V. M. Shevlyuga

Atomic scale observation of iodine layer compression on Cu(1 1 1)

The atomic structure of the iodine covered Cu(1 1 1) surface was studied by STM in combination with LEED, AES and TDS techniques. It was established that iodine adsorption on Cu(1 1 1) leads to the uniaxial compression of chemisorbed iodine layer. The structure of the saturated iodine coverage on Cu(1 1 1) was described in terms of formation of the striped super heavy domain walls, with the interatomic distances in the wall being smaller than van der Waals diameter of iodine.

Atomic structure of silver chloride formed on Ag(111) surface upon low temperature chlorination

The structure of AgCl formed in the course of chlorine adsorption on Ag(111) surface was studied by STM. For the first time atomic resolution STM images of silver chloride were obtained. The silver chloride was detected as small (30-60 A in diameter) islands located on the atomic terraces. The upper plane of the islands was found to be AgCl(111) with interatomic distance close to the bulk value. The chloride surface lattice was not rotated with respect to the substrate. A possible mechanism of AgCl growth on Ag(111) is discussed.

Adsorption of O 2 on Ag(111): Evidence of Local Oxide Formation

The atomic structure of the disordered phase formed by oxygen on Ag(111) at low coverage is determined by a combination of low-temperature scanning tunneling microscopy and density functional theory. We demonstrate that the previous assignment of the dark objects in STM to chemisorbed oxygen atoms is incorrect and incompatible with trefoil-like structures observed in atomic-resolution images in current work. In our model, each object is an oxidelike ring formed by six oxygen atoms around the vacancy in Ag(111).

Local structure of thin AgCl films on silver surface

The new structural results revising commonly accepted point of view on role of products of silver chlorination in catalytic reaction of ethylene epoxidation are presented. It was established that AgCl nucleation on Ag(111) mainly occurs on step edges but low temperature reaction could also start on atomic terraces. On atomically resolved images of AgCl nuclei surface obtained with scanning tunneling microscope, the specific features of atomic size were observed and attributed to silver atoms and atomic clusters. We predict that such silver cluster formation could explain a very high activity of chlorinated surface in ethylene epoxidation. Interatomic distances measured in thin AgCl films were found to be equal to corresponding bulk parameters. The photoelectron diffraction patterns observed upon Ag(100) chlorination were explained by the formation of AgCl(111) domains rotated on 90° with respect to each other.

Local structure determination for surface chlorination with EELFS

Abstract In this paper the results of an investigation of the local structure of a layer of molecular chlorine condensed on Si(1 0 0) at T = 110 K and an unbroken AgCl film evaporated on Ag(1 1 1) at T = 160 K are presented. It was found that the nearest neighbor ClAg distance in the AgCl film of 2.75 ± 0.05 A is in good agreement with bulk values (2.77 A). The ClCl distance of 2.00 ± 0.05 A in a chlorine film condensed on Si(1 0 0) is also close to the bulk value (1.98 A).

Structural transformations on an oxidized Ag(111) surface

The structure of the Ag(111) surface after the adsorption of molecular oxygen at a temperature of 300 K is studied by low-temperature scanning tunneling microscopy. It is established that local surface oxide is formed at the first stage of adsorption. The subsequent adsorption of O results in the appearance of new objects with a size of 3–8 Å and a height of 1.0–1.5 Å on the Ag(111) surface, which form quasi-ordered structures with increasing degree of coating. The heating of the system obtained up to 500 K leads to a structural transition resulting in the formation of single islands of the p (4×4) phase on the surface. Surface structures are identified by a simulation based on the density functional theory.

Adsorption of molecular oxygen on the Ag(111) surface: A combined temperature-programmed desorption and scanning tunneling microscopy study

The adsorption of O2 on Ag(111) between 300 and 500 K has been studied with temperature-programmed desorption (TPD) and scanning tunneling microscopy (STM). At the first stage of adsorption, the disordered local oxide phase (commonly looking in STM as an array of black spots) is formed on the surface irrespective of the substrate temperature. The maximum concentration of black spots was found to be ≈0.11 ML, which corresponds to an oxygen coverage of ≈0.66 ML. Taking into account that the nucleation of the Ag(111)-p(4 × 4)-O phase starts after the saturation of the disordered phase, one can conclude that its coverage is at least not less than 0.66 ML. The analysis of STM and TPD data shows that the thermodesorption peak (m/e = 32) at 570 K is related exclusively to the decomposition of the p(4 × 4) phase, while the local oxide phase does not contribute to desorption.

Electron-induced interaction of condensed chlorine with Si(100)

We have studied electron-induced interaction of molecular chlorine with Si(100) surface under ultra-high vacuum conditions. Without electron beam irradiation, chlorine was found to be adsorbed both dissociatively (first step) and molecularly (second step) at 100 K. Illumination of molecular chlorine-covered surface at 100K with electron beam (1–3 keV) led to the formation of silicon tetrachloride on the Si(100) surface. We established that the surface reaction, that is responsible for the SiCl4 formation, is initiated by Cl2 dissociation and appearance of the active chlorine radicals. Importance of the such effect is also discussed in a view of their possible applications for selective etching of semiconductor surfaces.

Structural transformations on Cu(110) under molecular iodine action

Interaction of molecular iodine with the Cu(110) surface is investigated by the methods of ultrahigh vacuum scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). It is found that at the coverage θ =0.5 monolayer (ML) iodine forms a simple commensurate c(2×2) lattice. Further exposure of iodine leads to uniaxial compression of the c(2×2) lattice along the 〈110〉 direction of the substrate. The STM data indicate that compression of the iodine layer proceeds through formation of striped domain walls. As the coverage is saturated at θ = 0.63 ML, iodine forms a uniformly compressed quasi-hexagonal structure. Further exposure of iodine on the Cu(110) surface results in growth of a copper iodide film. STM images of thin (7 to 20 Å) CuI films reveal, in addition to atomic modulation, a superstructure with a period of 90 to 100 Å consisting of double stripes. A structure model of the copper iodide surface allowing for CuI lattice contraction and formation of double stripe domain walls is proposed.