Meandering C$_{60}$ Chains on Organic Film Substrates

Meandering chains of C$_{60}$ molecules are observed following vapor deposition of C$_{60}$ on a variety of organic molecular films including pentacene and zinc phthalocyanine,as well as previously reported a-sexithiophene. Such filamentous structures are in complete contrast to C$_{60}$'s typical close-packed growth habit, but are reminiscent of dipole fluids. We present STM images and a statistical analysis of chain structures observed for 0.2 - 0.9 mL C$_{60}$ on the square ZnPc overlayer $(a=13.6\mathop A\limits^\circ ,b=14.5\mathop A\limits^\circ ,\alpha =89^\circ )$on Ag (111). Large islands of meandering C$_{60}$ islands indicate $>$50 nm C60 diffusion lengths on this ZnPc film. The C$_{60}$ chain length, $\ell \sim $ 20 nm, depends weakly on coverage, reflecting the $\sim $constant C$_{60}$ density within these islands, and exhibits multiple branches. Chain structures are compared to the molecular dynamics predictions of a 2-d dipole fluid with the C$_{60}$ - C$_{60}$ interaction described by the Girifalco Potential (1 eV attraction) with additional dipole terms. A vertical moment of $\sim $0.8 D simulates filament formation. The structure of the ZnPc support, not included in this simulation, appears to be a secondary consideration in these chain structures.