Modulated self-assembly of 4,4′-diphenyltetrathiafulvalene molecules on highly oriented pyrolytic graphite byn-tetradecane solvent

Abstract

We report the formation of a binary-component self-assembled monolayer (SAM) comprising4,4′-diphenyltetrathiafulvalene(DP-TTF) and n-tetradecane (n-C14H30) molecules with periodic strip-like phase separation structures on a highly orientedpyrolytic graphite (HOPG) surface. Scanning tunneling microscopy (STM) imaging revealsthat ordered DP-TTF single- and double-lamella are periodically tuned by orderedn-C14H30 single- and double-lamella, respectively. This finding can be qualitatively understood interms of a phase field model, in which the interplay of three ingredients, including freeenergy of the binary-component solution monolayer, phase boundary energy andsurface stress, determines the final equilibrium sizes of the ordered DP-TTF andn-C14H30 phases in the binary-component SAM. Furthermore, anisotropy of thesurface stress breaks the symmetry of the substrate and causes then-C14H30 molecules to arrange along preferential substrate⟨010⟩ directions. Theorientation of the n-C14H30 molecule stripes further guides the directions of the DP-TTF lamellar structures.In addition, scanning tunneling spectra (STS) of the individual DP-TTF andn-C14H30 molecules in the ordered monolayer show a remarkable difference inI(V) curves on the HOPG substrate.