Modeling of 1,4-cyclohexadiene adsorption thermodynamics on Si(001)-2 × 1 surface

Abstract

Abstract The adsorption thermodynamics of 1,4-cyclohexadiene on Si(001)-2 × 1 are studied in the framework of the lattice gas model with the Monte Carlo and transfer-matrix methods. Adsorption isotherms, heat capacity, entropy as function of chemical potential and adlayer density were calculated. It is shown that there are two ordered phases for the model under consideration. The first phase consists of the tetra-σ complexes and there is a low surface concentration of the adsorbed molecules. The second phase consists of the π-complexes and there is a high surface concentration. The ordered phase consisting of the di-σ complexes is absent within the constructed model. The obtained set of ordered phases and the sequence of its arising with growth of chemical potential are in close correspondence with the experimental data. In the framework of the model under consideration the observed behavior of 1,4-cyclohexadiene adsorption layer on Si(001)-2 × 1 is explained by the presence of three adsorption states with different surface area per molecule and repulsive lateral interactions. Thus, the transition between π- and di-σ configurations and the formation of the π-complex phase can be the thermodynamically driven effect.