以單壁奈米碳管為電極之單分子電性量測平台的製程與分析

Abstract

以單分子接合點(即「電極−分子−電極」)為基礎的分子電性量測平台，可在單分子的尺度下探討物理及化學現象的基本原理。如何製作結構穩定的「電極−分子−電極」以將分子連接至外部量測電路並擷取其電性訊號，是重要的課題。本研究以奈米碳管作為單分子接合點之電極，待測分子為兩端軸向頭基為乙腈(NCMe, acetonitrile)之直線型三核鎳金屬串錯合物[Ni3(dpa)4(NCMe)2][(PF6)2]，進行單分子元件製程及其電性量測的研究。 單分子元件製程的部分，藉由末端羧酸化之碳管的R-COO‒取代分子的頭基NCMe，使碳管與分子以配位共價鍵(dative covalent bond)的形式接合，形成「碳管–分子–碳管」的結構。為了將「碳管–分子–碳管」連接至外部電路進行電性量測，本實驗室以熱敏式掃描探針微影(thermal scanning probe lithography, t-SPL)製作「金屬電極–碳管–分子–碳管–金屬電極」的量測平台，而本研究解決了過去造成此製程良率低下的3個關鍵問題：(1)透過「碳管–分子–碳管」AFM影像疊加的方式輔助探針刻劃位置的設定，並校正刻劃位置的偏移，提升探針刻劃「碳管–分子–碳管」結構末端處之上層光阻的位置準確度；(2)增加探針單位像素的刻劃時間及重複刻劃次數，避免刻劃區域光阻殘留；(3)調整光阻軟烤溫度、時間及烘烤方式，避免顯影後光阻塌陷。 單分子元件電性量測的部分，為了探究分子電性隨溫度的變化，本研究共量測2個「金屬電極–碳管–分子–碳管–金屬電極」元件在不同溫度下(4~300 K)的I-V曲線，並觀察到元件電性隨溫度的變化含有來自碳管與金屬電極的貢獻。而10個不同的「金屬電極–碳管–金屬電極」之I-V曲線的量測結果，反映出連接分子的電極系統會造成每個元件之間的差異，使單分子元件電性量測結果的分析變得複雜，除了每根碳管本身性質的差異，也需考慮金屬電極的接觸電阻。以電性訊號為基礎的非彈性電子穿隧能譜(inelastic electron tunneling spectroscopy, IETS)可提供單分子元件振動能譜的資訊，有助於其電性量測結果的分析。然而，以實驗室過去量測參數所得之IETS無法獲得單分子尺度的結構資訊，故本研究進行量測參數的優化，調整量測系統中鎖相放大器之參考訊號的頻率和振幅、時間常數以及掃描速率後，成功測得符合Raman振動光譜之單根單壁奈米碳管的IETS，為後續單分子元件的結構鑑定提供了可靠的途徑。

Molecular electrical detection platforms based on single-molecule junctions (electrode–molecule–electrode, EME) enable monitoring of physical and chemical processes at the single-molecule level. Realization of stable EME structures and detection of molecular electrical signals in the structures are key scientific issues. In this study, electrodes are carbon nanotubes (CNTs), and molecules are [Ni3(dpa)4(NCMe)2][(PF6)2], one-dimension metal atoms with labile acetonitrile (NCMe) ligands in the axial positions, also known as extended metal-atom chains (EMACs). This study contains two parts: fabrication of single-molecule devices and measurement of molecular electrical signal. In the part of single-molecule device fabrication, to form CNT–molecule–CNT, R-COO‒ on the ends of the acid-functionalized CNTs are substituted for NCMe ligands of the molecules, and the CNT and the molecule can be connected via the dative covalent bond. For the purpose of connecting the CNT–molecule–CNT to external circuits for electrical measurement, thermal scanning probe lithography (t-SPL) is adopted to fabricate metal electrode–CNT–molecule–CNT–metal electrode platforms. In this study, three problems are solved that result in low yields of device fabrication in the past: (1) by overlaying AFM image of CNT–molecule–CNT and correcting patterning position shift, accuracy of position to CNT–molecule–CNT during t-SPL patterning was improved; (2) by increasing pixel time and field during t-SPL patterning, photoresist residues in the patterned area were prevented; (3) by optimizing the soft bake temperature, time and method, photoresist collapsing after developing was prevented. In the part of electrical signal measurement of the single-molecule devices, current-voltage (I-V) curves of two devices are measured at 300 K~4 K to investigate electrical properties of the molecules with temperature. The I-V curves show that the electrical characteristics is contributed not only from the molecule, but from the CNTs and the metal electrodes. To clarify the role of CNTs in the device, I-V curves of ten different metal electrode–CNT–metal electrode are measured, which implies that variability in the properties of each CNT electrode would complex the discussion on electrical measurements of the single-molecule devices. Besides, the contact resistance of the metal electrodes also needs to be considered. Inelastic electron tunneling spectroscopy (IETS) provides information about vibration modes of the single-molecule devices, facilitating analysis on device electrical measurements. However, the IETS derived from previous measurement parameters does not reflect information at the single-molecule level. Through optimization of measurement parameters, including reference frequency, modulation voltage, time constant, and scan rate, signals attributed to the vibration modes of single-walled carbon nanotube in the devices are successfully obtained from IETS. This IETS measurement system paves the way for characterization of molecular electrical properties in the single-molecule devices.