In general, there are two major factors affecting bandgaps in nanostructures:(i) the enhanced electron-electron interactions due to confinement and (ii) themodified self-energy of electrons due to the dielectric screening. While recenttheoretical studies on graphene nanoribbons (GNRs) report on the first effect,the effect of dielectric screening from the surrounding materials such assubstrates has not been thoroughly investigated. Using large-scale electronicstructure calculations based on the GW approach, we show that when GNRs aredeposited on substrates, bandgaps get strongly suppressed (by as much as 1 eV)even though the GNR-substrate interaction is weak.
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