The contents of this chapter are published in Advanced science 7 (4), 1900757, (2020). (Appendix C)
We used AgCN to further study whether the epitaxial growth like AuCN/graphene heterostructure is was the same or not.
Figure 30. Structure of AgCN on various hexagonal two-dimensional materials at (a) top view
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and (b) side view. (c) Low magnification TEM image. (d) Simulation model of 1D AgCN structure. (e) TEM SAED patterns image of AgCN wires images and (f) simulation image.
AgCN microwires were grown on graphene using a drop-cast method. Like AuCN, AgCN were also epitaxially grown on graphene as shown in Figure 30a, b. The direction of the AgCN wires, especially the cyanide chain axis, were characterized by TEM imaging. TEM low magnification image (Figure 30c) and the model of AgCN (Figure 30d) were well matched. As shown in Figure 30e, f, experimental observed SAED pattern was in good agreement with the simulated diffraction pattern of the AgCN crystal structure for the [100] zone axis.
Next, TEM image and SAED patterns were observed growing 1D cyanide chains on various hexagonal 2D materials.
Figure 31. Universal oriented AgCN wires on various hexagonal 2D materials. TEM diffraction pattern of AgCN on (a) graphene, (b) MoS2, (c) MoSe2, (d) MoTe2.
Through analysis of the diffraction patterns, AgCN wires axes were well aligned with the lattice ZZ direction of hexagonal various 2D crystals (graphene, MoS2, MoSe2, and MoTe2) regardless of the surface conditions and lattice constants (Figure 31).
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Figure 32. (a) Low mag TEM image of AuCN on h-BN. TEM diffraction pattern of AuCN on (b) h-BN, (c) graphene, (d) MoS2. (e) Low mag TEM image of Cu0.5Au05CN on graphene, TEM diffraction pattern of Cu0.5Au05CN on (f) graphene, (g) h-BN, (h) MoS2.
It was confirmed through the SAED pattern that the same tendency appeared in both AuCN and Cu0.5Au05CN as well as AgCN wires (Figure 32). In conclusion, through Figure 32, 33 we found the universal oriented van der Waals epitaxy of 1D cyanide chains on hexagonal 2D crystals.
AgCN microwires which was universal oriented epitaxy on 2D hexagonal crystals were useful way to determine the orientation of 2D crystals. And, those wires can be used to align marker for
fabricating the vertical 2D heterostructures with twist angle between each layer in stack. Vertical 2D structures have received significant interest due to their unique physical properties and new
functionality.88,119,120 In particular, the twist angle between 2D layers has been found to be one of the key parameters that largely control some important system properties and it is there-fore useful to establish a simple way to control the twist angle for vertical heterostructures.91-93,121
The fabrication of vertical heterostructures with controlling stacking angle is shown in Figure 33a.
In first, we used AgCN microwires as align marker in the progress of stacking. Graphene and h-BN were selected as vertical heterostructures for demonstration purposed. By protecting some part of the 2D crystal flakes using PDMS, AgCN wires were grown on the limited area of crystals, and partially pure part 2D crystals for fabricating heterostructures were obtained. The graphene/h-BN was
vertically stacked with the aid of a manipulator and the crystal orientation was identified by analyzing the rotational distribution of the AgCN wires. Two flakes were stacked so that the twist angle between graphene and h-BN was zero. This 2D vertical heterostructure was analyzed by transfer to TEM grid.
Figure 33b shows that the twist angle between graphene and h-BN was zero by TEM diffraction
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pattern images. The diffraction spots strongly overlapped because of little lattice mismatch. For relatively high index diffraction spots, h-BN and graphene signals were distinguished, as shown in Figure 33b. The h-BN and graphene (0-220) diffraction spots were identified, as shown in Figure 33c, and the lattice parameter difference (1.76 ± 0.01%) and twist angle (<0.1°) were measured. We also observed moiré peaks in addition to the h-BN and graphene diffraction spots. The simulated diffraction signals from the h-BN/graphene heterostructure with zero twist angle reproduced the observed diffraction pattern (Figure 33d). The h-BN/graphene moiré pat-tern was also directly observed using phase-contrast TEM imaging (Figure 33e, f) and the observed moiré periodicity was consistent with the calculated value from the twist angle and the measured lattice parameter difference (Figure 33g). The moiré periodicity was found to be uniform over the observed region, which
strongly suggests that the 2D interface between graphene and h-BN was pristine. These results demonstrate that the microwire-assisted identification of crystal orientation is a reliable method and allows the fabrication of vertical 2D heterostructures with high-precision control of twist angle.
Figure 33. Fabrication of vertical heterostructures with stacking angle control. (a) Schematic
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illustration of the overall process for fabrication of vertical 2D heterostructures with control of the stacking angle. (b) SAED of graphene/h-BN heterostructure with zero twist angle. The diffraction spots in the six color boxes are the zoomed-in diffraction signals inside the black center box. (c) Zoomed-in diffraction signal in one of the boxes in panel (b). (d) Simulated diffraction signal from graphene/h-BN with zero twist angle. (e) Moiré spacing as a function of twist angle between graphene and h-BN. Inset: the Moiré spacing over the range -0.5 to 0.5 twist angle. The blue dot indicates the experimentally measured moiré distance.