The initial stages of deposition of Cu on ZnO(101̄0) and MgO(001) surfaces studied by electron energy loss spectroscopy

The copper deposition on single crystal ZnO(101̄0) and MgO(001) surfaces has been studied by electron energy loss spectroscopy (EELS) in UHV at room temperature. The initial deposited Cu (well below one monolayer) induces a loss peak at about 2 eV on both oxide surfaces and at 4.3 eV on the MgO(001) surface. Based upon heat treatment and oxidation experiments the 2 eV structure is assigned to the electronic resonance of Cu(I) from the Cu deposit on the oxide matrix substrates. On the basis of the experiments the colour-center-related loss peaks, at 2.6 eV for MgO(001) and at 1.9 eV for ZnO(101̄0), are believed to be due to electronic resonance of a V⁻_s center, and the metal ion vacancies are suggested to be active centers which interact with the submonolayer copper deposits. Finally, the electronic energy loss spectra from the Cu-covered oxide surfaces are discussed in the framework of electronic band structures.     

Initial stage of oxidation of titanium

The electronic structure of titanium with different oxidation levels of the surface was studied by photoelectron spectroscopy and electron energy loss spectroscopy (EELS). The spectra of the valence band and the core level of Ti3p and EEL spectra were recorded for a titanium surface with oxidation exposures from 0.12 to 900 L at room temperature. In general, the titanium oxidation process is similar to that for zirconium, which was investigated earlier; however, certain features of the electronic structure that were not observed earlier were revealed for titanium. Even at high oxidation exposures of ∼1000 L, both O2p valence band and valence band corresponding to titanium metal states were observed. During consecutive oxidation, one can see four different positions of the Ti3p peak corresponding to titanium atoms with different valences. It is probable that, at the initial oxidation stage, islands of complex composition oxide were formed on the surface and grew into the bulk during further oxidation.     

The effect of adsorption of various gas molecules on the EEEL Sof the (100) face of tungsten

The effect of adsorption on the EELS of the tungsten (100) surface plane, for loss energies in the range of 2–25 eV, is investigated for a series of adsorbates (i.e. O₂, N₂, NO, CO and H₂). Low-lying sharp structures observed in the energy loss spectra seem to be associated with prominent structures in the local surface density of states. A growth of a peak at a loss energy of about 7.5 eV, observed after exposing the surface to any one of the gases studied, is attributed to the breakdown of optic-like selection rules by the adsorption process. The effect of adsorption on the various surface plasmon peaks seems to be adsorbate-specific.     

Characteristic electron energy loss structure for clean and oxidized chromium surfaces

AES and EELS spectra have been measured on clean and oxidized chromium surfaces. Auger peaks at 31.0 and 44.0 eV of the oxide are attributed to cross transitions between chromium and oxygen: {M_2,3(Cr)V(Cr)V(O)} and {M₁(Cr)L₁(O)V(Cr)} respectively. Core loss features are consistent with valence band structure with a newly observed loss peak at 41.1 eV for the oxidized surface being ascribed to a core exciton with binding energy E_b = 1.6 eV.     

Hydrogen on WO3(001)

Ultraviolet photoemission measurements of the (001) surface of WO₃ show that after exposure to atomic hydrogen new states appear in the energy gap region at 2.3 and 0.9 eV below the Fermi energy. A peak at 2.2 eV was also produced by hydrogen ion bombardment. To investigate whether these surface effects were due to the formation of hydrogen tungsten bronze H_xWO₃, comparison was made with spectra from the electronically similar sodium bronze. The form of the spectra were different both in the valence band and energy gap regions indicating that hydrogen adsorption or loss of oxygen from the surface rather than formation of H_xWO₃ is the major effect.     

Electronic structure of SiO2(111) thin film

The electronic structure of (111) surface of β-crystobalite is investigat ed using the empirical tight binding method. Our calculations identify surface states in the conduction band, band gap and valence band. The surface state formed from silicon-s and p_z orbitals, which is believed to account for the structure in the O K excitation spectra, lies in the band gap. It is seen that oxygen adsorption on the surface removes surface states and gives rise to a sharp peak at about — 3.8 eV below the valence band edge.     

Experimental and theoretical determination of the low-loss electron energy loss spectroscopy of LiMn2O4

The dielectric properties of cubic spinel-type LiMn₂O₄, used as cathode material in lithium ion secondary batteries, are studied by analyzing the low-loss region of the electron energy loss spectroscopy (EELS) spectrum in a transmission electron microscope. A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the generalized gradient approximation (GGA) is presented. The origins of interband transitions are identified in the electronic band structure, by calculating the partial imaginary part of the dielectric function and the partial density of states of Li, Mn and O. Good agreement with experimental spectra is observed which allowed interpreting main features of the EELS spectrum.     

XPS characterisation of in situ treated lanthanum oxide and hydroxide using tailored charge referencing and peak fitting procedures

A technique is described for deposition of gold nanoparticles under vacuum, enabling consistent energy referencing of X-ray photoelectron spectra obtained from lanthanum hydroxide La(OH)₃ and in situ treated lanthanum oxide La₂O₃ powders. A method is also presented for the separation of the overlapping lanthanum 3d and MNN peaks in X-ray photoelectron spectra acquired with Al Kα radiation. The lower satellite intensity in La(OH)₃ compared to La₂O₃ is related to the higher ionicity of the La–O bond in the former compared to the latter compound. The presence of an additional peak in the valence band spectrum of the hydroxide compared to the oxide is attributed to the O–H bond as indicated by density functional theory based calculations. A doublet in the O 1s peak of lanthanum oxide is associated to the presence of two distinct oxygen sites in the crystal structure of this compound.     

Excitation of lowest electronic states of the uracil molecule by slow electrons

The excitation of lowest electronic states of the uracil molecule in the gas phase has been studied by electron energy loss spectroscopy. Along with excitation of lowest singlet states, excitation of two lowest triplet states at 3.75 and 4.76 eV (±0.05 eV) and vibrational excitation of the molecule in two resonant ranges (1?C2 and 3?C4 eV) have been observed for the first time. The peak of the excitation band related to the lowest singlet state (5.50 eV) is found to be blueshifted by 0.4 eV in comparison with the optical absorption spectroscopy data. The threshold excitation spectra have been measured for the first time, with detection of electrons inelastically scattered by an angle of 180°. These spectra exhibit clear separation of the 5.50-eV-wide band into two bands, which are due to the excitation of the triplet 1³ A?? and singlet 1¹ A?? states.     

Growth and properties of oxide films on Zn(0001)

The initial stages of the formation of ZnO films on cleavage planes of Zn single crystals have been studied. The growth rate is found to be linear in pressure and independent of temperature over the range from 77 to 425 K. The growth law is consistent with the rate limiting step being the adsorption of oxygen rather than ion transport through the oxide layer. Both the atomic and electronic structure have been monitored during oxygen exposure. The LEED patterns and intensity-voltage data indicate that at low temperatures the oxide is either amrophous or a fine grained polycrystal while above room temperature it is single crystal ZnO epitaxially oriented on Zn(0001). The changes during oxygen exposure of the Zn M₂₃M₄₅M₄₅, M₂₃M₄₅V, M₂₃VV and corresponding M₁ Auger spectra have been studied in some detail. The details of the spectra are identified through comparison with calculated and measured band structures and with previous observations of LMM transitions. The relaxation energy associated with the two d-band holes in the final state (i.e., δE(M₄₅M₄₅)] is found to be ～9 eV in good agreement with a recent calculation. The corresponding relaxation energy for the oxide is ～13.5 eV. The development of the electron energy loss spectra with oxygen exposure has been followed for a range of primary energies. A growth model which is consistent with the LEED observations, Auger peak heights and lineshapes and energy loss spectra is that the oxide grows heterogeneously on the Zn surface with no distinguishable precursor adsorbed state. However, contrary to previous models, it is shown that the surface is completely covered by oxide at a mean thickness of 2–3 monolayers. LEED, Auger and energy loss data for the two polar cleavage faces of ZnO are presented for comparison with those from the oxide overlayer.     

Electron energy-loss spectroscopy of V2O5 nanofibers synthesized by electro-spinning

The dielectric properties of V₂O₅ nanofibers, synthesized by the electrospinning method, are studied by analyzing the low-loss region of the electron energy loss spectroscopy (EELS) in a transmission electron microscope. A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the Generalized Gradient Approximation (GGA) is presented, having found an excellent agreement between them. Although the experimental EELS has been acquired for the nanoparticles composing the fibers, and numerical calculations were carried out for bulk material, agreement between experimental and calculated results shows that no difference exists between the electronic structure in calculated bulk material and the nanoparticles. Furthermore, our results from EELS confirm that we accomplished the expected crystalline phase. The origins of interband transitions are identified in the electronic band structure by calculating the partial imaginary part of the dielectric function and the partial density of states.     

Modification of the electronic structure and formation of an accumulation layer in ultrathin Ba/n-GaN and Ba/n-AlGaN interfaces

The electronic structure of the n-GaN(0001) and Al _x Ga_{1 ? x} N(0001) (x = 0.16, 0.42) surfaces and the Ba/n-GaN and Ba/AlGaN interfaces is subjected to in situ photoemission investigations in the submonolayer Ba coverage range. The photoemission spectra of the valence band and the spectra of the surface states and the core 3d level of Ga, the 2p level of Al, and the 4d and 5p levels of Ba are studied during synchrotron excitation in the photon energy range 50–400 eV. A spectrum of the surface states in Al _x Ga_{1 ? x} N (x = 0.16, 0.42) is found. The electronic structure of the surface and the near-surface region is found to undergo substantial changes during the formation of the Ba/n-GaN and Ba/AlGaN interfaces. The effect of narrowing the photoemission spectrum in the valence band region from 10 to 2 eV is detected, and surface eigenstates are suppressed. The Ba adsorption is found to induce the appearance of a new photoemission peak in the bandgap at the Fermi level in the Ba/n-GaN and Ba/n-Al_0.16Ga_0.84N interfaces. The nature of this peak is found to be related to the creation of an accumulation layer due to a change in the near-surface potential and enriching band bending. The energy parameters of the potential well of the accumulation layer are shown to be controlled by the Ba coverage.     

A study of core and valence levels in β-PbO2 by hard X-ray photoemission

The core and valence levels of β-PbO₂ have been studied using hard X-ray photoemission spectroscopy (hν = 6000 eV and 7700 eV). The Pb 4f core levels display an asymmetric lineshape which may be fitted with components associated with screened and unscreened final states. It is found that intrinsic final state screening is suppressed in the near-surface region. A shift in the O 1s binding energy due to recoil effects is observed under excitation at 7700 eV. It is shown that conduction band states have substantial 6s character and are selectively enhanced in hard X-ray photoemission spectra. However, the maximum amplitude in the Pb 6s partial density of states is found at the bottom of the valence band and the associated photoemission peak shows the most pronounced enhancement in intensity at high photon energy.     

Electronic structure of PdO studied by photoemission (XPS,UPS)

In this paper we present the results of photoemission studies (XPS and UPS) performed on a polycrystalline surface of PdO. The electron density of states (EDOS) deduced both from XPS and UPS (He_I and He_II) are very similar. The valence band of PdO, which differs significantly from the Pd one, can be built up by four structures located at 0.5 eV, 2.2eV, 4.5 eV and 6.5 eV below E_F. The various electronic contributions (p or d) in the band are considered and, in order to explain our spectra, we discuss several hypothesis taking into account the possible existence of satellite lines or crystal field effects. Our XPS and UPS spectra show that the energy bands of PdO are narrow (～ 2–3 eV), moreover the energy shift of the core levels (|ΔE^F_B| = 2 eV) is important : these results suggest that the correlations between the d electrons may be important in PdO.     

Valence band photoemission spectroscopy of a ternary layered semiconductor: ZnIn2S4

UV photoemission spectroscopy (UPS) experiments have been carried out on the layer compound ZnIn₂S₄ employing several different photon energies in the range $\text{h}\text{?}{}_{\mathrm{\omega }}\text{= 9.5?21.2}\text{eV}$. The energy distribution curves (EDC's) exhibit four valence band density of states structures besides the Zn 3d peak. These five peaks appear 0.90 eV, 1.6 eV, 4.3 eV, 5.8 eV and 8.7 eV respectively below the top of the valence band, E_v. The atomic orbital character of the shallowest peak A appears different from that of the three deeper valence band peaks B, C and D and this is discussed in terms of the more or less pronounced ionic character of the intralayer chemical bonds. These results demonstrate that an overall understanding of the electronic states in complex structures can be achieved by an approach based on photoemission experiments and chemical bonding considerations which has been widely used in the past to study simple binary layer compounds.     

Cathodoluminescence of copper and nickel surfaces

Cathodoluminescence spectra of clean and oxidized Cu(100) and Ni(100) surfaces, using 60–1000 eV electron bombardment, has been measured. A broad peak at 310 nm has been observed for both clean Cu and Ni surfaces. This peak is attributed to radiative recombination of electrons from the point X'₄ with holes at the point X₅ of the energy band diagram. A narrow peak at 520 nm has been found to correlate with the growth of oxide on the Cu and Ni surfaces. This peak can be attributed to electronic transitions involving energy levels of the O^2- ions. The cathodoluminescence technique appears useful for studying the initial growth of metal oxides.     

A study of LaH3 by auger electron appearance potential spectroscopy

The Auger electron appearance potential spectra (AEAPS) of M_4,5 levels of lanthanum in the elemental state and in LaH₃ are presented. AEAPS measures the total differential secondary electron yield from the surface constituents as a function of incident electron energy. In the presence of a 3d hole, the 4f level is pulled down below the Fermi energy, E_F. In the metallic La spectrum this is seen as a shoulder on the low energy side of the main peak, but in the LaH₃ spectrum this shoulder disappears and is replaced by two peaks, again on the low energy side. The high binding energy satellite reported in the M₅ level XPS spectra in La insulating compounds has been attributed to the lowering of the 4f level above the ligand band. Since no high binding energy satellite is observed in the M_4,5 level AEAPS of LaH₃, the lowest energy peak is assigned as the transition of the 3d electron to the 4f level pulled down below the hydrogen induced band. The other peak is related to the partially filled hydrogen induced band below E_F. Tentative energy level diagrams for these transitions are proposed.     

Correlation of electronic and optical transitions: Mo(CO)6 adsorbed on clean and K-preadsorbed Si(111)7 × 7

Electron energy loss spectroscopy (EELS) is used to measure the electronic structure of adsorbed Mo(CO)₆ molecules on clean and potassium-preadsorbed Si(111)7 × 7 at 82 K. The electronic structure of physisorbed MoCCO)₆ is found to be not significantly (within 0.1 eV) perturbed from that of free molecules. The implication of electronic EELS measurements to surface photochemistry is discussed in this paper.     

Electronic excitations at oxygen deficient TiO2(110) surfaces: A study by EELS

TiO₂(110) surfaces with controlled oxygen deficiency introduced by 160 eV electron bombardment have been studied by XPS and EELS. Stoichiometry was monitored by the growth of core peaks due to Ti³⁺ states in XPS. Oxygen desorption is characterised by an initial cross section of 3 × 10⁻²¹ cm² that decreases with increasing oxygen loss, tending toward a limiting composition Ti₄O₇. The oxygen deficient surfaces display sub-bandgap excitations in electronic EELS, whilst in the vibrational region there is a selective downward shift and attenuation of the highest energy phonon loss. This is attributed to modification of the effective background dielectric constant by the defect excitations. Quantitative consideration of the changes in HREELS leads to an estimate of 0.1 for the oscillator strength of the defect electronic excitations. The high value supports the idea that electrons at oxygen deficient TiO₂ surfaces occupy states that are pulled down below the conduction band by polaronic self trapping.     

Comparison of light rare earths N4,5-level soft X-ray and auger electron appearance-potential spectra

Appearance-potential Spectroscopy (APS) probes the binding energy of core levels and local conduction band states of atoms in the surface region. Soft X-ray APS (SXAPS) and Auger electron APS (AEAPS), respectively, measure the differential X-ray fluorescence and secondary electron yields as a function of incident electron energy. We have obtained the N_4,5-level SXAPS and AEAPS spectra of La, Ce, Pr, Nd and Sm metals. Both spectra exhibit multiplet structure below the expected 4d excitation threshold and a broad, 10–20 eV wide peak above the threshold followed by small peaks of decreasing intensity. The data are used to gain an understanding of the decay mechanism following the excitation of the core levels in these spectroscopies. The strong similarity observed between the SXAPS and AEAPS indicates that the characteristic emission and not the bremsstrahlung dominates the spectral lineshape in APS.