A scanning tunneling microscopy study on the monolayer epitaxy of [cyano(ethoxycarbonyl) methylene]-2-ylidene-4,5-dimethyl-1,3-dithiole on graphite and its dynamic feature

The monolayer film of [cyano(ethoxycarbonyl)methylene]-2-ylidene-4,5-dimethyl-1,3-dithiole (CEMYDD) vacuum-deposited on a (0001) graphite surface was investigated by scanning tunneling microscopy (STM). High-resolution STM images showed that all of the molecules formed the same two-dimensional unit cell in each domain and that the unit cell was almost equivalent to that in the (011) plane of the second-harmonic-generation active β-form crystal. Therefore the CEMYDD molecules were arranged as polar domains with a size of several hundred nm. The crystallographic orientation of the unit cell in the film with respect to the graphite surface was found to have commensurate epitaxial coincidence. This commensurate structure was stable under STM measurements, except near the domain boundaries where the moiré-like, long-range, periodic contrast modulation due to the commensurate coincidence often varied while scanning with the STM tip. Although the arrangement of molecular images themselves moved only slightly, such a collective dynamic feature at boundaries could be detected from changes in the moiré-like contrast modulation.