STM study of the organic semiconductor PTCDA on highly-oriented pyrolytic graphite

Monolayer and multilayer films of the archetype organic semiconductor 3,4,9,10-perylenetetracarboxylic dianhydride are deposited at room temperature in ultrahigh vacuum on freshly cleaved highly-oriented pyrolytic graphite (HOPG) and analyzed by scanning tunneling microscopy (STM). STM images of monolayer films show a 'herringbone' structure with two molecules per unit cell and dimensions which are in good agreement with prior studies. High-resolution STM images acquired under a variety of bias voltages indicate an electronic inequivalence of the two molecules within the unit cell. Although the image of the most prominent of these molecules resembles a figure eight over a wide range of sample biases, a difference in shape between filled and empty states appears at the smallest bias voltages. These differences are compared to molecular orbital calculations for the highest occupied and lowest unoccupied molecular orbitals of an isolated molecule and found to correlate rather well. Moiré fringes are observed with much larger periodicity than those previously reported by Ludwig et al., and can be explained by the incommensurate nature of the overlayer, which grows quasi-epitaxially, constrained mostly by the symmetry of the substrate. The first reported results of STM on multilayer films is presented showing crystalline domains in multiples of 60°, the rotational symmetry of the HOPG (0001) surface. In most cases, edges of these domains are observed to preferentially align along the a-axis of the unit cell. Finally, given this evidence of stable tunneling with molecular resolution on monolayer and multilayer films, mechanisms for STM image contrast on PTCDA films are discussed.