A study of copper adsorption on (100) tungsten by high field microscopy

An increase in the work function of a copper layer approximately three monolayers deep on tungsten (100) can be thermally induced at temperatures above 540 K. Helium ion microscopy reveals a consequent but small increase in structural perfection of the tungsten substrate surface, which is akin to the rearrangement of gold, but the change in work function of the copper layer appears not to depend upon the rearrangement. Detailed investigation of changes in work function at (100) using a probe-hole field emission microscope shows that copper in the first monolayer increases φ₁₀₀ by 0.42 eV at monolayer coverage, probably by formation of an array of dipoles of moment μ₀ = (1.6 ± 0.5) × 10^?30 C m with polarizability α = 3.80 ± 1.8 ų. A large apparent increase in φ₁₀₀ of 1.6 eV takes place when a thicker copper layer is spread so that it surrounds and approaches (100), and this is ascribed to a reduction in the field enhancement factor β produced by the surrounding copper. Comparison of the present findings with those of Bauer et al. reveals substantial agreement on the behaviour of the first monolayer and two major differences in the behaviour of thicker layers: (i) an increase in φ₁₀₀ from 4.25 to 7.1 eV which we observe above 540 K, and (ii) absence of any evidence for breakup of third and higher layers to form crystallites. It is tentatively suggested that (i) results from band-structure changes accompanying structural reorganisation within the adsorbed layer, which take place under experimental conditions not used by Bauer et al., and that (ii) is due to the absence of steps in the probed area of (100) which can act as nucleating centres for crystallite formation on a macroscopic (100) surface.     

Bromine adsorption on Cu(111)

The dissociative chemisorption of molecular bromine on Cu(111) at 300 K has been studied using ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and work function change measurements. A (√3 × √3)R30° structure is formed initially at a bromine coverage of 0.33 ML. This then converts to a (9√3 × 9√3)R30° compression structure with a coverage of 0.41 ML. The coincidence distance of the compression structure is determined entirely by the van der Waals diameter of adsorbed bromine. The applicability of using the van der Waals diameters of the three halogens, Cl, Br and I, to predict the saturation compression structures on Cu(111), is discussed.     

An electron spectroscopic investigation of the adsorption of NO on Ni(111)

The adsorption, decomposition, and desorption of NO on the close packed Ni(111) surface have been investigated by XPS, XPS satellites, XAES, UPS, and LEED between 125 and 1000 K. At adsorption temperatures below 300 K a single molecular species (v) is formed with about unit sticking coefficient, which is interpreted as bridge-bonded; its saturation coverage is about 85% of that of CO, i.e. 0.5 relative to surface Ni atoms. Adsorption at 300 to 400 K yields dissociative adsorption (β) followed by molecular adsorption; above 400 K only dissociated species are formed. Upon heating, a full molecular layer dissociates only after some NO desorption (at 380–400 K), while dilute layers (below half coverage) dissociate already above 300 K without NO desorption. Together with quantitative findings this shows that for dissociation of one v-NO, the space of two is required. N₂ desorption from the β-layer occurs above 740 K; the oxygen staying behind diffuses into the crystal above 800 K. Readsorption of NO onto a β-layer or onto an oxygen precoverage at 125 K leads, besides to an α₁-state similar to v-NO, to another molecular state (α₂) which is interpreted as linearly bound. The resulting total coverage is considerably higher than in a virgin layer. This shows that the blocking of dissociation in a full v-layer is probably not due to β requiring the same sites, but to kinetic hindrance; an influence of β-induced surface reconstruction cannot be excluded, however. The LEED results agree with a previous report and are well compatible with the other results.     

Measurement of the adsorption energy on single crystal faces by field emission (hydrogen on tungsten)

As a preliminary study has shown¹), the isosteric energy of adsorption on single crystal faces can be measured, if the coverage is kept constant on controlling the field electron currents. This method has been improved (a) by the introduction of the probe hole technique, (b) by special cleaning procedures, and (c) by a more precise measurement of gas pressure and temperature. The method is used to obtain the adsorption energy q of hydrogen on the tungsten faces (100), (111), (112), (013), (122), (123) and (114). Initial q-values vary between 33 kcal/mole on (013) and 40 kcal/mole on (112). q decreases with coverage down to a few kcal/mole. The results indicate a relation between the adsorption states and the structure of each face.     

X-ray photoelectron spectroscopic study of the physical adsorption of xenon and the chemisorption of oxygen on tungsten (111)

X-ray photoelectron spectroscopy (ESCA) has been used to study the physical adsorption of Xe and the chemisorption of oxygen by W (111). An ultrahigh vacuum ESCA spectrometer has been modified such that thermal desorption behavior from the W (111) crystal can be directly compared with ESCA spectra of the adsorbed species. In addition, since the work function of a W (111) crystal covered with one monolayer of Xe is accurately known from previous work, the binding energy of the $\text{Xe}\text{(3}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ adsorbate level can be accurately compared to the gaseous $\text{Xe}\text{(3}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ level.When Xe is physisorbed to 1 monolayer the $\text{Xe}\text{(3}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ level exhibits a binding energy (relative to the vacuum level) which is 2.1 eV below that found for Xe (g). At lower Xe coverages the shift becomes monotonically greater, approaching 2.6 eV at a Xe coverage of 0.05. This 0.5 eV shift downward is accompanied by an increase of only 0.05 eV in adsorption energy as coverage decreases, and may be partially caused by the presence of ~ 10–20 % of extraneous adsorption sites other than W (111) which adsorb Xe with higher adsorption energy. The adsorption energy of Xe may also be increased by coadsorption of oxygen and the $\text{Xe}\text{(3}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ binding energy exhibits a corresponding shift downward as adsorbed oxygen coverage is increased to θ_o = 0.5. Electronic relaxation processes affecting the final state are dominant factors in determining the magnitude of the chemical shift upon adsorption, in agreement with the predictions of Shirley. The magnitude of the relaxation effect seems to be very sensitive to small changes in Xe adsorption energy. Similar effects have been seen for chemisorption of CO.The adsorption of O₂ at 120 K by W (111) yields a single broad O(1s) peak whose line-width decreases with increasing coverage. The final spectra at θ_o = 1 monolayer are very similar to those obtained at temperatures of 300 K or above on polycrystalline tungsten.     

Photo- and thermodesorption of helium on Pt(111)

In a detailed study of thermal desorption of monolayers of both 4He and 3He adsorbed on Pt(111) (binding energy about 9 meV), we have observed photodesorption induced by the blackbody radiation from a room temperature environment. This process proceeds independently of the thermal desorption. Theoretical treatments of both thermal and photodesorption are given and agree very well with the data in all important aspects. We conclude that the photodesorption is due to direct coupling of photons to the adsorbate.     

Ge adsorption on the Si(111) surface

Total energy calculations, performed for one monolayer of Ge adsorbed on Si(111), indicate that 1 × 1 models such as the atop site and hollow site adsorption geometries are unstable with respect to the formation of 2 × 1 Seiwatz chains of Ge adatoms. This result indicates that, for one monolayer coverage, Ge-Ge bonds are likely to form.     

The adsorption of beryllium on the tungsten (211) surface

The adsorption of beryllium on a W(211) surface was investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), work function change (Δφ) measurements and thermal desorption spectroscopy (TDS). Measurements were made at room temperature before and after annealing of the adsorbed layer. The work function change depends on the thermal history of the sample and can be explained in terms of a temperature-dependent adsorption process involving double layer formation.     

The adsorption of sulfur on the tungsten (110) surface

The adsorption and desorption of S on a W(110) surface is studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), work function change (δφ) measurements and thermal desorption spectroscopy (TDS). The evolution of the structure with coverage θ is quite different from that reported for S on Mo(110) and — at low coverages — from that of Te on W(110). At low coverages the structure indicates complex lateral interactions. The bonding changes with coverage similar to S on W(100). Evidence for more complex desorption kinetics than assumed in the past is presented.     

Nickel carbonyl adsorption on the Ni(111) surface

Overlayers formed by the adsorption of Ni(CO)₄ in CO on the Ni(111) surface at 100 K were characterized using high resolution electron energy loss spectroscopy and thermal desorption spectroscopy. At temperatures below 135 K, molecular nickel carbonyl adsorbs on the CO saturated Ni(111) surface as suggested by several observations. Vibrational transitions characteristic of molecular Ni(CO)₄ are dominant. The energy dependence of both the elastic and inelastic electron scattering cross sections are dramatically altered by Ni(CO)₄ adsorption. All of the mass spectrometer ionization fragments typical of molecular Ni(CO)₄ are observed in the narrow thermal desorption peak at 150 K. The inelastic scattering cross sections for both adsorbed nickel carbonyl and adsorbed CO on the Ni(111) surface suggest that a nonresonant dipole scattering mechanism is dominant.     

Coverage dependent adsorption site for sulfur on Ni(111)

Angular-resolved photoemission spectra have been observed from two ordered sulfur overlayers, p(2×2) and (√3 ×√3)R30°, representing different coverages on the Ni(111) surface. Utilization of selection rules applicable to photoemission along the sample normal permits an assignment of placement and composition of the adsorbate induced bands. The spectra show dramatic changes in peak positions between these two overlayers similar in magnitude to those observed on Ni(100). The shifts in peak positions are attributed to symmetry prescribed changes in the surface unit cell relating to the character of the bond. Arguments that these changes provide evidence that the bonding site of the adsorbate atom may change with coverage are presented.     

The influence of hyperpolarisability and field-gradient polarisability on field adsorption binding energies for He on W(111)

The effects of imaging-gas hyperpolarisability and field-gradient polarisabuity terms on field-adsorption binding energies have been explored. At best image fields for the noble imaging gases and molecular hydrogen, the correction to long-range binding energy is at most a few percent and may be neglected. At the tungsten evaporation field the correction is significant in the cases of Ar, Kr, H₂, and especially Xe. The system He on W(111) has been used as a paradigm in the investigation of short-range binding energy. The largest correction here is due to one of the field-gradient polarisabuity terms. In the field range of most relevance to the field-ion techniques the total correction is about 15–20%, and should not be neglected in detailed treatments.