3.1. Point defects in monolayer MoS2
Two-dimensional layered MoS2 is a typical semiconductor with a well-known cross-over from indirect to direct bandgap when the thickness decreases from bulk to monolayer, as a result of quantum confinement effect. Semiconducting MoS2 (E.g., 1.3–1.8 eV) can be promising building blocks of photodetectors, gas sensors, and opto/electronic devices. To synthesize MoS2 atomic layers in large scale, chemical vapor deposition (CVD) [16, 17] has been demonstrated as a feasible route to realize the scalable nanoelectronic applications based on large-size high-quality thin films. However, plenty of point defects and grain boundaries are still inevitably present in the atomic thin layers after the CVD growth.
3.1.1. Vacancies and antisite defects
Diverse intrinsic point defects in CVD-grown MoS2 were first systematically characterized by Zhou et al. [1] by using atomic resolution HAADF imaging. Due to the large intensity difference of Mo and S2 column in this Z-contrast mode, it is easy to directly assign the type of the point defects. Single-site sulfur vacancies were found to be the most common defects, including mono-vacancy (VS) and double-vacancy (VS2) with only one or two S atoms missing from the S sublattice. Other less common defects observed include extended Mo vacancies such as VMoS3 and VMoS6, and antisite defects with Mo atom replacing S2 column (MoS2) or S2 occupying the Mo site (S2Mo), but with a much lower frequency. Through HRTEM imaging, Komsa et al. [18] observed the structure of single vacancies VS and VS2 in monolayer MoS2 with atomic resolution, which could be readily created by electron beam irradiation at an acceleration voltage of 80 kV. Atomic S vacancies get generated and agglomerated in the monolayer under the electron beam irradiation. These vacancy sites could accommodate impurity atoms to form substitutional dopants, such as N, P, As, and Sb in V-A group behaving as acceptors and F, Cl, Br, I in VII-A group as donors, respectively. This electron beammediated substitutional doping could serve as a route to engineer the local electronic structure of TMDs.
Using atomically resolved ADF imaging, Jin et al. found plenty of antisite defects emerging in physical vapor-deposited (PVD) MoS2 monolayers. Figure 2 is an image gallery to demonstrate all types of antisite defects in monolayer MoS2 including Mo replacing S sublattice (MoS, MoS2, Mo2S2) and S substituting Mo sublattice (SMo, S2Mo). In the ADF imaging mode, these two different categories of antisites can be easily distinguished and even quantitatively analyzed. The experimental atomically resolved ADF-STEM images of antisites agree well with the simulated images based on density functional theory (DFT) relaxed structures. The DFT relaxed atomic model of antisite MoS in Fig. 2k still retains the three-fold symmetry, while antisite MoS2 (in Fig. 2l) has an obvious off-center characteristic because of the deviation of the antisite Mo atom from the center of the triangles formed by the three nearest-neighboring Mo atoms. The different structural symmetries between MoS and MoS2 give rise to their contrasting magnetic properties.
3.1.2. Defect species vs sample synthesis methods
Intrinsic structural defects emerge inevitably in the sample growth within finite time according to the thermodynamic theory. Hong et al. [19] found the primary point defects in monolayer MoS2 changed with the growth methods, physical vapor deposition (PVD) [11, 20–22], mechanical exfoliation (ME) [23] and chemical vapor deposition (CVD) [16, 17, 24, 25], as shown in Fig. 3. It is observed that antisite defects MoS2 with Mo replacing the S2 sublattice are the dominant point defects in PVD MoS2, while in ME and CVD monolayers, sulfur vacancies VS are the most common defects. Different atomic growth mechanisms [19] have been outlined to account for the difference in the primary defect species in the different growth methods.
The predominant defects such as antisite MoS2 and MoS in PVD samples are statistically analyzed at a density of (2.8 ± 0.3) × 1013cm−2 and 7.0 × 1012cm−2, corresponding to an atom percent of 0.8% and 0.21%, respectively. The dominant point defect VS vacancy in the CVD samples has a statistical concentration of (1.2 ±0.4) × 1013cm−2. As both concentrations of primary defects (vacancies or antisites) are remarkably high (0.8–0.2%), it is naturally expected that they will considerably tailor the electronic structures [19].
Figure 2. Atomic structures of antisite defects. (a)–(c) High-resolution ADF-STEM images of antisite MoS, MoS2, and Mo2S2, respectively. The former two antisites are dominant in PVD-synthesized MoS2 single layers. Scale bar: 0.5 nm. (d) and (e) Atomic structures of antisite defects SMo and S2Mo, respectively. (f)–(j) Simulated STEM images based on the theoretically relaxed structures of the corresponding point defects in (a)–(e). (k)–(t) Top and side views of DFT-relaxed atomic model of all antisite defects. Light blue: Mo atom; gold: S2 atoms. Reproduced from Jin et al. (2015) with permission.
3.1.3. Local magnetism induced by antisite defects
In the nonmagnetic MoS2, sulfur vacancies VS, VS2 will not induce any local magnetism [19] to the monolayer or multilayer. Through advanced first-principles calculation, Jin et al. found that only antisite MoS will give rise to local magnetism in monolayer MoS2, while antisite MoS2 is nonmagnetic. The calculated electronic structures of the most common antisites MoS and MoS2 are shown in Fig. 4. The defect states behave as nearly flat band dispersion within the intrinsic bandgap, indicating the excessive involvement of Mo d electrons (four Mo atoms within the ansite defect). Further, the density distribution of the extended electron wave function around the antisite forms a “superatom” with a radius of roughly 6 Å, representing the hybridization of Mo d and S p orbitals (Figs. 4(e) and 4(f)).
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