Highly Stable Yellow-emitting Carbon Dots-based Fluorescent Film For Realizing Laser Illumination

At present,light emitting diode(LED)is main solid-state light source used in illumination.However,it has a problem of "efficiency droop" at high input current density,which makes LED difficult to realize high brightness white light,severely restricting the development of LED in high-power solid-state illumination.Therefore,some researchers have focused their attentions on white light sources based on laser diode(LD).The luminous efficiency of LD is thousands of times that of LED,and there is no phenomenon of "efficiency droop".Moreover,LD will become a strong competitor in the field of solid-state illumination because of their advantages,including small emitting area,long lifetime,low beam divergence,and so on.The blue LD excites the yellow color converters to achieve white LD illumination,which has many advantages,such as low cost,easy control and stable performance.However,owing to the high energy and concentrated beam of LD,a lot of heat is generated when LD source irradiates the phosphors,which causes the rapid decadence of phosphors to induce fluorescence quenching.Therefore,the fluorescent materials for LD need to have high thermal stability.Currently,the fluorescent materials used in LD are mainly rare earth doped phosphors with good stability and mature synthesis technology.However,their high price,shortage of resources,and non-renewability make it urgent to develop an advanced phosphor with high thermal stability.Carbon dots(CDs)are a novel type of fluorescent materials,which have obvious advantages,including excellent photoluminescence,high stability,good biocompatibility,low cost,low toxicity,and so on.However,the photothermal stability of most CDs need to be improved for LD illumination.Therefore,this paper aims to construct highly stable yellow-emitting CDs and apply them in white LD devices.The specific research contents and results are as follows:(1)Application of highly stable yellow-emitting graphene quantum dots-based fluorescent film in LD illumination.Firstly,graphene quantum dots(GQDs)with bright green-emitting were obtained by a two-step solvothermal method using GO as starting material,N,N-dimethylformamide as solvent and hydrogen peroxide as oxidant.They are quasi-spherical with average particle size of 5.2 nm,and have a emission peak of 501 nm,quantum yield(QY)of 47.09%,and high crystallinity.Secondly,based on the concentration-dependent photoluminescence(PL)properties of GQDs,a yellow-emitting GQD solution with an emission peak of 574 nm was obtained.Then,a yellow-emitting fluorescent film with excellent photothermal stability was formed by mixing GQD solution and silane coupling agent KH-792.Finally,the fluorescent film was used as color converters to combine with a 450 nm LED chip to realize a white LED device with color coordinates of(0.33,0.34),correlated color temperature(CCT)of 5816 K,and color rendering index(CRI)of 77.Importantly,a white LD device,with color coordinate of(0.34,0.39),CCT of 5288 K,and CRI of 70,also be obtained by assembling the fluorescent film with a 405 nm LD.(2)Application of highly stable yellow-emitting carbon quantum dots-based fluorescent film in LD illumination.Firstly,the yellow-emitting carbon quantum dots(CQDs)with high crystallization degree and high QY were synthesized by one-step solvation thermal method with high sp2-conjugated pyromellitic acid and o-phenylenediamine as carbon and nitrogen sources.The CQDs have spherical shape with the average particle size of 2.9 nm,and an emission peak at 532 nm dominated by carbon nucleus emission,high crystallinity,and excitation-independent PL characteristics.Secondly,the CQD solution and silane coupling agent KH-792 were mixed and cured to form a fluorescent film with bright yellow-emitting and excellent photothermal stability.Finally,the fluorescent film was used as color converters to combine with a 420 nm LED chip to realize a white LED device with color coordinates of(0.30,0.42),color temperature of 6169 K,and color rendering index of 71.Meanwhile,a white LD device,with color coordinate of(0.31,0.32),CCT of 5971 K,and CRI of 71,also was achieved by assembling the CQD film with a 450 nm LD.In conclusion,using the carbon source with high sp2 conjugation as a starting material is conducive to the synthesis of CDs with a higher crystallization degree and improved photothermal stability.As a color converter,it can be combined with LED chip to realize white LED device,and can also be combined with LD to realize white laser illumination.This research not only effectively solves the problems of high price,resource shortage and non-renewability of rare earth phosphors,but also opens up a advanced application field of CDs,which confirms the potential of CDs as a novel type of LD fluorescent materials,and provides a new solution for white LD illumination.