Deterministic fabrication of graphene hexagonal boron nitride moiré superlattices
arxiv(2024)
Abstract
The electronic properties of moiré heterostructures depend sensitively on
the relative orientation between layers of the stack. For example,
near-magic-angle twisted bilayer graphene (TBG) commonly shows
superconductivity, yet a TBG sample with one of the graphene layers
rotationally aligned to a hexagonal Boron Nitride (hBN) cladding layer provided
the first experimental observation of orbital ferromagnetism. To create samples
with aligned graphene/hBN, researchers often align edges of exfoliated flakes
that appear straight in optical micrographs. However, graphene or hBN can
cleave along either zig-zag or armchair lattice directions, introducing a 30
degree ambiguity in the relative orientation of two flakes. By characterizing
the crystal lattice orientation of exfoliated flakes prior to stacking using
Raman and second-harmonic generation for graphene and hBN, respectively, we
unambiguously align monolayer graphene to hBN at a near-0 degree, not 30
degree, relative twist angle. We confirm this alignment by torsional force
microscopy (TFM) of the graphene/hBN moiré on an open-face stack, and then by
cryogenic transport measurements, after full encapsulation with a second,
non-aligned hBN layer. This work demonstrates a key step toward systematically
exploring the effects of the relative twist angle between dissimilar materials
within moiré heterostructures.
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