In-situ exfoliation method of large-area 2D materials

The success in studying 2D materials inherently relies on producing samples of large area, and high quality enough for the experimental conditions. Because their 2D nature surface sensitive techniques such as photoemission spectroscopy , tunneling microscopy and electron diffraction, that work in ultra high vacuum (UHV) environment are prime techniques that have been employed with great success in unveiling new properties of 2D materials but it requires samples to be free of adsorbates. The technique that most easily and readily yields 2dmaterials of highest quality is indubitably mechanical exfoliation from bulk grown samples, however as this technique is traditionally done in dedicated environment, the transfer of these samples into UHV setups requires some form of surface cleaning that tempers with the sample quality. In this article, we report on a simple and general method of \textit{in-situ} mechanical exfoliation directly in UHV that yields large-area single-layered films. By employing standard UHV cleaning techniques and by purpusedly exploiting the chemical affinity between the substrate and the sample we could yield large area exfoliation of transition metal dichalcogenides. Multiple transition metal dichalcogenides, both metallic and semiconducting, are exfoliated \textit{in-situ} onto Au and Ag, and Ge. Exfoliated flakes are found to be sub-milimeter size with excellent crystallinity and purity, as evidenced by angle-resolved photoemission spectroscopy, atomic force microscopy and low-energy electron diffraction. In addition, we demonstrate exfoliation of air-sensitive 2D materials and possibility of controlling the substrate-2D material twist angle.