Читать книгу Defects in Functional Materials - Группа авторов - Страница 22

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 (V_S) and double-vacancy (V_S2) with only one or two S atoms missing from the S sublattice. Other less common defects observed include extended Mo vacancies such as V_MoS3 and V_MoS6, and antisite defects with Mo atom replacing S2 column (Mo_S2) or S2 occupying the Mo site (S2_Mo), but with a much lower frequency. Through HRTEM imaging, Komsa et al. [18] observed the structure of single vacancies V_S and V_S2 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 (Mo_S, Mo_S2, Mo2_S2) and S substituting Mo sublattice (S_Mo, S2_Mo). 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 Mo_S in Fig. 2k still retains the three-fold symmetry, while antisite Mo_S2 (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 Mo_S and Mo_S2 give rise to their contrasting magnetic properties.