Читать книгу Defects in Functional Materials - Группа авторов - Страница 26

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 (T_Mo), above the center of the hexagon or the hollow site (H), and on top of S sites (T_S), 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 T_Mo 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 µ_B, 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 (T_Mo), at hexagon-center or hollow site (H), and on top of S site (T_S), 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 T_Mo, transition state TS, and metastable state H. (j) Detailed atomic dynamics of transition from T_Mo to H with top view and side view of the 3D atomic structure, respectively. Note the T_Mo1 → H → T_Mo2 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 ( in (b)) and then to another Mo vacancy V_Mo in (c). Note the vacancy migration is actually the motion of a neighboring Mo atom, shown in blue or purple. Reproduced from Jin et al. (2017) with permission.

The experimental ADF-STEM images of the ground-state T_Mo, first-metastable H, and second-metastable T_S 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 T_Mo1 → H → T_Mo2 with a migration barrier of 0.62 eV. Another secondary kinetic pathway T_Mo1 → T_S → T_Mo2 is also found but with a much lower frequency in the experimental observations, with a barrier calculated to be 1.1 eV. The T_Mo1 → H → T_Mo2 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 T_Mo and T_S. As both the energy barriers are not so large, thermal activation could still induce the surface migration but influenced by the electron beam irradiation.