Versatile polymer method to dry-flip two-dimensional moiré heterostructures for nanoscale surface characterization

The recent discovery of magic angle twisted bilayer graphene (MATBG), in which two sheets of monolayer graphene are precisely stacked to a specific angle, has opened up new opportunities in the field of topology, superconductivity, and other strongly correlated effects. Most conventional ways of preparing twisted bilayer devices require the use of high process temperatures and solvents and are not well suited for preparing samples which need to be flipped to be compatible with characterization techniques like scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES), piezoresponse force microscopy (PFM), scanning thermal microscopy (SThM), and magnetic force microscopy (MFM). Here, we demonstrate a very simple polymer-based method using polyvinyl chloride (PVC), which can be used for making flipped twisted bilayer graphene devices. This allowed us to produce flipped twisted samples without the need of any solvents and with high quality as confirmed by PFM. We believe that this dry flip technique can be readily extended to twist 2D materials beyond graphene, especially air-sensitive materials which require operation under inert atmosphere, where often solvents cannot be used.