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.     

The adsorption and nucleation of zirconium on tungsten field emitters

The adsorption of zirconium on clean tungsten, at residual gas pressures below 1 × 10⁻¹⁰ Torr, has been studied by field emission microscopy. It was necessary to outgas the Zr above 2400 °K to obtain reproducibility. The gases evolved during degassing of the zirconium were predominantly CO, CH₄ and H₂. Mass spectra of the evaporant flux showed no tungsten or oxide contamination after thorough outgassing. It was found that heating below the -β phase transition temperature of Zr (1135 °K) resulted in the formation of large cap-shaped nuclei on the high index planes, while above this temperature the deposit became smoothed on the {100} planes. A model is proposed involving a closepacked square array of Zr atoms on the W(100). We conclude that the above two temperature regions are eminently suitable for studying epitaxy following the initial growth processes of nucleation or of monolayer addition. The presence of contamination impeded both surface migration and nucleation and lowered the work function.     

Validity of the Fowler-Nordheim parameters measured for silicon adsorption on single crystal faces of tungsten

The adsorption of silicon on the (112), (111), (100), (110), (113) and (116) planes of tungsten field emitters has been studied. As with Ge and CdS adsorption, anomalous behaviour of the Fowler-Nordheim parameters A and φ was observed together with non-linearity in the Fowler-Nordheim plots. This non-linearity is not explicable in terms of the theories proposed for bulk semiconductors. The present results support the suggestion that field-induced Si migration from the W substrate is responsible for anomalies in the F-N parameters.     

The adsorption of activated nitrogen on platinum single crystal faces

The adsorption of activated nitrogen on a stepped Pt(S)-[9(111) × (111)] face was investigated by LEED, AES and flash desorption. Nitrogen was supplied to the crystal from a high frequency discharge tube. For comparison some orienting measurements were also carried out on smooth (111) and (100) platinum faces. Activated nitrogen is adsorbed at room temperature on all three faces up to about half a monolayer coverage. No additional LEED patterns indicating long range order of the adsorbed layer were found. By flash heating a small desorption peak at 120°C and a large peak between 175 and 230°C depending on the initial coverage were observed on the (111) type faces. The desorption can be described approximately by a second order rate law with an energy of activation of 25± 3 kcal/mole. No influence of surface steps on the properties of the adsorbed layer was detected. On the (100) face two coverage independent desorption maxima at 120 and 170°C of about equal intensities were found.     

Current fluctuations from various crystal faces of a clean tungsten field emitter

The temperature, frequency and current dependence of current fluctuations have been measured for the (310), (112) and (100) planes of a tungsten field emitter. The rms percent fluctuation of the current at 900 K was 8.7, 1.5 and 0.13% and the threshold temperature for current fluctuation was 300, 650 and 1000 K for the (310), (112) and (100) planes respectively. Based on the results reported here it is concluded that the current fluctuations arise from the generation of mobile atoms on net plane terraces.     

Field emission studies of selenium adsorption on tungsten and molybdenum

The adsorption of selenium on tungsten and molybdenum field emitters has been studied. Despite the topographical similarity of the two substrates there are marked differences in the migration and localised adsorption of the first two monolayers of selenium. Anomalous behaviour of the emission current and Fowler-Nordheim parameters is found for adsorption on both substrates. Measurements on single crystal planes indicate that this behaviour is not caused by localised growth or clustering effects. It is suggested that the model proposed by Nicolaou and Modinos to explain similar effects during germanium adsorption on tungsten is also applicable in the present case.     

Electron and ion emission of faces of a tungsten crystal

The emission properties of the close-packed (110) and porous (111) faces of a tungsten crystal were studied with band-shaped samples in planar devices at a residual gas pressure of 10^–9 torr. The experimental apparatus has a provision for simultaneous measurement of the emission currents from two samples. The orientation of the samples is determined by chemical etching and by x-ray diffraction. The most probable values of _(hkl) for the (110) and (111) faces were found to be 5. 30 ± 0. 03 and 4. 40 ± 0. 02 eV, respectively, by the method of thermionic emission and the method of positive surface ionization of lithium atoms. The peak observed at temperatures < 1500 °K on the logI⁺ = f(5040/T) curve for the (111) face is explained, following Dobretsov, in terms of an increase in the work function of this surface at low coverages (­ 1) by adsorbed lithium atoms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 7–11, June, 1970.     

Crystal face specificity of xenon adsorption on iridium field emitters

Fundamental problems of the adsorption of noble gas atoms on metal surfaces are discussed on the basis of new data of xenon adsorption on well-defined crystal faces of iridium. These data include surface potentials ( = changes in work function), heats of adsorption and their decrease with increasing coverage; they have been obtained by using a field emitter probe-hole assembly. It is found that the heat of adsorption Q_hkl is not simply additive in the number of Ir atoms contacting a Xe atom on a given site; in particular for the close-packed faces, Q₁₁₁ and Q₁₀₀ are relatively too high. Apparently, strong bonding is favoured by high work function of the adsorbing crystal face. This proves a significant contribution of a charge-transfer no-bond interaction to the adsorption bond. A model of Xe polarization by an electric surface field is rejected, as it predicts the wrong sign for the adsorption dipole. While at low coverage adsorption is confined to sites determined by the atomic topography of the adsorbing surface, several possibilities exist for high coverages. Either a two-dimensional close-packed layer is formed with little or no epitaxial relation to surface topography, or adsorption remains confined to certain sites. The present data favour the former possibility for atomically smooth faces in agreement with recent LEED results. For atomically rough faces however, the smallness of the decrease of Q_hkl with coverage seems to favour site adsorption even at high coverage. The latter result is of relevance for surface area determinations by means of “physical” adsorption.     

The effect of temperature and electrostatic field on tungsten and iridium field-ion emitters

Studies have been made of the effect of heating tungsten and iridium field-ion emitters in the presence of an applied electrostatic potential. Although the original intention was to induce plastic deformation as a result of field stresses, and hence to investigate surface slip step formation, it is clear from the image contrast observed that, rather than inducing slip, the procedure instead leads to the formation of fine-scale thermal facets. Earlier reported attempts to deliberately induce surface slip in this way must also be reinterpreted in terms of thermal facet formation.     

Adsorption of nitrogen on single crystal faces of iridium,studied by field emission microscopy

The adsorption of nitrogen on iridium was studied with a field emission microscope equipped with a probe-hole assembly to enable emission experiments on individual emitter regions. The adsorption of nitrogen is markedly face-specific. Temperature programmed desorption reveals three binding states: γ₁ on the (100) face with a maximum heat of adsorption of 7–8 kcal/mole, γ₂ on the regions around (110) with a maximum heat of adsorption of 10–11 kcal/mole and γ₃ on the roughest tip regions (210), (320), (531) and (731) with a maximum heat of adsorption of 13–14 kcal/mole. Nitrogen adsorbed in the γ₁ and γ₃ states causes a decrease, but in the γ₂ state a small increase, in the work function. These results are discussed in relation with data on nickel, palladium, platinum and rhodium. While nitrogen is only weakly adsorbed on all these metals there is a marked difference in the nature of the pertinent adsorption complex.     

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 oxygen on clean single crystal faces of aluminium

Auger electron spectroscopy and work function measurements have been used to study the interaction of clean Al(111) and Al(100) faces with oxygen at low pressure near room temperature. The results for the two faces differ strongly. Thus, the sticking probability of the (111) face decreases rapidly with coverage, while the work function increases slightly, by 0.1 eV at 200 L. In contrast, the sticking probability of the (100) faces goes through a maximum, whereas the work function decreases almost linearly with coverage, the total decrease at 200 L being 0.5–0.8 eV. The shape of the Al L_{2, 3}VV spectrum from oxidized Al(100) is independent of coverage, and it is in fact very similar to previously reported spectra from oxidized polycrystalline aluminium. The corresponding spectrum from Al(111) exhibits large changes with oxygen coverage and shows a previously unreported double peak at ~60 eV. The results are explained on the assumption that oxygen adsorbs randomly on the (111) face, and that thin (~5 Å) islands of Al₂O₃-like oxide form on the (100) face.     

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 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 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.     

Correlation of field emission flicker noise and work function for tungsten single crystal planes with adsorbed potassium

The flcker noise power of the (112), (110), (100), (111) and (122) planes as well of (110) vicinals of a tungsten field emitter has been measured as a function of potassium coverage. Distinct minima of the noise power for coverages that correspond exactly to the work function minima could be found. It seems that this coincidence might be explained — apart from big lateral interactions of the adparticles — by the stronger adsorbate-substrate interaction in a coherent adiayer than in an incoherent one and/or by an adlayer transition to the “metallic” state resulting in compensation of the ionic adsorbate charge by a common electron gas.     

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.     

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.