以金屬型單壁奈米碳管電極共價鍵結之單分子電性平台： 三核釕金屬串錯合物元件

Abstract

本實驗室過去在製作以單壁奈米碳管(single-walled carbon nanotubes, SWCNTs)為電極的元件時，並未對鍵結於兩碳管電極間的分子進行研究，因此尚無關於分子元件傳輸性質的探討。本研究以金屬型單壁奈米碳管作為電極，透過共價鍵建構單分子接點，克服在掃描式穿隧顯微鏡斷裂接點(scanning tunneling microscope break junction)技術下分子接點穩定性不足的問題，能對同一分子接點進行長時間的量測，以探討元件中與溫度相關的電荷傳輸行為。 本研究以三核釕金屬串[Ru3(dpa)4(CNPhNH2)2](PF6)2做為目標分子，其結構中以強配位能力之異氰基(isocyanide group)做為軸向配基，能夠提高分子穩定性，確保其在製程中維持結構的完整。實驗將奈米碳管溶於濃硫酸和濃硝酸之混合液，以超音波震盪，使碳管末端氧化形成羧基，接著透過N,N,N',N'-四甲基氯甲脒六氟磷酸鹽(chloro-N,N,N',N'-tetramethylformamidinium hexafluorophosphate)和N-甲基咪唑(N-methylimidazole)活化碳管之羧基，使其與金屬串末端之胺基進行醯胺化反應，並以紅外光譜與針尖增強拉曼光譜(tip-enhanced Raman spectroscopy)驗證碳管與分子間共價鍵之形成。 後續透過熱敏式掃描探針微影技術製作「碳管–分子–碳管」元件，並針對元件進行非彈性電子穿隧能譜量測，驗證分子接點之形成。實驗量測元件在4~300 K之I–V曲線，並觀察到傳輸行為隨著電壓和溫度產生對應的模式。在低溫時，元件隨著外加電壓變化展現出與溫度無關的非共振穿隧和共振穿隧；隨著溫度升高，熱激發傳輸成為主導機制。經過對照實驗和交流阻抗分析後，本研究指認在低溫下之穿隧表現源自於三核釕金屬串分子；而高溫時與溫度相關之傳輸行為則由金屬串分子以及元件中的接觸電阻所共同造成。

In our previous works, devices based on single-walled carbon nanotubes (SWCNTs) with a molecule bonded between two CNT electrodes had not been explored; consequently, the transport property of such system remained unclear. In this study, metallic SWCNTs were employed as electrodes to construct single-molecule junctions via covalent bonds. Unlike the scanning tunneling microscope break-junction technique, which suffers from junction instability, this approach enables long-term measurement on the same molecular junction, providing a stable platform for investigating temperature-dependent charge transport behavior at the molecular level. The triruthenium metal-string complex [Ru₃(dpa)₄(CNPhNH₂)₂](PF₆)₂ was selected as the target molecule. Its structure incorporates strongly coordinating isocyanide groups as axial ligands, which enhance the molecular stability during device fabrication. SWCNTs were treated in a mixture of concentrated sulfuric acid and nitric acid in an ultrasonic bath to introduce carboxyl groups at the tube ends. The resulting functional groups were subsequently activated by chloro-N,N,N',N'-tetramethylformamidinium hexafluorophosphate and N-methylimidazole, enabling amidation with the terminal amino groups of the metal-string complex, as confirmed by infrared spectroscopy and tip-enhanced Raman spectroscopy. “SWCNT–molecule–SWCNT” devices were subsequently fabricated via thermal scanning probe lithography. Inelastic electron tunneling spectroscopy verified the successful bridging of the triruthenium metal-string complex between SWCNT electrodes. I–V curves of the devices measured over a temperature range of 4~300 K revealed distinct transport behaviors that depended on temperature and bias voltage. At low temperatures, direct tunneling and resonant tunneling, exhibiting minimal temperature dependence, dominated the conduction. However, with increasing temperature, the current showed a pronounced temperature dependence, indicating that a thermally activated transport pathway became the predominant mechanism. Control experiments and AC impedance analysis suggested that tunneling features at low temperatures originated from the triruthenium metal-string complex, while the temperature-dependent transport behavior at higher temperatures likely arose from a combination of the molecule and contact resistance at the metal-CNT interface.