Inversion domain boundaries in MoSe2 layers

Abstract

Structural defects, including point defects, dislocation and planar defects, significantly affect the physical and chemical properties of low-dimensional materials, such as layered compounds. In particular, inversion domain boundary is an intrinsic defect surrounded by a 60° grain boundary, which significantly influences electronic transport properties. We study atomic structures of the inversion domain grain boundaries (IDBs) in layered transition metal dichalcogenides (MoSe₂ and MoS₂) obtained by an exfoliation method, based on the aberration-corrected scanning transmission electron microscopy observation and density functional theory (DFT) calculation. The atomic-scale observation shows that the grain boundaries consist of two different types of 4-fold ring point shared and 8-fold ring edge shared chains. The results of DFT calculations indicate that the inversion domain grain boundary behaves as a metallic one-dimensional chain embedded in the semiconducting MoSe₂ matrix with the occurrence of a new state within the band gap.

Fig. 1. Atomic structure of MoSe₂ and MoS₂. (a) Crystal structure of MoSe₂ 2H and 3R phases view along [001] (top) and [010] direction (bottom). (b) HAADF image of exfoliated trilayers MoSe₂ in 2H phase viewed along [001] direction. (c) HAADF image of exfoliated trilayers MoS₂ in 2H phase viewed along [001] direction. Scale bar, 10 Å.

Fig. 2. Inversion domains boundary in few-layer MoSe₂. (a) HAADF image of MoSe₂ with top part is a monolayer, middle part is a bilayer stacking, and bottom part is trilayer stacking, which are separated by blue and while lines. The superimposed atomic arrays on the inset of indicate the locations of atoms in first layer and second layer of 2H phase. The 3R phase appears inside the yellow and green dashed line. (b) Image intensity profile acquired along the gray line in (a). (c) The structure model of the observed region of bilayer stacking. (d) Atomic model of experimental structure of second layer shown in (c). The yellow and green lines indicate the grain boundary composed of 4- and 8-fold rings respectively. Two triangles indicate the orientation of two inversion domains.

Fig. 3. Simulated HAADF-STEM images of the IDBs in monolayer, bilayer, and trilayer MoSe₂. (a and b) The figure shows simulated images (a) based on the DFT-optimized structures (b) for the IDBs models. (c) HAADF image and the corresponding simulated image of bilayer MoSe₂ showing the 4-fold rings (yellow lines) and 8-fold rings (green lines).

Fig. 4. Inversion domains grain boundary in MoS₂. (a) HAADF image of MoS₂ nanosheets. The dark line indicates the stacking boundaries of 2H and 3R phase. (b) Image intensity profile acquired along the denoted line in (a). The red dashed lines indicate boundary position.

Fig. 5. The schematic illustration of the atomic growth of inversion domain. (a) Three-fold rotational defects with Mo–Se bonds rotating 60°; (b) 60° rotation of three pairs of Mo–Se (blue) bonds around the Mo atom (orange); (c) to form three octagons; (d) the creation of two vacancies of Se pairs (yellow) at the vicinity of octagons; (e) seven pairs of Mo–Se bond rotations 60° around the Mo atoms marked by orange color; (f) the heterostructure with 8|8 IDBs.

Fig. 6. Atomic structure of MoSe₂ 1T phase and simulated HAADF image. (a) Crystal structure of MoSe₂ 1T phase view along [001] and (b) [110] direction. (c) Bilayer MoSe₂ used to simulate HAADF image and (d) simulated HAADF image of 1T phase.

Fig. 7. DFT relaxed atomic models of the inversion domain boundaries in the monolayer MoSe₂. The solid black boxes represent the 4 × 7, 5 × 8, and 5 × 9 supercells, in which the 8|8-fold rings with two (a), three (b), and four (c) rings, respectively, are displayed in yellow, and the 4|4-fold rings with two rings are displayed in pink.

Fig. 8. DFT calculated the projected density of states (PDOS) from the domain center and the inversion domain boundaries in the monolayer MoSe₂. The PDOS were acquired from Mo (b) and Se atoms (c) marker in blue (domain), red (8|8-fold rings), and green (4|4-fold rings) boxes shown in the 6 × 10 supercell (a).