Search academic texts

Information × Registration Number 0213U002712, 0110U002256 , R & D reports Title Diffusion and phase transformations in copper monolayers on (112) tungsten facet popup.stage_title Head Loburets Anatoly Timofiyovich, Registration Date 05-02-2013 Organization Poltava National Technical University named after Yuri Kondratyuk popup.description2 The object of investigation is mono- and submonolayer films adsorbed on the faces of the atomic anisotropic crystals. The subject of the study is the phase transformations and the surface diffusion of the adsorbed films, depending on the atomic structure of the substrate, the nature of the interaction between adatoms and a padded, critical parameters and order. The purpose of research is experimental and theoretical study of the dynamic properties of monolayers of copper adsorbed on the (112) tungsten using simulation of surface diffusion in phase-nonhomogeneous films and study of phase equilibria in these films. Research methods and apparatus. The experimental part of the work is based on a study by the potential difference of two-dimensional distributions of local work functions during isothermal evolution of the adsorbed film at different temperatures. For this purpose contact-potential scanning microscope was been designed, manufactured and patented (Patent 23,087 UA, IPC N01J37/26. Bulletin. № 6, 2000). Due to using a specially conducted experiments calibration graphs showing the concentration dependence of the work function finishing the concentration distribution of the adsorbate were been obtained. The diffusion (or the like) shaped concentration profiles and their evolution at fixed temperatures were been obtained too. In the case of classical diffusion, these profiles were used to calculate the concentration dependence of the diffusion coefficients by the Boltzman-Matano method at different temperatures. Mathematical modeling of surface diffusion of the adsorbed submonolayer films in two-dimensional periodic potential of the substrate was performed by the method of molecular dynamics. Theoretical and practical results. Surface diffusion is determined by the phase state of the adsorbed film. When using atomic-perfect single crystal surface in ultrahigh vacuum, adsorbent phase state of submonolayer adsorbed films is reflected in the concentration distributions, formed in the diffusion zone of a heterogeneous film. It is shown that such distributions obtained at different temperatures below the critical one carry information about the degree of coverage of the coexisting phases. This hypothesis was confirmed experimentally by the example of the adsorption system Cu-W(112). The phase diagram of a two-dimensional adsorbed film was been plotted. The conditions for a dynamic self-organization in an open, nonlinear nonequilibrium system may arise in the diffusion zone of submonolayer heterogeneous adsorbed film at temperatures, approaching the critical one. It is shown that it is indeed possible under the condition that the desorption of the gas phase is much higher than a condensed one. In addition it is shown that in coadsorbed films with lateral dipole interaction of the adatoms surface diffusion is strongly suppressed by small amount of slow coadsorbate. Moreover, strontium and lithium can form a common structure, which is manifested in the diffusion characteristics. The novelty of these results is the fact that it was first shown: In a strongly nonequilibrium heterogeneous submonolayer adsorption system Cu-(112)W, the dynamic equilibrium between condensed and gas phases controlled by diffusion process can be established, if in the two-dimensional heterogeneous system adsorbate is readily desorbed from the gas phase at temperatures of intense diffusion, and the desorption from condensed phase is negligible. This leads to the self-organization of the film with the release of the dynamic structure in the form of high-gradient region of the concentration profile. Solitary concentration wave moves at a constant speed without changing its shape over time, but its height decreases with increasing temperature due to changes in the concentrations in the coexisting phases. This enabled for the first time to plot the state diagram of adsorbed film, which is formed by the Stranski-Krastanow mechanism under condition of transition from layer-by-layer growth to island one. In two-dimensional adsorption systems with an attractive interaction between adatoms, phase state change is analogous to phase transitions in three-dimensional systems and different from the results predicted by the theory. The second-order transitions in strongly anisotropic two-dimensional systems are characterized by the order parameters which are almost twice as much as the value for isotropic systems and theoretical values of the Ising model. At low temperatures, superstructures arise in the two-dimensional incommensurate with the substrate film of copper on a background of the hexagonal structures through mutually agreed periodic displacement of adsorbed atoms from their adsorption sites (one-dimensional soliton lattice is formed). In coadsorbed films Li-Sr even a few percent of slower Sr adatoms strongly inhibit the diffusion of lithium. This effect is caused both by the interaction between the coadsorbed particles, especially when lateral forces are of long-range nature, and the existence of collective mechanisms of surface diffusion. The significance of these results is that the obtained new information could be used to solve a number of practical problems associated with the creation of new materials, elements of modern solid-state micro-and nanoelectronics, with crystal growth, heterogeneous catalysis, powder metallurgy, etc. Efficiency of implementation: Results of the research enhances understanding of the mechanisms of atomic interaction and transport of atoms in the adsorbed films, the dynamics of the film growth and the influence of external factors, the dependence of these processes on the electronic structure of the atoms of the adsorbate phase state and structure of the two-dimensional systems and the substrate. From a practical point of view, the results are important for controlling the kinetics of surface transport of substances and, therefore, any processes that depend on it. It is shown that the use of phase effects in surface diffusion and appropriately selected coadsorbates can be an effective way to influence the kinetics of the surface mass transfer, and the influence of impurities on coadsorbed surface diffusion in the film can be used for the targeted modification of physical and chemical properties of the surfaces to create new nano-materials, electronic, chemical, and biological technologies. Proposed and realized a fundamental improvement in the scanning contact-potential microscope has an important methodological significance. Research results are used in the courses of lectures on such subjects as "Physical Chemistry", "Physical chemistry and chemistry of silicates", "Theoretical Foundations of phase transformations of hydrocarbons", "Concepts of modern science." Product Description popup.authors Заїка С.О. Лобурець А.Т. Сененко А.І. Сененко Н.Б. Стрижуес Д.В. popup.nrat_date 2020-04-02 Close