8/12/2023 0 Comments Mos2 lattice constantThe band structure calculation is performed by using the generalized gradient approximation method with Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional as implemented in the VASP. Additional ARPES measurements were performed at the Beamline 5-4, Stanford Synchrotron Radiation Lightsource (SSRL), SLAC National Accelerator Laboratory. Following the growth, a 20-min post-annealing process at the same temperature was performed. 24 The substrate temperature was set at 280 ☌ for growth. The substrate was BLG prepared by vacuum graphitization of 6H–SiC(0001). Mo (99.99% purity) and Te (99.999% purity) were evaporated by an e-beam evaporator and a Knudsen cell, respectively, with the flux ratio of ∼1:20. 1T′-MoTe 2 was grown by MBE with a base pressure of ∼2 × 10 −10 Torr. A characterization tool that can directly visualize the band structure and the size of the bandgap, such as angle-resolved photoemission spectroscopy (ARPES), 22,23 would provide a clearer insight on this aspect.īoth thin film growth and ARPES characterization of 1T′-MoTe 2 were performed at the Beamline 10.0.1, Advanced Light Source, Lawrence Berkeley National Laboratory. 21 Whether the SOC is strong enough to open a bulk gap is still not very clear. Optical absorption and transport measurements report a 60 meV bulk gap in few-layer mechanically exfoliated (ME) 1T′-MoTe 2, 20 while CVD-grown monolayer 1T′-MoTe 2 shows a metallic transport behavior. 3,19,20 However, as shown in Table I, the calculated bandgap size varies significantly depending on calculation methods, even to a degree that it is not clear whether 1T′-MoTe 2 is a semiconductor or a semimetal. 18 The former has been well established by the previous theoretical calculations. 3,19–21 From the electronic structure point of view, two critical elements of achieving a QSH state in group VI TMDCs are the band inversion caused by the structural distortion from the 1T phase to 1T′ phase and the 2D bulk bandgap induced by strong SOC. Many efforts have been devoted to achieve a QSH state in monolayer 1T′-MoTe 2. 17,18 It is of great interest whether it is possible to realize a QSH state in other 1T′ TMDCs and whether fundamental parameters such as bulk bandgap would be different from those of 1T′-WTe 2. The QSH phase in 1T′ TMDC has been recently realized only in 1T′-WTe 2. 14–16 The resulting transport properties exhibit quantized Hall conductance even in the absence of magnetic field, and spin-polarized edge current is expected to be useful for the spintronic applications. 3 A QSH insulator, or a two-dimensional (2D) topological insulator, is a topologically nontrivial quantum state, which is hallmarked by the helical edge state protected by time reversal symmetry and the bulk (as opposed to the edge) bandgap opening due to strong spin-orbit coupling (SOC). While its three-dimensional bulk form has been explored in terms of type-II Weyl semimetal, 10–13 the monolayer is predicted to host a quantum spin Hall (QSH) insulator. However, the strength of SOC is found to be insufficient to open a bandgap, which makes monolayer 1T′-MoTe 2 on bilayer graphene a semimetal.ġT′-MX 2 (M = Mo, W X = S, Se, Te) has gained particular interest regarding its topological properties. The SOC-induced breaking of the band degeneracy points between the valence and conduction bands is clearly observed by ARPES. Using in situ angle-resolved photoemission spectroscopy (ARPES), we have investigated the low-energy electronic structure and Fermi surface topology. Here we report a successful growth of high-quality monolayer 1T′-MoTe 2 on a bilayer graphene substrate through molecular beam epitaxy. However, the bandgap opening due to the strong spin-orbit coupling (SOC) is only verified for 1T′-WTe 2 recently and still debated for other TMDCs. The band inversion and the concomitant changes in the band topology induced by the structural distortion from 1T to 1T′ phases are well established. Monolayer transition metal dichalcogenides (TMDCs) in the 1T′ structural phase have drawn a great deal of attention due to the prediction of quantum spin Hall insulator states.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |