Light Emission in Graphene Nanoribbons: Insights from Ab-Initio Simulations

Graphene nanoribbons (GNRs) have attracted increasing attention in the last decade as a viable route for graphene-based opto-electronic applications, especially in view of the successful production of ultranarrow and atomically precise structures by means of bottom-up techniques. While GNR absorption properties have been addressed in depth [1, 2], emission properties are still largely unexplored. We here report on the optical response of finite-length GNRs as resulting from state-of-the-art ab initio calculations beyond mean field [3, 4]. Our results indicates that bulk-like excitations coexist with below-bandgap states localized at the GNR extremities, which are almost independent on the length. By investigating both the presence of defects and the coupling with a gold tip, our simulations allow us to identify unpredicted optical transitions in GNRs and to elucidate the origin of below-bandgap STM-induced light emission recently observed in suspended GNRs [3], providing a promising route for the realization of bright, robust, and controllable graphene-based light-emitting devices.

[1] Denk et al., Nat. Commun. 5, 4253 (2014).
[2] Soavi et al., Nat. Commun. 7, 11010 (2016).
[3] Chong et al., Nano Lett. 18, 175 (2018).
[4] Cardoso, Ferretti, Prezzi, EPJB, in press (2018).