  We present a first-principles calculation of the optical properties of
armchair-edged graphene nanoribbons (AGNRs) with many-electron effects
included. The reduced dimensionality of the AGNRs gives rise to an enhanced
electron-hole binding energy for both bright and dark exciton states (0.8-1.4
eV for GNRs with width w ~ 1.2 nm) and dramatically changes the optical spectra
owing to a near complete transfer of oscillator strength to the exciton states
from the continuum transitions. The characteristics of the excitons of the
three distinct families of AGNRs are compared and discussed. The enhanced
excitonic effects found here are expected to be of importance in optoelectronic
applications of graphene-based nanostructures.
