Fabricating clean 2D material devices in ultra-high vacuum

Two-dimensional (2D) materials exhibit a rich set of physical phenomena as a result of reduced dimensionality. Artificial heterostructures based on 2D materials further enrich the physics in flatlands. Most 2D materials are, however, prone to degradation in the presence of trace amount of oxygen or water vapor. Meanwhile, interfacial contamination introduced during the stacking process limits the quality of the heterostructures. To solve these problems, we design and build an ultra-high vacuum (UHV) system in which the entire fabrication process is performed under pressure lower than 5×10^-10 mbar. We demonstrate the functionality of this UHV fabrication system by exfoliating various 2D materials with atomically clean surfaces, and in-situ assembling van der Waals heterostructures without interfacial contamination.