Angle-resolved UV Photoelectron Spectroscopy Research Articles

Electronic structure of bimetallic Ni–Rh nanowires

The electronic structure of a bare Rh(553) surface and of a Ni-decorated Rh(553) surface has been investigated by angle-resolved UV photoelectron spectroscopy and density functional theory calculations. The self-assembly of Ni adatoms leads to the decoration of the steps of the Rh(553) surface with monoatomic Ni rows under suitable kinetic conditions, thus forming a regular array of pseudomorphic bimetallic Ni–Rh nanowires. The electronic structure of the clean Rh(553) surface has been compared to the one of the flat Rh(111) surface, and additional surface states localized at the step edges due to the lower coordination of the step atoms have been detected. The Ni wires are weakly hybridized with the Rh substrate states and are characterized by only weakly dispersing states. This leads to a strong narrowing of the d-band, which is argued to be the origin of the observed high chemical reactivity of the Ni–Rh nanowires.

Photoelectron spectroscopic studies of ultra-thin CuPc and PTCDA layers on Cu(1 0 0)

The initial interaction and interface formation between Cu(1 0 0) and the organic semiconductors 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and copper phtalocyanine (CuPc) has been studied by means of angle-resolved UV photoelectron spectroscopy (ARUPS). Both CuPc and PTCDA are known to adsorb strongly on the Cu(1 0 0) surface. The bonding interaction is revealed via interface-related structures in photoemission spectra. However, the spectra develop rather differently in the low coverage regime: while CuPc molecular states are observed below monolayer coverage, the first PTCDA layer is found to be reacted such that the adsorbate-induced emission is strongly modified relative that of intact PTCDA molecules. We find a number of structures that are neither PTCDA- nor Cu-derived, and that the oxygen-related component of the PTCDA spectrum is completely missing in spectra from the monolayer. Importantly for device applications, we find evidence of interfacial electronic states in the form of new peaks located in the former HOMO–LUMO gap for both molecules. In the case of PTCDA these support a chemisorptive bonding model, whereas in the case of CuPc we interpret the state as a monolayer-specific resonance.

Self organization of pentacene grown on Cu(1 1 9)

We have investigated the structural and electronic properties of a pentacene film (∼2 nm) grown on the Cu(1 1 9) substrate using scanning tunneling microscopy (STM) and synchrotron based angle-resolved UV photoelectron spectroscopy at room temperature. STM images indicate an ordered superstructure of pentacene molecules lying almost flat with respect to the substrate. From angular-resolved photoemission spectra, we have observed an energy-band dispersion of about 100 meV for the highest occupied molecular orbital in the direction perpendicular to the pentacene surface.

Intercalation of silver atoms under a graphite monolayer on Ni(111)

The process of silver intercalation under a graphite monolayer (GM) grown on the (111) nickel single-crystal face, GM/Ni(111), is studied. The experiments were conducted in ultrahigh vacuum. The systems were formed in situ in a vacuum chamber under direct monitoring of each stage in the formation of the systems by angle-resolved UV photoelectron spectroscopy and LEED. The possibility of silver intercalation in the GM/Ni(111) system was studied in the course of deposition of various amounts of the metal on the given subject with subsequent heat treatment. It was established that the process occurs optimally under cyclic alternation of the operations of adsorbate (Ag) deposition on the GM/Ni(111) surface and subsequent annealing of the system. In the intermediate stages of GM/Ag/Ni(111) formation, the GM on Ni(111) was found to exist in two phases. Ag intercalation under a graphite monolayer on Ni(111) at room temperature was verified.

Electronic transition of Au-induced atomic chains onSi(5 5 12): A strong similarity with theAu∕Si(5 5 7)system

We investigated in detail the electronic structures of the $\text{Si}(5 5 12)$ surface with Au adsorbates, which features one-dimensional (1D) atomic chains and a 1D metallic band. From angle-resolved UV photoelectron spectroscopy, we resolved that the nearly half-filled 1D band is actually composed of two parallel branches; one is metallic, but the other is insulating with an energy gap of $50\phantom{\rule{0.3em}{0ex}}\text{meV}$ below ${E}_{F}$ at room temperature. An energy gap evolves gradually from $270\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ on the metallic branch, saturating at $75\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with a size of $40\phantom{\rule{0.3em}{0ex}}\text{meV}$ below ${E}_{F}$, while the insulating branch is intact. These behaviors are essentially identical with those recently reported on the $\text{Au}∕\text{Si}(5 5 7)$ system [Phys. Rev. Lett. 91, 196403 (2003)]. The scanning tunneling microscopy imaging indicates that these surfaces possess common 1D atomic chain structures, while the interchain distance is different. This explains the very similar electronic structure and transition of the two systems.

Growth and electronic properties of thin Zn layers on Cu(1 1 1)

The growth and the electronic structure of thin Zn layers on Cu(1 1 1) in the coverage range 0⩽ Θ Zn⩽60 ML was studied by temperature-programmed desorption (TPD) of Zn and CO and angle resolved UV-photoelectron spectroscopy (ARUPS). CO titration experiments indicate two-dimensional growth for small amounts of Zn (up to 1 ML) after annealing to 400 K. Upon further annealing, the first Zn layer desorbs over a relatively wide temperature range between 500 and 640 K. For sub-monolayers of Zn an irreversible phase transition is found around 475 K which causes drastic changes in the shape of the CO-TPD spectra from these surfaces. The ARUPS data show a small but clear dispersion of the Zn 3d states for Zn layers above 0.5 ML. The transition at 475 K causes a narrowing of the Zn 3d bandwidth leaving the average binding energy unchanged. This indicates a larger separation of the Zn atoms in this surface arrangement which can be explained by a substitutional adsorption of Zn atoms.

Correlation between chemical properties and electronic structure of pseudomorphic Cu monolayers on Ni(1 1 1) and Ru(0 0 0 1)

The adsorption of CO and benzene on pseudomorphic Cu monolayers on Ni(1 1 1) and Ru(0 0 0 1) shows pronounced differences: For Cu/Ni(1 1 1) a behavior very similar to that of Cu(1 1 1) is observed, for Cu/Ru(0 0 0 1) the adsorbate substrate bond is significantly strengthened for both molecules, as is deduced from a shift of the desorption temperature to higher values. Based on angle resolved UV photoelectron spectroscopy using synchrotron radiation at BESSY and density functional calculations we have determined the two-dimensional band structure of the pseudomorphic Cu monolayers on Ni(1 1 1) and Ru(0 0 0 1). Our results show that for Cu/Ni(1 1 1) only weak coupling of the Cu 3d states to the Ni substrate states occurs. On the other hand, strong coupling is observed for Cu/Ru(0 0 0 1). This difference is made responsible for the strongly modified chemical properties of Cu/Ru(0 0 0 1) as compared to Cu/Ni(1 1 1).

Electronic structure and orientation of benzene adsorbed on a pseudomorphic Cu monolayer on Ru(0001)

The adsorption of benzene on a pseudomorphic Cu monolayer on Ru(0001) was studied by temperature-programmed desorption and angle-resolved UV photoelectron spectroscopy in combination with symmetry selection rules. Benzene is molecularly adsorbed on the pseudomorphic Cu monolayer on Ru(0001). Desorption of the first layer occurs in the temperature range between 155 and 340 K. In contrast to the clean Ru(0001) surface, no decomposition is observed upon heating. The analysis of the binding energies of the molecular benzene levels reveals a significant bonding shift of 1.1 eV of the benzene π levels, which is nearly as large as the value of 1.4 eV for the strongly chemisorbed saturated benzene layer on Ru(0001). The benzene molecules are azimuthally oriented with the CH bonds aligned along the [101̄0] direction. This geometry corresponds to an azimuthal rotation of 30° as compared to that on the clean Ru(0001) surface.

THE ORIENTATION OF BENZENE ON BIMETALLIC SURFACES

Angle-resolved UV photoelectron spectroscopy (ARUPS) using linearly polarized synchrotron radiation is used to determine the orientation of benzene molecules on two modifications of the pseudomorphic Ni/Cu(111) layer, the Ni-terminated adlayer and the Cu-terminated Ni sublayer. It is found that the molecules adsorb with their C–H bonds 30° off the close-packed substrate rows (σv orientation) on the Ni adlayer whereas no preferential orientation was found for the Cu-terminated Ni sublayer. For these and other close-packed mono- and bimetallic surfaces involving Cu, Ni and Ru, the correlation between adsorption geometry and reactivity of benzene is explored in connection with temperature-programmed desorption (TPD). Like for the Ni/Cu(111) sublayer, no preferential lateral orientation and a weak bond is found on most other Cu-terminated surfaces at 80 K. This goes along with the absence of any dissociation on these surfaces. Only on the stretched 1 ML Cu/Ru(0001) surface is the substrate–benzene bond strong enough to induce a clear preferential orientation (σd with C–H bonds parallel to the close-packed rows) but still too weak to induce dissociation. The same orientation was found for the saturated benzene layer on Ni(111) where the molecules also do not dissociate. Dissociation is, however, observed for the saturation coverage of benzene on the Ni/Cu(111) adlayer and for low benzene coverages on Ni(111). On these surfaces the azimuthal orientation of the benzene molecules is σv, i.e. rotated by 30° with respect to the close-packed rows.

Adsorption and thermal evolution of SO 2 on the Pt(110) surface

The adsorption and thermal evolution of SO 2 on a Pt(110) surface was studied by temperature programmed desorption, angle-resolved UV photoelectron spectroscopy and X-ray photoelectron spectroscopy. At 140 K two molecular SO 2 species are observed that are attributed to adsorption on different adsorption sites. Upon heating to 340 K atomic S and molecular SO 3 and SO 4 are formed with the amount of the various products depending on the initial SO 2 coverage. For high SO 2 coverages these reactions are accompanied by molecular SO 2 desorption. The SO 3 species decomposes upon further heating to 500 K leading to SO 2 desorption. SO 4, which is only formed for high initial SO 2 coverages, is stable up to this temperature but decomposes upon further heating to 650 K. In the same temperature range atomic sulphur and oxygen recombine to desorb as SO 2.

Electronic and geometric structure of NH3 on Ge(001) under equilibrium adsorption conditions.

The adsorption of ${\mathrm{NH}}_{3}$ on the Ge(001) surface is studied by angle-resolved UV-photoelectron spectroscopy using synchrotron radiation and by high-resolution low-energy electron diffraction (LEED) measurements. On the clean surface, the phase transition from 2\ifmmode\times\else\texttimes\fi{}1 to c(4\ifmmode\times\else\texttimes\fi{}2) is observed over a temperature range from 400 to 220 K. Measurements during ${\mathrm{NH}}_{3}$ admission at varying temperature in adsorption-desorption equilibrium yield a sequential occupation of several states. Up to 1/2 ML (one molecule per reconstruction dimer), ${\mathrm{NH}}_{3}$ is bound strongly on the dimer down atoms with the Ge-N axis perpendicular to the surface (\ensuremath{\alpha} state). The adsorption occurs via a mobile precursor state. The \ensuremath{\alpha} state is connected with a (2\ifmmode\times\else\texttimes\fi{}2) superstructure. Yet, the transition c(4\ifmmode\times\else\texttimes\fi{}2)\ensuremath{\rightarrow}(2\ifmmode\times\else\texttimes\fi{}2) starts already at very low coverages (0.01 ML) and is completed for about 0.04 ML. We propose that a long-range attractive interaction causes \ensuremath{\alpha}-${\mathrm{NH}}_{3}$-island formation and that a short-range repulsive interaction is responsible for a change to 2\ifmmode\times\else\texttimes\fi{}2 within the islands by a dimer flip of every second dimer row. At the island edges, this dimer flip continues over the clean part of the surface thus changing its structure to 2\ifmmode\times\else\texttimes\fi{}2 long before 1/2 ML saturation. Beyond 1/2 ML, the structure changes again. A good-quality 2\ifmmode\times\else\texttimes\fi{}1 structure is seen up to 4 ML indicating pseudomorphic growth up to this coverage. Beyond 4 ML, the adsorbate structure changes irreversibly to ${\mathrm{NH}}_{3}$-ice without any LEED pattern. \textcopyright{} 1996 The American Physical Society.

Final-state scattering in angle-resolved ultraviolet photoemission from copper.

The problem of direct transition intensities in angle-resolved UV photoelectron spectroscopy is addressed. We demonstrate that the angular distribution of intensities integrated over the full $3d$ band of copper is dominated by final-state scattering effects much like those observed in the diffraction of core level photoelectrons. These UV photoelectron diffraction effects are very sensitive to the angular momentum character of the valence orbitals that form the band states, and to the atomic structure of the surface layers. Specifically, we have performed measurements on Cu(111) and Cu(001) surfaces where we find excellent agreement of experimental angular distributions of integrated $d$ band emission excited by He I and He II radiation and single-scattering cluster calculations, involving emission from localized $d$ states, and including proper photon polarizations. At the same time the angle-resolved energy spectra show strong dispersion effects, reflecting the delocalized character of these band states. This duality may be a further indication for the localization of the valence hole upon photoemission.

Electronic Band Structure of Monolayer Bi on GaP(110)

The two-dimensional electronic band structure of monolayer Bi on GaP(110) has been mapped using angle-resolved UV photoelectron spectroscopy (ARUPS) with synchrotron radiation. Surface photovoltage effects are corrected for by simultaneous second-order core spectroscopy. From valence-band spectra along the four symmetry directions of the surface Brillouin zone at three photon energies it is possible to distinguish three states as surface related. The topmost band is found to be inside the fundamental band gap, at ca 0.75 eV above the bulk valence-band maximum. Comparison with other V/III-V(110) systems shows that this system is not significantly different, despite the relatively large size of the Bi atoms with respect to the GaP lattice; in a selective comparison with InP and GaAs it would appear that Bi-substrate anion bonding is a more important factor than strain.

Core-level shifts on clean and adsorbate-covered Si(113) surfaces.

The silicon 2p core level was investigated on the Si(113) surface using angle-resolved UV photoelectron spectroscopy and synchrotron radiation. For the clean 3\ifmmode\times\else\texttimes\fi{}2 reconstructed Si(113) surface several surface core-level shifts are observed, two of them shifted by 640 meV to lower and 760 meV to higher binding energies, respectively. They are attributed to adatomlike and dimerlike atoms, the main constituents in the Si(113) 3\ifmmode\times\else\texttimes\fi{}2 surface structure. A broad component close to the bulk emission is found and is assigned to the other atoms in the two topmost surface layers that are slighty shifted from their bulk position within the reconstruction. By adsorption of atomic hydrogen, these surface components can be quenched and others can be created. Monohydride and dihydride phases are identified.

The clean and hydrogenated Ge(113) surface studied by angle-resolved photo-electron spectroscopy

Angle-resolved UV photo-electron spectroscopy (ARUPS) was applied to the valence bands of the Ge(113) surface. Using the known Ge bulk band structure, the position of the valence-band maximum (VBM) relative to the Fermi level was measured. Electronic surface resonances were identified on the clean (3 × 2)-reconstructed surface. The dispersion of the surface resonances was studied in the main symmetry directions. It reflects the periodicity of the surface only along the [11̄0] azimuth but not along [332̄]. Adsorption of hydrogen leads to a change of the surface reconstruction to (3 × 1)-H connected with a quenching of the surface resonances found on the clean surface. Further hydrogenation changes the surface into a (1 × 1)-H structure and a dispersing H-induced state appears at −5.5 eV relative to the VBM.

Angle-resolved UV photoelectron spectroscopy of ethylene and benzene on nickel

A review of results obtained by Angle-Resolved UV-Photoelectron Spectroscopy (ARUPS) using linearly polarized synchrotron radiation is presented for two model systems, ethylene/Ni and benzene/Ni. It is shown that for these systems detailed conclusions concerning adsorbate/substrate and adsorbate/adsorbate interactions can be derived from ARUPS spectra using symmetry selection rules, and in combination with model calculations. In particular, electronic structure, bonding, orientation and symmetry of the adsorbates in dilute and saturated layers will be discussed. It is shown that at high adsorbate coverages lateral interactions in the adsorbate layer play a dominant role. Steric effects in densely packed layers can lead to a reorientation of the molecules as compared to the orientation of the single molecules. The ARUPS spectra of well ordered, densely packed layers exhibit significant (up to 2 eV) dispersion of the various adsorbate bands and allow detailed conclusions on two-dimensional adsorbate band structures.

On the structure and electronic properties of the GaAs(113̄)B surface

GaAs(113̄)B surfaces were prepared by ion bombardment and annealing (IBA) and by molecular beam epitaxy (MBE) using a single Knudsen cell filled with GaAs. These surfaces were investigated through low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and angle-resolved UV photoelectron spectroscopy (ARUPS) using photon energies of hv = 11.8 and 21.2 eV and synchrotron light. With both IBA and MBE, a 1 × 1 reconstructed surface was prepared which differs by a large amount of As in the surface layer. For the MBE 1 × 1 reconstructed surface, three different surface states or resonances were found at +0.3, −0.7 and −3.5 eV with respect to the valence band maximum which are assigned to As-derived dangling bonds and back bonds. The states at + 0.3 and $ ̄ 0.7 eV can be quenched by H adsorption. For the n-type sample the bands are bent upwards by 0.30 V at the surface.

Angle-resolved UV-photoelectron spectroscopy

Angle-resolved UV-photoelectron spectroscopy

Sulphur dioxide adsorption on the Ni(110) surface

The adsorption of SO 2 on Ni(110) has been studied in detail by temperature programmed desorption (TPD), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and angle resolved UV photoelectron spectroscopy (ARUPS) using linearly polarized synchrotron radiation. At 100 K SO 2 is found to chemisorb in molecular form on Ni(110) with a saturation coverage of 0.5 ML (1 ML = 1 molecule Ni-atom ) . The saturated chemisorbed layer exhibits a c(2 × 2) LEED pattern. At initial coverages below 0.25 ML, SO 2 completely decomposes upon heating to 750 K; for higher coverages molecular SO 2 desorption is also observed in a single peak between 300 and 400 K. From the angle resolved UPS spectra and symmetry selection rules we deduce the symmetry of the adsorbed SO 2 molecules for low coverages as C s (σ yz ), with the molecular O-S-O plane oriented perpendicular to the surface and aligned along the [001] azimuth (perpendicular to the close-packed substrate rows). Within the plane the molecule is tilted, which is attributed to an additional Ni-O interaction. In the saturated layer the symmetry is further reduced to C 1 indicating an additional tilting and/or rotation of the molecular plane with respect to the substrate.

Experimental and theoretical investigation of the electronic structure of silver deposited onto InSb(110) at 10 K.

Ag films of thickness z with 0.5\ensuremath{\le}z\ensuremath{\le}60 monolayers (ML) evaporated onto a cleaved InSb(110) substrate at 10 K have been investigated through angle-resolved uv photoelectron spectroscopy (ARUPS) using synchrotron radiation and low-energy electron diffraction (LEED). The band structure has been calculated for bcc and fcc Ag. Inclusion of spin-orbit interaction makes the band structure near \ensuremath{\Gamma} very similar for bcc and fcc structures, whereas in the outer part of the Brillouin zone towards L and N distinct differences exist. From comparison between calculations and measurements we find some evidence that at 10 K and using an InSb(110) substrate bcc Ag is formed for thicknesses 2.5\ensuremath{\le}z\ensuremath{\le}30 ML. The LEED pattern also indicates that bcc Ag is formed with Ag(110)=InSb(110) as it was recently observed by Aristov et al. Annealing films with zg5 ML to 300 K transforms these bcc films into fcc (111)-oriented Ag clusters. A 60-ML-thick annealed film exhibits as perfect ARUP spectra of the 4d bands as measured for a Ag(111) single-crystal surface, but exhibits no LEED pattern. For the thin bcc films a quantum-well state is observed.