The influence of surface oxides on photocurrents at anthracene electrodes

The photoelectric work function was measured for six faces of a molybdenum single crystal. The crystal was cleaned by repeated cycles of ion bombardment and annealing at 920K under ultrahigh vacuum conditions. The work functions were found to be:

$\text{Φ}{}_{110}\text{=4.95±0.02}\text{eV}\text{,}\text{Φ}{}_{111}\text{=4.55±0.02}\text{eV}\text{,}\text{Φ}{}_{332}\text{=4.55±0.02}\text{eV}\text{,}\text{Φ}{}_{100}\text{=4.53±0.02}\text{eV}\text{,}\text{Φ}{}_{114}\text{=4.50±0.04}\text{eV}\text{,}\text{Φ}{}_{112}\text{=4.36±0.03}\text{eV}\text{.}$

The homogeneity of the surfaces was tested by the Schottky effect of work function lowering at high collecting fields. Effects of adsorption from the ambient atmosphere at pressures of p<2×10^?10torr, thermal segregation of bulk impurities, and of Ne⁺ bombardment dosage and energy, were investigated under identical conditions for each crystal face. The anisotropic character of the surface was clearly revealed.     

Adsorption isotherms of oxygen on tungsten (100), (110) and (111) surfaces in the pressure range 10−9 to 10−4 torr and temperature range 1500 to 2600 k,as measured by AES

This paper is a continuation of a previous investigation of oxygen adsorption on tungsten at high temperature using Auger electron spectroscopy. In this paper the adsorption isotherms of oxygen on (100), (110) and (111) faces of tungsten are reported. It is shown that these isotherms can be described by an equation of the form pO₂ = AF(θ) exp [?q(θ)/ kT]. The coverage depended functions F(θ) and q(θ) evaluated from the isotherms are different for all three investigated faces. The isosteric adsorption energy q has following initial values at very low oxygen coverage: q₁₀₀ = 6.1 eV, q₁₁₀ = 6.8 eV and q₁₁₁ = 6.5 eV. Increasing the oxygen coverage has only small influence on q₁₁₁; it changes from the initial value to q₁₁₁ ≈ 6.0 eV at θ ≈ 0.3 and remains constant at this value up to θ ≈ 1. q₁₁₀ shows the strongest dependence on oxygen coverage. It decreases rapidly at low coverages, slowly at moderate coverages and reaches the value q₁₁₀ = 5.0 atθ ≈ 1. The variation of q₁₁₀ with increasing oxygen coverage is monotonie from the initial value to q₁₁₁ = 4.9 eV at θ ≈ 1. Assuming that the atomic oxygen is the dominant species leaving the tungsten surface at high temperatures the functions F(θ) are used to calculate the oxygen equilibration probability ζO₂ (high temperature sticking probability) as a function of oxygen coverage θ. The main characteristic of ζO₂(θ) for all three faces is that it shows a maximum for (100) and (111) faces at θ = 0.3 and for (110) face at θ = 0.55.     

High field microscopy of nickel on tungsten

Field emission and field ion microscopy have been used to study the properties of nickel layers adsorbed on tungsten, and the growth of nickel crystallites. The first monolayer of nickel has a maximum density of 0.97 ± 0.05 × 10¹⁹ atoms m^?2 and results in an increase in the work function which can be attributed to the formation of dipoles of moment μ₀ = 1.70 ± 0.08 × 10^?30Cm at zero coverage and polarizability $\text{α = 7.3 ± 0.05}\text{A}\text{?}{}^3$. Nickel desorbs from the tungsten surface with activation energy 4.22 ± 0.01 eV and second layer atoms desorb with activation energy 3.2 ± 0.02 eV. Surface diffusion of second and higher layers over clean tungsten layer is believed to proceed by the “unrolling carpet” mechanism, with activation energy 0.93 ± 0.03 eV in close agreement with measurements of surface self-diffusion of nickel. Nickel does not dissolve appreciably in single-crystal tungsten and we confirm that atomic disordering at the nickel-tungsten interface is confined within a few angstroms of the interface. Well-ordered crystallites can be grown from a central nuclear structure which develops on (110)W. Combination of field ion and field emission techniques indicate that the crystallites adopt the expected growth form, having surfaces comprising large low-index faces, and also serve to confirm that field emission images alone cannot be relied upon to give an indication of crystallite shape. Crystallites invariably form upon an adsorbed layer which is at least one atom thick but may be thicker depending upon conditions of growth. The growth of crystals in situ offers the possibility of generating well-ordered low-index planes of large area which are suitable for further study, but it has yet to be confirmed that they behave as surface planes of bulk nickel.     

The role of HH interactions in the formation of ordered structures on Ni and Pd single crystals

The interaction between H adatoms on a surface is calculated within the embedded cluster model of chemisorption. The model is first applied to the case of two H atoms on a free electron surface. The interaction energy is found to be an oscillatory function of the H-H separation R_ab. Application of the free electron model to the problem of chemisorption on transition metal surfaces leads to unphysical results with the prediction of formation of ordered H overlayers which are not observed in LEED experiments. We next include the l = 2 TM muffin tins. Results for H adsorption on the low index faces of Ni and Pd substrates are presented. Graphitic structures are predicted for the (111) faces of both Ni and Pd with the H atoms occupying both types of three-fold hollow sites on the surface. This agrees with the results of LEED experiments for H/Ni(111). Comparison with experiment is not possible in the case of H/Pd(111) owing to the lack of low temperature studies for that system. Zig-zag chains with the H atoms adsorbed in sites of three-fold coordination on alternate sides of the TM(110) rows are predicted for both Ni and Pd. This is in agreement with the results of He diffraction experiments for H/Ni(110). No structure determination has been done for H/Pd(110). Adsorption in the four-fold centre sites for H on the (100) faces of Ni and Pd is found to be unfavourable. The H atoms are expected to adsorb in sites of three-fold symmetry below the (100) surface for H on Pd with formation of a c(2 × 2) structure in agreement with the LEED observations. For H/Ni(100) the H atoms are believed to adsorb above the surface, away from the centre site and to bond to two surface Ni atoms. No short-range ordered structures are predicted in this case.     

Adsorption of potassium on iron

Adsorption of K on Fe(110), (100) and (111) surfaces was studied by means of LEED, AES, thermal desorption and work function measurements. The monolayer capacity is about 5.5 × 10¹⁴ K-atoms/cm² in all three cases. With Fe(111) an ordered 3 × 3 overlayer was found at fairly low coverages. The work function decreases to a minimum and the initial dipole moments were determined to μ₀ = 7.0 Debye for Fe(110), μ₀ = 4.4 Debye for K/Fe(100) and μ₀ = 3.9 Debye for K/Fe(111). The heat of adsorption decreases from its initial value (Fe(110): 57; Fe(100): 54; Fe(111): 52 kcal/mole) continuously with increasing coverage which parallels the continuous decrease of the dipole moment of the adsorbate complex.     

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.     

Dipole effects and the work functions of the single crystal LaB6

Both the interior dipole structure normal to the polar surface of a crystal and any dipole layer at the surface itself contribute to the surface work function. For LaB₆, the structures of the unreconstructed (100), (110) and (111) faces or the reconstructions believed to characterize the (110) and (111) surfaces would produce dipole layers and differences in work function among these surfaces which are at least an order of magnitude greater than the differences which have been measured. Modification in ionic charge at sites making up the outermost surface dipole layers can markedly affect the dipole contribution to the work functions and it is argued that such charge modification may occur as a general phenomenon in ionic crystals.     

Low energy electron diffraction and work function change studies of the adsorption of substituted aromatic molecules on the (111) and (100) crystal faces of platinum

The chemisorption of toluene, m-xylene, mesitylene, n-butylbenzene, t-butylbenzene, aniline, nitrobenzene and cyanobenzene were studied on the (111) and (100) crystal faces of platinum at low pressures (10^?9 to 10^?7 torr) and at temperatures of 20 to 300 °C by low energy electron diffraction and work function change measurements. After adsorption, reorientation of the molecules in the adsorbed layer is necessary to form the ordered structures. Molecules that have either higher rotational symmetry (mesitylene) or have only small size substituents on the benzene rings exhibit better ordering if the adsorption is carried out at low incident flux. The adsorbed layers are more ordered on the (111) crystal face than on the (100) crystal face of platinum. The work function changes upon adsorption rangs from ?1.4 eV for nitrobenzene to ?1.8 eV for aniline. Both the diffraction and work function change data indicate that, under the conditions of these experiments, all of the molecules chemisorb with their benzene ring parallel to the surface and interact with the metal surface primarily via the π-electrons in the benzene ring. The substituent groups play an important role in determining the ordering characteristics of the overlayers but do not markedly effect the strength of the chemical bond between the substrate and the adsorbate.     

Comment on “oscillatory indirect interaction between adsorbed atoms — Non-asymptotic behavior in tight-binding models at realistic parameters” by K.H. Lau and W. Kohn

A sensitive infrared reflectance accessory suitable for the study of surface films on medium size single crystals is described. Oxide films formed on the (100), (110) and (111) crystal faces of aluminum in air, at room temperature, display nearly identical behavior with films approximately 10 Å thick absorbing as a single band near 940 cm^?. After 10⁴ sec at 570 °C, in oxygen, films formed on these crystals begin to display differences in band characteristics and growth kinetics. Between 10⁴ and 4 × 10⁴ sec the rates of growth on the (110) and (111) crystal faces are much greater than on the (100) face. Beyond 4 × 10⁴ sec the growth rate on the (100) face increases while the (110) and (111) growth rates approach zero. Limiting thicknesses reached after 4 × 10⁵ sec approach 3.4 × 10², 2.1 × 10² and 2.2 × 10² Å for (100), (110) and (111) faces, respectively. Oxide compositional differences were reflected by the number and form of the infrared bands after 10⁴ sec of oxidation. After 5 × 10⁴ sec the (100) face oxide was composed of two and possibly three oxide species as evidenced by several bands. Differences in bandwidth and frequency were observed between the (110) and (111) oxide films. The significance of such differences is discussed.     

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB₆(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB₆(100), (110) and (111) clean surfaces. The surface states on LaB₆(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB₆(110) is increased to ～3.8 eV by an oxygen exposure of ～2 L. The surface states on LaB₆(111) disappear at an oxygen exposure of ～2 L where the work function has a maximum value of ～4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB₆(111) until an exposure of ～2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ～4.4 to ～3.8 eV until an oxygen exposure of ～100L. The initial sticking coefficient on LaB₆(110) has the highest value of ～1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB₆ surfaces.     

Final state band gap effects in UV photoemission from Cu(111)

The crystal momentum dependence of the final states involved in bulk photoemission from the sp-bands through a (111) surface of copper was investigated with a photon energy hν = 11.7 eV. Plane wave hybridization was observed and is described in terms of a 2-OPW model Hamiltonian with parameters determined to be V₀=7.1±0.1 eV for the inner potential and E_g = 1.6 ± 0.2 eV for the energy gap. The model is also shown to account for a rapid intensity variation with crystal momentum of the form |a_G(k)|², where a_G(k) is a plane wave amplitude of the final state wave function.     

Adsorption of hydrogen on palladium single crystal surfaces

The adsorption of hydrogen on clean Pd(110) and Pd(111) surfaces as well as on a Pd(111) surface with regular step arrays was studied by means of LEED, thermal desorption spectroscopy and contact potential measurements. Absorption in the bulk plays an important role but could be separated from the surface processes. With Pd(110) an ordered 1 × 2 structure and with Pd(111) a 1 × 1 structure was formed. Maximum work function increases of 0.36, 0.18 and 0.23 eV were determined with Pd(110), Pd(111) and the stepped surface, respectively, this quantity being influenced only by adsorbed hydrogen under the chosen conditions. The adsorption isotherms derived from contact potential data revealed that at low coverages θ ∞ √p_H2, indicating atomic adsorption. Initial heats of H₂ adsorption of 24.4 kcal/mole for Pd(110) and of 20.8 kcal/mole for Pd(111) were derived, in both cases E_ad being constant up to at least half the saturation coverage. With the stepped surface the adsorption energies coincide with those for Pd(111) at medium coverages, but increase with decreasing coverage by about 3 kcal/mole. D₂ is adsorbed on Pd(110) with an initial adsorption energy of 22.8 kcal/mole.     

Low-energy electron diffraction,Auger electron spectroscopy,and thermal desorption studies of chemisorbed CO and O2 on the (111) and stepped [6(111) × (100)] iridium surfaces

The adsorption of CO, O₂, and H₂O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H₂O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O₂ was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O₂ on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.     

Absolute coverages of CO and O on Pt(111); Comparison of saturation CO coverages on Pt(100), (110) and (111) surfaces

Nuclear microanalysis (NMA) has been used to determine the absolute coverages of oxygen and CO adsorbed on Pt(111). The saturation oxygen coverage at 300 K is 3.9 ± 0.4 × 10¹⁴ O atoms cm^?2 (θ = 0.26 ± 0.03), confirming the assignment of the LEED pattern as p(2 × 2). The saturation CO coverage at 300 K is 7.4 ± 0.3 × 10¹⁴ CO cm^?2 (θ = 0.49 ± 0.02). The low temperature saturation CO coverages on Pt(100), (110) and (111) surfaces are compared.     

Potentiodynamic desorption spectra of metallic monolayers of Cu,Bi, Pb,Tl, and Sb adsorbed at (111), (100), and (110) planes of gold electrodes

The formation of metallic adsorption layers was studied in solutions of Cu²⁺, Bi³⁺, Pb²⁺, Tl⁺ and Sb³⁺ at (111), (100) and (110) planes of gold single crystal electrodes. Potentiodynamic desorption spectra were recorded with a sweep rate of 20 mV s^?1 for all systems. Characteristic peak structures were obtained which depend strongly on the nature of the adsorbate as well as on the substrate orientation. The half width of the peaks indicates attraction and repulsion respectively for various systems. In most systems more than one peak was observed. This is explained by the formation of various ordered structures. At low coverages peak charge data obtained by integration of current/time curves yield surface concentrations which fit those of ordered structures well, e.g. c(2×2) on (100) or $\text{p(}\text{3}\text{×}\text{3}\text{)}$ R 30° on (111). The adsorption behaviour of the (110) plane is similar in all systems because atomic chains seem to be generally stable. Near the equilibrium potential of the correspondent metal electrode, ?_r = 0, a “mono-molecular” adsorption layer was found for Cu²⁺, Pb²⁺ and Bi³⁺. In the case of the small copper atom, a 1:1 adsorption was found for all planes. Larger atoms like bismuth and lead form epitactic layers at low coverages; at high coverages they form close-packed monolayers with surface concentrations independent of the substrate structure but decreasing with increasing adsorbate radius. The coulometric data for antimony and thallium are not so conclusive. Measurements with various sweep rates show that the adsorption reaction is a slow potential dependent process in various systems. The underpotential/work function correlation of Kolb, Gerischer and Przasnyski is discussed with respect to these experiments. It follows that this concept developed for polycrystalline electrodes is qualitatively valid for (110), but not clearly so for (100) and (111).     

Thermal accommodation coefficients by high speed vibration of solid samples: VI. O2 and N2 on As(111) and Sb(111) single crystal surfaces

Translational γ_t and rotational γ_r energy accommodation coefficients are measured using a high speed vibrational method described earlier. For N₂ on cleaved As(111) single crystal surfaces γ_t = 0.34 ± 0.03, γ_r = 0.03 ± 0.03; on cleaved Sb(111) single crystal surfaces γ_t = 0.49 ± 0.02, γ_r = 0.06 ± 0.04. For O₂ on As(111) γ_t = 0.39 ± 0.02, γ_r = 0.02 ± 0.02; on Sb(111) γ_t = 0.56 ± 0.05, γ_r = 0.07 ± 0.06. (Uncertainties are statistical estimates of precision. In addition, it is possible that the results are systematically low by up to 10%.) Comparison with N₂ and O₂ accommodation coefficients measured by the same method on other metal substrates suggests that the antimony surface is covered by chemisorbed oxygen but that the arsenic surface is somewhat cleaner. The results are consistent with oxidation of these semimetals occurring by way of physisorbed gas precursor states.     

The chemisorption of CO,CO2, C2H2, C2H4, H2 and NH3 on the clean Fe(100) and (111) crystal surfaces

The chemisorption of small molecules (CO, CO₂, C₂H₂, C₂H₄, H₂ and NH₃) has been studied on the clean Fe(110) and (111) crystal faces by low-energy electron diffraction (LEED) and thermal desorption. C₂H₄ and C₂H₂ yield the same sequence of surface structures that change with temperature and crystal orientation. CO and CO₂ chemisorption similarly results in the formation of the same types of surface structures that change with surface temperature and crystal orientation. Ammonia forms several ordered surface structures on both iron crystal faces. All of the molecules decompose as a function of temperature on the iron surfaces as indicated by the Auger and thermal desorption spectra.     

Field emission studies of oxygen on silver tips

Clean [111] oriented silver field emitting tips have been exposed to oxygen at 10^?3 Torr for 1 min at temperatures ranging from ? 170 to 200°C. From 50 to 200°C, an adsorption structure is formed that is stable in oxygen. The structure is characterized by intensely emitting regions on either side of enlarged {110}, {210} and {310} faces and a dark region in the (111)-{100} zone line directions. For adsorption from ? 170 to 200°C, the structure of the patterns depends distinctly on the adsorption temperature because the coverages are different and adsorption is activated. Oxygen adsorption at 10^?3 Torr for 1 min at 0°C causes an increase in the average work function of 1.15 eV. At 0°C, silver was exposed increasingly at 10^?6 Torr until 6100 L was reached. The work function increased progressively by 0.61 eV for this exposure. The {111}, {100}, {311}, {211} and {533} faces are attacked first. Then, the {110} faces are attacked followed by the {210} {310} and {320}. Heating of the adsorption layer formed at 0°C produced no changes in pattern and work function up to 100°C. Between 100 and 200°C, a strong decrease in work function and changes in the pattern result from oxygen penetration into the bulk.     

Space charge regions at silver halide surfaces: Experimental results for undoped AgCl

Vibrating capacitor measurements have been made of the variation with temperature of the surface potential of AgCl single crystals of surface orientation (100), (210), (110) and (111). The magnitude and temperature variation of the surface potential was found to be strongly dependent on the orientation of the free surface. The data are interpreted in terms of a model which attributes the existence of surface space charge potentials to differences in the binding energies of silver ions at surface sites relative to normal interior lattice sites. The results are summarized in terms of the Gibbs free energies ΔG_v^(hkl) for cation vacancy formation and give ΔG_v⁽¹¹⁰⁾ = + 0.01 eV + 5 kT, ΔG_v⁽¹⁰⁰⁾ = ? 0.2eV + 10 kT, ΔG_v⁽²¹⁰⁾ = = ? 0.8eV + 23kT, ΔG_v⁽¹¹¹⁾ = + 0.8 eV ? 20kT. These numbers have large absolute uncertainties but the relative values for the different orientations are reliable. The values of ΔG_v^hkl can be used to calculate the magnitude and sign of the surface space charge fields and allow some predictions on the formation of photographic latent images near free surfaces.