Recently, Prof. QU Songnan’s group at Changchun Institute of Optics and Fine Mechanics and Physics of Chinese Academy of Sciences (CAS) developed full color luminescence carbon dots (CDots) for the first time and achieved their composite phosphors with high fluorescence quantum efficiency for white light LEDs application through a convenient and fast method.

CDots possess a number of distinct merits such as low toxicity, good luminous performance, and biocompatibility, which renders them attractive for applications in bioimaging, sensor, catalysis, lasers, photoelectric devices and lighting. However, the lack of method to control the band-gap of CDots and aggregation induced quenching of CDots in the solid state still hinder the development and the application of CDots in solid state lighting.

QU’s group proposed an approach towards controlling the bandgap emissions of CDots to realize full-color emissive CDots from citric acid and urea. They employed three different solvents (water, glycerol and dimethylformamide) and their combinations in a solvothermal synthesis. The solvents govern the degree of dehydration and carbonization processes of precursors, which results in formation of differently sized sp2-conjugated domains within the prepared CDots, leading to different color of their emission ranging from blue to the red.

Besides, they employed a microwave assisted treatment of CDots dispersed in aqueous solutions of sodium silicate, which immobilizes well dispersed CDots in the silica network and effectively prevents aggregation of CDots and producing strongly luminescent inorganic CDot phosphors with photoluminescence quantum yields (PLQYs) in the range of 30-40% in the solid state. By covering the red and green CDot phosphors on blue emitting InGaN chips, White Light Emitting Diodes (WLEDs) were fabricated, solely based on CDot phosphors, with Commission Internationale de L'Eclairage (CIE) of (0.34, 0.31) and the color rendering index of 82.4.

Besides, they employed a microwave assisted treatment of CDots dispersed in aqueous solutions of sodium silicate. This method can immobilize well dispersed CDots in the silica network, effectively prevent aggregation of CDots and produce strongly luminescent inorganic CDot phosphors by the range of 30-40% in the solid state. By covering the red and green CDot phosphors on blue emitting InGaN chips, WLEDs were fabricated, solely based on CDot phosphors, with CIE of (0.34, 0.31) and the color rendering index of 82.4.

This study was beneficial to understand the luminescent mechanism of CDots and promote the application of CDots in the lighting devices field. It was published in Advanced Optical Materials.

This work was supported by the National Natural Science Foundation of China, Outstanding Young Scientists Program of CAS, Youth Innovation Promotion Association of CAS, Jilin Province Science and Technology Research Projects, Open Project Program of the State Key Laboratory of Supramolecular Structure and Materials in Jilin University.

Figure 1: (a) Full color luminescence CDots under day light (up) and UV light (down), (b) their corresponding fluorescence spectrum under excitation wavelength of 360 nm. (Image by CIOMP)

Figure 2: Scheme of the formation mechanism of inorganic functional CDot-based phosphors from the CDot-sodium silicate solution under microwave assisted treatment. (Image by CIOMP)