EPITAXIAL THIN-FILMS OF LARGE ORGANIC-MOLECULES - CHARACTERIZATION OF PHTHALOCYANINE AND CORONENE OVERLAYERS ON THE LAYERED SEMICONDUCTORS MOS2 AND SNS2

Ordered overlayers of large organic molecules have been studied on the basal planes of freshly cleaved metal dichalcogenides such as MoS2 or SnS2. Low-energy electron diffraction (LEED) has been used to characterize the growth of copper (CuPc) and chloroindium (InPcCl) phthalocyanine and coronene on these substrates, at coverages up to ca. two monolayers. InPcCl and CuPc films form square lattice nets on both SnS2 and MoS2, with lattice vectors aligned along (PHI = 0-degrees, MoS2) or near (PHI = +/- 4-degrees, SnS2) the principal lattice vectors of the metal dichalcogenide (0001) surface, resulting in 3-6 equivalent Pc domains. Ordered coronene films on MoS2 consist of two hexagonal lattice domains rotated by +/- 13.9-degrees from each principal axis of the (0001) surface Of MoS2. Modeling studies were conducted for adsorbed coronene on MoS2 and for InPcCl on both MoS2 and SnS2, where purely van der Waals forces were presumed to be responsible for the orientation of the epitaxial layers of these organic materials. By summing van der Waals interactions over surface lattices of up to nine molecules, it is possible to show that small differences in the binding site, repulsive and attractive interactions between molecules, and sulfur-sulfur spacing in the MoS2 or SnS2 basal plane may be responsible for the orientations of the first close-packed monolayers.