Characteristic Excitation Wavelength Dependence of Fluorescence Emissions in Carbon Quantum Dots

Carbon quantum dots (CDots), generally defined as small carbon nanoparticles with various surface passivation schemes, have emerged to represent a rapidly advancing and expanding research field. CDots are known for their bright and colorful fluorescence emissions, where the colorfulness is associated with the emissions being excitation wavelength dependent. In this work, CDots with 2,2'-(ethylenedioxy)bis(ethylamine) (EDA) for surface functionalization were studied systematically by using steady-state and time-resolved fluorescence methods. The observed fluorescence quantum yields are strongly excitation wavelength dependent, and the dependence apparently tracks closely the observed absorption profile of the EDA-CDots, whereas the excitation wavelength dependence of observed fluorescence lifetimes is much weaker, obviously decoupled from the quantum yields. Mechanistically, the presence of two sequential processes immediately following the photoexcitation of CDots leading to fluorescence is used to rationalize these effects, and the experimental results seem better explained by attributing one of the two processes to be primarily responsible for the characteristic excitation wavelength dependence. Significant implications of the mechanistic probing to the understanding of CDots as a new class of quantum dot-like fluorescent nanomaterials are discussed, and so are further challenges and opportunities.