STACKED SHEETS. “When two pieces of graphene sheets are vertically stacked together, it is known as bilayer graphene, and the twisting angle (θ) between the two sheets matters. Since the discovery of graphene, more than a decade of efforts have been invested to obtain a perfect Bernal-stacked bilayer graphene (θ = 0°) system because such a system possesses an electrically tunable bandgap (0–0.25 eV), a characteristic that does not exist in conventional materials, allowing innovations in ubiquitous electronics.”
https://link.springer.com/article/10.1557/s43577-021-00135-y