近年來因石墨烯的發現，使二維材料受到極高的關注。其中，二硫化鉬為令人注目的焦點之一[1,2]。主要是因為單層二硫化鉬的晶體結構與石墨烯類相似，能帶結構卻具有能隙。研究中發現當二硫化鉬從塊材轉變成薄膜時，能隙會由間接能隙轉成直接能隙，值也由1.29 eV變成1.9 eV [3-7]。這些性質讓大家認為單層二硫化鉬是極具應用潛力的新穎半導體材料。Yen-Hung Ho等人計算出，單層的二硫化鉬在外加強磁場下，因為Landau levels分裂的現象，導致能隙會隨著外加強磁場而有線性的變化[8]，這個現象令我們好奇，希望能以實驗來驗證或比對理論的預測。
由於實驗需要在強磁場下量測光譜，我們建立了一台在脈衝磁場下量測吸收光譜的系統。我們也和日本東京大學ISSP強磁場物性研究所合作，使用那邊設備做量測。在台灣我們量測了90 K到300 K的光譜來觀察趨勢，在日本則分別測量在液氦、液氮溫度及室溫的光譜來做比較。
單層二硫化鉬薄膜在可見光區具有兩個吸收峰，能隙大小分別為1.95 eV和2.08 eV[9]，由結果可以看出當溫度越低時，能隙會變大，這和別人有著類似的現象，主要的原因是因為晶格熱脹冷縮的現象導致的[10]。但在低溫區200 K以下的區域，我們觀察到有比較不平滑的巨量變化，我們目前不知道成因為何，但猜測可能是晶格在200 K附近有顯著變化所導致。磁場方面，我們在日本分別在4K與77K的溫度中，量測二硫化鉬在脈衝磁場環境下的吸收光譜。脈衝磁場的波峰值分別為8 T、29 T、52 T。經過數據處理分析後，在77 K的環境下所量測的結果，吸收峰可以觀察的出能隙有隨著磁場變強而產生微弱紅移的現象，4 K環境下所得到的結果則很難觀察到明顯的趨勢。77 K 時的結果基本與理論預測趨勢相符，但我們認為整體而言，不同磁場的數據仍太少，無法做出明確的判斷與結論。待新樣品成長完成後，會繼續利用台灣的強磁場系統完成更多實驗。

Recently, motivated by the discovery of graphene, two-dimensional materials have attracted more attention. MoS2 is one of focused two-dimensional material [1,2], owning two its gapped energy structure and the similar crystal structure with graphene. The energy structure of MoS2 changes from the indirect band gap of 1.29 eV to the direct band gap of 1.9 eV when it is thinned from bulk to monolayer [3-7]. Recently, Yen-Hung Ho et al. have studied the Landau levels split on monolayer MoS2 in high magnetic fields, which suggests the energy gap will show linear dependent with increase of magnetic field [8]. However, the high-field experimental evidence still lack.
To study the magnetic-field dependent of energy structure, we have constructed an optical spectrum system. Similar experiments were performed in a middle-pulsed-high-magnetic-field system that is located in the International MegaGauss Science Laboratory, The Institute for Solid State Physics, University of Tokyo, Japan. In Taiwan, we can measure the spectrum from 90 K to 300 K. In Japan, we measured only in 4 K, 77 K and room temperature.
In the literature, the monolayer MoS2 film have two absorption peaks, 1.95 and 2.08 eV, in the visible region [9]. The absorption peaks show blue shift when temperature decreases. This result is same as other one, the reason just caused by thermal expansion [10]. We observed that the temperature dependence of absorption peak showed dramatic change at T ~ 200 K, which cannot be explained by thermal expansion effect. The thermal-induced lattice anomaly could cause it. To know the origin of this behavior, more detailed experiments are needed.
We performed the magnetic-field-dependent optical measurements by using the pulse magnetic fields. With our home made coil design we acheieved peak field value of 8 T, 29 T and 52 T. When T = 77 K, the peaks behave red shift with increase of magnetic fields. In case of 4 K, the peaks show no tendency with magnetic field. Due to limited magnetic fields for the measured optical spectrum, it is hard to give a convincible conclusion on magnetic field effect of MoS2. After we synthesize new samples, we will perform further experiments in more different magnetic fields with the pulsed-magnetic-field system in Taiwan.

致謝 iii
摘要 iv
Abstract vi
圖目錄 x
第一章理論與材料 1
1.1半導體的基本特性 1
1.1.1晶體鍵結與能帶 1
1.1.2電傳導與能隙 2
1.1.3參雜半導體 3
1.2二維半導體材料的性質 4
1.3Spin-orbit coupling與Landau levels理論 8
1.3.1Spin-orbit coupling 8
1.3.2Landau levels 9
第二章樣品製備與實驗方法 10
2.1樣品製備 10
2.2低溫系統 12
2.3光學量測系統 14
2.3.1儀器設備 14
2.4強磁場量測系統 17
2.5系統整合 19
2.5.1系統的架設 19
2.5.2低溫中的光學量測 21
2.5.3脈衝磁場下的光學量測 23
第三章結果與討論 27
3.1基板的溫度及磁場效應 27
3.2室溫光譜及Lorentz分析 29
3.3樣品溫度效應 32
3.4樣品磁場效應 46
第四章總結 51
附錄 52
參考文獻 53

[1] L. Tao, H. Long, B. Zhou, S. F. Yu, S. P. Lau, Y. Chai, K. H. Fung, Y. H. Tsang, J. Yao, and D. Xu, “Preparation and characterization of few-layer MoS2 nanosheets and their good nonlinear optical responses in the PMMA matrix”, Nanoscale 6, 9713 (2014).
[2] A. H. Castro-Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, “The electronic properties of graphene”, Rev. Mod. Phys. 81, 109 (2009).
[3] K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically thin MoS2：a new direct-gap semiconductor”, Phys. Rev. Lett. 105, 136805 (2010).
[4] J. Qi, X. Li, X. Qian, and J. Feng, “Bandgap engineering of rippled MoS2 monolayer under external electric field”, Appl. Phys. Lett. 102, 173112 (2013).
[5] L. Yang, X. Cui, J. Y. Zhang, K. Wang, M. Shen, S. S. Zeng, S. Dayeh, L. Feng, and B. Xiang, “Lattice strain effects on the optical properties of MoS2 nanosheets” , Sci. Rep. 4, 5649 (2014).
[6] H. P. Komsa and A. V. Krasheninnikov, “Effects of confinement and environment on the electronic structure and exciton binding energy of MoS2 from first principle”, Phys. Rev. B 86, 241201(R) (2012).
[7] A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Y. Chim, G. Galli, and F. Wang, “Emerging photoluminescence in monolayers MoS2”, Nano. Lett. 10, 1271 (2010).
[8] Y. H. Ho, Y. H. Wang, and H. Y. Chen, “Magnetoelectronic and optical properties of a monolayer”, Phys. Rev. B 89, 155316 (2014).
[9] C. C. Shen, Y. T. Hsu, L. J. Li, and H. L. Liu, “Charge dynamics and electronic structures of monolayer MoS2 films grown by chemical vapor deposition”, Appl. Phys. Express 6, 125801 (2013).
[10] R. Soklaski, Y. Liang, and L. Yang, “Temperature effect on optical spectra of monolayer molybdenum disulfide”, Appl. Phys. Lett. 104, 193110 (2014).
[11] N. W. Ashcroft and N. D. Mermin, Solid state Physics, Holt, Rinehart and Winston, New York 1th (1976).
[12] A. K. Geim and I. V. Grigorieva, “Van der Waals heterostructures”, Nature 499, 419 (2013).
[13] V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, and M. S. Strano, “Liquid exfoliation of layered materials”, Science 340, 1420 (2013).
[14] Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Colemanand and M. S. Strano, “Electronics and optoelectronics of two-dimensional transition metal dichalcogenides”, Nat. Nanotechnol. 7, 699 (2012).
[15] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric Field Effect in Atomically Thin Carbon Films”, Science 306, 666 (2004).
[16] C. Lee, X. Wei, J.W. Kysar, and J. Hone, “Measurement of the Elastic properties and intrinsic strength of monolayer graphene”, Science 321, 385 (2008).
[17] K. F. Mak, M. Y. Sfeir, Y. Wu, C. H. Lui, J. A. Misewich, and T. F. Heinz, “ Measurement of the optical conductivity of graphene”, Phys. Rev. Lett. 101, 196 (2008).
[18] A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, and C. N. Lau, “Superior thermal conductivity of single-layered graphene”, Nano. Lett. 8, 902 (2008).
[19] J. H. Chen, C. Jang, S. Xiao, M. Ishigami, and M. S. Fuhrer, “Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO2”, Nat. Nanotechnol. 3, 206 (2008).
[20] M. I. Katsnelson, “Graphene-carbon-in-two-dimensions” , Mater. Today 10, 20 (2007).
[21] H. Ochoa and R. Roldan, “Spin-orbit-mediated spin relaxation in monolayer MoS2 ”, Phys. Rev B 87, 245421 (2013).
[22] K. F. Mak, K. He, C. Lee, G. H. Lee, J. Hone, T. F. Heinz, and J. Shan, “Tightly bound trions in monolayer MoS2”, Nat. Mater. 12, 207 (2013).
[23] K. Muraki, “Unraveling an exotic electronic state for error-free quantum computation”, NTT. Technol. Rev. 10, 10 (2012).
[24] K. K. Liu, W. Zhang, Y. H. Lee, Y. C. Lin, M. T. Chang, C. Y. Su, C. S. Chang, H. Li, Y. Shi, H. Zhang, C. S. Lai, and L. J. Li, “Growth of large-area and highly crystalline MoS2 thin layers on insulating substrates”, Nano. Lett. 12, 1538 (2012).
[25] A. M. Sanchez, D. Sangalli, K. Hummer, A. Marini, and L. Wirtz, “Effect of spin-orbit interaction on the optical spectra of single-layer, double-layer, and bulk MoS2”, Phys. Rev. B 88, 045412 (2013).
[26] A. Kuc, N. Zibouche, and T. Heine, “Influence of quantum confinement on the electronic structure of the transition metal sulfide TS2”, Phys. Rev. B 83, 245213 (2011).
[27] W. S. Yun, S. W. Han, S.C. Hong, I. G. Kim, and J. D. Lee, “Thickness and strain effects on electronic structures of transition metal dichalcogenides: 2H-MX2 semiconductors (M = Mo, W; X = S, Se, Te)”, Phys. Rev. B 85, 033305 (2012).
[28] T. Cheiwchanchamnangij, and W. R. L. Lambrecht, “Quasiparticle band structure calculation of monolayer, bilayer, and bulk MoS2”, Phys. Rev. B 85, 205302 (2012).
[29] D. Y. Qiu, F. H. da Jornada, and S. G. Louie, “Optical spectrum of MoS2: Many-body effects and diversity of exciton States”, Phys. Rev. Lett. 111, 216805 (2013).
[30] A. Ramasubramaniam, “Large excitonic effects in monolayers of molybdenum and tungsten dichalcogenides”, Phys. Rev. B 86, 115409 (2012).
[31] H. Ochoa, and R. Roldan, “Spin-orbit-mediated spin relaxation in monolayer MoS2”, Phys. Rev. B 87, 245421 (2013).
[32] T. Goto, Y. Kato, K. Uchida, and N. Miura, “Exciton absorption spectra of MoS2 crystals in high magnetic fields up to 150 T ”, J. Phys.: Condens. Matter 12, 6719 (2000).
[33] L.Yang, J. Deslippe, C. H. Park, M. L. Cohen, and G. Louie, “Excitonic Effects on the Optical Response of Graphene and Bilayer Graphene”, Phys. Rev. Lett. 103, 186802 (2009).
[34] S. Huang, Y. Liang, and L. Yang,“Exciton spectra in two-dimensional graphene derivatives ”, Phys. Rev. B 88, 075441 (2013).
[35] P. Cudazzo, C. Attaccalite, I. V. Tokatly, and A. Rubio, “Strong Charge-Transfer Excitonic Effects and the Bose-Einstein Exciton Condensate in Graphane”, Phys. Rev. Lett. 104, 226804 (2010).
[36] M. Rohlfing and S. G. Louie,“Electron-hole excitations in semiconductors and insulators”, Phys. Rev. Lett. 81, 2312 (1998).
[37] M. Rohlfing,“Electron-hole excitations and optical spectra from first principles”, Phys. Rev. B 62, 4927 (2000).
[38] L. Vina, S. Logothetidis, and M. Cardona, “Temperature dependence of the dielectric function of germanium”, Phys. Rev. B 30, 1984 (1984)