Adsorbate-induced States Research Articles

Influence of lateral configurations on the charge transfer between H− ions and Na/Ag(1 1 1) surfaces

This study investigates the electron transfer during lateral collisions between H− projectiles and Ag(111) surfaces with Na adsorbates. These interactions aim to probe the nearside region of the adsorbate. Compared to the on-top configuration, the lateral projected density of states reveals strong local effects such as perturbations of energy levels, adsorbate-induced states not identifiable from on-top, and image states enhancement. A study of dynamics reveals that, in addition to the projectile’s distance of closest approach and interaction time with surface, the projectile’s survival is influenced by the balance between electron capture and loss at the start of the exit trajectory.

Electronic shell structure and chemisorption on gold nanoparticles

We use density functional theory (DFT) to investigate the electronic structure and chemical properties of gold nanoparticles. Different structural families of clusters are compared. For up to 60 atoms we optimize structures using DFT-based simulated annealing. Cluster geometries are found to distort considerably, creating large band gaps at the Fermi level. For up to 200 atoms we consider structures generated with a simple EMT potential and clusters based on cuboctahedra and icosahedra. All types of cluster geometry exhibit jelliumlike electronic shell structure. We calculate adsorption energies of several atoms on the cuboctahedral clusters. Adsorption energies are found to vary abruptly at magic numbers. Using a Newns-Anderson model we find that the effect of magic numbers on adsorption energy can be understood from the location of adsorbate-induced states with respect to the cluster Fermi level.

REACTIVITY AND STABILITY OF OXYGEN VACANCIES ON TiO2 ANATASE (101) SURFACE: FIRST-PRINCIPLES CALCULATIONS

In this study, density-functional theory plane-wave pseudopotential method was employed to investigate several oxygen vacancies on TiO 2 anatase (101) surface. At first, a suitable defect-free slab model has been established by analyzing the surface energies and the atomic relaxations influenced by different technical parameters. The formation energies of different kinds of oxygen vacancies in the outermost layer have also been compared as well as the atomic displacement of the defective surfaces. It was found that the presence of bridging oxygen vacancy is more energetically favored and causes larger atomic displacement than other types of surface oxygen vacancies. The reactivity of oxygen vacancies has also been tested by both molecular and dissociated oxygen adsorption. Furthermore, we discussed the configurations and the electronic properties of O 2-adsorbed surface, and found that the appearance of oxygen adsorbate-induced states in the band gap can make the surface sensitive to visible light.

Silicon Surface Preparation and Passivation by Vapor Phase of Heavy Water

The well known wet chemical treatments of the silicon surface and its native oxidation in air cause a high density of interface states, which predominantly originate from dangling bonds strained bonds or from bonds, between adsorbates and silicon surface atoms. Therefore, a number of wet-chemical treatments have been developed for ultraclean processing in order to produce chemically and electronically passivated surfaces [1]. The saturation of dangling bonds by hydrogen removes the surface states and replaces them by adsorbate-induced states, which influence the surface band-bending [2]. The first thermal hydrogen desorption peak from a hydrogen passivated Si surface in vacuum or inert gas ambient can be detected at around 380°C [3,4]. Simultaneously the combination of the hydrogen atoms of neighboring dihydrides generates a pair of dangling bonds. At around 480-500°C dangling bonds are generated on the silicon surface by desorption of the remaining hydrogen [5]. At that moment the silicon surface becomes extremely reactive.

Electronic structure of point defects in controlled self-doping of theTiO2(110) surface: Combined photoemission spectroscopy and density functional theory study

Point defects in metal oxides such as ${\text{TiO}}_{2}$ are key to their applications in numerous technologies. The investigation of thermally induced nonstoichiometry in ${\text{TiO}}_{2}$ is complicated by the difficulties in preparing and determining a desired degree of nonstoichiometry. We study controlled self-doping of ${\text{TiO}}_{2}$ by adsorption of 1/8 and 1/16 monolayer Ti at the (110) surface using a combination of experimental and computational approaches to unravel the details of the adsorption process and the oxidation state of Ti. Upon adsorption of Ti, x-ray and ultraviolet photoemission spectroscopy (XPS and UPS) show formation of reduced Ti. Comparison of pure density functional theory (DFT) with experiment shows that pure DFT provides an inconsistent description of the electronic structure. To surmount this difficulty, we apply DFT corrected for on-site Coulomb interaction $(\text{DFT}+U)$ to describe reduced Ti ions. The optimal value of $U$ is 3 eV, determined from comparison of the computed $\text{Ti}\text{ }3d$ electronic density of states with the UPS data. $\text{DFT}+U$ and UPS show the appearance of a $\text{Ti}\text{ }3d$ adsorbate-induced state at 1.3 eV above the valence band and 1.0 eV below the conduction band. The computations show that the adsorbed Ti atom is oxidized to ${\text{Ti}}^{2+}$ and a fivefold coordinated surface Ti atom is reduced to ${\text{Ti}}^{3+}$, while the remaining electron is distributed among other surface Ti atoms. The UPS data are best fitted with reduced ${\text{Ti}}^{2+}$ and ${\text{Ti}}^{3+}$ ions. These results demonstrate that the complexity of doped metal oxides is best understood with a combination of experiment and appropriate computations.

Effect of impurities on Tamm-like lanthanide-metal surface states

The effects of isolated residual-gas adsorbates on the local electronic structure of the Dy(0001) surface were spatially mapped by scanning tunneling microscopy and spectroscopy at 12 K. Less than 15 A away from an adsorbate, a strong reduction of the intensity and a significant increase of the width of the majority component of the surface state due to impurity scattering were observed, with essentially no change of the minority component; this reflects a high lateral localization of the Tamm-like surface state. Furthermore, an adsorbate-induced state was found that behaves metastable.

Highly Efficient Ultrafast Energy Transfer into Molecules at Surface Step Sites

Ultrafast laser-induced desorption of NO and CO molecules on a platinum surface reveals that the rate of energy transfer between the laser-heated electrons and the adsorbate depends critically on the precise adsorbate location. An analysis based on a simple electronic friction model suggests that this femtosecond energy transfer occurs ∼3 times faster to adsorbates at step sites compared to adsorbates located on atomically flat surface regions. Density functional theory calculations of vibrational damping rates by electron−hole pair excitations corroborate the increased rate of energy transfer at step sites and show that it is caused primarily by an increased local density of adsorbate-induced states around the Fermi level.

Effects of adsorbate-induced states on the metastable He atom scattering from water- and benzene-adsorbed Cu(1 0 0)

The intensity of metastable helium (He*) atoms which survive during the scattering from water- and benzene-adsorbed Cu(1 0 0) surfaces was measured. The survival probability (SP) of He* was found to be sensitive to the electronic states at around the Fermi level, which is derived from the adsorbate/metal hybridization and extend toward the vacuum. The SP is likely to depend largely on the kinetic energy of the He* atoms. The kinetic energy dependence can be understood on the basis of the He* decay mechanism. Metastable-atom deexcitation spectroscopy (MDS) and surface electronic structure calculation have been used for discussing the results for the He* SP.

Hydrogen adsorption on Gd(0001)

The electronic structure of hydrogen adsorbate-induced states on Gd(0001) was investigated by means of photoelectron spectroscopy with linearly polarized radiation. The E vector of the incoming photon beam is rotatable. Clean and well-ordered rare-earth (0001) surfaces exhibit a highly localized surface state near the Fermi edge. After the adsorption of hydrogen, the surface state disappears and an additional sharp feature at about 4 eV binding energy is observed. For this latter d z 2 -like state, the ratio of the radial matrix elements as well as the relative phase shifts were determined to be R= R p/ R f=2.4±0.3 and δ f− δ p=310±10°, respectively. The removal of the Gd surface state by hydrogen adsorption was investigated by means of scanning tunneling microscopy (STM) and spectroscopy (STS). The removal of the surface state exhibits domain-like behavior, with surface steps acting as domain boundaries. The tunneling spectra reveal that hydrogen adsorption causes a dramatic reduction in the differential conductivity near the Fermi level.

Removal of the clock reconstruction of Ni(100)-(2×2)p4g-N by coadsorption of K: A spot-profile-analysis low-energy-electron-diffraction and angle-resolved ultraviolet-photoemission-spectroscopy study

Previous work has revealed that the $(2\ifmmode\times\else\texttimes\fi{}2)p4g$structure formed by adsorption of N on Ni(100) is a clock reconstruction. A coverage of 0.5 ML of N atoms located in alternate fourfold hollow sites on the surface results in an expansion of each hollow by means of rotation of the Ni atoms in alternate clockwise and counterclockwise directions. Here we report on the removal of this substrate reconstruction by coadsorption of K. The structural changes induced by the alkali metal on the N precovered Ni(100) surface have been monitored by means of spot-profile-analysis low-energy-electron diffraction. The chemisorption bonding of N on Ni(100) and K on Ni(100) has been investigated by means of angle-resolved ultraviolet-photoemission spectroscopy. The band structure of the adsorbate-induced states is presented. The changes in the dispersive behavior of the N-derived bands upon coadsorption of K have been related to the structural changes. Our data suggest a mechanism for the removal of the reconstruction based on the K-induced restoration of the screening of the repulsive forces between the topmost Ni atoms.

Electronic excitations of adsorbates analyzed with two-photon photoemission: [formula omitted

Photoinduced excitations of normally unoccupied electronic states of adsorbates are investigated with two-photon photoemission (2PPE). We report 2PPE spectra from an ordered (1.4 × 1.4)-CO layer on Cu(1 1 1) for excitation with 3.5 eV ≤ hv ≤ 4.5 eV photon energy. The energetic position, E kin, of a CO induced state is found to shift as ΔE kin = 1 × Δhv. This provides evidence for emission from a normally unoccupied state, located (3.3 ± 0.2) eV above the Fermi level. Based on the polarization dependence of the emission from this adsorbate-induced state it is identified as the 2π ∗ affinity level of chemisorbed CO on Cu(1 1 1) which is temporarily populated in the 2PPE process.

Adsorption of O, HCO2H and C6H5CO2H on Cu(110) studied using reflectance anisotropy. Chemical and structural influences on an optically active surface resonance

Real-time reflectance anisotropy (RA) transients have been recorded from a cu(110) surface during exposure to O2, O2–HCO2H and C6H5CO2H, using a simple HeNe laser-based system operating at 1.96 eV polarised parallel to first the [110] and then the [100] directions on the crystal surface. The marked changes in the RA response observed during the formation of the reconstructed overlayers is attributed to the quenching of an allowed optical transition of the surface, as identified using electron spectroscopy, by the process of adsorption and restructuring. However, the influence of new adsorbate-induced states on the RA response cannot be ruled out at present. The magnitude of the observed changes in RA are comparable for the formation of the p(2 × 1)O and (3 × 1)O/HCO2H structures and are tentatively assigned to a combination of the rapid quenching of an allowed surface transition and the possible modification of new adsorbate-induced states. For the formation of the c(8 × 2) benzoate overlayer the RA change is considerably larger, which is believed to be due to a more thorough quenching of the allowed surface transition than that which occurs for the other systems studied. For the formation of the initial stages of the (2 × 1)O overlayer, in particular, the RA response would be consistent with simple Langmuir adsorption kinetics providing that a model in which the RA response was directly proportional to coverage was applicable.

Thermally reversible band bending for Bi/GaAs(110): Photoemission and inverse-photoemission investigations.

Results of synchrotron-radiation photoemission, inverse-photoemission, and low-energy electron diffraction studies of the Bi/GaAs(110) interface over the temperature range 20 to 300 K are presented. At 300 K, the first Bi monolayer grows in zigzag chains along the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction to form an ordered (1\ifmmode\times\else\texttimes\fi{}1) overlayer. This overlayer is semiconducting with a gap of 0.7 eV. Continued deposition results in Bi island growth atop the initial monolayer (ML) and conversion from semiconducting to semimetallic character. Bi deposition at 50 K results in layer-by-layer growth with no long-range order. Temperature- and coverage-dependent studies of band bending show symmetric behavior for n- and p-type GaAs(110) with matched bulk dopant concentrations. For 300-K deposition, the Fermi level ${\mathit{E}}_{\mathit{F}}$ moves rapidly toward final positions 0.74 and 0.98 eV below the conduction-band minimum for n- and p-type GaAs doped at ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. For 50-K deposition, the bands remain nearly flat to coverages of 1--2 ML where ${\mathit{E}}_{\mathit{F}}$ moves rapidly toward midgap. Significantly, both techniques also show that Fermi-level movement is thermally reversible, such that the position of ${\mathit{E}}_{\mathit{F}}$ in the surface band gap depends on temperature. A small part of the ${\mathit{E}}_{\mathit{F}}$ movement is due to nonequilibrium processes involving the creation of electron-hole pairs by the incident photon or electron beam followed by the transport of the minority carriers to the surface region by the electric field in the depletion region. This surface voltage is most important at low temperature, and studies which vary the incident photon flux by over 3 orders of magnitude establish that the photovoltage has a small (\ensuremath{\le}100 meV), but non-negligible, effect on the band bending measured under ``normal'' experimental conditions. Hence, equilibrium processes control coupling of substrate and adsorbate-induced states in the GaAs band gap through the semiconductor depletion region and they determine the temperature-dependent interface band bending.

Electronic states around and within the bandgap for clusters on GaAs

Recently two types of states have been observed for Sb, Bi, and other adsorbates on GaAs(110). For ordered overlayers, photoemission and inverse photoemission show adsorbate-induced states of the kinds that one might expect for systems with good two-dimensional order. On the other hand, scanning tunneling spectroscopy measurements also show states associated with various types of imperfections. The former states are consistent with no Fermi-level pinning, whereas the latter states can provide a mechanism for Fermi-level pinning. Here we report theoretical studies of the electronic states associated with Sb and Bi clusters, around and within the GaAs band gap. Incomplete bonding leads to gap states, but the Sb clusters with the most nearly complete adatom-adatom bonding show semiconducting behavior.

Two-photon photoemission from adsorbate-induced states: Atomic oxygen on Cu(111).

Two-photon photoemission is observed from unoccupied and occupied states of atomic oxygen adsorbed on Cu(111). The unoccupied state is found as an intermediate in the two-photon process with an energy of 2.79\ifmmode\pm\else\textpm\fi{}0.07 eV above the Fermi level. The occupied state is excited by two photons and has a binding energy of 2.13\ifmmode\pm\else\textpm\fi{}0.04 eV below the Fermi level.

Azimuthal dependence of angle-resolved photoemission cross section from adsorbate-induced initial states.

We have investigated the azimuthal dependence of angle-resolved photoemission cross section for emission from surface atoms and molecules. Using group theory, we have determined the azimuthal dependence for emission normal to the surface from adsorbate-induced states of atoms and molecules situated at different symmetry locations.

Direct observation of adsorbate-induced band-gap states on GaAs(110).

Normally unoccupied adsorbate-induced states within the GaAs band gap were studied by measurement of ultraviolet-photoemission spectra from pulsed-laser-excited surfaces. Strikingly similar discrete structures within the gap were observed for submonolayer coverages of both chemisorbed oxygen and gold. The implications of these results for the understanding of Fermi-level pinning at interfaces are discussed.