161 research outputs found
Galore: Broadening and weighting for simulation of photoelectron spectroscopy
Galore simplifies and automates the process of simulating photoelectron spectra from ab
initio calculations. This replaces the tedious process of extracting and interpolating crosssectional weights from reference data and generates tabulated data or publication-ready
plots as needed. The broadening tools may also be used to obtain realistic simulated
spectra from a theoretical set of discrete lines (e.g. infrared or Raman spectroscopy)
High resolution X-ray photoemission study of nitrogen doped TiO 2 rutile single crystals
Abstract The electronic structure of nitrogen doped TiO 2 prepared by annealing single crystal rutile (1 1 0) substrates in NH 3 at elevated temperatures was investigated using high resolution X-ray photoelectron spectroscopy. NH 3 treatment at 600Ā°C introduced N into the TiO 2 lattice without concomitant surface reduction of the rutile phase. This doping leads to bandgap narrowing associated with the appearance of new N 2p electronic states above the O 2p band in valence region photoemission spectra. Surface modification at the higher temperature of 700Ā°C also produced bandgap narrowing but at the same time led to pronounced surface reduction
Photoemission study of the metal-insulator transition in VO_2/TiO_2(001) : Evidence for strong electron-electron and electron-phonon interaction
We have made a detailed temperature-dependent photoemission study of
VO_2/TiO_2(001) thin films, which show a metal-insulator transition at \sim 300
K. Clean surfaces were obtained by annealing the films in an oxygen atmosphere.
Spectral weight transfer between the coherent and incoherent parts accompanying
the metal-insulator transition was clearly observed. We also observed a
hysteretic behavior of the spectra for heating-cooling cycles. We have derived
the ``bulk'' spectrum of the metallic phase and found that it has a strong
incoherent part. The width of the coherent part is comparable to that given by
band-structure calculation in spite of its reduced spectral weight, indicating
that the momentum dependence of the self-energy is significant. This is
attributed to by ferromagnetic fluctuation arising from Hund's rule coupling
between different d orbitals as originally proposed by Zylbersztejn and Mott.
In the insulating phase, the width of the V 3d band shows strong temperature
dependence. We attribute this to electron-phonon interaction and have
reproduced it using the independent boson model with a very large coupling
constant.Comment: 7 pages, 7 figures, submitted to Phys. Rev.
Electronic structure of InāOā and Sn-doped InāOā by hard x-ray photoemission spectroscopy
The valence and core levels of InāOā and Sn-doped InāOā have been studied by hard x-ray photoemission spectroscopy (hĪ½=6000āeV) and by conventional AlāKĪ± (hĪ½=1486.6āeV) x-ray photoemission spectroscopy. The experimental spectra are compared with density-functional theory calculations. It is shown that structure deriving from electronic levels with significant In or Sn 5s character is selectively enhanced under 6000 eV excitation. This allows us to infer that conduction band states in Sn-doped samples and states at the bottom of the valence band both contain a pronounced In 5s contribution. The In 3d core line measured at hĪ½=1486.6āeV for both undoped and Sn-doped InāOā display an asymmetric lineshape, and may be fitted with two components associated with screened and unscreened final states. The In 3d core line spectra excited at hĪ½=6000āeV for the Sn-doped samples display pronounced shoulders and demand a fit with two components. The In 3d core line spectrum for the undoped sample can also be fitted with two components, although the relative intensity of the component associated with the screened final state is low, compared to excitation at 1486.6 eV. These results are consistent with a high concentration of carriers confined close to the surface of nominally undoped InāOā. This conclusion is in accord with the fact that a conduction band feature observed for undoped InāOā in AlāKĪ± x-ray photoemission is much weaker than expected in hard x-ray photoemission
Experimental and theoretical study of the electronic structures of lanthanide indium perovskites LnInO3
Ternary lanthanide indium oxides LnInO3 (Ln = La, Pr, Nd, Sm) were synthesized by high-temperature solid-state reaction and characterized by X-ray powder diffraction. Rietveld refinement of the powder patterns showed the LnInO3 materials to be orthorhombic perovskites belonging to the space group Pnma, based on almost-regular InO6 octahedra and highly distorted LnO12 polyhedra. Experimental structural data were compared with results from density functional theory (DFT) calculations employing a hybrid Hamiltonian. Valence region X-ray photoelectron and K-shell X-ray emission and absorption spectra of the LnInO3 compounds were simulated with the aid of the DFT calculations. Photoionization of lanthanide 4f orbitals gives rise to a complex final-state multiplet structure in the valence region for the 4fn compounds PrInO3, NdInO3, and SmInO3, and the overall photoemission spectral profiles were shown to be a superposition of final-state 4fnā1 terms onto the cross-section weighted partial densities of states from the other orbitals. The occupied 4f states are stabilized in moving across the series PrāNdāSm. Band gaps were measured using diffuse reflectance spectroscopy. These results demonstrated that the band gap of LaInO3 is 4.32 eV, in agreement with DFT calculations. This is significantly larger than a band gap of 2.2 eV first proposed in 1967 and based on the idea that In 4d states lie above the top of the O 2p valence band. However, both DFT and X-ray spectroscopy show that In 4d is a shallow core level located well below the bottom of the valence band. Band gaps greater than 4 eV were observed for NdInO3 and SmInO3, but a lower gap of 3.6 eV for PrInO3 was shown to arise from the occupied Pr 4f states lying above the main O 2p valence band
Nature of the Band Gap of InāOā Revealed by First-Principles Calculations and X-Ray Spectroscopy
Bulk and surface sensitive x-ray spectroscopic techniques are applied in tandem to show that the valence band edge for InāOā is found significantly closer to the bottom of the conduction band than expected on the basis of the widely quoted bulk band gap of 3.75 eV. First-principles theory shows that the upper valence bands of InāOā exhibit a small dispersion and the conduction band minimum is positioned at Ī. However, direct optical transitions give a minimal dipole intensity until 0.8 eV below the valence band maximum. The results set an upper limit on the fundamental band gap of 2.9 eV
Identification of Lone-Pair Surface States on Indium Oxide
Indium
oxide is widely used as a transparent electrode in optoelectronic
devices and as a photocatalyst with activity for reduction of CO<sub>2</sub>. However, very little is known about the structural and electronic
properties of its surfaces, particularly those prepared under reducing
conditions. In this report, directional ālone-pairā
surface states associated with filled 5s<sup>2</sup> orbitals have
been identified on vacuum-annealed In<sub>2</sub>O<sub>3</sub>(111)
through a combination of hard and soft X-ray photoemission spectroscopy
and density functional theory calculations. The lone pairs reside
on indium ad-atoms in a formal +1 oxidation state, each of which traps
two electrons into a localized hybrid orbital protruding away from
the surface and lying just above the valence band maximum in photoemission
spectra. The third electron associated with the ad-atoms is delocalized
into the conduction band, thus producing the surface electron accumulation
layer identified previously on vacuum-annealed In<sub>2</sub>O<sub>3</sub>(111) (1 Ć 1) surfaces. The surface structure is further
supported by low-energy electron diffraction, but there is no chemical
shift in indium core level X-ray photoelectron spectra between surface
InĀ(I) ad-atoms and bulk InĀ(III). The 5s<sup>2</sup> lone pairs confer
Lewis basicity on the surface In sites and may have a pronounced impact
on the catalytic or photocatalytic activity of reduced In<sub>2</sub>O<sub>3</sub>
Voxel-wise quantification of myocardial blood flow with cardiovascular magnetic resonance: effect of variations in methodology and validation with positron emission tomography
- ā¦