Fig. 1

Charge-transport characteristics of a graphene-porphyrin single-molecule junction. a Schematic representation of our device architecture: nanometre-separated graphene source and drain electrodes are used to contact the molecule, and a local gate electrode separated from the molecule by a thin layer of HfO₂ (10 nm thick) is used to shift the molecular energy levels. For clarity, the bulky side-groups are not shown. b The molecule M used in this study comprises of a porphyrin core (blue), with solubilising aryl side-groups on two of the porphyrin meso-positions (grey), and π-stacking anchor groups on the other two meso-positions (red); here THS is trihexylsilyl. c Charge stability diagram showing the differential conductance (G_b) as a function of bias voltage (V_b) and gate voltage (V_g) at 3.5 K; the actual gate voltage experienced by the molecule is only a fraction of the applied gate voltage because of the drop across the HfO₂ layer. The top panel shows the differential conductance of the top triangle as an average along the lines indicated by the arrows running parallel to the edge of the triangle. d Schematic representation of current flowing through our single-molecule transistor. The molecular DOS for reduction and oxidation processes are shown in red and blue, respectively, with electron-transfer rates shown as coloured areas. The Fermi-Dirac distributions f_S and f_D, are shown as the grey areas for source and drain, respectively. At negative bias voltage, electrons tunnel sequentially from the source via the molecule into the drain. For convenience, the bias voltage is drawn as applied symmetrically across the source and drain electrodes