Deterministic fabrication of graphene hexagonal boron nitride moir\'e superlattices

TL;DR

This paper presents a deterministic method for fabricating graphene/hBN moiré superlattices with precise alignment, enabling systematic exploration of twist angle effects on electronic properties.

Contribution

The authors develop a lattice orientation characterization technique to align graphene and hBN with near-zero degrees twist, reducing ambiguity and improving reproducibility in moiré heterostructure fabrication.

Findings

Unambiguous alignment of graphene to hBN at near-0° twist angle.

Confirmation of alignment via torsional force microscopy.

Transport measurements validate the fabrication method.

Abstract

The electronic properties of moir\'e 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 moir\'e 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 moir\'e heterostructures.