3. Atomically resolved ADF-STEM images to reveal the distribution of different point defects. (a) Antisite defects in PVD MoS2 monolayers. Scale bar: 1 nm. (b) Vacancies including VS and VS2 observed in ME monolayers, similar to those observed for the CVD sample. Scale bar: 1 nm. (c) and (d) Histograms of various point defects in PVD, CVD, and ME monolayers. ME samples are in green, PVD samples in red, and CVD in blue. Reproduced from Jin et al. (2015) with permission.
Figure 4. Electronic properties of the predominant antisite defects in the MoS2 monolayer. (a) Band structure and the corresponding density of states (DOS) of the antisite defect MoS2. The gray bands are from normal lattice sites, similar to the conduction band and valence band of a perfect monolayer, while the discrete red bands show the localized defects states. The DOS is projected onto the atoms around the defect (defect) and those in the middle plane of two adjacent defects (pure), respectively. The gray dash line indicates the position of the Fermi level. (b) and (c) Real-space distribution of the wave functions of the two defect states below and above the Fermi energy. (d) The band structure and DOS of antisite MoS, with a color scheme similar to that in (a) but the two spin components are colored in red (spin-up) and blue (spin-down), respectively. (e) Spin density of antisite MoS, as defined ρup − ρdown, charge densities ρup and ρdown are spin-resolved for spin-up and down components, which are represented by yellow and blue isosurfaces, respectively. (f) Spin-resolved real-space distribution of the wave function of the two marked defect states (state 3) in (d). The isosurface value in (b), (c), (e) and (f) is 0.001e · Bhor−3. Reproduced from Jin et al. (2015) with permission.
Antisite MoS2 is calculated to be nonmagnetic, as shown in Fig. 4(a). The calculated spin-polarized charge density of antisite MoS in Fig. 4(e) presents the magnetic structure with a total magnetic moment of 2µB. Figure 4(f) plots the spin-resolved distribution of a defect-induced state (state 3) marked in Fig. 4(d) to illustrate the origin of the magnetism. The occupied spin-up component (yellow isosurface) is mainly composed of dxy and dx2−y2 orbitals of the antisite Mo atom, while the unoccupied spin-down component (cyan isosurface) is projected onto dxy and dz2 orbitals of the surrounding Mo atoms, consistent with the total spin charge density shown in Fig.4(e).
The difference of magnetic MoS and nonmagnetic MoS2 can be well explained by crystal field theory and hybrid orbital theory. For three-fold symmetric MoS, the antisite Mo takes d4s hybridization with five orbitals, i.e.,s, dxz, dyz, dxy, and dx2−y2. The former three orbitals are filled by six electrons from the antisite Mo and the latter two orbitals are filled by two electrons from the adjacent Mo atoms. Hence, these two degenerate orbitals accommodate unpaired electrons whose spin directions are parallel due to the on-site 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
