Adsorption of tungsten on stepped tungsten surfaces studied by work function measurement

Work function measurements have been performed during the deposition of W on the (110)W plane and several stepped W surfaces with (110) terraces and different terrace width. For each sample the work function decreases with growing coverage. The total work function drop diminishes strongly with decreasing terrace width. The results are interpreted in terms of a reduced nucleation process on stepped surfaces as compared to the flat (110) plane. The step edges act as sinks for the deposited adatonis and cause in their proximity a “dead” zone for nuclei formation. Details of the work function change with coverage are discussed in terms of an edge roughening effect on stepped surfaces.     

Diffusion and desorption of barium and oxygen on tungsten dispenser cathode surfaces

Auger measurements have been made on the concentration of barium and oxygen diffusing out of a pore and slot onto the surface of a simulated tungsten dispenser cathode. Profiles of concentration vs distance from the source were obtained at different temperatures. It is found that at cathode operating temperatures very little gradient of oxygen concentration exists on the surface, and under steady state conditions Ba is the main diffusing species. Ba diffusion distances derived are consistent with previous measurements. However it is found that this distance increases with concentration in spite of the fact that the Ba surface lifetime decreases with concentration. Time dependent measurements made on a clean surface show that the rate of Ba and O build up is limited by the supply rate of oxygen to the surface. This supply rate is not diffusion limited but seems to be limited by the mechanism generating free oxygen. A comparison of emission microscope measurements on a tungsten matrix dispenser cathode with the simulated cathode data indicates that similar oxygen generation processes may be controlling the activation of these cathodes.     

Work function of stepped tungsten single crystal surfaces

The work function of regularly stepped tungsten single crystal planes has been investigated as function of terrace width and step orientation. The terraces were always formed by (110) planes. One set of samples exhibited step edges running parallel to the [001] direction but different terrace widths. Another set with almost equal terrace width showed step edges of different directions with correspondingly different edge structures. The work function measurements were performed using thermionic emission in the temperature range 2000–2800 K. The work function decreases linearly with step density for a given step orientation. Different step orientations give rise to slightly different work function reductions. The results are interpreted by attributing additional dipole moments to edge atoms. Different edge structures lead to different dipole moments. These findings are consistent with Smoluchowski's “smoothing effect” of the electron charge distribution caused by the structural arrangement of surface atoms. The temperature coefficient of the work function also depends strongly on step density and step orientation.     

Adsorption of transition metal monolayers on tungsten surfaces

In a simple self-consistent band model, we calculate the work function change when one adsorbs monolayers of transition metals on tungsten. The work function change with the coverage is in agreement with the experimental results. It shows a maximum for one monolayer and is nearly equal to the difference between the work functions of W and the adsorbed element for a coverage equal to a few monolayers. In all the cases, the charge transfer between the adatoms and the tungsten surface atoms are small. This point allows us to simply calculate the binding energies of the monolayer. The same model is also applied to discuss the diffusion of a monolayer into a bulk transition metal.     

Thermal desorption of silicon from polycrystalline tungsten surfaces

The adsorption of Si on polycrystalline tungsten surfaces was studied for the first time by means of thermal desorption. Instead of the main peak of Si ($\text{m}\text{e}\text{=28}$), the isotope ($\text{m}\text{e}\text{=30}$) contained naturally (～3%) was monitored by mass spectrometry. This method can reduce the contribution from CO to the mass signal of Si. Two desorption peaks were observed at 1480 and 1820 K. The activation energies for the desorption were estimated to be about 90 and 110 kcal/mol, respectively.     

Atomic imaging of oxygen desorption from tungsten trioxide

We report direct observations by high-resolution electron microscopy of oxygen desorption from tungsten trioxide. Clear evidence is found for layer epitaxial growth of metallic tungsten with (110)W|(100)WO₃ and [001]W and [001]WO₃ parallel to the electron beam, consistent with low-energy electron diffraction data on the low-temperature epitaxy of WO₃ on W. W[001] was always observed parallel to the electron beam independent of the surface normal. The results suggest that bulk and surface damage, and displacement processes, are similar.     

Lithium adsorption on hot oxygen covered tungsten surfaces

The mean residence times τ of lithium particles on oxygen covered tungsten surfaces were measured accurately over a wide temperature range (1200 < T < 1900 K) by the beam modulation technique with a lock-in analyzer. A predominant monocrystalline W(100) structure was obtained by recrystallization of a polycrystalline tungsten ribbon. The residence time was determined as a function of the oxygen coverage θ and the temperatureT of the surface. The desorption energy l and the preexponential factor τ₀, calculated from the Arrhenius equation, are not only dependent on the amount of adsorbed oxygen but also on the oxygen structure. Apparently the desorption parameters l and τ₀ are correlated. An increasing desorption energy l is connected to a decreasing “vibration period” τ₀ whereby the influence on the residence time is partly compensated.     

Thermal desorption of metals from tungsten single crystal surfaces

After a short review of the experimental methods used to determine desorption energies E and frequencies v the assumptions underlying the theoretical analysis of the experimental data are discussed. Then recent experimental results on the flash desorption of Cu, Ag and Au from clean, well characterized W {110} and {100} surfaces are presented and analysed in detail, in particular with respect to the coverage dependence. The results obtained this way clearly reveal the limitations of previous analysis methods and of the experimental technique per se (such as structure and phase changes below and in the temperature region in which desorption occurs). Furthermore the need for more theoretical work to understand the large changes of v and E with coverage and the so-called “compensation effect”, i.e. the relation between In v and E, becomes evident.     

Adsorption of single alkali atoms on tungsten using field emission and field desorption

Single adatom events have been detected and the binding energies and dipole moments of single sodium, potassium and cesium adatoms have been measured on the (110), (112) and (111) planes of tungsten in a probe hole field emission microscope. The Fowler-Nordheim formulation has been modified to include averaging effects implicit in probe hole measurements on single adsorbed atoms. The work function changes and consequent dipole moments associated with single alkali adatoms on a tungsten surface have been estimated. A model has been proposed to obtain binding energies from measurements of the field required to desorb a single alkali adatom. The results are in good agreement with current theoretical predictions for the adsorption of single alkali atoms on metals.     

Adsorption,diffusion and self-diffusion on tungsten surfaces with adsorbed palladium

The influence of a Pd adsorption layer on the surface self-diffusion of tungsten has been measured using a field emitter technique. By adsorption of 1.2 × 10¹⁵ Pd atoms/cm² — measured by high energy ion backscattering — the surface self-diffusion energy decreases from 70 to 54 kcal/mole, and the diffusivity from 0.95 to about 0.2 cm²/s, which corresponds to an increase of the diffusion coefficient by a factor 40 at 1600 K and 80 at 1400 K. Furthermore are determined (1) adsorption lifetimes in adsorption-desorption equilibria, (2) the isosteric adsorption heat (103 kcal/mole), and (3) the coefficient of Pd surface diffusion on W resulting in diffusion energies of 32 kcal/mole around (001) and 17 kcal/mole around (111).     

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.     

Adsorption on stepped substrates

An analysis is given of the behavior of an interface above a stepped substrate in the presence of an external pinning potential for the lattice solid-on-solid (SOS) interface model in 2D. Step-free energy including step-step interaction free energy is calculated, for large step separation. It is found it vanishes at temperatures lower thanT _w (wetting transition temperature), which is different from the case having only one step on a substrate where, as it is well known, step-free energy vanishes at the wetting transition.     

Relaxation of tungsten adatoms on tungsten surfaces

The relaxation energy of a tungsten atom on tungsten surfaces is estimated to be about 1 eV. It seems difficult to neglect the variation of that energy in the interaction between adatoms.     

LEED studies on stepped W surfaces

Steps on single crystal surfaces have been shown to cause changes of various physical properties and to influence the behaviour towards chemical reactions. A proper knowledge of the step structure is required for the understanding of these phenomena. The following investigation concentrates on a detailed evaluation of the LEED patterns of various stepped tungsten surfaces. A formula is given for determining the terrace width of ordered step arrays from any diffraction order. Step height, step orientation, terrace width and the sample orientation have been deduced from the LEED patterns and the experimental errors involved are being discussed. The step height can be determined within 1% and the terrace width within 1 to 3% depending on the step density. It is concluded that the determination of the sample orientation as obtained from the LEED pattern is at least as precise as by using the Laue X-ray back reflection technique.     

Structural phase transitions on stepped surfaces

We predict a new class of structural phase transitions which occur on a microscopic length scale on stepped surfaces. These include commensurate and incommensurate “step-step” transitions. They can occur either on clean equilibrium surfaces or can be adsorbate induced. We also present a Landau-Lifshitz classification of some of these structural phase transitions. Critical exponents characterizing these transitions can be experimentally determined by LEED measurements.     

Segregation at stepped surfaces

A theoretical model of segregation at a (120) stepped surface with ($\text{1}$10) monoatomic steps and terraces on the (110) plane of a body centered cubic alloy is presented. The model is based on pairwise interactions between nearest neighbours only and includes long and short range order effects. Results are presented for the concentration at different sites on the stepped surface as well as for the short range order parameters as functions of temperature.     

Adsorption of oxygen on low-index surfaces of Ti3Al alloy

Physics of the Solid State - The atomic and electronic structure of the three surfaces of Ti3Al alloy—(0001), ( $$1bar 100$$ ), and ( $$11bar 20$$ )—is calculated by the projector...     

Adsorption of oxygen on clean gaas surfaces investigated by ultraviolet photoemission

The adsorption of oxygen at exposures of up to 10⁵ L on differently oriented, ion-bombarded and annealed GaAs surfaces was investigated by UPS. Coverages θ_As for the clean surfaces and oxygen coverages for the oxygen exposed surfaces were estimated by additional SXPS measurements. It was concluded that at small exposures molecular and atomic adsorption are comparable in quantity and that atomic adsorption (oxidation) becomes maximum at θ_As, ≈ 0.2 for (111)Ga and (001) surfaces. Bonding of oxygen molecules should involve Ga sites. Specific bonding of oxygen atoms (O-Ga or O-As) was not indicated by the two stable UPS peaks as they occurred for arsenic coverages from 0 to 0.5 and did not shift their energy positions. They simply indicate the two states of the adsorbate atoms, single and double bonds, to substrate atoms. For the surfaces prepared here, monolayer coverage by oxygen was obtained at about 10¹⁰L. Likewise adsorption of H₂O was investigated.