Coulomb repulsion according to Hund’s rules. Thus, these two unpaired electrons lead to the magnetic moment of 2 µB. For antisite MoS2, the offcenter characteristic in structure gives rise to the absence of orbital dxz in the formation of hybridization. Hence, the antisite Mo takes d3s hybridization forming four hybridized orbitals, originated from s, dxy, dx2−y2, dyz, filled by eight electrons. As a result of the d3s hybridization, antisite MoS2 is non-magnetic. The structural symmetry breaking of antisite MoS2 makes a big difference in the magnetic properties [19], in contrast with the antisite MoS.
3.2. Capturing the dynamics of point defects in MoS2
Besides the high sub-atom spatial resolution for static imaging, TEM also has a moderate temporal resolution in the order of millisecond to second. Modern fast camera techniques have been developed to allow for ms-frame-rate recording of image slices together with atomic resolution. Recent advancements in both spatial and temporal dimension have brought the electron microscopy into the so-called 4D TEM era.
Atomic diffusion on surfaces and inside solids is the most elementary process in materials behaviors such as phase transition [26], nanomaterials growth [27–29], defect evolution, surface reconstruction, and heterogeneous catalysis [30]. Real-time TEM or STM would provide us a proper time window to directly observe the atomic migration or molecular dynamics which is of great significance in many of these material processes.
3.2.1. Mo adatom
In the monolayer MoS2 system, Jin et al. [31] used time-sequential ADF-STEM imaging to track the defects’ evolution and atomic migration. As shown in Fig. 5, this chemically sensitive ADF-STEM imaging visualizes an obvious time sequence of the hopping of Mo adatom on the monolayer substrate. Statistical analysis also indicates its random migration on the lattice without any directional preference. Three types of Mo adatom configurations were frequently observed: on top of Mo sites (TMo), above the center of the hexagon or the hollow site (H), and on top of S sites (TS), shown in Figs. 6(a)–6(f), respectively. They correspond to DFT-derived ground state, metastable configurations of Mo adatom in the surface migration on the monolayer. The statistical counts of all these states (Figs. 6(g) and (h)) agree well with the DFT-calculated stability sequence that TMo is the most stable ground-state configuration, H is the first metastable state, and TS is the second metastable state. All these adatom configurations have a three-fold symmetry structure with local magnetic moments > 2 µB, according to the DFT calculation. They are all highly spin polarized and localized mainly on the Mo adatom, with a minor contribution from the neighboring S atoms.
Figure 5. The migrating Mo adatom defects. (a)–(j) Experimental time sequential of ADF images of Mo adatom hopping as an example. Time interval: 3 s. Scale bar: 0.5 nm. Reproduced from Jin et al. (2017) with permission.
Figure 6. Different states of Mo adatom and their transition energetics. (a–f) Atomically resolved ADF images and structure models of different adsorption states on top of Mo site (TMo), at hexagon-center or hollow site (H), and on top of S site (TS), respectively. Scale bar: 0.5 nm. False color is used to better illustrate the adatom configuration. The relative energies of different adatom states are given. (g) Statistical counts of different adsorption states in (a), (c), (e). (h) Statistical dwell time of different adatom structures. (i) DFT revealed energetics for evolution between ground state TMo, transition state TS, and metastable state H. (j) Detailed atomic dynamics of transition from TMo to H with top view and side view of the 3D atomic structure, respectively. Note the TMo1 → H → TMo2 transition in (i) is symmetric, and hence, only the first half process (left red arrows in (i)) is drawn. Reproduced from Hong et al. (2017) with permission.
Figure 7. Atomic-scale migration of vacancy defects in monolayer MoS2. (a)–(d) Time-lapse series of experimental ADF-STEM images of an Mo vacancy. Scale bar: 0.5 nm. (e)–(h) DFT-relaxed atomic structures corresponding to the evolution in a–d from the Mo vacancy (VMo in (a)) to its metastable state (
The experimental ADF-STEM images of the ground-state TMo, first-metastable H, and second-metastable TS also provide important configurations as the input for the DFT calculation, to reveal more details of the structure transition and energetics involved in the surface migration. Figures 6(i) and 6(j) show the energy profile and corresponding structural evolution in the primary kinetic pathway TMo1 → H → TMo2 with a migration barrier of 0.62 eV. Another secondary kinetic pathway TMo1 → TS → TMo2 is also found but with a much lower frequency in the experimental observations, with a barrier calculated to be 1.1 eV. The TMo1 → H → TMo2 pathway on MoS2 surface is preferred and acts as the dominant pathway, due to the tendency of the d-electrons of the Mo adatom to form covalent bonds with the surface S in a most-stable triangular–prismatic or metastable octahedral coordination [32].
This difference in energy barriers for different adatom migration pathways also agrees well with the contrasting experimental statistics of TMo and TS. As both the energy barriers are not so large, thermal activation could still induce the surface migration but influenced by the electron beam irradiation.
3.2.2. Mo vacancy
Compared to the mobile Mo adatom defects, Mo vacancies are less frequently observed to migrate within the monolayer lattice. Jin et al. utilized high acceleration voltage to observe the evolution of the vacancies through time-elapsed ADF-STEM imaging series [31]. Figure 7 shows one time-sequential example of the migration of Mo vacancies with initial, metastable, and final states all imaged in one series of vacancy hopping. This Mo vacancy migration is actually the movement of neighboring Mo lattice atom. In the corresponding structure models in Figs.