以電化學方法研究直線型三核銠金屬串錯合物之導電性質與電流阻斷效應

Abstract

使用分子或原子作為奈米級電子元件，以量子力學的觀點評估元件在微觀下的性能，始終是分子電子學努力的目標。本研究以掃描式穿隧顯微術破裂接合法(scanning tunneling microscopy based break junction, STM-BJ)為量測平台，搭配電化學方法改變工作電極電位以調控待測分子的氧化價數，於室溫室壓下對分子兩端軸向頭基為氰根或硫氰酸根之直線型三核銠金屬串錯合物([Rh3(dpa)4(CN)2], [Rh3(dpa)4(NCS)2])進行單分子電性分析。[Rh3(dpa)4(CN)2]+分子的導電值-位移二維統計圖顯示，該分子呈現兩個導電平台，隨著金探針電極位移距離逐漸變大，由高導電平台往低導電平台延伸。以導電平台長度初步判斷，高導電平台源自單一分子之接合點構形，而推測低導電平台為兩個工作電極間形成分子-金原子-分子之接合點構形。由於該分子之頭基氰根與金探針電極之間的良好親和力，當金探針電極往上拉伸時，分子接合點斷裂於金探針前緣之金-金鍵，而非氮-金鍵，即該分子從金探針前緣抓出一顆金原子，第二個分子因而有機會橋接於其中。後續藉由不同分子濃度的實驗、是否額外添加氰化鉀於分子溶液之分子與氰根競爭關係的探討，實驗結果皆意味著該分子之低導電平台為分子-金原子-分子之接合點構形。此外，導電值軌跡圖中高、低導電平台間的不連續，顯示近似球狀結構的[Rh3(dpa)4(CN)2]金屬串分子需要足夠的空間才能靠近並形成第二次的分子-金-分子之結合點構形，即立體障礙(steric hindrance)所致。 電極-分子-電極接合點中，電極費米能階(Fermi level, EF)與分子前緣分子軌域(frontier molecular orbital, EFMO)之間的能階匹配程度是影響電子傳遞的重要因素。本研究對[Rh3(dpa)4(NCS)2]於不同工作電極電位進行I-Ebias掃描，並以diamond shape統計圖分析。結果顯示當工作電位靠近該分子的氧化還原電位，由於費米能階與前緣分子軌域的能階匹配，使得偏壓往負或往正方向掃描時，較小的偏壓視窗即可涵蓋一個分子軌域而使電流迅速上升，有較大的dI/dV值；反之，當工作電位尚未靠近氧化還原電位，無能階匹配的結果使得需要較大的偏壓視窗才可涵蓋一個分子軌域，因此電流上升速率較慢，有較小的dI/dV值，即電流阻斷(current blockade)的現象。

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.