| Summary: | 碩士 === 國立臺灣大學 === 化學研究所 === 107 === The ability to modulate and control charge transport though single-molecule junction devices is crucial to achieving the ultimate goal of molecular electronics: constructing real-world-applicable electronic components from single molecules. In this research, we explore the electrical characteristics of the prototypical EMACs (Extended Metal-Atom Chain), [Rh3(dpa)4(CN)2] and [Rh3(dpa)4(NCS)2], which are one-dimensional metal atoms helically coordinated by nitrogen atoms of four α-pyridylamine ligands. The conductance-displacement histogram of [Rh3(dpa)4(CN)2]+ exhibits the evolution of conductance during the elongation process, which is from the higher conductance to the lower conductance. We demonstrate the formation of molecule-Au-molecule chains operated by scanning tunneling microscopy based break junction techniques. As a result of the high affinity of CN anchoring group and gold, the formation of these chains are mediated by gold atoms that are pulled out of the electrodes and are being inserted between the [Rh3(dpa)4(CN)2]+ monomers. We show that the chaining process is critically determined by the molecular concentration of the liquid environment. In particularly, we performed that by the competitive behavior between ligand, which is cyanide ion here and molecule, by adding potassium cyanide into the liquid environment, the chaining process can be suppressed. Specifically, from conductance traces, we note that a noise space left between the high conductance plateau and the low conductance plateau, and it was attributed to the steric hindrance effect of the bulky [Rh3(dpa)4(CN)2]+ on the chain formation. Our finding of molecule-Au-molecule chains demonstrates an in-situ formation of one-dimensional coordination dimers in molecular junctions.
Furthermore, via the I-Ebias scans fixed at several working potentials, Ewk, and the consequent diamond shape of [Rh3(dpa)4(NCS)2], we observe current blockade at room temperature in hundreds of single-molecule junctions. When Ewk is very different from the redox potential of the molecule, E1/2, the transport occurs via a co-tunneling mechanism. This is the blockade regime. By increasing the tip bias in the negative or positive direction, the bias window is opened to include an unoccupied or occupied level, and sequential tunneling may come into play. The blockade is lifted and an increase in current is observed.
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