Single-molecule electronics with graphene nano-electrodes

Single-molecule electronics have attracted widespread attention for both basic scientific interests and potential in technological applications. However, development has been limited by the difficulty in fabricating robust nano-electrodes suitable for contacting individual molecules. Carbon based materials have recently emerged as alternative electrode materials and possess several distinct advantages over conventional gold based electrodes. This DPhil project is undertaken with the goal of developing graphene nano-electrodes and the subsequent fabrication and characterisation of graphene based single-molecule devices. By combining the best of two prevalent approaches for fabricating graphene nano-gaps: feedback controlled electroburning and plasma etching, it is possible to produce graphene nano-gap with sizes 1 − 2 nm. The fabrication procedure is performed at room temperature and in ambient conditions with a high yield. Furthermore, arrays can be produced which makes the technique suitable for integration with conventional semiconductor technologies for scalable applications. The graphene nano-electrodes are used to fabricate single-molecule transistors using porphyrin molecules. Due to the stability of the graphene nano-electrodes, the porphyrin single-molecule transistors show reproducible single-electron charging behaviour even at room temperature. High bias and gate transport spectroscopy can be performed where the excited energy spectrum of the molecule is measured. Graphene-fullerene single-molecule transistors are studied. We observe electron avalanche transport and redox-dependent Franck-Condon blockade as a result of the strong electron-vibron coupling and weak vibronic relaxation of the system. The vibrational modes of the molecule are found to be due to both intrinsic vibrational and center-of-mass motion as verified by transport spectroscopy, Raman spectroscopy and DFT calculations. The current stability diagram of our device compares well with a rate equation model from which we extract the electron-vibron coupling constant.