Elucidating the Role of Molecule-Electrode Interfacial Defects in Charge Tunneling Characteristics of Large-Area Junctions.

Interfacial chemistry at organic-inorganic contact critically determines the function of a wide range of molecular and organic electronic devices and other systems. The chemistry is, however, difficult to understand due to the lack of easily accessible in-operando spectroscopic techniques that permit access to interfacial structure on a molecular scale. Herein, we compare two analogous junctions formed with identical organic thin film and different liquid top-contacts (water droplet vs eutectic gallium indium alloy) and elucidate the puzzling interfacial characteristics. Specifically, we fine-tune the surface topography of the organic surface using mixed self-assembled monolayers (SAMs): single component SAM composed of rectifier (2,2'-bipyridyl-terminated n-undecanethiolate; denoted as SC11BIPY) is systematically diluted with nonrectifying n-alkanethiolates of different lengths (denoted as SC n where n = 8, 10, 12, 14, 16, 18). Characterization of the resulting mixed SAMs in wettability and tunneling currents with the two separate liquid top-contacts allows us to investigate the role of phase segregation and gauche defect in the SAM//liquid interfaces. The results reported here show the difference in length between SC11BIPY and SC n is translated into nanoscopic pits and gauche-conformer defects on the surface, and the difference in contact force-hydrostatic vs user pressures-and hence conformity of contact account for the difference in wettability and rectification behaviors. Our work provides an insight into the role of molecule-electrode interfacial defects in performance of molecular-scale electronic devices.

Deconvolution of Tunneling Current in Large-Area Junctions Formed with Mixed Self-Assembled Monolayers.

Whereas single-component self-assembled monolayers (SAMs) have served widely as organic components in molecular and organic electronics, how the performance of the device is influenced by the heterogeneity of monolayers has been little understood. This paper describes charge transport by quantum tunneling across mixed SAMs of n-alkanethiolates of different lengths formed on ultraflat template-stripped gold substrate. Electrical characterization using liquid metal comprising eutectic gallium-indium alloy reveals that the surface topography of monolayer largely depends on the difference in length between the thiolates and is translated into distribution of tunneling current density. As the length difference is more significant, more phase segregation takes place, leading to an increase in the modality of Gaussian fitting curves. Consequently, statistical analysis permits access to deconvolution of tunneling currents, mirroring the phase-segregated surface. Our work provides an insight into the role of surface topography in the performance of molecular-scale electronic devices.