Investigation of electronic transport through ultrathin carbon nanomembrane junctions by conductive probe atomic force microscopy and eutectic Ga–In top contacts

Abstract

Highly ordered self-assembled monolayers (SAMs) can be considered as functional building blocks for molecular electronics. Aromatic SAMs can be converted into a highly stable monolayer, i.e., carbon nanomembranes, via electron irradiation induced cross-linking. Here, we report the electronic transport characteristics of the pristine SAM of 4′-nitro-1,1′-biphenyl-4-thiol (NBPT) and the amino-terminated cross-linked monolayer prepared on Au/mica and Au/Si substrates with the use of a conductive probe atomic force microscope (CP-AFM) and a eutectic Ga–In (EGaIn) top electrode. The amino-terminated cross-linking monolayer exhibits a lower friction compared to the non-crosslinked SAM, as electron irradiation leads to the enhancement of both molecular rigidity and hydrophilicity. The electron irradiation effect on junction conductance was also directly observed by CP-AFM. Quantitative measurements and statistical analysis were performed by applying current–voltage spectroscopy in CP-AFM and EGaIn methods. Both methods demonstrate that the cross-linking of a NBPT–SAM leads to a decrease of conductance by more than one order of magnitude, which is attributed to a partial loss of aromaticity of the SAM as well as a partial decoupling of molecules from the Au substrate. Transition voltages were found to be significantly reduced for the cross-linked monolayer. The surface roughness effect on the transport characteristics has been addressed based on a comparison between two junction platforms.