Technical Effects of Molecule-Electrode Contacts in Graphene-Based Molecular Junctions

This study focuses on comparing methods for capturing and measuring the charge transport properties of single molecules in gold-graphene contact gaps. We have attempted to measure the single-molecule conductance of a series of 1,n-alkanedithiols (n = 4, 6, 8) tethered between a gold and a graphene contact with three different methods. The conducting probe atomic force microscopy break junction (CP-AFM BJ), scanning tunneling microscopy (STM) break junction (STM BJ), and STM-based I(s) techniques for forming molecular junctions with graphene lower contacts were compared. In each case, the upper contact was gold, with a gold-coated AFM probe in the CP-AFM BJ method and a gold STM tip for both the STM BJ and I(s) techniques. Both the CP-AFM BJ and the STM-based I(s) methods yielded similar values for the conductance decay constant values, with β_N = 0.56 and 0.40, respectively. In line with previous observations, these are much smaller than values recorded for the same alkanedithiol series in symmetric gold-molecule-gold junctions, where we find that β_N = 1.1. This clearly shows the impact of substituting one of the gold contacts for a graphene one. This observation has been previously rationalized as resulting from the breaking of the junction symmetry, the change in electrode-molecule coupling and energy level alignment. On the other hand, stable molecular junctions could not be formed using the STM BJ technique with graphene contacts, which may be because of transient instability in the gold tip contact after it has been pushed hard onto the graphene surface.