Fluctuation Effects of the Electric Field Induced by Water on a Graphene Dot Band Gap

Abstract

We investigate how a box of water molecules affects the HOMO-LUMO gap of a rectangular graphene nanodot (GND) with two zigzag and two armchair edges, using a combination of first principles and molecular mechanics, and also classical molecular dynamics. A GND is solvated in a periodic box of water molecules, and the HOMO-LUMO gap is computed for some snapshots taken from a molecular dynamics simulation. Although an isolated GND has a semiconductor state with degenerate α and β gaps, we find that, in a solvated GND, that degeneracy is broken and the gaps of both spins flavors oscillate following the time fluctuations in strength and direction of the electric field generated by the solvent at the edges. The average electric field generated by the water molecules causes an effect equivalent to applying a uniform electric field of 0.16 V/Å computed at the PBE level of theory. In particular, this field is not strong enough to change the GND semiconductor ground state to a half-metallic one in nanodots with dimensions smaller than 2.5 nm, as those studied here. These results can be useful in the design of sensors based on graphene, indicating that important fluctuations in the energy gap can occur if water molecules are present.