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This thesis investigates the design of a unimolecular donor-acceptor system (4TPA-C60) for the purpose of developing biomimetic Turin IETS sensors. The gas phase 4TPA-C60 molecule is calculated to have a localised double-well electronic structure, which is similar to that of a nanowire resonant tunnelling device. 4TPA-C60 decorated gold surfaces are prepared from scratch, and characterised at each step. STM imaging confirms successful grafting of the molecular species to the gold surface. Scanning tunneling spectroscopy performed on these molecular double-barrier tunnel junctions show a slightly asymmetric I(V ) profile, similar to predictions from ab initio calculations. DFT calculations also reveal that the behaviour of the device is strongly dependent on the supramolecular couplings at both metal/molecule interfaces. A new phenomenon is identified, where pinning of the LUMO to the HOMO states maintains the resonant transmission channel and prevents crossing of the frontier molecular orbitals at higher biases. The HOMO-LUMO pinning effect is determined to arise from charge accumulation on the C60 cage, due to smaller coupling at the C60/metal interface. By preventing crossing of the states, HOMO-LUMO pinning delays the onset of the NDR feature, resulting in a wider current peak and lower resolution of the sensor. Based on the charge transport mechanism, several alternative systems are proposed in which HOMO-LUMO pinning can be minimized. An endohedral fullerene derivative, 4TPA-F@C60, is found to be the most promising candidate, displaying much narrower NDR peaks. These findings not only help to improve future designs of molecular Turin IETS sensors, but also contribute significantly to our understanding in the broader field of molecular devices in general. |
