| Summary: | 博士 === 國立清華大學 === 化學系 === 98 === The realization of molecular electronics requires comprehension of single molecular I-V characteristics. Aside from the electron transporting properties of the molecular framework, the molecule-electrode binding contributes significantly to the contact resistance, Rn=0, and thus to the values of single-molecule resistance. Isothiocyanate (–NCS) and cyanate (–CN), versatile ligands for EMACs (Extended Metal-Atom Chains), can bind onto metal substrate to complete a metal-molecule-metal configuration for the external measurements. To isolate the contact effect of contact between headgroup and electrode from other factors, alkanediisothiocyanates and alkanedicyanates are studied because the large HOMO-LUMO gap of alkyl chains is not sensitive to the number of methylene units. The conductance at the single molecular level has long been expected to be matched strongly with the Fermi level of metal electrodes. Another factor to affect the electron transporting characteristic of EMACs is varying the metel center. By incorporating a diruthenium moiety into a string of nickel cores, the heteropentanuclear Ni-Ru-Ru-Ni-Ni EMAC has a single-molecule conductance of 6.3 ± 1.0 MΩ, 4-fold superior to that of the pentanickel analogue (23.3 ± 4.1 MΩ) at the ohmic region and results in NDR characteristics, unobserved for its analogues of pentanickel or pentaruthenium EMACs. The diruthenium unit is accounted for discrete HOMO levels that lead to the NDR behavior as a result of the energy alignment with the electrode Fermi. From the Landauer equation, we only focused on the resistance of molecular without the contact. The resistance of trinickel EMAC is 330 kΩ, one order less than hexamethylene.
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