Theoretical study of Ti and Cr as candidate assisted metals for mechanical exfoliation of monolayer transition metal dichalcogenides.

Because of its dimensional characteristics, two-dimensional (2D) materials exhibit many special properties. The key to researching their features is to prepare high-quality larger-area monolayer 2D materials. Metal-assisted mechanical exfoliation method offers the possibility. However, there is still an unclear mechanism. We take transition metal dichalcogenides (TMD) as examples to perform serials of first-principles calculations to study their interlayer interaction. The calculated results demonstrate that the main factors affecting the interlayer interaction of 2D TMD materials include their structural features and the elemental features. The smaller interlayer distance between the topmost two layers than that of bulk leads to the stronger interlayer bonding energy, which results in the difficulty to exfoliate the large-area monolayer 2D crystals successfully. Further, we selected Ti, Cr and Au as the assisted metal due to their enhanced density of states near the Fermi level to calculate the adhesion energies of TMD on their surfaces. We found that there exists strong adhesion energy between 2D crystal and metal, which stretches the spacing of the topmost two layers and leads to the electron density rearrangement. Therefore, metal assisted mechanical exfoliation facilitates the successful preparation of monolayer 2D crystal.