Two-dimensional (2D) transition metal dichalcogenides MX₂ (M = Mo, W, X = S, Se, Te) attracts enormous research interests in recent years. Its 2H phase possesses an indirect to direct bandgap transition in 2D limit, and thus shows great application potentials in optoelectronic devices. The 1T′ crystalline phase transition can drive the monolayer MX₂ to be a 2D topological insulator. Here we realized the molecular beam epitaxial (MBE) growth of both the 1T′ and 2H phase monolayer WSe₂ on bilayer graphene (BLG) substrate. The crystalline structures of these two phases were characterized using scanning tunneling microscopy. The monolayer 1T′-WSe₂ was found to be metastable, and can transform into 2H phase under post-annealing procedure. The phase transition temperature of 1T′-WSe₂ grown on BLG is lower than that of 1T′ phase grown on 2H-WSe₂ layers. This thermo-driven crystalline phase transition makes the monolayer WSe₂ to be an ideal platform for the controlling of topological phase transitions in 2D materials family.

近年来，二维（2D）过渡金属二卤化金属双金属卤化物MX ₂（M = Mo，W，X = S，Se，Te）引起了巨大的研究兴趣。它的2H相在2D极限内具有间接到直接的带隙跃迁，因此在光电器件中显示出巨大的应用潜力。1T'结晶相变可以驱动单层MX ₂成为2D拓扑绝缘体。在这里，我们实现了在双层石墨烯（BLG）衬底上1T'和2H相单层WSe ₂的分子束外延（MBE）生长。使用扫描隧道显微镜对这两个相的晶体结构进行表征。单层1T'-WSe ₂被发现是亚稳态的，并且可以在后退火过程中转变为2H相。在BLG上生长的1T'-WSe ₂的相变温度低于在2H-WSe ₂层上生长的1T'的相变温度。这种热驱动的晶体相变使单层WSe ₂成为控制2D材料系列中拓扑相变的理想平台。