Lattice Expansion in Seamless Bi layer Graphene Constrictions at High Bias

TL;DR

This study reveals significant lattice expansion in seamless bi-layer graphene constrictions at high bias, driven by impact ionization and strain, with implications for high-performance carbon-based electronic devices.

Contribution

First direct atomic-resolution observation of large, localized lattice expansion in bi-layer graphene constrictions under high bias conditions.

Findings

Lattice expansion approaches 5% uniaxially and exceeds 1% isotropically near failure.

Current density reaches 4×10^9 A/cm² in sub-10 nm constrictions.

Impact ionization and strain contribute to the observed lattice expansion.

Abstract

Our understanding of sp2 carbon nanostructures is still emerging and is important for the development of high performance all carbon devices. For example, in terms of the structural behavior of graphene or bi-layer graphene at high bias, little to nothing is known. To this end we investigated bi-layer graphene constrictions with closed edges (seamless) at high bias using in situ atomic resolution transmission electron microscopy. We directly observe a highly localized anomalously large lattice expansion inside the constriction. Both the current density and lattice expansion increase as the bi-layer graphene constriction narrows. As the constriction width decreases below 10 nm, shortly before failure, the current density rises to 4 \cdot 109 A cm-2 and the constriction exhibits a lattice expansion with a uniaxial component showing an expansion approaching 5 % and an isotropic component showing an expansion exceeding 1 %. The origin of the lattice expansion is hard to fully ascribe to thermal expansion. Impact ionization is a process in which charge carriers transfer from bonding states to antibonding states thus weakening bonds. The altered character of C-C bonds by impact ionization could explain the anomalously large lattice expansion we observe in seamless bi-layer graphene constrictions. Moreover, impact ionization might also contribute to the observed anisotropy in the lattice expansion, although strain is probably the predominant factor.