van der Waals Heterostructures with High Accuracy\nRotational Alignment

We describe the realization of van\nder Waals (vdW) heterostructures with accurate rotational alignment\nof individual layer crystal axes. We illustrate the approach by demonstrating\na Bernal-stacked bilayer graphene formed using successive transfers\nof monolayer graphene flakes. The Raman spectra of this artificial\nbilayer graphene possess a wide 2D band, which is best fit by four\nLorentzians, consistent with Bernal stacking. Scanning tunneling microscopy\nreveals no moiré pattern on the artificial bilayer graphene,\nand tunneling spectroscopy as a function of gate voltage reveals a\nconstant density of states, also in agreement with Bernal stacking.\nIn addition, electron transport probed in dual-gated samples reveals\na band gap opening as a function of transverse electric field. To\nillustrate the applicability of this technique to realize vdW heterostructuctures\nin which the functionality is critically dependent on rotational alignment,\nwe demonstrate resonant tunneling double bilayer graphene heterostructures\nseparated by hexagonal boron-nitride dielectric.