Thermal behavior of the roughened Cu(110) surface

The roughened Cu(110) surface was prepared by annealing the clean surface at various temperatures ranging from 700 to ∼1000 K. A significant drop in intensity of reflection anisotropy spectroscopy (RAS) peak at 2.1 eV photon energy as a function of increasing sample temperature was found for annealing above the roughening transition at 900 K. The observed change of 2.1 eV peak in RAS spectra is because of the surface state Fermi level shift due to temperature change. The RAS result is in good agreement with an unoccupied surface state energy using inverse photoemission spectroscopy (IPES). New IPES results indicate that the unoccupied surface state intensity decreases with increasing annealing temperature. It was also found that the unoccupied surface state was shifted. IPES results provide that the contributions of the surface state to surface optical properties at 2.1 eV are relevant for the RAS technique.     

Inverse photoemission spectroscopy of the unoccupied electronic structure of Na/Cu(110)

The unoccupied electronic states of Na thin films on a Cu(110) substrate have been measured by inverse photoemission spectroscopy (IPES). The IPES spectrum provides the intensity of the unoccupied states, which decreases with increasing Na coverage at off-normal incidence of the electron beam. The IPES spectra at 17 and 19 eV incident electron energies show a shift towards the Fermi level with increasing Na coverage for the peak at ∼7.8 eV.     

Electron density of states at the edge of the valence band of Cd0.88Fe0.12Se—A photoemission yield study

The electronic structures of Cd_0.88Fe_0.12Se and CdSe have been investigated by photoemission yield spectroscopy (PYS) in the photon energy range from 5 to 12 eV. The () surfaces were obtained by cleavage under ultrahigh vacuum (UHV) conditions. An Fe-related emission appeared at 0.58 eV above the valence band edge. The freshly cleaved surface of Cd_0.88Fe_0.12Se interacted with ambient atmosphere more strongly than CdSe crystal. Leaving the sample in an UHV chamber at room temperature enabled us to identify surface related features and to observe decrease of the ionization energy E_i, energy threshold E_d and the crystal affinity χ due to change of the surface conditions. Effective density of states, derived from the experimental spectra of Cd_0.88Fe_0.12Se exhibits, in contrast with CdSe, a surface-related feature degenerated with the bulk valence band.     

The unoccupied electronic structure characterization of hydrothermally grown ThO2 single crystals

Single crystals of thorium dioxide ThO₂, grown by the hydrothermal growth technique, have been investigated by ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and L₃, M₃, M₄, and M₅ X‐ray absorption near edge spectroscopy (XANES). The experimental band gap for large single crystals has been determined to be 6 eV to 7 eV, from UPS and IPES, in line with expectations. The combined UPS and IPES, place the Fermi level near the conduction band minimum, making these crystals n‐type, with extensive band tailing, suggesting an optical gap in the region of 4.8 eV for excitations from occupied to unoccupied edge states. Hybridization between the Th 6d/5f bands with O 2p is strongly implicated. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface resonance transition of roughened Cu(1 1 0)

The temperature dependence of the reflection anisotropy spectroscopy (RAS) of a Cu(1 1 0) surface has been studied over the temperature range 700-1000 K. Because of the roughening transition at 900 K, the bimodal feature at 4.2 eV for a clean surface shifted to 4.3 eV on annealing. A significant decrease in intensity of the same energy level was also observed with increasing annealing temperature. In the annealing temperature range 700-1000 K, anharmonic behavior is expected to be the predominant process of atomic disordering at the surface. Changes in the RAS of Cu(1 1 0) as a result of thermal processing can be understood in terms of the associated changes in surface states. The RAS signal for a surface resonance transition at 4.2 eV is associated with monoatomic [0 0 1] steps.     

Charge-transfer at silver/phthalocyanines interfaces

Valence band photoemission spectroscopy (VB-PES) and inverse photoemission spectroscopy (IPES) were employed to determine the occupied and unoccupied density of states upon silver deposition onto layers of two phthalocyanines (H₂Pc and CuPc). The two different Pc molecules give rise to very distinct behaviour already during the initial stage of silver deposition. While in the CuPc case no shift occurs in the energy levels, the H₂Pc highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are shifting simultaneously by 0.3 eV, i.e., the HOMO shifts away from the Fermi level while LUMO shifts towards the Fermi level. As the silver quantity increases the HOMO levels of both Pcs are shifting towards the Fermi level. When the Fermi level is resolved in the VB spectra, the characteristic features of H₂Pc and CuPc are smeared out to some extent. Shifts in HOMO and LUMO energy positions as well as changes in line shapes are discussed in terms of charge-transfer and chemical reactions at the interfaces.     

A single h-BN layer on Pt(1 1 1)

The structure and formation of an ultrathin hexagonal boron nitride (h-BN) film on Pt(1 1 1) has been studied by a combination of scanning tunneling microscopy, low energy electron diffraction, low energy electron microscopy, X-ray absorption and high resolution core level spectroscopy. The study shows that a single boron nitride layer is formed on Pt(1 1 1), resulting in a coincidence structure. High resolution scanning tunneling microscopy (STM) images of the h-BN ultrathin film display only one of the atomic species in the unit cell. Probing the boron and nitrogen related local density of states by near edge X-ray absorption fine structure measurements we conclude that the nitrogen sublattice is visible in STM images. The growth of the single hexagonal boron nitride layer by vapourized borazine in the pressure range of 1×10^-6–1×10^-8 at 800 °C is further studied by low energy electron microscopy, and reveals that the number of nucleation sites and the perfection of the growth is strongly pressure dependent. A model for the single, hexagonal, boron nitride layer on Pt(1 1 1) is proposed.     

Unoccupied band dispersion of Si(1 1 1):√3 × √3-Ag surface studied by time- and angle-resolved two-photon photoemission spectroscopy

Band dispersion and transient population of unoccupied electronic states on Si(1 1 1):√3 × √3-Ag surface have been studied by time-resolved (TR) and angle-resolved (AR) two-photon photoemission (2PPE) spectroscopy. The band dispersions originating from unoccupied electronic states have been identified from the comparison between AR-2PPE spectra and angle-resolved one-photon photoemission spectra with synchrotron radiation. A lifetime of unoccupied surface state has been determined from the TR-2PPE spectra.     

RAS: An efficient probe to characterize Si(0 0 1)-(2 × 1) surfaces

A complete inspection of the capabilities of reflectance anisotropy spectroscopy (RAS) in studying the adsorption of molecules or atoms on the Si(0 0 1)-(2 × 1) surface is presented. First, a direct comparison between RA spectra recorded on the clean Si(0 0 1)-(2 × 1) and the corresponding topography of the surface obtained using scanning tunneling microscopy (STM) allows us to quantify the mixing of the two domains that are present on the surface. Characteristic RA spectra recorded for oxygen, hydrogen, water, ethylene, benzene are compared to try to elucidate the origin of the optical structures. Quantitative and qualitative information can be obtained with RAS on the kinetics of adsorption, by monitoring the RA signal at a given energy versus the dose of adsorbate; two examples are presented: H₂/Si(0 0 1) and C₆H₆/Si(0 0 1). Very different behaviours in the adsorption processes are observed, making of this technique a versatile tool for further investigations of kinetics.     

Surface states,local bonding,and surface reconstruction: Na on Cu(110)

Occupied and unoccupied surface states on Na/Cu(110) have been investigated by photoemission and inverse photoemission. By measuring surface state positions at the same adsorbate coverage on the reconstructed and the unreconstructed surface it is demonstrated that the surface states contribute to the total energy balance of the Na induced missing row reconstruction. During the onset of the reconstruction an up-shift of the surface states depopulates an initially occupied CuCu bonding surface state and consequently destabilizes the topmost Cu atoms. The surface state wavefunctions are qualitatively analyzed in a thight-binding LCAO picture.     

Si 2p and O 1s photoemission from oxidized Si(0 0 1) surfaces depending on translational kinetic energy of incident O2 molecules

The influence of translational kinetic energy of incident O₂ molecules for the passive oxidation of the partially oxidized Si(0 0 1) surface has been studied by photoemission spectroscopy. The incident energy of O₂ molecules was controlled up to 3 eV by a supersonic molecular beam technique. Two incident energy thresholds (1.0 and 2.6 eV) were found out in accordance with the first-principle calculations. Si 2p and O 1s photoemission spectra measured at representative incident energies showed the incident energy induced oxidation at the backbonds of the dimer and the second layer (subsurface) Si atoms. Moreover, the difference of oxygen chemical bonds was found out to be as the low and the high binding energy components in the O 1s photoemission spectra. They were assigned to bridge sites oxygen and dangling bond sites oxygen, respectively.     

Large scale atomic ordering on uncovered GaAs(0 0 1) after InAs monolayer capping: Atomic structure of the (12 × 6) reconstruction

A well ordered c(8 × 2)-InAs monolayer is grown by molecular beam epitaxy (MBE) on a GaAs(0 0 1) substrate. After slow sublimation of this monolayer up to 560 °C, a homogeneously (n × 6) reconstructed GaAs surface is obtained. This surface is studied by scanning tunneling microscopy (STM) in UHV. This shows that it is well-ordered on a large scale with 200 nm long As dimer rows along and is also locally (12 × 6) reconstructed, the cell structure is proposed. We believe that this surface organization results from the specific As/Ga (0.7) surface atomic ratio obtained after the InAs monolayer growth and sublimation cycle.     

Electronic band structure of Cu3Au: An angle-resolved photoemission study along the [111]-direction

High-resolution normal photoemission (ARPE) spectra have been recorded for Cu₃Au(111) with the use of polarized synchrotron and rare-gas resonance radiation in the photon energy range from 9 to 27 eV. It is for the first time that dispersions of the gold-like bands have been found experimentally. Using a fully relativistic layer-KKR photoemission formalism, occupied and unoccupied bands as well as one-step-model photoemission spectra have been calculated. The comparison of calculated spectra with experimental ones and the observation of direct-transition resonances upon photon energy near the Brillouin zone-center reveal a shift of the unoccupied ground-state bands by about +2.5 eV (self-energy shift). The direct-transition structures in the experimental spectra have been exploited to determine the dispersions of the occupied bands along the [111] direction (A line in k space). In order to determine the wave vector of the experimental direct transitions we used as final state that calculated unoccupied band along [111], which also exists in pure copper and gold up to about 20 eV above the Fermi energy (“unfolded” band structure), shifted by + 2.5 eV. The experimental occupied bands with Cu character are in very good agreement with theory after shifting the latter by about 0.3 eV to lower energy, whereas somewhat bigger discrepancies exist for the gold-like bands.     

The electronic structure of from high energy spectroscopy

We report the results of a detailed study of the occupied and unoccupied electronic structure of dimers of the new heterofullerene by means of photoemission and electron energy-loss spectroscopy. A close similarity is found between the electronic structures of pristine and with an additional broadening of the spectra in the former due to the distortion of the fullerene cage caused both by dimerization and the chemical substitution. Both the occupied and unoccupied electronic states, as well as the interband transitions between them, attest to the high degree of molecular character retained in the solid state. Comparison of the shake-up structures in the and X-ray photoemission spectra confirm that the highest lying occupied states in the heterofullerene have a strong degree of N character, whereas the lowest lying unoccupied states have mainly C character. We also present the optical conductivity of the heterofullerene (derived from the loss function), which shows an optical gap of 1.4 eV, some 0.4 eV smaller than that of . Received: 25 August 1997 / Revised: 22 September 1997 / Accepted: 16 October 1997     

Observation of charge transfer states of F4-TCNQ on the 2-methylpropene chemisorbed Si(1 0 0)(2 × 1) surface

We have investigated the valence electronic states of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) on the 2-methylpropene chemisorbed Si(1 0 0)(2 × 1) surface using valence photoelectron spectroscopy. Since the electron affinity of condensed F₄-TCNQ is 5.24 eV and the energy from the valence band maximum of the 2-methylpropene saturated Si(1 0 0)(2 × 1) surface to the vacuum level is 4.1 eV, spontaneous charge transfer would be expected in the present system. At sub-monolayer coverage of F₄-TCNQ, characteristic peaks are observed at 1.1 and 2.5 eV below Fermi energy. The former peak is assigned to a singly occupied affinity level, and the latter is ascribed to a relaxed highest occupied molecular orbital of adsorbed F₄-TCNQ. The work function change is increased up to +1.3 eV as a function of F₄-TCNQ coverage. These results support the occurrence of charge transfer into F₄-TCNQ on the 2-methylpropene saturated Si(1 0 0)(2 × 1) surface.     

Density functional theory analysis of the Ni(1 1 0)c(2 × 2)-CN surface phase

Density functional theory slab calculations have been used to investigate the structure of the Ni(1 1 0)c(2 × 2)-CN adsorption phase. The results show excellent agreement with experimental quantitative determinations of this structure by photoelectron diffraction and low energy electron diffraction. In particular, they show that a lying-down orientation with the C–N axis along [0 0 1] perpendicular to the close-packed Ni rows on the surface is strongly favoured over end-on adsorption (with the C–N axis perpendicular to the surface). This geometry is also favoured over a lying-down geometry with the C–N axis aligned along the azimuth, as originally proposed for this system and supported by cluster calculations.     

Electronic structure of the molecular switch tetra-<Emphasis Type="Italic">tert</Emphasis>-butyl-azobenzene adsorbed on Ag(111)

Occupied and unoccupied electronic states in tetra-tert-butyl-azobenzene (TBA) absorbed on Ag(111) have been investigated by one-photon and two-photon photoemission spectroscopy. These measurements allow the quantitative determination of energetic positions of the highest occupied (HOMO) and the lowest unoccupied molecular orbital (LUMO) as well as the n=1 image potential state. The assignment of the electronic states are supported by quantum chemical calculations. Experimentally a HOMO–LUMO gap of 2.85 eV is observed, whereas the gap obtained from the calculated molecular orbital energies is 0.92 eV larger. This discrepancy can be explained by image charge screening. Furthermore, two unoccupied final states located 0.18 and 0.43 eV above the vacuum level, respectively, have been identified. PACS 73.20.-r; 74.25.Jb; 79.60.-i; 79.60.Dp; 68.43.Vx     

Surface gap and surface electronic states in CuGeO3 single crystal

The surface and bulk electronic excitations of CuGeO₃ are investigated by means of electron energy loss and polarized X-ray absorption spectroscopy. CuGeO₃ shows a surface charge transfer gap of about 3.0±0.3 eV. The unoccupied oxygen derived density of states, as probed by X-ray absorption at the O 1s edge, is in good agreement with recent many-body calculations.     

Electron and hole doping in NiO

We have investigated hole doped (by lithium) and electron-doped (by nickel metal) NiO with photoemission (PES), inverse photoemission (IPES) and low and high energy electron energy loss spectroscopy (EELS). Both types of doping create empty states approximately in the middle of the charge transfer gap of undoped NiO.     

Angle-resolved photoemission from surface states

The role of the evanescent part of the unoccupied complex band structure in photoemission from surface states is revealed. The frequency dependence of the emission intensity from two surface states on the (100) and (111) surfaces of Al in the photon energy range from 40 to 110 eV is explained within an ab initio one-step theory of photoemission. A novel embedding method to determine surface states is presented. A high sensitivity of surface states spectra to details of the surface potential barrier is predicted, which offers a way to efficiently monitor surface properties.