Rotational stability of twisted bilayer graphene

Moire superlattices form in twisted graphene bilayers due to periodic regions of commensurability, but truncation of the moire patterns affects the rotational stability of finite-sized sheets. Here, we report the stepwise untwisting of nanometer-sized bilayer graphene flakes at elevated temperatures, each step corresponding to a potential energy barrier formed by changes to the commensurability between the moire superlattice and flake size with twist angle. The number of locally stable energy states and their barrier energies scale with the flake size, allowing twisted graphene flakes of several tens of nanometers to remain thermally stable even at chemical vapor deposition temperatures.