AES-LEED-ellipsometry study of the kinetics of the interaction of methane with Ni(110)

The interaction of methane with Ni(110) was studied with AES, LEED and ellipsometry. Sticking coefficients were determined in the temperature range 298–600 K at methane pressures of 10^?4–10^?2 Torr. The carbon coverages were derived from Auger spectra by calibration with ellipsometry. At room temperature no detectable adsorption was observed without use of electron sources. In the temperature range 473–579 K the coverage versus exposure curves show an induction effect at low coverage followed by an almost linear increase up to a saturation coverage of about $\text{1}\text{3}$ monolayer of carbon. At these temperatures a Ni(110)-(2 × 3)-C structure was observed with streaks in the direction of constant h. The observed behaviour is explained with a nucleation and growth model in which mobile carbon species are captured at the edges of surface nickel carbide islands. At temperatures above 600 K carbon diffuses into the bulk and the Ni(110)-(4 × 5)-C superstructure is observed.     

Growth of extra-thin ordered aluminum films on Si(110) surfaces

The formation of surface structures upon Al deposition onto a Si(110) surface was studied by LEED and AES. The “4 × 6”, “1 × 9”, 2 × 1, 1 × 1 ordered Si(110)-Al surface phases and epitaxial Al(110) domains were observed depending on Al coverage and substrate temperature. The formation phase diagram was drawn for the Al/Si(110) system.     

Interaction of aluminum with iridium surface: Adsorption, desorption, dissolution, and formation of surface compounds

The interaction of aluminum with an iridium (111) surface was studied in ultrahigh vacuum by Auger electron spectroscopy over the broad temperature range 300–2000 K. At room temperature, layer-by-layer growth of an aluminum film was observed, with a monolayer forming in coherent relation to the substrate. Deposition at 1100–1300 K gives rise to the formation of surface aluminide Ir₄Al with an adatom concentration N _Al = (4.20 ± 0.15) × 10¹⁴ cm^?2. It was shown that aluminum escapes out of the surface aluminide by thermal desorption in the 1300–1700-K temperature interval, with the desorption activation energy changing from ～4.5 to ～5.7 eV as the coverage decreases from the value corresponding to the surface aluminide (taken for unity) down to zero.     

Oxidation and faceting of polycrystalline tungsten ribbons

We have measured the oxidation rate of tungsten and the evaporation rate of tungsten oxide in the temperature range from 900 to 1200 K at an oxygen pressure from 5 × 10^?4 to 5 × 10^?3 Torr. The oxidation rate increases steadily with coverage in the whole range studied. The evaporation rate decreases at high pressure and is strongly dependent on the initial conditions of the experiments. These kinetic measurements support a qualitative model of oxidation. The surface is composed of oxide islands surrounded by oxide-free regions covered only by chemisorbed oxygen atoms. On the bare regions beside the chemisorbed oxygen atoms we suppose the existence of a dilute chemisorbed oxide layer which can either enter the condensed oxide phase or evaporate. The number of the growing islands is set up at the beginning of the reaction and does not increase further. This model, consistent with kinetic results during oxidation, has been proposed first to explain results obtained by Auger electron spectroscopy and thermal desorption spectroscopy under vacuum. Faceting is particularly important in the early stages of the experiment because it can hinder the nucleation of the oxide which is a necessary step for growth. In a narrow range of temperature and oxygen pressure this inhibited nucleation leads to an enhanced evaporation rate so that the growth rate is lower. Recording this growth rate allows us to follow faceting. The parameters studied are the oxygen coverage and the temperature, experimental results are in agreement with LEED and RHEED results. Reconstruction and faceting are discussed and are believed to be caused by a smoothing of the surface during the chemisorption step.     

Surface superstructures of the Pb/Si(001) system

In the present work the Pb/Si(001) system has been studied with LEED (low-energy electron diffraction), AES (Auger electron spectroscopy), and EELS (electron energy loss spectroscopy). Five different surface superstructures, i.e., the 2 × 2, c(4 × 8), 4 × 1, 2 × 1 and c(4 × 4) were observed. Four of them are found for the first time, except for the 2 × 1. Their relationship has been investigated as a function of Pb coverage and annealing temperature. As a result, a complete phase diagram of the system has been determined. Upon annealing at 450°C the 2 × 2 superstructure undergoes an irreversible phase transformation to the c(4 × 8), while the c(4 × 4) reversibly transforms to 2 × 1 at 300°C. Strong influence of oxygen contamination on the surface superstructures has been observed.     

Irreversible structure transitions in Gd monolayers on Mo(112)

Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and contact potential difference (CPD) methods have been used to investigate the structure of Gd monolayers deposited on Mo(112) at T = 78 K and the changes upon annealing in a wide temperature range, up to the beginning of desorption. In the submonolayer coverage range (θ < 0.67), the film structures p(1.3×1) and p(2×1) already formed at T = 78 K, testifying that Gd adatoms possess some mobility at rather low temperatures. The p(1.3×1) structure was found to appear at 0.07 < θ < 0.25, but it irreversibly turned into the p(2×1) structure when the annealing temperature, T_an, exceeded 500 K. Above θ = 0.25, the p(2×1) structure emerged immediately at 78 K. Formation of step arrays was observed in the range of T_an = 500–1200 K and is attributed to surface alloying. The suggestion of surface alloying is corroborated by data on annealing induced variations of the work function and Auger peak of Gd. In the coverage range 0.5 < θ < 0.67, the phase p(2×1) was found to coexist with the phase c(1.5×2), which corresponds to a physical monolayer. No evidence of surface alloy in the complete monolayer was revealed. Distinction between ordering scenarios for the systems Gd/Mo(1 1 2) and Dy/Mo(112) is discussed.     

W(110)面二维氧化的低能电子显微镜研究

应用低能电子显微术对W（110）面二维氧化结构进行了初步研究．随着氧暴露量的增加，低能电子衍射图样由清洁表面（1×1）结构转变成p（2×1），再变为带有复杂衍射卫星点的p（2×2）结构．利用低能电子显微术的暗场像模式，对（00）束附近的分数衍射斑点进行了放大成像，发现表面由两种对比度相差很大的区域组成，它们就是具有不同方位取向的氧超结构畴区．两种畴区的分布与衬底表面缺陷特别是表面台阶有一定的关系，而且温度对这种钨表面的二维氧化起着重要作用 关键词：     

Change in adsorption bond length with coverage for KAl(111)

The local bond geometry of K adsorbed on Al(111) at low temperature has been studied by photoelectron diffraction (PED) as a function of K coverage. It is found that K atoms occupy on-top sites in the coverage range 0.05-0.4 monolayer and that the KAl bond length increases by 0.17 Å over this coverage range. The reliability of this result is supported by PED studies of the (√3 × √3)R30° structures formed by adsorption of one-third monolayer Na and K at 300 K, and K at 150 K, which give results in quantitative agreement with previous structure determinations by SEXAFS and LEED.     

Order-disorder transitions in a chemisorbed system: Hydrogen on Ni (111)

Atomic hydrogen chemisorbed on a Ni (111) surface forms at coverages between 0.3 and 0.6 an ordered 2 × 1-structure as observed by low energy electron diffraction (LEED). The intensity of the fractional-order LEED spots was measured at different coverages as a function of temperature. Continuous order-disorder transitions are found, the maximum transition temperature (270 K) being at θ = 0.5. The phase diagram, however, is asymmetric with respect to this coverage and can therefore presumably not be explained on the basis of (independent) pairwise interactions.     

Structure of ultra-thin Ti film on the Al(001) surface

The atomic structure of sub-monolayer amounts of Ti deposited on the Al(001) surface at room temperature has been investigated using low-energy electron diffraction (LEED) and low-energy ion scattering spectroscopy (LEIS). The Ti coverage was determined using Rutherford backscattering spectroscopy (RBS). Though a crisp LEED image is inherently difficult to obtain, the symmetry of the observed c(2 × 2) LEED images allows us to infer a structure which places Ti atoms in every other Al lattice site. Analysis of the LEIS azimuth- and polar-angle scan spectra has been done to determine the best structural model which supports the c(2 × 2) symmetry of the LEED image as well as LEIS experimental data. It was concluded that the best model consistent with the experimental data, puts Ti preferentially below the surface of the Al substrate at every other lattice site for sub-monolayer coverage of Ti on Al(001). As Ti coverage increases, the presence if Ti atoms in the surface layer also increases. Results of this study are relevant to research pertaining to the possible use of Ti as a catalyst in sodium alanate (NaAlH₄) in hydrogen storage applications.     

The growth of ice clusters on the Si(100)(2 × 1)-H(D) surface: Electron energy loss spectroscopy and thermal desorption studies

The growth of ice clusters on the Si(100)(2 × 1)-H surface has been investigated mainly by the use of high-resolution electron energy loss spectroscopy and thermal desorption spectroscopy. At 90 K, H₂O molecules are adsorbed on the (2 × 1)-H surface to form ice clusters by the hydrogen-bonding interaction. Four H₂O peaks are observed at 165, 185, 215 and 270 K in the thermal desorption spectra for the ice-covered surface. The peaks at 165 and 185 K correspond to the ice clusters and the peaks at 215 and 270 K to the strongly-bound H₂O species which play a role as the nucleation centers of the ice clusters desorbing as H₂O at 185 and 165 K, respectively.     

Characterization of oxygen,carbon, and sulfur adlayers on W(211)

Adlayers of oxygen, carbon, and sulfur on W(211) have been characterized by LEED, AES, TPD, and CO adsorption. Oxygen initially adsorbs on the W(211) surface forming p(2 × 1)O and p(1 × 1)O structures. Atomic oxygen is the only desorption product from these surfaces. This initial adsorption selectively inhibits CO dissociation in the CO(β₁) state. Increased oxidation leads to a p(1 × 1)O structure which totally inhibits CO dissociation. Volatile metal oxides desorb from the p(1 × 1)O surface at 1850 K. Oxidation of W(211) at 1200 K leads to reconstruction of the surface and formation of p(1 × n)O LEED patterns, 3 ? n ? 7. The reconstructed surface also inhibits CO dissociation and volatile metal oxides are observed to desorb at 1700 K, as well as at 1850 K. Carburization of the W(211) surface below 1000 K produced no ordered structures. Above 1000 K carburization produces a c(6 × 4)C which is suggested to result from a hexagonal tungsten carbide overlayer. CO dissociation is inhibited on the W(211)?c(6×4)C surface. Sulfur initially orders into a c(2 × 2)S structure on W(211). Increased coverage leads to a c(2×6)S structure and then a complex structure. Adsorbed sulfur reduces CO dissociation on W(211), but even at the highest sulfur coverages CO dissociation was observed. Sulfur was found to desorb as atomic S at 1850 K for sulfur coverages less than $\text{7}\text{6}$ monolayers. At higher sulfur coverages the dimer, S₂, was observed to desorb at 1700 K in addition to atomic sulfur desorption.     

Oxygen adsorption on clean Mo (100) surfaces

Oxygen adsorption on clean Mo (100) surfaces has been studied by LEED, AES, work function changes and energy loss spectroscopy. At room temperature, the oxygen uptake as determined by AES is linear up to one third of the saturation value. Data obtained with CO adsorption have been used to determine the oxygen coverage. With increasing oxygen exposure LEED shows three stages: a c (2 × 2) phase growing simultaneously with a (6 × 2) structure, a stage with (110) microfacets covered by two-dimensional structures and finally a p (3×1) structure together with a p (1×1) structure, probably due to an oxide phase. Even in the low temperature range (370–500 K) remarkable effects are observed: adsorption at 370 K produces a disordered c (4×4) structure which is followed by a (√5 × √5)?R 26° 33 structure. The same occurs when the inital c (2 × 2) structure formed at 295 K is heated above 370 K. Measurements of the work function indicate a minimum at the end of the c (2×2) structure, then a rapid increase and at saturation a value of about 1.5 V above that of the clean surface. Energy loss spectroscopy measurements point to an increase of the surface plasmon energy during the faceting stage. New transitions are observed which are due to new electronic levels induced by the adsorption. They are comparable with photoemission results on W and Mo.     

Adsorption and reaction of bromine with Ag(110)

The adsorption and reaction of Br₂ with Ag(110) was studied with Auger electron spectroscopy, LEED, work function measurements and thermal desorption spectroscopy in the temperature range of 130–1000 K. Depending on Br coverage and crystal temperature, four different adsorption and reaction states could be detected. For fractional monolayer coverages, chemisorbed Br(ad) is found to be the most stable species. This adsorption state saturates for θ(Br) ? 0.75. In the chemisorption stage, two LEED patterns, a p(2 × 1) with θ(Br) ? 0.5 and a c(4 × 2) with θ(Br) ? 0.75, were observed. For higher Br₂ exposures and T = 130 K a layer-by-layer growth of AgBr is detected. At higher temperature, T > 190 K, there is evidence for a transformation from a 2D growth mechanism of AgBr into a 3D agglomeration of larger AgBr cluster. Molecularly adsorbed.     

Numerical simulation of water vapor nucleation on electrically neutral nanoparticles

Atomic-level Monte Carlo simulations are performed to calculate the free energy, entropy, and work of nucleation for clusters of more than 6 × 10³ water molecules growing on silver iodide crystalline particles of size up to 4 nm at a temperature of 260 K. The Hamiltonian of the system includes explicit expressions for hydrogen bonding energy and Coulomb, dispersion, exchange, and polarization interactions. The work of nucleation exhibits complex behavior depending on the nucleation-site size. With increasing nanoparticle size, clusters become less stable and the probability of crystallization increases. Mutual polarization enhances the bonding between a cluster and a crystalline particle. Cluster growth on relatively large nanoparticles involves two stages characterized by two critical sizes: monolayer growth on the surface and growth normal to the surface. Spontaneous microdroplet polarization involving domain formation is found to occur at the crystal surface. The dependence of the ice-forming activity of an aerosol on particulate size observed in experiments is explained by combined effects of several competing factors, the dominant ones being the stabilizing and destabilizing effects of the nanoparticle electric field.     

Surface structural transition of adsorption of oxygen on Ag(100)

High resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED) were used to study the oxygen adsorption on Ag(100). An ordered c(2 × 2) superstructure occurs after low temperature adsorption, in which the stretching mode at 37 meV was observed. The energy loss at 30 meV is attributed to the ordered p(1 × 1) structure after the adsorption at room temperature. The structure transition from the c(2 × 2) to the p(1 × 1) has been observed when heating the adsorbed surface from low temperature to room temperature. Cooling of the adsorbed surface at room temperature down to 180 K results in the surface transition from the p(1 × 1) structure to the coexistence of the p(1 × 1) and c(2 × 2) structures.     

Examination of the distribution of the tensile deformation systems in tension and tension-creep of Ti-6Al-4V (wt.%) at 296 K and 728 K

The deformation behaviour of an α + β Ti–6Al–4V (wt.%) alloy was investigated during in situ deformation inside a scanning electron microscopy (SEM). Tensile experiments were performed at 296 and 728 K (~0.4Tm), while a tensile-creep experiment was performed at 728 K and 310 MPa (σ/σ_ys = 0.74). The active deformation systems were identified using electron backscattered diffraction-based slip-trace analysis and SEM images of the specimen surface. The distribution of the active deformation systems varied as a function of temperature. Basal slip deformation played a major role in the tensile deformation behaviour, and the relative activity of basal slip increased with increasing temperature. For the 296 K tension deformation, basal slip was less active than prismatic slip, whereas this was reversed at 728 K. Twinning was observed in both the 296 and 728 K tension experiments; however, no more than 4% of the total deformation systems observed was twins. The tension-creep experiment revealed no slip traces, however grain boundary ledge formation was observed, suggesting that grain boundary sliding was an active deformation mechanism. The results of this work were compared with those from previous studies on commercially pure Ti, Ti–5Al–2.5Sn (wt.%) and Ti–8Al–1Mo–1V (wt.%), and the effects of alloying on the deformation behaviour are discussed. The relative amount of basal slip activity increased with increasing Al content.     

Structural and dc conduction studies on PbTe thin films

Lead Telluride (PbTe) films of different thickness were prepared onto precleaned glass substrates under the pressure of 2?×?10^?5 Torr by thermal evaporation. X-ray diffraction technique, scanning electron microscopy, and current–voltage characteristics were used to characterize the films. The structural analysis of the films was carried using X-ray diffractometer. The surface morphology was analyzed by using scanning electron microscope. The dc electrical conduction mechanism in vacuum-evaporated Al/PbTe/Al thin film sandwich system in the thickness range 500–5,000 Å at different temperature (303–483 K) was found to be a modified Poole–Frenkel type. The results of variation of activation energy with applied voltage and thickness are discussed.     

The oxidation of Fe(111)

The oxidation of Fe(111) was studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS) and scanning tunnelling microscopy (STM). Oxidation of the crystal was found to be a very fast process, even at 200 K, and the Auger O signal saturation level is reached within ~ 50 × 10^? 6 mbar s. Annealing the oxidised surface at 773 K causes a significant decline in apparent surface oxygen concentration and produces a clear (6 × 6) LEED pattern, whereas after oxidation at ambient temperature no pattern was observed. STM results indicate that the oxygen signal was reduced due to the nucleation of large, but sparsely distributed oxide islands, leaving mainly the smooth (6 × 6) structure between the islands. The reactivity of the (6 × 6) layer towards methanol was investigated using temperature programmed desorption (TPD), which showed mainly decomposition to CO and CO₂, due to the production of formate intermediates on the surface. Interestingly, this removes the (6 × 6) structure by reduction, but it can be reformed from the sink of oxygen present in the large oxide islands simply by annealing at 773 K for a few minutes. The (6 × 6) appears to be a relatively stable, pseudo-oxide phase, that may be useful as a model oxide surface.     

The segregation of carbon to the (110) surface of a Fe-10at% Si single crystal

The kinetics of the surface segregation of carbon to the (110) surface of a Fe-10at%Si single crystal was studied in the temperature range 350–430°C by Auger electron spectroscopy. The results could not be explained by diffusion models. However, it was found that the rate determining process was graphite island growth on the surface of the crystal to attain a maximum coverage of 40%. The activation energy for the process is (230±20) kJ/mol and the hexagonal constant for the graphite overlayer is 0.245±0.002 nm. These results are in contrast with segregation results obtained for a Fe-6at%Si single crystal where bulk diffusion was the rate limiting process and a maximum carbon coverage of 12% was observed.