Controlling the Band Gap in Zigzag Graphene Nanoribbons with an Electric Field Induced by a Polar Molecule

Abstract

Graphene nanoribbons with both armchair- and zigzag-shaped hydrogen-passivated edges (AGNR and ZGNR) have band gaps which depend on the width of the ribbon. In particular, a ZGNR has localized electronic states at the edge which decay exponentially toward the center of the ribbon. Interestingly, application of a uniform external electric field (Eext) in the direction perpendicular to the edge of a ZGNR is capable of reducing the band gap for one spin state (beta) and opens the other spin state (alpha). Moreover, for a critical Eext the ZGNR becomes half-metallic. In the case of an 8-chain zigzag ribbon, the critical Eext is 2 V/nm within the local spin density approximation. Motivated by these findings, we study the influence on the gap of the electric field produced by a polar ad-molecule to the surface of an 8-zigzag ribbon. The formula units of the ad-molecules that we studied are NH3(CH)6CO2 and NH3(CH)10CO2. We show that within the generalized gradient approximation the band gap of 0.52 eV without ad-molecule is reduced to 0.27 eV for the beta-spin state and increased to 0.69 eV for the R-spin state. Also, combining the ad-molecule and Eext ) 1 V/nm parallel to the dipole moment of the ad-molecule induces a reduction of the beta-spin band gap and an increase for the R-spin band gap. For Eext ) -1 V/nm, antiparallel to the dipole moment of the ad-molecule, the band gap for both spin states is similar to the case without ad-molecule and Eext. These results suggest possible uses for the graphene nanoribbons as sensors or switching devices.