無軸向頭基金屬串錯合物[Rh3(dpa)4]2+以金屬—金屬交互作用作為單分子接合點的電子傳輸機制

Abstract

了解單分子電性對於設計奈米級電子元件十分重要。單分子導電值取決於前緣分子軌域和電極費米能階的能量差，以及分子和電極介面接觸的交互作用。金屬串錯合物具有直線型可延伸的結構，過去已有許多量測金屬串導電值的案例，儘管在分子長度增加後導電值並不會明顯下降，但是導電值有時仍不及有機共軛體系分子。其中一個影響導電值的原因為用於連接電極和分子的軸向頭基具有較大接觸電阻，因此在本研究中試圖透過移除金屬串軸向頭基由電極直接和金屬核接觸，以掃描式穿隧顯微術破裂接合法量測無軸向頭基直線型三核銠金屬串分子[Rh3(dpa)4]2+的導電值，並搭配電化學方法控制金電極工作電極電位，改變分子能階和電極費米能間的能障，進一步調控分子導電值。 在導電值位移統計圖中呈現三種導電平台(UHC、HC、LC)，且證明三種構型為獨立存在，其中LC構型的電子傳輸機制為空間傳輸，而HC構型包含鍵結傳輸貢獻。在電荷穩定性圖中分子在氧化還原電位(0.3 V及0.9 V)有較大的電流，為電子藉由兩步驟循序穿隧的方式傳輸至另一電極；另外在非分子氧化還原電位(0.6 V)也發現電流上升，由於此通道非由金屬核貢獻，在此電位下電子偏好由dpa傳輸至另一電極，與先前[Rh3(dpa)4(CN)2]觀察到的結果一致。理論計算中，在電極和分子作用後系統總能皆下降，且分子以軸向構型連接金電極較以側向構型連接穩定，表示分子和電極有機會以Au–Rh或Au–dpa的作用方式接合。穿透能譜中顯示軸向構型的穿透係數比側向構型高，且傳輸途徑顯示Au→Rh及Au→dpa的電子傳輸機制。另外比較平電極或粗糙電極，當金電極更靠近銠金屬時，穿透係數比起平電極更高。因此結合實驗與理論計算結果，推論UHC導電平台為軸向構型而金電極和銠距離接近可形成鍵結，HC導電平台為軸向構型而金電極和銠距離較遠僅有交互作用，LC導電平台為側向構型由金電極和dpa作用。 本研究結果提供另一種形成分子電極接合的設計，由金屬—金屬之間直接接觸並成功量測到分子電性，且具有多通道電子傳輸性質，更加提升單分子電子元件的導電值。

Understanding the single-molecule electron transport behaviors is essential to fabricate single-molecule-based nanodevices. The conductance of single-molecule junction can be altered by controlling the energy barrier between electrode Fermi level and frontier molecular orbital. Previous studies have shown that extended metal atom chains (EMAC) have lower decay constants than molecules with organic backbone. However, conductance of EMAC is often lower than that of conjugated system. One of the factors is that the axial group used to connect electrode and molecule may have greater contact resistance, which reduces the single-molecule conductance significantly. Therefore, in this study, the axial group of EMAC was removed so that electrode can contact with EMAC metal atom directly. The STM-bj method was used to measure conductance of [Rh3(dpa)4]2+, and a bipotentiostat was used to control the energy level alignment between electrode and molecule. The conductance-displacement histograms show three conductance plateaus as UHC, HC and LC junctions. These three types of junctions coexist rather than byproducts from the change of working potential. Flicker noise analysis shows that LC junction is pure through-space tunneling, while HC junction contains through-bond tunneling. The charge stability diagrams show that the currents near redox potential (0.3 V, 0.9 V) are higher, which indicates sequential tunneling process assisted by molecular redox level. Also, near non-redox level (0.6 V), another transport channel is shown. This channel is the direct tunneling channel through dpa rather than Rh, which agrees with previous studies of [Rh3(dpa)4(CN)2]. In theoretical calculations, the axial and lateral structure between gold electrode and molecule was used. The total energies of both structures are reduced after interaction, which indicates that both structures are possible to form molecular junctions. The transmission spectra show that the axial structure is more conductive than lateral one, and the electron transport pathways show Au→Rh and Au→dpa contribution. In addition, by comparing the plate and rough gold electrodes, the results show that the conductance is near resonant when Au–Rh distance is short enough in the rough electrode. Based on experimental and theoretical results, we propose that the UHC junction corresponds to axial structure where Au–Rh distance is close enough to form bond, the HC junction corresponds to axial structure where the electrode interacts with Rh and dpa through long-ranged interactions, and LC junction corresponds to lateral structure where the electrode interacts with dpa through long-ranged interactions. These findings give insights into molecular design with multiple transport pathways, which include direct Au−Rh contact and Au−dpa interactions, and enable more conductive multichannel devices.