Radiative Rate Modulation Reveals Near‐Unity Quantum Yield of Graphene Quantum Dots

A fundamental but difficult to assess photophysical property of quantum emitters is their fluorescence quantum yield. It describes how efficiently a fluorophore converts absorbed light into fluorescence. Conventional measurements of quantum yield are prone to errors when the sample contains also absorbing but non‐luminescent species. This is, however, commonly encountered in complex systems such as graphene quantum dots that are either optically inactive themselves or contain impurities formed during nanoparticle synthesis. Their presence can lead to a gross underestimation of the quantum yield of the luminescent species. Here, a plasmonic nanocavity‐based method is used to measure absolute quantum yields of graphene quantum dots by modulating their radiative rate. This method is insensitive to the presence of non‐luminescent species and allows to measure absolute values of quantum yield of the luminescent nanoparticles. The determined quantum yields of nearly 100% significantly exceed previously reported values. By comparing these values with those obtained with a comparative method, the average size and relative concentration of the non‐luminescent particles is determined. Thus, this nanocavity‐based method offers a new way for not only measuring absolute values of quantum yield, but also for assessing the fraction of optically inactive species within a sample. Fluorescence quantum yield of graphene quantum dots is their fundamental but difficult to assess property because of possible presence of absorbing species. A plasmonic nanocavity is used to measure absolute quantum yields of graphene quantum dots. By comparing the values with those obtained with a comparative method, the average size and relative concentration of the non‐luminescent particles is quantified.