Strong covalent bonding between two graphene layers

TL;DR

This paper demonstrates that directly stacked graphene layers can form strong covalent bonds at close proximity, significantly altering their electronic properties and stability compared to typical van der Waals interactions.

Contribution

It reveals a new metastable covalently bonded form of graphene bilayer with unique structural and electronic characteristics, differing from conventional van der Waals stacking.

Findings

Strong covalent bonds form at 0.156 nm separation.

Graphene's in-plane bonds weaken from double to single bonds.

Electronic structure varies from semimetal to semiconductor under stress.

Abstract

We show that two graphene layers stacked directly on top of each other (AA stacking) form strong chemical bonds when the distance between planes is 0.156 nm. Simultaneously, C-C in-plane bonds are considerably weakened from partial double-bond (0.141 nm) to single bond (0.154 nm). This polymorphic form of graphene bilayer is meta-stable w.r.t. the one bound by van der Waals forces at a larger separation (0.335 nm) with an activation energy of 0.16 eV/cell. Similarly to the structure found in hexaprismane, C forms four single bonds in a geometry mixing 90^{0} and 120^{0} angles. Intermediate separations between layers can be stabilized under external anisotropic stresses showing a rich electronic structure changing from semimetal at van der Waals distance, to metal when compressed, to wide gap semiconductor at the meta-stable minimum.