Molecular Self-Assembly in a Poorly Screened Environment: F(4)TCNQ on Graphene/BN

[Image: see text] We report a scanning tunneling microscopy and noncontact atomic force microscopy study of close-packed 2D islands of tetrafluorotetracyanoquinodimethane (F(4)TCNQ) molecules at the surface of a graphene layer supported by boron nitride. While F(4)TCNQ molecules are known to form cohesive 3D solids, the intermolecular interactions that are attractive for F(4)TCNQ in 3D are repulsive in 2D. Our experimental observation of cohesive molecular behavior for F(4)TCNQ on graphene is thus unexpected. This self-assembly behavior can be explained by a novel solid formation mechanism that occurs when charged molecules are placed in a poorly screened environment. As negatively charged molecules coalesce, the local work function increases, causing electrons to flow into the coalescing molecular island and increase its cohesive binding energy.