Valence Level Photoemission Research Articles

Evidence for surface derelaxation induced by metals on III-V compound semiconductors: Cs/InP(110)

High resolution In 4d core and valence level photoemission spectra from InP(110) show that a new peak is induced by Cs adsorption, and that the surface core level (SCL) peak shifts towards larger separations from the bulk line with Cs coverage. While the presence of the Cs-induced peak is related to charge transfer processes on the basis of spectra which show emission above the valence band maximum, the increase in SCL is interpreted, by recourse to tight-binding calculations, as due to a lifting of the bond angle rotation-relaxation of the clean surface. These findings provide a consistent interpretation of changes in the electronic structure of the surface upon alkali metal deposition, and a basis for a valid model function for investigations of Fermi level movement through core level line shape analysis.

Photoemission and electron-energy-loss-spectroscopy study of BaRuO3.

Core- and valence-level photoemission spectra of the hexagonal perovskite ${\mathrm{BaRuO}}_{3}$ are reported. Valence-region photoemission shows that ${\mathrm{BaRuO}}_{3}$ is a metallic material with a well-defined cutoff in the energy-distribution curve at the Fermi energy, while in electron-energy-loss spectroscopy a plasmon-loss peak is observed at 1.0 eV. Comparison with previously published data for ${\mathrm{CaRuO}}_{3}$ and ${\mathrm{SrRuO}}_{3}$ shows that electron correlation becomes progressively less important in the series ${\mathrm{CaRuO}}_{3}$-${\mathrm{SrRuO}}_{3}$-${\mathrm{BaRuO}}_{3}$.

Chemical and electronic properties of Pt in bimetallic surfaces: Photoemission and CO-chemisorption studies for Zn/Pt(111)

On platinum zinc surfaces at 150–300 K, CO adsorbs only on Pt sites. CO molecules bonded to these sites exhibit a significantly lower adsorption energy (4–8 kcal/mol) and a larger O 1s binding energy (0.2–0.4 eV) than molecules bonded to clean Pt(111). To explain these changes in the Pt↔CO interaction, we have examined the electronic properties of Zn/Pt(111) surfaces using core and valence level photoemission, and ab initio self-consistent-field calculations. At 80 K, Zn atoms deposited on Pt(111) remain on top of the surface. Above 400 K, alloy formation occurs. In general, the formation of Pt–Zn bonds produces a large depletion in the density of Pt 5d states around the Fermi level, with a shift in the centroid of the Pt 5d band and 4f core levels toward higher binding energy. This is accompanied by an important redistribution of charge, in which Pt loses 5d electrons and gains (6s,6p) electrons. The electronic perturbations induced by Zn on Pt reduce its CO-chemisorption ability by weakening the strength of the Pt(5d)–CO(2π*) bonding interactions. The changes in the Pt properties observed after bonding this metal to Zn are as large as those found for Pt bonded to early transition metals, and much larger than those found when Pt is bonded to late-transition metals. In the bimetallic surfaces, the perturbations in the Pt properties increase when the fraction of empty states in the valence band of the metal bonded to Pt rises.

Characterisation of bandgap states in V-doped TiO2 by high-resolution X-ray photoemission spectroscopy

Core- and valence-level photoemission spectra of V-doped TiO2 have been measured with monochromatic Al-Kα X-ray excitation. UHV annealing of as-presented samples leads to the appearance of a photoemission peak above the O 2p valence band within the bulk bandgap of TiO2. Simultaneous changes in V core-level spectra allow this peak to be identified as arising from electrons trapped on vanadium ions to generate localised VIV centres.

A photoemission study of Sb-doped TiO 2

The influence of Sb-doping on the electronic structure of TiO 2 (rutile) has been investigated by core and valence level photoemission spectroscopy, supplemented by high resolution electron microscopy, SQUID magnetometry and infrared reflectance spectroscopy. Pronounced segregation of Sb to polycrystalline TiO 2 surfaces is evident from core level XPS, with a surface Sb concentration that increases only weakly with nominal Sb doping level in the range between 1% and 10%. Segregated Sb ions may be imaged directly in high resolution transmission electron microscopy. Ultraviolet photoemission spectra reveal the presence of an Sb-induced state toward the bottom of the bulk bandgap. The experimental data are discussed in terms of a model involving bulk substitution of Sb(V) ions onto Ti(IV) sites, largely compensated by cation vacancies or anion interstitials. Electrons are trapped by surface Sb ions to generate an Sb(III) lone-pair-like surface state. The influence of Sb doping on the electronic structure of TiO 2 is contrasted with the behaviour of Sb in SnO 2.

Interactions in Bimetallic Bonding: Electronic and Chemical Properties of PdZn Surfaces

The properties of PdZn surfaces have been examined using thermal desorption mass spectroscopy, core- and valence-level photoemission, and CO chemisorption. The formation of Pd-Zn bonds increases the binding energy of the core and valence levels of Pd and reduces the binding energy of the core and valence levels of Zn. An equiatomic PdZn alloy exhibits a large depletion of Pd(4d) states near the Fermi level and a shift of [approximately] +0.7 eV in the binding energy of the Pd(3d) levels. Ab initio SCF calculations for the PdZn molecule and Pd[sub n]Zn[sub n] (n = 2-4) clusters show a decrease in the electron population of the Pd(4d) orbitals as a consequence of Pd(4d) [yields] Zn(4p) charge transfer and Pd(4d) [yields] Pd(5s,5p) rehybridization. These electron transfers reduce electron-electron repulsion within a Pd atom, shifting its core and valence levels toward higher binding energy. The electronic perturbations induced by Zn on Pd reduce the CO-chemisorption ability of Pd by weakening the Pd(4d)-CO(2[pi]) bonding interactions. PdZn surfaces that are rich in Zn show Pd-CO bonds that are 12-16 kcal/mol weaker than those seen on pure Pd surfaces. 57 refs., 12 figs., 1 tab.

Quantisation of valence states observed in small Ag islands on the GaAs(110) surface

The morphology of Ag on the cleaved (110) surface of GaAs has been modified by control of the growth conditions. Deposition at 300 K results in a partially ordered, rough surface, while at 100 K a disordered but uniform film results. High resolution core and valence level photoemission has been applied to monitor the annealing of the film deposited at low temperature. This results in the formation of uniform, crystalline Ag islands. The sp valence band of Ag appears as a sequence of separate peaks in the photoelectron emission spectrum. The number and separation of the peaks depend on the amount of metal deposited; a consequence of quantum confinement in these islands. The photon energy and emission angle dependence of the peak positions and intensities are related to the real space structure of the crystallites and to the position of the probed states in the extended bulk Ag Brillouin zone.

Photoemission study of the Cs⧸GaP(110) interface at low temperatures

The formation of the Cs⧸GaP(110) interface at low temperature has been studied using core and valence level photoemission. It is found that a nonmetallic first adsorption layer is followed by a transition to a metallic film. Valence level spectra show that Cs induces a new peak above the valence band maximum, which from the appearance of the core level spectra is assigned to adsorbate-substrate charge transfer. At higher coverages an intermediate species is identified on the basis of Cs 5p core level spectra. The metallic layer of Cs is characterised by metallic surface and bulk core level emission, the occurrence of plasmon satellites, and the emergence of a metallic Fermi edge. The relative separation of the different Cs core level line contributions are analysed through a Born-Haber cycle and qualitative arguments based on final state screening.

Development of Surface Acidity in Water-Oxidized Alumina Films: Ethylene Adsorption as a Function of Pretreatment Temperature

The structure and surface acidity of thin water-oxidized alumina films on Al(110) have been studied as a function of film treatment temperature and ethylene adsorption using X-ray photoelectron spectroscopy. The OH content of the oxides decreases monotonically with temperature while the crystallinity increases uniformly to form a polycrystalline [gamma]-alumina at 500[degree]C. Ethylene adsorbed on the oxides identifies the presence of acid sites with a maximum spectral intensity for alumina films pretreated between 350 and 400[degree]C. An interpretation is proposed in terms of a molecular orbital adsorbate-surface model based on the core level and valence level photoemission data to explain how chemisorbed molecular ethylene adsorbate may form from a [pi]-complex adduct on the acid sites. The results are consistent with the formation of a [sup +]C[sub 2]H[sub 5] carbenium ion. 31 refs., 10 figs., 3 tabs.

Photoemission and thermal desorption studies of Cs-Au and Li-Au films on Ru(001)

The properties of CsAu and LiAu alloy films on Ru(001) have been studied using temperature-programmed desorption and core- and valence-level photoemission. LiAu alloys have a larger thermal stability than CsAu alloys. For CsAu alloy films, thermal decomposition occurs between 400 and 500 K. In contrast, LiAu films are stable up to 600 K. In the decomposition process of the alkali-gold alloys, LiAu or CsAu bonds are replaced by AuAu bonds, and the alkali segregates to the surface of the film before desorbing, with Au remaining on top of the Ru substrate up to about 1050 K when desorption of the Au multilayer starts. The results of valence-band photoemission indicate that LiAu has metallic properties, showing no band gap in the region close to the Fermi level. On the other hand, the valence spectra for CsAu show the typical band gap expected for a semiconductor. The formation of LiAu and CsAu bonds produces a large shift toward higher binding energy in the centroid of the Au 5d band and a reduction in the Au(5 d 3 2 ) Au(5 d 5 2 ) separation. Alloys saturated with Cs or Li show a separation of ∽ 1.5 eV between the Au5 d 3 2 and 5 d 5 2 levels. For CsAu, the binding-energy shifts with respect to the Fermi edge are + 1.3 eV for the Au 4f levels and − 0.3 eV for the Cs 3d and 4d levels. The corresponding shifts with respect to the vacuum level are ∽ − 2.0 eV for the Au 4f levels and − 0.6 eV for the Cs 3d and 4d levels. The shift in the Au levels is a consequence of electron transfer from Cs to Au, whereas the shifts in the Cs levels are a result of a large contraction in the atomic volume of Cs. In LiAu, the binding-energy shifts with respect to the Fermi edge are + 0.4 eV for the Au 4f levels and ∽ 0 eV for the Li 1s level.

Na adsorption on Si(111)-(7×7) studied by scanning tunneling microscopy and photoemission

The Na-induced (3×1) reconstruction is investigated by core and valence level photoemission, as well as scanning tunneling microscopy. It is demonstrated that the conversion of the (7×7) into the (3×1) reconstruction involves a large amount of mass transport. The (3×1)-Na reconstruction is semiconducting. We observe a (7×7)/(3×1)-Na coexistence that can be interpreted as metal-semiconductor contact. The density of Na atoms in the (3×1) unit cell is determined to one atom per unit cell.

Photoemission studies of zinc-noble metal alloys: Zn–Cu, Zn–Ag, and Zn–Au films on Ru(001)

Zn and the noble metals alloy when coadsorbed on Ru(001). The properties of Zn–Cu, Zn–Ag, and Zn–Au alloys have been studied using core- and valence-level photoemission and temperature programmed desorption. Alloy formation induces only small shifts (−0.2 to −0.3 eV) in the position of the Zn 2p, 3s, and 3d levels. In contrast, the core and valence levels of the noble metals show large shifts toward higher binding energy. For small amounts of Cu, Ag, and Au dissolved in Zn multilayers, the shifts in the core levels of the nobel metals follow the sequence: Cu(2p3/2), 0.8 eV≊Ag(3d5/2), 0.8 eV<Au(4f7/2), 1.4 eV. The magnitude of the shift increases as the Pauling electronegativity of the noble metal increases. However, the sign of the shifts for the Cu(2p3/2), Ag(3d5/2), or Au(4f7/2) levels is not directly determined by the direction of charge transfer within the corresponding Zn-noble metal bond.

Overlayer-induced valence states, and evidence for charge transfer in Na/GaP(110) and Na/GaAs(110): A comparative photoemission study

Core and valence level photoemission experiments for Na overlayers on GaP(110) and GaAs(110) reveal new peaks which are interpreted in terms of Na–Ga charge transfer, as predicted by total energy calculations. New peaks in the region above the valence band are identified with the previously empty Ga surface state which is filled by the charge transfer process. The role of this gap emission in the pinning of the Fermi level at submonolayer to metallic coverages is discussed for both semiconductors.

The chemical properties of bimetallic surfaces: bonding between CO and Zn on Ru(001)

The adsorption of CO on Zn/Ru(001) surfaces has been studied using thermal desorption mass spectroscopy, core and valence level photoemission and Auger spectroscopy. At 80 K, CO does not adsorb on thick Zn films under ultra-high vacuum conditions. In contrast, adsorption of the molecule is observed on Zn atoms bonded to Ru(001). Bimetallic bonding induces a reduction of ∼ 0.5 eV in the binding energy of the core and valence levels of Zn. This shift probably increases the strength of the ZnCO bond. CO molecules adsorbed on Zn adatoms show an O(1s) binding energy close to 535.2 eV and a desorption temperature of ∼ 125 K. Both properties indicate that π-backbonding is very weak in the ZnCO bond. For CO chemisorption on Zn/Ru(001) surfaces, Zn blocks Ru sites approximately on a one-to-one basis and induces a weakening of the RuCO bond.

The chemical properties of bimetallic surfaces: bonding between CO and Zn on Ru(001)

The adsorption of CO on Zn/Ru(001) surfaces has been studied using thermal desorption mass spectroscopy, core and valence level photoemission and Auger spectroscopy. At 80 K, CO does not adsorb on thick Zn films under ultra-high vacuum conditions. In contrast, adsorption of the molecule is observed on Zn atoms bonded to Ru(001). Bimetallic bonding induces a reduction of ∼ 0.5 eV in the binding energy of the core and valence levels of Zn. This shift probably increases the strength of ZnCO bond. CO molecules adsorbed on Zn adatoms show an O(1s) binding energy close to 535.2 eV and a desorption temperature of ∼ 125 K. Both properties indicate that π-backbonding is very weak in the ZnCO bond. For CO chemisorption on Zn/Ru(001) surfaces, Zn blocks Ru sites approximately on a one-to-one basis and induces a weakening of the RuCO bond.

Interaction of oxygen with lithium-gold and cesium-gold films: a photoemission and thermal desorption study

The reaction of O[sub 2] with thick Li-Au or Cs-Au films supported on Ru(001) has been investigated using thermal desorption mass spectroscopy and core- and valence-level photoemission. The interaction of O[sub 2] with CsAu at 100 K produces O[sub 2][sup [minus]] (the only oxygen species at low O[sub 2] exposures) and O[sub 2][sup [minus]]. Annealing of the O[sub 2]/CsAu film to 300 K leads to the disappearance of O[sub 2] and the appearance of O[sup 2[minus]]. The adsorption of O[sub 2] on Li-Au alloys at 300 K produces the O[sub 2][sup [minus]] ion together with another oxygen species. The presence of oxygen shifts the Au(4f) and Cs(3d) levels of CsAu approximately 1 eV toward lower binding energy, with the final peak positions being very close to those observed for metallic Au and oxidized Cs. A similar result was observed for O[sub 2]/Li-Au. No evidence for bonding between the oxygen ions and Au was found. At T [le] 300 K, oxygen breaks the Au-alkali bonds to form Li-O or Cs-O and very small Au clusters that are dispersed on the film. Annealing to 500 K produces sintering of these Au clusters into large three-dimensional particles. The existence of Li-O or Cs-Omore » bonds increases the thermal stability of the alkali in the film. The Li-O and Cs-O compounds decompose at much higher temperatures than the LiAu and CsAu alloys. 28 refs., 10 figs., 2 tabs.« less

Ag/GaAs(110) revisited: A photoemission study

The Ag/GaAs(110) system, which serves as a model system for unreactive metal-semiconductor interfaces, is re-examined by means of high resolution valence and core level photo-emission. The origin of an adsorbate-induced new component on both Ga3d and As3d core level lines is discussed in terms of charge transfer between overlayer and substrate atoms, or an extra-atomic screening effect. The broadening of the Ag d-band is compared with the evolution and decreasing half width of the Fermi edge with Ag deposition at low substrate temperature. These observations are discussed in relation to the search for a criterion for the onset of metallicity in metal overlayers on semiconducting substrates.

Metal–metal bonding on surfaces: Zn–Au on Ru(001)

The properties of Zn, Au, and Zn–Au films on Ru(001) have been studied using thermal desorption mass spectroscopy (TDS) and core and valence-level photoemission. TDS spectra show desorption of Zn from Ru(001) at 450 (multilayer), 490 (second layer), and 720 K (first layer). The monolayer of Zn desorbs following zero-order kinetics with an activation energy for desorption of 36 kcal/mol. Photoemission studies show that the Zn(3s) and Zn(3d) levels of Zn bonded to Ru are shifted 0.5 eV toward lower binding energy with respect to the corresponding levels of pure metallic Zn. The direction of this binding-energy shift together with the results of CO-TDS indicate that Zn is withdrawing electrons from the Ru(001) surface. Such charge transfer is in part responsible for the large stability of the Ru–Zn bond. Au desorbs from Ru(001) at temperatures of 1175 (multilayer) and 1260 K (first layer). For a monolayer of Au deposited on Ru(001), the positions of the Au(4f ) and Au(5d) levels are very close to those of the surface atoms of Au(111), indicating that charge transfer between Au and Ru is minimal. A comparison of the desorption temperature for a monolayer of Zn or Au from Ru(001) with values previously reported for the desorption of monolayers of alkali (K and Cs), noble (Cu and Ag), and transition (Mn, Ni, and Pd) metals indicates that the strength of a metal–metal bond in a bimetallic surface depends on (1) the bulk cohesive energy of the individual metals (which gives an indication of the tendency of the element to form strong metal–metal bonds), and (2) the charge transfer within the bond. Zn and Au alloy when coadsorbed on Ru(001). Results for submonolayer coverages of Zn and Au show that the Zn–Ru and Zn–Au bonds in the trimetallic system are more stable than the corresponding bonds in Zn/Ru(001) or Zn–Au alloys. This phenomenon is probably caused by synergistic interactions in three center metal–metal bonding. The formation of ZnAu alloys induces a shift of ∼+1.0 eV in the Au(4f ) and Au(5d) levels, and −0.2 eV in the Zn(3s) and Zn(3d) levels. The shift in the core and valence levels of Au is caused by rehybridization of the Au(5d,6s,6p) levels, while the shift in the Zn levels comes from a contraction in volume.

A core and valence level photoemission study of Y 1− xPr xBa 2Cu 3O 7

Core and valence level photoemission spectra of the 123 oxides Y 1− x Pr x Ba 2Cu 3O 7 have been measured over the complete composition range 0< x<1. Ceramic samples were cleaned in situ in the spectrometer UHV system by annealing in oxygen to yield clean surfaces whose atomic composition mirrored that of the bulk. No major variations in core level binding energies as a function of composition parameter were apparent. Comparison of Pr 3d core level spectra of BaPrO 3 and BaPr 2O 4 with those of PrBa 2Cu 3O 7 suggest a Pr(III) valence state in the 123 compound. There was a progressive increase with Pr doping in the intensity of the photoemission structure excited with 21.2 eV radiation in the upper part of the valence band. This observation is discussed in relation to the Pr 4f/O 2p mixing thought to be responsible for the metal to non-metal transition in this system.

Bonding and electronic structure in high-Tc superconducting oxides: A case for the importance of the cations.

It is suggested that the chemical bonding in the high-${\mathit{T}}_{\mathit{c}}$ superconducting oxides is strongly influenced by the large fields induced by the very ionic cations (e.g., barium) and that the presence of such cations is a necessary requirement for superconductivity. The electronic effect of this field is thus to modify the interaction between the oxygen and copper atoms in the cuprate anion of the superconductor. The experimental evidence for these ideas come from a comparison of core- and valence-level photoemission spectra of the superconducting oxides with those of typical transition-metal oxides on the one hand and the very ionic oxides (e.g., BaO) on the other.