- 學位論文
電鍍碲化鉍之片狀結構成長機制研究
Structural Evolution of Disc-Like Bi2Te3 Grown by Electrodeposition
摘要
碲化鉍(Bi2Te3)是現今室溫下最普遍的商用熱電材料，由於其結構為非均向性(anisotropy)，所以在製備Bi2Te3時其結晶方向將會影響熱電性質的好壞。在製備Bi2Te3中，電鍍法是一種低成本、高鍍率以及製程簡單的技術，且生長的Bi2Te3晶粒有特定結晶方向。然而，目前對於電鍍Bi2Te3的成長結構尚無完整的研究，所以本實驗將對於此結構做一系列研究，以了解此種生長成特定結晶方向的機制。 本實驗主要探討電鍍法製備的Bi2Te3晶體形貌、晶體結構以及成長機制之關係。電鍍Bi2Te3晶體在SEM下觀察發現呈現片狀交叉結構，形成三角錐-四角錐的形狀。此片狀結構經由穿透式電子顯微鏡(TEM)分析確認此結構為一單晶，垂直其扁平面方向為[00.1]方向。電鍍Bi2Te3晶體片狀交叉結構推論與 及 雙晶結構生成有關。理論計算因雙晶結構形成的片狀物夾角為63.82o及77.5o，與根據SEM分析量測片狀物的夾角－65o以及77o大致相符。最後，根據X-ray繞射(XRD)分析得到電鍍Bi2Te3薄膜隨著電鍍的時間增長，具有愈強的{11.0}繞射峰，此結果是因為在Bi2Te3結構中基面(basal plane)是電阻率最低的部分，所以在電鍍過程中電子流主要沿著基面方向傳導，鉍與碲離子亦將沿此方向沉積，導致垂直於基面的{11.0}晶面成為優選的晶面方向。
並列摘要
Bismuth telluride (Bi2Te3) is the most well known thermoelectric material at near-room-temperature regime. Due to the anisotropic nature of the material, thermoelectric properties will be strongly dependent on its crystal orientation. Electrodeposition, one of many methods of preparing Bi2Te3, has the advantages of being low cost, high efficiency, and simple process, with the additional attribute of grain growth under a distinct crystallization mechanism. Findings in this research provide a thorough understanding of the structural evolution during the electrodeposition process. In this study, structural features and growth mechanism of electrodeposited Bi2Te3 grains are carefully investigated. From scanning electron microscopy (SEM), the interpenetrating discs exhibit a distinctly pyramidal shape with combined triangular and square bases. Further analysis with transmission tunneling electron microscopy (TEM) shows that each individual disk is a single crystal, with the c-direction [00.1] perpendicular to the flat surface of the crystal. The interpenetrating structure is speculated to be related to the twin structure of Bi2Te3 crystals with and twin plane, based on the similarities between the measured angles 65o and 77o from SEM micrographs and the theoretical calculated included angles 63.82o and 77.5o. Finally, the XRD diffraction patterns indicate that the intensity of {11.0} plane increases with increasing electrodeposition time. Such preferential growth plane is the result of electrons preferably traveling through the basal plane, which has the lowest electrical resistivity. Subsequently, bismuth and tellurium ions would likewise deposit along the same path near the basal plane, leading to the plane perpendicular to the basal plane {11.0} being the preferred orientation.