the confinement of the IDBs. Through this combination and comparison of the experiment/simulation results, each domain of the large-area bilayer can be unambiguously identified with one of the diverse stacking orders.
Figure 11. Diverse atomic structure of MoSe2 bilayer domains. (a) Experimental HAADF image of a typically continuous and uniform bilayer MoSe2. Those domains marked by triangles in the same color indicate the same stacking order. Scale bar: 2 nm. (b) Atomic model and the simulated ADF-STEM images of diverse bilayer stacking orders. The atomic structures of each domain in (a) are assigned by the comparison of experimental and simulated ADF images. Reproduced from Jin et al. (2017) with permission.
It’s expected that diverse stacking bilayer structures would have different electronic structures, and distinctive electronic density of states near the fermi level. To probe the dependence of electronic structure on the stacking order, scanning tunneling spectroscopy was thus utilized to measure the electronic states from the domain centers. As shown in Fig. 12(a), experimental STS spectra collected from different domains present three types of features: olive spectra with band tail (BT) states; the pronounced peak of valence band splitting into double peaks (DP) with an obvious separation; low-conductance (LC) spectra.
Figure 12. Distinctive electronic structures of the diverse bilayer domains. (a) DFT calculated LDOS of several typical bilayer stacking structures. (b) Experimental STS spectra measured at different domains. The inset is a STM image of the corresponding domains. The valence band edge is dependent on the stacking order. The different valence band DOS should arise from the diverse stacking orders of bilayer domains. (c)–(d) Band structures of the frequently observed stacking orders AB–V4 and AA–V3. Reproduced from Jin et al. (2017) with permission.
Three most common stackings were found in experiments: AB–0, AB–V4, AA–V3, whose electronic structure was calculated by DFT in Fig. 12. The calculated valence band DOS of AB-V4 (black curve) has an obvious double peak feature, and that of AA–V3 shows a band-tail structure and its bandgap get reduced, compared to the normal DOS of AB–0. The experimental STS in olive with band-tail feature can be assigned to the BT category, where the frequently observed AA–V3 is a typical stacking. And AB–V4 could be one possible stacking responsible for the observed “DP” spectra in black in Fig. 12(a). Further calculated band structure of AB–V4 in Fig. 12(c) shows that the VBM at K and the second valence band extreme at Γ with a 130-meV separation result in the double-peak feature of the valence band of AB–V4. While for AA–V3, the VBM was, however, found at Γ, nearly degenerated with the 59-meV-lower second valence band extreme at K. This band extreme crossover results in the band tail state, in AA–V3.
For various bilayer stackings, the relative energy increases linearly with the interlayer distance d, suggesting an attractive interlayer interaction [42]. The smallest interlayer distance d means the highest stability, as proved by the most common AB–0, AA–V3 with small d. The observed band tail state is a fingerprint for smaller-interlayer-distance stacking orders, in STS measurements. DFT-calculated K-Q and Γ-Q gaps increase exponentially with interlayer distance d, while the Γ-Q gap is more sensitive to interlayer distance. This is because the VB at Γ primarily comprises Mo dz2 and Se-pz orbitals and is more sensitive to the interlayer interaction than the VB at K point.
4. Summary
In this chapter, both ADF-STEM and STM/STS demonstrate powerful atomic resolution imaging capability in the direct probing of atomic defects in 2D transition metal dichalcogendies. Point defects such as vacancy and antisite, grain/domain boundaries have been characterized by atomically resolved ADF-STEM or STM imaging, together with spectroscopy to reveal the electronic states induced by defects and low-symmetry lattice-translational stackings. Time sequential STEM to track the atomic flow also elucidate the different states involved in defects’ evolution to deduce the primary kinetic pathways in the atomic migration.
In the 2D materials research, STEM/STM show their versatility in revealing the nanophysics of defects: both atomic characterization of the structures of defects and translational stackings and spectroscopic measurement of the electronic states induced.
Acknowledgments
JH and CJ acknowledge the financial support provided by the National Science Foundation of China under grant nos. 51772265, 51761165024 and 61721005, the Zhejiang Provincial Natural Science Foundation under Grant No. D19E020002, and the 111 project under no. B16042. MX acknowledges the support provided by a Collaborative Research Fund (C7036-17W) and a General Research Fund (No. 17327316) from the Research Grant Council, Hong Kong Special Administrative Region. CJ and MX also acknowledge the financial support provided by the NSFC/RGC joint research scheme (Nos. 51761165024 and N HKU732/17). The authors acknowledge Dr. Wei Huang, Feng Jiang, and Dr. Yipu Xia for their kind assistance in preparing this chapter.
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