A. Ciszewski

The interaction of water with surfaces of Pt and Ir field emitters

Abstract Some results of a study of the adsorption and thermal desorption of H 2 O on Pt and Ir surfaces, using field emission methods, are presented. At a substrate temperature of 78 K H 2 O admolecules reduced the total field emission current both from the Pt and Ir tips in a wide range of coverages whereas the local emission current, e.g. from the (001) and (113) faces of Ir, was increased. Water was desorbed from thermally cleaned Pt in the temperature range 140–160 K with an average desorption energy of E des = 54kJ/mol. The possible behavior of H 2 O molecules on the crystallographically diversified surfaces of Pt and Ir under the conditions of field emission is discussed.

Interaction of water with field emitter tips

The contribution of high-field techniques to solid-surface studies of the interaction between water and metal surfaces is reviewed. Particular attention has been paid to certain aspects of studies that concern the influence of high electric field on the adsorption layer of water in the multilayer and submonolayer coverage range. Recent results of investigations of the diffusion of water on metal surfaces are discussed.

Ni adsorption on the principal faces of a tungsten crystal : work function changes and adsorbate distribution

Abstract The field electron microscope was used to study work function changes as a function of the amount of Ni deposition on a W tip at 78 K and annealed at various temperatures from 78 to 870 K. The work function changes were measured for the (110), (112), (111) and (100) faces of the W crystal. Strong differences were found in the work function changes for these four faces. The surface density of the adsorbate corresponding to the extreme of work function change for each face was equal to the reticular density of the substrate surface. On the basis of this, the distribution of the Ni adatoms on the surface of the W single crystal under the conditions of thermal equilibrium was determined. The results are compared with already known properties of the Ni/W adsorption system.

Epitaxy of metals on metal substrates: the contribution of field electron emission microscopy

Abstract The contribution to the literature on the epitaxial growth of metals on metal substrates made by research using field electron emission microscopy are reviewed. In addition to a large amount of information about specific metal/metal pairs, some generalities have emerged. It is especially striking that in many cases of metal/metal epitaxy, the dominant factor determining the epitaxial relationship is the alignment of close-packed atomic rows in low-index crystallographic planes of each metal.

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

Abstract 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/A 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/A. A mechanism of the growth of indium crystals has been proposed.

Interaction of hydrogen with ultrathin titanium layers on tungsten

Abstract Hydrogen adsorption on titanium layers of an average thickness from 0.03 to 50 monolayers deposited on a tungsten substrate was investigated at liquid nitrogen temperature using the field electron emission microscopy techniques. It has been found that the interaction between titanium and hydrogen depends on the thickness and atomic structure of the Ti/W layer. For tightly arranged titanium layers isomorphous with the (011)W face the H–Ti interaction is weak and does not change the morphology and atomic structure of the layer. In the case of titanium layers isomorphous with the (001)W the H–Ti interaction dominates over the Ti–W interaction, beginning from the thinnest titanium layers deposited on the tungsten substrate. This results in a 3-dimensional nucleation of TiHx at the steps of the terraces of the (001)W face. For the titanium layers isomorphous with the (111)W and (112)W faces, the Ti–W interaction is stronger than the H–Ti interaction up to the thickness of 3 to 4 monolayers of titanium. Measurements of the average work function changes as a function of the H2 exposure indicate that, apart from the hydrogen adsorption state which raises the work function, there exists another one which decreases the work function. The latter state occurs in titanium layers isomorphous with the (111)W face for the coverage in the range from 0.65 to 2.7 titanium monolayers. Measurements of the local work function changes carried out for H2 adsorption on Ti/(111)W and Ti/(016)W indicate that this adsorption state cannot be recognized as the unstable β+ adsorption state observed during the hydrogen adsorption on a surface of bulk titanium [R. Duś, E. Nowicka, Z. Wolfram, Surf. Sci. 269/270 (1992) 545]. The one observed in this experiment is stable and is believed to result from the immersion of the adsorbed hydrogen atoms into the electron gas of the Ti–W interface.

Pd/GaN(0001) interface properties

This report concerns the properties of an interface formed between Pd films deposited onto the surface of (0001)-oriented n-type GaN at room temperature (RT) under ultrahigh vacuum. The surface of clean substrate and the stages of Pd-film growth were characterized in situ by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), ultraviolet photoelectron spectroscopy (UPS), and low energy electron diffraction (LEED).As-deposited Pd films are grainy, cover the substrate surface uniformly and reproduce its topography. Electron affinity of the clean n-GaN surface amounts to 3.1 eV. The work function of the Pd-film is equal to 5.3 eV. No chemical interaction has been found at the Pd/GaN interface formed at RT. The Schottky barrier height of the Pd/GaN contact is equal to 1.60 eV.

Adsorption and nucleation of hafnium on facets and stepped surface areas of tungsten

Abstract The adsorption of Hf deposited from vapor in ultra-high vacuum onto thermally cleaned surfaces of W has been studied using field electron emission microscopy. Three-dimensional nucleation and two-dimensional layer formation on structurally and topographically different areas of the substrate were observed for various annealing temperatures and various amounts of the deposited adsorbate. The dependence of the average work function on the adsorbate coverage as well as on the structural and morphological changes of the adsorption layer, involved by annealing at various temperatures, has been measured.

STM observation of steps and terraces on tungsten (2 1 1) surface

Thermally cleaned W(211) surface with 0.7 degrees miscut consists of (211) terraces separated by monoatomic steps. When the surface is exposed to oxygen and subsequently annealed at 1100-1900 K, the width of (211) terraces increases and multilayer steps are formed. Similar step bunching is observed during routine cleaning of the sample by annealing in oxygen and thermal flashing in ultra high vacuum. During such cleaning procedure islands of c(6 x 4) reconstruction are observed.

AFM studies of pits formation on KBr(1 0 0) during its dissolution by water

Abstract The formation of etch pits along screw dislocations on KBr(1 0 0) surface during its dissolution by water is investigated by means of atomic force microscopy (AFM). Clean KBr(1 0 0) is obtained by cleavage. A weak solution of water in isopropyl alcohol is used to investigate the etching in real time. Observations of the etch pit evolution with etching time show that concentration of atomic steps on the pit walls and dissolution rate of the walls vary up to complete dissolution of the screw dislocation. The screw dislocation removal stabilizes the dissolution, resulting in constant values of atomic steps concentration on the pit walls and their dissolution rate during further etching, which continues according to the crystal dissolution stepwave model. It was found that the movement of AFM scanning tip essentially affected the etching process.