Angle-resolved Ultraviolet Photoelectron Spectroscopy Research Articles

One-Dimensional Electronic States of Metal Surfaces Studied by High-Resolution Angle-Resolved Ultraviolet Photoelectron Spectroscopy using Synchrotron Radiation

Surface electronic states of a stepped Cu(755) surface were studied by angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation (SR-ARUPS). We have observed an isotropic free-electron-like surface state below the Fermi level. Unlike to the stepped Ni(755) surface with the same surface structure, no electronic state localized at the step edges was found. This experimental fact shows that the valence-band structure plays an important role in inducing step-localized states. In addition, we tried to prepare Co atomic chains on the stepped Cu(755) surface by vacuum deposition. However, the analysis of low-electron-energy-diffraction (LEED) patterns shows that the Co atoms grow in island-growth mode into epitaxial triangular nanoislands. The Co electronic structure in these Co nanoislands is investigated also by SR-ARUPS. Below 0.2 ML, two Co-derived zero-dimensional peaks were clearly resolved at around 0.3 and 1.1 eV. With coverage increasing, these peaks shift toward each other. This peak shift dependent on the Co coverage is explained in terms of the enhanced exchange splitting that is caused to reduce unavoidably increased Coulomb interaction between Co electrons with flat-band localized in thesmall Co nanoislands at low coverages.

Adsorption of methylsilane on copper surfaces

The adsorption of methylsilane on the (1 1 1), (1 1 0) and polycrystalline surfaces of copper is examined using vibrational electron energy loss spectroscopy, angle-resolved ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. On all surfaces, low exposures of methylsilane below −140 °C yield chemisorbed CH 3Si species, while undissociated methylsilane physisorbs at higher exposures. This physisorbed layer then dissociates at higher temperatures to leave CH 3Si on the surface by room temperature. Above room temperature, the VEELS and ARUP spectra suggest that Si–C bond cleavage occurs on the (1 1 1) surface, whereas the C–H bonds break to leave an SiC molecular species on the (1 1 0) surface. A surface–molecule bonding model is constructed to describe the adsorbate–substrate interactions and suggests that CH 3Si adsorbs in a hollow site on the (1 1 1) surface and in a long-bridge site on the (1 1 0) surface.

Intermolecular energy-band dispersion in PTCDA multilayers

The electronic structure of a well-oriented perylene-3,4,9,10-tetracarboxylic acid-dianhydride multilayer prepared on ${\mathrm{MoS}}_{2}$ single crystal surface were studied by angle-resolved ultraviolet photoemission spectroscopy using synchrotron radiation. From the photon energy dependence of normal emission spectra, we observed an intermolecular energy-band dispersion of about 0.2 eV for the highest occupied molecular orbital (HOMO) band of single $\ensuremath{\pi}$ character. The observed energy-band dispersion showed a cosine curve, which originates from the intermolecular $\ensuremath{\pi}\ensuremath{-}\ensuremath{\pi}$ interaction. Analyses using the tight-binding model gave that the transfer integral of about 0.05 eV for the $\ensuremath{\pi}\ensuremath{-}\ensuremath{\pi}$ interaction, the effective mass of HOMO hole ${m}_{h}^{*}{=5.28m}_{0},$ and the hole mobility ${\ensuremath{\mu}}_{h}>3.8{\mathrm{cm}}^{2}/\mathrm{V}\mathrm{}\mathrm{s}.$ This is the first observation of the intermolecular energy-band dispersion of a conventional single-component organic semiconductor only with the weak intermolecular van der Waals interaction.

A photoemission study of CoO-films on Au(1 1 1)

Angle-resolved ultraviolet photoemission spectroscopy has been used to study thin layers of Co and CoO on an Au(1 1 1) substrate. A bulk-like CoO layer was obtained for a nominal coverage of 7 ML Co, with Co evaporated in an oxygen atmosphere of 10 −4 Pa and the Au(1 1 1) substrate at 350 K. Low-energy electron diffraction of this layer exhibits a (1 × 1) structure, indicating an unreconstructed CoO(1 1 1) surface. Considering the instability of a bulk-like terminated CoO surface, we assume stabilization by OH termination. At low coverage, the CoO layer is oxygen deficient and exhibits metallic character. For the bulk-like CoO layers we found band-like oxygen states and no detectable dispersion for the states near the Fermi edge, which is in agreement with previous investigations of single-crystalline CoO.

Electronic States Localized at Surface Defects on Cu(755) Studied by Angle-Resolved Ultraviolet Photoelectron Spectroscopy Using Synchrotron Radiation

“Regularly stepped” and “defective” surfaces of Cu(755) were prepared by low- and high-temperature annealing, respectively, of a clean specimen. Electronic states on both surfaces were studied by angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation. On the defective Cu(755), we found a new photoelectron peak due to surface defects just below the Fermi level. The dispersion profile of the defect state is derived to be almost flat, which demonstrates the localized nature of the defects. High activity to oxygen adsorption of the defect state was revealed.

Growth of ordered bithiophene layers on the p(2 × 1)O reconstructed Cu(1 1 0) surface

Submonolayer to multilayer coverages of bithiophene on the clean Cu(1 1 0) and the Cu(1 1 0)–p(2 × 1)O reconstructed surface have been investigated comparatively with thermal desorption spectroscopy, angle resolved ultraviolet photoemission spectroscopy (ARUPS) and near edge X-ray absorption fine structure spectroscopy. On the clean Cu(1 1 0) substrate the bithiophene monolayer is strongly bound via the π-orbitals and flat lying. After the completion of the wetting layer, further growth occurs in islands with no evidence for a preferred molecular orientation or order. In contrast, on the oxygen reconstructed surface the wetting layer formed has a weaker interaction with the substrate and in this monolayer the molecular plane is tilted with respect to the surface. For the multilayer film on the Cu(1 1 0)–p(2 × 1)O reconstructed surface the ARUPS displays a distinct azimuthal anisotropy. This is suggested to arise from a template effect of the reconstruction.

Study on the stages of N, N-dibutyl–3,4,9,10-perylene tetracarboxylic diimide growth on Ru (0 0 0 1) surface

The stages of N, N-dialkyl-3,4,9,10-perylene tetracarboxylic diimide derivatives growth on Ru (0 0 0 1) surface has been studied with the ultraviolet photoemission spectroscopy measurements, three emission features of the organic material are located at 2.9, 4.8 and 6.7 eV, respectively below the Fermi level. The angle-resolved ultraviolet photoemission spectroscopy results suggest that the organic molecule is parallel to the substrate surface. Desorption of the organic material in multilayer occurs with warming the substrate. The decomposition of the adsorbed organic substance on Ru (0 0 0 1) surface is at roughly 125°C, the low decomposition temperature observed is attributed to the Ru substrate.

Surface electronic state on stepped Cu(755) studied by angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation

The surface electronic state on the stepped surface of Cu(755) has been investigated by means of angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation(SR-ARUPS). We have observed a free-electron-like surface state below the Fermi level. In spite of the anisotropy of the atomic arrangement due to steps, the surface state is shown to be isotropic since the dispersion profile and the peak shape are almost identical in the directions parallel and perpendicular to the steps. This result makes a clear contrast with the previous SR-ARUPS results on Ni(755) surfaces which have the surface structure similar to Cu(755). Those experimental evidences are discussed based upon the electron configurations of both metal substrates.

Electronic structure of theCu−O/Ag(110)(2×2)p2mgsurface

Reviewing the Slater-Koster parameters experimentally collected on various oxygen-adsorbed surfaces, we suggest a scaling law describing the Slater-Koster (SK) parameters between the oxygen and d-electron metals, instead of the conventional Harrison rule. We demonstrate an examination of this scaling law by the comparative study with the angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) measurement on the $\mathrm{Cu}\ensuremath{-}\mathrm{O}/\mathrm{Ag}(110)(2\ifmmode\times\else\texttimes\fi{}2)p2mg$ surface. First, the electronic structure of the $\mathrm{Cu}\ensuremath{-}\mathrm{O}/\mathrm{Ag}(110)(2\ifmmode\times\else\texttimes\fi{}2)p2mg$ surface is determined by ARUPS using synchrotron radiation. The band assignment of O2p and $\mathrm{Cu}3d$ is carefully done by means of the cross-section analysis for the 20 and 100 eV photon energy. The obtained electronic structure is interpreted in terms of the tight-binding calculation. In the calculation, we use SK parameters produced by the new scaling law. From the good agreement between the experiment and the calculation, we can see that the scaling law is practically useful.

Ultraviolet photoelectron spectroscopy of a methyl-terminated Si [formula omitted] surface

Angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) of a Si(1 1 1) surface regularly terminated by methyl groups is reported. The electrochemical methylation was performed in inert atmosphere. The cleanliness and 1×1 ordering of the surface were confirmed by Auger electron spectroscopy and reflection high-energy electron diffraction, respectively. The energy band diagram has been obtained from ARUPS and each energy level was assigned from the comparison with the results of hydrogen-terminated Si(1 1 1) and quantum chemical cluster calculation.

Effects of the stoichiometry and chemical order on the electronic structure and magnetic properties of Cr-Pt alloys probed by angular-resolved photoemission

The electronic structure of the intermetallic compound ${\mathrm{CrPt}}_{3}$ is studied by angle-resolved ultraviolet photoemission spectroscopy on (001) and (111) surfaces of molecular-beam-epitaxy--grown epitaxial films in the photon energy range of 21--150 eV. The surface structures and the chemical order are investigated by reflecting high-energy electron diffraction. The partial density of states of ${\mathrm{CrPt}}_{3}$ has been estimated using the photoionization dependence of the photoemission. The momentum dependence of the valence-band energies is determined for the $\ensuremath{\Gamma}\ensuremath{-}\mathrm{R}$ bulk high-symmetry line and the $\ensuremath{\Gamma}\ensuremath{-}M$ and $R\ensuremath{-}X$ directions using synchrotron radiation. The nature of these bands was identified by a comparison with band-structure calculations based on density-functional theory in the local spin-density approximation. A good agreement between measured and ab initio calculated energy bands is found. Band mapping has also been performed using the angle-variation method on (001) surfaces excited by the He I emission line. For variations of the degree of surface order, a simple relationship between the electronic structure and magnetic properties of the alloy is found, which is discussed in terms of a modeled band structure. The magnetic anisotropy of this alloy is found to relate to the density of states at the Fermi level as determined by photoemission.

Evolution of geometric and electronic structure in ultrathin In films on Cu(001)

We have studied the initial growth and the evolution of the atomic and electronic structure of ultrathin In films on Cu(001) at 110--300 K by scanning-tunneling microscopy, low-energy electron diffraction, Auger-electron spectroscopy, work-function measurement, and angle-resolved ultraviolet photoelectron spectroscopy. While the deposition at 110--200 K results in the In adsorption on Cu(001) terraces, In atoms deposited at 300 K are initially incorporated into the topmost layer of the substrate. At coverages between 0.26 and 0.35 ML $(1 \mathrm{ML}=1.53\ifmmode\times\else\texttimes\fi{}{10}^{15} {\mathrm{cm}}^{\ensuremath{-}2}),$ surface dealloying occurs and In atoms form an overlayer on top of the Cu(001) terrace. A variety of phases, including seven long-range-ordered ones, are formed depending on the deposition temperature (110--300 K) and the In coverage (0--1 ML). When In is deposited at 110--200 K, the surface shows two superstructures, which are metastable and transform irreversibly to other phases upon annealing. When In is deposited onto Cu(001) at 300 K, the surface exhibits three distinct superstructures different from those obtained at 110--200 K, and two of them undergo reversible phase transitions upon the temperature increase. For In deposition at 300 K, a surface resonance band is observed, whose intensity is highest around the completion of a monolayer, indicating that the resonance has a large amplitude at the In-Cu interface. Coverage-dependent photoemission data show that the resonance splits off from the Cu $4sp$ band at $\ensuremath{\sim}0.3 \mathrm{ML}$ and shows a nearly free electron like dispersion similar to that of the Cu $4sp$ band. A similar interface resonance state is observed for the surface grown at 110 K, while the dispersion is modulated due to the unmatched interface structure. The mechanism of formation of the surface resonance and its relevance to the evolution of the geometric structure is discussed.

Is the onset of high-temperature superconductivity associated with a dimensional crossover?

Angle-resolved ultraviolet photoemission spectroscopy (ARUPS) from high-T(c) superconductors shows an effective-mass renormalization and intense quasiparticle peaks close to the Fermi energy E(F), which change dramatically with temperature as T(c) is crossed. They are attributed to many-body effects, but their precise nature has been controversial until now. We find very similar spectral fingerprints, even with a similar temperature dependence albeit with much higher critical temperature, in a quasi-one-dimensional Br/Pt surface compound. The striking parallels support an interpretation based on spin-charge separation and are consistent with a dimensional crossover taking place at T(c).

Adsorption of SiH 4 on copper ( [formula omitted]) and ( [formula omitted]) surfaces

The adsorption of silane on Cu(1 1 1) and Cu(1 1 0) is examined using vibrational electron energy loss spectroscopy, angle-resolved ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. At room temperature, SiH adsorbs on Cu(1 1 1), whereas complete dissociation occurs on Cu(1 1 0) to leave adsorbed silicon. Below room temperature, an SiH x ( x=2 or 3) species exists on both surfaces. A surface-molecule bonding model is constructed to describe the adsorbate–substrate interactions and suggests that both Si and SiH adsorb in a substitutional or hollow site on the (1 1 1) surface and Si adsorbs in one of the same two sites on the (1 1 0) surface.

Oriented Growth of Model Molecules of Polyethylene and Poly(tetrafluoroethylene) (n-C44H90 and n-C24F50) and Angle-resolved UPS Study of Their Intramolecular Energy Band Dispersion (E=E(k)) Relation

ABSTRACTWe report on the recent progress of our continued effort for the preparation and characterization of well oriented films of tetratetracontane (n-C44H90) and perfluorotetracosane (n-C24F50), which are good model molecules of fundamental polymers, polyethylene (CH2)n and poly(tetrafluoroethylen) (CF2)n, vacuum-deposited on metal surfaces. When the surface of the metal substrate was oxidized, the deposited chain molecules are aligned with their molecular axes vertically standing on the surface, while they lie flat on the surface when atomically clean Cu(100) surface was used. In the case of n-C44H90 on Cu (100), low-energy electron diffraction (LEED) studies revealed that the molecules were also azimuthally aligned with the chain axis almost parallel with the <100> and <010> axis, and infrared reflection-absorption spectroscopy (IR-RAS) enabled detailed studies of the film structure at increasing film thickness._By applying the technique of angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) to these flat-lying molecules, the intramolecular energy-band dispersion relation could be determined, The results agreed well with the results deduced from the previous work using vertically standing molecules assuming an inner potential V0. of –5 eV, confirming the validity of the estimation of V0. Excellent agreement was found between the observed and simulated spectra using the independent-atomic-center (IAC) approximation. Also the value of the vacuum level shift by molecular deposition was deduced for TTC/Cu(100) system to be – 0.3 eV. As for n-C24F50, the E(k) relation deduced in our previous study using vertically standing molecules showed discrepancy with band calculations. The detailed reexamination of the experimental data using IAC calculations for the previous report gave more realistic estimation of V0, and the newly deduced E(k) relation using this data showed good correspondence with theoretical band calculations, thus removing the difficulty in the previous work.

Electronic band dispersion of vertically aligned multiwall-carbon nanotubes

We have investigated electronic band structure of the vertically aligned multiwall-carbon nanotubes using angle-resolved ultraviolet photoemission spectroscopy and density functional calculations. There exists a finite density of states at the Fermi level, a large one-dimensional subband dispersion, highly degenerate states, and a small momentum periodicity (with a lattice constant of 6.3 Å) in the band dispersion. This strongly suggests that the observed nanotubes are mostly metallic with a chirality of (4 k, k), where k is an integer, as predicted from the density functional calculations, although the chirality of the multiwalled nanotubes are likely to be mixed.

Growth of Ordered Hexakis-dodecyl-hexabenzocoronene Layers from Solution: A SFM and ARUPS Study

The layer growth of a polycyclic aromatic hydrocarbon moiety, hexakis-dodecyl-hexabenzocoronene (HBC-C12), from solution onto a conductive flat solid substrate has been studied. Scanning Force Microscopy (SFM), together with the analysis of the intensities of π-structures in spectra of Angle-Resolved Ultraviolet Photoelectron Spectroscopy (ARUPS) measurements, revealed that the HBC-C12 molecules can self-assemble as dry layers with the conjugated disklike molecules lying flat on the (0001) plane of highly oriented pyrolitic graphite (HOPG). By varying the rate of the molecular physisorption it was possible to orient these molecular architectures along preferential directions according to the symmetry of the substrate. Additionally, the film morphology is affected by the concentration of the solution. This indicates that the growth of these organic layers on HOPG is a kinetically governed process which, if carried out sufficiently slowly, leads to the growth of hetero-epitaxial crystallites.

Positive ion neutralisation on chlorine covered silver surfaces

We report a study of the effect of chlorine adsorption on the interaction of positive ions with the silver surfaces. Neutralisation rates of He + were measured and an ion scattering spectroscopy (ISS) study of Cl covered Ag(1 1 1) was performed. The Cl ISS peak remains vanishingly small during the Cl chemisorption submonolayer phase and then rapidly increases above 1.5 L. The neutralisation rate of He + is found to increase during the first phase and then decrease above 1.5 L, an effect that we attribute to AgCl island formation. This points out a chemical environment dependance of the Cl detection with ISS due to changes in particular quasi-resonant neutralisation rates involving Cl(3s) core levels. In the case of H +, Ne + and Ar + neutralisation on Ag(1 1 1) (1 1 0) and (1 0 0) a striking oscillation of the neutralisation rate is found for the three surfaces at low coverages. A first minimum is reached around 0.5 to 2/3 of the full coverage and a maximum is then attained at full coverage. Angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and metastable deexcitation spectroscopy (MDS) measurements allow one to describe these features in terms of the modifications in Auger neutralisation (AN) rates due to changes of the density of states as adsorption changes from smaller to higher packing of adsorbed Cl. Beyond the saturation limit the neutralisation rate is found to decrease again on Ag(1 1 1) but remains constant on Ag(1 1 0) and Ag(1 0 0). This is ascribed to the fact that, in the exposure range reported, AgCl formation occurs only on the (1 1 1) face. On the two other faces an ordered chemisorption phase remains stable until higher coverages.

Structural, electronic, and magnetic properties of thin V/Cu film

We studied the electronic and magnetic properties of the surface and interface of V/Cu alloys by means of the ab initio full-potential linear muffin-tin orbital (FP-LMTO) method. One monolayer (ML) V adsorbed on the Cu(1 0 0) and Cu(1 1 1) substrates is non-magnetic in contrast to the V MLs on Ag and Fe, which are magnetic. And also, the V ML embedded in bulk Cu are non-magnetic. In the 1-ML V sandwich of Cu/ V(1 1 1)/ Cu , although there is the very weak spin-polarization in the ferrimagnetic phase, the energy of the magnetic band-splitting turned out to be very low. We did not find the magnetic order of 1-ML V(1 0 0) embedded in bulk Ni, either. In summary, our result for the 1– 3 ML V embedded in bulk Cu provided no evidence for long-range magnetic ordering, either in-plane or out-of-plane. Our thin film calculations are compared with other theoretical results and with data measured by angle-resolved ultraviolet photoemission spectroscopy (ARUPS).

Surface electronic structure and molecular orientation of poly(9-vinylcarbazole) thin film:: ARUPS and NEXAFS

The molecular orientation at the surfaces of poly(9-vinylcarbazole) (PvCz) thin films was studied by angle-resolved ultraviolet photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The observed take-off angle ( θ) dependence of photoelectron intensities from top π band peaks clearly at larger θ than the calculated one for the three-dimensional isotropic random orientation model. The results indicate that there are more pendant groups with large tilt angles than the three-dimensional isotropic random orientation model, which is in good agreement with the result obtained from NEXAFS spectroscopy. The surface electronic states of PvCz may be rather dominated by σ(C–H) states at the pendant carbazole group than π states