Science.gov

Sample records for a-ga010 studiedby spa-leed

  1. Dy uniform film morphologies on graphene studied with SPA-LEED and STM

    SciTech Connect

    McDougall, D.; Hattab, H.; Hershberger, M. T.; Hupalo, M.; Horn von Hoegen, M.; Thiel, P. A.; Tringides, M. C.

    2016-07-01

    The use of graphene for microelectronics and spintronic applications requires strategies for metals to wet graphene and to grow layer-by-layer. This is especially important when metals will be used as electrical contacts or as spin filters. Extensive work in the literature so far has shown that this is very challenging, since practically all metals grow 3D, with multi-height islands forming easily. Reasons for the 3D morphology are the much weaker metal carbon bond when compared to the metal cohesive energy and the role of Coulomb repulsion of the poorly screened charges at the metal graphene interface. We employed the complementary techniques of SPA-LEED and STM to study the growth of Dy on graphene. It was found that under kinetic limitations it is possible to fully cover graphene with a bilayer Dy film, by growing well below room temperature in stepwise deposition experiments. Lastly, the Dy film, however, is amorphous but ways to crystallize it within the 2D morphology are possible, since long range order improves at higher growth temperature.

  2. Dy uniform film morphologies on graphene studied with SPA-LEED and STM

    SciTech Connect

    McDougall, D.; Hattab, H.; Hershberger, M. T.; Hupalo, M.; Horn von Hoegen, M.; Thiel, P. A.; Tringides, M. C.

    2016-07-01

    The use of graphene for microelectronics and spintronic applications requires strategies for metals to wet graphene and to grow layer-by-layer. This is especially important when metals will be used as electrical contacts or as spin filters. Extensive work in the literature so far has shown that this is very challenging, since practically all metals grow 3D, with multi-height islands forming easily. Reasons for the 3D morphology are the much weaker metal carbon bond when compared to the metal cohesive energy and the role of Coulomb repulsion of the poorly screened charges at the metal graphene interface. We employed the complementary techniques of SPA-LEED and STM to study the growth of Dy on graphene. It was found that under kinetic limitations it is possible to fully cover graphene with a bilayer Dy film, by growing well below room temperature in stepwise deposition experiments. Lastly, the Dy film, however, is amorphous but ways to crystallize it within the 2D morphology are possible, since long range order improves at higher growth temperature.

  3. Dy uniform film morphologies on graphene studied with SPA-LEED and STM

    DOE PAGES

    McDougall, D.; Hattab, H.; Hershberger, M. T.; ...

    2016-07-01

    The use of graphene for microelectronics and spintronic applications requires strategies for metals to wet graphene and to grow layer-by-layer. This is especially important when metals will be used as electrical contacts or as spin filters. Extensive work in the literature so far has shown that this is very challenging, since practically all metals grow 3D, with multi-height islands forming easily. Reasons for the 3D morphology are the much weaker metal carbon bond when compared to the metal cohesive energy and the role of Coulomb repulsion of the poorly screened charges at the metal graphene interface. We employed the complementarymore » techniques of SPA-LEED and STM to study the growth of Dy on graphene. It was found that under kinetic limitations it is possible to fully cover graphene with a bilayer Dy film, by growing well below room temperature in stepwise deposition experiments. Lastly, the Dy film, however, is amorphous but ways to crystallize it within the 2D morphology are possible, since long range order improves at higher growth temperature.« less

  4. A combined STM and SPA-LEED study of the "explosive" nucleation and collective diffusion in Pb/Si(111)

    NASA Astrophysics Data System (ADS)

    Hattab, H.; Hupalo, M.; Hershberger, M. T.; Horn von Hoegen, M.; Tringides, M. C.

    2016-04-01

    A novel type of very fast nucleation was recently found in Pb/Si(111) with 4- to 7-layer high islands becoming crystalline in an "explosive" way, when the Pb deposited amount in the wetting layer is compressed to θc 1.22 ML, well above the metallic Pb(111) density. This "explosive" nucleation is very different from classical nucleation when island growth is more gradual and islands grow in size by single adatom aggregation [8]. In order to identify the key parameters that control the nucleation we used scanning tunneling microscopy (STM) and spot profile analysis low energy electron diffraction (SPA-LEED). It was found that the number and duration of steps in iterative deposition used to approach θc and the flux rate have dramatic effects on the crystallization process. Larger depositions over shorter times induce greater spatial coverage fluctuations, so local areas can reach the critical coverage θc easier. This can trigger the collective motion of the wetting layer from far away to build the Pb islands "explosively". The SPA-LEED experiments show that even low flux experiments in iterative deposition experiments can trigger transfer of material to the superstable 7-layer islands, as seen from the stronger satellite rings close to the (00) spot.

  5. A combined STM and SPA-LEED study of the “explosive” nucleation and collective diffusion in Pb/Si(111)

    DOE PAGES

    Hattab, H.; Hupalo, M.; Hershberger, M. T.; ...

    2015-08-20

    A novel type of very fast nucleation was recently found in Pb/Si(111) with 4- to 7-layer high islands becoming crystalline in an “explosive” way, when the Pb deposited amount in the wetting layer is compressed to θc ~ 1.22 ML, well above the metallic Pb(111) density. This “explosive” nucleation is very different from classical nucleation when island growth is more gradual and islands grow in size by single adatom aggregation [8]. In order to identify the key parameters that control the nucleation we used scanning tunneling microscopy (STM) and spot profile analysis low energy electron diffraction (SPA-LEED). It was foundmore » that the number and duration of steps in iterative deposition used to approach θc and the flux rate have dramatic effects on the crystallization process. Larger depositions over shorter times induce greater spatial coverage fluctuations, so local areas can reach the critical coverage θc easier. This can trigger the collective motion of the wetting layer from far away to build the Pb islands “explosively”. Here, the SPA-LEED experiments show that even low flux experiments in iterative deposition experiments can trigger transfer of material to the superstable 7-layer islands, as seen from the stronger satellite rings close to the (00) spot.« less

  6. A combined STM and SPA-LEED study of the “explosive” nucleation and collective diffusion in Pb/Si(111)

    SciTech Connect

    Hattab, H.; Hupalo, M.; Hershberger, M. T.; Horn von Hoegen, M.; Tringides, M. C.

    2015-08-20

    A novel type of very fast nucleation was recently found in Pb/Si(111) with 4- to 7-layer high islands becoming crystalline in an “explosive” way, when the Pb deposited amount in the wetting layer is compressed to θc ~ 1.22 ML, well above the metallic Pb(111) density. This “explosive” nucleation is very different from classical nucleation when island growth is more gradual and islands grow in size by single adatom aggregation [8]. In order to identify the key parameters that control the nucleation we used scanning tunneling microscopy (STM) and spot profile analysis low energy electron diffraction (SPA-LEED). It was found that the number and duration of steps in iterative deposition used to approach θc and the flux rate have dramatic effects on the crystallization process. Larger depositions over shorter times induce greater spatial coverage fluctuations, so local areas can reach the critical coverage θc easier. This can trigger the collective motion of the wetting layer from far away to build the Pb islands “explosively”. Here, the SPA-LEED experiments show that even low flux experiments in iterative deposition experiments can trigger transfer of material to the superstable 7-layer islands, as seen from the stronger satellite rings close to the (00) spot.

  7. Probing the buried Pb/Si(111) interface with SPA LEED and STM on Si(111)-Pbα√3×√3.

    PubMed

    Yakes, M; Tringides, M C

    2011-06-30

    High resolution spot profile analysis low energy electron diffraction (SPA-LEED) and variable temperature scanning tunneling microscopy (STM) have been used to observe the growth of Pb on the Pb/Si(111)-α√3×√3 phase, which is driven by quantum size effects (QSE). A change in the rotation of the Pb grown islands with respect to the Si substrate has been observed with increasing coverage θ. At lower coverage, separated two-step islands are grown and are aligned with the [110] axis of the substrate. With increasing coverage above 1.5 ML, of the islands coalesce and form a bilayer, with additional islands grown on top. The preferred Pb island orientation changes to 5.6° with respect to the [110] direction. These changes at the metal/semiconductor buried interface are obtained both with SPA LEED and STM as changes to the period of the Moire pattern. The method of analysis of the corrugation period and rotation angle of the Moire pattern measured with diffraction and STM can be applied to obtain the structure of buried metal/substrate interfaces in other epitaxial systems.

  8. The dense α-√3 × √3Pb/Si(1 1 1) phase: A comprehensive STM and SPA-LEED study of ordering, phase transitions and interactions

    NASA Astrophysics Data System (ADS)

    Stepanovsky, S.; Yakes, M.; Yeh, V.; Hupalo, M.; Tringides, M. C.

    2006-04-01

    The T- θ phase diagram for the system Pb/Si(1 1 1) was determined in the coverage range 6/5 ML < θ < 4/3 ML from complementary STM and SPA-LEED experiments. This coverage is within the range where a "Devil's Staircase" (DS) has been realized. The numerous DS phases answer conflicting information in the Pb/Si(1 1 1) literature and update the previously published phase diagram. The measurements reveal the thermal stability of the different linear DS phases with the transition temperature found to be a function of phase period. Because of additional complexity in the experimental system (i.e. two-dimensionality and 3-fold symmetry) the linear DS phases transform at higher temperature into commensurate phases of 3-fold symmetry HIC (historically named "hexagonal incommensurate phase"). Different types of HIC phases have been discovered differing in the size of the supercell built out of √3 × √3 domains separated by domain walls of the √7 × √3 phase. The detailed structures of these HIC phases (coverage, binding site, twist angle, etc.) have been deduced from the comparison of STM images and diffraction patterns. After heating the system to even higher temperature the HIC phase transforms into the disordered phase. For sufficiently high coverage a SIC ("striped incommensurate phase" which is also built from √3 × √3 domains but meandering √7 × √3 domain walls) is observed which also disorders at high temperatures.

  9. SPA-LEED Study of the Morphology and Nucleation of a Novel Growth Mode and the ''devil's staircase'' on Pb/Si(111)

    SciTech Connect

    Yeh, Wang-Chi Vincent

    2003-01-01

    This thesis was developed to address the following questions for the Pb/Si(111) system: (1) Is it possible to control the nano-structure growth by changing the initial substrate; (2) is the nucleation theory applicable to the case of the 7-step growth mode; and (3) what phase or phases could be formed between coverage 6/5 ML and 4/3 ML? The first question was answered in chapter 2, different growth results were observed for different initial substrate, suggesting the possibility of controlling nano-structure growth by selecting the initial substrate. The applicability of nucleation theory was determined to be unclear in chapter 3, from the results that the saturation island density does not depend on deposition rate, in contrary to the prediction of nucleation theory. Chapter 4 revealed a novel ''devil's staircase'' in Pb/Si(111) within the coverage range 6/5 ML and 4/3 ML. Low temperature deposition experiments showed high order of self-organization in such a system. Theoretical studies are needed to understand such a low temperature behavior. In general, this thesis provides possibilities of controlling nano-structure growth, which can be possibly an indication for future application. It also raises interesting questions in fundamental researches: a modified theory of nucleation is needed, and a detailed study of low temperature behavior is required. Details of the conclusions in each of the chapters are collected in the following sections.

  10. Reactivity of Au nanoparticles supported over SiO2 and TiO2 studiedby ambient pressure photoelectron spectroscopy

    SciTech Connect

    Herranz, Tirma; Deng, Xingyi; Cabot, Andreu; Alivisatos, Paul; Liu, Zhi; Soler-Illia, Galo; Salmeron, Miquel

    2009-04-15

    The influence of the metal cluster size and the identity of the support on the reactivity of gold based catalysts have been studied in the CO oxidation reaction. To overcome the structural complexity of the supported catalysts, gold nanoparticles synthesized from colloidal chemistry with precisely controlled size have been used. Those particles were supported over SiO{sub 2} and TiO{sub 2} and their catalytic activity was measured in a flow reactor. The reaction rate was dependent on the particle size and the support, suggesting two reaction pathways in the CO oxidation reaction. In parallel, ambient pressure photoelectron spectroscopy (APPS) has been performed under reaction conditions using bidimensional model catalysts prepared upon supporting the Au nanoparticles over planar polycrystalline SiO{sub 2} and TiO{sub 2} thin films by means of the Langmuir-Blodgett (LB) technique to mimic the characteristic of the powder samples. In this way, the catalytically active surface was characterized under true reaction conditions, revealing that during CO oxidation gold remains in the metallic state.

  11. Oxygen chemisorption on Cu(19 19 1) studied by spot profile analysis low-energy electron diffraction

    SciTech Connect

    Brandstetter, T.; Draxler, M.; Hohage, M.; Zeppenfeld, P.

    2007-12-15

    Cu(110) and the vicinal Cu(19 19 1) surfaces were characterized by recording maps of the reciprocal space by means of spot profile analysis low-energy electron diffraction (SPA-LEED). For both surfaces, kinematic simulations were performed to get insight into the main features of the experimental data. Furthermore, it is shown that chemisorption of oxygen and subsequent annealing lead to the formation of a Cu-CuO stripe phase and induce faceting of the Cu(19 19 1) surface. The evolution from the clean Cu(19 19 1) surface to the coexistence of the (110) and (111) facets with increasing oxygen exposure was characterized by SPA-LEED.

  12. Effects of laser irradiation on the morphology of Cu(110)

    SciTech Connect

    Brandstetter, T.; Draxler, M.; Hohage, M.; Zeppenfeld, P.; Stehrer, T.; Heitz, J.; Georgiev, N.; Martinotti, D.; Ernst, H.-J.

    2008-07-15

    The effects of pulsed laser irradiation on the morphology of the Cu(110) surface were investigated by means of reflectance difference spectroscopy (RDS) and spot profile analysis low-energy electron diffraction (SPA-LEED). The laser light induces surface defects (adatoms and islands) as well as subsurface dislocation lines. The high surface mobility leads to efficient annealing of the surface defects even at room temperature, whereas the subsurface dislocation lines persist up to temperatures T>800 K. SPA-LEED profiles of the (00) diffraction spot from the laser irradiated surface suggest an anisotropic distribution of the subsurface line defects related to the geometry of the fcc easy glide system, which is corroborated by STM measurements. Comparative experiments using conventional Ar ion bombardment point out the distinctiveness of the morphological changes induced by laser irradiation.

  13. Evolution of Mound Morphology in Reversible Homoepitaxy on Cu(100)

    SciTech Connect

    Zuo, J.; Wendelken, J.

    1997-04-01

    Evolution of mound morphology in reversible homoepitaxy on Cu(100) was studied via spot-profile-analysis (SPA) LEED and scanning tunneling microscopy. The mound separation shows coarsening vs growth time with L(t){approximately}t{sup 1/4}, in support of theory based on capillarity between mounds. The growth ultimately reaches a steady state characterized by a selected mound angle of {approximately}5.6{degree}. We suggest that this results from a downhill current driven by step edge line tension in balance with an uphill current due to the Schwoebel barrier effect. Also, we have clarified the interpretation for the evolution of the SPA-LEED profile from a ring structure to a single time-invariant peak. {copyright} {ital 1997} {ital The American Physical Society}

  14. Growth Anisotropy and Pattern Formation during Metal Epitaxy

    NASA Astrophysics Data System (ADS)

    Poelsema, Bene; Jorritsma, Louis C.; Rosenfeld, Georg

    1997-03-01

    The homoepitaxial growth on Cu(001) has been investigated by He atom beam scattering (TEAS) and High Resolution Low Energy Electron Diffraction (SPA-LEED). In the temperature regime below 300 K the growth initially starts `layer-by-layer', but turns gradually into multi-layer growth at higher coverage. Analysis of SPA-LEED peak profiles reveals that as growth proceeds checkerboard-like arrangements of mound-like structures are formed. The sides of the mounds are composed of <11n>-facets. The origin of such an arrangement has been traced back to laterally anisotropic advance rates of adatomisland edges in combination with a finite energy barrier for descending from a step edge.

  15. Reciprocal space mapping by spot profile analyzing low energy electron diffraction

    SciTech Connect

    Meyer zu Heringdorf, Frank-J.; Horn-von Hoegen, Michael

    2005-08-15

    We present an experimental approach for the recording of two-dimensional reciprocal space maps using spot profile analyzing low energy electron diffraction (SPA-LEED). A specialized alignment procedure eliminates the shifting of LEED patterns on the screen which is commonly observed upon variation of the electron energy. After the alignment, a set of one-dimensional sections through the diffraction pattern is recorded at different energies. A freely available software tool is used to assemble the sections into a reciprocal space map. The necessary modifications of the Burr-Brown computer interface of the two Leybold and Omicron type SPA-LEED instruments are discussed and step-by-step instructions are given to adapt the SPA 4.1d software to the changed hardware. Au induced faceting of 4 deg. vicinal Si(001) is used as an example to demonstrate the technique.

  16. Chopped sample heating for quantitative profile analysis of low energy electron diffraction spots at high temperatures

    SciTech Connect

    Kury, P.; Zahl, P.; Horn-von Hoegen, M.; Voges, C.; Frischat, H.; Guenter, H.-L.; Pfnuer, H.; Henzler, M.

    2004-11-01

    Spot profile analysis low energy electron diffraction (SPA-LEED) is one of the most versatile and powerful methods for the determination of the structure and morphology of surfaces even at elevated temperatures. In setups where the sample is heated directly by an electric current, the resolution of the diffraction images at higher temperatures can be heavily degraded due to the inhomogeneous electric and magnetic fields around the sample. Here we present an easily applicable modification of the common data acquisition hardware of the SPA-LEED, which enables the system to work in a pulsed heating mode: Instead of heating the sample with a constant current, a square wave is used and electron counting is only performed when the current through the sample vanishes. Thus, undistorted diffration images can be acquired at high temperatures.

  17. Structure and morphology of the tenfold surface of decagonal Al{sub 71.8}Ni{sub 14.8}Co{sub 13.4} in its low-temperature random tiling type-I modification

    SciTech Connect

    Sharma, H.R.; Franke, K.J.; Theis, W.; Riemann, A.; Foelsch, S.; Rieder, K.H.; Gille, P.

    2004-12-15

    We have investigated the structure and morphology of the tenfold surface of decagonal Al{sub 71.8}Ni{sub 14.8}Co{sub 13.4} by highly surface sensitive He atom scattering (HAS), high resolution low energy electron diffraction(SPA-LEED), and low temperature scanning tunneling microscopy (STM). The SPA-LEED patterns reveal more than 500 individual diffraction spots in the k-vector range of vertical bar k{sub parallel} vertical bar <3 A{sup -1}. The positions of all observed diffraction spots agree with the surface projections of the reciprocal lattice structure of the type-I bulk phase. HAS shows identical spot positions as SPA-LEED, thus demonstrating a top surface layer with long range quasicrystalline order and a reciprocal lattice structure consistent with that of a bulk truncated surface. SPA-LEED peak widths are found to vary between different diffraction orders. Based on an analysis of a randomized Fibonacci sequence, this is linked to the random nature of the tiling of the type-I structure. STM measurements reveal a surface morphology characterized by rough single-height steps separating terraces with widths on the order of 100 A. Two different surface terminations are observed, a coarse and a fine one, frequently coexisting on single terraces. The fine structure termination directly reflects the atomic structure of a bulk truncated surface, allowing a random rhombic tiling to be identified. In order to compare diffraction, real-space data, and atomic structure models, the Patterson function and autocorrelation of the surface structure, respectively, are studied. This allows an understanding of the coarse structure termination as consisting of subunits of a few atoms each arranged statistically on sites defined by the atomic tiling of the bulk tenfold planes.

  18. Structure and morphology of the tenfold surface of decagonal Al71.8 Ni14.8 Co13.4 in its low-temperature random tiling type-I modification

    NASA Astrophysics Data System (ADS)

    Sharma, H. R.; Franke, K. J.; Theis, W.; Riemann, A.; Fölsch, S.; Gille, P.; Rieder, K. H.

    2004-12-01

    We have investigated the structure and morphology of the tenfold surface of decagonal Al71.8Ni14.8Co13.4 by highly surface sensitive He atom scattering (HAS), high resolution low energy electron diffraction(SPA-LEED), and low temperature scanning tunneling microscopy (STM). The SPA-LEED patterns reveal more than 500 individual diffraction spots in the k -vector range of ∣k‖∣<3Å-1 . The positions of all observed diffraction spots agree with the surface projections of the reciprocal lattice structure of the type-I bulk phase. HAS shows identical spot positions as SPA-LEED, thus demonstrating a top surface layer with long range quasicrystalline order and a reciprocal lattice structure consistent with that of a bulk truncated surface. SPA-LEED peak widths are found to vary between different diffraction orders. Based on an analysis of a randomized Fibonacci sequence, this is linked to the random nature of the tiling of the type-I structure. STM measurements reveal a surface morphology characterized by rough single-height steps separating terraces with widths on the order of 100Å . Two different surface terminations are observed, a coarse and a fine one, frequently coexisting on single terraces. The fine structure termination directly reflects the atomic structure of a bulk truncated surface, allowing a random rhombic tiling to be identified. In order to compare diffraction, real-space data, and atomic structure models, the Patterson function and autocorrelation of the surface structure, respectively, are studied. This allows an understanding of the coarse structure termination as consisting of subunits of a few atoms each arranged statistically on sites defined by the atomic tiling of the bulk tenfold planes.

  19. Isotropic thin PTCDA films on GaN(0 0 0 1)

    NASA Astrophysics Data System (ADS)

    Ahrens, Ch; Flege, J. I.; Jaye, C.; Fischer, D. A.; Schmidt, Th; Falta, J.

    2016-11-01

    The growth of 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA) on the Ga-polar GaN(0 0 0 1) surface has been studied by x-ray photoelectron spectroscopy (XPS), spot profile analysis low-energy electron diffraction (SPA-LEED), near edge x-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The stoichiometric ratios derived from XPS indicate that the molecules remain intact upon adsorption on the surface. Furthermore, no chemical shifts can be observed in the C 1s and O 1s core levels with progressing deposition of PTCDA, suggesting none or only weak interactions between the molecules and the substrate. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. High-resolution STM shows PTCDA islands of irregular shape on the sub-micron scale, and together with corresponding SPA-LEED data reveals a lateral ordering of the molecules that is compatible with the presence of (1 0 2) oriented PTCDA nano-crystals. SPA-LEED moreover clearly shows the presence of homogeneously distributed rotational domains of two-dimensionally isotropic PTCDA.

  20. Controlling atomistic processes on Pb films via quantum size effects and lattice rotation

    SciTech Connect

    Binz, Steven

    2012-01-01

    The two main techniques used to record the data in this dissertation were Spot Profile Analysis - Low Energy Electron Diffraction (SPA-LEED) and Scanning Tunneling Microscopy (STM). A specific data analysis technique for LEED data called G(S) curves is described in depth. G(S) curves can provide a great deal of structural information about the surface; including step heights, island size, and island separation. The effects of quantum size effects (QSE) on the diffusion and critical island sizes of Pb and In on Pb films are reported. Pb depositions on the 2D In phases {radical}3 and {radical}31 to see how the phases affect the Pb growth and its strong QSE are reported.

  1. Adatom-dimer interaction on the Si(001)-2 × 1 surface

    NASA Astrophysics Data System (ADS)

    Toh, C. P.; Ong, C. K.

    1994-02-01

    We use a modified form of the Stillinger-Weber potential to obtain the binding sites and diffusion barriers of a Si adatom in the vicinity of single F and B type dimers on the Si(001)-2 × 1 surface. We find that both kinds of dimer provide good sinks for adatoms and are therefore ideal nucleation sites, provided the temperature is not too high as to induce dimer breaking. Our results also show that adatoms can be trapped in non-lattice sites surrounding the F type dimer, leading to a disordering of the growing epitaxial film. Monte Carlo simulated annealing indicates that adatoms at these "defect" sites are vertically displaced with respect to those adsorbed on the epitaxial sites, giving rise to step structures that closely resemble those proposed by Falta and Henzler [Surf. Sci 269/270 (1992) 14] to account for their SPA-LEED results.

  2. On the microscopic structure of a nominal Ag(441) surface

    NASA Astrophysics Data System (ADS)

    Wagner, Thorsten; Fritz, Daniel Roman; Zimmerleiter, Robert; Zeppenfeld, Peter

    2017-07-01

    Regularly stepped (vicinal) surfaces provide a convenient means to control the number of defects of a surface. They can easily be prepared by a slight miscut of a low index surface. In the case of an fcc(n n 1) surface with small integer n, it is even expected that the large number of steps will dominate the surface properties. We are the first to study the Ag(441) surface with a combination of scanning tunneling microscopy (STM) and high resolution electron diffraction (SPA-LEED). The surface is found to consist of different building blocks, which can be either (331) or (551) microfacets. To unravel the actual morphology, we carried out simulations of the reciprocal space maps (RSMs) in the framework of the simple kinematic approximation.

  3. Growth of praseodymium oxide on Si(111) under oxygen-deficient conditions

    SciTech Connect

    Schaefer, A.; Zielasek, V.; Baeumer, M.; Schmidt, Th.; Schowalter, M.; Schulz, Ch.; Rosenauer, A.; Falta, J.; Sandell, A.; Seifarth, O.; Schroeder, T.; Wollschlaeger, J.

    2009-07-15

    Surface science studies of thin praseodymium oxide films grown on silicon substrates are of high interest in view of applications in such different fields as microelectronics and heterogeneous catalysis. In particular, a detailed characterization of the growth and the final structure of the films are mandatory to achieve a fundamental understanding of such topics as oxygen mobility and defect structure, and their role for the electronic and chemical properties. In this paper, the MBE growth of praseodymium oxide films on Si(111) substrates was investigated at low-deposition rates (0.06 nm/min) and low-oxygen partial pressures (p(O{sub 2})<1x10{sup -10} mbar). To obtain insight into the structure and chemical composition of the growing film, spot profile analyzing low-energy electron diffraction (SPA-LEED), transmission electron microscopy, and synchrotron radiation-based x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) were applied. SPA-LEED reveals the formation of an initial closed layer followed by continuous roughening and formation of ordered three-dimensional structures. This result is in contrast to observations at higher-deposition rates, were a layer-by-layer growth was reported. XAS and XPS provide evidence that a continuous reaction takes place in the growing Pr{sub 2}O{sub 3} film leading to the formation of silicate and silicide structures within the film. Combining all data, a consistent picture of the deposition of praseodymium oxide on Si(111) emerges which clearly shows that in contrast to higher-throughput molecular beam epitaxy conditions the reactivity of the growing film strongly influences the growth behavior at low-deposition rates and low pressures.

  4. Defects at semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Henzler, Martin

    1985-04-01

    Low Energy Electron Diffraction (LEED) is widely used for detection of periodicity at the surface and of atom arrangement within the unit cell. Experiments and results, however, are increasing, which use the spot profile analysis (SPA-LEED) for the study of nonperiodic surfaces. Here the kinematical approximation provides a wider range of validity than expected. For semiconductors defects are especially important, since the surface states in the gap are determined or strongly influenced by almost any kind of defects at the surface. Atomic steps at the interface {Si}/{SiO2} have been shown to be correlated with many electronic properties of MOS devices like mobility, interface states and fixed charge. The epitaxy on Si and GaAs has been studied with LEED and RHEED, showing the density of the nuclei during formation of a layer and the layer-by-layer growth. The formation of metal suicides in the monolayer range is accompanied by many different superstructures and other rearrangements. It is demonstrated, that the new high resolution instruments provide additional qualitative and quantitative informations on any kind of surface defects.

  5. Electron density contour smoothening for epitaxial Ag islands on Ag(100)

    NASA Astrophysics Data System (ADS)

    Bedrossian, Peter; Poelsema, Bene; Rosenfeld, Georg; Jorritsma, Louis C.; Lipkin, Nuphar N.; Comsa, George

    1995-07-01

    Rocking curves acquired with thermal energy He-atom scattering indicate that for small 2D Ag islands grown on Ag(100) at 200 K, the apparent step height normal to the surface is reduced by about 0.6 Å compared to the step height calculated on the basis of bulk separation. For higher growth temperatures this effect is not observed. SPA-LEED measurements indicate that this behavior cannot be explained with a corresponding relaxation of island atoms. Rather, it may be attributed to a smoothening of the electronic corrugation (Smoluchowski effect), which is probed by the He atoms and which can be expected to occur for small and/or closely spaced islands. The dependence on the growth temperature indicates that this effect is, indeed, related to the size and proximity of the islands, being most pronounced for closely spaced islands consisting of a few tens of atoms only. These findings suggest that the difference between the morphology of the electron density contours, probed by methods which are sensitive to the density of valence electrons, such as atom scattering or scanning tunneling microscopy, and the morphology at the atomic core level is significant for small metal structures on metal surfaces.

  6. Binding site H3 to T4 occupation switching and the Pb/Si(111) ``Devil's Staircase'' phase diagram

    NASA Astrophysics Data System (ADS)

    Yeh, V.; Yakes, M.; Hupalo, M.; Tringides, M. C.; Chvoj, Z.

    2006-03-01

    With SPA-LEED and STM it has been observed that there is a switching occupation from only H3 sites to H3 and T4 sites within the unit cell of the DS (``Devil's Staircase'') (n,m) linear phases at the (1,1) phase or theta=1.25ML. This is observed from the doubling of the linear phase period and the ``flipping'' of the triangle diffraction pattern. The transition temperature from linear to HIC shows a minimum at ˜120K for the (1,1) phase and follows a U-shaped curved in the whole DS range 6/5ML

  7. Studies of Dirac and Weyl fermions by angle resolved photoemission spectroscopy

    NASA Astrophysics Data System (ADS)

    Huang, Lunan

    This dissertation consists of three parts. First, we study magnetic domains in Nd2Fe14 B single crystals using high resolution magnetic force microscopy (MFM). In addition to the elongated, wavy nano-domains reported by a previous MFM study, we found that the micrometer size, star-shaped fractal pattern is constructed of an elongated network of nano-domains about 20 nm in width, with resolution-limited domain walls thinner than 2 nm. Second, we studied extra Dirac cones of multilayer graphene on SiC surface by ARPES and SPA-LEED. We discovered extra Dirac cones on Fermi surface due to SiC 6 x 6 and graphene 6√3 x 6√3 coincidence lattice on both single-layer and three-layer graphene sheets. We interpreted the position and intensity of the Dirac cone replicas, based on the scattering vectors from LEED patterns. We found the positions of replica Dirac cones are determined mostly by the 6 x 6 SiC superlattice even graphene layers grown thicker. Finally, we studied the electronic structure of MoTe2 by ARPES and experimentally confirmed the prediction of type II Weyl state in this material. By combining the result of Density Functional Theory calculations and Berry curvature calculations with out experimental data, we identified Fermi arcs, track states and Weyl points, all features predicted to exist in a type II Weyl semimetal. This material is an excellent playground for studies of exotic Fermions.

  8. Switching orientation of adsorbed molecules: Reverse domino on a metal surface

    NASA Astrophysics Data System (ADS)

    Braatz, C. R.; Esat, T.; Wagner, C.; Temirov, R.; Tautz, F. S.; Jakob, P.

    2016-01-01

    A thus far unknown phase of 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) on Ag(111), characterized by an all perpendicular orientation of the planar molecules and bound to the Ag substrate through the carboxyl oxygen atoms has been identified using infrared absorption spectroscopy and scanning tunneling microscopy. Its formation process requires second layer NTCDA to squeeze into empty spaces between relaxed monolayer NTCDA molecules. Remarkably, this process causes initially parallel oriented NTCDA to likewise adopt the new, highly inclined adsorption geometry. According to our SPA-LEED and STM findings, the new phase displays a distinct long range order and shows a pronounced tendency to form 1D rows or narrow islands. We suggest that extra NTCDA preferentially transforms into the upright configuration close to existing islands and attaches to them, i.e. the transformation process proceeds in a directed and recurrent manner (reverse domino scenario). Identical processing starting with a compressed NTCDA/Ag(111) monolayer leads to a purely parallel oriented bilayer, that is, the NTCDA monolayer phase is retained and merely acts as a passive template for bilayer NTCDA. The new vertical NTCDA phase represents an unusual molecular system with π-orbitals oriented parallel to a metal surface. A substantially reduced coupling of these orbitals to Ag(111) electronic levels is conjectured, which will have a major impact on intermolecular couplings and electronically excited state lifetimes.

  9. Surface reconstruction of Pt(001) quantitatively revisited

    NASA Astrophysics Data System (ADS)

    Hammer, R.; Meinel, K.; Krahn, O.; Widdra, W.

    2016-11-01

    The complex hexagonal reconstructions of the (001) surfaces of platinum and gold have been under debate for decades. Here, the structural details of the Pt(001) reconstruction have been quantitatively reinvestigated by combining the high resolving power of scanning tunneling microscopy (STM) and spot profile analysis low energy electron diffraction (SPA-LEED). In addition, LEED simulations based on a Moiré approach have been applied. Annealing temperatures around 850 °C yield a superstructure that approaches a commensurable c (26.6 ×118 ) substrate registry. It evolves from a Moiré-like buckling of a compressed hexagonal top layer (hex) where atomic rows of the hex run parallel to atomic rows of the square substrate. Annealing at 920 °C stimulates a continuous rotation of the hex where all angles between ±0.7° are simultaneously realized. At temperatures around 1080 °C, the nonrotated hex coexists with a hex that is rotated by about 0.75°. Annealing at temperatures around 1120 °C yield a locking of the hex in fixed rotation angles of 0.77°, 0.88°, and 0.94°. At temperatures around 1170 °C, the Pt(001)-hex-R 0.94° prevails as the energetically most favored form of the rotated hex.

  10. SPALEED Studies of the Growth of Zero to Mono-layer Graphene on SiC(0001)

    NASA Astrophysics Data System (ADS)

    Hupalo, M.; Hershberger, M. T.; Hattab, H.; McDougall, D. C.; Horn von Hoegen, M.; Tringides, M. C.

    The growth of graphene on SiC was studied in detail with SPA LEED to understand the transition from zero to monolayer graphene with increasing temperature starting at 1200°C. Both the changing diffraction spots with annealing and their line shapes are studied in detail until a fully completed monolayer is obtained with only 6x6 spots remaining. In particular we focus on two strong features not investigated previously: (i) superstructures spots at n/13 locations present between the specular and the graphene spots. These spots are possibly related to different coincidence lattices before graphene locks into its final 6x6 orientation. (ii) The presence of a very broad background intensity covering ~60% of the BZ both around the specular and graphene spots whose origin is still unknown. Detailed studies of the dependence of this background component on energy and comparison between the graphene and specular spots suggest that the origin is not due to the standard variation with electron energy, i.e. a g(s) curve caused by the topography. Throughout the literature this broad background has been seen in graphene grown in different types of substrates. We comment on possible reasons for the origin of the background. Ames Laboratory is operated by the US-DOE under Contract No. DE-AC02-07CH11358.

  11. Defects and inhomogeneities in Fe3O4(111) thin film growth on Pt(111)

    NASA Astrophysics Data System (ADS)

    Sala, A.; Marchetto, H.; Qin, Z.-H.; Shaikhutdinov, S.; Schmidt, Th.; Freund, H.-J.

    2012-10-01

    Growth and surface termination of a Fe3O4(111) thin film on a Pt(111) surface were examined by a combination of low-energy electron microscopy, selected area low-energy electron diffraction (LEED), and x-ray-induced photoemission electron microscopy. The film exhibits the predominance of one out of two possible rotational domains, independent of film thickness. The morphology strongly depends on preparation conditions, e.g., at high oxidation temperature FeO/Pt(111) domains are formed that prevent the closure of the thin film. Dynamical LEED analysis and spot-profile analysis LEED (SPA-LEED) show that the surface exposes ¼ monolayer of Fe over a close-packed oxygen layer only when the sample is subsequently annealed in ultrahigh vacuum at 900 K. In contrast, the as-prepared films grown by oxidation at 1000 K and subsequent cooling down in oxygen, additionally exhibit small FeOx agglomerates that rest upon the canonical surface termination. Their formation as a function of the various preparation conditions of the thin film is discussed.

  12. Studies of Dirac and Weyl fermions by angle resolved photoemission spectroscopy

    SciTech Connect

    Huang, Lunan

    2016-01-01

    This dissertation consists of three parts. First, we study magnetic domains in Nd2Fe14B single crystals using high resolution magnetic force microscopy (MFM). In addition to the elongated, wavy nano-domains reported by a previous MFM study, we found that the micrometer size, star-shaped fractal pattern is constructed of an elongated network of nano-domains about 20 nm in width, with resolution-limited domain walls thinner than 2 nm. Second, we studied extra Dirac cones of multilayer graphene on SiC surface by ARPES and SPA-LEED. We discovered extra Dirac cones on Fermi surface due to SiC 6 x 6 and graphene 6√ 3 6√ 3 coincidence lattice on both single-layer and three-layer graphene sheets. We interpreted the position and intensity of the Dirac cone replicas, based on the scattering vectors from LEED patterns. We found the positions of replica Dirac cones are determined mostly by the 6 6 SiC superlattice even graphene layers grown thicker. Finally, we studied the electronic structure of MoTe2 by ARPES and experimentally con rmed the prediction of type II Weyl state in this material. By combining the result of Density Functional Theory calculations and Berry curvature calculations with out experimental data, we identi ed Fermi arcs, track states and Weyl points, all features predicted to exist in a type II Weyl semimetal. This material is an excellent playground for studies of exotic Fermions.

  13. Nanostructure Control: Nucleation and Diffusion Studies for Predictable Ultra Thin Film Morphologies

    NASA Astrophysics Data System (ADS)

    Hershberger, Matthew Thomas

    This thesis covers PhD research on two systems with unique and interesting physics. The first system is lead (Pb) deposited on the silicon (111) surface with the 7x7 reconstruction. Pb and Si are mutually bulk insolubility resulting in this system being an ideal case for studying metal and semiconductor interactions. Initial Pb deposition causes an amorphous wetting layer to form across to surface. Continued deposition results in Pb(111) island growth. Classic literature has classified this system as the Stranski-Krastanov growth mode although the system is not near equilibrium conditions. Our research shows a growth mode distinctly different than classical expectations and begins a discussion of reclassifying diffusion and nucleation for systems far away from the well-studied equilibrium cases. The second system studied investigates the interactions of the Rare Earth metal Dysprosium (Dy) with a carbon based 2D lattice called graphene. Graphene is a 2D material composed of carbon atoms arranged in hexagons, similar to a honeycomb with carbon atoms at each corner. The graphene we used is grown epitaxially from a substrate of silicon carbide. This creates a multilayered playground to study how metals interact both on the surface of graphene and intercalated in between graphene layers. Many types of atoms have been studied in graphene systems, but the rare earths and in particular Dy have not been well investigated. This thesis contributes to the knowledge base of graphene on SiC structure and metal-graphene interactions. These systems have been investigated in ultra-high vacuum (UHV) environments with base pressures around 5.0x10-11 torr. The Pb/Si(111)-7x7 system was investigated with scanning tunneling microscopy (STM) and the Graphene/SiC system was investigated with both STM and Spot Profile Analyzing Low Energy Electron Diffraction (SPA-LEED).

  14. Step induced magnetic anisotropy of iron/tungsten

    NASA Astrophysics Data System (ADS)

    Mireles, Hector Cordova

    Surface Magneto Optic Kerr Effect (SMOKE) measurements of ultrathin layers of Fe on graded-step-density W(001) are used to test essential predictions of a hysteresis loop phase diagram model reported by Zangwill et al. [1]. The one-dimensional micromagetic model produces a rich variety of hysteresis loop shapes as well as predictions of switching field strengths that depend on two parameters: Λ = normalized step length and κ = normalized step-anisotropy energy. These parameters are varied in the experiment by using a graded-step-density W(001) surface (vicinal angle α range from 0°-15°), and by reducing the step anisotropy energy with chemisorption. Spot-Profile- Analysis Low Energy Electron Diffraction (SPA- LEED) is used to characterize the stepped surfaces. The experiments indicate that the Zangwill model successfully accounts for general trends in hysteresis loop shapes and switching field dependencies as a function of Λ and κ. Our measured switching fields, ( Hswitch) vs vicinality for 2 monolayers thick Fe films are generally compatible with results reported by Kawakami et al. [2] over the range 0° < α < 7°, although our results over this range yield a power law of Hswitch ~ αn where n = 2.6 +/- .1, which is different from the quadratic behavior Hswitch ~ α 2 obtained from their measurements and those predicted by the Néel model [3]. Above a critical vicinal angle, αq the step-induced anisotropy becomes ineffective and the loops revert back to the square shape observed on flat surfaces. The angle αq is dependent on the oxygen dosage on the sample as well as on the film thickness. Novel two- state switching is observed at selected vicinal angles, which is related to surface-step-induced anisotropy. While both the Néel model and the Zangwill model account for general trends in step-induced magnetic anisotropy, refinements in the micromagnetic model appear to be required to explain the observed phenomena in detail.

  15. Autoradiographic distribution of 5-HT7 receptors in the human brain using [3H]mesulergine: comparison to other mammalian species

    PubMed Central

    Martín-Cora, Francisco J; Pazos, Angel

    2003-01-01

    The main aim of this investigation was to delineate the distribution of the 5-HT7 receptor in human brain. Autoradiographic studies in guinea-pig and rat brain were also carried out in order to revisit and compare the anatomical distribution of 5-HT7 receptors in different mammalian species.Binding studies were performed in rat frontal cortex membranes using 10 nM [3H]mesulergine in the presence of raclopride (10 μM) and DOI (0.8 μM). Under these conditions, a binding site with pharmacological characteristics consistent with those of the 5-HT7 receptors was identified (rank order of binding affinity values: 5-CT>5-HT>5-MeOT>mesulergine ≈methiothepin>8-OH-DPAT=spiperone ≈(+)-butaclamol≫imipramine ≈(±)-pindolol≫ondansetron ≈clonidine ≈prazosin).The autoradiographic studies revealed that the anatomical distribution of 5-HT7 receptors throughout the human brain was heterogenous. High densities were found over the caudate and putamen nuclei, the pyramidal layer of the CA2 field of the hippocampus, the centromedial thalamic nucleus, and the dorsal raphe nucleus. The inner layer of the frontal cortex, the dentate gyrus of the hippocampus, the subthalamic nucleus and superior colliculus, among others, presented intermediate concentrations of 5-HT7 receptors. A similar brain anatomical distribution of 5-HT7 receptors was observed in all three mammalian species studied.By using [3H]mesulergine, we have mapped for the first time the anatomical distribution of 5-HT7 receptors in the human brain, overcoming the limitations previously found in radiometric studies with other radioligands, and also revisiting the distribution in guinea-pig and rat brain. PMID:14656806