Polymer–Free Assembly of Van der Waals Heterostructures

2D heterostructures present an optimal approach for the synthesis of novel materials with highly adjustable properties, including electron topology and correlation effects. Currently, the prevailing method for constructing van der Waals heterostructures involves the "tear and stack" technique, wherein individual 2D layers are stacked using a polymer stamp. However, this approach easily traps contaminants at the heterostructure interface, leading to limitations in atomic cleanliness and electronic performance of the resulting heterostructures. To address this issue, we have developed a novel polymer–free stacking method that employs flexible, transparent, and thermally stable metallized silicon nitride membranes (SiN). These SiN membranes enable precise and rapid transfer of 2D flakes to desired locations in various environments, including air, argon glovebox, and ultrahigh vacuum, at elevated temperatures of up to 300°C. The elevated temperature during the fabrication process facilitates the removal of surface–absorbed contaminants from the 2D heterostructure interfaces. To showcase the merits of this polymer–free transfer technique, we successfully fabricated and assessed distinct heterostructures. In initial experiments, we achieved hBN/graphene/hBN heterostructures exhibiting remarkably bubble–free regions, accompanied by mobility values reaching 3×10⁶ cm²V^-1s^-1. Moreover, we demonstrated the fabrication of a more intricate LED structure with a bubble-free area 25x30 µm, completed within a mere 10–minute timeframe. Furthermore, a twisted monolayer/bilayer graphene on an hBN substrate displayed minimal variation in twist angles, with deviations of merely 0.016° over a length of 10 μm. Lastly, the SiN membrane method enables the large–scale transfer of 2D materials synthesized via chemical vapour deposition (CVD) from the growth substrate, devoid of any polymer involvement.