Electronic Transport Properties of Molecular Graphyne

Thirty years ago there were two classic forms of carbon (diamond and graphite). Since then, three new carbon allotropes (buckyballs, carbon nanotubes and graphene), all involving only sp2 carbons, have been discovered. While there is much excitement about graphene, another interesting carbon-based material, graphyne, has been suggested. Graphyne is a two dimensional network of sp and sp2 carbon centers. While this material is not known in nature, it has been the subject of theoretical calculations which suggest exciting properties for graphynes beyond those of graphene. While macroscopic samples of graphyne are not currently available, molecular units of graphyne, called carbo-benzenes, have been synthesized by our collaborators in Toulouse. We report the single molecule electrical measurements on molecular carbo-benzenes, connected to gold electrodes via amine anchoring groups, determined using scanning tunneling microscope break junction (STM-BJ) methods. Our results show that these highly conjugated macro-aromatics with NH2 metal-anchoring functions are superior in conductivity (~100 nS) compared with molecules of similar length (1.94 nm). These experimental results are rationalized by high level calculations performed by our colleagues at Northwestern. Cutting one side of the macro-aromatic circuit results in a two orders of magnitude smaller junction conductance. Furthermore, single carbo-benzene-mediated junctions exhibit field-effect transistor behavior when an electrochemical gate potential is applied, opening the way for device applications.