Synthesis. Nonporous graphene for patterning was grown on copper foil (Alfa Aesar 46986; surface was milled to remove contamination). Graphene growth was performed at 1000°C in a custom-built quartz tube furnace (Graphene Square Inc., Republic of Korea). The copper foil was annealed at 950°C for 1 hour flowing 5 sccm of H2 at 70 mtorr. Subsequently, 40 sccm of CH4 was flown for 30 min to synthesize full-coverage graphene on the Cu foil at 100 mtorr.

Transfer to glass substrate. The graphene sample was then spin-coated with PMMA (anisole, 2 weight %) to yield a 150-nm-thick film and baked for 5 min on a hot plate at 180°C. Ammonium persulfate solution (0.5 M; Sigma-Aldrich) was used as a copper etchant, and the PMMA/graphene/copper was floating-etched until the copper was removed entirely. The sample was then rinsed for 30 min in DI water and fished with another sample of graphene on copper to yield the double layer. The sample was again etched and rinsed in the same fashion. A glass slide [large cover glass (thickness, 190 to 250 μm), Ted Pella] was used to fish the double-layer PMMA/graphene. The sample was let dry in ambient and placed on a hot plate for 30 min to reduce wrinkling. The PMMA was subsequently removed in acetone and rinsed in isopropanol.

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