Ti L2 Research Articles

X-ray Diffraction and X-ray Spectroscopy Studies of Cobalt-Doped Anatase TiO2:Co Nanopowders

Co-doped TiO2 is one of the most extensively studied oxides for applying as dilute magnetic semiconductors due to its room temperature magnetism. Here we present results of the studies of Ti0.97Co0.03O2 nanopowders synthesized by hydrothermal method by means of X-ray diffraction, soft X-ray absorption spectroscopy (Ti L2,3 and Co L2,3 spectra), hard X-ray absorption spectroscopy (Co K spectra), and 1s3p resonant inelastic X-ray scattering at the Co K edge. According to X-ray diffraction data and Ti L2,3 X-ray absorption spectra, all the samples before the thermal treatment exhibit anatase structure with substantial amount of amorphous phase. After annealing the Ti0.97Co0.03O2 samples in vacuum or hydrogen at 700oC, the anatase structure persists while amorphous phase contribution is eliminated. Surface sensitive soft X-ray absorption Co L2,3 spectroscopy revealed only Co2+ ions tetrahedrally coordinated by oxygen ions and no sign of metallic Co. Co2+ tetrahedral sites (instead of typical octahedral ones) ...

Rapid thermal annealing induced modification in structural and electronic structure properties of Ti0.95Co0.05O2⿿δ thin films

Abstract Thin film of Ti0.95Co0.05O2-δ was deposited on Si (100) using PLD method and annealed in O2 and N2 environment. Raman spectra confirm that all the films have rutile structure. Surface morphology indicates that the surface roughness and grain size increase with annealing. The electronic structure studied by NEXAFS spectroscopy at O K, Ti L3,2 and Co L3,2-edges revealed that peak intensities decrease significantly for the film annealed N2 environment. The ligand-field splitting estimated from the energy difference between the t2g and eg features in O K-edge spectra were 2.71 eV for as-deposited and O2 annealed film, whereas reduced more than double (⿼1.32 eV) for the film annealed in N2. Atomic multiplet calculations and experimentally observed NEXAFS spectra at Co L3,2-edge and Ti L3,2-edge confirm that Co present in 2+ and Ti in +4 valence state, whereas the multiplet structures of O2 annealed film looks similar to Co metal.

Electronic structure modification and Fermi level shifting in niobium-doped anatase titanium dioxide thin films: a comparative study of NEXAFS, work function and stiffening of phonons.

The electronic structure and tuning of work function (WF) by electronic excitations (EEs) induced by swift heavy ions (SHIs) in anatase niobium-doped titanium dioxide (NTO) thin films is reported. The densities of EEs were varied using 80 MeV O, 130 MeV Ni and 120 MeV Ag ions for irradiation. The EE-induced modifications in electronic structure were studied by O K-edge and Ti L3,2 edge absorption spectra using near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The reduction of hybridized O 2p and Ti 3d unoccupied states in the conduction band with a decrease in energy of the crystal field strength of ∼ 480 meV and the correlated effect on the decrease in the WF value of ∼ 520 meV upon increasing the total energy deposition in the lattice are evident from the study of NEXAFS and scanning Kelvin probe microscopy (SKPM), respectively. The observed stiffening in the low frequency Raman mode (LFRM) of ∼ 9 cm(-1) further validates the electronic structure modification under the influence of EE-induced strain in TiO6 octahedra. The reduction of hybridized valence states, stiffening behavior of LFRM and decrease in WF by nano-crystallization followed by amorphization and defects in NTO lattice are explained in terms of continuous, discontinuous amorphous ion tracks containing intestinally created defects and non-stoichiometry in the lattice. These studies are very appropriate for better insights of electronic structure modification during phase transformation and controlled Fermi level shifting, which plays a crucial role in controlling the charge carrier injection efficiency in opto-electronic applications and also provides a deeper understanding of the involved physical processes.

Electronic and magnetic phenomena at the interface of LaAlO3 and Ru doped SrTiO3

We have investigated the effect of Ru doping the SrTiO3 (STO) side of the LaAlO3/STO (LAO/STO) interface. The metallic behavior at the interface is remarkably robust to defects and disorder. Despite spin moment contribution from Ru ions, we see no evidence of magnetic ordering at the Ti L3,2 edge in either doped or undoped interfaces using X-ray magnetic circular dichroism. Magnetotransport measurements also do not show any evidence of magnetic scattering beyond that observed in undoped LAO/STO interfaces. Insertion of more than 7 unit cells of Ru doped STO at the interface suppresses metallic conductivity with a surprisingly sharp metal insulator transition. A similar metal-insulator transition is observed when a homoepitaxial STO film is grown on the single crystal substrate before LAO deposition. Together, our results indicate that ferromagnetism is not intrinsic to the interface, magnetic Ru dopants are not significant sources of scattering, and that cation vacancy formation alone cannot explain the insulating behavior observed in thick homoepitaxial LAO/STO/STO bilayers.

Electronic structure and x-ray magnetic circular dichroism of Mn-doped TiO2

The electronic structure of (Ti,Mn)O2 diluted magnetic semiconductors was investigated theoretically from first principles using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure was obtained with the local spin-density approximation taking into account strong Coulomb correlations in the frame of the LSDA + U approximation. The x-ray absorption spectra and x-ray magnetic circular dichroism spectra at the Mn and Ti L2,3 and O K edges were investigated theoretically from first principles. The origin of the XMCD spectra in these compounds was examined. The calculated results are compared with available experimental data.

Electronic structure of octahedrally coordinated Cr in CrxTiX2 (X = Se, Te) and TixCr1−xSe2

Abstract Influence of the covalence, octahedral distortion and the translation symmetry of the chromium ions in octahedral coordination on the electronic structure for compounds under investigation was clarified. Cr L 2,3 and Ti L 2.3 X-ray absorption spectra (XAS) along with atomic multiplet calculations and resonant Ti 2p-3d and Cr 2p-3d photoelectron spectra of valence band (RXPS) in the intercalated titanium dichalcogenides Cr x TiX 2 (X = Se, Te) and in the substitution compound Ti x Cr 1− x Se 2 show the charge transfer between chromium and titanium sublattices.

Electron-Phonon Coupling in the Bulk of Anatase TiO2 Measured by Resonant Inelastic X-Ray Spectroscopy.

We investigate the polaronic ground state of anatase TiO2 by bulk-sensitive resonant inelastic x-ray spectroscopy (RIXS) at the Ti L3 edge. We find that the formation of the polaron cloud involves a single 95 meV phonon along the c axis, in addition to the 108 meV ab-plane mode previously identified by photoemission. The coupling strength to both modes is the same within error bars, and it is unaffected by the carrier density. These data establish RIXS as a directional bulk-sensitive probe of electron-phonon coupling in solids.

Role of Interface Structure and Chemistry in Resistive Switching of NiO Nanocrystals on SrTiO3

Nickel oxide (NiO) nanocrystals epitaxially grown on (001) strontium titanate (SrTiO3) single crystal substrates were characterized to investigate interface morphology and chemistry. Aberration corrected high angle annular dark field scanning transmission electron microscopy reveals the interface between the NiO nanocrystals and the underlying SrTiO3 substrate to be rough, irregular, and have a lower average atomic number than the substrate or the nanocrystal. Energy dispersive x-ray spectroscopy and electron energy loss spectroscopy confirm both chemical disorder and a shift of the energy of the Ti L2,3 peaks. Analysis of the O K edge profiles in conjunction with this shift, implies the presence of oxygen vacancies at the interface. This sheds light into the origin of the previously postulated minority carriers’ model to explain resistive switching in NiO [J. Sullaphen, K. Bogle, X. Cheng, J. M. Gregg, and N. Valanoor, Appl. Phys. Lett. 100, 203115 (2012)].

Electronic structure and magnetic properties of Co doped TiO2 thin films using X-ray absorption spectroscopy

Abstract The thin film of Ti 0.95 Co 0.05 O 2− δ has been grown on LaAlO 3 (100) substrate using pulsed laser deposition method. X-ray diffraction, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, X-ray magnetic circular dichroism (XMCD) and magnetic hysteresis loop measurements were used to fully understand the origin of ferromagnetism. Our structural analysis reveals a single phase nature of the film and excludes the presence of any secondary phase. NEXAFS spectra collected at Ti L 3,2 , and Co L 3,2 -edge infer that Co and Ti ions are in 2+ and 4+ valence states, respectively. Multiplet calculation performed at Co L 3,2 -edge also support the experimental observations and shows that Co ions are in 2+ valence state in O h (octahedral) symmetry. Zero field cooled and field cooled magnetization infer that T C of Ti 0.95 Co 0.05 O 2− δ film is above room temperature. DC magnetization hysteresis loop study and XMCD measurement at Co L 3,2 -edge reflect that Ti 0.95 Co 0.05 O 2− δ film exhibits ferromagnetic ordering at room temperature.

Structural and Optical Interplay of Palladium-Modified TiO2 Nanoheterostructure

The electronic structure and optical properties of Pd-modified TiO2 nanotubes (NTs) with a vertically aligned nanotubular structure grown by a two-step electrochemical anodization method have been studied using X-ray spectroscopy. X-ray absorption near-edge structure (XANES) at the Ti L3,2- and O K-edges was used to investigate the TiO2 NTs before and after Pd modification. It was found that Pd nanoparticles (NPs) are uniformly coated on the NT surface. The Pd L3-edge of the deposited Pd NPs shows a more intense whiteline and a blue shift for the Pd L3-edge absorption threshold relative to Pd metal, indicating charge depletion from the Pd 4d orbital as a result NP formation. The lattice of Pd is slightly contracted upon NP formation, although it remains fcc as revealed by extended X-ray absorption fine structure (EXAFS) analysis at the Pd K-edge. X-ray-excited optical luminescence (XEOL) together with XANES with element and site specificity was used to study the optical luminescence from TiO2 NTs. It was ...

Mn2+ Substitutional Doping of TiO2 Nanoribbons: A Three-Step Approach

An in situ doping approach was successfully employed for synthesis of Mn2+-doped sodium titanate nanoribbons, which were used as a precursor for preparation of TiO2 nanoribbons with homogeneous distribution of Mn2+ ions. The comprehensive structural characterization using powder X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) provided compelling evidence that the Mn2+ ion predominantly substitutes the Ti4+ ion at octahedral coordination sites in bulk. Measurements performed on individual nanoribbons using near edge X-ray absorption fine structure spectromicroscopy revealed that the strong alkaline environment required for the formation of sodium titanate nanoribbons did not affect the manganese oxidation state. In the next two steps, the ion exchange process in HCl(aq) solution followed by the thermal treatment in air, lead to the formation of Mn2+ doped TiO2 nanoribbons. Analysis of the manganese content by X-ray photoelectron spectroscopy of several TiO2 nanoribbon samples calcined in the temperature range from 400 to 700 °C as well as analysis performed at the Ti L2,3 and Mn L2,3 edges with electron energy loss spectroscopy (EELS) showed that calcination at elevated temperatures induced the diffusion of manganese ions toward the nanoribbons’ surface. However, transformation of anatase nanoribbons to rutile nanoparticles, this process started at around 580 °C, was also accompanied by the partial oxidation of Mn2+ to Mn3+ and Mn4+. Manganese atoms that diffused to the TiO2 surface preferentially formed MnOx clusters as observed from characteristic electron paramagnetic resonance spectra and EELS measurements. In addition, the presence of Mn2+ reduced the beginning of phase transformation from anatase to rutile to near 120 °C.

ChemInform Abstract: Energy‐Enhanced Atomic Layer Deposition for More Process and Precursor Versatility

AbstractReview: 201 refs.

Role of interface structure and chemistry in resistive switching of NiO nanocrystals on SrTiO3

Nickel oxide (NiO) nanocrystals epitaxially grown on (001) strontium titanate (SrTiO3) single crystal substrates were characterized to investigate interface morphology and chemistry. Aberration corrected high angle annular dark field scanning transmission electron microscopy reveals the interface between the NiO nanocrystals and the underlying SrTiO3 substrate to be rough, irregular, and have a lower average atomic number than the substrate or the nanocrystal. Energy dispersive x-ray spectroscopy and electron energy loss spectroscopy confirm both chemical disorder and a shift of the energy of the Ti L2,3 peaks. Analysis of the O K edge profiles in conjunction with this shift, implies the presence of oxygen vacancies at the interface. This sheds light into the origin of the previously postulated minority carriers’ model to explain resistive switching in NiO [J. Sullaphen, K. Bogle, X. Cheng, J. M. Gregg, and N. Valanoor, Appl. Phys. Lett. 100, 203115 (2012)].

XANES observations of the inhibition and promotion of anatase and rutile phases in silver containing films

X-ray absorption near-edge structure (XANES) spectroscopy has been applied to characterize the phase evolution of Ag–TiO2 films subjected to annealing treatments up to 1000 °C. Silver has been theoretically predicted as a rutile promoter in anatase powders lowering the transition temperature, however, no systematic studies on films have been reported yet. In this work, the Ti L2,3- and O K-edge in XANES spectra were used as fingerprints of the phase structure of the films. The results show that silver atoms strongly affect the transition of the initially amorphous films to the anatase structure, delaying the crystallization temperature up to 400 °C and, at the same time, promoting the subsequent phase transformation from anatase to rutile at temperatures as low as 600 °C. These results have been confirmed by X-Ray Diffraction (XRD) analysis as well as by High Resolution TEM images (HRTEM) of films.

Atomic scale investigation of a PbTiO3/SrRuO3/DyScO3 heterostructure

An epitaxial PbTiO3 thin film grown on self-organized crystalline SrRuO3 nanowires deposited on a DyScO3 substrate with ordered DyO and ScO2 chemical terminations is investigated by transmission electron microscopy. In this PbTiO3/SrRuO3/DyScO3 heterostructure, the SrRuO3 nanowires are assumed to grow on only one type of substrate termination. Here, we report on the structure, morphology, and chemical composition analysis of this heterostructure. Electron energy loss spectroscopy reveals the exact termination sequence in this complex structure. The energy loss near-edge structure of the Ti–L2,3, Sc–L2,3, and O K edges shows intrinsic interfacial electronic reconstruction. Furthermore, PbTiO3 domain walls are observed to start at the end of the nanowires resulting in atomic steps on the film surface.

The formation of Ti–O tetrahedra and band gap reduction in SiO2 via pulsed ion implantation

Titanium ions are implanted into amorphous SiO2 at two different fluences using pulsed ion implantation, and the resulting samples are annealed. Bulk sensitive soft X-ray absorption spectroscopy of the Ti L2,3 edge reveal strikingly different spectra for the two fluences. Spectral simulations using multiplet crystal field theory show clearly that for low fluence the Ti ions have a local octahedral coordination, while at higher fluence the formation of Ti4+–O tetrahedra dominates. Using O K-edge X-ray absorption and emission, the effect of the Ti states on the valence and conduction bands of the host SiO2 is revealed. With the introduction of Ti tetrahedra, the band gap reduces from about 8 eV to just over 4 eV, due entirely to the Ti 3d conduction band states. These results demonstrate the possibility to obtain Ti–O tetrahedra in silica by Ti ion implantation and a suitable thermal treatment, clarify the mechanism of band gap reduction with Ti doping in SiO2, and demonstrate the sensitivity of L-edge X-ray absorption with a multiplet crystal field theory analysis to the Ti coordination of novel materials.

Labeling interacting configurations through an analysis of excitation dynamics in a resonant photoemission experiment: the case of rutile TiO2

A detailed study of resonant photoemission at Ti L2,3 edges of insulating rutile TiO2−x thin film is presented. Pure TiO2 resonating structures, defect-related resonances, resonant Raman–Auger and normal LVV Auger emissions are tracked, including an unpredicted two-hole correlated satellite below the non-bonding part of the valence band. The analysis of excitation dynamics unambiguously addresses the origin of these features and, in particular, the extent of charge transfer effects on the Ti–O bonding in the valence band of rutile, disclosing further applications to the more general case of, formally, d0 oxides.

Electronic structure of Ti–Ni alloys: An XPS and NEXAFS study

The paper reports on a study of the electronic structure of binary homogeneous alloys Ti55Ni45, Ti50Ni50, and Ti45Ni55 by X-ray photoelectron and near-edge X-ray absorption spectroscopies. The experimental data obtained were used to derive the occupied and unoccupied d-states density redistribution upon formation of the Ti–Ni alloys. The Ti and Ni L2,3 X-ray absorption edge intensity measured permitted calculation of the variation of the titanium and nickel d-shell occupancy induced by formation of the Ti50Ni50, Ti55Ni45 and Ti45Ni55 alloys. It is shown that the occupation of the nickel d-band decreases by 0.13 el./atom in formation of the above alloys and practically does not depend on the ratio of the constituent elements, whereas that of the titanium d-shell is strongly dependent on the atomic composition of the alloy. The charge transfer between the components was found to be insignificant; this implies that the change in occupancy of the nickel and titanium d-shells originates from intratomic redistribution of electron density between the d- and sp-states. The Ti50Ni50 equiatomic alloy is unique in that it has the lowest titanium d-shell occupancy and, hence, the largest number of sp-electrons. It is shown that the increasing Ni content in the TiNi alloys leads to a noticeable gain of titanium d-electrons. It is assumed that the additional d-electrons can form a direct bonds, which increases the elastic moduli and, consequently, reduce the temperature of martensitic transformation in Ni-rich TiNi alloys.

Resonant photoemission at the L3 absorption edge of Mn and Ti and the electronic structure of 1T-Mn0.2TiSe2

Resonant valence band x-ray photoelectron spectra (ResPES) excited near the 2p3/2 core level energies, 2p x-ray photoelectron spectra (XPS) and L3,2 x-ray absorption spectra (XAS) of Ti and Mn in single crystals of 1T-Mn0.2TiSe2 were studied for the first time. The ionic–covalent character of the bonds formed by the Mn atoms with the neighboring Se atoms in the octahedral coordination is established. From the XPS and XAS measurements compared with the results of atomic multiplet calculations of Ti and Mn L3,2 XAS, it is found that the Ti atoms are in the ionic state of 4 + and the Mn atoms are in the state of 2 + . In ResPES of Mn0.2TiSe2 excited near the Ti 2p3/2 and Mn 2p3/2 absorption edges the Ti 3d and Mn 3d bands at binding energies just below the Fermi level are observed. According to theoretical calculations of E(k) the Ti 3d states are localized in the vicinity of the Γ point and the Mn 3d states are localized along the direction K–Γ–M in the Brillouin zone of the crystal.

Modifications in structural and electronic properties of TiO2 thin films using swift heavy ion irradiation

We report on the structural and electronic properties of swift heavy ion (SHI) irradiated pristine TiO2 thin films, deposited by radio frequency magnetron sputtering on sapphire substrates. The high resolution x-ray diffraction and Raman measurements show a structural phase transition from anatase to admixture of brookite and rutile phases of TiO2 with increasing SHI fluence followed by a significant distortion in the TiO6 octahedra. The modification in the electronic structure stimulated by SHI irradiation has been investigated using x-ray absorption (XAS) experiments at the O K and Ti L3,2 absorption edges. The O K edge spectra clearly indicate the splitting of the pre-edge spectral features having t2g and eg symmetry bands due to structural disorder/distortion induced by irradiation. The intensity of the SHI generated components at the O K edge increases monotonically, which can be correlated to the modification in unoccupancies associated with O 2 p orbitals hybridized with Ti 3 d states. The XAS spectra at the Ti L3,2 edge further authenticate that SHI creates a controlled structural disorder/distortion in the TiO6 octahedra.