Preparation Of Multicolor Fluorescent Carbon Quantum Dots And Its Application In Solid State Lighting

At present,commercial rare earth yellow phosphors are used as color converters in solid-state lighting,because of the lack of effective red light components,their color temperature is high and the color rendering index is low,while rare earths are expensive as a limited strategic resource.Need to develop a material with high effective red luminescence content and rich in source.Hence,a series of multi-color luminescent carbon quantum dots were designed and studied their photoluminescence properties and structural characteristics and explored their application in solid-state lighting.(1)the orange-blue two-color carbon quantum dots were synthesized in one step by using solvothermal method with p-phenylenediamine as carbon source and formamide as solvent.The orange luminescence carbon dot is strongly absorbed in the ultraviolet and visible light regions,and the blue ones are strongly absorbed only in the ultraviolet region.The prepared carbon dot solution is dispersed in an aqueous solution of different pH values,and it is found that the pH value has no influence on it.The structural model and luminescence mechanism of carbon dots are proposed,the surface defects of a large number of hydrophilic groups grafted on the surface of spherical carbon dots lead to orange luminescence emission at 580 nm wavelength.With the increase of temperature or the reaction time,the shedding of surface groups reveals the carbon core of the sp2 type graphene-like carbon core structure,resulting in blue luminescence emission at a wavelength of 414 nm.The coupling of carbon dots and polymer produces a transparent film,which is used as a color converter in solid-state illumination to achieve white light emission,significantly improves the color temperature,color rendering index and other photoelectric parameters.(2)red luminescence carbon dots were synthesized by solvothermal method using p-phenylenediamine as carbon source and isopropanol as solvent.It is found to have excitation-independent properties,strong absorption in the ultraviolet and visible regions,and its lifetime curve does not decrease with changes in emission wavelength.By TEM,XPS,FT-IR,Raman characterization,it consists of π-conjugated carbon core and a surface defect formed by grafting a large number of hydrophilic groups to produces red luminescence emission.It was found that changing the experimental conditions had no effect on luminescence.By investigating the effect of pH on the emission/absorption of carbon dots,it was found that the red luminesecence of carbon dots was actually caused by N-related defect states,and the introduction of H+and OH-has a great influence on these surface emission states,confirming that the surface defect state induced red luminescence in the carbon dots of this experiment.The prepared red carbon dots and the polymer were coupled to prepare a carbon dot film as a color converter to change the LED from cool white light to warm white light,which significantly improved the color temperature and color rendering index and reached the level of commercial phosphor powder.(3)yellow fluorescent carbon quantum dots were synthesized by one-step hydrothermal method using o-phenylenediamine as carbon source and nitrogen source and water as solvent.Transmission electron microscopy(TEM)demonstrates that the carbon dots are monodisperse and have no uniform size distribution(2-3 nm).High-resolution TEM(HRTEM)micrographs confirm the crystal characteristics of well-resolved lattice fringes of carbon dots.Further study of the effect of solvent(DMF,ethanol,formamide,methanol,water)on the emission wavelength of synthetic carbon dots.When water is replaced by other solvents,the emission wavelength of the carbon dots moves in the short-wavelength direction,indicating that the water in the hydrothermal reaction is a good solvent for forming carbon dots in the system.The yellow carbon dots exhibit Excitation-Independent luminescence.Although the emission intensity changes,the peak position does not change significantly with the change of the excitation light wavelength,indicating that the luminescence of the carbon dots may originate from a single emission state.The yellow carbon dots were found by TEM,XPS,FT-IR characterization.These results indicate that the carbon dots are composed of the carbon core of the graphite structure and the amorphous carbon on the surface,which are covered by the N-and O-heterocyclic chemical structures and abundant amino and hydroxyl functional groups.The prepared carbon dots and polymer were coupled to prepare a yellow fluorescent film as a color converter,a pure carbon dot film-based white LED was successfully prepared.It was found that its photoelectric parameters reached an ideal level and realized white light emission.