Field electron emission from an aluminum oxide covered tungsten {112} plane at temperatures up to 1700°K

Field electron emission from an aluminum oxide mono-layer, face specifically adsorbed on a tungsten {112} plane, was measured between 1670°K and room temperature and at local field strength ranging from 17 MV/cm to 35 MV/cm. In a Fowler-Nordheim plot straight lines were obtained up to about 820°K. The work function at 650°K was determined to be 3.2 eV. The experimental values were compared with those determined by Christov's unified theory of field and thermionic emission. Deviations are attributed to the change in the shape of the potential barrier, which is caused by the aluminum oxide adsorption.     

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.     

Field emission study of ammonia adsorption and catalytic decomposition on individual molybdenum planes

A probe-hole field emission microscope was used to investigate the crystallographic specificity of ammonia adsorption at 200 and 300 K on (110), (100), (211) and (111) molybdenum crystal planes. Chemisorbed NH₃ causes a large work function decrease, especially at 200 K in agreement with an associative adsorption model which can also explain that this decrease is more important on the crystal planes of highest work function (At 200 K, Δφ = ?2.25 eV on Mo(110) compared to Δφ = ?1.55 eV on Mo (111). The decomposition of NH₃ was followed by measuring the work function changes for stepwise heating of the Mo tip covered with NH₃ at 200 K. On the four studied planes NH₃ decomposition and H₂ desorption are completed at about 400 K. Δφ changes above 400 K depend on the crystal plane and have been related to two different nitrogen surface states. No inactive plane towards NH₃ adsorption and decomposition has been found but the noted crystallographic anisotropy in this low pressure study is relevant to the structure sensitive character of the NH₃ decomposition and synthesis reactions.     

A model for chemisorption of nitrogen on the {100} face of tungsten

A theoretical model for the adsorption of nitrogen on the {100} face of tungsten is given which is similar to an earlier model proposed by King and Wells to explain the dependence of the sticking coefficient on the covering fraction. The present model differs from the earlier one in that it attributes disorder in the chemisorbed adlayer to the irreversible nature of the adlayer formation process rather than to equilibrium entropic considerations. In contrast to the equilibrium model, the present one successfully accounts for the experimentally observed, temperature independence of the adlayer disorder, and, in particular leads to prediction of a temperature independent, effective saturation covering fraction at which chemisorption ceases. A simple one-dimensional model of the adlayer formation process is developed which leads to an estimate of the saturation covering fraction, independent of any adjustable parameter, which is within 5% of the experimentally observed value. Other manifestations of the non-equilibrium nature of the adlayer formation process are also discussed.     

Field emission spectroscopy of gold on the (110) and (211) planes of tungsten

Adsorption of Au up to monolayer amounts on the (110) plane of a tungsten field emitter leads to a decrease in work function, and also to a decrease in effective emitting area. Energy distributions of field emitted electrons show no structure attributable to Au. These results can be explained by postulating that there is no tunneling from Au atoms or from beneath Au atoms. The most probable reason for this is that in (110) tunneling matrix elements from outward normal tungsten d-orbitals are stronger than the tunneling matrix element from the non-directional 6s Au orbital. Experiments on the (211) plane show behavior similar to that previously found on (111), i.e. a work function increase but strong emission from Au atoms.     

Properties of ultrathin layers of palladium on a tungsten {110} surface

The growth mode, structure, thermal stability and work function of Pd films up to several monolayers in thickness on a W{110} surface are studied. At room temperature, layer-by-layer growth occurs, at least up to 4 layers. Layers in excess of the first monolayer are metastable and agglomerate at substrate temperatures above 700 K. Pd-W and Pd-Pd interactions within the first layer are investigated with thermal desorption techniques. The binding energy for single Pd atoms to the substrate is 3.6 eV/atom, the energy per lateral bond between nearest Pd atoms is about 0.05–0.12 eV/bond dependent upon range of interaction. These data are based on a phase transition occurring during the desorption of the first layer. The desorption of Pd in excess of the first monolayer is similar to the sublimation of bulk Pd.     

Study of adsorption of titanium on tungsten and rhenium by field electron emission

The deposition and layer growth of titanium, evaporated on to tungsten and rhenium field emitter tips has been studied, and field emission average work-functions measured at varying titanium layer thicknesses from less than a monolayer to several monolayers. Titanium evaporated on to a clean tungsten tip at 293 K in the submonolayer region tended to collect preferentially on atomically rough planes. With further deposition, a “pseudo-clean” emission pattern was obtained at about a monolayer coverage when the titanium atoms were concluded to be in epitaxial registry with the BCC substrate; at about this coverage the work-function for the system passed through a minimum. Further titanium deposition in excess of three monolayers resulted in a progressive roughening of the surface due to microcrystallinity: brief heating (~ 1 min, 1300 K) of such a surface resulted in a well ordered layer identified as ß-titanium (BCC) presumably epitaxed to the tungsten. This epitaxed layer remained indefinitely stable at 293 K.     

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.     

The co-adsorption of copper and oxygen on a tungsten {100} surface

The co-adsorption of Cu on O₂ and a W{100}surface is studied by Auger electron spectroscopy (AES), thermal desorption (TD), low energy electron diffraction (LEED) and by work function change (δø) measurements. It is shown that the presence of Cu on the surface initially decreases s_O, the sticking coefficient of O₂. For longer oxygen exposures and for higher adsorption temperatures, θ_O reaches values larger than those on the clean surface for the same O₂ exposure. Except at the highest θ_O values and temperatures, the sticcking coefficient for copper, s_Cu, is unity and is independent of the oxygen coverage θ_O in the range studied (0 ? θ_O ? 2). Co-adsorption at room temperatures does not produce any long range order while co-adsorption at elevated temperature leads to the ordered structures (1 × 1), p(2 × 1), p(2 × 2) and c(2 × 2). The saturation coverage of the two dimensional co-adsorbate at 800 K is given by the relation $\text{θ}{}_{\mathrm{Cu}}\text{+}\text{8}\text{5}\text{θ}{}_{\mathrm{O}}\text{= 2}$. The work function is a complicated function of θ_O and θ_Cu and is determined predominantly by the temperature at which oxygen is adsorbed. At high temperatures the sequence of adsorption has no influence, in contrast to the room temperature behavior.     

Splitting of dislocations in b.c.c. metals on {110} planes

Three possible splittings of a screw dislocation on {110} planes of a metal with a b.c.c. lattice are proposed, their total elastic energy is calculated and the dependence of the splitting width on the stacking fault energy is studied.We wish to express our thanks to B. esták, CSc., and S. Libovický for valuable comments and M. Jiincová for her help in carrying out the numerical calculations.     

Nucleation and orientation of graphite{0001} layers on Pt{110}

Investigations by field electron microscopy revealed that graphite{0;0001} layers grow preferentially on Pt{110}. Layer nucleation and initial growth occurs in the vicinal area of {110} along the 〈112〉 and the 〈100〉 zones and not on the unstepped {110} plane itself. The growth rate in the vicinal area is determined by the epitaxial misfit parallel to the step edges. An alignment of the graphite layer with the underlying {110} is observed. It corresponds to graphite〈101̄0〉 ∥ Pt〈100〉, which coincides with the calculated orientation of smallest epitaxial misfit. The change of relative emission from {110} upon adsorption of graphite is discussed.     

The adsorption site of the surface formate species chemisorbed on Ni{110}

The selection rules pertinent to the dipole scattering of electrons involved in the vibrational excitation of surfaces are exploited to provide information on the nature of the adsorption site occupied by the surface formate species chemisorbed on a Ni{110} single crystal surface. In particular, attention is concentrated on the observation, in specular scattering geometry, of a surface phonon mode which has its origin at the S̄ point of the clean surface Brillouin zone, and clearly indicates that short bridge sites are occupied.     

The adsorption and decomposition of formaldehyde by tungsten

A hot, clean tungsten surface was highly effective in decomposing formaldehyde vapour to carbon monoxide and hydrogen. At temperatures above 1300 K about 30% of the impinging molecules decomposed. At lower temperatures the reactivity was reduced, probably as the result of inhibition by the adsorbed carbon monoxide. Pre-adsorbed oxygen affected the reactivity in an unusual way. At low coverages of oxygen (θ < 0.4) and intermediate filament temperatures (T < 1000 K) no effect was observed, whereas under more extreme conditions (θ > 0.5 or T > 1200 K) a nearly linear decline in activity was recorded. These results are attributed to a similarity between the inhibiting effects of adsorbed oxygen and carbon monoxide.     

On the adsorption of oxygen on the Mo{110} surface and its vicinals

The adsorption of oxygen on a Mo{110} surface and four vicinal planes with steps parallel to 〈100〉, 〈110〉 and 〈111〉 directions, is studied by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The work has the goal to elucidate whether O adsorption on Mo{110} is really as different from O adsorption on W{110} as reported, and what is the cause of the apparent discrepancies between the various reported sticking coefficients on stepped surfaces.     

Photon and electron stimulated desorption of adsorbates from W {100}

We have performed photon and electron stimulated desorption (PSD and ESD) studies of F⁺, Cl⁺ and O⁺ ions from a W {100} surface and have measured the kinetic energies for a small range of emission angles of these ions as well as the effect of different electron and photon excitation energies. We find that the PSD and ESD processes show essentially the same threshold energies and produce the same ion energy and angle distribution, and so evidently involve the same mechanism. For the case of O⁺ we also find clear evidence that substrate core-level excitation initiates the desorption process as proposed by Knotek and Feibelman for adsorbates bonded to surface atoms with a maximal valency configuration. The substrate levels are not seen to play a role in the F⁺ and Cl⁺ desorption, indicating a very different type of bonding for these species on the W surface.     

Electron beam interactions with CO on W {100} studied by Auger electron spectroscopy

The interaction of 2500 eV electrons with carbon monoxide chemisorbed on tungsten {100} was investigated by rapid-scan Auger electron spectroscopy. When no α state was present the O and C signals from the β state of CO were invariant during electron bombardment, giving an upper limit estimate for the electron stimulated desorption cross section, Q_β of 2 × 10^?21 cm². With the crystal at room temperature and saturated with CO, however, electron-beam induced accumulation of carbon was observed and characterised, the rate of the process being independent of CO pressure at pressures above 2 × 10^?8 Torr. At 450 K the rate was found to be pressure dependent up to at least 6 × 10^?7 Torr. A model is proposed for the accumulation process, which is based on electron beam dissociation of α₂-CO to form adsorbed carbon and gaseous O and the creation of new sites for further α₂-CO adsorption; it is in quantitative agreement with the results and yields a cross section for ESD of α₂-CO (Q_α2 = 1.55×10^?18cm²) in close agreement with direct measurements.     

Core level photoemission study of the adsorption of iodine on Ni{100}

Synchrotron radiation photoemission from the Ni 3p and I 4d core levels has been studied for iodine adsorbed on Ni{100} in a range of different phases. In the range of chemisorbed structures involving overlayer compression from coverages θ of about 0.3 to 0.4 the I 4d photoemission binding energies decrease by ~0.3 eV, while further compression to a c(2 × 2) structure leads to a further decrease by ~0.4 eV. Low temperature adsorption of molecular iodine shows a binding energy of 0.1 eV lower still. In the chemisorbed coverage range these core level shifts are found to be substantially less than the I-I repulsive energy interactions and are associated with the through-metal component of this interaction, while the residual component concealed from the photoemission experiments is attributed to through-space overlap interaction. Other processes which may contribute to the observed binding energy shifts, including Coulomb interactions in both initial and final states, are discussed in some detail.     

Orientation and bonding of adsorbed CH3NCO on Pt{110} and Cu{110} from vibrational and photoelectron spectroscopies

The bonding and orientation of CH{in3}NCO on Pt{110} and Cu{110} are studied by HREELS, ARUPS, AES, and LEED. CH{in3}NCO is found to adsorb nondissociatively on both Cu{110} and Pt{110} at T = 160 K, bonding primarily through the 2π a′ orbital with the NCO group lying down on the surface and the methyl group largely unperturbed. We propose that the absence of a strong HREELS band at about 2290 cm^?1, which is the liquid phase frequency for the NCO asymmetric stretching mode, combined with the presence of strong bands between 1000 and 1450 cm^?1 provides a “finger-print” for NCO bonded to the surface in a lying-down configuration.