Two-dimensional transition-metal dichalcogenide materials: Toward an age of atomic-scale photonics
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Introduction Two-dimensional (2D) transition dichalcogenide (TMDC) materials have recently emerged as a topical area of physical science and engineering (see the Introductory article in this issue of MRS Bulletin). While interest was initially inspired by the discovery of graphene, a monolayer of carbon atoms, 2D TMDC materials bear two significant differences from graphene. First, compared with the relatively simple composition and structure of graphene, 2D TMDC materials feature rich compositional and structural variations.1 They have a general chemical formula of MX2 where M can be a transitionmetal element such as Ta, Nb, Mo, and W, and X can be S, Se, or Te. Two-dimensional TMDC materials also have multiple polymorphs in crystalline structures, including 1H or 1T for monolayers and 1T, 2H, and 3R for a few layers. This variation in composition and structure may enable broad tunability in functionalities that cannot be obtained with graphene. Second, unlike graphene, which is semi-metallic without a bandgap by nature or with a trivial bandgap (570 meV5 and 0.44 eV,36 respectively. The binding energy of the A exciton in monolayer WS2 is reported to be 0.7 eV37 and 0.32 eV36,38 by two different groups, the latter might underestimate the binding energy given that monolayer WS2 is expected Figure 2. Excitonic states in monolayer MoS2. (a) Spectral absorption of monolayer MoS2 to have a stronger exciton binding energy than with the A, B, and C excitons labeled as shown. (b) The band structure of monolayer MoS2 monolayer MoS2, as indicated by the much with arrows indicating the interband transitions associated with the A, B, and C excitons. (b) Reprinted with permission from Reference 21. © 2014 Nature Publishing Group. higher luminescent efficiency. No experimenNote: E, energy; Eg, bandgap energy. tal results are available for the Bohr radius of excitons yet, partially due to the lack of accuconduction band at the K point,26 while the C exciton is associrate information for the dielectric constant of the atomically ated with the transition from the valence band to the conducthin materials. tion band at some part of the Brillouin zone between the K and Most of the current work is focused on the A exciton, but Γ points7,27 (Figure 2b). how the results could be different for the other excitons is not Many of the interesting properties of the excitons are rooted clear. Additionally, the definition of the exciton Bohr radius, in the atomic orbitals involved. For instance, the band edges which assumes excitons to be spherical, is only valid for at the K point are mainly contributed by Mo 4d orbitals, and materials systems that are isotropic or reasonably isotropic hence the positions of A and B excitons are less sensitive to compared to the Bohr radius. However, the shape of the the number of layers than the C exciton.28 Additionally, the exciton in 2D TMDCs is expected to be anisotropic due to strong spin–orbit coupling of transition-metal orbitals (such as the extremely anisotropic dimension of the materials. Some Mo 4d or
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