High-yield Regulation Of High Fluorescence Quantum Yield Carbon Quantum Dots

White light-emitting diodes(white LED)are the first choice for nextgeneration lighting equipments owing to their long life,small size,high luminous efficiency and low energy consumption.Compared with traditional fluorescent materials(such as rare earth phosphor and semiconductor quantum dots)for white LED,carbon quantum dots(CQDs)have the advantages of good water solubility,excellent optical properties,unique photo-bleaching resistance,environmental friendliness and low price,is expected to be a costless and toxicityless fluorescent material for white LED.However,there are two basic problems in the white LED application of CQDs.First,the fluorescence quantum yield(QY)of CQDs is low,and can not meet the brightness and efficiency requirements of white LED;second,the product yield(PY)of CQDs is low,limiting their large-scale production.Therefore,this work adopted hetero-atom doping and the material with spatial steric effect or carbon-carbon bonds as the reaction material to prepare CQDs with both high QY and high PY.The detailed research contents and results are as follows:(1)Aiming at the low QY and low PY problems of CQDs,citric acid rich in oxygen-containing functional group was used as carbon source,and the silane coupling agent KH-792 with spatial stereo effect was used as nitrogen dopant,one-step hydrothermal method was adopted to obtain blue CQDs(b-CQDs)with high QY and high PY.By optimizing the reaction temperature,reaction duration and acumulative dialysis duration,the optimal reaction parameters were reaction temperature of 200°C,and reaction duration of 6 h,acumulative dialysis duration of 3 h.The synthesized b-CQDs were spherical,with average particle size of 2.57 nm.Researches showed that: citric acid is rich in oxygen-containing functional groups,along with KH-792 as a nitrogen dopant,increasing the electron cloud density of CQDs' surface,and thus promoting the high QY of CQDs;KH-792 has a large three-dimensional structure,which is conducive to the structural stability of CQDs,further ensuring the high PY of CQDs.bCQDs were used as a phosphor for white LED with a color rendering index(CRI)of 84,a correlated color temperature(CCT)of 8285 K,and color coordinates of(0.29,0.32).The prepared white LED belongs to cool white light,which is suitable for office and outdoor lighting.(2)The preliminary results in our group indicated that the fluorescence of the CQDs is mainly derived from surface state luminescence.Therefore,in order to explore the influence of the raw materials' structure on the QY and PY of CQDs,CQDs were synthesized using raw materials with different structural compositions,and their structures and properties were characterized.The factors affecting QY and PY of CQDs were explored.The results showed that the more the content of oxygen-containing functional groups(C=O)and nitrogen-containing functional groups(pyrrole N),the higher the QY values of synthetic CQDs;the higher the carbon atom content,the higher the PY value of the synthesized CQDs within the scope of this study.(3)For the low QY and low PY problems of long-wavelength CQDs,2,7-dihydroxynaphthalene with high carbon atom content was used as carbon source,ethylenediamine was used as nitrogen dopant,one-step solvothermal method was adopted to obtain green CQDs(g-CQDs)with high QY and high PY.The QY and PY are up to 62.98% and 70.90%,respectively.The synthesized g-CQDs were spherical,with an average particle size of 3.31 nm.By optimizing the amount of ethylenediamine,the reaction temperature and the reaction duration,the optimum reaction parameters were the amount of ethylenediamine of 4 mL(when the 2,7-dihydroxynaphthalene is 0.4 g),the reaction temperature of 180°C,and the reaction duration of 12 h.Researches have shown that ethylenediamine as a nitrogen dopant,increases the electron cloud density on the surface of gCQDs,contributing to the high QY of g-CQDs;2,7-dihydroxynaphthalene has a large number of carbon atoms,which is beneficial to the structural stability of g-CQDs,further ensuring the high PY of g-CQDs.g-CQDs were used as a phosphor for white LED with a CRI of 87,color coordinates of(0.45,0.37),and a CCT of 2520 K.The prepared white LED belongs to warm white light,suitable for office lighting.Through the above results,two strategies,using hetero-atom doping and a material with spatial steric effect or high carbon atom content as a reaction material,were used to synthesize blue and green CQDs with high QY and high PY.The white LEDs made by the two classes of CQDs are suitable for outdoor and indoor lighting,respectively.The reaction materials used in this paper are low-cost and reaction method is simple.This research provides a certain data foundation and theoretical analysis for the mass preparation and commercial applications of CQDs.