Precisely Controlled Up/Down‐Conversion Liquid and Solid State Photoluminescence of Carbon Dots

In this work, an efficient strategy is proposed to obtain single‐type carbon dots with precisely controlled up/down‐conversion photoluminescence under both liquid and solid states. Tunable fluorescence from green to red color can be achieved via either changing the solvents or matrices composition, or adjusting the concentration of carbon dots. In particular, the photoluminescence of the carbon dots can be linearly modulated by adjusting the polarity of the solvent, which is attributed to the surface‐state fluorescence mechanism. That is, the increase of solvent polarity causes the decrease of the highest occupied molecular orbital–lowest unoccupied molecular orbital energy levels, and therefore leads to the bathochromic shift of fluorescent emission. Meanwhile, the as‐prepared carbon dots and their hybrids possess excellent thermal stability. Thus, the obtained carbon dots are employed into different solid matrices as multicolor phosphors to fabricate white light‐emitting diodes that are suitable for indoor lighting. In addition, the realization of precisely controlled photoluminescence from the single‐type carbon dots will have wide applications in luminescent fields.