Tungsten Single Crystal Surfaces Research Articles

Photoemission and RHEED study of the supported Pt and Au epitaxial alloy clusters

In this paper, we investigated formation and structure of gold, platinum and Pt–Au bimetallic clusters on tungsten oxide surface. Crystallographic structure and orientation were investigated by reflection high energy electron diffraction, chemical composition and cluster's electronic state by synchrotron radiation photoelectron spectroscopy. Small amount of Au and Pt was deposited on WO3 (111) surface prepared by RF plasma oxidation and thermal annealing of (110) tungsten single crystal surface. Formation of 3D Pt or Au clusters with (111) epitaxial plane was observed. On the contrary to Au, strong metal substrate interaction between Pt and oxide was found, leading to the encapsulation of Pt clusters. Formation of Au–Pt bimetallic clusters was investigated at the temperature range of 160–870K. It was found that cluster structure depends on the substrate temperature and order of materials deposition. At the temperatures around 570K, metals formed bimetallic clusters with core–shell structure. Annealing to the temperatures over 770K led to the decomposition of Pt–Au clusters and formation of Pt–W alloy.

Plasma flow interaction with ITER divertor related surfaces

It has been found that the plasma flow generated by quasistationary plasma accelerators can be used for simulation of high energy plasma interaction with different materials of interest for fusion experiments. It is especially important for the studies of the processes such as ELMs (edge localized modes), plasma disruptions and VDEs (vertical displacement events), during which a significant part of the confined hot plasma is lost from the core to the SOL (scrape off layer) enveloping the core region. Experiments using plasma guns have been used to assess erosion from disruptions and ELMs. Namely, in this experiment modification of different targets, like tungsten, molybdenum, CFC and silicon single crystal surface by the action of hydrogen and nitrogen quasistationary compression plasma flow (CPF) generated by magnetoplasma compressor (MPC) has been studied. MPC plasma flow with standard parameters (1 MJ/m2 in 0.1 ms) can be used for simulation of transient peak thermal loads during Type I ELMs and disruptions. Analysis of the targets erosion, brittle destruction, melting processes, and dust formation has been performed. These surface phenomena are results of specific conditions during CPF interaction with target surface. The investigations are related to the fundamental aspects of high energy plasma flow interaction with different material of interest for fusion. One of the purposes is a study of competition between melting and cleavage of treated solid surface. The other is investigation of plasma interaction with first wall and divertor component materials related to the ITER experiment.

Production of a 10 eV–1000 eV neutral particle beam using surface neutralization

For the calibration of low-energy neutral particle mass spectrometers a neutralization stage for a charged-particle beam was built. The incident positive ion beam is neutralized upon reflection at grazing incidence off a highly polished tungsten single crystal surface. With protons as primary species the device produced a neutral hydrogen beam in the energy range of 10 eV–1000 eV depending on incident ion-beam energy with a centre energy of 85% to 89% and an energy width of 11% to 15% FWHM of the incident ion energy. The angular spread of the neutral beam was limited by apertures to 4.6° × 18° FWHM. The overall transmission of the device was found to be between 0.005 and 0.013.

The energy of fracture of a film of carbon monoxide adsorbed on a tungsten single crystal surface

The adsorption and desorption of carbon monoxide were studied on a tungsten single crystal in a field electron microscope. The most interesting effects were observed for the CO desorption from the crystal surface regions containing the {100} W cubic faces. In the course of desorption from these regions, the adsorbed electropositive film, retaining sharp boundaries, shrank toward the poles. The desorption energy value (3.4±0.1 eV), determined using the Arrhenius plots for the boundary velocity, is indicative of a considerable strength of the adsorbed CO film. Behavior of the CO phase desorbing from the {100} W regions can be consistently explained by assuming that this phase comprises a network of adsorbed CO dipoles oriented perpendicularly to the crystal surface, with the carbon atoms facing vacuum.

LEED and XPS studies of [formula omitted] surface in comparison with W(110) surface

Because the ZrO W(100) surface has a remarkably low work function, it has been widely applied to thermal field emitters for electron microscopes. Nevertheless, it seems that the lowering mechanism of the low work function has not been clearly understood. Especially, the question “why the lowering occurs only on the (100) surfaces” is left without answer. In situ analyses of LEED, XPS and work function measurement have been systematically performed under an intention to understand the lowering mechanism of the work function. In this study, a comparative analyses of the behaviours between (100) and (110) tungsten surfaces, due to the deposition of zirconium and the following heat treatment in oxygen ambient, will be given. Several monolayers of Zr were vacuum deposited on clean tungsten single crystal surfaces, followed by heat treatment in an oxygen ambient. In the case of the (100) surface, the work function once increased to 5.5 eV and dramatically decreased to 2.8 eV by the following heat treatment in UHV. The heat treatment in oxygen ambient once shifted the XPS peak for Zr3d 5 2 to a high binding energy and the following heat treatment in UHV shifted back the peak to an intermediate position. In the case of the (110) surface, the XPS peak for Zr showed almost the same behaviour as the (100) surface, but the work function decreased hardly. The results of a LEED study will be given also.

Low-energy electron loss spectroscopy of Eu, Gd, and Tb: The valence region

The energy losses of slow electrons reflected from Eu, Gd and Tb layers on tungsten single-crystal surfaces are studied as a function of adsorbate concentration and primary energy in order to understand the complex loss spectra of rare earths.

The interaction of gold with the surface of a cylindrical tungsten single crystal

Gold layers on the surface of a cylindrical tungsten single crystal were investigated by LEED and work function change measurements. Work function changes during gold adsorption on (110), (111), (112), (115) and (001) faces, and structures deduced from LEED patterns for (110), (112), (332) and (001) faces are presented with a common coverage scale. Upon heating the gold-covered crystal up to 1000 K, faceting occurs with faceted regions as large as 20°, 13° and 4° around the [112], [110] and [332] directions, respectively. The diffusion of the gold layer was studied by the movement of the sharp boundary across the cylindrical crystal surface at a temperature of 1000 K. The diffusion rate was found to be much higher in the (112) region than in the (001) region.

The interaction of gold with the surface of a cylindrical tungsten single crystal

The interaction of gold with the surface of a cylindrical tungsten single crystal

The structure of ultrathin iron films on tungsten single-crystal surfaces

Ultrathin iron films vapour deposited onto the surface of a cylindrical tungsten single crystal are discussed. Results from work function change, Auger electron spectroscopic and low energy electron diffraction techniques are combined for a comparison of the initial stages of film growth on four low index planes. Advantage is taken of the opportunity to evaporate onto and simultaneously to make measurements on all surface orientations of the 〈110〉 zone.

The interaction of KCl with tungsten single crystal surfaces

The interaction of KCl with tungsten single crystal surfaces

The interaction of KCl with tungsten single crystal surfaces

The interaction of KCl vapor with W single crystal surfaces is studied with the aim of obtaining information on the adsorption of ionically bonded molecules. The atomically smoothest and roughest surfaces, (110) and (111), are used. The adsorbate is characterized by LEED, thermal desorption spectroscopy (TDS) and by photoemission experiments which can be related to known work function data. It is found that the adsorbate is undissociated on W(110) and to a large extent also on W(111) at room temperature and that no ordered structures are formed. Electron bombardment causes KCl dissociation and Cl desorption which is complete with sufficiently high electron doses. Close-packed K monolayers can be formed this way.

Chemisorption of oxygen on tungsten (100): combined molecular beam and scanning electron beam study

A molecular beam technique has been used to deposit accurately controlled patches of oxygen onto the (100) surface of a tungsten single crystal, rigorously cleaned under ultra-high vacuum conditions, and a detailed examination of the spatial behaviour of these patches was made during thermal treatment using a low-resolution scanning electron beam to probe the surface. It is estimated that as little as 0.01 of a monolayer of oxygen can be detected with the electron probe. The work function was studied as a function of thermal treatment for various preadsorbed amounts and a useful correlation made with a model for oxygen adsorption on tungsten developed from flash desorption studies. The scanning electron probe revealed a novel one-dimensional boundary effect during thermal treatment of the crystal after oxygen preadsorption.

LEED Study of Halogen Adsorption on Tungsten-Two Dimensional Shear Structures

The dissociative chemisorption of iodine and bromine on tungsten single crystal surfaces has been studied by LEED. Iodine adsorption on W(110) produced long chain structures which could be divided into three families of phases (β1, β2 and β3) distinguishable by the arrangement of atoms within the chain. Continuous changes in coverage occurred by sheets of these wellordered chain structures shearing relative to each other to produce packing faults at the resulting shear line. Although bromine on W(110) produced two comparable chain structures, shearing did not occur with changing coverage. Iodine on W(100) failed to produce either chain or shear structures. The lateral adatom interactions responsible for the stability of these structures is discussed.

Angular dependence of clusters sputtered from a tungsten single crystal surface

Abstract The anisotropy of positively charged clusters sputtered with 150 keV Ar+ ions from a tungsten single crystal has been investigated with a mass and energy spectrometer. Not only the monomer W+ but also the clusters W+ 2, W+ 3 and W+ 4 (so far investigated) show maxima of emission along close packed directions. The maximum cluster yield along a <111> direction normalized to the yield along a random direction increases with increasing cluster size and cluster energy, although the absolute cluster yield decreases. An interpretation of the experimental results on the basis of a simple statistical model of cluster formation kinetics is presented which explains the yield profiles of the anisotropic contribution fairly well.

Chemisorption of CO on tungsten (100): combined flash desorption and electron stimulated desorption study. I

The chemisorption of CO on a tungsten (100) single crystal surface has been studied at adsorption temperatures of 295 K and 195 K. A combination of flash desorption and electron stimulated desorption (ESD) techniques has been used. In CO layers produced at 295 K, the well-known α-CO state has been shown to be a superposition of two states, designated α 1 and α 2. The α 1-CO state exists preferentially at higher α-CO coverages and is characterized by a predominant ion yield of CO + upon 100 eV electron bombardment. The α 2-CO state yields O +, and consists of a number of overlapping states possessing a range of desorption activation energies. During thermal desorption of α-CO, a stoichiometric state interconversion process occurs in which α 1-CO converts to more strongly-held α 2-CO as surface coverage decreases. Similar conversion behavior is observed when the surface layer is depopulated by ESD. Adsorption of CO at 195 K resembles adsorption at 295 K, except that a small additional α-CO state, designated α 0, and yielding CO + on ESD, is populated. Structural models are presented for α 1-CO and α 2-CO, and the role of surface species' structure in determining ionic fragmentation probabilities is discussed.

Rheed studies of the initial stages of the oxidation of tungsten

Reflection mode high energy electron diffraction (RHEED) has been used in an ultra high vacuum environment to study the initial stages of the oxidation of (100) and (110) tungsten single crystal surfaces. On exposing the (110) tungsten surface to oxygen at temperatures below 1025°K, nuclei of tungsten trioxide grow. The epitaxial relationships are metal (110)//oxide (11 1 ) and metal [001]// oxide [1 1 0]. The nuclei may be removed by heating the surface in a good vacuum to 100 °K. The growth of nuclei on the (100) tungsten surface is complicated by the faceting of that surface. On unfaceted surfaces tungsten trioxide nuclei form after small exposures of oxygen, but again only at temperatures below 1025 °K. On faceted surfaces much larger doses of oxygen are required. In both cases the epitaxial relationships are metal (100)//oxide {100} ± 9° and metal [011]//oxide <100> ± 7.5°. It is shown that the (100) W/oxide system can be described in terms of the metal (110)/oxide system.

The work function variation during caesium deposition on the (100) and (110) single crystal surfaces of tungsten

An improved vibrating capacitor technique has been used to follow work function changes during the adsorption of caesium on (100) and (110) oriented tungsten single crystal surfaces at 300 K. In the limit of zero coverage the dipole moments of Cs-W(100) and Cs-W(110) were respectively 15.8 ± 0.8 Debyedouble dagger and 19.9 ± 1 Debye. The minimum and full coverage work functions were respectively 1.795 ± 0.01 eV and 1.82 ± 0.01 eV for the (100) face and 1.975 ± 0.01 eV and 2.07 ± 0.01 eV for the (110) face. Excellent agreement exists between these results and those of Fehrs from retarding field experiments but comparison with data from field emission experiments (Swanson et al., Gavrilyuk et al.) is only qualitative.

Work function changes due to the adsorption of chlorine, bromine and iodine on tungsten single crystal surfaces

The adsorption of chlorine, bromine and iodine onto large oriented tungsten single crystals has been studied using the Kelvin contact potential difference technique. Work function changes have been determined for the (110), (100) and (111) orientations as a function of the number of molecules incident on each surface. The surface potentials for atomic adsorption were, for chlorine, +0.26, −0.58 and −1.03 V respectively, for bromine + 0.32, −0.80 and −0.88V and for iodine +0,78, +0.18 and +0.13 V. In some cases adsorption at higher coverages revealed the presence of weakly bound phases which invariably caused a decrease in the surface potential. A qualitative interpretation of the results for atomic adsorption is discussed in terms of a simple dipole model in which three contributions to the surface potential are considered. These arise from charge transfer between the adsorbate and the tungsten surface, polarization of the adsorbed halogen in the metal's external field and the smoothing of the electronic charge distribution in the adsorbed film. With such a simple model it is possible to explain qualitatively the variation in magnitude of the dipole and its reversal between different crystal planes for the same adsorbate.

A study of facetting of tungsten single crystal surfaces

Thermal facetting of the (100), (112), and (111) surfaces of tungsten to {110} planes is observed in the presence of oxygen at pressures between 10 −8 and 10 −6 Torr using a vibrating capacitor work function probe in conjunction with low energy electron diffraction (LEED). The (110) surface is stable against facetting under the same conditions. The work function of the facetted surfaces for the three planes indicated is the same as that of an oxygen covered (110) surface and is equal to 6.45 eV. A (112) surface containing periodic {110} facets only 20 Å wide has the same 6.45 eV work function as an oxygen covered (110) plane. A diffraction analysis is presented which allows the determination of the indices of the facet planes from the voltages at which very intense primary maxima occur and the symmetry of the pattern at intermediate voltages. For simple facet planes, the primary maxima correspond to bulk Laue reflections. The kinetics of facet growth on the (112) surface are characterized by a low temperature W-O coupled diffusion controlled region and a high temperature region controlled by oxygen supply to dissociation sites.

1518. Investigation of sodium adsorption on the surface of tungsten single crystal, in an electron field emission microscope: E V Klimenko and V K Medvedev,Fiz Tverd Tela, 10 (7), July 1968, 1986–1990 ( in Russian)

1518. Investigation of sodium adsorption on the surface of tungsten single crystal, in an electron field emission microscope: E V Klimenko and V K Medvedev,Fiz Tverd Tela, 10 (7), July 1968, 1986–1990 ( in Russian)