Clean assembly of van der Waals heterostructures using silicon nitride membranes

Van der Waals heterostructures are fabricated by layer-by-layer assembly of individual two-dimensional materials and can be used to create a wide range of electronic devices. However, current assembly techniques typically use polymeric supports, which limit the cleanliness-and thus the electronic performance-of such devices. Here, we report a polymer-free technique for assembling van der Waals heterostructures using flexible silicon nitride membranes. Eliminating the polymeric supports allows the heterostructures to be fabricated in harsher environmental conditions (incompatible with a polymer) such as at temperatures of up to 600 degrees C, in organic solvents and in ultra-high vacuum. The resulting heterostructures have high-quality interfaces without interlayer contamination and exhibit strong electronic and optoelectronic behaviour. We use the technique to assemble twisted-graphene heterostructures in ultra-high vacuum, resulting in a tenfold improvement in moire superlattice homogeneity compared to conventional transfer techniques. Membranes made of metal-coated silicon nitride can be used to assemble van der Waals heterostructures without a polymer support layer, thus improving cleanliness and allowing assembly at more extreme temperature and vacuum conditions.