Effect of molecular symmetry and packing efficiency on the epitaxial growth modes of monomolecular film on graphite studied by STM: the case of 2,4,6-tris(4,5-dimethyl-1,3-dithiol-2-ylidene)-1,3,5-cyclohexanetrione

The monomolecular film of 2,4,6-tris(4,5-dimethyl-1,3-dithiol-2-ylidene)-1,3,5-cyclohexanetrione (HMTDTCHTO) vacuum-deposited on a (0 0 0 1) surface of graphite was observed by scanning tunneling microscopy, in order to reveal its growth mode and structure. The unit lattice of the HMTDTCHTO monomolecular layer is incommensurate to the substrate lattice. The flat lying molecules form an ordered close-packed arrangement in the film on the graphite. The results are compared with those of 2,4,6-tris(1,3-dithiol-2-ylidene)-1,3,5-cyclohexanetrione (TDTCHTO). The growth modes of some monomolecular films are examined from a viewpoint of two-dimensional packing efficiency and molecular symmetry.