Since the introduction of the bulk heterostructure concept, efficiencies of organic photovoltaic devices have improved markedly. This has spurred intense interest in controlling chemical morphologies to achieve more efficient charge separation. Unfortunately, there is surprisingly little guidance on how to achieve vertical nanophase separation in small-molecule materials. Basic studies of interface formation and nanophase separation in molecular materials are needed to understand how local chemical forces direct chemical morphology.

We present STM investigations of interface-formation and nanophase separation in binary films of zinc phthalocyanine (ZnPc) and C60 on Ag(111) and Au(111) supports. Physical vapor deposition provides exquisite control of the growth kinetics, allowing access to both metastable and thermodynamic phases. We demonstrate the controlled formation of ZnPc:C601-D and 2-D interfaces with distinctive molecular orientations and packing densities. Of particular interest is the formation of filamentous C60structures at the ZnPc surface. C60chains of single-molecular width wander the ZnPc substrate without registration to the underlying ZnPc template, islanding into a disordered chain phase. (Similar structures are also observed on Pn and 6-T surfaces.) These structures are reminiscent of dipole fluids (albeit of single　 molecular widths!) We present detailed measurements and analysis of C60 wandering chain formation on ZnPc/Ag (111) and ZnPc/Au (111) substrates. We then explore the physical origin of these structures through simulations with a model potential that incorporates short-range C60 – C60 attraction and a long-range dipolar repulsion. From simulations of realized structures, we　 estimate the effective dipole needed for chain formation.　 We account for unexpected magnitude through the combined influences of the charge-transfer (interface) moment and the induced moment in these highly polarizable materials.

**This work has been supported by the UMD MRSEC (DMR 0520471) and NSF Surface and Analytical Chemistry(CHE0750203)