Valence Band Features Research Articles

Al-Pt intermetallic compounds: HAXPES study.

Intermetallic compounds in the Al-Pt system were systematically studied via hard X-ray photoelectron spectroscopy, focusing on the positions of Pt 4f and Al 2s core levels and valence band features. On one hand, with increasing Al content, the Pt 4f core levels shift towards higher binding energies (BE), revealing the influence of the atomic interactions (chemical bonding) on the electronic state of Pt. On the other hand, the charge transfer from Al to Pt increases with increasing Al content in Al-Pt compounds. These two facts cannot be combined using the standard "chemical shift" approach. Computational analysis reveals that higher negative effective charges of Pt atoms are accompanied by reduced occupancy of Pt 5d orbitals, leading to the limited availability of these electrons for the screening of the 4f core hole and this in turn explains the experimentally observed shift of 4f core levels to higher BE.

Electronic Structure Evolution from Metallic Vanadium to Metallic VxOy: A NAPPES Study for O2 + V Gas–Solid Interaction

Gas–solid interactions between molecular oxygen and metallic vanadium surfaces and the systematic evolution in the electronic structure of vanadium oxide (VOx) surfaces have been explored in the present work by near-ambient pressure photoelectron spectroscopy (NAPPES). The current article studies the evolution of various oxides of vanadium as a function of partial pressure of O2 (ultrahigh vacuum to 1 mbar), temperature (298–875 K), and the exposure time to oxygen (up to 18 h). Valence-band (VB) and core-level spectral measurements recorded with UV (He–I = 21.2 eV) and Al Kα (1486.6 eV) photons, respectively, show interesting changes. (1) Oxidation is limited to the top layers of vanadium at 298 K and up to a partial pressure of 1 mbar O2. About 50% of vanadium gets oxidized, and the remaining amount exists as metal within the top 10 nm. (2) Metallic vanadium disappears above 625 K, and it is predominantly oxidized to a mixture of V4+ and V5+ oxidation states at a 0.1 mbar partial pressure of O2. Points 1 and 2 suggest the predominantly thermodynamically controlled nature of vanadium oxidation through oxygen diffusion into the subsurface and bulk layers. (3) The Fermi-level (EF) feature observed first at ≥725 K at a 0.1 mbar O2 pressure demonstrates the formation of metallic VO2; however, its metallic nature is preserved even at ambient temperature due to interweaving nanodomains of VOx with VO2. (4) Only partial conversion of surface layers to V5+ (V2O5) along with VO2 and V2O3 (within the probing depth of 8–10 nm by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS)) was observed even after prolonged heating (18 h) in 1 mbar O2 pressure. (5) The nature of the surface changes between metal and semiconducting/insulator oxides is substantiated by the observation of changes in work function (ϕ) and EF features. Typical VB features and Fermi intensity of V-metal and vanadium oxides were observed, and the results were corroborated with core-level and VB spectra. The present results extend the capabilities of NAPPES to explore the electronic structure evolution as a function of reaction conditions and underscore its relevance to areas such as heterogeneous catalysis and sensing.

Investigating the valence-band bound states in GaAs/GaAsSb/GaAs V-shaped quantum wells: The role of intense laser fields and position-dependent effective mass

This theoretical study investigated the valence-band electronic features in GaAs/GaAsSb/GaAs V-shaped quantum wells. Accordingly, an analytical relation has been extended on the position-dependent effective mass in GaAsSb well region as a function of well geometry. The Finite Element Method and the effective mass approximation have been used to investigate the combined effects due to a non-resonant high-frequency intense laser field and an applied electric field. This study considered the effects of geometric and structural parameters on the laser-dressed confining potential profile, bound states, and the corresponding wave functions. The performed studies have shown that the laser-dressing could convert a V-shaped potential profile to a parabolic-like potential and also a single-well to a double-well. The combined effects of intense laser and electric fields have caused a conversion from a symmetric well to a larger and shallower asymmetric structure. It has been found from the valence-band energy levels that pronounced blueshifts and redshifts were obtained in the electromagnetic energy spectrum with increasing the strength of the intense laser and applied electric fields, respectively. The numerical results have indicated that the electronic properties in valence-band quantum wells were modulated according to the purpose through varying the well half-width and the antimony concentration. Such adjustable valence-band-based features can be beneficial in the design and fabrication of the next generation equipment used in the optoelectronic domain.

An unusual Cu/Te hybridization in the Cu0.3ZrTe2 intercalation compound

The electronic structure of the novel intercalation compound Cu0.3ZrTe2 was analyzed using the X-ray photoelectron spectroscopy, resonant photoelectron spectroscopy and DFT calculations. The resonant photoemission enhancement of the valence band features of Cu0.3ZrTe2 at the Zr 3p→4d and Cu 3p→3d photo absorption thresholds was studied. The tetrahedral coordination of the Cu atoms by the Te atoms was shown to be responsible for the strong chemical bonding of the Cu atoms within the ZrTe2 lattice through the Cu 3d/Te 5p hybridization. These findings were confirmed by theoretical density of states (DOS) calculations performed for CuxZrTe2 (x = 0.25 and 0.5) with the Cu atoms tetrahedrally and octahedrally coordinated by Te atoms. It was concluded that the substitution of atoms in the CuxTiSe2 system with heavier and polarizable ones (Se→Te, Ti→Zr), i.e. an increase in the lattice polarizability, destabilizes the octahedral coordination of the Cu atoms by the Se/Te ones. This limits the stability of the phases characterized by quantum states with a charge density wave and superconductivity.

Investigating the effect of position-dependent effective mass on the valence-band electronic states of GaAs/GaAsSb/GaAs parabolic quantum wells modulated by intense laser fields

This article has theoretically investigated the valence-band electronic properties of GaAs/GaAsSb/GaAs parabolic quantum well structures. To this end, an analytical expression has been developed to describe the position-dependent effective mass in the GaAsSb parabolic well region as a function of well geometry. Simultaneous effects of a non-resonant high-frequency intense laser field and an applied electric field have been studied using the Finite Element Method within the framework of the effective mass approximation. The laser-dressed confinement potential profile, bound states, and the corresponding wave functions have been examined as a function of geometric and structural parameters of GaAsSb well. The obtained numerical findings have shown that the well geometry was modulated to achieve a Gaussian-like profile from a parabolic shape and a double-well structure from a single-well one under intense laser irradiations. Simultaneous effects of intense laser and electric fields have indicated the formation of asymmetric geometric profiles from symmetric quantum well structures. The analysis of valence-band energy states has revealed pronounced blueshifts and redshifts in the electromagnetic energy spectrum, increasing the strength of the intense laser and applied electric fields. Studies have demonstrated that the valence-band electronic features were adjusted to alter the well half-width and the antimony concentration. These tunable valence-band-based properties can be profitable in designing and applying new devices operating in the optoelectronic field.

Surface Chemistry of Gold Nanoparticles Produced by Laser Ablation in Pure and Saline Water.

Pulsed laser ablation in liquid (PLAL) is a powerful method for producing nanoparticle colloids with a long-term stability despite the absence of stabilizing organic agents. The colloid stability involves different reactivities and chemical equilibria with complex ionic-specific effects at the nanoparticle/solvent interface which must be strongly influenced by their chemical composition. In this work, the surface composition of PLAL-produced gold nanoparticles in alkaline and saline (NaBr) water is investigated by X-ray photoelectron spectroscopy on free-flying nanoparticles, exempt from any substrate or radiation damage artifact. The Au 4f photoelectron spectra with a depth profiling investigation are used to evaluate the degree of nanoparticle surface oxidation. In alkaline water, the results preclude any surface oxidation contrary to the case of nanoparticles produced in NaBr solution. In addition, the analysis of Br 3d core-level photoelectron spectra agrees with a clear signature of Br on the nanoparticle surface, which is confirmed by a specific valence band feature. This experimental study is supported by DFT calculations, evaluating the energy balance of halide adsorption on different configurations of gold surfaces including oxidation or adsorbed salts.

Synthesis and electronic properties of InSe bi-layer on Si(111)

In this paper, we report on the transient phase that forms at the initial stages of InSe growth on Si(111) surface. Using scanning tunneling microscopy and angle resolved photoemission spectroscopy observation and density-functional-theory (DFT) calculations we found that it consists of In-Se bi-layer that is half of the authentic InSe quadruple layer (QL). It has indirect band gap of ~1.2 eV and the parabolic shape of the valence band in contrast to the Mexican-hat shape of the valence band observed for InSe QL. Parity analysis of the valence bands suggests inversion of the Se pz and px,y orbitals near the Γ¯ point. We attribute the observed features of the InSe valence bands to the charge transfer from the InSe bi-layer to the Si(111) substrate.

Spectro-microscopy characterization of CoPc-Au(111) interface

Abstract Co-phthalocyanine molecules have wide range of optoelectronics and sensor applications. Motivated by the importance of this material, we studied the adsorption of CoPc thin to thick films on Au(111) surface by using ultraviolet photoelectron spectroscopy (UPS) coupled with angle resolved UPS (AR-UPS) and scanning tunnelling microscopy (STM). Molecular spectral emissions are found to be at 5.13, 7.94, and 10.32 eV below the Fermi level. Observed UPS and AR-UPS measurements show both changes and shifting in valence band features. The work functions of clean Au(111) surface and thick CoPc film are found circa 5.32 eV and 5.25 eV, respectively. Both the work function variation and shifting of molecular features dictate strong adsorbate-substrate interaction. AR-UPS measurements further illustrates that CoPc molecules lie flat on the Au(111) surface. Our STM results disclose that at sub-monolayer (Sub-ML, 0.4–0.8 ML) coverage and room temperature, CoPc molecules tend to adsorb in random fashions; which reveal a strong interaction at the CoPc-Au(111) interface. However, at the ML coverage and elevated temperature (~520K), long range pattern with the oblique structure is observed. Beyond ML coverage, existence of mixed phases consisting of well arranged patterns and molecular clusters have been observed. Overall, our STM results show that depending on the molecular thickness, CoPc molecules form three types of organized oblique, rectangular and square geometries which is consistent with the findings of CuPc and CoPc on Au(111) surface. Our findings may seek potential use in the organic molecular based electronic devices.

Comparison of optoelectronic properties of epitaxial and non-epitaxial GaN nanostructures

Structural, optical and surface properties of epitaxially grown 2D GaN nanowall network using Molecular Beam Epitaxy have been compared to those of non-epitaxially grown single-crystalline 1D GaN nanowires using Chemical Vapour Deposition. The kinetics of growth mechanisms and formed morphology are shown to significantly influence the respective band structures and consequently their luminescence properties. X-ray diffraction and Raman spectroscopy reveal that the epitaxial 2D nanowall network experiences a hydrostatic strain in addition to a compressive strain, whereas non-epitaxial 1D nanowires possess a morphology-dependent tensile/compressive strain and a negligible hydrostatic strain. Slightly blue-shifted photoluminescence emission from both these nanostructures is markedly enhanced compared to that from an epilayer. The epitaxial nanowall network exhibits the highest enhancement in the band edge emission among them. X-ray photoelectron spectroscopy spectra show shifts in the valence band features and in the hybridization of shallow core levels. Using the XRD, Raman, PL and XPS data, a variation in the band structure of these differently kinetically formed GaN nanostructures is also sketched.

Compressively-strained GaSb nanowires with core-shell heterostructures

GaSb-based nanowires in a gate-all-around geometry are good candidates for binary p-type transistors, however they require the introduction of compressive strain to enhance the transport properties. Here, we for the first time demonstrate epitaxial GaSb-GaAsxSb1−x core-shell nanowires with a compressively strained core. Both axial and hydrostatic strain in GaSb core have been measured by X-ray diffraction (XRD) and Raman scattering, respectively. The optimal sample, almost without plastic relaxation, has an axial strain of −0.88% and a hydrostatic strain of −1.46%, leading to a noticeable effect where the light hole band is calculated to be 33.4 meV above the heavy hole band at the Γ-point. This valence band feature offers more light holes to contribute the transport process, and thus may provide enhanced hole mobility by reducing both the interband scattering and the hole effective mass. Our results show that lattice-mismatched epitaxial core-shell heterostructures of high quality can also be realized in the promising yet demanding GaSb-based system.

Accurate surface band bending determination on Ga-polar n-type GaN films by fitting x-ray valence band photoemission spectrum

The surface band bending in Ga-polar n-type GaN surfaces, as well as the effect of Si doping levels and in situ Ar+ ion processing on band bending, was systematically investigated. To precisely determine the valence band maximum (VBM) of GaN beyond instrumental and material surface environments by XPS, a valence band feature fitting procedure based on photoemission spectra and theoretical densities of states has been developed. Poisson calculation with quadratic depletion approximation on surface potential has been used to model the band bending and further correct the VBM energy. Then, the actual surface band bending was correctly evaluated. Upward band bending of 1.55 ± 0.03 eV with highly Si doped n-GaN, which is about 0.88 eV higher than that of the moderately doped sample, was found. After in situ Ar+ plasma treatment, the varying degree of band bending was observed distinctly depending on the Si doping density. The surface components associated with the Ga/N ratio and Ga–O bonding concentration on the n-GaN surface have been used to evaluate the contribution to surface band bending.

Carrier Transport in the Valence Band of nBn III–V Superlattice Infrared Detectors

Mid-wavelength infrared detectors have been fabricated in the nBn configuration using the InAs/InAsSb superlattice as the absorber. Possible impediments in the valence band can interfere with the transport of holes that represent the signal. We demonstrate that the thermal activation energy of the photocurrent density, as a function of the applied bias voltage, can be a very sensitive probe of the valence band features. We identify and measure two types of impediments, the hole-block due to a band misalignment and the localization sites formed by fluctuations in the superlattice layer thicknesses. The latter are found to dominate the temperature dependence of the hole mobility. Our inferred localization characteristics are consistent with published results obtained by other techniques.

Three-dimensional band structure and surface electron accumulation of rs-CdxZn1\u2212xO studied by angle-resolved photoemission spectroscopy

Three-dimensional band structure of rock-salt (rs) CdxZn1−xO (x = 1.0, 0.83, and 0.60) have been determined by angle-resolved photoemission spectroscopy (ARPES) using synchrotron radiation. Valence-band features shift to higher binding energy with Zn content, while the conduction band position does not depend strongly on Zn content. An increase of the indirect band gap with Zn-doping is larger than that of the direct band gap, reflecting a weaker hybridization between Zn 3d and O 2p than that between Cd 4d and O 2p. Two-dimensional electronic states due to the quantization along surface normal direction are formed in the surface accumulation layer and show non-parabolic dispersions. Binding energy of the quantized two-dimensional state is well reproduced using an accumulation potential with the observed surface band bending and the characteristic width of about 30 Å.

Investigation of Mn 3d derived states in La0.2Sr0.8MnO3

We investigate the Mn 3d derived states in La0.2Sr0.8MnO3 to understand the temperature dependent spectral weight transfer observed in the valence band spectra. Towards this, we have used DFT and DFT+U calculations and x-ray and Mn 3p to 3d resonant photoemission spectroscopic techniques. Our results show that the calculation gives better representation of the experimental valence band spectra for on site Coulomb interaction energy U = 5 eV. The room temperature resonant photoemission technique reveal that the valence band feature ∼2 eV is of mainly 3dn character ( is the ligand hole formed by ligand to metal charge transfer) and the feature ∼5 eV is predominantly of O 2p character. As the sample enters the low temperature tetragonal and insulating phase, the Mn 3d character of feature ∼2 eV increases and of feature ∼5 eV, it decreases.

Strain Induced Modifications of Valence Band Features in Fe3O4 Epitaxial Heterostructures

We report systematic investigations of lattice mismatch strain and the strain relaxation induced modifications on the valence band electronic structure of the epitaxial Fe3O4/Si (100) and Fe3O4/MgO (100) heterostructures. The Fe3O4 films on Si (100) and MgO (100) substrates were investigated though the angle-integrated photoemission spectroscopy (AIPES) at room temperature in the energy range 45-65 eV. Depending on the strain state of the films, the Raman modes and the Verwey transition temperature show deviations from the corresponding bulk values. The valence band feature at 2.7 eV (3.5 eV) shifts towards (away) the Fermi level with an increase (decrease) in strain resulting in decrease (increase) of density of states (DOS) of the minority spin Fe(A) 3d-eg band (majority spin Fe(B) 3d-t2g band). The effect is more pronounced in the case of films on MgO (100) substrate in comparison to the films on Si (100) substrate.

Metallic Cobalt to Spinel Co3O4—Electronic Structure Evolution by Near-Ambient Pressure Photoelectron Spectroscopy

In the present study, valence band (VB) and core level photoelectron spectroscopy was carried out to investigate the electronic structural changes from polycrystalline Co to spinel Co3O4, via CoO at near ambient pressures (NAP; ∼0.1 mbar). O2–Co and H2–CoOx gas–solid oxidative and reductive interactions, respectively, have been explored with UV photons (He–I) or low kinetic energy electrons (≤16 eV) under NAP conditions. Typical VB features of Co metal, CoO, Co3O4, and a mixed phase between any two adjacent features were observed and well corroborated with core level changes. Very significant and characteristic changes were observed with Co 3d features in the VB for each stage from Co oxidation to Co3O4 as well as Co3O4 reduction to CoO. Co3O4 and CoO can be reversibly obtained by alternating the conditions between 0.1 mbar of H2 at 650 K and 0.1 mbar of O2 at 400 K, respectively. A meaningful correlation is observed between the changes in work function with cation oxidation state; small changes in the st...

Electronic structure studies of Fe doped Eu 2 O 3 thin film using x-ray absorption and resonant photoemission spectroscopy

Herein, we have investigated the electronic properties of Fe doped Eu2O3 thin film using x-ray absorption spectroscopy (XAS), valence band spectroscopy and resonant photoemission spectroscopy (RPES). The Fe L3, 2 edge XAS spectra confirms the “3+” ionic state of the Fe in the film. The valence band spectrum of Fe doped film shows an enhancement in the intensity of feature “A” close to Fermi level and origin of another feature “E” at 11.1 eV. The valence band spectroscopy measurements have been performed at the energies in the Eu 4d → 4f and Fe 3p → 3d photo absorption region that clearly disentangles the features in valence band to be of either Eu and Fe hybridized with O or to be of Eu and O only.

Gap states in the electronic structure of SnO2 single crystals and amorphous SnOx thin films

The electronic structure of a SnO2 single crystal is determined by employing resonant photoelectron spectroscopy. We determine the core level, valence band, and X-ray absorption (XAS) data and compare these with those of amorphous SnOx thin films. We find similar properties concerning the data of the core levels, the valence band features, and the absorption data at the O1s edge. We find strong signals arising from intrinsic in-gap states and discuss their origin in terms of polaronic and charge-transfer defects. We deduce from the XAS data recorded at the Sn3d edge that the Sn4d10 ground state has contributions of 4d9 and 4d8 states due to configuration interaction. We identify localized electronic states depending on the strength of the 4d-5s5p interaction and of the O2p-to-Sn4d charge-transfer processes, both appear separated from the extended band-like states of the conduction band. For the amorphous SnOx thin films, significant differences are found only in the absorption data at the Sn3d-edge due to a stronger localization of the in-gap states.

Investigating crystalline-polarity-dependent electronic structures of GaN by hard x-ray photoemission and ab-initio calculations

Crystalline-polarity-dependent electronic structures of gallium nitride (GaN) were studied by photoemission spectroscopy (PES) using soft and hard x-rays with different linear polarizations. A peak located near the valence band (VB) maximum was enhanced for a (0001) surface compared with that for a (0001¯) surface regardless of photon energy. Comparison of the VB density of states obtained by ab-initio calculations with the observed VB-PES spectra indicates that the crystalline-polarity dependence is associated with the Ga 4p and N 2p states. The most plausible origin of the crystalline-polarity-dependent VB feature is based on the photoemission phenomena of electrons in the pz-orbitals due to spontaneous electric polarization along the c-axis of GaN.

Hole-induced insulator-to-metal transition inLa1−xSrxCrO3epitaxial films

We have investigated the evolution of the electronic properties of La1-xSrxCrO3 (for the full range of x) epitaxial films deposited by molecular beam epitaxy (MBE) using x-ray diffraction, x-ray photoemission spectroscopy, Rutherford backscattering spectrometry, x-ray absorption spectroscopy, electrical transport, and ab initio modeling. LaCrO3 is an antiferromagnetic insulator whereas SrCrO3 is a metal. Substituting Sr2+ for La3+ in LaCrO3 effectively dopes holes into the top of valence band, leading to Cr4+ (3d2) local electron configurations. Core-level and valence-band features monotonically shift to lower binding energy with increasing x, indicating downward movement of the Fermi level toward the valence band maximum. The material becomes a p-type semiconductor at lower doping levels and an insulator-to-metal transition is observed at x greater than or equal to 0.65, but only when the films are deposited with in-plane compression via lattice-mismatched heteroepitaxy. Valence band x-ray photoemission spectroscopy reveals diminution of electronic state density at the Cr 3d t2g-derived top of the valence band while O K-edge x-ray absorption spectroscopy shows the development of a new unoccupied state above the Fermi level as holes are doped into LaCrO3. The evolution of these bands with Sr concentration is accurately captured using density functional theory with a Hubbard U correction of 3.0 eV (DFT + U). Resistivity data in the semiconducting regime (x less than or equal to 0.50) do not fit perfectly well to either a polaron hopping or band conduction model, but are best interpreted in terms of a hybrid model. The activation energies extracted from these fits are well reproduced by DFT + U.