P–T diagrams of the system of CH3(CH2)n−1 self-assembled on the Au(111) crystal surface

Abstract

Equilibrium states of the system of self-assembled monolayers (SAM’s) of n-alkanethiol molecules CH3(CH2)n−1 chemiabsorbed on the Au(111) crystal surface are considered in relation to temperature and external pressure applied normally to the surface. Couplings between the atoms (C, H) of the n-alkanethiols are approximated both by the Morse potential and by the Lennard-Jones one. Couplings between the n-alkanethiols and the crystal surface are approximated by the 12-3 potential. Because of the symmetry Z2 of the n-alkanethiols in the tilted state the system is reduced to the Ising model on the triangular lattice with two competing exchange parameters. Calculated p–T diagrams of the tilted system include the following phases: para, ferro, incommensurate, and structure 2×1. The incommensurate phase results from competition between the exchange parameters. It is shown that for some specific choices of the coupling constants the p–T diagram has tricritical points. The temperatures of phase transitions and equilibrium tilt, twist, and azimuthal angles depending on the coupling constants are found. All phase transitions are of the first order. Temperature behavior of heat capacity is calculated.