Superlattices formed by electrodeposition of silver on iodine-pretreated Pt(111); studies by LEED,Auger spectroscopy and electrochemistry

Reported are studies by LEED and Auger spectroscopy of silver layers electrodeposited on well-characterized Pt(111) surfaces from aqueous solution. Prior to electrodeposition. the Pt(111) surface was treated with I₂ vapor to form the Pt(111) ($\text{7}\text{×}\text{7}$)R19.1°-I superlattice which protected the Pt and Ag surfaces from attack by the electrolyte and residual gases. Electrodeposition of silver occurred in four distinct ranges of electrode potential. Ordered layers having (3 × 3) and (18 × 18) (coincidence lattice) LEED patterns were formed at all coverages from the onset of deposition to the highest coverages studied, about twenty equivalent atomic layers. Formation of ordered Ag layers has therefore been demonstrated, at least for deposits of limited thickness. Auger spectra revealed that for deposits of a few atomic layers. The iodine layer remained attached to the surface during multiple cycles of electrodeposition and dissolution of silver from iodine-free solution. Each peak of the voltammetric current-potential scan produced a change in the LEED pattern.     

The adsorption of chlorine and chloridation of Ag(111)

The adsorption of chlorine on the Ag(111) surface has been studied using LEED, Auger and temperature programmed desorption. Chlorine adsorbs dissociately with an initial sticking probability of ~ 0.4, and a precursor state is implicated in the chemisorption process. The chlorine appears to form a close-packed monolayer with the same packing density as in AgCl(111), and is epitaxially related to the substrate mesh. Chlorine continues to adsorb above a monolayer in coverage, though the sticking probability drops precipitately, being ~ 0.01 after the adsorption of 5 monolayers at 300 K. There is little increase in the chlorine Auger signal above one monolayer coverage at 300 K, but when adsorption is carried out at 240 K the chlorine signal is more than doubled. This is interpreted as being due to the formation of a layer structure of alternate Cl and Ag layers at the lower temperature, while adsorption at 300 K results in dissolution of subsurface Cl into the bulk of the crystal. Upon heating, the low temperature layer structure is destroyed, the chlorine signal diminishes to a limiting value at 450 K equivalent to the value for one adsorbed monolayer — apparently due to the dissolution of the near surface Cl layers into the bulk. However, the chlorine re-emerges at the surface at ~ 600 K, probably due to an exothermic heat of solution of Cl in the silver lattice. Desorption from the multilayers peaks at 670 K and both AgCl and Ag are desorbed coincidently with kinetics identical to those for the sublimation of bulk AgCl (ΔH = 235 kJ mol^?1, ΔS = 90 JK^?1 mol^?1). After the multilayers have desorbed, the final Cl layer desorbs in a higher temperature peak ( ~ 760 K) as AgCl (no silver desorption) which shows complex desorption kinetics indicative of the strong influence of a precursor state in the desorption process.     

Adsorption-desorption properties,coadsorption, and surface structural chemistry of chlorine and oxygen on Ag(331)

At 300 K oxygen chemisorbs on Ag(331) with a low sticking probability, and the surface eventually facets to form a (110)?(2 × 1) O structure with ΔΦ = +0.7 eV. This facetting is completely reversible upon O₂ desorption at ~570 K. The electron impact properties of the adlayer, together with the LEED and desorption data, suggest that the transition from the (110) facetted surface to the (331) surface occurs at an oxygen coverage of about two-thirds the saturation value. Chemisorbed oxygen reacts rapidly with gaseous CO at 300 K, the reaction probability per impinging CO molecule being ~0.1. At 300 K chlorine adsorbs via a mobile precursor state and with a sticking probability of unity. The surface saturates to form a (6 × 1) structure with ΔΦ = +1.6 eV. This is interpreted in terms of a buckled close-packed layer of Cl atoms whose interatomic spacing is similar to those for Cl overlayers on Ag(111) and Ag(100). Desorption occurs exclusively as Cl atoms with E_d ~ 213 kJ mol^?1; a comparison of the Auger, ΔΦ, and desorption data suggests that the Cl adlayer undergoes significant depolarisation at high coverages. The interaction of chlorine with the oxygen predosed surface, and the converse oxygen-chlorine reaction are examined.     

Effect of surface reconstruction on the adsorption of Ge on clean Si(111)

The effect of ultrahigh vacuum deposition of Ge below and at monolayer (ML) coverage onto a 7 × 7 reconstructed clean Si(111) surface held at room temperature is studied by low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS). The results are compared to those obtained on 2 × 1 reconstructed clean Si(111) : (i) the Si dangling bond states are replaced by Ge dangling bond states at submonolayer coverages in both cases; (ii) the 7 × 7 reconstruction persists below 1 ML, it is not replaced by a ? 3 × ? 3 R30° at 1/3ML as it was on the 2 × 1; and (iii) the coverage below 1 ML is not homogeneous on the 7 × 7 reconstruction. This behaviour can be explained by the influence of the inhomogeneties associated with the 7 × 7 reconstruction.     

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).     

Leed and Aes study of the Au/AuPb2 interface

The adsorption of lead on gold at room temperature in UHV conditions has been studied by LEED and AES. We review some of the data obtained on the Au(100), (111), and (110) faces, published elsewhere, and we give some new experimental results on the stepped Au(S) [n(100) × (111)] (with n = 3, 4, 5, and 6) faces. On all these faces, as lead is deposited on the gold substrate it first forms a monolayer of lead, then a compound AuPb₂. Using the LEED and Auger data we give a model of the epitaxy with a layer-by-layer growth mechanism. We propose a model which involves a transition alloy wich forms at the interface Au/AuPb₂. This model is in agreement with the LEED diagrams observed before the one corresponding to bulk AuPb₂. In the case of the epitaxy of lead on gold (100), we calculate the Auger peak-to-peak ] heights of the gold (72 eV) and lead (93 eV) transitions versus coverage. We obtain good agreement with the experimental data, assuming that the first and last layers of the alloy are lead monolayers and diffusion of lead in gold as well as gold in lead.     

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.     

Ultrahigh resolution electron microscopy of the surface structure of (110) Ag2O

An ultrahigh resolution transmission electron microscope and diffraction investigation of the residual oxidized silver on the surface of (111) Au films has shown that it is possible to resolve a monolayer coverage of silver atoms and the atomic structure of crystalline Ag₂O surfaces. Localized patches often under 100 Å in lateral extent show a reoriented (110) oxide which often exhibits a (2 × 1) surface superstructure. The structure of various different layer coverages has been analyzed from the high resolution micrographs via digital image processing and model building to interpret surface morphology.     

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.     

Adsorption of oxygen on silver single crystal surfaces

The adsorption of oxygen on Ag(110), (111), and (100) surfaces has been investigated by LEED, Auger electron spectroscopy (AES), and by the measurement of work function changes and of kinetics, at and above room temperature and at oxygen pressures up to 10^?5Torr. Extreme conditions of cleanliness were necessary to exclude the disturbing influences, which seem to have plagued earlier measurements. Extensive results were obtained on the (110) face. Adsorption proceeds with an initial sticking coefficient of about 3 × 10^?3 at 300 K, which drops very rapidly with coverage. Dissociative adsorption via a precursor is inferred. The work function change is strictly proportional to coverage and can therefore be used to follow adsorption and desorption kinetics; at saturation, ΔΦ ≈ 0.85 eV. Adsorption proceeds by the growth of chains of oxygen atoms perpendicular to the grooves of the surface. The chains keep maximum separation by repulsive lateral interactions, leading to a consecutive series of (n × 1) superstructures in LEED, with n running from 7 to 2. The initial heat of adsorption is found to be 40 kcal/mol. Complicated desorption kinetics are found in temperature-programmed and isothermal desorption measurements. The results are discussed in terms of structural and kinetic models. Very small and irreproducible effects were observed on the (111) face which is interpreted in terms of a general inertness of the close-packed face and of some adsorption at irregularities. On the (100) face, oxygen adsorbs in a disordered structure; from ΔΦ measurements two adsorption states are inferred, between which a temperature-dependent equilibrium seems to exist.     

Fe在Ru(1010)表面上沉积的LEED，ARUPS研究

Fe在Ru(1010)表面上的沉积,经过LEED和ARUPS的测量研究.结果表明,经过退火Fe在Ru(1010)表面根据覆盖度大小分别形成(1×1)和P(2×1)结构.当覆盖度大于10个单层时,表面结构不再保持与衬底相关的晶格常数与对称性,形成fcc铁的(111)外延结构.铁覆盖度较小时,费米能级附近的价电子能级与清洁时相似,外延fcc铁表面则形成了与表面有关的价电子能级. 关键词：     

Binding states and decomposition of NO on single crystal planes of Pt

Nitric oxide desorption and reaction kinetics are compared on the (111), (110),and (100) planes of platinum using temperature programmed desorption mass spectrometry. NO exhibits large crystallographic anisotropies with the (100) plane having stronger bonding and much higher decomposition activity than the (110) or (111) planes. The desorption activation energies for the major tightly bound states are 36, 33.5, and 25 kcal mole^?1 on the (100), (110), and (111) planes respectively. Pre-exponential factors for these states on the (110) and (111) planes are 1 × 10^16±0.5s^?1. The major tightly bound state on the (100) plane dissociates to yield 50% N₂ and O₂, but all other states all planes desorb without significant decomposition. The fraction decomposed is less than 2% on the Pt(111) surface.     

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.     

Surface diffusion of Pb on clean Si surfaces

Pb diffusion on clean Si(111), (100), and (110) surfaces was studied by Auger electron spectroscopy and low energy electron diffraction in the temperature range from 100 to 300°C. It is shown that lead transport along sillicon surfaces takes place via the mechanism of solid-phase spreading with a sharp moving boundary. The temperature dependence of the Pb diffusion coefficients on Si(111), (100) and (110) surfaces have been obtained. A Si(110)-4×2-Pb surface structure has been observed for the first time.     

Xenon monolayer structures on copper and silver

LEED studies of xenon monolayers at 77K on (111), (100) and (110) faces of copper and (111), (110) and (211) faces of silver show that the xenon atoms are hexagonally close-packed (or nearly so) on each surface, and that the surface area per adatom is about 17Ų. The adsorbate layer is epitaxially related to the substrate but is in full registry only on Cu (111). Surface potential values are consistent with those already reported for annealed polycrystalline films of copper and silver indicating that the latter are not specifically related to surface roughness.     

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.     

RHEED study of In-induced superstructures on Ge(111) surfaces

When submonolayer and monolayer amounts of indium were deposited onto clean Ge(111) surfaces at room temperature and then heated, (13 × 2√3), (12 × 2√3), (11 × 2√3), (10 × 2√3), (4√3 × 4√3) R30°-related, (√31 × √31) R(±9°), (√61 × √61) R(30 ± 4°) and (4.3 × 4.3) structures appeared on the surfaces at fixed In coverages and at fixed surface temperatures. General intensity features of superlattice reflections are derived from intensity estimations by eye of superlattice spots in their RHEED patterns, and some structural characteristics of the superstructures are clarified from the analysis of the general intensity features. The former four superstructures are long-period (2 × 2)-related antiphase structures whose period changes, depending on the coverage. The wavevector characterizing the (13 × 2√3) structure, which appears at the smallest coverage, almost coincides with those of structural fluctuation emerging at the clean Ge(111) (1 × 1) surface around 350°C. The coincidence suggests that the longperiod (2 × 2)-related antiphase structures have a close relationship to the structural fluctuation and, besides, to the (2 × 8) structure in their origin.     

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 and desorption of oxygen on stepped tungsten surfaces

The adsorption and desorption of oxygen on stepped tungsten surfaces with orientations close to the (110) orientation and steps parallel to the most densely packed crystal direction ([111]) is studied with low energy electron diffraction, Auger electron spectroscopy, work function measurements and thermal desorption spectroscopy. With increasing deviation from the (110) orientation, an increasing preference for the formation of the p(2 × 1) domain with the densely packed direction parallel to the steps is noted. The adsorption kinetics does not differ markedly from that on the flat (110) surface, however the desorption behaviour at low coverages (θ < 0.3) is quite different. The results are interpreted in terms of the dissociation of a mobile precursor at terrace and step sites, the competition between the two domains during their growth and a step-induced premature transition to the complex structure observed on flat (110) surfaces at $\text{θ ? 8}$. The steps are believed to play also a significant role in desorption.     

Co chemisorption on PtCu surfaces III. CO on PtCu(111)

Selected thermal desorption and valence band photoemission data on the chemisorption of CO on PtCu(111) surfaces are presented. The main objective is to make a comparison with CO chemisorption on an annealed (1 × 3) reconstructed Pt_0.98Cu_0.02(110) surface. The (111) alloy surfaces are unreconstructed (1 × 1) surfaces, with average near-surface Cu concentrations ranging from ? 7.5% to ? 20% as indicated by the Cu 920 eV Auger signal. It is observed that the effect of alloying Pt(111) with Cu is to progressively lower the desorption peak temperature and hence the free energy of CO desorption from Pt sites. A second observation is that the energy distribution of the Cu 3d-derived states is little affected by CO adsorption on Cu sites at 155 K. Both these results offer a contrast to the results for CO/Pt_0.98Cu_0.02(110) reported earlier.