Nanostructured surfaces

Collective diffusion mechanisms
in phase inhomogeneous adsorbed films
Loburets A.T.1, Zayika S.O.1
1National University «Yuri Kondratyuk Poltava Polytechnic». 
Pershotravnevyi Prospect, 24, Poltava-36011, Ukraine. 
E-mail: anatollob@gmail.com

A two-dimensional mathematical model of the adsorption system with a random initial adatom configuration is developed. To find the parameters of the lateral interaction with the substrate, the information from earlier real experiments in the systems Li-W(110) and Li-W(112) was used [1, 2]. The obtained results of the study of the mathematical model properties are related to its equilibrium state, which was evaluated by achieving the reproducibility of the values of the collective diffusion coefficients after the series of heating.
Our goal was to explore the mechanisms of surface diffusion at the atomic level in combination with the thermodynamic characteristics of surface diffusion (self-diffusion coefficients, activation energy, pre-exponential factor in the diffusion equation) and the dependences of these parameters on the coverage degree in monolayer. Strong concentration dependence D(θ) and the presence of specific features on isotherms lnD(θ) (sharp maxima and minima) were revealed. This allowed to select the region of adsorbed film heterogeneity on the area lnD(θ), θ, construct a complete binodal, find the critical temperature of the system under study and estimate the existence region of the critical value of the coverage degree θС. It is demonstrated that the midline of binodal (rectilinear diameter) is a straight line at low temperature, but it begins to deviate towards high concentrations when approaching the critical temperature. The structures formed in films at temperatures below the critical one were studied. It is shown that they correspond to long-period structures in the real Li-W(112) system obtained with LEED method by other authors. Note that in real experiments, the equilibrium surface diffusion study is associated with significant difficulties. After all, equilibrium measurements are based on the relaxation of small concentration fluctuations depended on the temperature and the structure of the films, which are difficult to measure directly. This can be done by exploring a mathematical model which is based on the molecular dynamics method and real interaction potentials.

1.	Loburets A.T., Naumovets A.G., Vedula Yu.S. Surface diffusion of lithium on (011) face of tungsten // Surface Science.-1982.-120.-P. 347-366.
2.	Loburets A.T., Senenko N.B., Vedula Yu.S, Naumovets A.G. Surface diffusion in lithium submonolayer films on the (112) surface of tungsten // Ukr. J. Phys.-2005.-50, N 8.-P. 805-815.