單分子導電性之研究：分子與電極間接觸電阻及五核鎳釕異核金屬串分子之負微分電阻性質

Studies of Single-Molecule Conductance: Headgroup-Electrode Contact and Negative Differential Resistance Behavior of [Ru2Ni3(tpda)4(NCS)2] Complex

傅明棟

指導教授 ： 劉瑞雄 ； 陳俊顯

國立清華大學/理學院/化學系/博士(2009年)

摘要

本研究工作採掃瞄穿隧破裂接合法(scanning tunneling microscopy break junction)，研究金屬串錯合物的單分子電性。影響分子導電性的因子包括分子末端官能基、電極材料與分子主體，為了探究這些因素對導電值的影響，我們將研究工作分為"末端官能基與電極的接觸"及"分子主體的電阻"兩大類。對於研究"接觸電阻"的策略是採HOMO-LUMO能量差值大特性的飽和烷鏈取代金屬串錯合物複雜的分子主體，測量飽和烷雙異硫氰基與雙氰基分子在金、鈀和鉑三種金屬電極的導電值，研究末端官能基與電極間的接觸電阻(contact resistance)，並且比較電極-頭基組合對分子導電性的影響。結果顯示以鈀和鉑電極所量測這兩種飽和烷分子的結果優於金電極，特別是異硫氰基搭配鉑電極的接觸電阻最低，推測是末端官能基與金屬電極之間耦合程度的差別，影響了電子穿透此界面的難易度，密度泛函理論的計算也吻合於實驗結果。此外，我們摒除接觸電阻的影響後，求得三核鉻金屬串分子主體的電阻為112 kohm、三核鎳金屬串分子主體的電阻為330 kohm，而長度略短了0.5 Å的六亞甲基為6.5 Mohm。顯示出即使鍵序為0的鎳金屬串分子，其電阻仍比碳-碳單鍵的六亞甲基低了一個數量級。
本研究探討的兩種異核金屬串分子[Ru2Ni(dpa)4(NCS)2]及[Ru2Ni3(tpda)4(NCS)2]，其導電值分別為0.67 (± 0.13) microS (或8.6 (± 1.7) × 10-3 G0) 及0.16 (± 0.02) microS (或2.0 (± 0.3) × 10-3 G0)，G0 = 2e2/h, ~77.4 microS或(12.9 kohm)−1。藉由改變金屬串錯合物中心金屬原子的組成，影響了金屬-金屬的鍵序，進而調控分子的導電性。我們發現[Ru2Ni3(tpda)4(NCS)2]有負微分電阻現象，是因為電極的費米能階與分子軌域能量的匹配而進行電子的共振穿隧，在本論文以密度泛函理論計算分子軌域的能量加以說明。

關鍵字

導電性 ； 接觸電阻 ； 負微分電阻 ； 金屬串分子

並列摘要

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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