In single-molecule transistors, we observe inelastic cotunneling features that correspond energetically to vibrational excitations of the molecule, as determined by Raman and infrared spectroscopy. This is a form of inelastic electron tunneling spectroscopy of single molecules, with the transistor geometry allowing in situ tuning of the electronic states via a gate electrode. The vibrational features shift and change shape as the electronic levels are tuned near resonance, indicating significant modification of the vibrational states. When the molecule contains an unpaired electron, we also observe vibrational satellite features around the Kondo resonance.

No abstract

Scaling laws and universality are often associated with systems exhibiting emergent phenomena possessing a characteristic energy scale. We report nonequilibrium transport measurements on two different types of single-molecule transistor (SMT) devices in the Kondo regime. The conductance at low bias and temperature adheres to a scaling function characterized by two parameters. This result, analogous to that reported recently in semiconductor dots with Kondo temperatures two orders of magnitude lower, demonstrates the universality of this scaling form. We compare the extracted values of the scaling coefficients to previous experimental and theoretical results.

We report electronic transport measurements of single-molecule transistor devices incorporating bipyridyl-dinitro oligophenylene-ethynylene dithiol (BPDN-DT), a molecule known to exhibit conductance switching in other measurement configurations. We observe hysteretic conductance switching in 8% of devices with measurable currents, and find that dependence of the switching properties on gate voltage is rare when compared to other single-molecule transistor devices. This suggests that polaron formation is unlikely to be responsible for switching in these devices. We discuss this and alternative switching mechanisms.

Single-molecule transistors (SMTs) incorporating individual small molecules are unique tools for examining the fundamental physics and chemistry of electronic transport in molecular systems at the single nanometer scale. We describe the fabrication and characterization of such devices, and the synthesis and surface attachment chemistry of novel transition metal complexes that have been incorporated into such SMTs. We present gate-modulated inelastic electron tunneling vibrational spectroscopy of single molecules, strong Kondo physics (T K $ 75 K) as evidence of excellent molecule/electrode electronic coupling, and a demonstration that covalent attachment chemistry can produce SMTs that survive repeated thermal cycling to room temperature. We conclude with a look ahead at the prospects for these nanoscale systems.