Novel Techniques for the Assembly of Twisted Bilayer Graphene*

Van der Waals materials are structures that exhibit strong in-plane covalent bonding. The similar lattice structure of monolayer and many-layer Van der Waals materials, such as graphene and hexagonal Boron Nitride (h-BN), produce periodically repeating interference patterns known as moiré patterns. Interestingly, the geometry displayed by the moiré pattern of twisted bilayer graphene (TBG) allows for fascinating electronic phenomena when monolayer graphene flakes are rotated to a relative uniform 'magic angle' of ~ 1.1°. However, assembling hBN-encapsulated TBG heterostructures using polymer-based stamps, strong Van der Waals interactions, and precision micro-manipulation is subject to a severe lack of reproducibility between samples. Current assembly processes lead to inhomogeneity such as lattice strain, interfacial bubbles, and variations in relative twist angle. Here, we propose a novel process flow including thermally-induced lattice relaxation and a stamp-to-stamp transfer technique that provides graphene rigid support from the hBN flake. Ultimately, improving the process by which TBG is assembled promises to reduce inhomogeneities and enable further investigation of the exotic superconducting and insulating states that magic-angle TBG exhibits.