Making Graphene Nanoribbons Photoluminescent.

We demonstrate the alignment-preserving transfer of parallel graphene nanoribbons (GNRs) onto insulating substrates. The photophysics of such samples is characterized by polarized Raman and photoluminescence (PL) spectroscopies. The Raman scattered light and the PL are polarized along the GNR axis. The Raman cross section as a function of excitation energy has distinct excitonic peaks associated with transitions between the one-dimensional parabolic subbands. We find that the PL of GNRs is intrinsically low but can be strongly enhanced by blue laser irradiation in ambient conditions or hydrogenation in ultrahigh vacuum. These functionalization routes cause the formation of sp3 defects in GNRs. We demonstrate the laser writing of luminescent patterns in GNR films for maskless lithography by the controlled generation of defects. Our findings set the stage for further exploration of the optical properties of GNRs on insulating substrates and in device geometries.