半導體矽化鉻和碲化鎘奈米結構之合成及應用研究

本篇論文主要在探討矽化鉻及碲化鎘奈米結構材料的生長方式，鑑定分析和物理特性。由於他們的半導體特性加上有多樣的實際應用，這兩種材料在近年來受到廣泛的研究。接著這篇論文主要針對於這兩者在設計和開發新的功能性奈米元件。最後也提出一些未來相關的潛在應用。由矽化鉻及氧化矽組成的奈米電纜成功藉由常壓氣相沉積方式合成出來，且不需要任何的金屬催化劑。我們是利用氯化鉻蒸氣直接和矽基板做反應。結構分析確認出這些奈米電纜是沿著[001]生長。我們並意外發現這些電纜具有獨特的室溫鐵磁及形狀非等向異性。利用第一原理去跑模擬分析發現在電纜介面處的鉻原子有相當高的飽和磁化量 – 2.202波爾磁子，相對的在矽化鉻中間的鉻原子幾乎不具有任何磁化量 (0.002波爾磁子)。實際量測到的磁化量和模擬計算出來的值很接近。我們推論這異常高的磁化量主要來自於兩個原因，一是在介面處的鉻原子因有許多未鍵結鍵，此會貢獻很多淨的磁矩，另一個原因是一維奈米材料有高的表面積比，表面效應會讓整體磁性更顯著。除此之外，被去掉氧化層的矽化鉻奈米線同樣也具有磁性以及磁化量大小會正比於奈米電纜的介面面積，這個研究結果相信會對未來稀磁性半導體元件的開發有顯著的幫助。第二部分在討討單根矽化鉻奈米線的熱電和機械性質。採用哈曼量測方式，直徑70奈米的熱電優質為0.3，而表面被粗糙化的熱電優質更高達0.35 (增加15個百分比)，原因是表面粗糙化會降低材料的熱傳導性進而提升熱電效應。熱電優質並隨著奈米線直徑遞減而上升。另一方面，單根矽化鉻奈米線的平均楊氏模數高達225 GPa，這顯著改善的熱電性質和優越的機械強度對於之後的先進熱電元件必有實際應用的價值。論文的第三部分主要在探討開發閃鋅礦結構的奈米發電機。製備方式為常見的水熱法，我們合成出閃鋅礦和纖鋅礦共存的微米和奈米碲化鎘線。經由水平包裝後，此奈米發電機的電量輸出高達0.3 V和40 nA並且不會隨著震動頻率而改變，成功證明同樣具有非對稱中心的閃鋅礦結構材料也有壓電性質。這閃鋅礦材料可用於新能源開發方面的自主供電系統。

關鍵字

矽化鉻；碲化鎘；奈米結構

並列摘要

Synthesis, characterizations, and physical properties of CrSi2 and CdTe-based nanostructures have been investigated. Both materials have attracted intensive research effort in recent years, owing to their semiconducting properties and various practical applications. The design and development in functional nanodevices based on these two materials are of current interest. Furthermore, the possible further advanced applications of these structures are also proposed. Free-standing CrSi2/SiO2 nanocables have been synthesized via a simple atmospheric pressure chemical vapor deposition (APCVD) method. High quality nanocables were produced by a chemical vapor transport based method without using metal catalysts. The nanocables were formed by a direct reaction of CrCl2 vapor and a Si substrate in one single step. Structural characterization confirms the core of these nanocables to be hexagonal CrSi2, grown in the [001] direction. The room temperature ferromagnetism in CrSi2/SiO2 nanocables is discovered for the first time and the hysteresis loops show shape anisotropy effect when the applied magnetic field is perpendicular or parallel to the substrate. Through first-principles calculation, we found that the Cr atoms at the interface between CrSi2 and SiO2 layers possess a significant high saturation magnetization up to 2.202 B, while the Cr atoms in the middle of CrSi2 layer have a negligible magnetic moment (0.002 B). The calculated total saturation magnetization of the measured sample is close to the measured value obtained by superconducting quantum interference device (SQUID). The room temperature ferromagnetism in CrSi2/SiO2 nanocables is attributed to unpaired Cr atoms at the interface and high surface-to-volume ratio of these 1D nanostructures. Surface spins of pure CrSi2 nanowires after the removal of outer SiO2 layer also contribute large magnetic moment. Moreover, a comparison of measured values for CrSi2/SiO2 nanocables with different aspect ratios indicates that the magnetization is indeed proportional to interface area. The results obtained from the present fundamental studies shall lend substantial support to the development of future dilute magnetic semiconductor devices. Thermoelectric and mechanical properties of an individual CrSi2 nanowiwre were investigated. ZT value ~0.30+/-0.01 for a single CrSi2 NW with ~70 nm in diameter can be directly assessed by employing Harmon method. A remarkable enhancement of ~15 % for ZT value up to ~0.35+/-0.01 can be achieved due to reduced thermal conductivity by roughening the surface of NW because of higher surface-to-volume ratio. ZT value was found to increase with decreasing diameter. On the other hand, elastic modulus of CrSi2 NWs is first investigated in the present study. The elastic modulus was found to be independent of diameter and the averaged modulus value of ~225 GPa is obtained. The enhancement of the thermoelectric properties of CrSi2 nanowires with robust mechanical properties may lead to their practical applications in advanced thermoelectric devices in the future. A zinc blende structure based nanogenerator were developed. Here, free-standing CdTe microwires/nanowires (MWs/NWs) have been synthesized with a facile one-step hydrothermal method. The structural analysis shows that the synthesized materials (which consist of wires and particles) are composed of multiple phases with much more zinc blende CdTe than wurtzite CdTe. This coexistence of two phases was also confirmed using high resolution transmission electron microscopy (HRTEM). A laterally packaged nanogenerator (NG) can generate up to 0.3 V and 40 nA when strain is applied on the individual MW. Due to the high stability, the MW can be used in piezoelectric applications under various circumstances. Through these combined properties, zinc blende-based CdTe material appears to be promising for application in self-powered system in energy harvesting.

並列關鍵字

無資料

參考文獻

Chapter 1 Nanotechnology

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半導體矽化鉻和碲化鎘奈米結構之合成及應用研究

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