Bilayer graphene: gap tunability and edge properties

TL;DR

This paper investigates how an external electric field can tune the electronic gap in bilayer graphene and explores the unique edge states that arise at zigzag boundaries, with implications for nanotechnology applications.

Contribution

It provides a detailed analysis of gap tunability in bilayer graphene considering screening effects and introduces a new type of edge state enabled by the bilayer structure.

Findings

A tunable gap between zero and mid-infrared energies can be achieved.

Bilayer graphene supports unique edge states at zigzag edges.

A new edge state with enhanced bulk penetration causes band crossing inside the gap.

Abstract

Bilayer graphene -- two coupled single graphene layers stacked as in graphite -- provides the only known semiconductor with a gap that can be tuned externally through electric field effect. Here we use a tight binding approach to study how the gap changes with the applied electric field. Within a parallel plate capacitor model and taking into account screening of the external field, we describe real back gated and/or chemically doped bilayer devices. We show that a gap between zero and midinfrared energies can be induced and externally tuned in these devices, making bilayer graphene very appealing from the point of view of applications. However, applications to nanotechnology require careful treatment of the effect of sample boundaries. This being particularly true in graphene, where the presence of edge states at zero energy -- the Fermi level of the undoped system -- has been extensively reported. Here we show that also bilayer graphene supports surface states localized at zigzag edges. The presence of two layers, however, allows for a new type of edge state which shows an enhanced penetration into the bulk and gives rise to band crossing phenomenon inside the gap of the biased bilayer system.