Formation and characterization of the Cu2O overlayer on Zn-terminated ZnO(0 0 0 1)

Low-energy electron diffraction, X-ray photoelectron spectroscopy and synchrotron-radiation-excited angle-resolved photoelectron spectroscopy have been used to characterize Cu-oxide overlayers on the Zn-terminated ZnO(0 0 0 1) surface. Deposition of Cu on the ZnO(0 0 0 1)-Zn surface results in the formation of Cu clusters with (1 1 1) top terraces. Oxidation of these clusters by annealing at 650 K in O₂ atmosphere (1.3 × 10⁻⁴ Pa) leads to an ordered Cu₂O overlayer with (1 1 1) orientation. Good crystallinity of the Cu₂O(1 1 1) overlayer is proved by energy dispersion of one of Cu₂O valence bands. The Cu₂O(1 1 1) film exhibits a strong p-type semiconducting nature with the valence band maximum (VBM) of 0.1 eV below the Fermi level. The VBM of ZnO at the Cu₂O(1 1 1)/ZnO(0 0 0 1)-Zn interface is estimated to be 2.4 eV, yielding the valence-band offset of 2.3 eV.     

Configuration of pentacene (C22H14) films on Si(1 0 0)-2 × 1 studied by NEXAFS

Pentacene films on Si(1 0 0)-(2 × 1) surface at 300 K were investigated using near edge X-ray absorption fine structure (NEXAFS) at the carbon K-edge. NEXAFS spectra show that pentacene molecules are chemisorbed on the Si(1 0 0)-(2 × 1) surface for monolayer with flat-laying and predominantly physisorbed on the Si(1 0 0)-(2 × 1) surface for multilayer films with an upright molecular orientation. Absorption angle of pentacene molecules were measured through π^∗ transition. The angles between the double bond and the silicon surface were 35-55°, 65° and 76° at monolayer, 24 and 48 nm pentacene deposited on the Si(1 0 0) surface, respectively. We observed that the intermediate flat-laying phase is favored for monolayer coverage, while the films of molecules standing perpendicular to the Si(1 0 0) surface are favored for multilayer coverage.     

CO2 adsorption on Cr(1 1 0) and Cr2O3(0 0 0 1)/Cr(1 1 0)

We attempt to correlate qualitatively the surface structure with the chemical activity for a metal surface, Cr(1 1 0), and one of its surface oxides, Cr₂O₃(0 0 0 1)/Cr(1 1 0). The kinetics and dynamics of CO₂ adsorption have been studied by low energy electron diffraction (LEED), Aug er electron spectroscopy (AES), and thermal desorption spectroscopy (TDS), as well as adsorption probability measurements conducted for impact energies of E_i = 0.1-1.1 eV and adsorption temperatures of T_s = 92-135 K. The Cr(1 1 0) surface is characterized by a square shaped LEED pattern, contamination free Cr AES, and a single dominant TDS peak (binding energy E_d = 33.3 kJ/mol, first order pre-exponential 1 × 10¹³ s⁻¹). The oxide exhibits a hexagonal shaped LEED pattern, Cr AES with an additional O-line, and two TDS peaks (E_d = 39.5 and 30.5 kJ/mol). The initial adsorption probability, S₀, is independent of T_s for both systems and decreases exponentially from 0.69 to 0.22 for Cr(1 1 0) with increasing E_i, with S₀ smaller by ∼0.15 for the surface oxide. The coverage dependence of the adsorption probability, S(Θ), at low E_i is approx. independent of coverage (Kisliuk-shape) and increases initially at large E_i with coverage (adsorbate-assisted adsorption). CO₂ physisorbs on both systems and the adsorption is non-activated and precursor mediated. Monte Carlo simulations (MCS) have been used to parameterize the beam scattering data. The coverage dependence of E_d has been obtained by means of a Redhead analysis of the TDS curves.     

Electrochemical scanning tunneling microscopy examination of the structures of benzenethiol molecules adsorbed on Au(1 0 0) and Pt(1 0 0) electrodes

In situ electrochemical scanning tunneling microscopy (STM) has been used to examine the structures of benzenethiol adlayers on Au(1 0 0) and Pt(1 0 0) electrodes in 0.1 M HClO₄, revealing the formation of well-ordered adlattices of Au(1 0 0)-(√2 × √5) between 0.2 and 0.9 V and Pt(1 0 0)-(√2 × √2)R45° between 0 and 0.5 V (versus reversible hydrogen electrode), respectively. The coverage of Au(1 0 0)-(√2 × √5) is 0.33, which is identical to those observed for upright alkanethiol admolecules on Au(1 1 1). In comparison, the coverage of Pt(1 0 0)-(√2 × √2)R45° - benzenethiol is 0.5, much higher than those of thiol molecules on gold surfaces. This result suggests that benzenethiol admolecules on Pt(1 0 0) could stand even more upright than those on Au(1 0 0). All benzenethiol admolecules were imaged by the STM as protrusions with equal corrugation heights, suggesting identical molecular registries on Au(1 0 0) and Pt(1 0 0) electrodes, respectively. Modulation of the potential of a benzenethiol-coated Au(1 0 0) electrode resulted in irreversible desorption of admolecules at E ? 0.1 V (vs. reversible hydrogen electrode) and oxidation of admolecules at E ? 0.9 V. In contrast, benzenethiol admolecule was not desorbed from Pt(1 0 0) at potentials as negative as the onset of hydrogen evolution. Raising the potential rendered deposition of more benzenethiol molecules before oxidation of admolecules commenced at E > 0.9 V.     

Growth and morphology of the epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) Trilayers

Growth and surface morphology of epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) trilayers constituting a magnetic tunnel junction were investigated by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). STM reveals a grain-like growth mode of MgO on Fe(1 1 0) resulting in dense MgO(1 1 1) films at room temperature as well as at 250 °C. As observed by STM, initial deposition of MgO leads to a partial oxidation of the Fe(1 1 0) surface which is confirmed by Auger electron spectroscopy. The top Fe layer deposited on MgO(1 1 1) at room temperature is relatively rough consisting of clusters which can be transformed by annealing to an atomically flat epitaxial Fe(1 1 0) film.     

Isomeric effects on room-temperature chemisorption and thermal evolution of iso-, cis- and trans-dichloroethylene on Si(1 1 1)7 × 7

The room-temperature adsorption and thermal evolution of iso-, cis- and trans-dichloroethylene (DCE) on Si(1 1 1)7 × 7 have been studied by vibrational electron energy loss spectroscopy and thermal desorption spectrometry (TDS). The presence of the Si-Cl stretch at 510 cm⁻¹ suggests that, upon adsorption, all three isomers dissociate via C-Cl bond breakage on the 7 × 7 surface to form mono-σ bonded chlorovinyl , which could, in the case of iso-DCE, further dechlorinate to vinylidene (:CCH₂) upon insertion into the back-bond. The higher saturation exposure for the Si-Cl stretch at 510 cm⁻¹ observed for cis- and trans-DCE than iso-DCE suggests that Cl dissociation via the CHCl group in the cis and trans isomers is less readily than the CCl₂ group in iso-DCE. Our TDS data show remarkable similarities in both molecular desorption near 360 K and thermal evolution of the respective adstructures for all three isomers on Si(1 1 1)7 × 7. In particular, upon annealing to 450 K, the mono-σ bonded chlorovinyl adspecies is found to further dechlorinate to either vinylene di-σ bonded to the Si surface or acetylene to be released from the surface. Above 580 K, vinylene could also become gaseous acetylene or undergo H abstraction to produce hydrocarbon or SiC fragments. All three DCE isomers also exhibit TDS features attributable to an etching product SiCl₂ at 800-950 K and recombinative desorption products HCl at 700-900 K and H₂ at 650-820 K. The stronger Cl-derived TDS signals and Si-Cl stretch at 510 cm⁻¹ over 450-820 K for trans-DCE than those for cis-DCE indicate stronger dechlorination for trans-DCE than cis-DCE, which could be due to less steric hindrance resulting from the formation of the chlorovinyl adspecies for trans-DCE during the initial adsorption/dechlorination process. Finally, our density functional calculations qualitatively support the thermodynamic feasibility and relative stabilities of the proposed adstructures involving chlorovinyl, vinylidene, and vinylene adspecies.     

Low temperature adsorption of oxygen on reduced V2O3(0 0 0 1) surfaces

Well ordered V₂O₃(0 0 0 1) films were prepared on Au(1 1 1) and W(1 1 0) substrates. These films are terminated by a layer of vanadyl groups under typical UHV conditions. Reduction by electron bombardment may remove the oxygen atoms of the vanadyl layer, leading to a surface terminated by vanadium atoms. The interaction of oxygen with the reduced V₂O₃(0 0 0 1) surface has been studied in the temperature range from 80 to 610 K. Thermal desorption spectroscopy (TDS), infrared reflection absorption spectroscopy (IRAS), high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) were used to study the adsorbed oxygen species. Low temperature adsorption of oxygen on reduced V₂O₃(0 0 0 1) occurs both dissociatively and molecularly. At 90 K a negatively charged molecular oxygen species is observed. Upon annealing the adsorbed oxygen species dissociates, re-oxidizing the reduced surface by the formation of vanadyl species. Density functional theory was employed to calculate the structure and the vibrational frequencies of the O₂ species on the surface. Using both cluster and periodic models, the surface species could be identified as η²-peroxo () lying flat on surface, bonded to the surface vanadium atoms. Although the O-O vibrational normal mode involves motions almost parallel to the surface, it can be detected by infrared spectroscopy because it is connected with a change of the dipole moment perpendicular to the surface.     

Dissociative adsorption and thermal evolution of dichloroethylenes on Ni(1 0 0)

The room temperature (RT) adsorption and thermal evolution of cis- and trans-dichloroethylene (DCE) and their structural isomer, iso-DCE, on Ni(1 0 0) have been studied by vibrational electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES) and thermal desorption spectrometry (TDS). For RT adsorption, both cis- and trans-DCE exhibit very similar EELS features that are different from those found for iso-DCE. These differences indicate the formation of different fragments upon RT adsorption. In particular, the primary adspecies for cis- and trans-DCE are ethane-1,1,2,2-tetrayl () and acetylide-like () adspecies along with a small amount of chlorovinyl adspecies, while ethylylidyne () is the more plausible adspecies for iso-DCE. The differences in the adstructures upon dissociative adsorption at RT underline the important isomeric effects. Furthermore, both AES and TDS results for all three DCE isomers show that most of the Cl atoms produced by dechlorination remain on the surface and its surface concentration remains unchanged upon annealing the samples above 500 K. Upon further annealing to 550 K, the EELS spectra of all three isomers exhibit a broad feature near 1600 cm⁻¹, which suggests the formation of carbon clusters on the surface. The presence of surface Cl atoms therefore appears to prevent the CC bond cleavage during thermal evolution of the adspecies on Ni(1 0 0).     

Formation of doubly excited He atoms during scattering of He ions from a clean and oxygen covered Ni(1 1 0) surface

The formation of doubly excited states of He atoms during collisions of He²⁺ ions with energies between 60 eV and 1 keV with a Ni(1 1 0) surface is studied via Auger electron spectroscopy. We observe that the electron spectra from autoionization of doubly excited states of 2s², 2s2p, 2p² configurations show a pronounced dependence on the coverage of the target surface with oxygen. For a controlled O₂ adsorption on the Ni(1 1 0) surface we can explain the resulting changes in the electron spectra by the modification of the work function of the target surface. Thermal desorption and dissolution into the bulk of surface contaminations at elevated temperatures provides an alternative interpretation of recent work where the local electron spin polarization of a Ni(1 1 0) surface was deduced from changes in the electron spectra as function of target temperature.     

Scanning tunneling microscopy luminescence from nanoscale surface of GaAs(1 1 0)

Scanning tunneling microscopy luminescence (STML) was induced from the nanometer scale surfaces of cleaved n-type and p-type GaAs(1 1 0) wafers by using of an ITO-coated optical fiber probe in an ultrahigh-vacuum chamber. The STML from n-type GaAs(1 1 0) surface was induced under negative sample bias when the applied bias exceeds a threshold voltage around −1.5 V. Whereas the STML from p-type GaAs(1 1 0) surface was induced under positive sample bias when the applied bias exceeds a threshold voltage around +1.5 V. The excitation energies at the threshold voltages are consistent with the band gap of GaAs (1.42 eV) at 295 K. The typical quantum efficiencies for n-type and p-type GaAs are about 3 × 10⁻⁵ and 2 × 10⁻⁴ photons/electron, respectively. The observed STML from are attributed to a radiative recombination of electron-hole pairs generated by a hole injection for n-type GaAs under negative sample bias and an electron injection for p-type GaAs under positive sample bias, respectively.     

Adsorption of water on thin V2O3(0 0 0 1) films

V₂O₃(0 0 0 1) films have been grown epitaxially on Au(1 1 1) and W(1 1 0). Under typical UHV conditions these films are terminated by a layer of vanadyl groups as has been shown previously [A.-C. Dupuis, M. Abu Haija, B. Richter, H. Kuhlenbeck, H.-J. Freund, V₂O₃(0 0 0 1) on Au(1 1 1) and W(1 1 0): growth, termination and electronic structure, Surf. Sci. 539 (2003) 99]. Electron irradiation may remove the oxygen atoms of this layer. H₂O adsorption on the vanadyl terminated surface and on the reduced surface has been studied with thermal desorption spectroscopy (TDS), vibrational spectroscopy (IRAS) and electron spectroscopy (XPS) using light from the BESSY II electron storage ring in Berlin. It is shown that water molecules interact only weakly with the vanadyl terminated surface: water is adsorbed molecularly and desorbs below room temperature. On the reduced surface water partially dissociates and forms a layer of hydroxyl groups which may be detected on the surface up to T ∼ 600 K. Below ∼330 K also co-adsorbed molecular water is detected. The water dissociation products desorb as molecular water which means that they recombine before desorption. No sign of surface re-oxidation could be detected after desorption, indicating that the dissociation products desorb completely.     

Adsorption and reactivity of SO2 on Ir(1 1 1) and Rh(1 1 1)

The adsorption and reactivity of SO₂ on the Ir(1 1 1) and Rh(1 1 1) surfaces were studied by surface science techniques. X-ray photoelectron spectroscopy measurements showed that SO₂ was molecularly adsorbed on both the Ir(1 1 1) surface and the Rh(1 1 1) surface at 200 K. Adsorbed SO₂ on the Ir(1 1 1) surface disproportionated to atomic sulfur and SO₃ at 300 K, whereas adsorbed SO₂ on the Rh(1 1 1) surface dissociated to atomic sulfur and oxygen above 250 K. Only atomic sulfur was present on both surfaces above 500 K, but the formation process and structure of the adsorbed atomic sulfur on Ir(1 1 1) were different from those on Rh(1 1 1). On Ir(1 1 1), atomic sulfur reacted with surface oxygen and was completely removed from the surface, whereas on Rh(1 1 1), sulfur did not react with oxygen.     

Infrared reflection absorption spectral study for CO adsorption on Pd/Pt(1 1 1) bimetallic surfaces

Infrared reflection absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on 0.15 nm-thick-0.6 nm-thick Pd-deposited Pt(1 1 1) bimetallic surfaces: Pd_x/Pt(1 1 1) (where x is the Pd thickness in nanometers) fabricated using molecular beam epitaxial method at substrate temperatures of 343 K, 473 K, and 673 K. Reflection high-energy electron diffraction (RHEED) measurements for Pd_0.15-0.6 nm/Pt(1 1 1) surfaces fabricated at 343 K showed that Pd grows epitaxially on a clean Pt(1 1 1), having an almost identical lattice constant of Pt(1 1 1). The 1.0 L CO exposure to the clean Pt(1 1 1) at room temperature yielded linearly bonded and bridge-bonded CO-Pt bands at 2093 and 1855 cm⁻¹. The CO-Pt band intensities for the CO-exposed Pd_x/Pt(1 1 1) surfaces decreased with increasing Pd thickness. For Pd_0.3 nm/Pt(1 1 1) deposited at 343 K, the 1933 cm⁻¹ band caused by bridge-bonded CO-Pd enhanced the spectral intensity. The linear-bonded CO-Pt band (2090 cm⁻¹) almost disappeared and the bridge-bonded CO-Pd band dominated the spectra for Pd_0.6 nm/Pt(1 1 1). With increasing substrate temperature during the Pd depositions, the relative band intensities of the CO-Pt/CO-Pd increased. For the Pd_0.3 nm/Pt(1 1 1) deposited at 673 K, the linear-bonded CO-Pt and bridge-bonded CO-Pd bands are located respectively at 2071 and 1928 cm⁻¹. The temperature-programmed desorption (TPD) spectrum for the 673 K-deposited Pd_0.3 nm/Pt(1 1 1) showed that a desorption signal for the adsorbed CO on the Pt sites decreased in intensity and shifted ca. 20 K to a lower temperature than those for the clean Pt(1 1 1). We discuss the CO adsorption behavior on well-defined Pd-deposited Pt(1 1 1) bimetallic surfaces.     

A density functional theory study of ethylene adsorption on Ni10(1 1 1), Ni13(1 0 0) and Ni10(1 1 0) surface cluster models and Ni13 nanocluster

Ethylene adsorption was studied by use of DFT/B3LYP with basis set 6-31G(d,p) in Gaussian’03 software. It was found that ethylene has adsorbed molecularly on all clusters with π adsorption mode. Relative energy values were calculated to be −50.86 kcal/mol, −20.48 kcal/mol, −32.44 kcal/mol and −39.27 kcal/mol for Ni₁₃ nanocluster, Ni₁₀(1 1 1), Ni₁₃(1 0 0) and Ni₁₀(1 1 0) surface cluster models, respectively. Ethylene adsorption energy is inversely proportional to Ni coordination number when Ni₁₀(1 1 1), Ni₁₃(1 0 0) and Ni₁₀(1 1 0) cluster models and Ni₁₃ nanocluster are compared with each other.     

An IRAS study of CO bonding on Sn/Pt(1 1 1) surface alloys at maximal pressures of 10 Torr

The adsorption of CO on Pt(1 1 1), (2 × 2) and (√3 × √3)R30° Sn/Pt(1 1 1) surface alloys has been studied using temperature programmed desorption (TPD), low energy electron diffraction (LEED) and infrared reflection adsorption spectroscopy (IRAS). The presence of Sn in the surface layer of Pt(1 1 1) reduces the binding energy of CO by a few kcal/mol. IRAS data show two C-O stretching frequencies, ∼2100 and ∼1860 cm⁻¹, corresponding to atop and bridge bonded species, respectively. Bridge bonded stretching frequencies are only observed for Pt(1 1 1) and (2 × 2) Sn/Pt(1 1 1) alloy surfaces. A slight coverage dependence of the vibrational frequencies is observed for the three surfaces. High pressure IRAS experiments over a broad temperature range show no indication of bridge bonded CO on any of the three surfaces. Direct CO adsorption on Sn sites is not observed over the measured temperature and pressure ranges.     

Li ion neutralization on Ag layers grown on Cu(1 1 1)

Electron transfer processes in the neutralization of Li⁺ ions on Ag layers grown on Cu(1 1 1) are investigated in quest of quantum confinement effects. Neutralization probabilities in the scattering of Li⁺ for incident ion energies in the 300 eV to 2 keV range are reported for Ag coverages ranging from 0.15 ML to 5 ML. Results are compared to those for Ag(1 1 1) and Cu(1 1 1) surfaces of bulk crystals. Although existing studies of the characteristics of Ag layers on Cu(1 1 1) indicate significant differences in electronic structure as a function of film thickness, the electron transfer probabilities we measure are found to be very close to those for bulk Ag(1 1 1). These results are commented on the basis of existing models and earlier studies of Li ion neutralization on various metals.     

Adsorption of organic molecules by photochemical reaction on Cl:Si(1 1 1) and H:Si(1 1 1) evaluated by HREELS

The covalent attachment of alkyl groups to silicon surfaces, via carbon-silicon bond formation, has been attempted using gas-surface reactions starting from Cl-terminated Si(1 1 1) or H:Si(1 1 1) under ultraviolet light irradiation. The formation of Cl-terminated Si(1 1 1) and its resulting stability were examined prior to deposition of organic molecules. High-resolution electron energy loss spectroscopy (HREELS) was utilized for detecting surface-bound adsorbates. The detection of photo-deposited organic species on Cl:Si(1 1 1) from gas-phase CH₄ or CH₂CH₂ was not significant. On H:Si(1 1 1), it was evident that after the photoreaction with gas-phase C₂H₅Cl, C₂H₅ groups were chemically bonded to the surface Si atoms through single covalent bonds. The C₂H₅ groups were thermally stable at temperatures below 600 K. Alkyl monolayers prepared on silicon surfaces by dry process will lead to a new prospective technology of nanoscale fabrication and biochemical applications.     

Carbon monoxide adsorption on the metastable fcc-Co/Cu(1 0 0) surface

The adsorption properties of CO on the epitaxial five-monolayer Co/Cu(1 0 0) system, where the Co overlayer has stabilized in the metastable fcc-phase, are reported. This system is known to exhibit metallic quantum well (MQW) states at energies 1 eV or greater above the Fermi level, which may influence CO adsorption. The CO/fcc-Co/Cu(1 0 0) system was explored with low energy electron diffraction (LEED), inverse photoemission (IPE), reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Upon CO adsorption, a new feature is observed in IPE at 4.4 eV above E_F and is interpreted as the CO 2π^∗ level. When adsorbed at room temperature, TPD exhibits a CO desorption peak at ∼355 K, while low temperature adsorption reveals additional binding configurations with TPD features at ∼220 K and ∼265 K. These TPD peak temperatures are correlated with different C-O stretch vibrational frequencies observed in the IR spectra. The adsorption properties of this surface are compared to those of the surfaces of single crystal hcp-Co, as well as other metastable thin film systems.     

In-situ study of the catalytic oxidation of CO on a Pt(1 1 0) surface using ambient pressure X-ray photoelectron spectroscopy

CO and O₂ co-adsorption and the catalytic oxidation of CO on a Pt(1 1 0) surface under various pressures of CO and O₂ (up to 250 mTorr) are studied using ambient pressure X-ray photoelectron spectroscopy (APXPS) and mass spectrometry. There is no surface oxide formation on Pt under our reaction conditions. CO oxidation in this pressure (<500 mTorr), O₂ to CO ratio (<10), and temperature (150 °C) regime is consistent with the Langmuir-Hinshelwood reaction mechanism. Our findings provide in-situ surface chemical composition data of the catalytic oxidation of CO on Pt(1 1 0) at total pressures below 1 Torr.     

CO oxidation over Ru(0 0 0 1) at near-atmospheric pressures: From chemisorbed oxygen to RuO2

RuO₂(1 1 0) was formed on Ru(0 0 0 1) under oxygen-rich reaction conditions at 550 K and high pressures. This phase was also synthesized using pure O₂ and high reaction temperatures. Subsequently the RuO₂ was subjected to CO oxidation reaction at stoichiometric and net reducing conditions at near-atmospheric pressures. Both in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and post-reaction Auger electron spectroscopy (AES) measurements indicate that RuO₂ gradually converts to a surface oxide and then to a chemisorbed oxygen phase. Reaction kinetics shows that the chemisorbed oxygen phase has the highest reactivity due to a smaller CO binding energy to this surface. These results also show that a chemisorbed oxygen phase is the thermodynamically stable phase under stoichiometric and reducing reaction conditions. Under net oxidizing conditions, RuO₂ displays high reactivity at relatively low temperatures (?450 K). We propose that this high reactivity involves a very reactive surface oxygen species, possibly a weakly bound, atomic oxygen or an active molecular O₂ species. RuO₂ deactivates gradually under oxidizing reaction conditions. Post-reaction AES measurements reveal that this deactivation is caused by a surface carbonaceous species, most likely carbonate, that dissociates above 500 K.