Preparation Of Solid-state Red Light-emitting Carbon Dots Based On High Boiling Point Solvents And Their Application In White Light LED

Carbon dots(CDs)is an advanced carbon nanomaterial with the size of less than 10 nm.In recent years,CDs has been widely used in the photoelectric fields due to their advantages,including low cost,simple preparation,good biocompatibility with low toxicity and adjustable optical properties.Solid-state fluorescent CDs has been widely used in light emitting diodes(LED)devices as phosphors.However,the emission of solid-state fluorescent CDs is mostly concentrated in the short-wavelength regions such as blue and green,and quantum yield(QY)is generally low,the color rendering index(CRI)of white LED devices is often not high,which limits their actual application in LED devices.This thesis aims at the preparation of solid red light emission fluorescence CDs by choosing a high boiling point solvent(N,N-Dimethyl formamide(DMF)or formamide),which is beneficial for improve the carbonization degree of CDs to facilitate them light wavelength redshift.They were prepared by the simple quick one-step microwave method using phloroglucinol with conjugated structure of benzene ring as the carbon source,urea and ammonium fluoride as additives.Then,white LED devices were obtained by Ultraviolet(UV)chip combining with phosphors,which consist of as-prepard solid red CDs mixing with the blue and green CDs previously synthesized by the research group.The main research contents and results are as follows:1.Preparation of solid red CDs based on DMF solvent and their application in white LED.Firstly,the solid-state red CDs(R-CDs-D)was obtained by one-step microwave method with DMF and water as the solvent by adjusting the reaction parameters.The optimal reaction conditions were as follows: phlorogluogl:urea:ammonium fluoride is 3:1:6,the volume ratio of DMF is 50%,reaction time is 15 min,reaction power is 800 W.The best emission wavelength of R-CDs-D is about 625 nm with QY of 3.2%,and the fluorescence intensity remains about 96% after 60 min of UV lamp irradiation.The weight loss rate of RCDs-D is only 3% at 200 ℃,which shows good photothermal stability.Then,a red LED with international commission on lighting color coordinates(CIE)of(0.67,0.33)was prepared by a composite fluorescent film of R-CDs-D and polyvinylpyrrolidone(PVP),A white LED device,with CIE of(0.32,0.33),correlated color temperature(CCT)of 6239 K,and CRI of91,was obtained by combining R-CDs-D with blue and green CDs prepared by our research group.2.Preparation of solid red CDs based on formamide solvent and their application in white LED.In order to further achieve the solid red CDs with longer wavelength and higher QY,adopting the mixture of the higher boiling point formamide and water as solvent,keeping3:1:6 of phloroglucinol,urea and ammonium fluoride as raw material,solid red CDs(R-CDsF)were prepared by one-step microwave method(reactive power 800 W)through regulating the reaction solvent mixture ratio and reaction time.The optimum reaction parameters are formamide volume ratio of 5%and reaction time of 3 min.The reaction time is greatly shortened,and the proportion of organic solvent is greatly reduced,which reduces the preparation cost of red CDs.The best emission wavelength of R-CDs-F is about 640 nm,QY is 3.8%,and the fluorescence intensity of R-CDs-F remains 98%after 60 min of UV lamp irradiation.Their weight loss rate is only 5%at 200℃,showing good photothermal stability.Then,R-CDs-F was combined with PVP to obtain red LED with CIE of(0.62,0.38),and white LED devices are prepared by combining above-mentioned blue and green CDs,the CIE is(0.31,0.31),CCT is 6874 K,and the CRI is 92.In summary,by increasing the boiling point of reaction solvent,the optimal wavelength of solid red emission CDs are redshifted at short microwave reaction time with same raw material ratio,and their solid QY is further improved.In addition,white LED with CRI of 92 was obtained by combining the red CDs and same blue and green CDs under UV excitation.This thesis provides a novel idea and an advanced way for the preparation of solid red CDs.