摘要
在過渡金屬二硫族化物中，鎝二硫族化物TcX2，其中 X 為硫(S)，硒(Se)或碲(Te)，其中鎝是在過渡金屬二硫族化物研究中最少的材料之一。透過第一原理計算，我們系統地研究 TcX2塊材和單層結構的 1T'', 1T', 1T 和 2H 結構的穩定性、電子特性以及熱電特性，並且於材料上通過氫原子單面和雙面吸附了解性質上的轉變。從形成能計算結果，我們觀察到 TcX2塊材和單層結構以1T''結構呈現。此外，這些塊材和單層的聲子能譜結果可以推斷 1T''結構在熱力學上是穩定的，表示了 TcTe2在實驗上合成的可能性。在電子特性的部分，塊材1T''結構的TcTe2，TcSe2 和 TcS2 的能帶能隙為間接能隙，分別為0.37、1.01和1.19電子伏特。對於單層結構，維持間接能隙，但能隙變大，分別為 1.21、1.60和1.87 電子伏特。此外，1T''單層結構可能擁有良好的熱電材料性質。最後，我們發現 TcS2 單層結構通過氫原子吸附導致結構相變，從 1T'' 轉變為 1T'結構，而 TcSe2 和 TcTe2 保持 1T''結構。這些發現為 TcX2 的結構穩定性、強大的電子特性和優異的熱電特性提供了重要的證據，而這些特性將可用於光電和熱電的相關應用。

Abstract
Among the transition metal dichalcogenides, technetium dichalcogenides (TcX2 where X = S, Se, or Te) is one of the least investigated materials. Using first-principles calculations, we systematically studied the structural stability and electronic properties of TcX2 in 1T’’, 1T’, 1T, and 2H phases for the bulk and monolayer structures, as well as the thermoelectric properties, and functionalization through single-sided and double-sided hydrogenation of the monolayer phase. From the formation energy calculations, we observed that the most energetically favorable bulk and monolayer phase of TcX2 is the 1T’’ structure. Moreover, the corresponding bulk and monolayer phonon dispersion results revealed that the 1T’’ phase is thermodynamically stable. This finding implies the possible synthesis of TcTe2. With regards to the electronic properties, bulk 1T’’-TcTe2, TcSe2, and TcS2 exhibit indirect band gaps of 0.37, 1.01, and 1.19 eV, respectively. For the monolayer phase, increased indirect band gaps of 1.21, 1.64, and 1.87 eV, respectively, were observed. Moreover, the thermoelectric properties for 1T’’ monolayers show that these materials have a good potential as thermoelectric materials. Finally, we found that the functionalization of TcS2 monolayer through hydrogenation resulted in a structural phase transition from 1T’’ to 1T’, while TcSe2 and TcTe2 remained to be 1T’’. Our findings provide crucial evidence regarding the structural stability, robust electronic properties, and excellent thermoelectric properties of TcX2 for potential optoelectronic and thermoelectric applications.

Table of contents
論文審定書..................................................................... i
Acknowledgements........................................................ ii
摘要................................................................................. iii
Abstract........................................................................... iv
Table of content.............................................................. v
List of figures.................................................................. vii
List of tables.................................................................... ix
Chapter I.......................................................................... 1
Introduction.................................................................... 1
1.1. Two-Dimensional (2D) Materials............................. 1
1.2. Transition Metals Dichalcogenides (TMDs)............. 2
1.3. Technetium Dichalcogenides (TcX2, X=S, Se, or Te) ......................................................................................... 3
Chapter II........................................................................ 6
Methodology................................................................... 6
2.1. Computational Procedure...................................... 6
2.2. Hybrid Functional Calculations.............................. 7
2.3. Dudarev GGA+U Calculations................................. 8
Chapter III....................................................................... 9
Results and Discussion.................................................. 9
3.1. Structures and Stability of Bulk TcX2..................... 9
3.2. Electronic Properties of Bulk TcX2......................... 14
3.3. Structures and Stability Monolayer TcX2............... 17
3.4. Electronic Properties of Monolayer TcX2............... 19
3.5. Thermoelectric Properties of Monolayer TcX2...... 22
3.6. Hydrogenation of Monolayer of TcX2.................... 28
Chapter IV....................................................................... 34
Conclusion..................................................................... 34
References..................................................................... 36
Appendix........................................................................ 45

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