A high-resolution X-ray photoemission study of the total valence-band densities of states of GaSe and BiI3

The total valence band denstiy of states spectra for the semiconducting layered compounds GaSe and BiI₃ were obtained by X-ray photoemission spectroscopy. The results for GaSe were used to test recent band structure calculations and are compared with earlier photoemission results. The BiI₃ data are the first experimental determination of the total valence band density of states for this compound. Since no calculations exist for BiI₃, tentative assignments were made.     

The electronic structure of ZrSe32

The energy bands of the semiconductor ZrSe₃ have been evaluated using the KKR method. The general features of the electronic structure are expected to pertain to other transition metal trichalcogenides which have similar trigonal prismatic coordination. The calculated density of states agrees well with X-ray photoemission spectra for the valence band. The joint density of states has been evaluated and is discussed in terms of optical measurements.     

Photoemission study of the density of valence states of amorphous As2S3 and Sb2S3

The densities of valence states of amorphous As₂S₃ and Sb₂S₃ have been investigated by means of X-ray photoemission and ultraviolet photoemission spectroscopy. The spectra are interpreted on the basis of existing band structure calculations.     

Surface electronic structure of the organic superconductor κ-(ET)2Cu(NCS)2 studied via photoemission microscopy

The surface electronic structure of cleaved single crystals of the organic superconductor κ-(ET)₂Cu(NCS)₂ has been studied using photoemission microscopy. Two types of cleaved surfaces were observed, displaying different valence band photoemission spectra and different spectral behavior near the Fermi level, E_F. In particular, spectra from one surface type display relatively broad spectral features in the valence band and finite spectral intensity at E_F, while spectra from the other surface type show well-defined valence band emission features and zero photoemission intensity at E_F. We propose that the spectral differences are due to a very short electron mean free path in this material, and our results are used to explain the differences between previously published photoemission spectra from this superconductor. We also report the results of an investigation of the electronic structure of defects in this material.     

The bonding of oxygen to Al(111)

Oxidation of the Al(111) surface is a two-stage process in which the formation of an ordered oxygen overlayer precedes the creation of a bulk-like amorphous oxide. An electronic structure calculation is reported here for the clean and oxygen-covered Al(111) surface and for bulk A1₂O₃. The calculation uses an atomic-orbital basis and the metal surface is modelled by an infinite two-dimensional crystal, containing seven layers of aluminium atoms. Oxygen atoms occupy three-fold sites, with an Al-O separation of 1.9 Å. The oxygen 2p resonance in the (1 × 1) chemisorbed overlayer is about 3 eV wide, compared to 1.9 eV for an equivalent isolated layer of oxygen atoms unhybridized with the metal. The valence band of A1₂O₃ is about 1.5 eV wider than the chemisorbed oxygen resonance, but in both cases most of the states are concentrated in the top 1.5 eV of the band. The results are related to available ultraviolet photoemission spectra, including the recent angular-resolved spectra of Martinson and Flodström.     

Correlation of conductivity and angle integrated valence band photoemission characteristics in single crystal iron perovskites for 300 K < T < 800 K: Comparison of surface and bulk sensitive methods

A single crystal monolith of La_0.9Sr_0.1FeO₃ and thin pulsed laser deposited film of La_0.8Sr_0.2Fe_0.8Ni_0.2O₃ were subject to angle integrated valence band photoemission spectroscopy in ultra high vacuum and conductivity experiments in ambient air at temperatures from 300 K to 800 K. Except for several sputtering and annealing cycles, the specimens were not prepared in situ. Peculiar changes in the temperature dependent, bulk representative conductivity profile as a result of reversible phase transitions, and irreversible chemical changes are semi-quantitatively reflected by the intensity variation in the more surface representative valence band spectra near the Fermi energy. X-ray photoelectron diffraction images reflect the symmetry as expected from bulk iron perovskites. The correlation of spectral details in the valence band photoemission spectra (VB PES) and details of the conductivity during temperature variation suggest that valuable information on electronic structure and transport properties of complex materials may be obtained without in situ preparation.     

Valence bands of layer structure transition metal chalcogenides

The valence band structure of representative MX₂ layer structure compounds has been obtained by X-ray photoemission with monochromatized radiation. Chalcogen s and p and metal d band are identified and their width and position obtained. The results are compared with u.v. and He II photoemission and with recent band structure calculations.     

Photoemission study of the Laves-phase compounds YMn2 and Y0.97Sc0.03Mn2

A small amount of Sc substitution for Y in the antiferromagnetic metal YMn₂ results in a paramagnetic metal with a strongly enhanced electronic specific heat. We have studied the electronic structure of YMn₂ and Y_0.97Sc_0.03Mn₂ by photoemission spectroscopy. The spectra of YMn₂ taken above and below the Néel temperature did not show appreciable difference, and the spectra of YMn₂ and Y_0.97Sc_0.03Mn₂ were similar to each other. The photoemission spectra of the antiferromagnetic phase are well explained by band-structure calculation on the antiferromagnetic state while those of the paramagnetic phase are not explained by band-structure calculation on the paramagnetic state. These observations suggest that there are strong antiferromagnetic correlations in the paramagnetic phase. For the paramagnetic phase, agreement between experiment and calculation could be considerably improved by applying a model self-energy correction to the band density of states.     

Ultraviolet photoemission from polymeric sulfur nitride polycrystalline films and single crystals

The (SN)_x valence band structure, for polycrystalline films as well as for single crystal samples, has been studied using He I and He II resonance radiation. In angle-resolved photoemission energy distributions from single crystals, structure in the spectra is selectively enhanced offering a possibility of assigning the photoemission as originating from particular regions of the Brillouin Zone. The observed onset of photoemission 0.2 eV below the Fermi edge is discussed.     

Band structure and cluster model calculations of LaNiO3 compared to photoemission,O 1s X-ray absorption,and optical absorption spectra

We studied the electronic structure of LaNiO₃ using band structure and cluster model calculations. This compound is a paramagnetic metal with a R3?c rhombohedral structure. The band structure was calculated using the generalized gradient approximation (GGA). The cluster model was solved using the configuration interactions (CI) many-body method. We present results for the density of states (DOS), the spectral weight, and the dielectric function ${\epsilon }_2$. The calculations are compared to previous photoemission (PES), O 1s X-ray absorption (XAS), as well as optical absorption spectra. Both band structure and cluster model results are in good agreement with the experimental data. We point out that this concordance is very rare and far from trivial; we argue that this may be due to the unusual characteristics of the ground state of LaNiO₃.     

Resonant-photoemission identification of the valence states of NiPS3

We monitored the resonant behavior of the Ni d satellite peaks in the valence band photoemission spectra of NiPS₃ at photon energies immediately below and immediately above the Ni3p threshold. The observed resonance gives an unequivocal identification of the satellite peaks and of the corresponding main Ni d features. The study of the electronic structure of this material and of the related compounds FePS₃ and HgPS₃ was extended to unoccupied states by means of partial-yield synchrotron-radiation photoemission spectroscopy.     

Electronic structure of tetragonal tungsten bronzes and electrochromic oxides

X-ray photoemission spectra of the band structures of WO₃, crystalline H _x WO₃ and the tetragonal and cubic bronzes M _x WO₃ (M=Li, Na) exhibit great similarity. In the bronzes tungsten 5d conduction band states are occupied. The tungsten 4f core level spectra of these materials have an unusual, but characteristic structure attributed to a combination of final state screening and hydrogen or alkali ion neighbor effects. The band structure of amorphous electrochromic WO₃ films differs in characteristic ways from that of the crystalline bronzes.     

Electronic structure of Ga1−xCrxN investigated by photoemission spectroscopy

We have obtained the Cr 3d-like energy states, which located in the band gap of GaN by means of resonant photoemission spectroscopy. In the difference spectrum between the valence band photoemission spectra of non-doped GaN and that of the Ga_0.937Cr_0.063N, we observed the new energy state, in band gap, consists of Cr 3d-like and N 2p-like component by strong hybridization.     

Surface charging effects on valence band spectra in X-ray photoemission: Crystalline and Amorphous As2S3

In X-ray photoemission (XPS) studies on insulators, strong electric fields associated with surface charging can perturb the observed spectra. We find that the standard technique of flooding with thermal electrons to neutralize the net charge does not eliminate this effect for valence band spectra, but that the use of thin or (for photoconductors) illuminated samples does solve the problem. These conclusions are demonstrated by experiments revealing new structure in, and real differences between, the XPS valence band spectra of crystalline and amorphous As₂S₃.     

The electronic structure of SrTiO3 from a direct-transition analysis of angle-resolved photoemission data

Angle-resolved photoemission spectra of SrTiO₃(100) have been recorded in the range 18eV⩽ hν⩽ 50eV. The results have been interpreted using a direct transition model and a free-electron like final state, yielding empirical Δ-line dispersion relations. While in overall agreement with available band structure calculations, significant discrepancies are revealed. In addition, resonance photoemission above the Ti 3p threshold has been used to identify regions of Ti 3d mixing in the mainly 0 2p valence bands, the results being consistent with theoretical models.     

Photoelectron spectroscopy studies of the band structure of silver halides

X-ray photoelectron spectroscopy has been used to probe the valence bands of the silver halides. Previous ultraviolet photoemission and optical absorption experiments together with the theoretical band structure calculations form the basis for interpretation of the spectra. Density of states maxima from the halogen p levels are clearly resolved from those of the silver 4d states. Additional splittings due to k space symmetry are observable and in the case of AgCl and AgBr, give excellent conformation of existing band structure calculations. The role of spin-orbit coupling is shown to be unimportant in determining the primary shape of the photoelectron spectrum. The spectrum of AgF shows the inversion of silver 4d and halogen 2p levels suggested by the optical absorption spectra and predicted by several recent band calculations. A new interpretation is proposed for the AgF optical spectrum in which the observed excitons are due to the forbidden Γ₁₂→ Γ₁ and Γ_25′ → Γ₁ tr the high temperature body centered cubic structure. These spectra clearly show the broadening of the I 5p levels which causes the band gap to decrease above the transition temperature.     

The magnetic moment of iron in GdFe intermetallic compounds

Using the multiplet splitting in Fe containing compounds as observed in photoemission the magnetic moment of Fe in GdFe₂, GdFe₃, and Gd₂Fe₁₇ was determined to be (1.76±0.02)μ_B independent of composition. Charge transfer as a cause of this reduction in the Fe moment compared to Fe metal has been excluded. Valence band spectra support a model in which a reduced exchange splitting of the Fe 3d states is responsible for the smaller moment.     

Electronic structure and bonding in CaH2: Experiment and theory

Synchrotron radiation photoemission studies of Ca and CaH₂ reveal changes in the electronic structure and bonding which directly reflect metal-hydrogen interactions. The results indicate similarities between the saline dihydrides and the transition metal dihydrides, particularly the energy location and width of the bonding band states. Combined with APW band calculations for CaH₂ in the CaF₂ structure, they provide new insight regarding bonding in hydrides.     

Photoemission investigations on nanostructured TiO2 grown by cluster assembling

Nanostructured titanium dioxide (ns-TiO₂) films were grown by supersonic cluster beam deposition method. Transmission electron microscopy demonstrated that films are mainly composed by TiO₂ nanocrystals embedded in an amorphous TiO₂ phase while their electronic structure was studied by photoemission spectroscopy. The cluster assembled ns-TiO₂ films are expected to exhibit several structural and chemical defects owing to the large surface to volume ratio of the deposited clusters. Ultraviolet photoemission spectra (hv = 50 eV) from the valence band unveil the presence of a restrained amount of surface Ti 3d defect states in the band gap, whereas Ti 2p core level X-ray photoelectron (hv = 630 eV) spectra do not manifestly disclose these defects.