White-light Emission of Single Carbon Dots Prepared by Hydrothermal Carbonization of Poly(diallyldimethylammonium Chloride): Applications to Fabrication of White-light-emitting Films

Different-sized carbon dots (CDs) with full-color emissions have immerse potentials as a novel class of light source in the field of light-emitting diodes (LED). However, few studies have been devoted to the development of the one-step process for preparing white-light-emitting CDs (WLECDs). Herein, we present a facile and one-pot synthesis of the WLECDs through microwave-assisted hydrothermal carbonization of poly(diallyldimethylammonium chloride) (PDDA). The as-synthesized WLECDs had a round shape with a mean particle size of 2.22 nm and their zeta potential reached up to 47 mV. Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy revealed the appearance of nitrogen and oxygen-containing functional groups on the CD surface, generating many surface state emissive traps. Additionally, photoluminescence spectroscopy showed that the CDs exhibited excitation-dependent surface-state emission and excitation-independent core-state emission. When excited at 350 nm, an aqueous solution of the WLECDs emitted white light with an absolute quantum yield of 11% and a correlated color temperature of 5999 K at Commission International de l'Eclairage (CIE) coordinates of (0.321, 0.348). Single-particle photoluminescence spectroscopy demonstrated that the WLECDs still possessed broadband white-light emission from 400 to 800 nm at a single particle level. Furthermore, a white-light-emitting polymer composite film excited by 365-nm UV light was fabricated by embedding the WLECDs into a polyvinyl alcohol matrix. This flexible solid-state film showed a correlated color temperature of 7023 K at CIE coordinates of (0.303, 0.332) and. Given that the WELCDs have highly positive charges, the fabrication of a white-light-illuminating film was successfully conducted by layer-by-layer assembly of the WELCD and poly(4-styrenesulfonic acid).