Structure analysis of W(0 0 1)2 × 1-O surface at room and liquid nitrogen temperatures

Surface structure of O-adsorbed W(0 0 1) surface after annealing to 1200 K has been analyzed by low energy electron diffraction at 77 K as well as at room temperature. The optimum structure has tungsten missing rows and oxygen double rows. Furthermore, the R-factor is minimized at the structure that O atoms are adsorbed on one of the two different threefold hollow sites of the (1 1 0) facet appearing on the W(0 0 1)2 × 1 with missing row. However, the results suggest that two domains of O atoms adsorbed on both the two different threefold hollow sites coexist. Then, I-V curves have been analyzed as a function of the mixing ratio of the two domains having different O adsorption sites at room and low temperatures. The energy difference between these two sites has been estimated to be 6.5 meV from the temperature dependence of the mixing ratio.     

Substitutional adsorption geometry for Pb(1 1 1)-(√3 × √3)R30°-K

Intermixed structures for alkalis (larger than Li) on close-packed substrates have previously been observed only on Al(1 1 1). This study shows that K forms an ordered intermixed structure on Pb(1 1 1). The structures of clean Pb(1 1 1) and Pb(1 1 1)-(√3 × √3)R30°-K were studied using dynamical low-energy electron diffraction (LEED). The clean Pb(1 1 1) surface at 47 K was found to be a relaxed version of the bulk structure, in agreement with an earlier study of the same surface [Y.S. Li, F. Jona, P.M. Marcus, Phys. Rev. B 43 (1991) 6337]. At room temperature, adsorption of K on this surface results in a (√3 × √3)R30° structure, which was shown using dynamical LEED to consist of K atoms substituted in surface vacancies. The K-Pb bond length was found to be 3.62 ± 0.3 Å, with no significant change to the Pb interlayer spacings.     

Growth of Co nanoparticles on a nanostructured θ-Al2O3 film on CoAl(1 0 0)

We have investigated the growth of Co nanoparticles on θ-Al₂O₃/CoAl(1 0 0) by means of Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (EELS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Due to Volmer-Weber growth, Co forms particles with a mean diameter of approximately 2.5 nm and height of 0.8 nm. Even on the entirely covered oxide, there is no Ostwald ripening and Co particles stay structurally isolated. The nanoparticles exhibit a small size distribution and tend to form chains, as predetermined by the streak structure of the oxide template. For sufficient high coverages Co-core-CoO-shell nanoparticles may be evidenced, which is explained as a result of surfactant oxygen. The nanostructured particles may open the door to numerous applications, such as in catalysis and magnetoelectronic applications, where large areas of ordered nanodots are desired.     

Surface pre-melting and surface flattening of Bi nanofilms on Si(1 1 1)-7 × 7

We report on the in situ observation of temperature-driven drastic morphology evolution and surface pre-melting of the Bi(0 0 1) nanofilm deposited on the Si(1 1 1)-7 × 7 surface by use of spot-profile-analyzing low-energy electron diffraction (SPA-LEED). Surface step density of the single-crystalline, epitaxial Bi(0 0 1) film decreases above 350 K in a critical manner. On annealed Bi(0 0 1) films, we have detected surface pre-melting with a transition temperature of 350 K, which yields reversible diffraction intensity drop in addition to the harmonic Debye-Waller behavior. The observed surface flattening of the as-deposited film is driven by the increased amount of mobile adatoms created through the surface pre-melting.     

CuPc molecules adsorbed on Au(1 1 0)-(1 × 2): growth morphology and evolution of valence band states

We present the growth morphology, the long-range ordering, and the evolution of the valence band electronic states of ultrathin films of copper phthalocyanine (CuPc) deposited on the Au(1 1 0)-(1 × 2) reconstructed surface, as a function of the organic molecule coverage. The low energy electron diffraction patterns present a (5 × 3) reconstruction from the early adsorption stages. High-resolution UV photoelectron spectroscopy data show the disappearance of the Au surface states related to the (1 × 2) reconstruction, and the presence of new electronic features related to the molecule-substrate interaction and to the CuPc molecular states. The CuPc highest occupied molecular orbital gradually emerges in the valence band, while the interface electronic states are quenched, upon increasing the coverage.     

Adsorption of Pb on NiAl(1 1 0): energetics and structure

The adsorption of Pb onto a NiAl(1 1 0) single crystal surface at 300 K has been studied by Auger electron spectroscopy (AES), Low energy electron diffraction (LEED), molecular beam/surface scattering and single crystal adsorption calorimetry (SCAC). AES indicates a Stranski-Krastanov growth mode, i.e., Pb initially grows on NiAl(1 1 0) two-dimensionally until the first layer completes at 0.89 ML, where a superstructure is observed by LEED, followed by 3D islanding. Measurements of the Pb gas that does not stick indicate that Pb sticks on NiAl(1 1 0) with an initial probability of 0.99. The initial heat of adsorption of Pb on NiAl(1 1 0) is 249 ± 10 kJ/mol. Due to the repulsive interactions between Pb adatoms, the heat of adsorption decreases within the first layer to a value identical to the heat of sublimation of bulk Pb (195 kJ/mol), where it remains at higher coverages. This first application of adsorption calorimetry on such a thick sample (75 μm versus 0.2-8 μm previously) demonstrates that adsorption calorimetry can be extended to a wider range of surfaces, since this thickness can be achieved with nearly any single crystal material by simple mechanical thinning.     

An ordered surface ternary alloy of a c(6 × 4) structure formed on Cu(0 0 1) by substitutional coadsorption of Mg and Bi

A c(6 × 4) structure formed on Cu(0 0 1) by the coadsorption of Mg and Bi atoms at room temperature has been determined by a tensor low energy electron diffraction analysis. It is an ordered surface ternary alloy with a thickness of single layer, in which Mg, Bi and Cu atoms are mixed in the top layer. In the primitive unit cell, there are one Mg, four Bi, six Cu atoms and one vacancy in the top layer, and substituted Mg and Bi atoms form MgBi₄ plane clusters being arranged in the c(6 × 4) order. Structural parameters show that Mg-Bi bond distances in the MgBi₄ cluster are 3.01 and 3.07 Å, which are shorter than the summation of metallic radii of Mg and Bi. It is concluded that a direct, attractive interaction between Mg and Bi atoms plays critical role in the formation of the c(6 × 4) structure.     

RT growth of acetonitrile and acrylonitrile on Si(0 0 1)-2 × 1 studied by XPS and LEED

In this work we show the adsorption of acetonitrile (CH₃CN) and acrylonitrile (CH₂CHCN) on Si(0 0 1)-2 × 1 at room temperature by increasing the molecular doses. Especially, by means of XPS and LEED data, we stress the action of these molecules on the silicon surface locating the dangling-bonds quasi-saturation within 10 L. The shortage of nitrogen XPS signal and some anomalies in carbon spectra point to an invading action from a traditional X-ray source (Al-K_α line) against chemisorbed molecules. In particular, we think that a long exposure to this radiation could break carbon-silicon bonds changing some adsorption geometries and making desorb molecular fragments.     

Oxygen-induced nano-faceting of Ir(2 1 0)

The adsorption of oxygen and the nanometer-scale faceting induced by oxygen have been studied on Ir(2 1 0). Oxygen is found to chemisorb dissociatively on Ir(2 1 0) at room temperature. The molecular desorption process is complex, as revealed by a detailed kinetic analysis of desorption spectra. Pyramid-shaped facets with {3 1 1} and (1 1 0) orientations are formed on the oxygen-covered Ir(2 1 0) surface when annealed to T?600 K. The surface remains faceted for substrate temperatures T<850 K. For T>850 K, the substrate structure reverts to the oxygen-covered (2 1 0) planar state and does so reversibly, provided that oxygen is not lost due to desorption or via chemical reactions upon which the planar (2 1 0) structure remains. A clean faceted surface was prepared through the use of low temperature surface cleaning methods: using CO oxidation, or reaction of H₂ to form H₂O, oxygen can be removed from the surface while preserving (“freezing”) the faceted structure. The resulting clean faceted surface remains stable for T<600 K. For temperatures above this value, the surface irreversibly relaxes to the planar state.     

The adsorption of CCl4 on Ag(1 1 1): Carbene and CC bond formation

The adsorption of CCl₄ on Ag(1 1 1) has been investigated from 100 K to 300 K using absolute sticking probability measurement, temperature programmed desorption, Auger electron spectroscopy, low energy electron diffraction, ultra-violet photoelectron spectroscopy and X-ray photoelectron spectroscopy. At 100 K, CCl₄ adsorbs molecularly with a sticking probability of 1.0, forming a (3 × 3) adsorption structure. At 300 K the following overall reaction occurs,

2CCl_4(g)→4Cl_(chem)+C₂Cl_4(g),     

Structure and composition of the NiPd(1 1 0) surface

The NiPd(1 1 0) alloy surface was studied using low energy electron diffraction to measure the structure and composition of the first three atomic layers. The surface layer is highly enriched in Pd and has a significantly buckled structure. The second layer is also buckled, with displacements even larger than the surface layer. The second layer also exhibits intralayer segregation (chemical ordering), with alternate close-packed rows of atoms being Ni enriched and Pd enriched. The third layer has a structure and composition close to that of the bulk alloy. These results are compared with results for the other low index faces of NiPd, the extensive literature on NiPt alloy surfaces, and the growing body of theoretical literature for NiPd alloy surfaces.     

Me-PTCDI thin film deposition on InSb(1 1 1)A

The evolution of N,N′-dimethylperylene-3,4,9,10-dicarboxyimide (Me-PTCDI) thin films formed by vapour deposition on InSb(1 1 1)A substrates has been studied by X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED). XPS studies of the Me-PTCDI covered surface indicate that no significant interaction occurs at sub-monolayer coverage when compared to multilayer Me-PTCDI films. HREELS studies suggest only a weak interaction as evidenced by very small changes in the frequencies of several molecular vibrational modes. LEED patterns show the Me-PTCDI overlayer adopts a structure commensurate with the underlying InSb(1 1 1)A substrate surface and that can be rationalised by van der Waals intermolecular energy calculations for the Me-PTCDI unit cell. The results are consistent with a weak interaction at the Me-PTCDI/InSb interface, the formation of the commensurate structure being sufficient to overcome the small energetic penalty associated with deviation from the calculated intermolecular interaction energy minimum.     

Structure and stability of Sb/Au(1 1 0)-c(2 × 2) surface phase

Adsorption of 0.5 monolayers (ML) of Sb on the Au(1 1 0) surface resulted in the formation of a c(2 × 2) surface reconstruction. Analysis of surface X-ray diffraction data by a direct method revealed the existence of an ordered substitutional surface alloy, with every other hollow site occupied by Au and Sb atoms. Quantitative conventional χ² refinement showed a contraction of 0.12 ± 0.03 Å in the spacing of the first Au layer to the second, an expansion of 0.13 ± 0.03 Å in the second-to-third layer distance, and an inward Sb displacement (rumpling) of 0.21 ± 0.04 Å. This surface phase proved to be extremely robust, with the long-range order of this arrangement remaining up to substrate temperatures of 900 K.     

Structural and statistical analysis of Yb/Si(1 1 1) and Eu/Si(1 1 1) reconstructions

We have performed the structural and statistical analysis of Yb/Si(1 1 1) and Eu/Si(1 1 1) surfaces in the submonolayer regime utilizing low-energy electron diffraction and scanning tunneling microscopy (STM). The almost identical series of one-dimensional chain structures (e.g., 3 × 2/3 × 1, 5 × 1, 7 × 1, 9 × 1, and 2 × 1 phases) are found in order of increasing metal coverage for both adsorbed systems, however, only the Eu/Si system reveals the ‘√3’-like reconstruction before the 2 × 1 endpoint phase. The atomic models of chain structures are proposed and discussed. In particular, our results suggest the odd-order n×1 (n=5,7,9,…) intermediate reconstructions to incorporate the Seiwatz chains and honeycomb chains with the proportion of m:1, where . The statistical analysis of STM images is carried out to examine the correlation of atomic rows on Eu/Si and Yb/Si surfaces. It is found that Eu stabilizes more ordered row configuration compared to Yb, which can be explained in terms of indirect electronic interaction of atomic chains or/and different magnetic properties of adsorbed species.     

SrTiO3(0 0 1) reconstructions: the (2 × 2) to c(4 × 4) transition

Scanning tunneling microscopy (STM) is used to investigate the (0 0 1) surface structure of Nb doped SrTiO₃ single crystals annealed in ultra high vacuum (UHV). Atomically resolved images of the (2 × 2) reconstructed surface are obtained after annealing a chemically etched sample. With further annealing dotted row domains appear, which coexist with the (2 × 2) reconstruction. The expansion of these domains with further annealing gives rise to the formation of a TiO₂ enriched c(4 × 4) reconstruction.     

Growth of Ni-Al alloys on Ni(1 1 1): (I) Formation of epitaxial Ni3Al from ultra-thin Al deposits

In this paper we describe the alloying process of ultra-thin Al layers (below 8 × 10¹⁵ Al/cm²) deposited on Ni(1 1 1). For this purpose Auger electron spectroscopy, low energy electron diffraction, and ion beam analysis-channelling measurements have been performed in situ in an ultra-high vacuum chamber. Al deposits formed at low temperature (about 130 K) are strained defective crystalline layers retaining the substrate orientation. Alloying takes place, with very progressive Ni enrichment, in a very broad temperature range between 250 K and 570 K. This feature shows that diffusion of the alloy species is more and more difficult when the Ni concentration increases. At 570 K a crystallographically and chemically ordered Ni₃Al phase is formed, and its order continuously improves upon annealing, up to 750 K. We have shown by ion beam methods that this alloy is three-dimensional, extending up to 16 (1 1 1) planes for the thickest deposits. The Ni₃Al phase can also be obtained directly by Al deposition at 750 K, but its crystalline quality is lower and the layer is probably formed of grains elongated along 〈1 1 −2〉 directions. The Al content of the thin Ni₃Al layers formed mostly dissolves in the bulk above 800 K. However a small amount of Al remains segregated at the Ni crystal surface.     

Ultra-thin Si overlayers on the TiO2 (1 1 0)-(1 × 2) surface: Growth mode and electronic properties

The growth of thin subnanometric silicon films on TiO₂ (1 1 0)-(1 × 2) reconstructed surfaces at room temperature (RT) has been studied in situ by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS), Auger electron and electron-energy-loss spectroscopies (AES and ELS), quantitative low energy electron diffraction (LEED-IV), and scanning tunneling microscopy (STM). For Si coverage up to one monolayer, a heterogeneous layer is formed. Its composition consists of a mixture of different suboxides SiO_x (1 < x ? 2) on top of a further reduced TiO₂ surface. Upon Si coverage, the characteristic (1 × 2) LEED pattern from the substrate is completely attenuated, indicating absence of long-range order. Annealing the SiO_x overlayer results in the formation of suboxides with different stoichiometry. The LEED pattern recovers the characteristic TiO₂ (1 1 0)-(1 × 2) diagram. LEED I-V curves from both, substrate and overlayer, indicate the formation of nanometric sized SiO_x clusters.     

Epitaxial C60 thin films on Bi(0 0 0 1)

We have used the Bi(0 0 0 1)/Si(1 1 1) template to grow highly ordered C₆₀ epitaxial thin films and analyzed them using scanning tunneling microscopy and low-energy electron microscopy. The in situ low-energy electron microscope investigations show that the initial nucleation of the C₆₀ islands on the surface takes place at surface defects, such as domain boundaries and multiple steps. The in-plane lattice parameters of this C₆₀ film turns out to be the same as that of the bulk fcc(1 1 1) C₆₀. The line-on-line epitaxial structure is realized in spite of a weak interaction between the C₆₀ molecules and Bi(0 0 0 1) surface, while scanning tunneling spectroscopy indicates that there is a negligible charge transfer between the molecules and the surface.     

Formation of oxygen-induced Si(1 1 3)-3 × 2 facets on the Si(5 5 12) surface

We report the formation of Si(1 1 3)-3 × 2 facets upon exposing oxygens on the Si(5 5 12) surface at an elevated temperature. These facets are found to form only for a limited range of oxygen exposure and exhibit a well-defined 3 × 2 LEED pattern. We also find the surface electronic state unique only to the facets in the valence band. The spectral feature of these electronic states and the behavior of a (1/3 1/2) LEED spot upon oxygen contents in the facets indicate that the formation is a heterogeneous mixture of the clean Si(1 1 3) facets free of oxygens and other facets containing oxygen atoms.     

(2 × 1)-Na surface reconstruction induced by NaCl dissociation on Ag(1 1 0) during LEED analysis

This study first reports the initial growth stages of sodium chloride (NaCl) on Ag(1 1 0) at room temperature. NaCl grows in bi-layer mode along its [1 0 0] axis and gives rise to (4 × 1) and (1 × 2) reconstructed domains for coverages lower than two monolayers (ML), a minimal thickness inducing a bi-dimensional closed film. In addition, a 10 ML NaCl film has been examined by low energy electron diffraction (LEED). LEED analysis leads to the dissociation of the NaCl deposit in a few minutes. The NaCl dissociation implies Cl desorption from the surface and Na remaining on it. The residual Na is arranged in the form of a (2 × 1) surface reconstruction and is found to be strongly bounded to the Ag substrate. These findings have been established by using the X-ray photoelectron spectroscopy technique.