found that the position and the intensity of the Ti 3d band gap emission were not significantly altered upon evaporation of Pt. In addition, the valence band maximum of TiO 2 remains unchanged. These findings indicate the absence of an electronic charge transfer due to the high work function of TiO 2(100)-(1 × 3) of 5.4 eV, in line with a simple metal-semiconductor contact theory.

A combined AES, resonant photoemission and EELS study of in-situ grown titanium nitride

Samples of titanium nitride were grown in an Ar N atmosphere with various Ar N gas ratios using reactive DC magnetron deposition onto stainless steel and silicon substrates. The samples were transferred to the analysis position in-situ and were studied there using AES, EELS and synchrotron-radiation excited UPS. Using AES, the composition of the samples was found to vary with the Ar N gas ratio, as expected. Changes in the hybridised Ti 3 d N 2p band seen via UPS agreed with the theoretical prediction that the band narrows as the nitrogen concentration is reduced. A large resonance in the Ti 3d band was observed at ∼73 eV, which is probably associated with the Ti 3s level. The UPS and EELS data is consistent with vacancies being present on the titanium sublattice. The EELS data displayed low-energy loss peaks which shifted to higher loss energies as the nitrogen concentration in the film was lowered. These loss peaks can be explained by a screened Drude plasmon loss rather than by intraband transitions. The intensity of the loss peaks was much weaker than previously reported, with attendant implications for explanations of the shape of the Ti 2p photoelectron peak observed in XPS.

Transport properties of BaTiO 3−δ under high pressure

We have measured the electrical resistivity of doped ferroelectric single crystal BaTiO 3−δ under high pressure up to 8.0 GPa in the temperature range between 4.2 and 295 K. By applying pressure above 6.5 GPa metallic conduction was observed down to 4.2 K. A T 2-dependence of resistivity was found in the pressure-induced metallic phase below 100 K, suggesting that electrons in the Ti 3d band of BaTiO 3−δ have a nature similar to strongly correlated electron systems.

Theoretical analysis of STM experiments at rutile TiO 2 surfaces

A first-principles atomic orbital-based electronic structure method is used to investigate the low index surfaces of rutile titanium dioxide (TiO 2). The method is relatively cheap in computational terms, making it attractive for the study of oxide surfaces, many of which undergo large reconstructions, and may be governed by the presence of oxygen vacancy defects. Calculated surface charge densities are presented for low-index surfaces of TiO 2, and the relation of these results to experimental scanning tunnelling microscopy images is discussed. Atomic resolution images at these surfaces tend to be produced at positive bias, probing states which largely consist of unoccupied Ti 3d bands, with a small contribution from O 2p. These experiments are particularly interesting since the O atoms tend to sit up to 1 Å above the Ti atoms, so providing a play-off between electronic and geometric structure in image formation.

Impurity levels and nonlinear optical properties of doped BaTiO3 from extended cluster LDA calculations

The electronic structures of some transition-metal ions doping the ferroelectric oxide BaTiO3 are calculated through the density functional theory framework (LCAO-LSD ADF method) on extended clusters [XO6Ba8Ti6] embedded in a punctual charge set depicting the crystalline environment. The transition ions X are Fen+(n = 2, 3, 4, 5), Ti3+, Nb4+, and Nb5+. Some related defects like Fe(SINGLE BOND)Vo, where Vo stands for a lacunar oxygen site, are also investigated through a similar process. The positions of impurity levels insides the O2p(SINGLE BOND)Ti3d band gap are obtained from eigenvalues and related optical transition or ionization energies are calculated using excited states. This allows us to discuss the photorefractive effect present in such doped materials. The EPR fine-structure parameters a and D are also derived for Fe3+ in cubic and tetragonal symmetries from crystal field calculations using parameters drawn from molecular orbital results. © 1997 John Wiley & Sons, Inc.

Intrinsic defects and Pt atoms at TiO 2(110) surfaces: a valence band spectroscopy study using synchrotron radiation

The effect of Pt atoms on the Ti 3d emission of Ti 3+ states at TiO 2(110) surfaces has been studied by means of valence band spectroscopy using synchrotron radiation with excitation energies between 6 and 80 eV. Clean (1 × 1) surfaces were prepared by Ar + sputtering and annealing at T = 970 K as checked with low energy electron diffraction (LEED). Intrinsic defects were formed by weak sputtering and identified by the Ti 3d band gap photoemission below the Fermi level. Pt atoms were evaporated in situ using a resistively heated Pt filament. We find that even submonolayer coverages of Pt lead to a pronounced damping of the Ti 3d emission which is attributed to an electron transfer from localized Ti 3+ states to Pt atoms at the surface.

Inelastic x-ray scattering from TiC and Ti single crystals.

Inelastic x-ray scattering spectra were measured with incident x rays having an energy near 8 keV for a range of momentum transfers q extending from 0.70 to 2.0 \AA{} $^{\mathrm{\ensuremath{-}}1}$. We find spectral features corresponding to valence electron excitations, as well as Raman scattering corresponding to excitation from the 3p core states to the 3d valence states of Ti. The spectral features corresponding to valence electron excitations lie in the regime conventionally interpreted as a bulk plasmon together with intraband and interband transitions. We compare our data for TiC at q=1.05 \AA{} $^{\mathrm{\ensuremath{-}}1}$ with calculations for the dynamical dielectric constant \ensuremath{\varepsilon}(q,\ensuremath{\omega}) based on local-density theory. The agreement between the data and calculations permits us to interpret certain features in the spectra as arising from band transitions out of the strongly hybridized C 2p--Ti 3d band. The energy loss position of the Raman scattering peak is independent of momentum transfer for both TiC and Ti and lies at 48 eV for Ti. This value is above the highest-lying x-ray ultraviolet absorption resonance for atomic Ti at 43.5 eV. This shift in the data to higher energy transfer is consistent with electron energy loss spectra and photoemission results reported in the literature and indicates that important physical processes are not encompassed within existing theoretical approaches to x-ray Raman scattering. \textcopyright{} 1996 The American Physical Society.

The electronic structure of Ti 4O 7 studied by resonant photoemission

We present and discuss resonant photoemission (PES) spectra of Ti 4O 7 taken at 50 and 300 K. The PES spectra taken in the metallic phase at 300 K show Ti 3d bands at the Fermi level and O 2p bands at higher binding energies. The intensity of the Ti 3d bands increases for photon energies above 40 eV. This effect is attributed to a constructive interference with the Ti 3p → 3d resonance which is confirmed by the corresponding constant-initial-state (CIS) spectra. The Ti 3d bands shift approximately 0.25 eV towards higher binding energies in the low temperature semiconducting phase at 50 K. This shift reflects the opening of the band gap and provides a rough estimate of the electronic contribution to the formation energy of the Ti 3+-Ti 3+ pairs in the low-temperature phase.

Changes in the electronic structure of Ti4O7 across the semiconductor-semiconductor-metal transitions.

We present and discuss photoemission (PES) and O 1s x-ray-absorption spectra (XAS) of ${\mathrm{Ti}}_{4}$${\mathrm{O}}_{7}$ taken at different temperatures in the range 50--300 K. The PES taken at 300 K show Ti 3d bands at the Fermi level and O 2p bands at higher binding energies. The Ti 3d bands shift approximately 0.25 eV towards higher binding energies in the low-temperature semiconducting phase (50 K). The O 1s XAS are related, via the corresponding metal-oxygen hybridization, to the unoccupied electronic states in the conduction band. The XAS taken at 300 K reflect Ti 3d bands at threshold and Ti 4sp bands at higher photon energies. The Ti 3d bands are split by crystal-field effects into ${\mathit{t}}_{2\mathit{g}}$ and ${\mathit{e}}_{\mathit{g}}$ subbands and shift approximately 0.45 eV towards higher photon energies in the low-temperature semiconducting phase (80 K). The XAS are basically temperature independent in both the low-temperature semiconducting phase and in the high-temperature metallic phase. The main changes in the O 1s XAS appear rather suddenly at the semiconductor-metal transition around 150 K.

Electronic properties of ultrathin Cu and Fe films on TiO2(110) studied by photoemission and inverse photoemission.

We have studied the electronic properties of the Cu/${\mathrm{TiO}}_{2}$(110) and Fe/${\mathrm{TiO}}_{2}$(110) interfaces using in situ ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPS). The chemical interaction between these two transition metals and the substrate was monitored by examining O 2p and band-gap emission in UPS, and the unoccupied Ti 3d band in IPS. For Cu film thicknesses ranging from 0.1 to 80 \AA{}, there is no sign of overlayer oxidation or substrate reduction in either spectroscopy indicating that there is no significant charge transfer between Cu and the substrate. In contrast, strong interaction between Fe and ${\mathrm{TiO}}_{2}$(110) has been found, especially at low coverages. Charge transfer between the Fe overlayer and the substrate takes place at the very initial stage of deposition resulting in oxidation of the adsorbed Fe atoms and reduction of the Ti ions on the surface. Although the band-gap emission due to the formation of Ti 3d states is usually obscured by the Fe 3d band in UPS, IPS clearly shows a ${\mathrm{Ti}}^{3+}$ induced state at \ensuremath{\sim}3 eV above the Fermi level as seen on defective ${\mathrm{TiO}}_{2}$(110). The UPS spectrum from an oxygen-exposed Fe overlayer indicates formation of an iron-oxide layer composed of FeO and ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ phases.

Electron-correlation effects on the properties of LaxSr1-xTiO3.

A theoretical study of the electron-correlation effects on the properties of ${\mathrm{La}}_{\mathit{x}}$${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{TiO}}_{3}$ is presented. Using a single band repulsive Hubbard model employing the Gutzwiller approximation we study the effect of electron correlations on the effective-mass, band narrowing, plasma frequency, and spin susceptibility. The doping dependence of these properties are found to be qualitatively in agreement with recent experimental studies on ${\mathrm{La}}_{\mathit{x}}$${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{TiO}}_{3}$. We have also proposed a possibility for inducing superconductivity using the idea of proximity effect. The induced superconducting gap in ${\mathrm{La}}_{\mathit{x}}$${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{TiO}}_{3}$ is predicted to be maximum near the half-filling of the Ti 3d band.

An inverse photoemission study of empty electronic states in TiO 2

Empty electronic states in rutile (TiO 2) have been studied by inverse photoemission spectroscopy at (110) and (001) surfaces. The spectra reveal a 2 eV splitting of the empty Ti 3d bands into π ∗ and σ ∗ components, with an overall Ti 3d bandwidth of order 6 eV. These values are in good agreement with periodic ab initio Hartree-Fock calculations.

Cu L<sub>2,3</sub> White Lines of Cu Compounds Studied by Electron Energy Loss Spectroscopy

Cu L 2.3 near edge fine structures of Cu compounds were investigated by electron energy loss spectroscopy. In Cu 60 Ti 40 , small but sharp white lines at the Cu L 2,3 edge were observed. The white lines were attributed to the charge transfer between the Cu-3d band and Ti-3d band. Similar white lines were observed in Al 63 Cu 24 Fe 13 , but the full width at half the maximum (FWHM) of the lines are much wider than those of Cu 60 Ti 40 . In the energy loss spectra of Al-Cu alloys, a white line at the L 3 edge with a wide FWHM was also observed, and it was found that the height of the L 3 line of the Al-Cu alloys at about 930 eV tends to increase with the increase of Al content

Adhesion and bonding of polar and non-polar SiC surfaces to Ti(0001)

The atom superposition and electron delocalization molecular orbital calculations have been performed for studying the interfacial bonding and adhesion of some polar and nonpolar surfaces of α- and β-SiC with the close-packed Ti(0001) surface, using large cluster models and idealized structures. Polar SiC surfaces studied in this work are the Si- and C-terminated (111) and (100) planes and the nonpolar surfaces with equal concentration of Si and C, i.e., β-SiC(110), α-SiC(101̄0) and α-SiC(112̄0). For the unrelaxed polar SiC surfaces, the average Si/Ti and C/Ti interfacial bond strengths are maximum for the β-SiC(110)/Ti(0001) interface due to two dangling surface state orbitals on each surface atom in place of one on the (111) SiC surfaces. The Si- and C-terminated (100) surfaces undergo dimerization. Adhesion energies for the (111) and (100) surfaces to Ti(0001) are comparable when surface relaxation and reconstruction effects are considered. Nonpolar α- and β-SiC surfaces bind comparatively weakly to titanium, and their adhesion energies to Ti(0001) are all approximately equal. In each case of interfacial bonding the sp hybridized dangling orbitals on Si and C surface atoms are stabilized by bond formation with Ti 3d band orbitals.

Transport and magnetic properties of La xSr 1-xTiO 3

We report the transport and magnetic properties of La x Sr l-x TiO 3 . Antiferromagnetic ordering of LaTi0 3 with the Neel temperature T N ≃ 110 K disappears in the presence of less than one percent of Sr. For a wide range of x, the resistivity exhibits a strong decrease for temperatures below about 40 K, which continues all the way down to 35 mK, although there is no magnetic anomaly. For x less than about 0.2, the transport properties are interpreted in terms of the presence of a narrow impurity band, which resides 20 - 200 meV below the bottom of the Ti 3d bands.

The role of Ti 3d electrons in the formation of titanium silicide and oxide

The effect of the change in Ti 3d band occupancy, as Ti chemically binds with oxygen and silicon, on the Auger Spectra and Slow Electron Energy Loss Spectra has been monitored. Along with the changes due to shallow core levels, plasmon energies, etc., the spectral features that involve the quasiatomic transition [3p → 3d], are seen to manifest the electron density in the Ti 3d valence band and consequently reflect the mode of charge transfer in the formation of the respective chemical bonds.

Electronic structure of TiSi2.

The linear augmented-plane-wave method has been applied to calculate the electronic structure and properties of the orthorhombic metastable base-centered (C49) and equilibrium face-centered (C54) phases of ${\mathrm{TiSi}}_{2}$. Despite the structural differences, both polytypes exhibit similar electronic properties, including broad low-lying Si 3s-3p bands, partially filled Ti 3d bands, and density-of-states minima within the 3d-band manifold near ${E}_{F}$. The high residual resistivity of C49 films in comparison with those having the C54 structure (\ensuremath{\sim}27 versus 0.7 \ensuremath{\mu}\ensuremath{\Omega} cm) is analyzed with the use of the calculated Drude plasma energy and Fermi velocity for the two polytypes. It is concluded that the comparatively short scattering length of the C49 phase (${l}_{c}$\ensuremath{\approxeq}90 A\r{} versus \ensuremath{\sim}990 A\r{} for C54 ${\mathrm{TiSi}}_{2}$) is an extrinsic property, caused primarily by the presence of heavily faulted microstructure in the C49 films.

Materials with cationic valence and conduction bands for photoelectrolysis of water

Two types of materials in which the highest filled and lowest empty electron bands are mainly cationic have been investigated, viz. Bi 4Ti 3O 12 and Bi 2WO 6 with a filled Bi 6s band and empty Ti(W) 3d(5d) bands and NiNb 2O 6 and CoNb 2O 6 with filled narrow transition metal bands and empty Nb 4d bands. Bi 4Ti 3O 12 shows weak photocurrents in a photoelectrochemical water decomposition cell, whereas Bi 2WO 6 does not. Also NiNb 2O 6 and CoNb 2O 6 behave differently. Whereas NiNb 2O 6 shows photocurrents, CoNb 2O 6 does not. These results are discussed in terms of bulk properties. The absence of a photocurrent is related to the formation of small polarons with high activation energy for migration. In agreement with this the compound Bi 2WO 6 shows luminescence.