Energy gap in graphene nanoribbons with structured external electric potentials

TL;DR

This paper demonstrates that structured external electric potentials can induce a spectral gap in graphene nanoribbons, turning them from metallic to insulating, with the gap's stability analyzed against edge disorder.

Contribution

It introduces a method to open and control an energy gap in graphene nanoribbons using edge-specific electrostatic potentials, supported by Dirac-fermion and tight-binding models.

Findings

External potentials can induce a spectral gap in graphene nanoribbons.

The maximum energy gap is approximately 0.12 eV for a 15 nm wide ribbon.

The spectral gap remains stable against edge disorder if the potential exceeds a certain threshold.

Abstract

The electronic properties of graphene zig-zag nanoribbons with electrostatic potentials along the edges are investigated. Using the Dirac-fermion approach, we calculate the energy spectrum of an infinitely long nanoribbon of finite width $w$, terminated by Dirichlet boundary conditions in the transverse direction. We show that a structured external potential that acts within the edge regions of the ribbon, can induce a spectral gap and thus switches the nanoribbon from metallic to insulating behavior. The basic mechanism of this effect is the selective influence of the external potentials on the spinorial wavefunctions that are topological in nature and localized along the boundary of the graphene nanoribbon. Within this single particle description, the maximal obtainable energy gap is $E_{\rm max}\propto \pi\hbar v_{\rm F}/w$, i.e., $\approx 0.12$\,eV for $w=$15\,nm. The stability of the spectral gap against edge disorder and the effect of disorder on the two-terminal conductance is studied numerically within a tight-binding lattice model. We find that the energy gap persists as long as the applied external effective potential is larger than $\simeq 0.55\times W$, where $W$ is a measure of the disorder strength. We argue that there is a transport gap due to localization effects even in the absence of a spectral gap.