Single-Layer Graphene Exfoliation and Hbn Growth

hBN single crystals are grown under high pressure and high temperature, as detailed in ref. ^{80 (link)}. The graphite is first cleaved using adhesive tape. The Si + SiO₂ substrate is then exposed to an oxygen plasma (100 W, 360 s). The surface of the tape is brought into contact with the SiO₂ substrate, which is then placed on a hot plate at 100 °C for 2 min, before the tape is removed. Heating the substrate allows us to achieve large (>100 μm) SLG flakes, whereas flakes produced without heating are typically <50 μm in size, in agreement with findings of ref. ^{50 (link)}. For the exfoliation of hBN, no plasma treatment of the SiO₂ surface is used, as we find this has no effect on the flakes’ lateral size. Polymer-contaminated samples are produced by first exfoliating SLG and subsequently depositing PMMA (8% in Anisole, 495 K molecular weight) via spin coating at 4000 rpm for 60 s. PMMA is then removed by acetone and isopropyl alcohol.