Observation of true c(8 × 2) symmetry in scanning tunnelling microscopy images of the clean InSb(001) surface

Filled and empty state scanning tunnelling microscopy images of the sputtered and annealed InSb(001) surface are presented. The sputter-anneal preparation generates a surface with two distinct phases. The dominant phase possesses a unit cell with true c(8 × 2) symmetry, whereas the other phase is attributed to an asymmetric 1 × 3 reconstruction. The presence of a c(8 × 2) unit cell in filled state images is in contrast to previous reports, which identified only a 4 × 1 unit cell. The true c(8 × 2) symmetry further indicates, the available structural model is used as a guide, that the current interpretation of features in filled state images is incorrect. This result may necessitate a reevaluation of the structural model for the InSb(001)-c(8 × 2) surface.     

X-ray and electron beam modification of thiophene overlayers on TiO2(100)1 × 1 and 1 × 3

The effects of electron and X-ray beams on thiophene overlayers on TiO₂(100) 1 × 1 and 1 × 3 surfaces have been investigated using AES, UPS and XPS. Mg K X-rays were found to polymerise a thiophene multilayer condensed at 120 K. The evidence points to a substrate-secondary-electron mediated process. A 3 keV electron beam also modifies a condensed thiophene overlayer, probably by polymerisation.     

Surface electronic structure of the organic superconductor κ-(ET)2Cu(NCS)2 studied via photoemission microscopy

The surface electronic structure of cleaved single crystals of the organic superconductor κ-(ET)₂Cu(NCS)₂ has been studied using photoemission microscopy. Two types of cleaved surfaces were observed, displaying different valence band photoemission spectra and different spectral behavior near the Fermi level, E_F. In particular, spectra from one surface type display relatively broad spectral features in the valence band and finite spectral intensity at E_F, while spectra from the other surface type show well-defined valence band emission features and zero photoemission intensity at E_F. We propose that the spectral differences are due to a very short electron mean free path in this material, and our results are used to explain the differences between previously published photoemission spectra from this superconductor. We also report the results of an investigation of the electronic structure of defects in this material.     

Surface defects created by low energy (20 < E < 240 eV) ion bombardment of Ge(001)

Surface defects created on Ge(001) exposed to low energy Xe ions are characterized by in situ scanning tunneling microscopy (STM). The temperature of the sample during ion bombardment is 165 C and ion energies range from 20 to 240 eV. The ion collisions create defects (vacancies and adatoms) which nucleate and form vacancy and adatom islands. For fixed total vacancy creation, the vacancy island number density increases with increasing ion energy: the vacancy island number density is 1.6 × 10⁻²⁰ cm⁻² for 40 eV ion bombardment and increases to 4.4 × 10⁻²⁰ cm⁻² for 240 eV ion bombardment. The increased nucleation rate for vacancies is attributed to clustering of defects. The sputtering yield of Ge(001) is also measured by STM. The sputtering yield for 20 eV ions is approximately 10⁻³ per ion but the net yield for surface defects (sum of adatoms and vacancies) is an order of magnitude higher, 10⁻², due to adatom-vacancy pair creation.     

Structures of sulfur on TiO2(1 1 0) determined by scanning tunneling microscopy, X-ray photoelectron spectroscopy and low-energy electron diffraction

The temperature dependent adsorption of sulfur on TiO₂(1 1 0) has been studied with X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). Sulfur adsorbs dissociatively at room temperature and binds to fivefold coordinated Ti atoms. Upon heating to 120°C, 80% of the sulfur desorbs and the S 2p peak position changes from 164.3±0.1 to 162.5±0.1 eV. This peak shift corresponds to a change of the adsorption site to the position of the bridging oxygen atoms of TiO₂(1 1 0). Further heating causes little change in S coverage and XPS binding energies, up to a temperature of 430°C where most of the S desorbs and the S 2p peak shifts back to higher binding energy. Sulfur adsorption at 150°C, 200°C, and 300°C leads to a rich variety of structures and adsorption sites as observed with LEED and STM. At low coverages, sulfur occupies the position of the bridging oxygen atoms. At 200°C these S atoms arrange in a (3×1) superstructure. For adsorption between 300°C and 400°C a (3×3) and (4×1) LEED pattern is observed for intermediate and saturation coverage, respectively. Adsorption at elevated temperature reduces the substrate as indicated by a strong Ti³⁺ shoulder in the XPS Ti 2p_3/2 peak, with up to 15.6% of the total peak area for the (4×1) structure. STM of different coverages adsorbed at 400°C indicates structural features consisting of two single S atoms placed next to each other along the [0 0 1] direction at the position of the in-plane oxygen atoms. The (3×3) and the (4×1) structure are formed by different arrangements of these S pairs.     

Room and high-temperature scanning tunnelling microscopy and spectroscopy (HT-STM/STS) investigations of surface nanomodifications created on the TiO2(1 1 0) surface

High-temperature scanning tunnelling microscopy, scanning tunnelling spectroscopy and current imaging tunnelling spectroscopy (HT-STM/STS/CITS) were used to study the topographic and electronic structures changes due to surface modifications of the TiO₂(1 1 0) surface caused by the STM tip. In situ high-temperature STM results showed that the created modifications were stable even at elevated temperatures. The STS/CITS results showed the presence of energy gap below the Fermi level on the untreated regions. The disappearance of energy gap below the Fermi level on the modifications created by the tip was observed. It is assumed that the presence of the tip can change the chemical stoichiometry of the surface from TiO_2−x towards Ti₂O₃.     

Scanning tunnelling microscopy and spectroscopy of the reduced TiO2(1 0 0) surface

Scanning tunnelling microscopy and current imaging tunnelling spectroscopy were used to study the topographic and electronic structure of a reduced TiO₂(1 0 0) surface. The STM results showed that the TiO₂(1 0 0) surface is capable to form (1 × 7) reconstruction which can transform to (1 × 3) reconstruction due to reoxidation of the surface. The CITS results showed that the (1 × 7) reconstruction is much more metallic in compared to the (1 × 3) reconstruction showing pronounced surface states at energy 1.3 eV and 0.8 eV below the Fermi level and at energy 1.0-1.2 eV above the Fermi level.     

Photoelectric emission from the negative electron affinity (100) diamond surface — exciton effects

Investigation of the photoelectric yield characteristics of the negative electron affinity NEA (100) diamond surface (produced by hydrogen plasma exposure) finds a striking similarity to the emission from NEA (111) diamond surfaces. Work on NEA diamond (111) surfaces has revealed the important role (sometimes dominant) of photoexcited excitons in the photoelectric yield. Comparison of the electron emission properties among surfaces with different crystallographic orientations can give further insight into electron emission mechanisms. We find that the kinetic energy distribution, the photon energy dependence of photoelectric yield, and the photon energy dependence of constant final state measurements of NEA (100) diamond are characteristically similar to the emission from NEA (111) diamond surfaces. Observed differences appear related to the change in band bending at the surface.     

Surface termination, composition and reconstruction of Fe3O4(001) and γ-Fe2O3(001)

We address a current controversy concerning the nature of the surfaces of Fe₃O₄(001) and γ-Fe₂O₃(001) grown on MgO(001) by molecular beam epitaxy. Despite recent claims to the contrary, we show that γ-Fe₂O₃(001) unambiguously exhibits a (1×1) surface net, in contrast to Fe₃O₄(001), which assumes a R45 reconstruction. In addition, we present high-energy-resolution Fe 2p and O 1s core-level photoelectron spectra obtained at both normal and grazing emission for γ-Fe₂O₃(001) and Fe₃O₄(001). These spectra show that the Fe₃O₄(001) surface has a higher Fe(III)/Fe(II) ratio than the bulk, and that the asymmetry in the O 1s line shape for Fe₃O₄(001) is due to final state effects rather than the presence of a surface oxygen or hydroxyl species.     

STM and XPS characterisation of vacuum annealed nanocrystalline WO3 films

Scanning tunnelling microscopy and X-ray Photoelectron Spectroscopy were conducted on magnetron sputtered WO₃ thin films, following a sequence of ultra high vacuum anneals from 100 °C to 900 °C. Annealing from 100 °C to 400 °C induced an upward surface band bending of about 0.3 eV, attributed to the oxygen migration from the bulk to the surface, but no changes in the surface topography. Chemical changes occurred from 600 °C to 800 °C, associated with the formation of secondary oxide species. STM imaging showed that the film surface consists of amorphous particles 35 nm in size up to 600 °C, while higher temperatures resulted in an increase in particle size. Crystallisation of the nanoparticles started to occur after annealing at 600 °C. The implications in terms of gas sensing are discussed.     

Hydroxylation-induced modifications of the Al2O3/NiAl(0 0 1) surface at low water vapour pressure

STM, STS, LEED and XPS data for crystalline θ-Al₂O₃ and non-crystalline Al₂O₃ ultra-thin films grown on NiAl(0 0 1) at 1025 K and exposed to water vapour at low pressure (1 × 10⁻⁷-1 × 10⁻⁵ mbar) and room temperature are reported. Water dissociation is observed at low pressure. This reactivity is assigned to the presence of a high density of coordinatively unsaturated cationic sites at the surface of the oxide film. The hydroxyl/hydroxide groups cannot be directly identify by their XPS binding energy, which is interpreted as resulting from the high BE positions of the oxide anions (O_1s signal at 532.5-532.8 eV). However the XPS intensities give evidence of an uptake of oxygen accompanied by an increase of the surface coverage by Al³⁺ cations, and a decrease of the concentration in metallic Al at the alloy interface. A value of ∼2 for the oxygen to aluminium ions surface concentration ratio indicates the formation of an oxy-hydroxide (AlO_xOH_y with x + y ∼ 2) hydroxylation product. STM and LEED show the amorphisation and roughening of the oxide film. At P(H₂O) = 1 × 10⁻⁷ mbar, only the surface of the oxide film is modified, with formation of nodules of ∼2 nm lateral size covering homogeneously the surface. STS shows that essentially the valence band is modified with an increase of the density of states at the band edge. With increasing pressure, hydroxylation is amplified, leading to an increased coverage of the alloy by oxy-hydroxide products and to the formation of larger nodules (∼7 nm) of amorphous oxy-hydroxide. Roughening and loss of the nanostructure indicate a propagation of the reaction that modifies the bulk structure of the oxide film. Amorphisation can be reverted to crystallization by annealing under UHV at 1025 K when the surface of the oxide film has been modified, but not when the bulk structure has been modified.     

Molecular dynamics simulation of the (0 0 0 1) α-Al2O3 and α-Cr2O3 surfaces

A simple, rigid pair-potential model is applied to investigate the dynamics of the (0 0 0 1) α-Al₂O₃ and α-Cr₂O₃ surfaces using the molecular dynamics technique. The simulations employ a two-stage equilibration process: in the first stage the simulation-cell size is determined via the constant-stress ensemble, and in the second stage the equilibration of the size-corrected simulation cell is continued in the canonical ensemble. The thermal expansion coefficients of bulk alumina and chromia are evaluated as a function of temperature. Furthermore, the surface relaxation and mean-square displacement of the atoms versus depth into the slab are calculated, and their behaviour in the surface region analysed in detail. The calculations show that even moderate temperatures (∼400 °C) give rise to displacements of the atoms at the surface which are similar to the lattice mismatch between α-alumina and chromia. This will help in the initial nucleation stage during thin film growth, and thus facilitate the deposition of α-Al₂O₃ on (0 0 0 1) α-Cr₂O₃ templates.     

Reaction of O2 with the boron-terminated TaB2(0 0 0 1) surface

X-ray photoelectron spectroscopy has been used to study the clean TaB₂(0 0 0 1) surface and its reaction with O₂. In agreement with previous studies, XPS indicates that the clean surface is boron terminated. The topmost boron layer shows a chemically shifted B 1s peak at 187.1 eV compared to a B 1s peak at 188.6 eV for boron layers below the surface. The 187.1-188.6 eV peak intensity ratio and its variation with angle between the crystal normal and the detector is well described by a simple theoretical model based on an independently calculated electron inelastic mean free path of 15.7 Å for TaB₂. The dissociative sticking probability of O₂ on the boron-terminated TaB₂(0 0 0 1) surface is lower by a factor of 10⁴ than for the metal-terminated HfB₂(0 0 0 1) surface.     

High resolution XPS study of the large-band-gap semiconductor stibnite (Sb2S3): Structural contributions and surface reconstruction

Conventional X-ray photoelectron spectroscopy (XPS) and synchrotron radiation XPS (SRXPS) were used to probe the chemical state properties of stibnite (Sb₂S₃), a large-band-gap semiconductor of complex structure. The conventional spectra were obtained with a Kratos Axis Ultra XPS with magnetic confinement charge neutralization, which is very effective in minimizing both uniform charging and differential charging on this large-band-gap semiconductor. The narrow linewidths (much narrower than previously obtained) for single doublet fits (e.g. Sb 4d_5/2 of 0.57 eV and S 2p_3/2 of 0.63 eV) enabled the observation of a small peak on the low binding energy side of the Sb 3d and Sb 4d lines. With the aid of the very surface-sensitive Sb 4d SRXPS spectra, these low energy peaks are assigned to small Sb metal clusters at the surface after cleavage; the signal for these clusters increases with X-ray dose on the sample.A detailed analysis of the Sb 4d and S 2p linewidths concludes that the Sb 4d_5/2 linewidth is larger than expected based on the inherent linewidth of the instrument and the Sb 4d lifetime width, and on comparison with the As 3d linewidth (0.52 eV) for the analogous As₂S₃. Also, the S 2p_3/2 linewidth is substantially broader than the Sb 4d_5/2 linewidth. These larger than expected linewidths are due to two structurally distinct Sb atoms and three structurally distinct S atoms in the Sb₂S₃ crystal structure. Accordingly, the Sb 4d and S 2p spectra have been fitted to two and three doublets respectively, and the linewidth for all peaks is 0.53 eV. Using recent molecular orbital calculations, the doublets have been assigned to the different structural Sb and S sites.     

Observation of new two-dimensional periodic structures in the confined regions of the reconstructed Pt(100) surface

New 2D periodic structures have been observed by STM in the regions of the reconstructed Pt(100) surface which are confined by domain boundaries or lattice steps. These structures can be seen only in a very narrow energy window near the Fermi level, and they are strongly correlated to the original atomic arrangements of the surface. These structures arise most probably from a modification in the distribution of the electronic density of states which is strongly coupled to the ion cores of the surface to produce a periodic shift in the atomic rearrangements in order to minimize the strain and free energy of the surface.     

Atomic geometry and electronic properties of the Ge(1 1 1)2 × 1-Sb surface studied by scanning tunneling microscopy/spectroscopy and core-level photoemission

By scanning tunneling microscopy and spectroscopy (STM/S) and high-resolution core-level photoemission using synchrotron radiation, we have investigated the atomic structure and electronic properties of Sb-induced 2 × 1 reconstruction on Ge(1 1 1). Our results support well the zigzag-chain model proposed for this phase in the literature; in particular, the STM images visualize the Sb zigzag (Seiwatz) chain in a real space, and the STS I-V spectrum suggests this surface to be semiconducting, in good agreement with the atomic configuration proposed. However, a closer inspection of the STM results does not support the buckling of Sb chains reported in earlier studies. Moreover, the analysis of the Sb 4d core-level line shape of the (2 × 1) reconstruction shows that the bonding state of the Sb atoms is very similar, suggesting an unbuckled Seiwatz chain. In addition, the Ge 3d core-level emission reveals only one component, giving evidence for the ideal bulk-terminated structure of the Ge substrate.     

Structural properties of Bi-terminated GaAs(0 0 1) surface

Electronic and structural properties of Bi-terminated reconstructions on GaAs(0 0 1) surface have been studied by scanning tunneling microscopy (STM) and synchrotron radiation core-level spectroscopy. A 2-3 monolayer thick Bi-layer was evaporated on a Ga-terminated GaAs(0 0 1) surface. By heating the surface, the reconstruction changed from (2 × 1) to (2 × 4). The α2 phase with one top Bi dimer and one As or Bi dimer in the third atomic layer per surface unit cell is proposed to explain the STM images of the Bi/GaAs(0 0 1)(2 × 4) surface heated at 400 °C. Bi 5d photoemission from the Bi/GaAs(2 × 4) consisted of two components suggesting two different bonding sites for Bi atoms on the (2 × 4) surface. The variation of the surface sensitivity of the photoemission induced no changes in the intensities of the components indicating that the origins of both components lie in the first surface layer.     

NO adsorption on the Rh(110) surface: kinetics and composition of the adlayer studied by fast XPS

The adsorption and dissociation of NO on the Rh(110) surface were studied by synchrotron radiation X-ray photoemission spectroscopy at temperatures in the range 210–370 K. The O 1s or N 1s spectra were collected every 14 s while the surface was continuously exposed to a steady NO gas pressure. The difference in the binding energies for the atomic oxygen (O 1s ≤530.2 eV), atomic nitrogen (N 1s 397.2 eV) and molecular upright bonded NO molecules (O 1s ≥531.0 eV and N 1s 400 eV) allowed us to distinguish these surface species and to follow the evolution of the adsorbate layer. In addition to these dominating surface species a new species, characterized by O 1s binding energy of 530.7 eV and N 1s binding energy similar to that of the atomic nitrogen, was detected within a narrow coverage range. This state is tentatively assigned to a “lying down” NO bonding configuration, detectable at the timescale of the measurements. The uptake plots, constructed using the integrated intensity of the deconvoluted O 1s and N 1s spectra, are used to elucidate the effect of the reaction temperature and surface coverage and composition on the kinetics of dissociative and molecular NO adsorption of Rh(110).     

Surface chemistry of HfI4 on Si(1 0 0)-(2 × 1) studied by core level photoelectron spectroscopy

The chemistry of HfI₄ adsorbed on the Si(1 0 0)-(2 × 1) surface has been studied by core level photoelectron spectroscopy in ultra-high vacuum. Two stable surface intermediates are identified: HfI₃ and HfI₂, both of which remain upon heating to 690 K. The dissociation of HfI₄ is accompanied by the formation of SiI. In addition, HfI₄ is observed up to 300 K. Complete desorption of iodine occurs in the temperature regime 690-780 K. Deposition of HfI₄ at 870 K results in a layer consisting of metallic Hf, whereas deposition at 1120 K results in the formation of Hf silicide. The results indicate that the metallic Hf formed at 870 K is in the form of particles. Oxidation of this film by O₂ at low pressure does not result in complete Hf oxidation. This suggests that complete oxidation of Hf is a critical step when using HfI₄ as precursor in atomic layer deposition.     

Adsorption and reactions of CH2I2 on Ru(001) surface

The adsorption and dissociation of CH₂I₂ were studied at 110 K with the aim of generating CH₂ species on the Ru(001) surface. The methods used included X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and work function measurements. Adsorption of CH₂I₂ is characterized by a work function decrease (0.96 eV at monolayer), indicating that adsorbed CH₂I₂ has a positive outward dipole moment. Three adsorption states were distinguished: a multilayer (T_p=200 K), a weakly bonded state (T_p=220 K) and an irreversibly adsorbed state. A new feature is the formation of CH₃I, which desorbs with T_p=160 K. The adsorption of CH₂I₂ at 110 K is dissociative at submonolayer, but molecular at higher coverages. Dissociation of the monolayer to CH₂ and I proceeded at 198–230 K, as indicated by a shift in the I(3d_5/2) binding energy from 620.6 eV to 619.9 eV. A fraction of adsorbed CH₂ is self-hydrogenated into CH₄ (T_p=220 K), and another one is coupled to di-σ-bonded ethylene, which — instead of desorption — is converted to ethylidyne at 220–300 K. Illumination of the adsorbed CH₂I₂ initiated the dissociation of CH₂I₂ monolayer even at 110 K, and affected the reaction pathways of CH₂.