| White light-emitting diodes?LED?,owing to their high luminous efficiency,small volume,long lifetime,low energy consumption and environmental friendliness,have been developed to be the most promising solid-state lighting devices.Compared with rare earth-based phosphors and semiconductor quantum dots phosphors,carbon dots?CDs?have been investigated in the field of LED lighting and displays as a novel generation of phosphor because of their excellent properties,including water solubility,low toxicity,strong absorption and wide emission in the deep ultraviolet region,excellent photoelectronic conversion ability,high electron mobility and superior fluorescent stability.To solve the problems of CD phosphor such as dull emission wavelength,low quantum yield?QY?and solid-state quenching resulting from aggregation,this thesis aims at realizing controllable synthesis of CDs with high QY,solid-state fluorescence and multicolor emission by a simple and rapid microwave-assisted hydrothermal method with optimizing synthesis technique using carbon source with different structures,including o-phenylenediamine?o-PD?,starch and citric acid?CA?.The micromorphology,surface structure and optical properties of CDs were characterized and analyzed,for promoting solid-state fluorescence of CD phosphors and their application in the field of white LED.The main contents and results of this thesis are as follows:?1?At present,most commercial yellow phosphors for white LED consist of non-renewable rare-earth materials or toxic heavy metal elements.In order to explore a low-cost,high-efficiency and environmentally friendly yellow fluorescent phosphor,yellow fluorescent CDs?Y-CDs?were synthesized by a one-pot microwave-assisted hydrothermal approach using o-PD as carbon source and urea as nitrogen source.The influences of reaction temperature and reaction time on optical property of Y-CDs were discussed.In the range of reaction parameters in this study,the optimal reaction parameters are as follows:reaction temperature is 140?and reaction time is 10 min.As-synthesized spherical Y-CDs with QY of 4.23%have the average diameter of 2.42±0.27 nm,good hydrophilic ability and the average fluorescent lifetime of 1.42±0.05 ns.It is suggested that o-PD is the primary source for forming carbon nucleus.The benzene ring with conjugated effect and NH2 groups with electron-donating ability in o-PD are expected to increase electronic cloud density,which is beneficial for synthesizing Y-CDs with low band gap,and thus realizing the long-wavelength emission.Urea as a nitrogen dopant effectively improves the QY of CDs.The as-prepared Y-CDs show an excitation wavelength-independent PL emission,good thermal ability and resistance against acid and alkali.Fluorescent Y-CDs in solid state were obtained by choosing polyvinyl alcohol?PVA?as dispersion matrix and encapsulation material,and Y-CDs were combined with blue chip(?ex=420 nm)to fabricate white LED,which realizing cold white light emission.This white LED has the color coordinates of?0.30,0.30?and the correlated color temperature?CCT?of 7915 K.?2?To obtain high efficiency and low cost white LED by UV excitation,with environmentally friendly and easily available biomass material??starch as carbon source,blue fluorescent CDs were synthesized by a one-pot microwave-assisted hydrothermal approach.The influences of reaction time,reaction temperature and the concentration of starch solution on fluorescent property of CDs were discussed.The optimal reaction parameters are as follows:reaction time is 30 min,reaction temperature is 220?co nacnedn ttrhaetion of starch solution is 1.0 mg/mL.As-synthesized spherical CDs with the average diameter of 3.0 nm have QY of 2.46%and the emission wavelength of 433 nm(?ex=360 nm).Nitrogen-doped CDs?N-CDs?were obtained by introducing ethylenediamine with nitrogen atom,and the QY of N-CDs doubled that of CDs.Near-spherical N-CDs with the average diameter of 4.5 nm have the emission wavelength of 460 nm(?ex=360 nm),and solid-stat N-CDs emit blue-green fluorescence.Besides,theCDsandN-CDshaveexcitation wavelength-dependent luminescence and excellent water solubility,and the N-CDs have larger red-green-blue?RGB?spectral component ratio and longer lifetime than CDs.To realize the solid-state fluorescence,CDs were dispersed in starch to form CDs/starch composite.Together with good thermal ability,white LED based on CDs/starch composite and N-CDs phosphor by using PVA as encapsulation material are achieved with CIE coordinates of?0.38,0.45?and?0.33,0.35?,and CCT of 4329 K and 5437 K,respectively.?3?To solve the aggregation-caused quenching?ACQ?effect and obtain solid-state fluorescence CDs for white LED,organosilane functionalized CDs?Si-CDs?with QY of 25.6%and RGB spectral component ratio of 64.9%were synthesized by a one-pot microwave-assisted hydrothermal approach using CA as carbon source and N-??-aminoethyl?-?-aminopropyl trimethoxysilane?KH-792?as distance barrier chains.The influences of the molar ratio of CA to KH-792 and reaction temperature on QY and RGB ratio of Si-CDs were discussed.The optimal reaction parameters are as follows:the molar ratio of reactants is 1:5 and reaction temperature is 180-?sy.nt hAe ssized quasi-spherical Si-CDs have the average diameter of 5?7 nm,an amorphous structure and good water solubility.Si-CDs solution with average fluorescent lifetime of 14.02±0.05 ns,emit blue-white light under UV light and shows excitation wavelength-independent fluorescent behavior.The QY of the solid-state Si-CDs emitting blue fluorescence reaches 65.8%,which is 2.5-fold that of their solution.This is because that Si-CDs particle can support each other to maintain an appropriate interval owing to the prominent steric hindrance of long chains of KH-792,which weakens the effect of aggregation and decreases?–?interaction,and thus gives the strong fluorescent powder.Together with their good film-forming ability,thermal stability and UV resistance,Si-CDs were applied in the field of fluorescent film,backlight displays and LED devices.White LED was fabricated using Si-CDs phosphor combined with UV LED chip,and exhibited cold white light with color coordinates of?0.32,0.36?and CCT of6071 K.To sum up,raw materials with conjugated structure is benficial for the synthesis of CDs with long wavelength emission.The introduction of nitrogen atom and modification of organosilane functional groups are helpful to improve the QY of CDs.Besides,the long chains of KH-792 with prominet steric hindrance can avoid the ACQ effect of solid-state CDs.Therefore,high QY,solid-state fluorescence and multicolor emission of CDs were prepared and realized by using carbon sources with different structures and optimizing synthesis technique. |
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