Unraveling the origin of near-infrared emission in carbon dots by ultrafast spectroscopy

Carbon dots (CDs) are unique light-emissive nanoparticles that are valuable for various applications. However, their complex chemical structures and the limited research interest in their visible-light emission hinder the understanding of their emission structures. Herein, we report the existence of near-infrared (NIR) emissive states originating from the graphitic cores in CDs, which exhibit a completely different behavior from their well-known visible emissive states. Using ultrafast spectroscopy and density functional theory (DFT) calculations, we elucidate the emission mechanism and suggest that small (1-2 nm) graphitic subregions in CDs produce the NIR emissive states, which are rarely affected by surface chemistry changes. Our proposed mechanism and its universality are investigated comprehensively by a comparative analysis with graphene oxide and other types of CDs obtained by different synthetic methods. Finally, we propose a comprehensive emission structure of CDs and redefine the role of structural components in visible-to-NIR emission.