Gigantic Binding Energy and Ultrafast Formation of Excitons in Graphene Nanoribbons

Due to its semimetal characteristics, transistors built on graphene are reported to show an on-off ratio which is too low for practical applications. A bandgap opening in graphene can be achieved by tailoring graphene into one-dimensional nano structures, i.e. graphene nanoribbons (GNRs). Upon controlling the width and edge structures, the size of the GNRs bandgap can be tuned precisely, which renders structurally defined GNRs a promising class of materials for optoelectronic and electronic applications. While advances have been made on the GNR synthesis and theoretical calculations sides, limited work has been conducted to investigate its static optical properties, particularly exciton effect in GNRs, and even fewer time-resolved experiments have been done to monitor the ultrafast carrier dynamics (e.g. the dynamic exciton formation process). Here, by employing THz spectroscopy we report a gigantic exciton effect with binding energy up to 700 meV in solution-dispersed GNRs (with a width of 1.7 nm in structure and an optical bandgap of 1.6 eV), illustrating the intrinsically strong Coulomb interactions between photogenerated electron and holes. Following theoretical calculation, we obtain an exciton binding energy of our GNRs of 550 meV, in good agreement with the experimental results. Further, our theoretical result reveals that the exciton states is strongly confined in space, indicating their molecule-like, Frenkel exciton characteristic. As a direct consequence of gigantic exciton binding and strong localization of the exciton excitation, an extremely fast and efficient exciton formation within 0.8ps in GNRs have been observed. As such, our results demonstrate not only fundamental aspects of excitons in GNRs (gigantic binding energy, localized nature, and ultrafast exciton formation), but also highlight the great promise of GNR for optoelectronic devices.