Bi0.5Sb1.5Te3薄膜之熱電性質研究和Sb2Te3奈米片的拓樸超導性質

Abstract

本博士論文包涵兩個研究主題和一個熱電特性量測技術補充資料。兩個研究主題集中於低維熱電和拓撲超導特性的研究，前者研究Bi0.5Sb1.5Te3薄膜，後者則研究Sb2Te3奈米片，熱電特性測量技術用於測量單腿AgSbTe2裝置的熱電性能。 Bi0.5Sb1.5Te3薄膜的研究，主要是通過調整電子和聲子的傳輸特性來提高ZT值。使用RF濺射法製備了一系列厚度分別為0.125、0.25、0.5和1微米的Bi0.5Sb1.5Te3薄膜，其微觀結構分析顯示薄膜的生長形態與薄膜厚度有高度相關。值得注意的是，0.5微米厚的薄膜在400 K時功率因子達到了18.1 μWcm-1K-2，並具有低至0.3 Wm-1K-1的熱傳導率，實現了接近室溫的最大ZT值1.86。 Sb2Te3奈米片的研究，在氮化矽基板上製備了一系列厚度為5到27奈米的超薄薄膜，此研究與作者周柏豪、牟中瑜教授合作，根據其提出的理論及計算，此薄型拓撲絕緣體表面電子間的相互配對形成的新型拓撲超導體。本研究顯示，Sb2Te3奈米片在a-b平面上是通過層與層範凡德瓦力結合的。我們實驗結果表明，由於表面狄拉克錐間耦合而形成的配對對稱性，是由自p_x+〖ip〗_y配對主導，形成了一類新的拓撲超導體。 第三部份為補充資料，熱電裝置效率的研究，為了直接測量熱電材料的輸出功率以獲得效率，最可靠的方法是直接測量其輸出功率，而不是藉由測量各別的熱電參數計算ZT值。本研究利用四點探針法測量單腿AgSbTe2的熱電轉換效率。這種四點探針法能有效減少測量過程中線阻和接觸電阻的影響，此研究在156 K的溫差下，樣品測量出6.4 mW的輸出功率，且有2.1 %的轉換效率，與傳統單獨量測熱電參數計算的ZT值一致。 綜合以上研究，本論文為高性能熱電系統的設計與製造提供了重要的科學見解和技術突破。透過這些先進材料的開發，我們朝著實現高效能的熱電轉換技術邁進，為未來可持續能源解決方案的實現奠定了堅實的基礎。此外本研究展示了超薄單晶Sb2Te3奈米薄片的新型拓撲超導性，提供了拓撲材料研究的新方向。

The doctoral thesis encompasses two thematic research topics and one supplementary thermoelectric (TE) characterization technique. The two research topics focus on the study of low-dimensional thermoelectric and topological superconducting properties. The former is on thermoelectric Bi0.5Sb1.5Te3 thin film, and the latter is on Sb2Te3 nanoflakes. The thermoelectric characterization technique is used to measure the thermoelectric performance of a single-leg AgSbTe2 device. The study on thermoelectric Bi0.5Sb1.5Te3 thin film aims to examine the ZT enhancement through the decoupling of the transport properties of electrons and phonons. A series of Bi0.5Sb1.5Te3 thin films with thicknesses of 0.125, 0.25, 0.5, and 1 μm were fabricated by RF sputtering. Detailed microstructural analysis revealed that the growth morphology of the films is highly correlated with the film thickness. Notably, the 0.5 μm film achieved a power factor of 18.1 μWcm-1K-2 at 400 K and a thermal conductivity as low as 0.3 Wm-1K-1, resulting in a maximum figure of merit ZT of 1.86 near room temperature. In the study of Sb2Te3 nanoflakes, a series of ultrathin films grown on silicon nitride substrates with thicknesses ranging from 5 to 27 nm were fabricated. This research was conducted in collaboration with Po-Hao Chou, Chung-Yu Mou, et al., based on their proposed theory and calculations. A new class of topological superconductivity was formed by inter-surface pairing of surface electrons in the thin topological insulators. This study demonstrated that Sb2Te3 nanosheets are bonded through van der Waals forces in the a-b planes, layer-by-layer. Our results indicate that the pairing symmetry due to the coupling of inter-surface Dirac cones is unambiguously dominated by the spinful p_x+〖ip〗_y pairing and forms a new class of topological superconductors. This supplementary information pertains to the study of thermoelectric device efficiency. Instead of measuring separate TE properties to calculate the ZT value, the most reliable way to characterize the performance of a TE material is to directly measure its output power to gain efficiency. The study utilized the four-point probe method to measure the thermoelectric conversion efficiency on a single-leg AgSbTe2 device as an example. This four-point probe method can effectively reduce the influence of line and contact resistance during measurements. At a temperature difference of 156 K, the sample exhibited a power output of 6.4 mW, corresponding to a conversion efficiency of 2.1%, which is consistent with the ZT value obtained from conventional individual TE parameters. In summary, this thesis provides important scientific insights and technological breakthroughs for the design and fabrication of low-dimensional high-performance thermoelectric systems. Through the development of these advanced materials, we are moving towards achieving high-efficiency thermoelectric performance technologies, laying a solid foundation for the realization of future sustainable energy solutions. Additionally, this research demonstrates a new class of topological superconductivity in ultrathin single-crystal Sb2Te3 nanoflakes, offering a new direction for topological material studies.