Intercalation of graphene on iridium with samarium atoms

Intercalation of graphene on Ir (111) with Sm atoms is studied by methods of thermal desorption spectroscopy and thermionic emission. It is shown that adsorption of samarium at T = 300 K on graphene to concentrations of N ≤ 6 × 10¹⁴ atoms cm^–2 followed by heating of the substrate leads to practically complete escape of adsorbate underneath the graphene layer. At N > 6 × 10¹⁴ atoms cm^–2 and increasing temperature, a fraction of adsorbate remains on graphene in the form of two-dimensional “gas” and samarium islands and are desorbed in the range of temperatures of 1000–1200 K. Samarium remaining under the graphene is desorbed from the surface in the temperature range 1200–2150 K. Model conceptions for the samarium–graphene–iridium system in a wide temperature range are developed.     

Electronic structure of pyrolytic graphite by auger electron spectroscopy

We have used Auger electron spectroscopy to study the effect of defects on the surface electronic states of graphite. We modeled the effects of outer layer inclusions composed of carbon and cesium atoms. The experimental Auger spectra are compared with theoretical calculations of the density of states in the valence band of graphite. An increase in defect density deforms the energy bands. Chelyabinsk State Teachers University. Translated from Izvestiya Vysshikh Uchebnykh, Zavedenii, Fizika, No. 10, pp 84–88, October, 1997.     

Field electron emission and field desorption of cesium from graphene

Field electron emission and field desorption of cesium ions from a monatomic graphene film on Ir and graphene clusters in amorphous carbon are investigated using field electron microscopy and continuous-mode field desorption microscopy. The deposition of cesium on amorphous carbon with graphite clusters leads to inversion of the emission (i.e., emission from the emission centers disappears against the back-ground of uniform emission from the previously nonemitting surface). In both systems, ion current pulses are observed during field desorption in a stationary electric field. During field desorption from the graphene film, current pulses of Cs⁺ ions with a duration shorter than 0.1 s appear from the plane faces of the iridium point. During desorption from graphite clusters, ion current pulses form a pattern of “collapsing rings” on the screen. Possible mechanisms of the observed processes are considered using the model of cesium intercalation by graphite and by the graphene layer and the desorption of Cs atoms under the action of the electric field, as well as the “flip” of the dipole moment during the cesium intercalation.     

Cesium adsorption on the β2-GaAs(001) surface

The results of first-principles calculations of the cesium adsorption energy on the β2-GaAs(001) surface performed within approaches of the density functional theory are presented for two possible terminations of the surface. It is shown that, among the considered high-symmetry positions, the energy-preferred position for cesium is position T ₃ when the surface layer contains arsenic and position T ₄ for gallium terminated surface. Cesium introduces insignificant perturbations in the positions of surface-layer atoms, and surface dimers do not break even in the case of adsorption at the dimer bridge and top positions. It is shown that cesium bonding to the GaAs (001) substrate can be explained by sp hybridization of arsenic and gallium orbitals as well as by formation of cesium states mixed with delocalized states of a clean surface. At low coverage, more preferable adsorbate sites are those with nearest neighbor arsenic atoms for both surface terminations.     

Hydrogen chemisorption on silicon (100) 2 × 1

Changes in the valence band density of states (DOS) of a (100) silicon surface that accompany he chemisorption of atomic hydrogen onto that surface are deduced from a study of the changes in the L_2,3VV Auger lineshape. Complementary changes in the conduction band DOS are inferred from changes in L_2,3VV-core-level characteristic loss spectra (CLS). The chemisorbed hydrogen layer is identified as the dihydride phase from low energy electron diffraction measurements. Upon hydrogen adsorption the DOS at the top of the valence band decreases and new energy levels associated with the Si-H bonds appear lower in the band. Assuming that the Auger signal from the hydrogen covered sample consists of a superposition of a signal from silicon atoms bonded to hydrogen in the dihydride layer and an elemental-Si signal from the substrate, a N(E) difference spectrum with features due only to the dihydride is obtained by subtracting the background corrected, loss deconvoluted L_2,3VV signal for a clean (100)Si surface rom the corresponding signal for the hydrogen covered surface. Comparisons of the energy position of the major peak in this difference spectrum with that of the main peak in a gas phase silane Si-L_2,3VV spectrum, and of the corresponding Auger energy calculated empirically, indicate a hole—hole interaction energy of ~8 eV for the two-hole final state in the gaseous system and zero for the dihydride surface system. Hydrogen induced changes in the conduction band DOS are less apparent than those of the valence band DOS with only the possibility of a decrease in the DOS at the bottom of the conduction band being inferred from the CLS measurements. Electron stimulated desorption of hydrogen from the dihydride layer is adduced from changes in the Auger lineshape under electron beam irradiation of the surface. Hydrogen induced changes in the near-elastic electron energy loss spectra (ELS) are also reported and compared with previously published ELS results.     

Intercalation of two-dimensional graphite films on metals by atoms and molecules

An analysis is made of some general laws governing a new physical effect, i.e., the spontaneous penetration of particles (atoms, C₆₀ molecules) adsorbed on a two-dimensional graphite film on a metal (Ir, Re, Pt, Mo,...) to beneath the graphite film (intercalation). It is shown that atoms having low ionization potentials (Cs, K, Na) intercalate a two-dimensional graphite film on iridium at T=300–400K with an efficiency χ≈0.5, accumulating beneath the film to a concentration of up to a monolayer. Atoms having high ionization potentials (Si, Pt, Ni, C, Mo, etc.) intercalate a two-dimensional graphite film on iridium at T≈1000K with an efficiency, χ≈1, forming beneath the film a thick intercalate layer which is strongly bonded chemically to the metal substrate but is probably weakly bonded to the graphite monolayer by van der Waals forces. The presence of a graphite “lid” impeding the escape of atoms from the intercalated state up to record high temperatures T∼2000K leads to superefficient diffusion (with an efficiency close to one) of various atoms (Cs, K) into the bulk of the substrate (Re, Ir). Zh. Tekh. Fiz. 69, 72–75 (September 1999)     

Graphene traps for cesium atoms

A significant increase in the surface concentration of cesium atoms intercalated under graphene islands on rhodium has been revealed when annealing the adsorbed layer in a ultrahigh vacuum. Heating leads to a decrease in the area of graphene islands due to the solution of carbon atoms in the metal bulk. At the same time, the edge carbon atoms in the islands, which are coupled with the surface by chemisorptive forces, prevent the leakage of the alkali metal from under the islands. This leads to the significant compression of cesium and to an increase in its surface concentration under the islands by a factor of almost 10. The desorption of cesium is observed only after the complete thermal destruction of graphene islands.     

Adsorption stage in the formation of Eu-Si(111) thin-film structures

The adsorption stage in the formation of the Eu-Si(111) interface has been studied within a broad temperature range by thermal and isothermal desorption spectroscopy, low-energy-electron diffraction, Auger electron spectroscopy, and the contact potential difference method. It is shown that the ordering of an adsorbed europium film is accompanied by silicon surface reconstruction throughout the coverage range studied, 0<θ≤1.8. This self-organized process is also shown to be thermally activated. Ordered adsorbed europium layers have been found to be made up of 2D islands, whose structure depends on the amount of the metal deposited on the surface. The energy required to remove atoms from an island to vacuum has been determined. This energy decreases with decreasing 2D lattice constant of the islands. This pattern of its variation is accounted for, in the final count, by the decrease of the number of the Si surface atoms not bound directly to Eu atoms.     

Anomalous changes in electron emission during adsorption of alkali metals on a carbon film

An anomalous change was discovered in the field of electron emission during the adsorption of alkali metal atoms on the surface of an amorphous carbon film. The phenomenon involves the disappearance of electron emission from graphite nanoclusters that were local emission sources before the deposition of cesium. The observed effect is explained on the basis of surface diffusion processes of cesium atoms in a nonuniform electric field and intercalation graphite nanostructures by cesium.     

The formation of a surface iodide on Ni{100} and adsorption of I2 at low temperatures

Iodine adsorption on clean Ni[100] has been investigated using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). At temperatures below 340 K. a saturated surface of adsorbed iodine atoms in a c(2 × 2) structure is observed. Adsorption of iodine on clean Ni{100} at temperatures in exces of 370 K forms a structure identified as a single layer of the layered compound NiI₂ on the metal substrate. Solid iodine is shown to grow epitaxially on both the c(2 × 2) chemisorbed surface and the surface iodide at temperatures less than 185 K. Heating to 185 < T < 226 K leaves a physisorbed molecular iodine layer, while on returning to room temperature the original c(2 × 2) or iodide is restored.     

Thermodynamics of xenon adsorption on Pd(s)[8(100) × (110)]: From steps to multilayers

The thermodynamic properties of the adsorption of xenon on the stepped Pd(s)[8(100)×(110)] surface have been studied over a wide range of pressure (5×10^?11 to 1×10^?4 Torr) and temperature (40–140 K). We have measured adsorption isobars using AES in order to evaluate the surface coverage. By choosing pressure and temperature we have studied under equilibrium conditions, the successive adsorption of xenon on the steps and on the terraces until the first layer is formed, the condensation of the second layer as well as the formation of xenon multilayers. For a small range of pressure and temperature, adsorption takes place only on the atomic steps. The LEED pattern shows that only every other site along the steps is occupied. The extrapolated initial heat of adsorption for steps is E_ad^S = 10.2 kcal/mol, decreasing monotonically by about 2 kcal/mol as the relative coverage of the step sites increases. The dipole moment of the Xe atoms adsorbed on steps is 1.12 D. During adsorption on the terraces the LEED observations suggest that the xenon adlayer is non-localized up to completion of the hexagonally close packed monolayer. The initial heat of adsorption on the terraces, E_ad^T is 8.2 kcal/mol and decreases continuously to a value of 6.9 kcal/mol for a complete monolayer due to lateral repulsive interactions between the adsorbed xenon atoms. The induced dipole moment of Xe on terraces is reduced to 0.49 D. The 5p_{$\text{1}\text{2}$} binding energy of Xe adsorbed on terrace sites is 0.3 eV smaller than that of Xe occuping step sites. The differential molar entropy of the adsorbed layer on the terraces as a function of coverage compares fairly well with the calculated value for an ideally mobile two-dimensional gas. No indication of the growth of two-dimensional xenon islands has been found under these conditions. The isosteric heat of adsorption for the second layer is E_ad^sec = 5.8 kcal/mol independently of the coverage. The condensation of the second layer is a first order two-dimensional gas ? two-dimensional solid phase transition in opposition to the continuous nature of the adsorption of the first layer (extending over a wide range of temperature for a given pressure). The induced dipole moment is further reduced for the Xe second layer to a value of 0.11 D. Finally, the condensation of multilayers proceeds with a latent heat of transformation of E_cond = 3.8 kcal/mol in excellent agreement with the known bulk value for the heat of sublimation of xenon. The line shape of the NVV low energy Auger transitions of xenon or the UPS binding energies of the Xe 5p_{$\text{3}\text{2}$,$\text{1}\text{2}$} spectra allow a clear distinction between first, second and higher layer Xe atoms. We have also established the temperature/pressure conditions for equilibrium between first, second and bulk xenon layers, i.e. a so-called “roughening point”.     

Reversible superstructural transitions on the GaAs(001) surface under the selective effect of iodine and cesium

The selective interaction of the iodine and cesium atoms with the GaAs(001) surface, which leads to a decrease in the bond energy of the Ga and As surface atoms, respectively, owing to the redistribution of the electron density in the near-surface region under the effect of electronegative and electropositive adsorbates, has been experimentally investigated. This selective interaction makes it possible to remove alternately the Ga and As monolayers in the iodine and cesium adsorption followed by heating at T ≤ 450°C and, thus, to implement reversible low-temperature transitions between the Ga-and As-stabilized superstructures, as well as the atomic layer etching of the semiconductor with the physically ultimate monolayer accuracy.     

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.     

Electron spectroscopy of surfaces by impact of metastable He atoms: CO on Pd(110)

Deexcitation of metastable He1 2¹S (excitation energy E1 = 20.6 eV) or 2³S (E* =19.8 eV) atoms at a clean Pd(110) surface proceeds through a two-stage process (resonance ionization + Auger neutralization, RI + AN). The measured electron energy distribution reflects the self-convolution of the local density of states of the outmost atomic layer. A CO adlayer suppresses the RI step and the spectra are caused by Auger deexcitation (Penning ionization). Comparison with corresponding UPS data allows identification of the valence orbitals of the adsorbate. Emission up to the Fermi level is ascribed to contributions from the 5σ level. The effectively available excitation energy in front of the adlayer is lowered by 0.5 eV. Extensive data on the variation of the intensities from the adsorbate valence levels with angle of incidence as well as of emission are presented and are analyzed in terms of an empirical model.     

Interaction of Co with an Fe(111) surface

Adsorption of CO on Fe(111) below 300 K causes the appearance of three different non-dissociated species as distinguished by their CO stretch frequencies of about 1530 cm^-1 (a), 1800 cm^-1 (b), and 2000 cm^-1 (c). At T ? 220 K the b-state is first filled up and saturates after 1.5 L exposure; upon increasing the temperature it partly desorbs around 400 K and partly dissociates. Recombination of the C and O atoms followed by CO desorption takes place at about 800 K. Above 1.5 L exposure the a- and c-states are occupied simultaneously; in the thermal desorption spectrum in turn they show up as a relatively broad shoulder at ～ 340 K, which indicates similar adsorption energies for these two species. Saturation of the surface is reached after about 6 L exposure, which is paralleled by a continuous work function increase of up to Δφ = 1.6 eV. A high background intensity in the LEED pattern suggests substantial disorder in the adlayer. Evaluation of the TDS data yields about 2:1 population of the b- and (c + a)-states. The unusual low CO frequency of the a-state finds its analogues in reports on CO adsorption at stepped surfaces, as well as with complex compounds where the π-orbitals of the ligand directly interact with a neighboring metal atom. This species is therefore identified with adsorption in the “deep hollow” sites on the rather open Fe(111) surface. The b-state is tentatively attributed to the “shallow hollow” sites, and the c-state to adsorption on the “on top” sites.     

Décomposition thermique du monoxide de carbone sur une surface de molybdène: Formation de couches superficielles de carbone et de carbure

We have investigated the decomposition of carbon monoxide on polycrystalline and (001), (110) monocrystalline molybdenum surfaces. This study was performed by massspectrometry, for thermal desorption studies, Auger electron spectrometry (AES), low energy electron diffraction (LEED) and photoelectron spectroscopy (ESCA). By heating the clean Mo surface in CO or by heating the Mo surface covered with CO, the dissociation of chemisorbed CO leads to a build-up of carbon layer which inhibits the subsequent adsorption. Two distinct types of fine structure are associated with the KLL line of carbon Auger spectra. If the Mo surface is heated at a temperature between 300 and 1500 K, the Auger peak is characteristic of a “graphite layer”. If the Mo surface is heated at a temperature up to 2000 K, the Auger peak is characteristic of a “carbure” layer. This “carbure layer” give rise to a surstructure which agrees with a Mo₂C surface layer and was also investigated by ESCA. Chemical shifts of (1s) C and (3d) Mo photoemission bands were observed and attributed to the bounding between Mo and C atoms in the Mo₂C layer.     

Characterization of doped diamond-like carbon films deposited by hot wire plasma sputtering of graphite

Diamond-like carbon (DLC) films doped with nitrogen and oxygen were deposited on silicon(100) and polytetrafluoroethylene (PTFE) substrates by hot wire plasma sputtering of graphite. The morphology and chemical composition of deposited films has been characterized by scanning electron microscopy, XPS, Auger, FTIR spectroscopy and micro-Raman scattering. Plasmon loss structure accompanying the XPS C 1s peak and electron energy loss spectroscopy (EELS) in reflection mode was used to study the fraction of sp³ bonded C atoms and the density of valence electrons. Raman spectra show two basic C–C bands around 1575 cm^-1 (G line) and 1360 cm^-1 (D line) . Auger depth profiling spectroscopy was used to measure the spatial distributions of C, N and O atoms in the surface layer of DLC films. The fraction of sp³ bonded atoms of about 40% was detected in DLC films by XPS plasmon loss and EELS techniques. Nitrile and iso-nitrile groups observed in FTIR spectra demonstrated the existence of sp bonded carbon in doped DLC films. The typical for DLC films specific density 1.7–1.8 g/cm³ was obtained from EELS and XPS data. PACS 52.77.Dq; 81.65.-b; 82.80.Pv     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

Cesium ion emission from a graphene surface on iridium in a high-intensity electric field

Features of cesium ion emission from a graphene film on iridium in a 10⁷–10⁸ V cm^?1 electric field of are studied. Field electron and desorption images demonstrate that the film consists of regions (islands) 20–100 nm in size. Intercalated cesium lies under the film. There is continuous desorption of cesium ions from the defects in the graphene film and the regions of where islands abut one another. Cesium ions arrive at the desorption sites from the intercalated state.