A field emission study of silver on rhenium

Field emission measurements of the change in average work function ?f of rhenium with adsorbed silver indicate that a rhenium-silver dipole forms with silver positive, of moment μ₀=5.2±1.5 ×10^?30 C m and polarizability $\text{α=29±12}\text{A}\text{?}{}^3$. Measurement of the rate of thermal desorption yields a mean binding energy of 2.31 ± 0.04 eV for sub-monolayer silver and 2.69±0.04 eV for a 2.5 monolayer deposit. Changes in work function induced by adsorption of silver on low-index rhenium plane surfaces are characterised by the formation of well-defined states and in this, silver resembles gold. These states are thought to result from a relatively large difference between the binding energy of adatoms on the low-index planes and on the surrounding surfaces, and this differnce is maintained when the surfaces are covered with silver. At the lowest coverage, silver is believed to be absent from all four observed planes and the measured rise in work function is thought to be apparent and to result from a decrease in field strength on the plane due to extension of the plane area by surrounding adsorbed silver. The structures adopted by silver overlayers are not known, but it is argued that on (101?0) and (101?1?) the final state at high coverage has the Ag(111) surface structure. On (112?0) and (112?2?) the silver layer at high coverage is thought to have either Ag(110) or Ag(100) surface structure. The structures of intermediate states found on all four low-index planes remain unkown. Field emission spectroscopy shows that emission from clean (101?0) is free-electron like and confirms earlier observations that emission from (202?1) is not. Spectroscopy also reveals a feature in the spectrum from silver on (101?0) which may be identified with a known surface state on Ag(111), thus providing some support for the assignment of Ag(111) to the surface structure of thick silver layers (> 3 monolayers) on (101?0).     

Investigation of the temperature dependent changes of work function induced by adsorption of Ag on W(011)

The adsorption of silver on the (011) face of tungsten has been studied at temperatures from 78 to 1200 K. Investigations were performed using LEED, AES and work function measurements. The work function changes appeared to depend on the nature of the adlayer, and its arrangement with respect to the substrate. An absolute coverage scale was determined for room temperature, and this scale was related to the work function changes observed at this temperature. An interpretation of the obtained curves of the work function changes is presented.     

Adsorption of copper on tungsten; measurements on single crystal planes

Work function changes caused by depositing and spreading of copper were measured on the (110), (100), (111) and (211) faces of tungsten crystal by means of field emission microscope. On (110) and (100) faces the work function for low coverage decreases vith increasing amount of copper deposited, passes through a minimum, then increases foi higher coverage and saturates for a thick layer. On (111) and (211) faces in the low coverage range, the work function increases when the amount of adsorbed copper increases. After reaching a maximum value the changes in work function on these planes have the same character as on (110) and (100) planes, i.e. the work function drops down when the coverage increases, passes through a minimum, increases again and saturates for a thick layer. It is proposed to connect the increase of work function on (111) and (211) faces at low coverage with the loose structure of the substrate surface. The adsorption of copper on these planes causes the smoothing of the crystal surface and this can lead to enhancement of the work function. The performed calibration of the coverage shows that changes in work function are occurring during formation of the first layer of copper adatoms.     

The growth mode of aluminium on silver (111)

The geometric and electronic structures occuring during the growth of Al on a single crystal Ag(111) surface have been studied using a combination of low energy electron diffraction (LEED), Auger electron spectroscopy (AES), energy loss spectroscopy (ELS) and work function measurements. The Auger signal versus deposition time plots, which were used to monitor the growth mode, are shown to behave in an identical fashion to that expected for layer-by-layer (Frank-van der Merwe) growth. LEED was used to determine the lateral periodicity of thin Al films and shows that Al forms, at very small coverages, 2D islands which have the same structure as the Ag(111) substrate and which grow together to form the first monolayer. At substrate temperatures of 150 K a well defined (1 × 1) structure with the same orientation as the underlying Ag(111) can be seen up to at least 12 ML. After completion of the third monolayer the ELS spectrum approached that observed for bulk aluminium. At a coverage of 3 ML the work function decreases by 0.4 eV from the clean silver value.     

Field desorption of Potassium from (001), (011) and (112) Tungsten planes

Field desorption of potassium from the (001), (011) and (112) tungsten planes in the temperature range 80–140 K and in the field range 0.10–0.55 V/Å has been investigated using the field emission micrscope with Faraday cage and a rotatable emitter. Generally good agreement with the results, obtained previously by other authors for other adsorbates and for the whole emitter tip, was found. However the temperature dependence of the desorption field is different for some crystal planes. The binding energy at zero coverage and zero field was determined. The values of 1.95 eV, 1.75 eV and 2.0 eV were obtained for the (001), (011) and (112) planes, respectively. The degree of desorption was also measured as a function of desorbing field and a significant difference of this dependence for the (001) and (011) planes was found.     

Chemisorption of oxygen on the silver (110) surface

In the present work the interaction between oxygen and the silver (110) surface is investigated, mainly using LEED-Auger techniques and thermal desorption spectra. The formation and stability of adsorption layers is studied after exposures at pressures from 10^?3 to 1 torr. At maximum coverage, a (2 × 1) superstructure is formed which is stable up to the desorption temperature. At lower coverage, (3 × 1) and (4 × 1) superstructures are also observed. On the basis of the experimental evidence, tentative models for these structures are presented and discussed.     

Photoemission studies of the alkali-covered Ge (111) surface

Photoelectron energy distribution measurements have been used to monitor changes in the surface potential of the Ge(111) surface as a function of cesium and sodium coverage. The specimen was sputter-cleaned, intrinsic (45 ohm-cm) germanium. It was found that the surface was p-type throughout the entire stable coverage range. In the case of cesium deposition, strong evidence of emission from occupied surface states was observed for coverages less than 0.15 monolayer. Exposure of the cesium-covered specimen to oxygen caused the surface to become less p-type. Structure in the surface potential versus coverage curve for sodium correlates with structure in the work function versus coverage curve and with a surface reconstruction observed by LEED.     

Adsorption of carbon monoxide on silver/palladium alloys

The adsorption of carbon monoxide on epitaxial (100) and (111) planes of Ag/Pd alloys with definite surface compositions has been studied by means of LEED, Auger electron spectroscopy and work function measurements. The formation of ordered adsorbed structures is prevented by even small amounts of silver in the surfaces. The maximum variation of the work function with CO adsorption bears no simple relationship to the surface composition. From measured adsorption isotherms the isosteric heats of adsorption have been evaluated. For CO adsorption on pure Pd planes the adsorption energies E_ad are either constant or decrease slowly up to high coverages, whereas a continuous decrease was observed with the alloys indicating the energetical heterogeneity. The results are discussed on the basis of our knowledge about the nature of the CO chemisorption and about the electronic structure of Ag/Pd alloys.     

Characterization of Ag adsorption on TiC（001） substrate： an ab initio study

Ag adsorptions at 0.25-3 monolayer （ML） coverage on a perfect TIC（001） surface and at 0.25 ML coverage on C vacancy are separately investigated by using the pseudopotential-based density functional theory. The preferential adsorption sites and the adsorption-induced modifications of electronic structures of both the substrate and adsorbate are analysed. Through the analyses of adsorption energy, ideal work of separation, interface distance, projected local density of states, and the difference electron density, the characteristic evolution of the adatom-surface bonding as a function of the amount of deposited silver is studied. The nature of the Ag/TiC bonding changes as the coverage increases from 0.25 to 3 MLs. Unlike physisorption in an Ag/MgO system, polar covalent component contributes to the Ag/TiC interfacial adhesion in most cases, however, for the case of 1-3 ML coverage, an additional electrostatic interaction between the absorption layer and the substrate should be taken into account. The value of ideal work of separation, 1.55 J/m^2, for a 3-ML-thick adlayer accords well with other calculations. The calculations predict that Ag does not wet TIC（001） surface and prefers a three-dimensional growth mode in the absence of kinetic factor. This work reports on a clear site and coverage dependence of the measurable physical parameters, which would benefit the understanding of Ag/TiC（001） interface and the analysis of experimental data.     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

Cu adsorption on Pt(111) and its effects on Chemisorption: A comparison with electrochemistry

The growth modes and interaction of vapor-deposited Cu on a clean Pt(111) surface have been monitored by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and work function measurements. The LEED data indicate that below 475 K Cu grows in p(1 × 1) islands in the first monolayer with the interatomic Cu spacing the same as the Pt(111) substrate. The second monolayer of Cu grows in epitaxial, rotationally commensurate Cu(111) planes with the CuCu distance the same as bulk Cu. For substrate temperatures below ～ 475 K, the variation of work function and “cross-over beam voltage” with Cu coverage show characteristic features at one monolayer that are quite useful for calibration of θ_Cu. Above 525 K, Cu-Pt alloy formation was observed in AES and LEED data. Thermal desorption spectroscopy of H₂ and CO has demonstrated that simple site blocking of the Pt(111) surface by vapor-deposited Cu occurs linearly with chemisorption being essentially eliminated at θ_Cu = 1.0–1.15. Conclusions drawn from this work correlate very favorably with the well-known effects of under potentially deposited copper on the electrochemistry of the H₂2H⁺ couple at platinum electrodes.     

Chlorine monolayers on the low-index faces of silver

Chlorine chemisorption layers were obtained on the (100), (110) and (111) faces of silver single crystals by means of the reaction with dichloroethane. The structure and stability of the surface phases was studied mainly by LEED and Auger spectroscopy. On the (100) face, a c(2 × 2) superstructure was found at all coverages, which was interpreted as a reconstructed layer with a structure similar to that of the AgCl (100) planes. On the other two faces, different superstructures were observed, depending on the coverage. At low coverages, (2 × 1) and (√3 × √3)-R 30° superstructures were found on the (110) and (111) faces, respectively, which seem to be better interprétable as reconstructed layers. At maximum coverages, c(4 × 2) and (3 × 3) superstructures were observed respectively on the two faces. These phases were interpreted as mixed layers with a packing of the silver and chlorine atoms similar to that of the AgCl (111) planes. The proposed models are in agreement with the different thermal stabilities of the layers. The models of the maximum-coverage structures correspond to about the same chlorine amount on the three faces, in agreement with the Auger results. The structures are also discussed in relation to the known epitactic relations for the growth of AgCl on silver.     

Comparative study of adsorption characteristics of Cs on the GaN （0001） and GaN （0001） surfaces

The adsorption characteristics of Cs on GaN （0001） and GaN （0001） surfaces with a coverage from 1/4 to 1 monolayer have been investigated using the density functional theory with a plane-wave uttrasoft pseudopotential method based on first-principles calculations. The results show that the most stable position of the Cs adatom on the GaN （0001） surface is at the N-bridge site for 1/4 monolayer coverage. As the coverage of Cs atoms at the N-bridge site is increased, the adsorption energy reduces. As the Cs atoms achieve saturation, the adsorption is no longer stable when the coverage is 3/4 monolayer. The work function achieves its minimum value when the Cs adatom coverage is 2/4 monolayer, and then rises with Cs atomic coverage. The most stable position of Cs adatoms on the GaN （000i） surface is at H3 site for 1/4 monolayer coverage. As the Cs atomic coverage at H3 site is increased, the adsorption energy reduces, and the adsorption is still stable when the Cs adatom coverage is 1 monolayer. The work function reduces persistently, and does not rise with the increase of Cs coverage.     

Adsorption of carbon monoxide on tungsten (210)

Thermal desorption spectra taken after adsorption of carbon monoxide at room temperature on W(210) show sequential formation with increasing coverage of strongly bound β₂ and β₁ binding states, correlated to the sequential formation of P(2 × 1) and (1 × 1) adsorbate structures as observed by LEED. Adsorption at room temperature gives a poorly ordered arrangement of adsorbed CO molecules, but well-ordered structures are produced by subsequent anneal. For adsorption without anneal the work function increases monotonically with coverage to a maximum of Δφ = + 0.70 eV at saturation coverage of 1 monolayer. For adsorption followed by anneal the work function dependence upon coverage is less simple, with even a decrease of work function at coverages less than a quarter monolayer. LEED intensity-voltage measurements from P(2 × 1)CO and P(2 × 1)N structures suggest that CO molecules occupy the sites of 4-fold symmetry upon which nitrogen is believed to be adsorbed. The distinction between the β₂ and β₁ states of adsorbed CO is attributed to heterogeneity induced by the reduction in binding energy of a CO molecule when its nearest-neighbor sites are occupied.     

Infrared-visible sum-frequency generation spectra of CO on Ni(100)

Infrared-visible sum-frequency generation (SFG) spectra of the stretching region of carbon monoxide adsorbed on Ni(100) are measured as a function of overlayer coverage and substrate temperature. The frequency and linewidth of the CO vibrational resonance in SFG have a similar dependence on coverage and temperature to that observed in linear absorption and emission spectroscopies. The lineshape of the SFG resonance, however, is influenced by the nonlinear response of the underlying metal surface. A simple model indicates that the nonresonant response of the substrate is out of phase with respect to that of the resonant CO response.     

Electronic structure and adsorption properties of the system Cs/O/W(110)

Comparative studies are carried out of the Cs/O/W(110) and Cs/W(110) adsorption systems. The method of threshold photoemission spectroscopy is used to study the work function and electronic structure in the energy region near the Fermi level as functions of the sub-monolayer cesium coverage. A significant increase of the saturation cesium coverage is observed on the O/W(110) surface. A new adsorption-induced surface band is observed in the electronic spectrum of the system Cs/O/W(110) with a binding energy ∼0.7 eV. For coverages of about one monolayer metallization of the adsorbed layer is observed. It is shown that the electronic structures of the systems Cs/O/W(110) and Cs/W(110) are similar for low coverages. A difference in the adsorption properties for these two systems occurs for coverages close to one monolayer, which is explained by the creation of new interaction centers of the Cs adatoms on the W(110) surface in the presence of oxygen. Fiz. Tverd. Tela (St. Petersburg) 39, 1683–1686 (September 1997)     

氧在Nb(110)表面吸附的第一性原理研究

通过第一性原理赝势平面波方法研究了氧在Nb(110)表面的吸附性质随覆盖度变化规律. O在Nb(110)表面最稳定吸附位是洞位,次稳定吸附位是长桥位. 在长桥位吸附时, O诱导Nb(110)表面功函数随覆盖度的增加而几乎线性增加;但当O在洞位吸附时, 与干净Nb表面相比, 覆盖度为0.75 ML和1.0 ML时功函数增加, 而覆盖度为0.25 ML和0.5 ML时功函数减小.通过对面平均电荷密度分布和偶极矩变化的讨论, 解释了由吸附导致功函数复杂变化的原因.通过对表面原子结构和态密度分析, 讨论了O在Nb表面吸附时引起表面原子结构变化以及O和Nb(110)表面原子的相互作用.     

O在Au(111)表面吸附的密度泛函理论研究

应用密度泛函理论，本文系统地研究了O在Au(111)表面上的吸附能、吸附结构、功函数、电子密度和投影态密度，给出了覆盖度从0.11ML到1.0ML的范围内，O的吸附特性随覆盖度变化的规律.研究发现O的稳定吸附位为3重面心立方(fcc)洞位，O在fcc洞位的吸附能对覆盖度比较敏感，其值随着覆盖度的增加而减小；O诱导Au(111)表面功函数的变化量与覆盖度成近线性关系，原因是Au表面电子向O偏移，形成表面偶极子；O—Au的相互作用形成成键态和反键态，且反键态都被占据，造成O—Au键很弱，O吸附能较小. 关键词： 表面吸附 Au(111)表面 密度泛函理论 电子特性     

Potassium adsorption on graphite(0001)

Potassium adsorption on graphite has been studied with emphasis on the two-dimensional K adlayer below one monolayer. Data are presented for the work function versus coverage, high-resolution electron energy loss spectroscopy (HREELS) vibrational spectra of K-adlayers, low energy electron diffraction and ultraviolet photoemission spectroscopy (UPS) spectra at different coverages. The data provide information regarding the vibrational properties of the K-adlayer, the metallization of the adlayer at submonolayer coverages, and the charge transfer from the K adatoms to the graphite substrate. Analysis of the work function, HREELS, and UPS data provides a qualitatively consistent picture of the charge state of the K adatoms, where at low coverages, below a critical coverage θ_c (θ_c=0.2–0.3), the K adatoms are dispersed and (partially) ionized, whereas at θ>θ_c islands of a metallic 2×2 K phase develops that coexist with the dispersed a K adatoms up to θ=1. We show that it is possible to understand the variation of the work function data based on a two-phase model without invoking a depolarization mechanism of adjacent dipoles, as is normally done for alkali-metal adsorption on metal surfaces. Similarly, the intensity variation as a function of coverage of the energy loss peak at 17 meV observed in HREELS, and the photoemission peak at E_b=0.5 eV seen in UPS can be understood from a two-phase model. A tentative explanation is presented that connects apparent discrepancies in the literature concerning the electronic structure of the K adlayer. In particular, a new assignment of the K-induced states near the Fermi level is proposed.     

Work function of adsorption systems potassium on tantalum and on molybdenum

Work functions φ_hkl of thermally annealed and potassium covered tantalum and molybdenum as a function of potassium surface density on (011), (112), (100) and (111) planes of these metals have been measured using a field emission microscope. The measurements have been performed at liquid nitrogen temperature (immobile layers). The work function decreases linearly at first, then more slowly, passes through a minimum, and then attains a constant value. Quantitative data on the dependence of φ_hklon surface density of potassium, N_hkl, for tantalum, molybdenum and tungsten have been compared. The principal results of the observations are: (i) for K on Ta, Mo, and W the work function minimum exhibits no distinct dependence on the type of substrate, however, it proves to depend on crystallographic direction of the latter; (ii) the values of the high coverage limit work function are approximately equal for one type of metal planes; (iii) the values of the high coverage limit surface densities of potassium adsorbed on Ta(011), Mo(011) and W(011) surfaces are approximately equal to the surface density of the (011) plane of bulk potassium crystal.