Stress-Induced Striped Domains at the C$_{60}$-Pentacene Interface*

C$_{60}$:pentacene heterojunctions have been recently employed in functional organic photovoltaic devices [1]. In order to develop a detailed structural understanding of these junctions, we have made STM observations of the growth of pentacene on top of a monolayer film of C$_{60}$ on Ag(111). We observe pattern formation in the first pentacene layer due to the tensile stress arising from interactions with the underlying C$_{60}$. The stress results in a striped pattern of alternating commensurate and incommensurate domains of pentacene with respect to the C$_{60}$ monolayer. The incommensurate domains appear as bright regions 3.6 $\pm $ 0.6 nm in width and are discussed using a 1D Frenkel-Kontorova model. *Supported by the NIST Center of Nanomanufacturing and Metrology and the University of Maryland MRSEC via DMR-05-20471.[1] Yoo et al., Appl. Phys. Lett. 85, 5472 (2004).