本研究主要是探討過渡金屬二硫族化物中具有電荷密度波特性的單層二鍗化鈦與單層二硒化鈦的混和與置換。實驗方法為使用掃描穿隧式顯微鏡(STM)，在低溫(LT)及室溫(RT)的STM下觀察兩種樣品：其一為透過分子磊晶之方式蒸鍍硒元素，將二鍗化鈦置換為各占一半比例之二鍗化鈦與二硒化鈦樣品，我們將透過LT-STM於77 K下觀察其表面形貌；其二為單層二鍗化鈦/雙層石墨烯樣品，以分子磊晶之方式多次蒸鍍硒元素，將二鍗化鈦置換為二硒化鈦，並以RT-STM觀察其表面變化。

在LT-STM結果如下：(1)於77 K掃描溫度，二硒化鈦的CDW特性在佔據態影像中顯得較為強烈，而二鍗化鈦的相變溫度為92 ± 3 K，其電荷密度波現象較難觀測；(2)單層二鍗化鈦與二硒化鈦的橫向應變異質接面存在因晶格失配而產生的錯位核心，其深度約300 pm，考慮二鍗化鈦與二硒化鈦具有6.5％的晶格失配，我們在此核心處發現錯排，錯位核心的發現不同於以往橫向應變異質接面情況；(3)二硒化鈦幾乎分布在薄膜邊緣處，而二鍗化鈦邊緣像是手指形狀般地被二硒化鈦所置換；(4)STM具有局域性特性，由此發現二鍗化鈦或二硒化鈦的單位晶格向量沒有一定與石墨稀的單位晶格向量一致，此在ARPES或是RHEED上並沒有觀察到。

在RT-STM實驗結果如下：透過蒸鍍硒元素，表面出現隨時間變動的不穩定二維島嶼，經過統計，這些二維島嶼的總面積與蒸鍍硒量成正比，我們認為其為已置換的二硒化鈦。總而言之，單層中獨特的電荷密度波現象與橫向應變異質接面引發了有趣的現象，這些成果將提供給未來研究過渡金屬二硫化物的學者們有用的幫助。

Here we report a study of the single-layer TiTe2 and TiSe2 mixed film. Both TiTe2 and TisSe2 are transition metal dichalcogenide (TMD) materials and characterized by charge density wave (CDW) in TMD materials. Using scanning tunneling microscopy (STM) at both 77 and 300 K, we observed the atomic details how TiTe2 domains on a bi-layer graphene are replaced by TiSe2, under the Se deposition.

The results in LT-STM experiment are as follows: (1) At 77 K scanning temperature, the appearance of the CDW on TiSe2 (TCDW = 232 K) is more obvious in occupied-states images. In contrast, the lower phase transition temperature of TiTe2 (TCDW = 92 K) makes the charge density wave phenomenon more difficult to observe. (2) There exists a lattice-misfit core between at the heterojunction of TiTe2 and TiSe2 domains due to a lattice mismatch of 6.5%. The core depth measures about 300 pm. These cores are to release the lattice misfit and have not been found at the interfaces of TMD films before. (3) Most of the TiSe2 areas are at the edge of the TiTe2 domains. (4) STM images also show that the unit cell vectors of TiTe2 and TiSe2 are not all be in the same direction as the unit cell vector of graphene.

On the other hand, the results in RT-STM experiment are as follows: Upon the evaporation of selenium, unstable islands appear on the surface and they change with time. The surface areas of these unstable islands increases monotonically with the amount of Se deposition; we considered they are TiSe2. The unique charge density wave phenomenon and the lattice-misfit lateral strained heterojunctions in the mixed TMD single-layer cause interesting phenomenon. The results of this study will shine light to the further understanding of TMD two-dimensional materials.

1 Xiaofeng Qian, J. L., Liang Fu, Ju Li. SCIENCE 346 (2014).
2 Chen, P. Charge density wave transition in single-layer titanium diselenide. Nat. Commun. 6, doi:10.1038/ncomms9943 (2015).
3 Alidoust, N. B., G.; Xu, S. Y.; Sankar, R.; Neupane, M.; Liu, C.; & Belopolski, I. Q., D. X.; Denlinger, J. D.; Chou, F. C.; Hasan, M. Z. Nat. Commun 5 (2014).
4 Das, S. C., H.-Y.; Penumatcha, A. V.; Appenzeller, J. .Nano Lett 13 (2013).
5 Yuan, H. B., M. S.; Morimoto, K.; Wu, S.; Nomura, K.;, Yang, B.-J. S., H.; Suzuki, R.; Toh, M.; Kloc, C.; Xu, X.; Arita, & R.; Nagaosa, N. I., Y. Nat. Phys 9 (2013).
6 Komesu, T. L., D.; Zhang, X.; Ma, Q.; Schwier, E. F.; Kojima, Y.;, Zheng, M. I., H.; Shimada, K.; Taniguchi, M.; Bartels, L.; & Rahman, T. S. D., P. A. .Appl. Phys. Lett 105, 241602 (2014).
7 Ross, J. S. K., P.; Jones, A. M.; Ghimire, N. J.; Yan, J.;, Mandrus, D. G. T., T.; Watanabe, K.; Kitamura, K.; Yao, W.; & Cobden, D. H. X., X. .Nat. Nanotechnol 9, (2014).
8 Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically thin MoS2: A new direct-gap semiconductor.Phys.Rev.Lett.105,doi:10.1103/PhysRevLett.105.136805 (2010).
9 Splendiani, A. Emerging photoluminescence in monolayer MoS2. Nano Lett. 10, doi:10.1021/nl903868w (2010).
10 Duan, X. D., Wang, C., Pan, A. L., Yu, R. Q. & Duan, X. F. Two-dimensional transition metal dichalcogenides as atomically thin semiconductors: opportunities and challenges. Chem. Soc. Rev. 44, doi:10.1039/C5CS00507H (2015).
11 Liu, G. B., Xiao, D., Yao, Y. G., Xu, X. D. & Yao, W. Electronic structures and theoretical modelling of two-dimensional group-VIB transition metal dichalcogenides. Chem. Soc. Rev. 44, doi:10.1039/C4CS00301B (2015).
12 Mak, K. F. & Shan, J. Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides. Nat Photon. 10, doi:10.1038/nphoton.2015.282 (2016).
13 Huang, C. Lateral heterojunctions within monolayer MoSe2–WSe2 semiconductors. Nat. Mater. 13, doi:10.1038/nmat4064 (2014).
14 Gong, Y. J. Vertical and in-plane heterostructures from WS2/MoS2 monolayers. Nat. Mater. 13, doi:10.1038/nmat4091 (2014).
15 Duan, X. Lateral epitaxial growth of two-dimensional layered semiconductor heterojunctions. Nat. Nanotech. 9, doi:10.1038/nnano.2014.222 (2014).
16 Li, M. Y. Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface. Science 349, doi:10.1126/science.aab4097 (2015).
17 Boer, D. K. G. Titanium ditelluride: Band structure, photoemission, and electrical and magnetic properties. Phys. Rev. B 29, doi:10.1103/PhysRevB.29.6797 (1984).
18 Koike, Y., Okamura, M., Nakanomyo, T. & Fukase, T. Log T Dependence of Resistivity and Negative Magnetoresistance in the Layered Compound TiTe2. J. Phys. Soc. Jpn 52, doi:10.1143/JPSJ.52.597 (1983).
19 Allen, P. & Chetty, N. TiTe2: Inconsistency between transport properties and photoemission results. Phys. Rev. B 50 (1994).
20 Salvo, F. J., Moncton, D. E. & Waszczak, J. V. Electronic properties and superlattice formation in the semimetal TiSe2. Phys. Rev. B 14, doi:10.1103/PhysRevB.14.4321 (1976).
21 Rossnagel, K. On the origin of charge-density waves in select layered transition-metal dichalcogenides. J. Phys. Condens. Matter. 23, doi:10.1088/0953-8984/23/21/213001 (2011).
22 Chen, P. et al. Emergence of charge density waves and a pseudogap in single-layer TiTe2. Nat Commun 8, 516, doi:10.1038/s41467-017-00641-1 (2017).
23 Heer, W. A. d. Solid State Communications 143 (2007).
24 H Hibino, S. T., S Mizuno and H Kageshima. Journal of Physics D: Applied Physics 45 (2012).
25 Chhowalla, M. et al. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nat Chem 5, 263-275, doi:10.1038/nchem.1589 (2013).
26 Geim, A. K. & Grigorieva, I. V. Van der Waals heterostructures. Nature 499, 419-425, doi:10.1038/nature12385 (2013).
27 Hossain, M. et al. Recent Advances in Two-Dimensional Materials with Charge Density Waves: Synthesis, Characterization and Applications. Crystals 7, 298, doi:10.3390/cryst7100298 (2017).
28 J. Chaussy, P. H., J.C. Lasjaunias, P. Monceau and G. Waysa. Phase transitions in NbSe3. Solid State Communications 20 (1976).
29 Kuo, Y. K. et al. Ionic size and atomic disorder effects on the charge-density-wave transitions inR5Ir4Si10(R=Dy−Lu). Physical Review B 67, doi:10.1103/PhysRevB.67.195101 (2003).
30 Zhang, C. et al. Strain distributions and their influence on electronic structures of WSe2-MoS2 laterally strained heterojunctions. Nat Nanotechnol 13 doi:10.1038/s41565-017-0022-x (2018).
31 Liu, L. Heteroepitaxial growth of two-dimensional hexagonal boron nitride templated by graphene edges. Science 343 (2014).
32 Han, G. H. Continuous growth of hexagonal graphene and boron nitride in-plane heterostructures by atmospheric pressure chemical vapor deposition. Acs Nano 7 (2013).
33 Sutter, P., Cortes, R., Lahiri, J. & Sutter, E. Interface formation in monolayer graphene-boron nitride heterostructures. Nano Lett. 12 (2012).
34 Ci, L. Atomic layers of hybridized boron nitride and graphene domains. Nat. Mater. 9 (2010).
35 Gao, Y. Toward single-layer uniform hexagonal boron nitride–graphene patchworks with zigzag linking edges. Nano Lett. 13 (2013).
36 Kim, S. M. Synthesis of patched or stacked graphene and hBN flakes: a route to hybrid structure discovery. Nano Lett. 13 (2013).
37 Park, J. Spatially resolved one-dimensional boundary states in graphene–hexagonal boron nitride planar heterostructures. Nat. Commun. 5, doi:10.1038/ncomms6403 (2014).
38 Khomyakov, P. A. First-principles study of the interaction and charge transfer between graphene and metals. Phys. Rev. B 79, 195425 (2009).
39 Arnaud, Y. & Chevreton, M. Etude comparative des composés TiX2 (X=S, Se, Te). Structures de TiTe2 et TiSeTe. J. Solid State Chem. 39, doi:10.1016/0022-4596(81)90336-4 (1981).
40 Wang, Q. et al. Large-scale uniform bilayer graphene prepared by vacuum graphitization of 6H-SiC(0001) substrates. J Phys Condens Matter 25, 095002, doi:10.1088/0953-8984/25/9/095002 (2013).
41 Ukpong, A. M. First principles study of van der Waals heterobilayers. Computational Condensed Matter 2, 1-10, doi:10.1016/j.cocom.2014.11.004 (2015).