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.