Fabrication of Blister-free van der Waals Heterostructure

Abstract

Van der Waals heterostructure consists of a stack of two or more 2-dimension materials held together by van der Waals force. Such heterostructures allow us to fabricate novel materials by simply stacking various layered materials to create new functionalities as well as observe emergent phenomena which may not exist in the underlying layered materials. In this project, we develop laboratory equipments and recipe for fabrication of blister-free van der Waals heterostructure using pick-up transfer method. Blisters can occur during transferring and stacking 2D material where they could reduce electron mobility. We apply tear-and-stack procedures to create twisted bilayer graphene (TBG) with a controllable twist angle from a singlelayer graphene sheet. In particular, we aim at fabricating TBG at a magic twist angle of 1.05° which has been shown to exhibit strongly correlated phases such as superconductivity and Mott insulator. Here we employ two types of polymer, Polypropylene Carbonate (PPC) and Poly(bisphenol A carbonate) (PC), for transfer process. We find that PC polymer improves the yield of blister-free TBG because of a much slower speed during the tear-step, unlike the PPC polymer which requires a sudden tear, causing graphene to damage. In addition, the PC polymer can withstand a temperature up to 180°C, compared to 120°C of PPC, which allows for an effective removal of blisters during stacking process. A complete TBG device is then characterized by optical microscope and atomic force microscope to verify blister-free area on the device. Finally, Raman spectroscopy is employed to confirm the small twist angle of TBG.