Abstract
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.
Highlights
As a member of 2D materials family, TMDCs MX2 has distinct electronic structures with various crystalline structures, attracts enormous research interests in recent years
The 2H phase is most stable with lowest total energy, while the 1T′ phase is metastable with local minimum energy
Even though the total energy of 1T′-WSe2 is higher than that of 2H phase (Fig. 1g), a 1T′-WSe2 monolayer can be formed on the substrate when we controlled the substrate temperature at 250 °C during the growth
Summary
The growth of the WSe2 films was performed in a combined MBE-STM ultra-high vacuum (UHV) system with base pressure of ~1.5 × 10−10 mbar. The high purity Se (99.9995%) was evaporated from a standard Knudsen cell. Both the flux of W and Se was calibrated by depositing them on a Si(111)-7 × 7 reconstructive surface using in-situ reflection high-energy diffraction (RHEED) and STM monitoring. Raman scattering measurements were performed using a home-built confocal microscope equipped with a grating spectrometer and a liquid-nitrogen-cooled charge coupled device from Princeton Instruments. The calculations were performed with Vienna ab-initio Simulation Package (VASP)[32]. The transition barrier between the two phases was calculated using Variable-Cell Nudged-Elastic-Band (VCNEB) method implemented in USPEX code[34] combining with VASP. For preventing the contamination and oxidization of the film during the transferring, an ~20 nm Se capping layer was deposited on the surface before taking the sample out of the UHV chamber[35]
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