X-Ray photoelectron valence band spectra from semiconductors Bi2Te3 and Sb2Te3

Angular resolved X-ray photoelectron spectra (ARXPS) are measured for valence bands of the layer semiconductors Bi₂Te₃ and Sb₂Te₃. Starting from simple models taking into consideration the atomic construction, photoionization cross sections and the crystal structure the spectra can be interpreted. Important conclusions are drawn on the electronic structure of these semiconducting compounds.     

Characterizations of a-Se based photodetectors using X-ray photoelectron spectroscopy and Raman spectroscopy

The ‘PLUMBICON’ was one of the first successful imaging tubes using amorphous selenium (a-Se) and many followed. Significant properties of a-Se based imaging tubes have been rediscovered through the invention of the ‘HARP (high-gain avalanche rushing amorphous photoconductor)’, but its operational mechanism and the physics, however are yet poorly described. Previously, we have fabricated photodetectors using nitrogen (N)-doped diamond as a cold cathode and a-Se as a photoconducting target, which successfully responded to light illumination, The device performance,in this case, deteriorates after continuous use largely due to the degradation of a-Se. In this paper, a-Se and amorphous arsenic selenide (a-As₂Se₃) films have been deposited Stoichiometry has been determined by XPS (X-ray photoelectron spectroscopy) followed by Raman spectroscopy characterization. We have found that even an extremely weak incident laser power causes sample degradation during signal accumulation. We speculate that either the incident laser itself and/or the temperature rise due to illumination causes the phase transition in a-Se films. In addition, when As is added into the film, the phase transition leading the degradation is hardly observed, implying that As affects the formation of crystalline Se making chemical bonds in the crystallographic network stronger.     

X-ray photoelectron spectroscopy of sodium phosphate glasses

X-ray photoelectron spectroscopy (XPS, ESCA) has been applied to sodium phosphate glasses with compositions between 15 and 50 mol% Na₂O and for the purpose of comparison also to crystalline compounds containing oxygen and phosphorus. From the measurements a discrimination between bridging and non-bridging oxygen atoms was possible. The method provides a quantitative technique for structural analysis of phosphate glasses.     

Study of nanostructured materials by optical and photoelectron spectroscopy

The results of investigations of the electronic energy structure and physicochemical properties of nanostructured systems by optical and photoelectron spectroscopy using synchrotron radiation are briefly reviewed. The quantum-size effect in thin metal films and surface lateral superlattices are analyzed. The effect of structural defects on the chemical state of metal clusters on the surface of oxides in catalytically active systems is discussed. The results of the study of the specific features of chemical bonding in solid-state nanostructures within the quasi-molecular approach are presented. The possibility of using biological structures (proteins) as matrices for the formation of ensembles of nanoclusters is considered.     

X-ray photoelectron spectroscopy of silica in theory and experiment

Fourier deconvolved X-ray photoelectron spectroscopy (XPS) valence band spectra obtained from crystalline and amorphous silica, used in conjuction with the results of quantum chemical calculations of the SiO₄ tetrahedral unit and other spectrometric measurements (soft X-ray emission, UV absorption and reflectivity, photoconductivity, photoinjection and energy loss spectroscopy), suggest a reinterpretation of the electronic band structure of silica that is consistent with all the data. A unique method for pinning the Fermi level of insulators to that of a metal calibrant is described, resulting in the ability to obtain absolute binding energies of the electronic levels in wide bandgap insulators. Observe peaks in UV reflectivity and energy loss spectra of silica are all assigned to direct interband transitions, and no excitonic states need be involved to explain the data. Upper and lower limits for the bandgap of dry crystalline (α-quartz) and amorphous (Corning Code 7940 glass) silica are adjusted downward from the 8.9 eV bandgap proposed by DiStefano and Eastman [1] to 7.8–5.55 eV for α-quartz and 7.3–5.05 eV for fused silica, respectively. This in no way compromises the obvious insulating properties of silica in MOS devices, since the conductivity is governed by the high barrier height (～3.8 eV in the case of gold) for metal-insulator electron transfer. The lowered bandgap results from increased low-energy electron density in the valence band, which we ascribe to the 1t₁ molecular orbital predicted by various quantum formalisms, but heretofore not detected experimentally in bulk (thick) silica. Disappearances of this orbital and rearrangement of the non-binding 5t₂ and 1e orbitals in silicas rich in silanols (OH), as may be the case for thin-film silica on Si metals, would increase the bandgap to 8.3 eV, in better agreement with previous determinations.     

X-ray photoelectron spectroscopy of erbium-activated-silica–hafnia waveguides

X-ray photoelectron spectroscopy (XPS) has been used in the study of sol gel-derived Er³⁺-activated xHfO₂–(100 − x)SiO₂ (x = 10, 20, 30, 40, 50 mol) planar waveguides. The analysis of Si 2p and O 1s core lines were related to the Hf/Si molar ratio to assess the role of hafnia in modifying the silica network. Increasing the HfO₂ content brings about a change of the Si 2p and O 1s binding energy respect to those from pure silica. This trend is explained with a formation of hafnium silicate in the matrix with successive phase separation between HfO₂ and SiO₂ rich phases. XPS results show that hafnia is well dispersed in the silica matrix for molar concentration below 30%. Formation of pure HfO₂ domains was detected at higher hafnia concentrations in agreement with previous spectroscopic analyses.     

X-ray photoelectron spectroscopy of bulk glassy Se under Ar ion bombardment

It has been found that Ar ion bombardment produces reversible changes in the XPS structure of the valence band of glassy Se. The result of ion bombardment is that the 4 p bonding and non-bonding peaks merge to give a single broad peak at a binding energy similar to that of the non-bonding peak. This merger of the bonding and non-bonding peaks occurs in density of state calculations when the dihedral angle between adjacent bonds is allowe to take any angle. It is concluded, therefore, that ion bombardment introduces a random dihedral angle into the glassy structure, which subsequently relaxes to give the fixed dihedral angle, but with a random direction which density of states calculations and diffraction work indicate as the structure of the glass.     

The structure of germanosilicate glasses,studied by X-ray photoelectron spectroscopy

O1s photoelectron spectra of several alkali silicate, germanate and germanosilicate glasses are given. The concentration ratio of bridging and non-bridging oxygen atoms is determined from these spectra. In contrast to silicate glasses, in germanate glasses non-bridging oxygen atoms are only observed when the alkali oxide mole fraction exceeds 0.18. The introduction of SiO₂ in the germanate glasses drastically alters the ratio of fourfold coordinated to sixfold coordinated germanium atoms.     

X-ray photoelectron spectroscopy (XPS) and magnetic susceptibility studies of vanadium phosphate glasses

Vanadium phosphate glasses with the nominal chemical composition [(V₂O₅)_x(P₂O₅)_1?x], where x = 0.30, 0.40, 0.50, and 0.60, have been prepared and investigated by X-ray photoelectron spectroscopy (XPS) and magnetization measurements. Asymmetries found in the O 1s, P 2p, and V 2p core level spectra indicate the presence of primarily P–O–P, P–O–V, and V–O–V structural bonds, a spin–orbit splitting of the P 2p core level, and more than one valence state of V ions being present. The magnetic susceptibility data for these glasses follow a Curie–Weiss behavior which also indicates the presence of some V ions existing in a magnetic state, i.e., a valence state other than that of the non-magnetic V⁵⁺. From qualitative comparisons of the abundance of the bridging oxygen or P–O–P sites as determined from the areas under the various O 1s peaks with the abundances of differing phosphate structural groups associated with the presence of different valence states of the vanadium ions, a glass structure model consisting of a mixture of vanadate phosphate phases is proposed for these glass samples. These include V₂O₅, VOPO₄, (VO)₂P₂O₇, VO(PO₃), and V(PO₃)₃ with the abundance of orthophosphate (PO₄)^3? units increasing with increasing vanadium content.     

Tin valence and local environments in silicate glasses as determined from X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) was used to characterize the tin (Sn) environments in four borosilicate glass nuclear waste formulations, two silicate float glasses, and three potassium aluminosilicate glasses. Sn K-edge XAS data of most glasses investigated indicate Sn⁴⁺O₆ units with average Sn-O distances near 2.03 Å. XAS data for a float glass fabricated under reducing conditions show a mixture of Sn⁴⁺O₆ and Sn²⁺O₄ sites. XAS data for three glasses indicate Sn-Sn distances ranging from 3.43 to 3.53 Å, that suggest Sn⁴⁺O₆ units linking with each other, while the 4.96 Å Sn-Sn distance for one waste glass suggests clustering of unlinked Sn⁴⁺O₆ units.     

Investigation of gap states in phosphorous-doped ultra-thin a-Si:H by near-UV photoelectron spectroscopy

A variant of photoelectron spectroscopy with near-UV light excitation was established and applied to an n-type doping series of ultra-thin a-Si:H layers (layer thickness ～10 nm). Using this technique, the position of the surface Fermi level E_Fs is obtained and the density of recombination active defect states in the a-Si:H band gap down to ～10¹⁵ states/cm³ can be detected. Defect densities are generally about one order of magnitude higher than in the bulk of thicker (several 100 nm) layers, and the minimum achievable distance of E_Fs from the conduction band is ～360 mV for doping with 10⁴ ppm PH₃. The optimum doping for the fabrication of solar cells is almost one order of magnitude lower. This discrepancy may be explained by enhanced recombination at the a-Si:H/c-Si interface at high doping levels, and in addition by an efficient recombination pathway where charge carriers tunnel from c-Si via a-Si:H band tail states into the a-Si:H and subsequently recombine at dangling bond states.     

Effect of electron irradiation on Na-K silicate glass investigated using X-ray photoelectron spectroscopy and pattern recognition method

Alkali silicate glasses and melts play an important role in material science. Electron interaction with glasses is important for radioactive waste deposition, where electrons of various energies lead to irreversible changes. These changes are caused mainly by ionization and ballistic interaction of electrons with atoms, introducing structural disorder, changes in atomic composition and chemical state, accompanied by alkali ions diffusion. The Na-K silicate glass (5Na₂O · 10K₂O · 85SiO₂), pristine and electron irradiated (doses from 25 C m⁻² to 20 236 C m⁻²) are investigated using X-ray photoelectron spectroscopy (XPS) and the pattern recognition (PR) and fitting procedures. Changes of composition and chemical state of atoms dependent on electron dose are analyzed. At low doses (100-300 C m⁻²), decrease followed by increase of O and Si concentrations was observed. Surface segregation, probable desorption, and in-bulk diffusion of K and Na ions (doses of about 50 C m⁻² and 2000 C m⁻², respectively) were observed. This was accompanied by changes in the chemical state of K atom, where with an electron dose increasing content of elemental K form accompanied by decreasing potassium peroxide form were observed. No difference in chemical state of Si and O atoms was visible under electron irradiation dose to 20 236 C m⁻², within the sensitivity of the applied method.     

Magnetic properties and X‐ray photoelectron spectroscopy of nanocrystalline Ho3Al5O12

Nanocrystalline holmium aluminium garnet (Ho₃Al₅O₁₂) has been prepared for the first time by modified Pechini's reaction after sintering the precursor gel at 1223 K. The nanomaterial has been characterized by X‐ray diffraction (XRD), selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). The XRD pattern confirms the formation of single‐phase Ho₃Al₅O₁₂; the average size of the nanoparticles has also been determined. X‐ray photoelectron spectroscopy (XPS) has been used to study the chemical composition and bonding in the as‐prepared samples. The binding energies of core‐level electrons in Ho, Al and O in the title material have been found slightly shifted compared to the values of the respective elements. DC magnetic susceptibility has been measured in the temperature range 2 – 260 K. Low effective magnetic moment of Ho³⁺, μ_eff = 1.35 µ_B and Weiss constant have been derived from the inverse magnetic susceptibility–temperature linear plot. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Combined investigations of NiAl alloys by means of X-ray photoelectron spectroscopy,compton gamma scattering and positron annihilation

The results of measurements of Compton profiles, photoelectron spectra, positron annihilation curves, and lifetime spectra of several NiAl alloys are discussed in terms of bonding and stability in transition metal aluminides. The interpretation of experimental Compton profiles suggests a transfer of nearly-free electrons from Al sites to more localized d-like states on Ni-sites. The transfer increases with Al contents up to 50 atomic percent. From X-ray photoelectron spectra in the case of Al-rich alloys a different behaviour follows. The results support a model established by Wenger. Following this in the composition range above 50 percent Al no further charge is transferred to Ni cells. The Ni d states seem to be occupied in the Al-rich alloys. A preferential annihilation of positrons in Ni cells is though to have its origin in the enhanced negative charge of these cells caused by charge transfer.     

Silver valence and local environments in borosilicate and calcium aluminoborate waste glasses as determined from X-ray absorption spectroscopy

Silver K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data were collected and analyzed to characterize silver (Ag) environments in borosilicate and Ca-aluminoborate glass formulations developed as potential candidates for the immobilization of certain nuclear wastes. Silver is found in some nuclear waste streams and must be encapsulated in glass during waste vitrification processes. A related concern deals with phase separation within these glasses and whether colloidal silver would be present in the glass melt, which could present processing issues, or in the waste glass product. Characterization of the silver environments provides useful information for optimizing the silver incorporation ability of such glasses. Data were also gathered on four crystalline standards: Ag-foil, Ag₂O, argentojarosite (AgFe₃(SO₄)₂(OH)₆), and AgO. XANES data indicate Ag⁺ as the dominant species in the glasses. XANES and EXAFS data show that the average Ag environment in the Ca-aluminoborate glass is different compared with those in the two borosilicate glasses investigated. EXAFS analyses show that Ag in the borosilicate glasses is coordinated by two oxygens in a similar environment to that in crystalline Ag₂O, except that the associated Ag–O distances are approximately 0.10 Å longer in the glass. Silver in the Ca-aluminoborate glass may be within one highly disordered site, or possibly, several different sites, where the average Ag–O distance, coordination number, and Debye–Waller factor are larger than those determined for the borosilicate glasses. Despite their relatively high silver contents, there is no evidence from XANES or EXAFS of colloidal silver in the glasses investigated.     

Structural transformations in solid and liquid n-butanol from FTIR spectroscopy

Abstract

FTIR spectra of n-butanol were registered for temperatures from ?100 °С to 40 °С during heating of the sample and from 40 °С to ?170 °С during its cooling. Structural changes at phase transitions solid–liquid and liquid–solid were detected. At n-butanol cooling below the melting point the initial crystalline structure was not achieved. Instead of it, a super-cooled liquid or amorphous phase was obtained. Quantum-chemical simulation of 27 n-butanol rotational conformers were provided via ROCBS-QB3 method. According to the calculations, the most stable conformer is TG′t. Comparison of calculated in the anharmonic approximation IR spectra of possible monomers and dimers of n-butanol with experimentally registered spectra showed that there are no monomers and dimers in the studied sample, and the structure of n-butanol in the condensed state is formed by bigger clusters.     

Time resolved fluorescence and anisotropy of 1-pyrene butyric acid and pyranine as probes of solvent organization in sucrose solutions

The microenvironment of solute-solvent interactions in aqueous sucrose solutions, ranging from 0 to 80 wt% in concentration, was investigated using time resolved fluorescence techniques. The fluorescence lifetimes and rotational correlation times of the trace fluorescent probes, PBA (1-pyrene butyric acid) and pyranine (trisodium 8-hydroxy-1,3,6-pyrenetrisulfonate), were measured in sucrose solutions. The behavior of the fluorescence lifetimes and the increase in the rotational correlation times with increasing sucrose concentration provided no evidence for the formation of water exclusive solute clusters in the metastable solutions. Instead, the results indicated the formation of a network of hydrogen bonding interactions between dispersed sucrose molecules.