The Synthesis Of Long Wavelength Fluorescent Carbon Dots With High Quantum Yield For Warm White LED

Light-emitting diodes(LEDs)have attracted wide attention owing to their long lifetime,high energy efficiency,and low energy consumption compared with conventional light sources.Carbon quantum dots(CQDs)are considered as a promising new type of LED phosphor with the advantages of environmental friendliness,low cost,good light and heat stability and excellent optical properties.They have wide application prospects in photoelectric devices.CQDs are zero-dimensional carbon nanomaterials with size below 10 nm,which contain elements such as C,H,O and N.They are also one class of the most popular new carbon nanomaterials following fullerene,carbon nanotubes,and graphene because of their distinct benefits such as easy functionalization,good light stability,low biological toxicity,good biocompatibility,and low preparation cost.At present,great progress has been made regarding the preparation of CQDs and their application in white LED.Most of the CQDs prepared emit blue fluorescence and the fluorescence quantum yield(QY)reaches as high as more than 90%.However,the correlated color temperature of the prepared white LED is over 5000 K,which belongs to the category of cool white light and is not suitable for occasions needing warm white light such as indoor lighting,since the lack of the long wavelength fluorescence centered at 500 nm or more.At the same time,long-wavelength fluorescent CQDs suffer from low QY and narrow spectrum,which are not conducive to the improvement of luminous efficiency and high color rendering index of the LED based on single matrix CQDs.Based on these challenges present in CQDs above,the dissertation aims at the preparation of fluorescent CQDs with long wavelengths above 500 nm and higher QY and further use of them as single-substrate phosphor for the preparation of warm white LED.Although the regulation mechanism of CQDs fluorescence is not well clear,large amount of evidence has shown that,large particle size,large conjugated sp~2-domain,high N-doping would contribute to long wavelength fluorescence emission of CQDs.Therefore,the long-wavelength fluorescent CQDs with emission wavelength above 500nm are expected to be prepared by solvothermal method from the carbon source and solvent which can make the generated CQDs have large sp~2-conjugation,large particle size,high nitrogen content.The main research content and results are as follows:(1)Green fluorescent CQDs were prepared by solvothermal method with pyroic acid with large conjugated sp~2-domain as carbon source and N,N-dimethylformamide(DMF)as solvent.The emission peak position centered at 520 nm with excitation-independent optical properties.The green fluorescent CQDs has an average particle size of 11.9 nm with large conjugated sp~2-domain.By screening the reaction time and reaction temperature,the optimal parameters were found to be:reaction time 14 h,reaction temperature 180°C.The QY of the green emissive CQDs prepared under the optimal parameters reached as high as 16.82%.Furthermore,the green fluorescent CQDs were applied to the preparation of white LED.The color coordinates of the white LEDs obtained were(0.39,0.49)and the correlated color temperature was 4323 K.which belong to the category of warm white light and are suitable for indoor lighting and other warm white light applications.(2)Orange fluorescent CQDs were prepared by solvothermal method with 2,7-dihydroxynaphthalene with larger conjugated sp~2-domain as the carbon source,DMF as the nitrogen source and ethanol as solvent.The orange emissive CQDs exhibited two emission peaks centered at 531 and584 nm and the proportion of blue-red light was as high as 82.05%.They exhibited excitation-independent optical properties with the QY as high as 26.03%,and excellent UV resistance and thermal stability.The color coordinates of the resulting LED device based on the orange emissive CQDs as single-substrate phosphor were(0.41,0.39),the color temperature was 3330 K and the color rendering index reached as high as91,which are suitable for indoor lighting and other warm white light applications.Through the studies above,we choose large sp~2 conjugated carbon source and solvent to realize the preparation of long-wavelength fluorescent CQDs with emission peak centered at more than 500 nm by solvothermal method.The resulting CQDs were used for the preparation of LEDs combined with UV chips.The color temperature of the obtained white LED was lower than 4500 K,achieving warm white light emission.