Charge transport and rectification in molecular junctions formed with carbon-based electrodes

Molecular junctions formed using the scanning-tunneling-microscope–based break-junction technique (STM-BJ) have provided unique insight into charge transport at the nanoscale. In most prior work, the same metal, typically Au, Pt, or Ag, is used for both tip and substrate. For such noble metal electrodes, the density of electronic states is approximately constant within a narrow energy window relevant to charge transport. Here, we form molecular junctions using the STM-BJ technique, with an Au metal tip and a microfabricated graphite substrate, and measure the conductance of a series of graphite/amine-terminated oligophenyl/Au molecular junctions. The remarkable mechanical strength of graphite and the single-crystal properties of our substrates allow measurements over few thousand junctions without any change in the surface properties. We show that conductance decays exponentially with molecular backbone length with a decay constant that is essentially the same as that for measurements with two Au electrodes. More importantly, despite the inherent symmetry of the oligophenylamines, we observe rectification in these junctions. State-of-art ab initio conductance calculations are in good agreement with experiment, and explain the rectification. We show that the highly energy-dependent graphite density of states contributes variations in transmission that, when coupled with an asymmetric voltage drop across the junction, leads to the observed rectification. Together, our measurements and calculations show how functionality may emerge from hybrid molecular-scale devices purposefully designed with different electrodes beyond the so-called “wide band limit,” opening up the possibility of assembling molecular junctions with dissimilar electrodes using layered 2D materials.Recent interest in understanding charge transport in molecular-scale devices and at metal/organic interfaces has led to innovations in both experimental and theoretical techniques designed to probe such devices (1, 2). Molecular junctions in a metal–molecule–metal motif using a variety of metals including Au, Ag, Pt, Al, and Cu have been studied extensively (3–7), contributing significantly to our understanding of the fundamental principles required to realize molecular-scale electronic components such as rectifiers or switches (8–14). However, the nanogap electrodes using such metals are mechanically unstable due to the high atomic mobility of metal atoms (15–18) and all except for Au oxidize easily under ambient conditions (6). Furthermore, the electrode density of states near the Fermi energy is typically nearly energy independent. This results in molecular junctions formed with metals having rather smooth and featureless transmission probabilities around the Fermi energy, limiting their applications. Carbon-based electrodes such as graphite have remarkable mechanical strength as well as a nonconstant highly dispersive density of states near its Fermi energy (19). In addition, molecules can be bonded covalently to carbon-based materials and can also bind through a van der Waals-based π–π stacking interaction (20). However, to date, such materials have not been used to create molecular junctions using the scanning-tunneling-microscope–based break-junction technique (STM-BJ). All-carbon electrodes have been used in the past, including carbon nanotubes and graphene (21–23); however, such devices are not easy to fabricate and characterize electronically with a statistically significant method. Moreover, there have been no computational studies on such junctions aimed at understanding the relation between charge transport and electrode properties.Here, we measure the conductance of a series of graphite/amine-terminated oligophenyl/Au molecular junctions using the STM-BJ technique (4). We show that the conductance of this series decays exponentially with molecular backbone length with a decay constant that is essentially the same as that for measurements with Au electrodes. We show further that these molecular junctions rectify (14, 24), due to an asymmetry in the coupling of the molecule with the Au and graphite electrodes. The nature and magnitude of the rectification is directly connected to the nonconstant density of states of graphite near the Fermi level. The trends from self-energy–corrected density functional theory calculations are in agreement with our experimental results; specifically, we find that junction conductance decreases as the junction is elongated, as the angle between the molecule and the graphite substrate increases. These measurements and calculations together demonstrate new classes of molecular junctions with dissimilar electrodes using layered 2D electrodes.