Applied Surface Science (v.254, #14)
by Robert Kucharczyk; Marek Nowicki (4217).
Adsorption of chlorophenol on the Cu(1 1 1) surface: A first-principles density functional theory study by Mohammednoor Altarawneh; Marian W. Radny; Phillip V. Smith; John C. Mackie; Eric M. Kennedy; Bogdan Z. Dlugogorski (4218-4224).
The interaction between a 2-chlorophenol (C6H4OHCl) molecule and the Cu(1 1 1) surface has been investigated using density functional theory as an initial step in gaining a better understanding of the catalyzed formation of dioxin compounds on a clean copper surface. The 2-chlorophenol molecule is found to form several weakly bonded, horizontally and vertically oriented configurations. Dissociative modes have also been investigated. For the latter, the formation of phenyl and benzyne fragments is found to be more energetically favourable than the formation of 2-chlorophenoxy radicals.
Keywords: Density functional calculations; Models of surface chemical reactions; Chemisorption; Copper;
Direct interactions between metal nanoparticles and support: STM studies of Pd on TiO2(1 1 0) by Michael Bowker; Elodie Fourré (4225-4229).
We have fabricated ultra-nanoparticulate model catalysts of Pd on TiO2(1 1 0) using metal vapour deposition (MVD) to form particles in the size range 1–50 nm, which can be imaged at very high spatial resolution (and in some cases at atomic resolution) using scanning tunnelling microscopy (STM). Using these methods we are able to identify the atomic level mechanism responsible for certain phenomena in catalysis, for which molecular level models have previously been proposed from macroscopic measurements. In this paper we address two such phenomena, namely spillover and the so-called strong metal–support interaction (SMSI) effect. Oxygen spillover from Pd particles to the titania support occurs due to the fast adsorption of oxygen on Pd compared with titania, and is driven by reaction with Ti3+ ions in the vicinity of the particles. The SMSI state is identified at atomic resolution as being due to the appearance of Ti at the surface of the Pd particles. These Ti layers are partially oxidised and form very well defined structures of two main types—a rectangular lattice and hexagonal unit cells of large dimension. These layers passivate the surface for the adsorption of CO.
Keywords: Model catalysts; Pd nanoparticles; Oxygen spillover; SMSI; Strong metal–support interaction; STM;
Nanostructured metal surfaces as quasi-one-dimensional model systems by E. Dona; P. Amann; M. Cordin; A. Menzel; E. Bertel (4230-4237).
The physics of quasi-one-dimensional systems deviates considerably from that of higher dimensional systems and is not yet completely understood. The phase diagram is surprisingly complicated and technologically highly interesting. This presentation focuses on the Pt(1 1 0) surface and halogen/Pt(1 1 0) adsorption systems as models for quasi-one-dimensional behaviour. The dimensional reduction results in a reduced screening and the onset of correlation effects despite the presence of the metallic bulk. Anomalous features in the electronic structure as probed by angle-resolved photoemission are observed and reveal indeed a significant correlation in the system. The phase diagram of the halogen/Pt(1 1 0) adsorption system is studied in detail by scanning tunnelling microscopy (STM) and low-energy electron diffraction (LEED) spot profile analysis. Analysis of the experimental data reveals the presence of fluctuations over a surprisingly large range of the phase diagram. Counter-intuitively, the long-range order of a uniform room temperature phase is destructed as the temperature is lowered. At 50 K three different phases coexist in an anisotropic glassy state. The anomalous loss of long-range order upon cooling is attributed to an electronically driven instability.
Keywords: Surface states; Correlation; Charge density waves; Phase transitions; Phase separation;
Different approaches to adjusting band offsets at intermolecular interfaces by P.A. Dowben; Jie Xiao; Bo Xu; Andrei Sokolov; B. Doudin (4238-4244).
We show that at the hetero-molecular interface, the molecular band offsets can be modified by either adjusting the dopant or dopant concentration of one or both molecular layers or by changing the dipole orientation at the intermolecular interface. Photoemission studies reveal the changes in electronic structure and diode devices exhibit conduction properties that are altered in response to changes to the molecular band offsets. As a demonstration, thin film copper phthalocyanine to crystalline ferroelectric copolymer poly(vinylidene fluoride with trifluoroethylene) heterojunction diodes, without additional dopants, are compared with doped polyaniline layers on the crystalline ferroelectric copolymer poly(vinylidene fluoride with trifluoroethylene).
Keywords: Organic heterojunctions; Ferroelectric polymers; Molecular orbital alignment;
Determination of deuterium adsorption site on palladium(1 0 0) using low energy ion recoil spectroscopy by I. Kambali; D.J. O’Connor; M.J. Gladys; M.A. Karolewski (4245-4250).
Ion beam analysis has been recently applied to study the adsorption phenomena of some adsorbates on metal surfaces. In this paper, surface recoils created by low energy Ne+ ions are employed to study the adsorption site of deuterium (D) atoms on Pd(1 0 0). This technique is extremely surface sensitive with the capacity for atomic layer depth resolution. From azimuthal angle observations of Pd(1 0 0) specimen, it was found that at room temperature, D was adsorbed in the fourfold hollow site of Pd(1 0 0) at a height of 0.25 ± 0.05 Å above the surface. The adsorbate remains in the hollow site at all temperatures to 383 K though the vertical height above the surface is found to depend on coverage and for the first time evidence is found of a transition to a p(2 × 2) structure for the adsorbate. There is no evidence of D sitting in the Pd(1 0 0) subsurface at room and higher temperatures.
Keywords: Hydrogen adsorption site; Low energy ion scattering; Palladium;
Scanning tunneling microscopy and spectroscopy investigations of copper phthalocyanine adsorbed on Al2O3/Ni3Al(1 1 1) by M. Moors; A. Krupski; S. Degen; M. Kralj; C. Becker; K. Wandelt (4251-4257).
Low temperature scanning tunneling microscopy (LT-STM) and scanning tunneling spectroscopy (STS) have been used to investigate adsorbed copper phthalocyanine (C32H16N8Cu) molecules on an ordered ultrathin Al2O3 film on the Ni3Al(1 1 1) surface as a function of coverage and annealing temperature. For sub-monolayer coverage and a deposition temperature of 140 K two different planar molecular adsorption configurations rotated by 30° with respect to each other were observed with submolecular resolution in the STM images. The template effect of the underlying oxide film on the CuPc orientation, however, is only weak and negligible at higher coverages. For θ CuPc ≈ 1 ML, before completion of the first layer, the growth of a second layer was already observed. The measured spacing of 3.5 Å between first and second layer corresponds to the distance between the layers in the α-modification of crystalline CuPc. The molecules deposited at 140 K are thermally stable upon prolonged annealing to temperatures up to 250 K. By the use of STS the lowest unoccupied molecular orbital (LUMO) of the adsorbed copper phthalocyanine molecules has been identified at an energy of 1.2 eV above EF. The lateral distribution of the electronic states of the CuPc has been analyzed and mapped by STS.
Keywords: Copper phthalocyanine; Alumina film; Scanning tunneling microscopy and spectroscopy (STM/STS);
Efficient channel of the associative oxygen desorption from Pt(1 1 1) by I.N. Yakovkin; N.V. Petrova (4258-4262).
Mechanism of the associative desorption of oxygen from the Pt(1 1 1) surface has been studied on atomic level by means of density-functional calculations. Key to the association of two oxygen adatoms into the O2 molecule is the excitation of one of the adatoms to on-top site, where it becomes essentially neutral. The related redistribution of the electronic density at the O adatom leads to the appearance of the lateral attraction with the other O atom, thus providing an efficient channel for associative desorption. Calculated local densities of states resemble the transformation of the electronic structure of adsorbed O adatoms from the reactive to bound state in the course of association.
Keywords: Associative desorption; Oxygen; Low-index transition metal surfaces; DFT;
Electron spectroscopy studies of surface In–Ag alloy formation on the tungsten surface by A. Bukaluk; M. Trzciński; K. Okulewicz (4263-4267).
XPS and UPS investigations of ultrathin films of In/Ag and Ag/In, deposited onto the W(1 1 0) surface in the ultrahigh vacuum conditions have been performed. Indium and silver films were formed by “in-situ” evaporation on W(1 1 0) substrate. XPS and UPS studies have been performed by means of SCIENTA ESCA200 instrument. The changes of In4d core-level and Ag4d valence band emissions with increasing Ag and In coverage were monitored to observe the energy shift and shape of the spin–orbit doublet of In4d and Ag4d lines in the Ag/In/W and In/Ag/W systems. UPS (HeI and HeII) measurements were supported by XPS AlKα measurements of In3d and W4p levels, as well as by investigations of Ag3d levels. XPS and UPS data allowed to evaluate the coverage and make conclusions concerning intermixing and surface alloying in the In/Ag/W and Ag/In/W systems. W(1 1 0) substrate can be cleaned after each deposition by thermal desorption and no alloying in the In/W and Ag/W systems is observed.
Keywords: Crystal surfaces; Photoelectron spectroscopy; Metallic films; Indium; Silver; Tungsten; Alloys;
Scanning tunneling microscopy investigations of silicon carbide nanowires by A. Busiakiewicz; Z. Klusek; A. Huczko; P.J. Kowalczyk; P. Dąbrowski; W. Kozłowski; S. Cudziło; P.K. Datta; W. Olejniczak (4268-4272).
Scanning tunneling microscopy (STM) images have been obtained from the surfaces of silicon carbide nanowires produced in the thermolysis-induced carbonization of halocarbons (combustion synthesis). The morphology of the nanowires shows trench-like features perpendicular to the fibres’ axis, which is assigned to the existence of microfacets on their sidewalls. For the first time high-resolution STM images of the SiC nanowires are presented. The results are in agreement with the previous reports suggesting the presence of microfacets on the SiC whiskers’ surface.
Keywords: Silicon carbide; Nanowires; Nanofibres; Scanning tunneling microscopy;
Preparation and tribological tests of thin fluoroorganic films by M. Cichomski; J. Grobelny; G. Celichowski (4273-4278).
Adhesion, friction and consequent wear of sliding surfaces are the basic problems that limit the performance and reliability of microelectromechanical devices. Lubrication of these nano- and microscale contacts is different from traditional lubricants. Self-assembled monolayers (SAMs) chemically bonded to the substrate are considered to be the best solution of lubrication. The majority of these monolayers are hydrophobic providing low friction, adhesion and wear.Chemical vapor deposition was used to grow a fluorosilane film on silicon Si(1 0 0) and a condensed monolayer of 3-mercaptopropyltrimethoxysilane (MPTMS) on Au(1 1 1). The films were characterized by means of a contact angle analyzer for hydrophobicity, and time-of-flight secondary ion mass spectrometry (ToF-SIMS) for identification of thin fluoroorganic monolayers deposited on silica surfaces and condensed monolayer MPTMS. Adhesion and friction measurements were performed using atomic force microscopy (AFM) and compared with measurements performed using a microtribometer operating in millinewton (mN) normal load range. Nanotribological measurements indicated that silica and MPTMS modified by fluorosilanes have the lowest friction coefficient and indicated a decrease friction coefficient with increasing fluoric alkyl chain length.
Keywords: Fluorosilanes; ToF-SIMS; AFM; Nanotribology; Microtribology; SAMs;
The Na-adsorbed Ge(0 0 1) surface: Structure and STM image simulations by Barbara Czech; Barbara Stankiewicz (4279-4285).
Sodium adsorbed on the Ge(0 0 1) surface causes reconstruction of the surface with the type of reconstruction depending on the amount of the adsorbate. We present theoretical investigations of the structure and electronic properties of Na-adsorbed Ge(0 0 1) for the coverage of 0.5 monolayer using the combination of two methods: a plane-wave basis method and a local-orbital minimal-basis method. Two possible minimum-total-energy atomic configurations have been found, namely, the Na/Ge(0 0 1)-p(2 × 1) and Na/Ge(0 0 1)-p(4 × 1) reconstructions. The surface electronic structure for all calculated configurations occurs to be metallic. Our investigations are completed by a simulation of STM images for the obtained atomic structures.
Keywords: Germanium; Alkali metals; Adsorption; Density functional calculations; Surface relaxation and reconstruction;
Hydrogen adsorption on rhodium: Hydride formed on the surface of thin rhodium films by R. Duś; E. Nowicka; R. Nowakowski (4286-4291).
H2 interaction with thin Rh films deposited on Pyrex glass under UHV conditions has been studied by simultaneous measurement of work function changes ΔΦ and hydrogen pressure P, at selected constant temperatures: 78 and 298 K. Prior to the adsorption experiments the thin film topography was illustrated using the AFM and STM methods. The influence of hydrogen adsorption on the resistance of thin Rh film was examined in the course of an independent experiment. The number of sites accessible for adsorption on the thin Rh film surface was found determining population of oxygen adatoms within the monolayer at 78 K, when incorporation of these adspecies below the surface is negligible. It was established that at all examined temperatures hydrogen adsorption led to coverage Θ approaching 1 under equilibrium pressure below 10−3 Pa, increasing the work function. Under higher H2 pressure an additional uptake of hydrogen leading to Θ ∼ 1.68 at 298 K, and to Θ ∼ 2 at 78 K is reached. On this surface at low temperatures there exist weakly bound, reversibly adsorbed, positively charged adspecies characteristic for hydrogen adsorption on transition metal hydrides. The change of thin Rh film resistance caused by hydrogen adsorption was not measurable.
Keywords: Thin rhodium films; Hydrogen adsorption;
Morphology of alkali halide thin films by F. Gołek; P. Mazur; Z. Ryszka; S. Zuber (4292-4296).
Two alkali halide adsorbate–substrate systems were investigated by atomic force microscopy (AFM) working in contact mode. Adsorbate film orientation relative to the substrate was determined from the arrangement of the atomic steps of the substrate and the edges of the forming islands. In this work we present experimental results obtained for systems: NaCl/LiF(0 0 1) and LiF/NaCl(0 0 1), which exhibit a strong tendency of the self-assembly into regular structures.
Keywords: Alkali halides; Nanostructures; Thin films;
The iron oxidation state in Mg–Al–Fe mixed oxides derived from layered double hydroxides: An XPS study by M. Hadnadjev; T. Vulic; R. Marinkovic-Neducin; Y. Suchorski; H. Weiss (4297-4302).
Mixed oxides with large surface area and high thermal stability can be obtained by thermal treatment of the layered double hydroxides (LDH). Mg–Al–Fe mixed oxide samples with varying Mg/Al ratio and 5 mol.% of Fe were prepared in this way and the iron oxidation state (Feox) in these compounds was studied by X-ray photoelectron spectroscopy (XPS), using a calibration based on the relation of Feox to the splitting between the O 1s and Fe 2p3/2 centroids. The XPS results confirm Fe3+ as a dominant oxidation state in the studied mixed oxides. A vacuum-induced reduction of iron in the Fe2O3 and Mg–Al–Fe oxide samples has been observed and an influence of the Mg:Al ratio on this effect in mixed oxides has been detected. The role of the local variations of the electron density distribution in the close neighbourhood of the surface oxygen atoms in the mixed oxides in the reduction processes is discussed.
Keywords: X-ray photoelectron spectroscopy; Layered double hydroxides; Mixed oxides; Iron oxide;
Structural investigations of TiO2:Tb thin films by X-ray diffraction and atomic force microscopy by D. Kaczmarek; J. Domaradzki; D. Wojcieszak; R. Wasielewski; A. Borkowska; E.L. Prociow; A. Ciszewski (4303-4307).
In this work, structural investigations of TiO2 thin films doped with Tb at the amount of 0.4, 2 and 2.6 at.% have been outlined. Thin films were deposited on Si and SiO2 substrates by high energy reactive magnetron sputtering from mosaic Ti–Tb target. The influence of Tb dopant amount, post-annealing treatment and kind of applied substrate on microstructure has been discussed. Thin films were investigated by means of X-ray diffraction (XRD) and atomic force microscopy (AFM). XRD analysis revealed the existence of crystalline TiO2 in anatase and rutile forms, depending on Tb amount in examined samples. AFM images show that as-deposited samples with 0.4 at.% concentration of terbium (anatase structure) have bigger crystallites as compared to 2% and 2.6 at.% of Tb (rutile structure). The additional annealing at 1070 K results in a mixed anatase (77%) and rutile (23%) structure.
Keywords: TiO2; Thin film; Sputtering; Terbium; AFM;
Gd-doping of HfO2 by Ihor Ketsman; Ya.B. Losovyj; A. Sokolov; Jinke Tang; Zhenjun Wang; M.L. Natta; J.I. Brand; P.A. Dowben (4308-4312).
An increase in the density of states between the oxygen 2p bands and the Fermi level is seen with increasing Gd concentrations. In addition, for the Gd-doped HfO2 films, the Gd 4f photoexcitation peak at 5.5 eV below the valence band maximum was identified using resonant photoemission. Electrical measurements show pronounced rectification properties for lightly-doped Gd:HfO2 films on p-Si and for heavily-doped Gd:HfO2 films on n-Si, suggesting a crossover from n-type to p-type behavior with increasing doping level. In addition, there is an increase in the reverse bias current with neutron irradiation.
Keywords: Oxide dielectric layers; Hafnium oxide; Gd doping; Resonant photoemission;
Photoelectron diffraction effect in the highly ordered Si(5 5 7)/Pb surface by M. Kisiel; K. Skrobas; M. Jałochowski (4313-4317).
The electronic structure of vicinal Si(5 5 7) surface covered with 2 ML Pb, ordered after annealing at 640 K as found previously [C. Tegenkamp, Z. Kallassy, H.-L. Gunter, V. Zielasek, H. Pfnür, Eur. Phys. J. B 43 (2005) 557; C. Tegenkamp, Z. Kallassy, H. Pfnür, H.-L. Gunter, V. Zielasek, M. Henzler, Phys. Rev. Lett. 95 (2005) 176804], is studied with angle-resolved photoemission spectroscopy (ARPES) at a temperature of 130 K. The spectra show a superposition of Pb-induced electronic surface states and the Si(1 1 1) bulk band states. The observed splitting of a Si bulk band suggests photoelectron diffraction on the one-dimensional grid of the vicinal surface.
Keywords: ARPES; Vicinal surface; Si(5 5 7); Final state diffraction;
First principles study of Si(3 3 5)–Au surface by Mariusz Krawiec (4318-4321).
The structural and electronic properties of gold decorated Si(335) surface are studied by means of density-functional calculations. The resulting structural model indicates that the Au atoms substitute some of the Si atoms in the middle of the terrace in the surface layer. Calculated electronic band structure near the Fermi energy features two metallic bands, one coming from the step edge Si atoms and the other one having its origin in hybridization between the Au and neighboring Si atoms in the middle of the terrace. The obtained electronic bands remain in good agreement with photoemission data.
Keywords: Density functional calculations; Silicon; High index surfaces; Surface structure; Photoelectron spectroscopy;
The bulk band structure and inner potential of layered In4Se3 by Jing Liu; Ya.B. Losovyj; Takashi Komesu; P.A. Dowben; L. Makinistian; E.A. Albanesi; A.G. Petukhov; P. Galiy; Ya. Fiyala (4322-4325).
The layered In4Se3 system does have a bulk band structure (i.e. discernable and significant band dispersion) perpendicular to the cleavage plane. Band widths (the extent of dispersion) of 300 meV or more are observed, for In-p and Se-p weighted bands within the valence region, and is indicative of a bulk band structure. Two-dimensionality of state is clearly not conserved, and there must exist interactions between layers sufficient to support a bulk band structure.
Keywords: Layered semiconductors; Bulk band structure; Photoemission; FP-LAPW;
The gold and oxygen (3 × 1) structures on W(1 1 2) by Ya.B. Losovyj; Ihor Ketsman; N. Lozova; John Scott; P.A. Dowben; I.N. Yakovkin; S.M. Zuber (4326-4331).
The adsorption of two very different adsorbates, gold and oxygen, induce the formation of a (3 × 1) surface structure on both W(1 1 2) and Mo(1 1 2). In spite of similar adsorbate unit cells, the surface electronic structure, derived from photoemission, exhibits pronounced differences for the two adsorbates. Indeed, both experiment and simulations indicate substantial changes in electronic structures of (1 × 1) and (3 × 1) gold overlayers supported by highly anisotropic (1 1 2) plane. We speculate that (3 × 1) is a favored periodicity in the atomic rearrangement of the (1 1 2) surfaces of molybdenum and tungsten due in part as a result of the initial state band structure of these surfaces.
Keywords: Surface electronic structure of W(1 1 2); Gold; Oxygen; Adsorption; Synchrotron radiation; Photoemission;
Current patterning of 6H–SiC(0 0 0 1) surface by AFM by P. Mazur; M. Grodzicki; S. Zuber; A. Ciszewski (4332-4335).
The Atomic Force Microscope (AFM) with the conducting cantilever has been used as a tool for controlled printing the well-defined shapes of conductive paths on the 6H–SiC(0 0 0 1) surface as well as paths connecting the shapes. For clean 6H–SiC(0 0 0 1) samples the metal-tip/sample contact is of the diode type. The conditions have been found (tip/sample voltage, current) for which the local morphology of the surface is modified during current flow between the tip and the sample. Such a modified surface shows quite a different conduction type of the tip/sample surface contact than that of the unmodified surface.
Keywords: Silicon carbide; AFM; Surface patterning;
Effects of thermal treatment on the morphology of Alq3 thin layers on Si(1 1 1) substrate by P. Mazur; S. Zuber (4336-4339).
Alq3 thin layers were vapor deposited onto a single crystal of Si(1 1 1) and the morphology of the surface was investigated by the scanning tunneling microscope under ultrahigh vacuum conditions. The STM imaging showed considerable influence of the thermal processing onto the topography of the sample. Slowly raising the sample temperature to ∼160 °C caused a complete desorption of Alq3 molecules and uncovering the clean surface of Si(1 1 1). A fast rise of the temperature (flashing) to ∼600 °C led to decomposition of the Alq3 and resulted in remnants of a carbon-rich surface species. Then heating or flashing this surface to a temperature in excess of 1000 °C brought about the occurrence of regular shape object on the Si(1 1 1) surface.
Keywords: Alq3; Scanning tunneling microscopy; Silicon carbonization;
The growth of Au/Pd/alumina/Cu–Al system studied by SRPES by Slavomír Nemšák; Jiří Libra; Tomáš Skála; Karel Mašek; Michal Škoda; Miloš Cabala; Vladimír Matolín (4340-4345).
An ultra-thin alumina layer grown on Cu–9at.%Al (1 1 1) surface was studied using synchrotron radiation photoelectron spectroscopy (SRPES), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). By deconvolving SRPES spectra of the Al 2p doublet, four components belonging to metallic as well as oxide phases were recognized. Pd–Au alloy formation was confirmed by SRPES measurement during Pd and Au deposition. The study of the system's thermal stability reveals diffusion of Pd and Au atoms through the alumina layer. While Au atoms start to diffuse under the alumina layer at 670 K, Pd atoms are forming Pd–Al surface alloy at this temperature. The diffusion of Pd atoms through alumina occurs when sample was heated over 770 K. Alumina layer was stable even after heating the sample at 870 K, but its structure was corrupted probably due to the diffusion of metal atoms.
Keywords: XPS; LEED; SRPES; Palladium; Gold; Alumina; Alloys;
Surface and bulk phenomena in the process of GdH x (0.01 < x < 3) formation in thin Gd films: Transition from metal to transparent semiconductor by E. Nowicka; R. Nowakowski; R. Duś (4346-4351).
The aim of this work was to answer the question whether in the process of gadolinium hydride formation the negatively charged hydrogen adspecies arise directly on the surface at some coverage, or a transition from protonic to anionic hydrogen occurs in the bulk. Thin gadolinium films were deposited on glass under UHV conditions and transformed “in situ” into GdH x (0.01 < x < 3) by introducing H2 in successive calibrated doses. Work function changes ΔΦ and pressure P were recorded continuously. Knowing the weight of the thin Gd film and the amount of hydrogen consumed, the atomic ratio H/Gd could be determined and correlated with ΔΦ and P at every step of the process. Parallel experiments were performed measuring the resistance and optical transparency of thin gadolinium films deposited under identical conditions during their transition into GdH x (0.01 < x < 3). It was found that in the process of GdH x formation, light-reflecting metallic thin Gd film is transferred into transparent trihydride while its resistance increases by several orders of magnitude. At low coverage, positively polarized hydrogen adspecies arise, penetrating quickly into the bulk. When the average H/Gd concentration approaches 1, negatively charged hydrogen adspecies appear directly on the surface, slowly penetrating into the bulk.
Keywords: Thin gadolinium film; Metal hydride; Switchable mirrors;
Monte Carlo study of oxidation of the 3C–SiC(0 0 1) 3 × 2 surface by S. Owczarek; E. Wachowicz; A. Kiejna (4352-4356).
Oxygen atoms adsorption and oxidation of the Si-rich 3C–SiC(0 0 1) 3 × 2 surface was investigated using Monte Carlo simulations. The adatom interactions were described using a lattice-gas model with interaction energies determined by density functional theory calculations. Simulations were performed for oxygen coverages ranging from 5 to 30%. Oxygen adatom structures tend to form clusters on the surface which are similar to those seen in scanning tunneling microscopy.
Keywords: Adsorption; Oxidation; Silicon carbide; Monte Carlo method; Pair interaction;
A theoretical study of structural and electronic properties of a missing dimer defect on Si- and C-terminated SiC(0 0 1) by Barbara Pieczyrak; Leszek Jurczyszyn (4357-4364).
We present a theoretical study of the geometrical and electronic properties of the C- and Si-terminated β -SiC(0 0 1) surfaces in the vicinity of the missing dimer defect. The experimental results suggest that the atomic structures of these two surfaces may be considerably modified by external stress. In our present study we have considered the possible influence of this factor on the surface geometry of both systems. We have shown that the structural differences between the C- and Si-terminated surfaces lead to their different behaviour in the presence of a missing dimer and applied stress. In the case of the C-terminated c( 2 × 2 ) surface, the missing dimer defect causes the buckling of the adjacent carbon dimers lying in the line of the defect (dimer atoms adjacent to the defect have vertical positions lower by 0.18 Å). This effect becomes more pronounced in the presence of compressive stress — the stress of 8% leads to the buckling of these two dimers of around 0.5 Å. The vertical positions of silicon atoms located directly below the defect were increased by 0.2 Å. We have also found that the missing dimer influences the structure of the carbon dimers on the neighbouring lines of dimers. Contrary to the C-terminated surface, the missing dimer defect on the Si-terminated SiC(0 0 1)-p( 2 × 1 ) surface remains neutral for silicon dimers located in the line of defect, i.e. the dimers do not change their geometrical properties in unstrained structure nor in the presence of a tensile stress. On the other hand, this defect modifies considerably the geometry of the dimers from the two neighbouring lines of dimers by reducing their bond lengths and vertical positions. Changes in the geometrical properties of the second neighbour dimers (with respect to the defect) in these two lines are also noticeable. Moreover, we have found that the presence of a missing dimer modifies significantly the positions of the adjacent subsurface carbon atoms.
Keywords: Semiconductor surfaces; Silicon carbide; Missing dimer; Density functional calculations;
Field ionization of free helium atoms: Correlation between the kinetic energy of ionized atoms and probability of their field ionization by J. Piskur; L. Borg; A. Stupnik; M. Leisch; W.E. Ernst; B. Holst (4365-4369).
In this paper the correlation between the kinetic energy of helium atoms and the probability of field ionization is investigated by exploiting the narrow velocity distribution of supersonic molecular beams. Field ionization measurements were carried out on supersonic helium beams at 298 K and 95 K corresponding to energies of about 65 meV and 20 meV, respectively, for the individual atoms. The field ionization was performed with a tungsten tip, radius of curvature 12 nm, kept at room temperature. The ionization probability was found to increase by about a factor 10 when the beam was cooled from 298 K to 95 K. The results presented in this paper are of importance for improving the understanding of field ionization and for the development of a new detector for helium and other molecular beams.
Keywords: Field ion detection; Field ionization of helium atoms; Supersonic helium beam; (Field) ionization probability;
On the importance of long range summation of oscillatory interactions in Monte Carlo modeling by J.M. Rogowska (4370-4374).
Kinetic and standard Monte Carlo (kMC and sMC) simulation of the system, where a pair-interaction energy is an oscillatory function of interatomic distance, requires a very careful selection of a cutoff radius r cut of the pair-interactions. As an example, the homoepitaxy of Cu on Cu(1 1 1) is investigated. The surface-state mediated interaction between the Cu adatoms has a very long range and oscillates between attraction and repulsion as a function of the adatom–adatom distance. The simulations reveal that, at 15 K and 0.03 monolayer coverage, the Cu adatoms self-assemble into a dilute nanostructure with a weak local hexagonal order, where the average separation between adatoms equals 11.7 Å. The nanostructure consists of islands and chains of adatoms. The simulated structure of adlayer surprisingly strongly depends on the cutoff radius applied to the Monte Carlo model. The tendency to the dilute island formation strengthens and weakens with the same periodicity vs. r cut as the pair-interaction energy vs. interatomic distance. The submonolayer morphology stabilizes when r cut becomes longer than 50 Å.
Keywords: Kinetic Monte Carlo simulation; Standard Monte Carlo simulation; Homoepitaxy Cu/Cu(1 1 1); Surface-state mediated interactions; Nanostructure;
Sn interaction with the CeO2(1 1 1) system: Bimetallic bonding and ceria reduction by M. Škoda; M. Cabala; V. Cháb; K.C. Prince; L. Sedláček; T. Skála; F. Šutara; V. Matolín (4375-4379).
A tin layer 0.8 nm thick was deposited onto the CeO2(1 1 1) surface by molecular beam deposition at a temperature of 520 K. The interaction of tin with cerium oxide (ceria) was investigated by X-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS) and resonant photoelectron spectroscopy (RPES). The strong tin–ceria interaction led to the formation of a homogeneous bulk Ce–Sn–O mixed oxide system. The bulk compound formation is accompanied by partial Ce4+ → Ce3+ reduction, observed as a giant 4f resonance enhancement of the Ce3+ species. CeO2 and SnO2 oxides were formed after oxygen treatment at 520 K. The study proved the existence of strong Ce–Sn interaction and charge transfer from Sn to the Ce–O complex that lead to a weakening of the cerium–oxygen bond, and consequently, to the formation of oxygen deficient active sites on the ceria surface. This behavior can be a key for understanding the higher catalytic activity of the SnO x /CeO x mixed oxide catalysts as compared with the individual pure oxides.
Keywords: Resonant photoemission; RPES; XPS; LEED; Ceria; Tin; Reduction; Bimetallic interaction; Bimetallic bonding;
Simulations of iodine adsorbed Ge(0 0 1) surface and its STM images by B. Stankiewicz (4380-4385).
A layer of iodine at Ge(0 0 1) surface develops an ordered structure of iodine atoms bound to Ge dimers. Here are discussed atomic structures of Ge(0 0 1) surface covered by 0.25 monolayer of iodine. The p( 2 × 4 ), p( 2 × 2 ), c( 2 × 4 ) and p( 1 × 4 ) surface structures are found in calculations. The structure with two iodine atoms of the dissociated I2 molecule adsorbed at both ends of the same germanium dimer is found to be energetically favourable over iodine adsorption at neighbouring dimers. Simulated STM images of the obtained surface structures are presented and compared with experimental data.
Keywords: Germanium; Density functional calculations; Surface relaxation and reconstruction; Adsorption; STM images;
Interaction of hydrogen with palladium surface: FIM and FEM studies by A. Tomaszewska; A. Ciszewski; Z.M. Stępień (4386-4390).
In the present paper, we focus on the geometrical and electronic changes in palladium surface structure which appeared during its interaction with hydrogen in the presence of an external electric field. The interaction process was examined by using the field ion microscopy (FIM) as well as the field emission microscopy (FEM) techniques. In order to study the geometrical changes in substrate surface structure, the distance distribution function (DDF) was constructed on the basis of FIM patterns of both a clean and hydrogen-covered palladium surface. The electronic changes were examined by the measurement of the total energy distribution (TED) of electrons emitted from the palladium tip surface. The most pronounce examples of such changes are an expansion of the equilibrium interatomic distance in palladium surface and a shift of the Fermi level of the metal. These changes may be explained among others by palladium hydrides formation. This process is the most efficient if the field strength exceeds 23 V/nm.
Keywords: Palladium; Hydrogen; Field ion microscopy (FIM); Field emission microscopy (FEM); Total electron emission energy distribution;
Growth and crystalline structure of Co layers on Cu(0 0 1) by D. Turko; I. Morawski; M. Nowicki (4391-4395).
The crystalline structure of Co layers deposited on the Cu(0 0 1) surface was investigated with the use of the directional elastic peak electron spectroscopy (DEPES). For clean Cu(0 0 1) the experimental DEPES profiles obtained for different energies of the primary electron beam exhibit intensity maxima corresponding to the close packed rows of atoms. The Auger peak kinetics recorded during continuous Co deposition suggest the layer-by-layer growth mode. The DEPES profiles recorded for 10 monolayers (ML) of Co on Cu(0 0 1) reflect a short-range order in the adsorbate. Intensity maxima observed in the DEPES profiles for Co along [1 0 0], [0 1 0], and [1 1 0] azimuths of Cu(0 0 1) are characteristic of the face centered cubic (fcc) Co(0 0 1) layers. Low-intensity reflections and considerable background intensities were found in the low energy electron diffraction (LEED) patterns recorded from 10 ML of Co, which indicates a weak long-range order in the adsorbate. The adsorption of about 20 ML of Co results in considerable background contribution to DEPES. No reflections but a large background were observed with the use of LEED for this layer. The heating of the Co/Cu(0 0 1) system at T = 770 K leads to an increase of the short- and long-range order in the overlayer, observed in the DEPES profiles and LEED patterns, respectively. The theoretical DEPES profiles were obtained with the use of a multiple scattering approximation. A very good agreement between experimental and theoretical scans was found for the clean and covered copper substrate. The latter proves the epitaxial growth of Co layers on Cu(0 0 1).
Keywords: Cobalt; Copper; Electron–solid interactions; Scattering; Low index single crystal surface; Multiple scattering (MS) calculations; Crystalline structure; Auger electron spectroscopy (AES); Directional elastic peak electron spectroscopy (DEPES);
Surface characterization of TiO2 thin films obtained by high-energy reactive magnetron sputtering by R. Wasielewski; J. Domaradzki; D. Wojcieszak; D. Kaczmarek; A. Borkowska; E.L. Prociow; A. Ciszewski (4396-4400).
This paper presents the results of surface characterization of TiO2 thin films deposited on different substrates by the use of high-energy reactive magnetron sputtering. Structural investigations carried out by X-ray diffraction (XRD) and atomic force microscopy (AFM) have shown a strong influence of both the substrate type, and its placement in the deposition chamber (relative to the sputtering target), on the structural properties of the films. In all cases, there is evidence for pseudoepitaxial growth. XRD examination showed existence of TiO2-rutile phase with preferred (1 1 0) orientation and AFM measurements revealed nanocrystalline structure directly after deposition. X-ray photoelectron spectroscopy analysis showed that the TiO2 films have stoichiometric composition.
Keywords: Rutile; Pseudoepitaxy; Reactive sputtering; Hot target; Titanium dioxide;
Structural analysis of In/Ag, In/Cu and In/Pd thin films on tungsten by ellipsometric, XRD and AES methods by A.A. Wronkowska; A. Wronkowski; A. Bukaluk; M. Trzcinski; K. Okulewicz; Ł. Skowroński (4401-4407).
Compositional, microstructural and optical properties of In/Cu, In/Ag and In/Pd thin films evaporated on W substrate in a vacuum were investigated by means of Auger electron spectroscopy, X-ray diffractometry, scanning electron microscopy, and spectroscopic ellipsometry methods. Thicknesses of deposited pure metal layers were adjusted to atomic concentration ratios In:Ag = 1:2, In:Cu = 2:1 and In:Pd = 3:1. Interdiffusion of metals and creation of intermetallic compounds AgIn2, Ag2In and CuIn2were detected at room temperature. Phase transformation and changes in the surface morphology due to annealing of samples at 393 K for 60 min were revealed. Combined investigations indicated a layered structure of In/Ag films. A tendency of island-like structure formation was stronger for In/Cu and In/Pd films. The complex dielectric functions 〈 ϵ ˜ ( E ) 〉 = 〈 ϵ 1 ( E ) 〉 + i 〈 ϵ 2 ( E ) 〉 of In/Ag, In/Cu and In/Pd composite layers were determined from spectroellipsometric data.
Keywords: Interdiffusion; Intermetallic compounds; Spectroscopic ellipsometry; X-ray diffractometry; Auger electron spectroscopy; Scanning electron microscopy;
Influence of substrate morphology on Pb growth by R. Zdyb (4408-4413).
The crystallographic structure and morphology of Pb layers in their early stage of growth on Si(3 3 5) surface are studied with Reflection High Electron Energy Diffraction (RHEED) and specific resistivity techniques. The vicinal Si(3 3 5), with different surface morphology controlled by the amount of predeposited Au, was used as a substrate. Changes in the substrate morphology, from disordered step distribution through a perfectly ordered Si(3 3 5) to a hill-and-valley structure consisting of wide (1 1 1) terraces and high Miller index facets, cause switching between one- and two-dimensional growth of the Pb structures.
Keywords: One-dimensional structures; Si(3 3 5); Pb; Specific resistivity; RHEED;