Origins of Efficient Multiemission Luminescence in Carbon Dots

Although several groups have synthesized dual-emission carbon dots (D-CDs) and paid more attention to their emission centers in recent years, the luminescence mechanism of D-CDs is still unclear. Here, we synthesized multiemission CDs via the one-pot hydrothermal method. The prepared CDs exhibit three fluorescence (FL) emissions (370, 425, and 505 nm) under one-wavelength excitation of 320 nm. In addition, various emission origins dominate the unique multiemission property, which is illuminated by means of the radiation decay process, CD internal graphite domains, and abundant functional group characterization. The characterization results fully indicate that there are three energy states: band gap state (carbon-core state), surface defect state, and molecular state. In addition, the peculiar concentration-dependent and polarity-modulated properties further support the explanation of CD multienergy states. Strikingly, the CD FL quantum yield is up to 50.68%. As CDs can show bright green FL excited over 400 nm, the color of concentrated CD solution incredibly turns from maroon to green in visible light observed from different angles. Our findings mainly account for this luminescence mechanism of CDs. This work utilizes time-resolved emission spectra for the first time to distinguish and identify multiemission origins, which would provide scientific inspiration for finding effective ways to regulate or control these luminescence processes.