▎ 摘　　要

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 (E-ext) 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 E-ext the ZGNR becomes half-metallic. In the case of an 8-chain zigzag ribbon, the critical Ex, 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)(6)CO2 and NH3(CH)(10)CO2. 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 alpha-spin state. Also, combining the ad-molecule and E-ext = 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 alpha-spin band gap. For E-ext = -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 E-ext. These results suggest possible uses for the graphene nanoribbons as sensors or switching devices.