Epitaxial films of 1,4-dithioketo-3,6-diphenyl-pyrrolo-[3,4c]-pyrrole (DTPP) on graphite (0001) were investigated by scanning tunneling microscopy (STM). In the STM images, one-dimensional periodic modulation of image contrast was observed in addition to each molecular image, which was interpreted as moire fringe caused by the interaction with a graphite lattice. Following to mathematical analysis, such one-dimensional periodic modulation was concluded to appear only under the condition that every lattice point of the deposited film is on an (m, n)-line of the substrate lattice for certain integers m and n. Using this property, the epitaxial orientation of DTPP with respect to the substrate lattice was determined precisely. The resulting orientation exhibits no exact commensurate relation with graphite, but every lattice point of DTPP was found to lie on a lattice-line parallel to the b-axis ((1, 0)-line) of graphite.
