  Systematic ab initio calculations show that the energy gap of boron nitride
(BN) nanoribbons (BNNRs) with zigzag or armchair edges can be significantly
reduced by a transverse electric field and completely closed at a critical
field which decreases with increasing ribbon width. In addition, a distinct gap
modulation in the ribbons with zigzag edges is presented when a reversed
electric field is applied. In a weak field, the gap reduction of the BNNRs with
zigzag edges originates from the field-induced energy level shifts of the
spatially separated edge-states, while the gap reduction of the BNNRs with
armchair edges arises from the Stark effect. As the field gets stronger, the
energy gaps of both types of the BNNRs gradually close due to the field-induced
motion of nearly free electron states. Without the applied fields, the energy
gap modulation by varying ribbon width is rather limited.
