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Thermoelectric properties in crystalline Bi2Te3 nanowires and thin films
Chen, Shang Chien
Chen, Yang Yuan
Chen, Shang Chien
|Issue Date: ||2011-10-05 14:44:22 (UTC+8)|
|Abstract: ||碲化鉍((Bi2Te3)是熱電材料轉換效率較高的元件，其優質係數ZT值約為1。希望藉由奈米的量子效應提升它的熱電性質，我們製作一系列低維度的奈米線和薄膜來進行研究。本實驗使用的碲化鉍奈米線乃利用薄膜樣品與基板的熱膨脹係數不同，經由熱處理在碲化鉍的薄膜上長出奈米線。由掃描式電子式顯微鏡和穿隧式電子顯微鏡可以觀察到菱形晶胞(Rhombohedral unit cell)結構的碲化鉍奈米線沿著(110)方向生長，直徑約150-330 nm長度約20-30 μm。將碲化鉍奈米線轉移到矽晶片上，運用半導體製程中的熱蒸鍍(Evaporator)以及電子束曝光系統(E-Beam writer)製作電極、熱電偶和加熱器來量測席貝克(Seebeck) 係數、電傳導率和熱傳導率。最後成功的製作與量測出p型(107 μV/k) 和n型(-52.8 μV/k) 的奈米線，雖然其席貝克係數小於塊材，但奈米線的熱傳導率低於塊材兩倍以上，研究發現最好的碲化鉍奈米線的熱電優值(ZT value) 可達1.18略大於塊材。|
碲化鉍薄膜是以分子束磊晶 (Molecular Beam epitaxy)成長，分子束磊晶是在高真空下以物理的方式將高純度的材料4N (99.99%)將原子傳遞至基板上進行沉積反應形成，鍍率可低於0.1 nm/秒以下，因此可以製備出高品質的薄膜樣品，製造出各種不同比例的Bi-Te的薄膜。藉由X光繞射儀可以得知薄膜是菱形晶胞結構並且延著(0,0,l)的平面所成長。並用熱電偶成功的量測出薄膜的席貝克係數在室溫下座落於80-80 μV/k，電阻率5-30 μΩ-m，計算出功率因子(power factor)最高可達2000 μW/mK^2，與塊材相比低於一半，但是薄膜的熱傳導率同樣也低於塊材兩倍以上。最後得到最佳的碲化鉍薄膜的熱電優值(ZT value) 可達到1.01等同於塊材。
Bismuth telluride (Bi2Te3) is the thermoelectric material used for high-efficiency energy conversion. The figure of merit ZT of bulk is around 1. To study the promising positive effects on the thermoelectric properties, low dimensional nanowires and thin films of Bi2Te3 were prepared and measurements were performed. Here the method applied to nanowires growth on Bi2Te3 thin films is the mismatch of thermal expansion between substrate and thin films. By annealing at 300-350℃ for a week, the nanowires were grown on the thin films. Rhombohedral structure of Bi2Te3 nanowires with diameter ~150-330 nm and length ~20-30 μm grew along (110) direction was confirmed by Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction Pattern (SAED). To measure the Seebeck coefficient, electrical conductivity and thermal conductivity, Bi2Te3 nanowires were moved to silicon chips. Electrodes, thermometers and heaters were fabricated through thermal evaporation and E-Beam lithography processes. We successfully grew p-type(107 μV/k) and n-type(-52.8 μV/k) nanowires. Although Seebeck coefficient of nanowires is smaller than that of bulks, its thermal conductivity is less than half of that of bulks. The best ZT value of nanowires we obtained was 1.18, which was slightly larger than that of the bulks.
Molecular beam epitaxy (MBE) is a technique to grow Bi2Te3 thin films under extremely high vacuum, which is undergoing a physical vapor deposition to atomically grow thin films layer by layer. Due to the deposition rate is lower than 0.1 nm/s, we can deposit the high-quality thin films and adjust the ratio between bismuth and telluride. Rhombohedral structure of thin films grew along (110) plane was confirmed by X-Ray Diffraction (XRD). The Seebeck coefficient (80-80 μV/k) and electrical resistivity (5-30 μΩ-m) in room temperature are obtained by the thermocouples. The highest power factor can reach to 2000 μW/mK^2. While the power factor of thin films is about half of bulk ‘s value, the thermal conductivity of thin films is also half of that of bulks. The best ZT value of thin films obtained was nearly as same as that of bulks, 1.01.
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|Source URI: ||http://thesis.lib.nccu.edu.tw/record/#G0098755009|
|Data Type: ||thesis|
|Appears in Collections:||[應用物理研究所 ] 學位論文|
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