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.     

Adsorption of lead on low-index planes of tungsten

Using probe-hole field emission microscopy the effect of adsorbed lead on the work function of the 100 and 110 planes of tungsten hasbeen studied and compared with the findings of Bauer et al. who studied the same system using LEED/Auger techniques. The effect of lead on the average work function $\text{}\text{?}$gf and that of (211) is also reported. Sub-monolayer lead increases φ(211) and this is ascribed to formation of a lead-tungsten dipole, the lead being negatively charged, with dipole moment 0.035 × 10^?30 C-m and polarizability 2.0 ų. On (110) lead reduces φ and behaves as a dipole with positively charged lead of moment 0.15 × 10^?30 C-m and polarizability 2.5 ų. φ(100) is also observed to decrease at low coverages equilibrated at low temperatures. This contrasts with Bauer's findings but is considered to result from failure of the Fowler—Nordheim model. With increasing lead coverage on all planes φ(hkl) tends to a constant value φ_sat. By comparison with Bauer et al. we can identify φ_sat on (110) as a compressed monolayer of lead. Likewise φ_sat produced by low temperature (～450 K) spreading on (100) is also associated with a compressed (1 × 1) structure. The lower value of φ(100) produced at higher temperatures (～850 K) is identified with the microfacetted surface observed by Bauer et al. Lead is observed to be absent from (110) when mean adatom densities as high as 8 × 10¹⁴ atoms cm^?2 are thermally equilibrated, and this is shown to result from the relatively low binding energy of lead on (110). The general agreement between the present findings and those of Bauer lends confidence to the belief that both techniques can detect the same behaviour despite the very large (10¹⁰) difference in the size of the area examined.     

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.     

Metal-support interactions in systems palladium deposited on oxidized tungsten surfaces

The morphology of the palladium (Pd) overlayers on oxidized tungsten (W) tips has been studied by Field Emission Microscopy (FEM). The effect of thermal treatment on the interaction of Pd with the support and chemisorption of CO on variously treated Pd-containing samples has been investigated. The results are discussed in relation to complementary macroscopic experiments by synchrotron radiation excited photoelectron spectroscopy (SRPES) and thermally programmed desorption (TPD) of carbon monoxide (CO) on a polycrystalline W foil. A distinct influence of support pre-oxidation on the Pd layer growth has been demonstrated. Two types of oxidized supports have been used: tungsten with oxygen pre-adsorbed at room temperature (RT) and then heated to 700 K (WO_x/W (RT) system) and tungsten oxidized at 1300 K (WO_x/W (1300 K) system) in situ. The surface of WO_x/W (1300 K) sample is fully oxidized in contrast to WO_x/W (RT), where the presence of un-oxidized patches has been demonstrated by SRPES measurements. A Pd layer grows on the WO_x/W (RT) surface mostly on the densely populated planes (1 1 0) and (2 1 1) of the W tip. Heating of this system up to 700 K results in disaggregation of the original Pd layer. Pd clusters on the tungsten tip oxidized at 1300 K are localized on the atomically rough (1 1 1) plane. The observed differences in CO adsorption on the aforementioned types of investigated samples can be attributed to differences in the chemical nature of their surfaces.     

Adsorption of copper on single crystal planes of tungsten

Properties of copper condensed on selected areas of a thermally cleaned tungsten surface have been studied by probe hole field emission microscopy. Interpretation of the φ_hkl?$\text{}\text{?}$gq relationship observed on (211), (111) and (310) for adsorption at T > 300 K is based on hardsphere models of the plane surfaces upon which the first monolayer of copper raises φ_ithkl, the second reduces it to a minimum value, and the third achieves $\text{}\text{?}$gf_sat on each plane. At T > 300 K the relatively low binding energy of copper on (110) prevents is population below ～2$\text{}\text{?}$gq as previously observed for lead, and the plateaux in the φ₁₁₀?$\text{}\text{?}$gq curves are thought to result from the difficulty of nucleating the first and second monolayers of copper on (110). Comparison of our observations with those made by LEED/Auger techniques emphasise significant differences between the substrates used, in that, on field emitters (110) is step-free and surrounded by a sink/ source of adatoms, while the LEED specimen is stepped but has no comparable local sink/ source. The initial changes in φ are ascribed to formation of an adsorbate-substrate dipole whose sign and magnitude is controlled by electron equilibration between the substrate metal and a broadened and partly-filled resonance level lying approximately 5.2 eV below the vacuum level. Measurement of the total energy distribution of electrons field emitted from (110) and (132) supports this picture which contrasts with that of Polanski and Sidorski who consider the dipole sign and strength to be controlled principally by adsite geometry. Low activation energies characterise surface transport which is controlled by one-plane processes, and in some cases transport across the probed area is controlled by processes of relatively high activation energy which take place outside the examined plane.     

Adsorption of carbon monoxide on tungsten (210)

Thermal desorption spectra taken after adsorption of carbon monoxide at room temperature on W(210) show sequential formation with increasing coverage of strongly bound β₂ and β₁ binding states, correlated to the sequential formation of P(2 × 1) and (1 × 1) adsorbate structures as observed by LEED. Adsorption at room temperature gives a poorly ordered arrangement of adsorbed CO molecules, but well-ordered structures are produced by subsequent anneal. For adsorption without anneal the work function increases monotonically with coverage to a maximum of Δφ = + 0.70 eV at saturation coverage of 1 monolayer. For adsorption followed by anneal the work function dependence upon coverage is less simple, with even a decrease of work function at coverages less than a quarter monolayer. LEED intensity-voltage measurements from P(2 × 1)CO and P(2 × 1)N structures suggest that CO molecules occupy the sites of 4-fold symmetry upon which nitrogen is believed to be adsorbed. The distinction between the β₂ and β₁ states of adsorbed CO is attributed to heterogeneity induced by the reduction in binding energy of a CO molecule when its nearest-neighbor sites are occupied.     

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.     

Electron-stimulated desorption of europium atoms from an oxidized tungsten surface

The yield of europium atoms in electron-stimulated desorption from Eu layers adsorbed on the surface of oxidized tungsten was studied with a surface-ionization detector as a function of the incident-electron energy, surface coverage by europium, and degree of tungsten oxidation. The yield of Eu atoms measured as a function of electron energy exhibits a distinct resonant character with peaks at electron energies corresponding to europium and tungsten core-level ionization energies. The peaks associated with the europium ionization reach a maximum intensity at europium coverages less than 0.1 and decrease subsequently to zero with increasing coverage, while the peaks due to tungsten ionization pass through the maximum intensity at a monolayer europium coverage. The coverage corresponding to the maximum europium atom yield grows with increasing tungsten oxidation. The results obtained are accounted for by the formation of the europium and tungsten core excitons. In the first case, the particles desorb in the reverse motion toward the surface of the oxidized tungsten; in the second, they desorb as a result of repulsion between the tungsten core exciton and the EuO molecule.     

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.     

Adsorption of silane and methylsilane on gold surfaces

The adsorption of silane and methylsilane on the (1 1 0) and polycrystalline surfaces of gold is examined using vibrational electron energy loss spectroscopy (VEELS), angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and X-ray photoelectron spectroscopy (XPS). Adsorption of silane onto the Au(1 1 0) surface at low temperatures is dissociative and yields an SiH₂ and possibly also SiH₃ surface species. Further dissociation occurs at room temperature to yield adsorbed SiH, which is tilted on the surface, with complete dissociation to Si occurring by 110 °C. The similarity in the UP spectra for silane adsorbed on the polycrystalline sample suggests that the same surface species are present over that temperature range. Above 200 °C, spectral changes suggest rearrangement of the Si atoms, which, by 350 °C, have diffused into the bulk. Adsorption of methylsilane onto the (1 1 0) surface at low temperatures initially produces adsorbed CH₃SiH or CH₃SiH₂, with undissociated methylsilane physisorbing at higher exposures. By room temperature, desorption and decomposition leaves (or direct adsorption yields) only adsorbed CH₃Si. After further heating, the hydrogen-carbon bonds of the CH₃ group break to leave an adsorbed SiC species. On the polycrystalline surface, methylsilane adsorption is the same at low temperatures as on (1 1 0). In contrast to the latter, though, the UP spectra indicate that direct exposures at room temperature yield adsorbed Si or SiC initially, with CH₃Si again adsorbing at higher exposures. Upon further heating to 330 °C, little if any methyl-groups remain on the surface and the Si has started to diffuse into the bulk.     

Adsorption of polyelectrolytes on silica and gold surfaces

The results of a study that helps understand the mechanisms of adsorption of polyelectrolytes on particles, using numerical simulation methods, specifically the one known as dissipative particle dynamics are reported here. The adsorption of cationic polyelectrolytes of two different polymerisation degrees interacting with two types of surfaces, one made of gold and the other of silica, is predicted and compared. We find that a more negatively charged wall does not necessarily adsorb more cationic polyelectrolytes because the electrostatic repulsion between the wall and the polyelectrolytes is stronger. Additionally, intra-chain repulsion plays an important role, because the largest polyelectrolyte chains have larger excluded volume than the shorter ones. In regard to the adsorption dependence on the polyelectrolyte polymerisation degree, we find that the excluded volume drives the adsorption throughout the intra-chain electrostatic repulsion, because the SiO₂ surface is strongly negative. These results are expected to be useful for several nanotechnological applications of current interest, such as in gene therapy and in the improvement of drug delivering mechanisms.     

Cross sections for electron-stimulated desorption of alkali-metal atoms from adlayers on oxidized tungsten

Earlier cross-section measurements of electron-stimulated desorption (ESD) of Li, Na, K, and Cs atoms from adlayers on oxidized tungsten are analyzed with respect to substrate temperature and degree of oxidation. It is conjectured that the ESD cross sections are determined by the ratio of the rates of neutral alkali-metal re-ionization on the surface and of relaxation of the O⁺ charge in the substrate. A comparison with experiment revealed the dependence of the re-ionization rate of an alkali-metal adatom on its size and mass, as well as the dependence of the O⁺ charge relaxation rate on substrate temperature and degree of oxidation. The relation of the charge relaxation time to the substrate band structure is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1491–1497 (August 1997)     

Samarium atom yield under electron-stimulated desorption from oxidized tungsten surface

The yield of samarium (Sm) atoms under electron stimulated desorption from Sm layers adsorbed on the surface of oxidized tungsten was studied as a function of incident electron energy, surface coverage by samarium, degree of tungsten oxidation, and substrate temperature. The measurements were conducted by the time-of-flight technique with a surface ionization detector in the substrate temperature interval from 140 to 600 K. The yield vs. incident electron energy function has a resonance character. Overlapping resonance peaks of Sm atoms are observed at electron energies of 34 and 46 eV, which may be related to excitation of the Sm 5p and 5s levels. The Sm yield is a complex function of samarium coverage and substrate temperature. Sm atom peaks occur only in the Sm coverage range from 0 to 0.2 monolayers (ML), in which the yield passes through a maximum. The shape of the yield temperature dependence is a function of Sm coverage. For low Sm cover-ages (<0.07 ML), the yield decreases slowly with the temperature increasing to 270 K, after which it drops to zero at temperatures above 360 K. At higher coverages, the Sm yield passes through a maximum with increasing temperature and additional peaks appear at electron energies of 42, 54, and 84 eV, which can be assigned to excitation of the tungsten 5p and 5s levels. These peaks are most likely associated with desorption of SmO molecules, whose yield reaches a maximum at an Sm coverage of about 1 ML.     

Electron-stimulated desorption of europium atoms from the surface of oxidized tungsten

An analysis of the yield q of europium atoms is made, and scenarios of electron-stimulated desorption are put forward. Expressions are obtained for the dependence of q on the coverage of oxidized tungsten by europium atoms.     

Adsorption of gold on low index planes of rhenium

On atomically rough areas of a thermally cleaned rhenium field emitter, adsorbed gold behaves like it does on tungsten. The average work function $\text{}\text{?}$gf increases at low average gold coverage $\text{}\text{?}$gq due to formation of gold-rhenium dipoles, and at high coverage a structural transformation in the gold layer leads to a $\text{}\text{?}$gq-independent work function. Broadly similar behaviour is found for gold on the low-index planes of tungsten, but on low-index rhenium planes gold behaves rather differently. When thermally cleaned at > 2200 K and annealed below 800 K, the work function, φ(clean), of (101&#x0304;1&#x0304;) takes one of two values 5.25 ± 0.04 eV, and 5.36 ± 0.04 eV, which are tentatively attributed to the two possible structures of this plane. Similar behaviour is expected and observed for (101&#x0304;0),but the values taken by φ(clean) are not well defined. Both forms of (101&#x0304;1&#x0304;) are thought to undergo reconstruction above 800 K forming a single structure with φ(clean) = 5.55 ± 0.03 eV. (112&#x0304;0) and (112&#x0304;2&#x0304;) each have only one possible structure, and in keeping with this, φ(clean) has a single well-defined value for each plane. The flatness of (101&#x0304;1&#x0304;) and (101&#x0304;0) leads to field reduction at their centres which produces an increase in their measured work functions by up to 10%. The initial increase in φ produced by gold condensed at 78 K and spread at low equilibration temperatures T_s on (112&#x0304;2&#x0304;), (101&#x0304;1&#x0304;) and (112&#x0304;0) is attributed to gold-rhenium dipoles, which, on the latter two planes approximate to the Topping model, giving dipoles characterised by μ₀(1011) = 0.1 × 10^?30 C-m with α = 10 ų and μ₀(112&#x0304;0) = 0.32 × 10^?30 C-m with α = 22 ų, where μ₀ is the zero-coverage dipole moment and α its polarizability. Failure of the Topping model on (112&#x0304;2&#x0304;) is attributed to its atomically rough structure. No dipole effect is seen on (101&#x0304;0). Energy spectroscopy of electrons field emitted at (202&#x0304;1&#x0304;) and (101&#x0304;1&#x0304;) demonstrates the non-free character of electrons in rhenium, while the small effect of adsorbed gold strengthens the belief that gold is bound through a greatly broadened 6s level centred 5.6 eV below the Fermi level and the dipolar nature of the bond supports this model. At higher values of T_s and $\text{}\text{?}$gq gold appears to form states which are well-characterised by a coverage-independent work function. (101&#x0304;0), (101&#x0304;1&#x0304;) and (112&#x0304;0) each form two such states, one in the range 2 < $\text{}\text{?}$gq < 4 (state 1), and the second at $\text{}\text{?}$gq > 4 (state 2). The atomic radii of gold and rhenium are thought to be sufficiently similar to allow the possibility that state 1 is a replication of the Re plane structure by gold. The high work function and thermal stability of state 2, taken together with the observed temperature dependence of the transformation of state 1 to state 2, encourages the belief that state 2 results from atomic rearrangement of state 1 into a close-packed Au(111) structure. State 2 also forms on (112&#x0304;2&#x0304;) and the absence of state 1 on this plane suggests some surface alloying at coverages below 4 $\text{}\text{?}$gq.     

Origin and activity of oxidized gold in water-gas-shift catalysis

As a promising route for large-scale H2 production, the water-gas-shift reaction (WGS, CO + H(2)O-->CO(2) + H(2)) on ceria-supported Au catalysts is of enormous potential in efforts to move towards a hydrogen economy. Recent research suggests that this reaction is in fact catalyzed by Au cations instead of the conventionally regarded metallic Au particles. Here density functional theory calculations demonstrate that the presence of empty localized nonbonding f states in CeO2 permits the oxidation of Au, enabling subsequent CO adsorption. A feasible reaction pathway leading to H2 production is identified, and it is concluded that four to six atom Au clusters at the O-vacancy sites of ceria catalyze the WGS reaction.     

Adsorption of thin films of titanium on tungsten (111) surface

The structural and electronic properties of titanium-covered (111) surface of tungsten are studied via first-principles density-functional calculations. The dominant role played by substrate–adlayer interactions in the formation of thin films of Ti on W(111) explains the observed isomorphous growth for Ti depositions up to a full physical overlayer, with adatoms most favorably arranged in substrate-lattice-continuation sites. The optimized atomic configurations indicate substantial interplane relaxations following the contraction–contraction–expansion pattern, characteristic also of the clean W(111) substrate. Modification of the surface electronic properties of the Ti/W(111) system is examined for increasing adsorbate coverage with emphasis placed on the formation and localization properties of adsorbate-induced resonances.