Binary-Component Self-Assembled Monolayer Comprising Tetrathiafulvalene and n-Tetradecane Molecules with Periodic Ordered Phase Separation Structures on a Highly Oriented Pyrolytic Graphite Surface

Abstract

A binary-component self-assembled monolayer (SAM) comprising tetrathiafulvalene (TTF) and n-tetradecane (n-C(14)H(30)) molecules has been investigated by a scanning tunneling microscope (STM) on a highly oriented pyrolytic graphite (HOPG) surface at room temperature. High-resolution STM images of the SAM reveal that the two different kinds of molecules spontaneously form periodic ordered strip-like phase separation structure on the HOPG substrate. The phenomenon can be qualitatively understood in terms of a phase field model, in which the interplay of three ingredients, including free energy of the binary component solution monolayer, phase boundary energy, and surface stress, is suggested to play an important role in determining the equilibrium sizes of strip-like domains of the TTF and n-C(14)H(30) in the ordered phase separation structure. In addition, scanning tunneling spectrum (STS) measurements have also been performed by operating an STM tip to locate on the individual TTF or n-C(14)H(30) molecule in the SAM on the HOPG substrate. The STS at the TTF molecule shows a distinct rectifying behavior, while at the n-C(14)H(30) molecule it shows an intrinsic small current increase with the change of bias voltage and a slight asymmetry.