- 學位論文
單根多壁奈米碳管之電子傳輸特性及一維鍺基三元氧化物奈米結構之研究
On the Electrical Transport Properties of Individual Multi-Walled Carbon Nanotubes and the Study of One-Dimensional Ge-Based Ternary Oxide Nanostructures
摘要
自從奈米碳管發現以來，一維奈米材料一直受到廣泛地注意。其中不僅是因為基礎科學上的興趣，更重要的是這些材料在分子光電元件上的發展具有相當大的潛力。在這個領域裡，奈米碳管本身及一維氧化物奈米材料無疑是最引人注目的研究主題之中的兩個。本論文之研究內容即與這兩個主題相關，並可區分成兩個主要部分。第一部份處理的是單根多壁奈米碳管的電子傳輸特性，第二部分的研究則以一維鍺基三元氧化物奈米結構為主。 論文第一部份所研究的多壁奈米碳管有兩種，分別稱為無序型及缺陷型多壁奈米碳管。就無序型多壁奈米碳管而言，其電阻率隨溫度降低而提高，並在4.2 到295 K的測量溫度內呈現T½的相關性。這樣的相關性可以Al'tshuler-Aronov模型解釋之，亦即在一無序系統中，電子間的交互作用會使得費米能級附近的電子能態密度產生負值的修正，從而使低溫時的電阻增加。此外值得注意的是，在本實驗中此模型適用的溫度範圍非常大，這在其他無序材料系統中是從未見過的，同時也彰顯出其電子散射時間短至10-15秒的數量級。至於缺陷型多壁奈米碳管，其與Nb電極間的接觸電阻大約在kΩ的層級。其電導隨溫度上升而提高，此溫度之相關性可用一簡單之雙能帶模型解釋之。再者，我們在大多數樣品的電流—電壓曲線中發現了一個很奇特的遲滯現象。此遲滯現象出現在室溫及低溫，而且在遲滯發生之後，樣品的電阻通常會增加。在文獻上，這樣的遲滯現象尚未在奈米碳管中被報導過。由於此遲滯所需的誘發電流及消耗功率都很小，故相當適用於分子記憶元件。 在第二部分關於一維鍺基三元氧化物奈米結構的研究中，我們首先報告單晶Zn2GeO4奈米柱的奇特成長現象。實驗發現在室溫下將熱蒸鍍法製備所得來的含鋅鍺奈米晶浸入水中一段時間之後，Zn2GeO4奈米柱會漸漸形成。由於含鋅鍺奈米晶表面的鍺原子，在水中的化學穩定性非常差，奈米晶會逐漸轉變成非晶形的縐折狀團聚膜。此非晶形團聚膜由鋅、鍺、氧三種成分構成。隨著時間的增長，單晶Zn2GeO4奈米柱會在此團聚膜裡直接孕核並逐漸成長出來。此奈米柱的直徑分佈從數十個到超過一百奈米不等，而從陰極發光譜分析發現，Zn2GeO4奈米柱的發光能譜處於藍綠光區間。這種特別的成長機制，在製備類似一維鍺基三元氧化物奈米結構的方法上提出了一個新的思考方向。 我們進一步觀測到若以適當比例將純鍺奈米晶或二氧化鍺的水溶液加入氫氧化鈣水溶液中，可合成Ca5Ge2O9的水合物奈米線。將純鍺奈米晶加入氫氧化鈣水溶液後，鍺奈米晶會逐漸溶解釋出鍺離子，並進一步與溶液中的鈣離子反應形成Ca5Ge2O9的水合物奈米線。若將鍺奈米晶換成二氧化鍺水溶液，亦可得到相同的產物，但其反應完成時間較短，這是因為此實驗不需歷經鍺離子釋出的過程。此水合物奈米線的直徑分佈從數十個到數百奈米。而經過加熱到400oC去水之後，可以得到非晶的Ca-Ge-O奈米線。同樣的方法亦可用來合成鍶與鍺比例為1比1的水合鍶鍺氧化物奈米線，只要將氫氧化鈣水溶液換成氫氧化鍶水溶液即可。這種水合鍶鍺氧化物奈米線的合成過程與化學性質與Ca5Ge2O9水合物奈米線非常相似。經過400oC去水之後，同樣可以得到非晶的Sr-Ge-O奈米線。這兩種非晶形奈米線的發光光譜相當類似，均介於300到550奈米波長的藍—紫外光區段，而發光峰值則在380奈米。此光激發光經推論為肇因於和鍺相關的發光源。以上這些非晶形氧化物奈米線的製備方式，將可提供未來在製備一維非晶奈米材料新的實驗方法。
並列摘要
Since the discovery of carbon nanotubes (CNTs), one-dimensional (1-D) nanomaterials have attracted a lot of attention due to not only fundamental scientific interests but also their potential applications in molecular optoelectronic devices. In this research field of 1-D nanomaterials, CNT and 1-D oxide nanomaterials are no doubt two of the most popular topics of investigation. The present study is in association with the above two subject materials, and can be divided into two major parts. The first part deals with the electrical transport properties of individual multi-walled CNTs (MWCNTs), and the second part focuses on the study of 1-D Ge-based ternary oxide nanostructures. In the investigation of electrical transport of individual MWCNTs, two kinds of MWCNTs were studied: one is disordered MWCNT, and the other is defective MWCNT. In the case of disordered MWCNTs, the intrinsic tube resistivity increased with decreasing temperature, and showed a T½ dependence in the temperature range of 4.2-295 K. The experimental finding can be well interpreted in terms of Al'tshuler-Aronov model in which strong electron-electron interaction leads to a singular negative correction to the single particle density of electronic states near the Fermi level for a disordered system, thus resulting in increased resistivity at low temperatures. Such a wide fitting range of temperature for T½ dependence has never been reported for other materials, implying an extremely short carrier scattering time in the order of fentosecond. As for the defective MWCNTs, the contact resistance was in the order of several kΩ using Nb leads, and the temperature dependence of tube conductance could be explained using a simple two band model. An unexpected electrical hysteresis was observed in the I-V curves of most of these MWCNT devices at low temperatures as well as room temperature, and it was generally observed that the resistance increased after hysteresis. Such a phenomenon has not been reported for CNTs in the literature so far. With both low inducing current and power consumption, this hysteresis is quite suitable for application in molecular memory devices. In the second part of the study on 1-D Ge-based ternary oxide nanostructures, we first demonstrated a novel growth phenomenon of Zn2GeO4 nanorods from Zn-containing Ge nanoparticles (ZCGNs) prepared by a vapor condensation technique. Zn2GeO4 nanorods were formed by aging these ZCGNs in water at room temperature. Due to the poor chemical stability of Ge surface in water, the ZCGNs first underwent a structural transformation into wrinkled amorphous membranes composed of Zn, Ge, and oxygen. After further aging, single-crystalline Zn2GeO4 nanorods nucleated directly from the amorphous membranes, and then continued to grow in the aqueous environment. These nanorods with the diameter ranging from several tens to more than 100 nm exhibited a blue-green luminescence peaked at 450 nm. This unique transformation route may provide a new thinking for preparing similar 1-D Ge-based ternary oxides or other 1-D nanomaterials. Besides the aforementioned experimental finding, it was further found that hydrated Ca5Ge2O9 nanowires could be synthesized by either immersing pure Ge nanoparticles or adding GeO2 aqueous solution into Ca(OH)2 aqueous solution at a stoichiometry of Ca:Ge = 5:2. In the first case, the Ge nanoparticles dissolved gradually in the solution, and the released Ge ions reacted rapidly with the calcium ions to form hydrated Ca5Ge2O9 nanowires. The reaction was completed in 10 min. In the second case, the reaction rate was increased due to the Ge ions already present in the aqueous solution. The diameter of these hydrated nanowires varied from several tens to more than 100 nm. After dehydrating the nanowires at 400oC, amorphous Ca-Ge-O nanowires were obtained. Hydrated strontium germanate nanowires with the atomic ratio of Sr to Ge at 1 could also be synthesized by the same approach in which the Ca(OH)2 aqueous solution was replaced by Sr(OH)2•8H2O aqueous solution. Both the formation process and chemistry of the hydrated strontium germanate nanowires were very similar to those of hydrated Ca5Ge2O9 ones. With subsequent dehydrating these nanowires at 400oC, amorphous Sr-Ge-O nanowires could also be obtained. Both amorphous Ca-Ge-O and Sr-Ge-O nanowires exhibited a very similar blue-violet luminescence. The emission band distributed from 300 to 550 nm, with the main peak locating at 380 nm. Ge-associated luminescence centers are proposed to be responsible for this emission. The formation of these amorphous nanowires served as a new approach to prepare amorphous one-dimensional nanomaterials.