Point Defects in Twisted Bilayer Graphene: A Density Functional Theory Study

TL;DR

This study uses density functional theory to analyze how point defects like Stone-Wales and monovacancies affect the electronic properties of twisted bilayer graphene, revealing weak interlayer coupling but significant electronic structure modifications.

Contribution

It provides a detailed first-principles analysis of defect energetics and electronic effects in twisted bilayer graphene, highlighting the impact of defects on band structure and magnetic properties.

Findings

Defect formation energies are weakly sensitive to stacking and defect site.

Stone-Wales defects cause Dirac cone shifts and mini gaps in TBLG.

Monovacancies induce a 0.25 eV Dirac cone shift and magnetic moments.

Abstract

We have used ab initio density functional theory, incorporating van der Waals corrections, to study twisted bilayer graphene (TBLG) where Stone-Wales defects or monovacancies are introduced in one of the layers. We compare these results to those for defects in single layer graphene or Bernal stacked graphene. The energetics of defect formation is not very sensitive to the stacking of the layers or the specific site at which the defect is created, suggesting a weak interlayer coupling. However signatures of the interlayer coupling are manifested clearly in the electronic band structure. For the "$\gamma\gamma$" Stone Wales defect in TBLG, we observe two Dirac cones that are shifted in both momentum space and energy. This up/down shift in energy results from the combined effect of a charge transfer between the two graphene layers, and a chemical interaction between the layers, which mimics the effects of a transverse electric field. Charge density plots show that states near the Dirac points have significant admixture between the two layers. For Stone Wales defects at other sites in TBLG, this basic structure is modified by the creation of mini gaps at energy crossings. For a monovacancy, the Dirac cone of the pristine layer is shifted up in energy by $\sim0.25$ eV due to a combination of the requirements of the equilibration of Fermi energy between the two layers with different numbers of electrons, charge transfer, and chemical interactions. Both kinds of defects increase the density of states at the Fermi level. The monovacancy also results in spin polarization, with magnetic moments on the defect of 1.2 - 1.8 $\mu_B$.