單壁奈米碳管的合成及其機電、電子、光電性質之量測分析

Synthesis of Single-Walled Carbon Nanotubes and Characterizations of the Electromechanical, Electronic and Optoelectronic Properties

翁政輝(Cheng-Hui Weng)

指導教授 ： 柳克強 ； 蔡春鴻

國立清華大學/原子科學院/工程與系統科學系/博士(2008年)

https://doi.org/10.6843/NTHU.2008.00416

摘要

Abstract
Single-walled carbon nanotubes (SWNTs) are quasi-one-dimensional nanomaterials and ideal one-dimensional quantum systems for fundamental research. SWNTs are also promising candidates for future applications like nanoscale electronics, photonics, electromechanical devices and sensors. Recent study showed that the optical and electrical characteristics of SWNTs are extremely sensitive to the environmental conditions, and such environmental interferences can be mostly avoided by suspending the SWNTs in air. The synthesis and the characterization of suspended SWNTs have attracted a great deal of interest in the recent years.
The first task in this study is the synthesis of high-yield suspended SWNTs by catalytic chemical vapor deposition (CCVD) methods. For the first time, two novel processes, including the direct synthesis of SWNTs across vertically-aligned carbon nanofibers (CNFs) and the silicon nanostructures (Si-ns), are developed. The CNF and Si-ns templates are formed via the bottom-up and top-down approaches, i.e., by the plasmas-enhanced CVD growth of CNFs followed by post-treatment, and the plasma assisted substrate-etching with nanomasks to form Si-ns, respectively. The yields can be optimized by simply tuning the plasma process conditions, e.g., single isolated SWNT suspended over a wide distance of ~ 10 贡m, whose micro-Raman spectra exhibit the intrinsic properties of SWNTs at a single nanotube level.
In the second task, in situ scanning electron microscopy (SEM) is employed for the electromechanical analysis of the SWNTs suspended across CNFs. SEM-based method is used to observe the electrostatic coupling in a real-time and non-contact view. The adhesion properties at the SWNT-CNF junctions are estimated by a simplified model, where the results suggest a strong non-van der Waals force interaction at the interface, which can be attributed to the strong chemical bonds at the SWNT-CNF junctions and amorphous carbon layers clamping around tips of the CNFs.
The fabrication and electrical characteristics of SWNT-based electronic devices for three kinds of applications are investigated in the third part. For in situ assembly of the “few-SWNT devices,” the short-channel devices are applied for high-performance p-type field-effect transistors, or, if metallic or bundled SWNTs are present, interconnects with a current-carrying capability of ~109 A/cm2. For thin-film type transparent conductors, CVD approach using alcohols is developed. Electrical characteristics indicate the Ohmic conduction, which is dominated by the tube-tube resistance in the channel-area. For thin-film type SWNT-network transistors (SNFETs), solution-based separation processes are adopted to obtain the enrichments of semiconducting SWNTs. Further purification by interfacial trapping to remove the bundles is found crucial to achieve high performance SNFETs with high mobility and reasonably high current-ON/OFF ratios.
Optoelectronic switching behaviors upon cyclic photoexcitations of photosensitive SWNT-polymer composites and the governing mechanisms are investigated in the last part. Firstly, two-terminal electrical measurements of the SWNT-network resistors on quartz substrates are used to reveal the native interactions between the SWNTs and the opposite types of polymers. Further study using electrostatic force micorscopy provides direct evidence for the mechanism governing the photoresponses of the SWNT-polymer composites.

關鍵字

單壁奈米碳管 ； 化學氣相沈積 ； 機電性質 ； 單壁奈米碳管網絡 ； 電子元件性質 ； 光敏單壁奈米碳管-聚合物之複合物 ； 光電特性

並列摘要

單壁奈米碳管為準一維的奈米材料，亦為基礎研究中理想的一維量子系統。單壁奈米碳管被認為有許多的未來應用，包括有電子元件、光電元件、機電系統及感測器等。近期的研究指出，單壁奈米碳管其電學上及光學上的性質對其外在的條件相當的敏感，並可利用將單壁奈米碳管懸掛於空氣中避免諸如此類的環境干擾。因此，懸掛的單壁奈米碳管的合成與分析在近期吸引許多研究學者的注意。
本研究論文主要分為四個主題：首先利用獨創的技術以電漿前或後處理的方式，利用催化劑式化學氣相沈積方法來選擇性直接合成單壁奈米碳管於垂直基版的奈米碳纖維及矽奈米結構上；可藉由電漿參數的調變來最佳化懸掛單壁奈米碳管的量率。而後，利用臨場掃瞄式電子顯微鏡，以靜電力吸引的方式來探求懸掛單壁奈米碳管與其支撐-奈米碳纖維頂端之間的作用力；使用一個簡化的模型分析後，發現到單壁奈米碳管與奈米碳纖維其間之作用力遠大於分子間的凡德瓦爾力，推測並解釋其強接觸力的機制為化學性鍵結的形成。另外，並以三種不同的方式來製作奈米碳管電子元件，包括臨場製作具有少根碳管的場效電晶體及高承載電流之連接導線、以化學氣相沈積方法製備具高穿透率之單壁奈米碳管導電薄膜、以及利用奈米碳管分離及純化技術所製作之高效能薄膜式電晶體等。最後，利用單壁奈米碳管與光敏聚合物之混合作為光敏感之複合材料，在單晶石英基版上之單壁奈米碳管網絡塗佈有光敏聚合物之電阻元件上可以量測單壁奈米碳管與光敏聚合物間原有的反應；除此之外，更進一步利用靜電力顯微術來研究其光反應之機制，其研究結果提供了“光激發引致直接的電荷轉移”及“基版引致電荷捕捉之靜電力效應”等光響應機制的直接證據。

並列關鍵字

Single-walled carbon nanotubes (SWNTs) ； Chemical vapor deposition (CVD) ； Electromechanical property ； SWNT-networks ； Electronic property ； Photo-sensitive SWNT-polymer composites ； Optoelectronic property

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