「電極-單根奈米碳管-電極」結構之製作:DNA 輔助碳管分散與熱敏式掃描探針顯微術

「金屬電極－目標分子－金屬電極」(metal–molecule–metal, MMM)的結構被廣泛地應用於建構單分子電性的量測平台，文獻研究中多以破裂接合法及電遷徙法製作MMM 橋接系統。以破裂接合法及電遷徙法製作面臨兩大問題：分子長度與電極間距的差異及分子與電極間為化學吸附。電極間距需與分子長度匹配，才有機會使分子橋接於電極間。再者，分子和電極間的作用力為化學吸附，在量測過程中，分子容易脫附造成斷路。為了調控電極間隙及電極與分子間的接合強度，我們選用可修飾且尺寸與分子匹配之單根單壁奈米碳管作為電極材料，期許以化學合成方式先形成共價鍵連結之MMM 結構再連結到外部電路。不同於碳管簇末端可與多個分子鍵結，單根單壁奈米碳管的末端只會鍵結單一分子，符合單分子電性量測的標準。實驗室過去以有機溶劑分散單壁奈米碳管之效果有限，分散後多為碳管簇而非單根碳管。而利用低濃度碳管溶液製備元件雖然有機會觀察到單根碳管，但碳管密度太低，在元件之狹縫中難以被觀察。因此，本研究以單股DNA 包覆酸化之單壁奈米碳管，顯著地增加碳管簇分散成單根碳管之機率(約87%)，並且有效地使單根單壁奈米碳管分散在水溶液中。另外，實驗室先前使用熱敏式掃描探針顯微鏡(thermal scanning probe microscope)製作固定碳管的金屬電極以連接到外部電路，其製作過程中光阻塗佈不均導致成功率偏低。我們調控塗佈光阻參數，以提升製程良率。並使用優化後的製程製作「鉑電極－單根單壁奈米碳管－鉑電極」(Pt–SWCNT–Pt)結構，量測到碳管電阻隨溫度變化趨勢。藉由非彈性穿隧能譜觀測到單根單壁奈米碳管之D band、G band (石墨類物質以sp2 混成的C–C 鍵振動)及G’ band (D band 之二倍頻)。本研究聚焦於單股DNA 分散單根單壁奈米碳管、改良t-SPL 參數、單根單壁奈米碳管之電性量測及非彈性穿隧能譜之擷取，期許為往後「鉑電極－單壁奈米碳管－目標分子－單壁奈米碳管－鉑電極」(Pt–SWCNT–molecule–SWCNT–Pt)之分子電性量測平台打下基礎。

關鍵字

熱敏式掃描探針顯微術；碳管分散；非彈性穿隧能譜

並列摘要

The Metal–Molecule–Metal (MMM) junctions are widely utilized to measure the electrical properties of single molecules and constructed mostly by break-junction and electromigration method in literature precedents. However, the size of gap and target molecules are different, and the attachment is constructed on the basis of chemical adsorption, resulting in the instability of device. To fix these issues, single single-walled carbon nanotube (single SWCNT) replaces metal as electrode material due to its nano diameter where we envisaged to form covalent bond with molecule in construction of MMM junction. Compared with SWCNT branches, single SWCNT forms one bond with molecule. However, it is challenging that the carbon nanotubes are prone to form carbon branches in organic solvents and water. Therefore, we introduced single-strand DNA in SWCNT solution to suppress the aggregation of SWCNT by the strong π-π interaction between single-strand DNA and single SWCNT, improving the ratio of single SWCNT to 87% significantly. Also, we modified the fabrication using thermal scanning probe lithography (t-SPL) and photoresist with better yield. The molecular junction with Pt–single SWCNT–Pt is therefore constructed and the temperature-dependent resistance of single SWCNT can be obtained. Also, the D band, G band (C–C vibration in sp2-hybridized graphene) and G’ band (duplicated band from D band) of single SWCNT can be observed in inelastic electron tunneling spectroscopy (IETS). Our effort involves DNA-assisted dispersion of single SWCNT, modification of t-SPL, focusing on the electrical properties of single SWCNT and identification of single SWCNT in IETS. This research manifests a systematic, rational approach to obtain the characterization of single SWCNT, which is beneficial for further applications such as Pt–SWCNT–molecule–SWCNT–Pt molecular junction.