Synthesis And Properties Of Solid-State Carbon Dots-based Phosphors And Their Application On White LED

As a kind of potential luminescent nanomaterial,carbon dots(CDs)are widely used in anti-counterfeiting,biological imaging,white light-emitting diode(WLED)and other fields due to their excellent luminescent properties,simple synthesis method,good stability,and biocompatibility.Due to the diversity and complexity of the surface states of CDs,the fluorescence mechanism is not clear.Meanwhile,the transformation of CDs from solution to solid is often affected by the direct?-?interaction and fluorescence resonance energy transfer(FRET)in the solid-state CDs,which leads to the generation of solid-state aggregation-induced luminescence quenching effect,and greatly limits its application in the field of optoelectronic devices.In view of the above problems,solid-state CDs-based phosphors were used as the research object,and solid-state CDs-based phosphors with high stability were successfully prepared by different processes and raw materials in this paper.In terms of fluorescence mechanism,pure and single functional group raw material was used to prepare solid-state CDs-based phosphors with pure surface,which eliminated the interference of other factors.In terms of solid-state fluorescence performance,through the adjustment of raw materials,synthesis process and the selection of matrix materials,the long-wavelength multi-peak emission,excellent stability and high photoluminescence quantum yield(PLQY)of solid-state CDs-based phosphors were realized,thereby solving the problem of fluorescence quenching of CDs in aggregation,and its application prospect in white LED devices were also discussed.The detailed research results are as follows:1.Using polyvinyl alcohol(PVA)with pure surface functional group as raw material and matrix carrier of CDs to prepare yellow CDs@PVA phosphor(Y-CDs)by one-step hydrothermal method in alkaline medium,and explore the fluorescence mechanism.By optimizing the molecular weight of precursor,reaction temperature,sodium hydroxide content and other parameters,yellow Y-CDs phosphor with the best optical performance was obtained.The experimental results show that the participation of sodium hydroxide accelerated the oxidation and decomposition of PVA,and increased the surface oxidation degree of CDs,causing the fluorescence emission to shift from blue to yellow.The long-chain structure of partially reacted PVA serves as a matrix carrier to wrap the CDs,which effectively increases the distance among CDs particles,and avoids fluorescence quenching caused by the aggregation of CDs,so that the CDs maintain fluorescence emission in solid-state.As-prepared yellow Y-CDs phosphor exhibits good photo stability.When it combines with a blue LED chip,the fabricated WLED exhibits the color rendering index(CRI)of 68.7 and CIE coordinates of(0.27,0.26).2.In order to achieve long-wavelength emission and high thermal stability of solid-state CDs-based phosphors,a suitable matrix material was sought to prepare phosphors with high stability.Using citric acid and urea as raw materials,green light emitting CDs were synthesized by one-step solvothermal method,and then they were compounded with calcium hydroxide matrix material to prepare a new type of red-and green-emitting CDs@Ca(OH)₂ phosphor(RG-CDs)with PLQY9.67%.By analyzing the performance,morphology,structure,surface group analysis and theoretical calculation of CDs and RG-CDs dual-emission phosphor,it is found that the addition of calcium hydroxide introduces a large number of hydroxyl groups with strong electrons donating property.The electron cloud density in the conjugated system increases,the highest occupied molecular(HOMO)energy level increases and the band gap decreases,which eventually leads to the formation of green and red emission in RG-CDs phosphor,and shows excellent thermal stability and water stability.The single-matrix WLED with high color rendering was obtained by combining it with the blue chip.The correlated color temperature(CCT)is 4870 K,the luminous efficiency is 19.43 lm/W,the CRI is 92,and the CIE is(0.34,0.31),which is close to normal white light(0.33,0.33).In this work,calcium hydroxide was used as a matrix material to combine with CDs to successfully obtain stable performance RG-CDs dual emission phosphors,which provides a new idea for the research of solid CDs-based phosphors.3.In order to explore the role of hydroxyl in the synthesis of CDs,using citric acid,urea and potassium hydroxide as precursors,blue fluorescent CDs were prepared by one-step solvothermal method.Similarly,calcium hydroxide was used as the matrix material to combine with blue-emitting CDs.As a result,a novel CDs@Ca(OH)₂ solid-state phosphor(RGB-CDs)with PLQY 7.40%was successfully acquired and exhibits the exciting white light emission.Through systematically investigating its fluorescent properties,microstructure,surface functional groups and corresponding theoretical calculation,the possible mechanism for the formation of trichromatic fluorescence is revealed.Potassium hydroxide firstly reacts with citric acid in the raw material to form potassium citrate before generating CDs,which makes the content of graphite nitrogen reduce,and the energy gap increase,resulting in the enhanced blue emission.After combining with calcium hydroxide,the long-wavelength emission of CDs is dramatically enhanced due to the introduction of more hydroxyl groups and a reduced energy gap.Finally,a single-matrix RGB-CDs phosphor with white light emission was successfully realized.When it was combined with a UV LED chip,a WLED with CRI of 84.2,CIE(0.32,0.32)and CCT of 6058 K is obtained,indicating of its potential application in the field of optoelectronic devices.4.To improve the PLQY of solid-state CDs-based phosphors,green-emitting fluorescent CDs with high PLQY(35.5%)were prepared simply and quickly by using the high-energy microwave method.The CDs and Ca(OH)₂ were compounded to obtain a green-emittting CDs@Ca(OH)₂ phosphor(G-CDs)with high PLQY of 48.16%,which is about 5 times higher than the previously prepared red-and green-emitting G-CDs phosphor.Through detailed analysis of the experimental results,it is found that the increase of carbonization and the deprotonation ability of the solvent play a key role in the improvement of PLQY.Mixing this phosphor and K₂Si F₆:Mn red phosphor to prepare a light conversion layer and coating on the blue LED chip to obtain a WLED device with CRI of 80,CCT of 7893 K,CIE coordinate of(0.30,0.29),and light efficiency of 16.2 lm/W.