Phase transitions in a system of CH3(CH2)n-1 self-assembled on the Au(111) crystal surface.

Abstract

The $n$-alkanethiols $\mathrm{C}{\mathrm{H}}_{3}{(\mathrm{C}{\mathrm{H}}_{2})}_{n\ensuremath{-}1}$ on the crystal surface of gold (111) are considered. Couplings between atoms (C, H) of the $n$-alkanethiols were modeled by the Morse potential, and couplings between the $n$-alkanethiols and the crystal surface were modeled by 12-3 potentials. Because of symmetry ${Z}_{2}$ of the $n$-alkanethiols in a tilted state, the system can be reduced to the Ising model on the triangular lattice, the phase diagram of which can be explicitly considered. Depending on the choice of the coupling parameters, the ordered state of the system may form an incommensurate or 2\ifmmode\times\else\texttimes\fi{}1 superstructure. The temperatures of the phase transition, equilibrium tilting, twisting, and azimuthal angles are found to depend on the Morse coupling constants.