Highly Efficient Bulk-Crystal-Sized Exfoliation of 2D Materials under Ultra-High Vacuum

Two-dimensional (2D) materials represent the ultimate limit of miniaturization along the vertical direction, where several fascinating quantum mechanical effects—that are otherwise inaccessible—appear, generating widespread interest in fundamental and applied research. However, the preparation of scalable monolayers is a requirement that is hindered because of the lack of suitable synthesis and manipulation techniques. Here, we report a facile approach for obtaining bulk-crystal-sized 2D monolayers with a yield of almost 100% under ultra-high vacuum (UHV) conditions. We have exfoliated MoS2 on Au and Ag substrates to demonstrate the exfoliation process, where the spectroscopic measurements suggest a very strong dispersive interaction between the metal substrate and MoS2, resulting in a high strain field on the topmost layer of the bulk crystal, which is substantially relaxed in the subsequent layer. This vertically inhomogeneous distribution of the strain field reduced the magnitude of interlayer van der Waals interactions, resulting in the high selectivity of monolayer exfoliation. Along with the scalable exfoliation of monolayers, our method also paves a route to obtain 2D monolayers of materials that are unstable under ambient conditions.