二維過渡金屬二硫屬化物（TMDs）具有廣為人知的通用性和可調整的物理特性，因此擁有潛在的應用。在這裡，我們使用第一原理計算方法，探索鉿二硫屬化物（HfX2，X = S，Se或Te）塊材和薄膜（從1層到10層）結構的原子和電子性質。從我們的計算發現，薄膜和塊材最穩定的結構均為1T。 鉿二化碲(HfTe2)的塊材與層狀結構的電子特性是半金屬，而鉿二化硫(HfS2)和鉿二化硒(HfSe2)是絕緣體。鉿二化硫和鉿二化硒薄膜能隙都顯示隨層數厚度增加而減小的現象，而鉿二化碲薄膜則隨著層數增加保持在半金屬狀態。此外，在三層鉿二化硫的價帶最高處觀察到范霍夫奇異點（van Hove singularity, vHs），而且可以通過施加雙軸向應變提升此現象強度，此結果表示系統可能具有二維超導性。最後，鉿二硫屬化物能帶結構計算，與塊材和單層的角分辨光電子能譜(Angle resolved photoemission spectroscopy, ARPES)實驗數據相符。從我們的結果確實發現，鉿二硫屬化物通過控制薄膜厚度對為未來的潛在應用提供了靈敏且可調整的電子特性。

Two-dimensional transition metal dichalcogenides (TMDs) have become well-known due to their versatile and tunable physical properties for potential applications. Here, we explored the atomic and electronic properties of bulk and thin film (from 1 up to 10 layers) structures of hafnium dichalcogenides (HfX2, X = S, Se, or Te) using first-principles calculations. Our calculations reveal that the most stable phase is 1T for both thin films and bulk. The bulk and layered structures of HfTe2 are semi-metallic while those of HfS2 and HfSe2 are insulating. Both HfS2 and HfSe2 thin films exhibit a decreasing band gap with increasing thickness while HfTe2 thin films remain semi-metallic with increasing number of layers. Moreover, van Hove singularity (vHs) is observed in 3L-HfS2 at the valence band maximum which can be further enhanced by applying biaxial strain, suggesting possible superconductivity. Finally, our band structure for HfX2 are in good agreement with the recent experimental ARPES data. Our results indeed show that HfX2 has sensitive and tunable electronic properties through film thickness control for future potential applications.

論文審定書 i
Acknowledgements ii
摘要 iii
Abstract iv
Table of Contents v
List of Figures vii
List of Tables x
Chapter 1 1
Introduction 1
Chapter 2 4
Computational Methods 4
2.1 Vienna Ab-initio Simulation Package (VASP) 4
2.2 Hybrid Functional Calculation 5
2.3 Dudarev GGA + U Calculation 6
Chapter 3 7
Results and Discussion 7
3.1 Structural and Electronic Properties of Bulk HfX2 7
3.2 Structural and Electronic Properties of Layered HfX2 13
3.3 Presence of van Hove singularity (vHS) in 3L-HfS2 20
3.4 Comparison of the ARPES data to the calculated surface band structures 22
Chapter 4 26
Conclusion 26
References 27
Appendix 32

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