Self-trapped exciton emission from carbon dots investigated by polarization anisotropy of photoluminescence and photoexcitation

Carbon dots have attracted tremendous attention because of their intrinsic advantages that open up opportunities to replace traditional fluorescent materials in various application fields. However, until now, the emission mechanism from carbon dots has been controversial, substantially hindering the extensive exploitation of these materials. Here, we explore systematically the essential emission behavior of carbon dots by using polarization anisotropy spectroscopy, electric-field modulation spectroscopy, and time-resolved photoluminescence measurements. We probe the momentum evolution dynamics and evaluate the decay process of the photoexcited hot carriers, which manifest characteristics that are distinct from band edge emission. We provide clear evidence that carbon dot emission originates from radiative recombination of self-trapped excitons, where the mobilization of the carriers is largely impeded due to the existence of a strong local potential field and thus the relaxation of the hot carriers is strongly suppressed. Based on the self-trapped exciton model, all the optical properties of carbon dots inferred from both steady-state and time-resolved optical spectroscopy can be interpreted consistently. Our investigation provides an alternative insight into the emission mechanisms of carbon dots, which may improve our understanding of these novel materials.