Crystallographic anisotropy in chemisorption: Nitrogen on tungsten single crystal planes

A molecular beam technique for the determination of sticking probabilities and surface coverages was used in earlier work to investigate the adsorption of nitrogen on tungsten {110}, {111} and {100} single crystal planes. In the present paper these studies have been extended to the {310}, {320} and {411} planes. Absolute sticking probabilities and adatom surface coverages are reported for crystal temperatures between 90 K and 960 K. Crystallographic anisotropy in this system is exemplified by zero coverage sticking probabilities with the crystal at room temperature: {110}, 1̃0^?2; {111}, 0.08; {411}, 0.4; {100}, 0.59; {310}, 0.72; {320}, 0.73. Results for planes on the [001] zone are quantitatively described by a general model developed for adsorption on stepped planes as an extension to the precursor-state order-disorder model for adsorption kinetics of King and Wells. It is shown that nitrogen dissociation only takes place at vacant pairs of {100} sites, but that subsequently the chemisorbed adatoms so formed may migrate out onto {110} terraces. The results are critically analysed in terms of the available LEED and work function data for nitrogen on tungsten single crystal planes, and the general model developed by Adams and Germer.     

The epitaxy of gold on (110) tungsten studied by leed

Growth of gold condensed on the (110) plane of tungsten has been studied using LEED and AES. Three ordered surface structures were observed when condensation takes place at or above 700 K, and no detectable order is seen below this temperature. Structure 1 is developed as the coverage approaches one monolayer and has gold atoms held in the W(110) array with a resultant 2% reduction in gold atom diameter. The second gold layer adopts the Au(111) structure with $\text{Au}\text{[1}\text{2}\text{1]}$ rotated by 2.5° from $\text{W}\text{[1}\text{1}\text{0]}$ and the first gold layer may also be constrained to adopt this structure. Deposition of more gold produces three dimensional crystallites with Au(111)∥W(110) which are double-positioned with their 121 directions parallel to the 121 directions of tungsten. Addition of half a monolayer of oxygen before condensation, completely prevents formation of structures 1 and 2. Instead, at coverages of 3 or more monolayers, three dimensional crystallites develop with Au(111) ∥ W(110) and $\text{Au}\text{[1}\text{2}\text{1] ∥}\text{W}\text{[1}\text{1}\text{0]}$. This behaviour is compared with the reported behaviour of copper and silver on W(110).     

Field emission studies of oxygen on silver tips

Clean [111] oriented silver field emitting tips have been exposed to oxygen at 10^?3 Torr for 1 min at temperatures ranging from ? 170 to 200°C. From 50 to 200°C, an adsorption structure is formed that is stable in oxygen. The structure is characterized by intensely emitting regions on either side of enlarged {110}, {210} and {310} faces and a dark region in the (111)-{100} zone line directions. For adsorption from ? 170 to 200°C, the structure of the patterns depends distinctly on the adsorption temperature because the coverages are different and adsorption is activated. Oxygen adsorption at 10^?3 Torr for 1 min at 0°C causes an increase in the average work function of 1.15 eV. At 0°C, silver was exposed increasingly at 10^?6 Torr until 6100 L was reached. The work function increased progressively by 0.61 eV for this exposure. The {111}, {100}, {311}, {211} and {533} faces are attacked first. Then, the {110} faces are attacked followed by the {210} {310} and {320}. Heating of the adsorption layer formed at 0°C produced no changes in pattern and work function up to 100°C. Between 100 and 200°C, a strong decrease in work function and changes in the pattern result from oxygen penetration into the bulk.     

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.     

Field electron emission study of nitrogen adsorbed on individual tungsten planes

The adsorption of nitrogen on 211, 111, 100 and 110 tungsten planes has been studied by means of the probe-hole emission technique over a wide range of temperatures. The field emission tube was attached to a molecular beam system. This technique enabled deposition of strictly controlled doses of nitrogen. It has been found that on the 211 plane three states of nitrogen γ, α and β exist. In the γ state molecules of opposite polarity are present. These correspond to the γ⁺ and γ^?. The α state undergoes transformation at about 300 K to a more stable β state. β nitrogen leaves the 211 plane through surface diffusion in the temperature region 600–700K. Results obtained on the 111 plane in the low temperature region confirm previous findings on the existence of γ and α states. At higher temperatures the concentration of nitrogen in the β state increases as a result of migration from the 211 plane. There is some evidence as to the existence of two high energetic states of nitrogen on the 111 and 100 planes. On the 110 plane only partial results were obtained due to field desorption.     

Oxidation of tungsten single crystals in the presence of a high-electric field

The effects of high-electric fields on oxidation of tungsten single crystals in 6 × 10^?4 torr of oxygen at 1200–1500 °K were studied by field emission and transmission electron microscopy. Exposure of field emitters to oxygen in the absence of a field resulted in the build-up of emitter tips. Oxidation under the application of a negative or positive field, on the other hand, involved plane faceting and formation of oxide crystallites. Plane faceting was recognized to occur on the {111} and the {112} regions, showing the facetings of the {111} and the {112} planes into the {110} planes, whereas, crystallite formation seemed to take place selectively on the {100} regions. It was suggested by field emission microscopy that negative fields have an additional effect which causes the growth of an oxide crystal on the (110) plane. Transmission electron microscopy of an emitter oxidized in a negative field actually revealed a tiny oxide crystal with a size of ～ 300 Å grown on the developed (110) plane. The crystal exhibited a triangular shadow image strongly indicating an external pyramid-like form.     

Growth and desorption of indium epitaxial layers investigated by high field microscopy

Growth of indium single crystals on tungsten field emission tips was carried out by deposition of indium from vapour in ultra high vacuum, using substrate temperatures in the range of 293–420 K. Two different tungsten tips were used as the substrate: a perfect W single crystal in one case and a bi-crystal with a distinct grain boundary in the other. No influence of the grain boundary on the epitaxial growth was found. Two orientation relationships were observed mostly: {111}In ∥ {110}W with 〈110〉In ∥ 〈111〉W and {111}In ∥ {100}W with 〈110〉In ∥ 〈110〉W. In the first case the growth was initiated by the indium nucleus created on the ledges of the {110}W plane. A field strength of 0.9 V/Å was found for the evaporation field of indium. The field strength of the desorption of In-W interfacial layer atoms was found to be 4.4–5.2 V/Å. A mechanism of the growth of indium crystals has been proposed.     

Surface self-diffusion behavior of individual tungsten adatoms on rhombohedral clusters

The diffusion of single tungsten adatoms on the surfaces of rhombohedral clusters is studied by means of molecular dynamics and the embedded atom method. The energy barriers for the adatom diffusing across and along the step edge between a {110} facet and a neighboring {110} facet are calculated using the nudged elastic band method. We notice that the tungsten adatom diffusion across the step edge has a much higher barrier than that for face-centered cubic metal clusters. The result shows that diffusion from the {110} facet to a neighboring {110} facet could not take place at low temperatures. In addition, the calculated energy barrier for an adatom diffusing along the step edge is lower than that for an adatom on the flat (110) surface. The results show that the adatom could diffuse easily along the step edge, and could be trapped by the facet corner. Taking all of this evidence together, we infer that the {110} facet starts to grow from the facet corner, and then along the step edge, and finally toward the {110} facet center. So the tungsten rhombohedron can grow epitaxially along the {110} facet one facet at a time and the rhombohedron should be the stable structure for both large and small tungsten clusters.     

Field electron emission changes by face specific adsorption of aluminum oxide and corresponding decomposition products on tungsten {110} and {100} planes

After deposition of aluminum oxide on a tungsten field emission microscope (FEM) tip and stepwise heating, three stages of emission changes were observed on {100}. Stages I and II cause work function decreases of 0.28 and 0.07 eV, respectively. Stage III is characterized by a large increase (Δ??+3 eV). The changes are discussed in terms of interaction of decomposition products (oxygen and aluminum) and adsorption of aluminum oxide. On {110} only a single aluminum oxide layer growth, which results in a work function decrease to ?=4.69 eV, is observed. The field electron emission from this layer was measured between 1400°K and room temperature. The experimental values were compared with those determined from Christov's unified theory of field and thermionic emission. The {110} layer values coincide with those obtained earlier from an aluminum oxide covered tungsten {112}.     

Field ion microscope observations of surface rearrangement and oxide formation at clean rhenium surfaces heated in oxygen

The interaction between oxygen and clean, well-ordered rhenium surfaces has been investigated in the field ion microscope over a wide range of temperatures. The surface structure of vacuum annealed specimens was extensively modified by the presence of adsorbed oxygen. At partial monolayer coverages, surface metal atom rearrangement was inhibited, particularly at {112?2}, but at coverages near saturation, facets were produced at {101?0} at a lower temperature than on the clean surface, resulting in the exposure of an increased proportion of closepacked surface structures and suggesting a change in the rearrangement mechanism at high coverages. The production of facets at the major poles was temperature and coverage dependent as a result of the varying influence of adsorbed oxygen on surface free energy. A decrease in oxygen coverage was observed at 1200–1300 K on near-saturated surfaces, which may be associated with the formation of volatile ReO₃ from regions where oxygen concentration exceeds a critical value. There was little evidence of the production of a discrete surface oxide phase formed on specimens heated in the presence of gas phase oxygen, but a reduction in specimen radius as a result of volatile oxide formation was observed at pressures greater than 10^?4 Torr and temperatures above 1400 K. It is concluded that the final structure of rhenium surfaces heated in oxygen is dependent upon the rate of oxide formation, the effective oxygen coverage during oxidation, and the extent of surface rearrangement.     

Field-ion microscope observation of ordering in the near-surface of Ni4Mo alloy using in situ annealing

Field-ion microscope observations were made on Ni₄Mo alloy to investigate surface effects on ordering during in situ annealing after quenching from high temperature α phase region. Surface ordering occurred preferentially at {200}_FCC and/or {111}_FCC facets (at 880–835°C). Surface layers with low degree of order ranging from ten to a few tens ångströms in thickness were formed (at 760–720°C). The degree of order in relatively large ordered domains decreased near the surface (at 825–810°C). In the interior of the specimens at a range of depths 100–300 Å from the surface, ordering behavior was essentially the same as that for bulk specimens (at 825–720°C). Homogeneously nucleated ordered domains coexisted with relatively large heterogeneously nucleated ones in the interior of the specimen (at 670°C). Certain relatively low index (FCC) planes faceted (at 880-720°C). Faceting originating from β structure (ordered structure) occurred on the surface (at 670°C). In addition, in situ formed deformation twin was observed (at 810–825°C).     

Surface energy anisotropy of tungsten

Field-ion microscopy was used to study the faceting behavior and/or surface energy anisotropy of tungsten in vacuum and in hydrogen. In vacuum below 1700 K the activation energy for {110} facet growth agreed with values previously reported for surface diffusion on tungsten. The observed anisotropy values at 0.5 T_m, where T_m is the absolute melting temperature of tungsten (～3680 K), were different from those previously reported at higher temperatures and more nearly agreed with broken bond calculations based on Mie potential using m = 5, n = 8, and a 1.5% lattice expansion. Hydrogen appeared to have a negligible effect on surface energy anisotropy, but did preferentially increase surface diffusion rates on {310} regions.     

LEED,Auger and work function study of iodine adsorbed on W(110)

The adsorption of iodine on a W(110) surface has been studied by LEED, Auger and work function changes. LEED has revealed several phases which desorb in different temperature regimes and are accordingly designated γ, α, β₁, β₂, and β₃. The γ and α phases exhibited p(2 × 2) and p(2 × 1) surface nets respectively with coherently positioned antiphase boundaries which produced a splitting of selected LEED beams. The separation between the antiphase boundaries of the α phase increased with decreasing coverage. Both the γ and α phases were associated with molecularly absorbed iodine. The three β phases were associated with dissociatively adsorbed iodine which formed chain structures on the surface with the arrangement of iodine atoms within each chain being unique to the particular phase. Continuous changes in coverage then occurred by sheets of these chains shearing to produce packing faults at the resulting shear lines. This shearing process occurred coherently in the β₁ and β₃ phases and incoherently in the β₂ phases. In the former cases, the effect was seen by the continuous movement of coherent LEED beams with changing coverage. A phase diagram was constructed to describe the relative coverages and thermal stability of the phases. The characteristic Auger electron emission of iodine was observed at 495 eV and used to estimate the surface coverage. The work function was found to decrease by 0.4 eV with the adsorption of the first half monolayer and remained unchanged with further adsorption.     

Chlorine monolayers on the low-index faces of silver

Chlorine chemisorption layers were obtained on the (100), (110) and (111) faces of silver single crystals by means of the reaction with dichloroethane. The structure and stability of the surface phases was studied mainly by LEED and Auger spectroscopy. On the (100) face, a c(2 × 2) superstructure was found at all coverages, which was interpreted as a reconstructed layer with a structure similar to that of the AgCl (100) planes. On the other two faces, different superstructures were observed, depending on the coverage. At low coverages, (2 × 1) and (√3 × √3)-R 30° superstructures were found on the (110) and (111) faces, respectively, which seem to be better interprétable as reconstructed layers. At maximum coverages, c(4 × 2) and (3 × 3) superstructures were observed respectively on the two faces. These phases were interpreted as mixed layers with a packing of the silver and chlorine atoms similar to that of the AgCl (111) planes. The proposed models are in agreement with the different thermal stabilities of the layers. The models of the maximum-coverage structures correspond to about the same chlorine amount on the three faces, in agreement with the Auger results. The structures are also discussed in relation to the known epitactic relations for the growth of AgCl on silver.     

The thermal accommodation of helium and argon on a tungsten wire,deduced from the heat lost by the wire

Experimental data are presented on the heat imparted by a polycrystalline tungsten wire of preferred orientation (such that the [110] direction was approximately parallel to the wire axis) to helium and argon at pressures of order 10^?3 torr. Values of the accommodation coefficient (α) are deduced, the temperature of the gases being 300°K and that of the tungsten ranging from 1073 to 1785°K. Over this temperature range the values of α for helium and argon are 0.018 and 0.25 respectively, being virtually independent of the temperature difference between the gases and the tungsten.     

The adsorption of CO on the (100) plane of tungsten; thermal and work function measurements

Thermal desorption and work function measurements indicate that a largely molecular layer, with some dissociation, is formed at 80–100 K, with an increase in work function of 0.55 eV. The coverage in this layer is 11.5 × 10¹⁴ molecules/cm², or CO/W = 1.15. On heating, equal amounts of a β precursor, possibly dissociated, and a molecular α species are formed at ≈300 K, with abundances of 5 × 10¹⁴ molecules/cm² each. The α desorption is complete at 360 K. The β precursor evolves on heating without desorption in the range 400–700 K as indicated by work function decreases, to β-CO, which is almost certainly dissociated. This change occurs at lower temperatures for low coverages. Thermal desorption shows 3 peaks, which have been traditionally labelled β_{1, β2}, and β₃ at 930, 1070, and 1375 K. Of these only β₃ corresponds to a well defined state. Readsorption after heating to 950 or 1150 K results in a doubly peaked spectrum at 1070 and 1375 K. The β₁ and β₂ peaks obey complex desorption kinetics, probably corresponding to desorption and rearrangement. The coverage of β₃ is 2.5 × 10¹⁴ molecules/cm², suggesting that the c(2 × 2) LEED pattern corresponds to occupany of every other unit cell by a C or an O atom. For coverages ? 1.5 × 10¹⁴ molecules/cm² β₃ desorption obeys second order kinetics with an activation energy of 83 ± 3 kcal/mole. For β₃ the work function decreases from the clean W value by 0.1 eV, suggesting adsorption of C and O in the center of the W unit mesh, below the surface layer of W atoms. Readsorption on β and β precursor layers leads to formation of electropositive α-CO, with a multiply peaked thermal desorption spectrum, indicating the existence of different binding sites. Adsorption-heatingreadsorption, -heating-readsorption sequences indicate that additional changes in the α desorption spectrum occur, suggesting reconstruction in the β layer.     

Dynamic mechanical characterization of relaxations in poly(oxymethylene), miscible blends,and oriented filaments

Multifrequency dynamic mechanical analysis (DMA) data were obtained for molded poly(oxymethylene) (POM) and its blends from-150°C to 150°C. Because of the high crystallinity, the assignment of the glass transition in POM has been controversial in the literature. Low and high glass transition temperature (T _g) phenolated compounds, including poly(vinyl phenol), were found to be miscible with POM. The shift of the β transition in the POM blends favors an assignment of the β transition detected at ?3°C(1 Hz), not the ?80°C γ transition, as the T _g in semicrystalline POM because the latter is invariant with diluent. The peak at the β transition in pure POM is weak and can only be seen clearly by DMA measurements on samples that have not “aged” at ambient temperature. This is further evidence that the β transition arises from a cooperative glass-transition-like motion. The γ transition is not influenced by aging because it is due to a concerted localized main chain motion. The β transition of an oriented POM filament can be seen in the DMA flexural loss spectrum at-18°C (1 Hz), but not in a tensile loss spectrum. The broad a relaxation was detected at about 110°C (1 Hz) in molded POM and its blends, while it was shifted to about 135°C in the higher crystallinity, oriented system. The α peak is also independent of diluent, consistent with a crystalline origin for this transition, as was proposed earlier.     

The adsorption of carbon monoxide on Cu(110)-Ni surfaces prepared by dissociation of nickel carbonyl

Cu(110)-Ni surface alloys were prepared by dissociation of nickel carbonyl on clean Cu(110). The adsorption of CO is reversible in the temperature region of 22–200°C and the pressure range of 5 × 10^?8-0.7 Torr, as monitored with ellipsometry and AES. The amount of adsorbed CO depends on the amount of preadsorbed oxygen but not on the amount of carbon present at the surface. The isosteric heat of adsorption decreases from 31 ± 3kcal/mole to 18 ± 2 kcal/mole with increasing CO coverage (up to θ = 0.14θ_max) but is constant for higher coverages (up to θ = 0.4θ _max).     

Dual path surface reconstruction in the H/Ni (110) system

Reconstruction of a Ni (110) surface under the influence of adsorbed hydrogen atoms can proceed in two different ways: Below 180 K a 2×1 lattice-gas structure with θ_H = 1.0 transforms cooperatively into a two-dimensional 1×2 structure by additional uptake of hydrogen up to θ_H = 1.5. At higher temperatures, activated local transformation into a more stable one-dimensional structure starts already at low coverages.     

Relation between surface structures and sputtering ratios of copper single crystals

Polished (100) Cu crystals have been bombarded at target temperatures of 204 K, 294 K and 456 K by 10 and 20 keV Ne⁺ ions up to a total dose of 1.7 × 10¹⁹ ions/cm². The plane of incidence was chosen to be a {100} plane perpendicular to the surface. Measurements have been performed for incident angles between 36° and 44° with respect to the surface normal. In this angular interval the sputtering ratio and the surface structure have been studied by weightloss and replica electron microscope techniques respectively. At target temperatures of 204 K and 294 K an anomaly was observed in the curve of the sputtering ratio versus angle of incidence. A small peak appears where the curve slopes towards the 〈110〉 minimum. The position and height of the peak is a function of target temperature and ion energy.

This sputtering submaximum is accompanied by the formation of {100} orientated furrows perpendicular to the ion beam. The nucleation of this relief is tentatively discussed in terms of local deviations from perfection of the surface, which might be due to a singularity in the production of focusing collisions influencing the damage structure. The growth of the furrows and the submaximum in the sputtering ratio are discussed in terms of the angle between the ion beam and the characteristic {110} side of the furrows.

These sputtering and faceting phenomena have not been observed at 20 keV Ar⁺ ion bombardment nor generally under bombardment at a target temperature of 456 K.