Built-in Bernal gap in large-angle-twisted monolayer-bilayer graphene

TL;DR

This paper reports the discovery of an intrinsic Bernal gap in a large-angle-twisted monolayer-bilayer graphene stack, caused by structural asymmetry and proximity effects, without external electric fields.

Contribution

It demonstrates the existence of a built-in Bernal gap in twisted graphene layers synthesized via CVD, expanding the understanding of intrinsic electronic properties in layered 2D materials.

Findings

Intrinsic Bernal gap of ~10 meV confirmed by thermal-activation measurements.

Large twist angle (~30°) decouples electronic bands but structural asymmetry induces a gap.

Built-in asymmetry can be compensated by a small displacement field.

Abstract

Atomically thin materials offer multiple opportunities for layer-by-layer control of their electronic properties. While monolayer graphene (MLG) is a zero-gap system, Bernal-stacked bilayer graphene (BLG) acquires a finite band gap when the symmetry between the layers' potential energy is broken, usually, via a displacement electric field applied in double-gate devices. Here, we introduce a twistronic stack comprising both MLG and BLG, synthesized via chemical vapor deposition, showing a Bernal gap in the absence of external fields. Although a large ($\sim30^{\circ}$) twist angle decouples the MLG and BLG electronic bands near Fermi level, proximity-induced energy shifts in the outermost layers result in a built-in asymmetry, which requires a displacement field of $0.14$ V/nm to be compensated. The latter corresponds to a $\sim10$ meV intrinsic BLG gap, a value confirmed by our thermal-activation measurements. The present results highlight the role of structural asymmetry and encapsulating environment, expanding the engineering toolbox for monolithically-grown graphene multilayers.