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.     

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.     

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.     

Short-range order evolution in S-rich Ge-S glasses by X-ray photoelectron spectroscopy

The structure of binary Ge_xS_100 − x chalcogenide glasses (10 < x < 30) is determined by high-resolution X-ray photoelectron spectroscopy (XPS). On the basis of compositional dependence of fitting parameters for Ge and S core level XPS spectra, the ratio between edge- and corner-shared tetrahedra is determined. It is shown that short-range order of these glasses includes fragments of high-temperature crystalline form of GeS₂. When subjected to X-irradiation, the structure of investigated glasses appears to become more homogeneous than that of the as-prepared samples.     

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 and X-ray photoelectron spectra of vanadium oxides

X-ray (XS) and X-ray photoelectron (XPS) spectra are reported for vanadium oxides. Because of the multivalent character of vanadium in the oxide system high quality measurements can be used for chemical shift investigation. Both inner level and valence band spectroscopy give information on the electronic structure and their systematic change with increasing oxidation state. The experimental results are discussed favourable in terms of molecular orbital theory (MO-theory). The complete set of XS and XPS data reported here for V-oxides allows the identification of unknown vanadium oxidation states too.     

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.     

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.     

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.     

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.     

Urbach rule in photoelectron emission from surface states of low-sized silica

Spectral and temperature dependences of photoelectron emission from surface (interface) states of SiO₂ films and nanocompacts in the near UV region were studied. They were interpreted in terms of ‘frozen phonons’ conception and modified three-stage electron emission model. The electron emission analog of optical Urbach rule for low-sized silica was proposed. Optical absorption stage was recognized as the dominant phase of electron emission process. The structure and energy parameters of studied samples were obtained. It was revealed that thin and thick SiO₂ films have similar structure parameters while their effective potential barriers differ because of different electron transport conditions. High value of static structural disorder in compacted SiO₂ nanopowders is attributed to peculiarities of their ultradispersed structure.     

Structural investigation of SnO-B2O3 glasses by solid-state NMR and X-ray photoelectron spectroscopy

Local structure of the SnO-B₂O₃ glasses was investigated using several spectroscopic techniques. ¹¹B MAS-NMR spectra suggested that BO₄ tetrahedral units maximized at around the composition with 50 mol% SnO. The BO₄ units were still present at compositions with high SnO content (67 mol% SnO), suggesting that SnO acted not only as a network modifier but also as a network former. O1s photoelectron spectra revealed that the addition of small amounts of SnO formed non-bridging oxygens (NBO) (B-O?Sn) and the amounts of NBO increased with an increase in SnO content. ¹¹⁹Sn Mössbauer spectra indicated that Sn was present only as Sn(II) in the glasses. The structure of the SnO-B₂O₃ glasses was compared with that of conventional alkali borate glasses and lead borate glasses. The thermal and viscous properties of these glasses were discussed on the basis of the glass structure revealed in the present study.     

Investigation of cobalt–iron alloy nanoparticles in silica matrix by X-ray diffraction and Mössbauer spectroscopy

Nanocrystals of Co_100−xFe_x with different nominal iron content (x=50, 70, 100) embedded in amorphous silica matrix were prepared by the sol–gel method and thermal treatment in hydrogen atmosphere. Different salts (acetates and nitrates) were used as metal precursors. The samples were characterized by X-ray diffraction and Mössbauer spectroscopy in order to verify the formation of the Fe–Co alloy and investigate its structure and composition. XRD patterns only for acetate samples exhibit peaks belonging to a body centered cubic phase, expected for the Fe–Co alloy at the investigated compositions, and also the lattice parameter values confirm the alloy formation. The average particle size obtained by the Warren–Averbach analysis turned out 9.6(5) nm. In agreement with these results, Mössbauer spectra show the formation of an ordered component with isomer shift and hyperfine fields typical of the Fe–Co alloy only for acetate samples.     

Characterization of ALCVD-Al2O3 and ZrO2 layer using X-ray photoelectron spectroscopy

The atomic layer chemical vapor deposition (ALCVD) deposited Al₂O₃ and ZrO₂ films were investigated by ex situ X-ray photoelectron spectroscopy. The thickness dependence of band gap and valence band alignment was determined for these two dielectric layers. For layers thicker than 0.9 nm (Al₂O₃) or 0.6 nm (ZrO₂), the band gaps of the Al₂O₃ and ZrO₂ films deposited by ALCVD are 6.7±0.2 and 5.6±0.2 eV, respectively. The valence band offsets at the Al₂O₃/Si and ZrO₂/Si interface are determined to be 2.9±0.2 and 2.5±0.2 eV, respectively. Finally, the escape depths of Al 2p in Al₂O₃ and Zr 3p3 in ZrO₂ are 2.7 and 2.0 nm, respectively.     

An X-ray absorption spectroscopy study of FeCo alloy nanoparticles embedded in ordered cubic mesoporous silica (SBA-16)

Nanocomposites containing FeCo alloy nanoparticles dispersed in a highly ordered cubic mesoporous silica (SBA-16) matrix were prepared using two different synthetic methods, co-precipitation and impregnation. Extended X-ray Absorption Spectroscopy (EXAFS) technique at both Fe and Co K-edges was used to investigate the structure of FeCo nanoparticles and the presence of additional disordered oxide phases. EXAFS technique gives evidence of differences in the oxidation degree of the FeCo nanoparticles depending on the synthetic method used.     

Time resolved valence band photoelectron spectroscopy of liquid palladium and molybdenum

Photoelectron spectroscopy measurements on liquid transition metals are not feasible by conventional steady state experiments because of the elevated vapor pressures of these metals at the melting point. However, dynamic experiments with surface melting induced by pulsed laser irradiation combined with time resolved photoelectron spectroscopy make these experiments possible for the first time. Valence band spectra of Pd and Mo have been obtained by this method in both the high temperature solid phase and the liquid phase under identical experimental conditions. Both the elements reveal distinct changes in the valence band spectra on melting. The shapes of the 4d bands of the two metals in the liquid phase are in good agreement with the calculated spectra based on the ab initio density of states (DOS) calculations.     

Luminescence and absorption spectroscopy of Sn-related impurity centers in silica

We report an experimental study on the absorption and luminescence spectra of oxygen deficient point defects in Sn-doped silica. The absorption band at 4.9 eV (B_2β band) and the two related photoluminescence bands at ∼4.2 eV (singlet–singlet emission, S₁ → S₀) and at ∼3.2 eV (triplet–singlet emission, T₁ → S₀), linked by a thermally activated T₁ → S₁ inter-system crossing process (ISC), are studied as a function of temperature from 300 to 20 K. This approach allows us to investigate the dynamics properties of the matrix in the surroundings of the point defects and the effects of local disorder on the two relaxation processes from S₁: the radiative channel to S₀ and the ISC process to T₁. We observe that the S₁ → S₀ decay kinetics at higher temperatures do not follow a single-exponential law and the ISC rate shows a temperature dependence that cannot be rationalized by a single activation process, suggesting the presence of a complex landscape of configurational energies. The comparison with analogous data for Ge-doped silica reveals that the local dynamics of the matrix, the defect–matrix electron–phonon coupling, and the ISC rate dispersion are not substantially modified by the isoelectronic and isostructural substitution Sn–Ge. On the contrary, the Sn-related ISC process is ∼5 times more efficient than the Ge-related one. Since we observed that the coupling with local phonons increases the ISC efficiency by four order of magnitudes in the investigated temperature range, the reported data strongly suggest that, even if the presence of the spin–orbit coupling is needed for ISC processes, it has not play a primary role in the ISC processes in silica, where it acts as a homogenous and temperature-independent scale factor.