A LEED study of UO2(∼100) vicinal surfaces

The ordering and faceting properties of UO₂(～100) vicinal surfaces have been studied via LEED and Auger measurements. The measurements have demonstrated a reduced tendency for step ordering on UO₂(～100) vicinal surfaces when compared to step ordering on UO₂(～111) vicinal surfaces. The UO₂(～100) vicinal surfaces were observed to decompose irreversibly into low-index facets, including prominent (100) facets, at temperatures below those needed for creation of lowest index faceting on UO₂(～111) vicinal surfaces. These properties suggest that (100) terraces, in contrast to (111) terraces, act as surface diffusion barriers that limit longrange surface communication while growing at the expense of intermediate faceting stages.     

Morphology of Cn thin films (50 ⩽ n < 60) on graphite: Inference of energy dissipation during hyperthermal deposition

Low energy cluster beam deposition, LECBD, under UHV conditions has been used to generate thin films comprising monodispersed non-IPR fullerenes, C_n, 50 ? n < 60, on pyrolithic graphite surfaces (HOPG). The morphology of the resulting C_n deposits has subsequently been studied by ex situ atomic force microscopy. Deposition experiments were carried out under nominally normal incidence and at hyperthermal incident kinetic energies, E₀, varied between 1 and 40 eV. Surface temperatures during deposition, T_s, were varied from 300 K to the desorption onset of ～700 K. Initial sticking of C_n cages is governed by the lateral density of step edges, which act as pinning and nucleation centres for migrating cages. Consequently in the early deposition stages, the surface exhibits large areas of empty terraces, while the step edges themselves are well-decorated. The terraces in turn become decorated by dendritic C_n islands in later deposition stages. Both, the mean size of these 2D islands and the mean distance between nearest islands, δ, scale with the size of the terraces. When increasing the primary kinetic energy, the fractal-like islands become smaller and less dendritic in shape. The mean initial sticking coefficient decays exponentially with increasing E₀. The island topography has also been found to depend sensitively on the deposition temperature. Instead of the dendritic/fractal islands generated at room temperature, densely packed islands terminated by smooth rims are observed upon deposition at elevated temperatures. We rationalize our findings in terms of a three step deposition process involving: (i) conversion of perpendicular E₀ into hyperthermal surface parallel gliding/sliding motion, (ii) friction–dissipation of this surface-parallel kinetic energy within an (unexpectedly large) mean free path Λ followed by (iii) thermal diffusion. Λ, is observed to scale with E₀ and T_s.     

Spontaneous composition modulation during Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te(301) molecular beam epitaxy

The spontaneous composition modulation of a solid solution in Cd _x Hg_1?x Te during molecular beam epitaxy at CdTe/ZnTe/GaAs(301) substrates has been discovered. The study of the micromorphology of the film surfaces in an atomic-force microscope, together with the study of the film microstructure in a transmission electron microscope, has revealed that a periodic system of macrosteps oriented along the [010] direction and separated by (100) terraces is formed during epitaxial growth at temperatures higher than the optimal value on a Cd _x Hg_1?x Te(301) surface. The growth of the film of homogeneous composition is only possible at the fronts of macrosteps and it is not possible at (100) terraces, where a layered structure grows with composition modulated along the [100] direction having period of several tens of angström. The observed phenomenon is explained by decreased adsorption of mercury atoms in the (100) plane.     

Surface structure and solidification morphology of aluminum nanoclusters

Classical molecular dynamics simulation with embedded atom method potential had been performed to investigate the surface structure and solidification morphology of aluminum nanoclusters Al_n (n=256, 604, 1220 and 2048). It is found that Al cluster surfaces are comprised of (1 1 1) and (0 0 1) crystal planes. (1 1 0) crystal plane is not found on Al cluster surfaces in our simulation. On the surfaces of smaller Al clusters (n=256 and 604), (1 1 1) crystal planes are dominant. On larger Al clusters (n=1220 and 2048), (1 1 1) planes are still dominant but (0 0 1) planes cannot be neglected. Atomic density on cluster (1 1 1)/(0 0 1) surface is smaller/larger than the corresponding value on bulk surface. Computational analysis on total surface area and surface energies indicates that the total surface energy of an ideal Al nanocluster has the minimum value when (0 0 1) planes occupy 25% of the total surface area. We predict that a melted Al cluster will be a truncated octahedron after equilibrium solidification.     

LEED studies of vicinal surfaces of germanium

Clean germanium surfaces inclined at small angles to (111), (100) and (110) planes were investigated by LEED. Surfaces with orientations close to (111) and (100) are stepped and regular steps are retained in the whole temperature range investigated.Steps with (111) terraces and edges towards $\text{[2}\text{1}\text{1}\text{]}$ have a height of about one interplanar distance d₁₁₁ at all temperatures, and steps with edges towards $\text{[}\text{2}\text{11]}$ have a height of about two interplanar distances below 500°C and of about one interplanar distance above 500°C. Steps with (100) terraces and edges in the [011] direction have a height about two interplanar distances d₁₀₀. The surfaces with orientations close to (110) are facetted at room temperature. The (17 15 1) facets are present on the surfaces oriented in the $\text{[}\text{1}\text{10]}$ zone and the (10 9 2) facets on the surfaces oriented in the [001] zone. At high temperatures (about 480 and 770°C respectively) a reversible structural reconstruction of these surfaces into stepped ones takes place.     

Electronic properties of TiC(100) and polar TiC(111) surfaces

The energy bands of films of TiC have been calculated using the linear-combination-of-atomic-orbitals method with parameters obtained by a fit to the bulk band structure. The Madelung potentials and charge redistribution have been determined self-consistently. For the neutral TiC(100) surface, the density of states (DOS) is similar to that of the bulk. For the non-neutral Ti-covered TiC(111) surface, Ti 3d-derived surface states appear around the Fermi energy E_F. The long-range electric field produced by the polar surfaces is screened by the charge redistribution, and the polar surfaces are stabilized. Characteristic features of TiC(111) compared to other surfaces of TiC are attributed to the high surface DOS at E_F.     

SrTiO3(100) step sites as catalytic centers for H2O dissociation

Features arising from surface Sr(Ti) atoms on the SrO(TiO₂) terraces of SrTiO₃ (100) have been resolved in core-level photoemission spectra, which provides new insights into the surface electronic structure and reactivity of transition metal oxides. The surface Sr 3d (Ti3s) features lie to lower (higher) initial energy of the bulk-derived peaks by ca. 1.0 eV (1.7 eV), being consistent with the expected enhancement of covalent bonding in the TiO₂-terrace surface. Step-sites, which connect the two types of terrace, are found to act as catalytic centers for H₂O dissociation.     

A pocket model for the surface tension of compressed nuclei

The surface structure of compressed semi-infinite isoscalar nuclear matter is studied starting from a simple energy density functional and adding constraints equivalent to some external pressure. For a wide class of compression modes we derive analytical expressions for the surface tension σ as a function of the (de)compressed bulk density n_c. The σ(n₀) = 0 theorem is verified for all modes. The model is tested with numerical calculations using realistic Skyrme-ETF energy density functionals including spin-orbit and effective mass contributions. They confirm the simple analytical model results almost quantitatively. The mode which leads to an antiscaled density (increasing surface diffuseness with increasing bulk density) leads to minimum values for σ around n₀. The implications of the static model for the compression modes in finite nuclei are discussed.     

Effects of evolving surface morphology on yield during focused ion beam milling of carbon

We investigate evolving surface morphology during focused ion beam bombardment of C and determine its effects on sputter yield over a large range of ion dose (10¹⁷-10¹⁹ ions/cm²) and incidence angles (Θ = 0-80°). Carbon bombarded by 20 keV Ga⁺ either retains a smooth sputtered surface or develops one of two rough surface morphologies (sinusoidal ripples or steps/terraces) depending on the angle of ion incidence. For conditions that lead to smooth sputter-eroded surfaces there is no change in yield with ion dose after erosion of the solid commences. However, for all conditions that lead to surface roughening we observe coarsening of morphology with increased ion dose and a concomitant decrease in yield. A decrease in yield occurs as surface ripples increase wavelength and, for large Θ, as step/terrace morphologies evolve. The yield also decreases with dose as rippled surfaces transition to have steps and terraces at Θ = 75°. Similar trends of decreasing yield are found for H₂O-assisted focused ion beam milling. The effects of changing surface morphology on yield are explained by the varying incidence angles exposed to the high-energy beam.     

Structural specificity in CO and H2 oxidation over single crystal copper surfaces

The reaction rates of CO and H₂ with preadsorbed oxygen on copper surfaces in the 〈110〉 zone presented by a cylindrical single crystal are found to display strong structural sensitivity. In both cases the highest reaction rates are observed for surface structures involving both steps and low index face terraces, and in particular surfaces vicinal to {311} and between either {111} or {100} are found to exhibit the highest rates of reaction. Possible mechanisms for the reaction are discussed, and the data are extrapolated to investigate the steady state oxidation of CO in a CO/O₂ gas mixture over copper surfaces in the 〈110〉 zone. The semi-empirical model describing the steady state oxidation reaction predicts that this reaction also exhibits marked crystallographic anisotropy at CO/O₂ partial pressures chosen to maximise the reaction rate.     

Leed studies of vicinal surfaces of silicon

Clean silicon surfaces inclined at small angles to (111), (100) and (110) planes were investigated by LEED. Surfaces oriented at low angles to the (111) plane contain steps with edges towards [2&#x0304;11] or [21&#x0304;1&#x0304;]. Steps with edges towards [2&#x0304;11] have a height of two interplanar distances d₁₁₁ at low temperatures. At 800°C the reversible reconstruction of this step array into the steps of monolayer height takes place. Steps with edges towards [21&#x0304;1&#x0304;] can be seen at low temperatures only. They are of monolayer height and disappear at annealing in vacuum. Surfaces oriented at low angles to the (100) plane contain steps with (100) terraces and have a height of about two interplanar distances d₁₀₀. Surfaces at low angles to (110) planes are facetted and contain facets of the (47 35 7) type. The information about surface self-diffusion of silicon may be obtained using the kinetic data of structural reconstructions on surfaces close to (111) at different temperatures.     

Linear scaling of the interlayer relaxations of the vicinal Cu(p,p,p − 2) surfaces with the number of atom-rows in the terraces

We investigate the multilayer relaxation trends in the vicinal Cu(p,p,p − 2) surfaces employing the all-electron full-potential linearized augmented plane-wave method. Calculations are performed for the (3 3 1), (2 2 1), (5 5 3), (3 3 2), (7 7 5), and (4 4 3) surfaces, which have 3, 4, 5, 6, 7, and 8 atom-rows in the terrace, respectively. The following trends are identified: (i) The interlayer relaxations perpendicular to the surface scale almost linearly with the number of atom-rows in the terraces. (ii) The nearest-neighbor distances do not depend on the surface termination, but only on the local coordination. (iii) For Cu(p,p,p − 2) in which the topmost n surface layers have nearest-neighbor coordination smaller than the bulk Cu (calculated for the unrelaxed surfaces), the topmost (n − 1) interlayer spacings (d₁₂, … , d_n−1,n) contract compared with the unrelaxed spacing, while the nth interlayer spacing (d_n,n+1) expands. The next (n − 2) interlayer spacings (d_n+1,n+2, … , d_{2n−2,2n−1}) contract, while the interlayer spacing indicated by d_2n−1,2n expands. A similar rule was found for the relaxations parallel to the surfaces. These trends provide a better understanding of the atomic structure of vicinal Cu surfaces.     

The voigt-type magneto-kerr effect in cubic semiconductors having ellipsoidal constant energy surfaces

A plane-wave analysis is given for the Voigt orientation of an applied static magnetic field ?B in cubic semiconductors having ellipsoidal constant energy surfaces. The effect is shown to be highly anisotropic with respect to the orientation of ?B (magneto-anisotropy) as well as to the initial polarization direction of the incident wave with respect to ?B (polarization anisotropy). The character and extent of the magneto-anisotropy depends not only on the location of the energy extrema, but on the anisotropy of the energy surfaces and on the energy dependence and anisotropy of the scattering processes.Calculations are given for the cases of propagation along 〈100〉 and 〈110〉 crystallographic directions in n-type germanium. High-field effects and infrared (IR) behavior are considered; however, the low-field quadratic range for microwaves in high-loss material is emphasized. A useful approximation for the high-loss case is given. Particular attention is given to the potential diagnostic applications of the effect.Room-temperature measurements of the Voigt-type Kerr effect for the TE₁₁ circular-waveguide mode at 35 GHz in n-type Ge are presented and compared with plane-wave calculations for propagation along 〈100〉 and 〈110〉 directions. Good quantitative agreement is obtained when the plane-wave calculations are reduced by the same mode-reduction factor as is applicable for the Faraday orientation.     

Surface enhanced covalency and Madelung potentials in Nb doped SrTiO3 (100), (110) and (111) single crystals

The influence of surface enhanced covalency on the Madelung potential is experimentally investigated using angle-resolved photoemission for (100), (110) and (111) SrTiO₃ surfaces after annealing in UHV at 630 °C. Deconvolution of the core level spectra (O 1s, Sr 3d and Ti 2p) distinguishes bulk and surface components, which are interpreted in terms of surface enhanced covalency (SEC). By comparing the experimentally measured binding energies with theoretical calculations developed in the framework of the Localized-Hole Point-Ion Model, we quantitatively determine the effective electron occupancy at bulk and surface Sr and Ti sites. Our results confirm the essentially ionic character of Sr–O bond and the partially covalent character of Ti–O bond in bulk STO. The cation Ti and Sr electron occupation is greater for all the three surfaces than in the bulk. Surface covalency shifts the Madelung potential at the surface by ΔE_M. ΔE_M is a minimum for the (111) surface, and increases through (100), attaining a maximum for (110). The angle-resolved valence band spectra and the work function values also confirm this trend. The results are consistent with d–d charge fluctuations dominating at the surface, whereas metal-ligand charge transfers are more energetically favourable in the bulk.     

Laser ultraviolet fragmentation of homogeneous (CF3I) n clusters in a molecular beam and (CF3I) n clusters inside of large (Xe) m clusters or on their surface

The fragmentation of homogeneous (CF₃I) _n clusters (where n ≤ 45 is the average number of molecules in a cluster) in a molecular beam, as well as (CF₃I) _n clusters inside of large (Xe) _m clusters (where m ≥ 100 is the average number of atoms in a cluster) or on their surface, by laser ultraviolet radiation has been studied. It has been found that the indicated three types of (CF₃I) _n clusters have different stabilities with respect to fragmentation and strongly different dependences of the fragmentation probability on the energy of ultraviolet radiation. Fragmentation at low energies and the weakest energy dependence of the probability of fragmentation are observed for homogeneous clusters, a stronger dependence is characteristic of (CF₃I) _n clusters localized inside (Xe) _m clusters, and the strongest dependence is observed for (CF₃I) _n clusters on the surface of (Xe) _m clusters. Possible reasons for such a character of the observed dependences have been discussed.     

Thermal activation measurement of positron binding energies at surfaces

The formation of positronium by low-energy positrons incident on “clean” metal surfaces is thermally activated by increasing temperature. The activation energy E_a has been measured for a number of surfaces. E_a is understood as the energy required to form positronium (binding energy 1/2Ry) from positrons bound at the surface by an energy E_b, E_a = E_b+φ_-? 1/2Ry, where φ_- is the electron work function. Representative values of E_b derived are Al(100):3.03(5) eV; Al(110):2.92(4) eV; Cu(111):2.80(5) eV.     

Selenium adsorption on Cs-covered Si(100) 2×1 surfaces

This report involves the study of Se adsorption on caesiated Si(100) 2×1 surfaces in ultra high vacuum (UHV) using low energy electron diffraction, Auger electron spectroscopy, thermal desorption spectroscopy and work function measurements. Selenium atoms on Cs/Si(100) 2×1 surface adsorb initially on uncaesiated portions of Si and subsequently on the Cs overlayer. The presence of Se increases the binding energy of Cs on Si(100). For Cs and Se coverages above 0.5 ml CsSe and Cs_xSe_ySi_z, compound formation was observed. The coadsorption of Se and Cs induces a high degree of surface disorder, while desorption most probably causes surface etching. The presence of Cs on Si(100) 2×1 surfaces prevents the diffusion of Se into the Si substrate and greatly suppresses the formation of SiSe₂ and SiSe₃, detected when Se is adsorbed on clean Si(100) 2×1 surfaces.     

Low energy electron diffraction and auger electron spectroscopy studies of the structure of adsorbed gases on solid surfaces

Low energy electron diffraction (LEED) studies of the structure of adsorbed molecules on crystal surfaces revealed that ordered surface structures predominate under most conditions of the experiments. In the absence of chemical reactions with the substrate, the degree of ordering depends on the heats of adsorption, ΔH_ads, and the activation energies for surface diffusion, ΔE_D1. Since ΔH_ads is usually markedly larger than ΔE_D1, small changes of substrate temperature facilitate ordering without appreciable increase in desorption rates. The surface structures of adsorbed gases that have been reported so far have been tabulated. For molecules whose size is compatible with the interatomic distance of the substrate, rules of ordering can be proposed that permit prediction of the structure of the adsorbed layer that is likely to form. These rules indicate close packing due to attractive interactions in the adsorbed layer, and that the rotational multiplicity of the substrate is likely to be maintained by the adsorbate structure. When molecules whose dimensions are larger than the substrate interatomic distance are adsorbed, the conditions that control ordering are more complex and simple rules may not be readily applicable.The surface structures of adsorbed gases have also been studied on high Miller Index substrate surfaces. These surfaces are characterized by ordered steps separated by terraces of low index surface orientation. Many gases have different ordering characteristics on stepped surfaces than on low index crystal faces due to the stronger substrate-adsorbate interactions in these surfaces. The dissociation of diatomic molecules at steps induces the formation of new types of surface structures (frequently one-dimensional) and the dehydrogenation of hydrocarbons at steps induces the formation of ordered carbonaceous surface structures that would not nucleate on low index substrate planes.So far, mostly work function changes upon adsorption gave indication of the magnitude of charge transfer upon adsorption and on forming of new surface chemical bonds. Most recently, chemical shifts of the Auger transitions of the substrate atoms and of the adsorbed molecules upon chemisorption, have been found to provide additional information on charge redistribution during adsorption.     

The adsorption of CO,O2, and H2 on Pt: I. Thermal desorption spectroscopy studies

Thermal desorption spectroscopy (TDS) has been used to study the chemisorption of CO, O₂, and h₂ on Pt. It has been found that TDS is quite sensitive to local surface structure. Three single crystal and two polycrystalline Pt surfaces were studied. One single crystal was cut to expose the smooth, hexagonally close-packed plane of the fee Pt crystal (the (111) surface). The other two single crystals were cut to expose stepped surfaces consisting of smooth, hexagonally close-packed terraces six atoms wide separated by one atom high steps (the 6(111) × (100) and 6(111) × (111) surfaces). Only one predominant desorption state was observed for CO and H adsorbed on the smooth (111) single crystal surface, while two predominant desorption states were observed for these gases adsorbed on the stepped single crystal surfaces. The low temperature desorption states on the stepped surfaces are attributed to desorption from the terraces, while the high temperature desorption states are attributed to desorption from the steps. TDS of CO from the polycrystalline foils exhibited some desorption states which were similar to those observed on the stepped single crystal surfaces, indicating the presence of adsorption sites on the polycrystalline foils that were similar to the terrace and step sites on the stepped single crystals. In general, these results suggest a high density of defect sites on the polycrystalline foils which can not be attributed simply to adsorption at grain boundaries. Oxygen was found to adsorb well on the stepped single crystals and on the polycrystalline foils, but not on the smooth (111) single crystal, under the conditions of these experiments. This is attributed to a higher sticking probability for dissociative O₂ adsorption at steps or defects than on terraces.     

Interface capacitance of nano-patterned electrodes

By employing numerical solutions of the Poisson–Boltzmann equation we have studied the interface capacitance of flat electrodes with stripes of different potentials of zero charge ?_pzc. The results depend on the ratio of the width of the stripes l to the dielectric screening length in the electrolyte, the Debye length d_Debye, as well as on the difference Δ?_pzc in relation k_BT/e. As expected, the capacitance of a striped surface has its minimum at the mean potential of the surface if l/d_Debye << 1 and displays two minima if l/d_Debye >> 1. An unexpected result is that for Δ?_pzc ? 0.2V, the transition between the two extreme cases does not occur when l ? d_Debye, but rather when l > 10d_Debye. As a consequence, a single minimum in the capacitance is observed for dilute electrolytes even for 100 nm wide stripes. The capacitance at the minimum is however higher than for homogeneous surfaces. Furthermore, the potential at the minimum deviates significantly from the potential of zero mean charge on the surface if l > 3d_Debye and Δ?_pzc is larger than about 4k_BT/e. The capacitance of stepped, partially reconstructed Au(11n) surfaces is discussed as an example. Consequences for Parsons–Zobel-plots of the capacitances of inhomogeneous surfaces are likewise discussed.