Domain wall arrays in twisted bilayer graphene

Graphene is composed of a single atomic layer of carbon arranged in a hexagonal lattice. First isolated in 2004 it is now the subject of a vibrant and dynamic field of research due to its many astounding properties for example, extremely high electron mobility and high strength. In fact, graphene is the flagship for a whole class of interesting materials defined by strong in-plane bonds between atoms but only weak van der Waals forces between planes. Such van der Waals (vdW) materials are therefore easily exfoliated, meaning that individual planes or layers can be separated from bulk crystals which has led to the discovery of a vast range of novel phenomena due to the unique atomically-thin two-dimensional (2D) crystalline structure.Recently there has been a surge of research interest around twisted bilayer graphene, in which a small misalignment between two layers of graphene produces a large scale moiré pattern of domains and domain walls arranged in a regular array. When the relative rotation between layers is 1.1°, the so-called “magic angle”, and the sample cooled to below 2 K, superconductivity emerges. A crucial aspect of this unexpected result is that the moiré pattern domain array must be finely tuned, meaning the structural order dictates the appearance of superconductivity. This proposal is therefore concerned with the study of the moiré pattern domain structure and its relation to superconductivity. To achieve this goal a method to directly image the domains and domain walls of the moiré pattern will be developed through use of piezoresponse force microscopy (PFM) for which preliminary studies have been performed and are promising. Furthermore, bilayer graphene nanostructures will be produced by a novel form of atomic force microscopy (AFM) lithography which will allow for engineering of the magic angle through local mechanical rotational manipulation of the top graphene layer with a AFM probe tip which would not otherwise be possible. The combination of the ability to image the moiré pattern domain array and to engineer magic angle conditions sets the stage for thorough investigation of superconductivity in this system.Moreover, we believe this opens the prospect of extending the approach to other 2D materials systems such as twisted bilayer WSe2 and MoS2 where perhaps unexplored opportunities will give rise to new phenomena.