Measuring electrical conduction through an individual molecule is not a straightforward task. Part of the problem lies in the impossibility to obtain electrodes separated by just a few nanometers (< 3 nm) using current lithographic techniques. Scanning probe microscopy techniques, e.g., scanning tunneling microscopy (STM), have been widely used to study the conduction through individual molecules deposited on a metallic surface. Unfortunately, these two-terminal transistor techniques lack the possibility of electrically gating the molecules, limiting a complete spectroscopic determination of the energy landscape. Alternative approaches involve the on-chip fabrication of nm-size gap electrodes, leading to three-terminal transistor devices which allow electrical gating. Three-terminal SET devices fabricated by electromigration are routinely used in the del Barco group to study electrical transport through nanoscale magnetic systems. Although excellent for SET spectroscopy, the fabrication of these transistors results in a lack of control of the molecule’s disposition within the junction region, which is crucial when studying highly anisotropic systems. In addition, the formation of the nanogaps by electromigration breaking of a nanowire requires high local current densities which can be excessively aggressive for some molecules.
In this project, we plan to circumvent these limitations by following a novel approach. We will integrate graphene-based transistors with an STM to produce a gateable three-terminal SET spectroscopy tool that will allow for measurements of electrical transport through molecules deposited on the graphene surface. The new approach will permit access to a large number of molecules deposited on the surface of the graphene transistor, eliminating the low yield, lack of orientation control and aggressiveness associated with electromigrated junctions. The proposed technique will allow the electrical gating of the molecular charge states through the graphene layer, which does not completely screen the electric field generated by an underneath back-gate. In the proposed SET spectroscopy measurements, the electrical current flows between the source (i.e., STM tip) and drain (graphene layer) electrodes through a sequential tunneling process whenever a molecular electronic level lies between the Fermi energies of the electrodes (see figure above). For small bias voltages no current flows though the device because the excited molecular levels are not available to accept conduction electrons. This regime is known as the Coulomb blockade. As the bias voltage is further increased, excited states open new conduction channels through the device. The position of these current steps can be tuned by a gate electrode potential VG. This technique allows for the determination of the level structure of an individual molecule and thus constitutes a powerful spectroscopic technique to study the energy landscape of isolated molecules.
Currently working on this project:
Collaborators in this project:
Physics:
Jens Martin (NUS, Singapore)
Masa Ishigami (UCF, Orlando, USA)
Chemistry:
Christian Nijhuis (NUS, Singapore)