∼1 nm. From the STS measurements (see Fig. 10(f)), one notes that these IDBs introduce mid-gap states at the Fermi level; thus, they act as metallic channels, where charge density wave transition may occur at a low temperature [33, 42], and thus lead to the intensity modulations. Another cause of the modulations has been suggested to reflect the quantum confinement effect of the metallic states or the Tomonaga-Luttinger liquid [33–35].
Figure 9. (a) STM image (size: 50 × 50 nm2, bias: −1V) of an as-grown MoSe2 on HOPG. (b) A close-up STM image (size: 13 × 13 nm2, bias: −1.46V) revealing the twin lines associated with each defect and the intensity undulations along the lines. Reproduced from H.J. Liu et al. (2014) with permission.
Figure 10(a) shows the atomically resolved ADF-STEM imaging of monolayer MoSe2 grown by MBE, where Se2 columns are brighter than the Mo lattices [42]. Its fast Fourier transform (FFT, Fig. 10(a) inset) presents abnormally sharp lines connecting the first-order diffraction spot, indicating the existence of some ultranarrow and long line defects emerging in the monolayer MoSe2. And also, these line defects have a specific directional distribution rather than in random directions. These line defects are actually IDBs, highlighted in blue (Fig. 10(b)) among those golden-colored triangular domains. Nanostripe-like IDBs are connecting with each other in a network form. Figure 10(c) provides a closer look at the atomic resolution ADF-STEM image of an IDB defect, where obvious four-membered rings are arranged in the zigzag direction and both domains are in a mirror symmetry in structure, as shown in Fig. 10(d). This DFT-relaxed IDB structure shows significant reorganization of Mo–Se bond lengths, which enlarges the horizontal Se–Se distance from 5.67 Å (d0) to 6.16 Å (d1). Then each IDB induces an uncommon lateral shift of 0.49 Å to the original Se–Se period of 5.67 Å, a fractional lattice translation, giving rise to diverse stacking orders if two such IDB-nested monolayers stack onto each other.
Figure 10. Inversion domain boundaries in MBE-grown monolayer MoSe2. (a) Atomically resolved ADF–STEM image of monolayer MoSe2. The inset FFT shows the quasi-periodicity of the ultra-narrow and long nanostructures. Scale bar: 2 nm. (b) False colored domains and boundaries. These dense inversion domain boundaries connect with each other like a wagon wheel. Scale bar: 2 nm. (c) Experimental and simulated ADF-STEM images of the boundary. Scale bar: 0.5 nm. (d) DFT relaxed atomic model of the boundary where orange balls represent Se atoms and cyan ones represent Mo atoms. (e) ADF-STEM intensity profiles along the long sides of the rectangular stripes marked in (c). (f) DFT-calculated DOS and experimental STS spectra from the domain center and the boundary. The calculated DOS were from boundary or domain Mo atoms highlighted in blue and red in (d), respectively, since Se atoms make negligible contribution to the DOS around the pristine bandgap. Reproduced from Jin et al. (2017) with permission.
The slight deviation of the symmetry of the ADF-STEM intensity in Fig. 10(c) is due to the presence of unintentional residual aberration such as three fold astigmatism A2 in the focused electron probe. Considering this residual aberration in electron optics, quantitative ADF-STEM image was simulated in the lower panel of Fig. 10(c) and compared with the experimental image in the upper panel. Figure 10(e) shows the intensity line profiles extracted along the long sides of the stripes in the experimental and simulated images, both in a high consistency, confirming an Se2-core boundary structure. In others words, neighboring triangular domains share the same line of Se2 columns to form the Se2-core IDBs everywhere [42].
Scanning tunneling spectra from the IDBs and domain center in Fig. 10(f) show distinctive characteristics especially around Fermi energy. The STS from the domain center is almost similar to the density of states (DOS) of a normal semiconductor with a bandgap of ∼2.0 eV, while that of IDBs has a remarkable midgap state at −0.41 eV and another two peaks at −1.8 eV and 0.6 eV. This metallic midgap state around Fermi level is characteristic of the IDB defects. The DFT-calculated DOS of IDB and the domain center both agree well with the experimental STS in the midgap states and bandgap characteristics, except the slight undrestimation of the bandgap.
3.4. Stacking-band structure diversity in bilayer MoSe2
Network-like IDBs will induce fractional lattice translation to the adjacent domains. If two layers with IDBs stack together, then diverse stacking orders will inevitably appear, which occurs exactly in the MBE-grown bilayers [42]. Figure 11(a) shows the atomically resolved ADF-STEM image of a typical MoSe2 bilayer without interlayer rotation. Random size of the non-periodic domains in a relatively large area rules out the possibility of Moiré patterns but confirms that they are stacking-dependent domains. This is quite different from the lattice-mismatch-induced Moiré stacking orders in hetero-bilayers [43].
After careful checking of the triangular bilayer domains, the difference in ADF-STEM imaging of different domains indicates distinctive stacking orders in the bilayer [42]. To specify the detailed stacking structure in each domain, construction of the stacking model and image simulation will be necessary. Starting from the initial high-symmetry AB–0 and AA–0 (Fig. 11(b)) configurations, the upper layer is shifted horizontally (H1, H2) or vertically (V1–V7), both parallel to the fixed bottom layer to yield various types of stacking orders, as shown in Figs. 11(b). The corresponding simulated ADF-STEM images demonstrate clearly the distinctiveness and diversity of these stacking sequences. Note that AB–0 and AA–V3 (Fig. 11(b)) stackings are actually bilayer structures in the well-known 2H and 3R phases, respectively. These simulated ADF-STEM images of each stacking structure act as fingerprints of each stacking sequence, and hence, can be directly compared with the experimental image in Fig. 11(a). Each domain can be assigned with a stacking order when the experimental images match the simulated images. As shown in Fig. 11(a), each domain is marked by triangles in different colors. Each color indicates one type of stacking order with its stacking name marked, as AB–V4, AB–V6, etc. Besides the high-symmetry configuration AB–0 and AA–V3, all the other experimentally observed
