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Abstract: In this study, using density functional theory and the SIESTA computationalcode, we investigate the electronic and optical properties of the armchair graphenenanoribbons and the armchair boron nitride nanoribbons of width 25 in the presence of atransverse external electric field. We have observed that in the absence of the electricfield, these structures are semiconductors with a direct energy band at Γ point andapplying electric field on them causes to change in the band structure, increasing theband gap and even eliminating the band gap. Increasing the intensity of the applyingfield on the graphene nanoribbons reduces the distance between the maximum of thehighest valence band and the minimum of the lowest conduction band and shifts theconvergence of these two bands in K space from the Γ point to the X point. The energyband gap of the boron nitride nanoribbons also has been decreased from 4.46 eV to lessthan 32.6 meV in presence of a transverse electric field of intensity about 0.30 V/Angand a semiconductor-metal transition was observed in the presence of the strongerfields.Next, we investigate the effect of the transverse electric field on the optical propertiesof both nanoribbons. Of course, in order to study the optical behavior of these systems,we apply only a radiation with the parallel polarization. According to the changes thatthe electric field makes on the band structure, we observed changes in the location andintensity of the optical graphs peaks. Also with increasing the intensity of the field, weobserve a significant increase in the static dielectric constant and the plasmonicbehavior of these structures.