Study On Preparation And Properties Of High Thermal Stability Carbon Dots And Their Fluorescent Films

In recent years,carbon dots（CDs）have been widely used as fluorescent materials in light-emitting diodes（LEDs）due to their advantages,such as easy functionalization,mild reaction conditions,good optical properties,and environmental friendliness,showing their application potential in the field of solid-state lighting.However,compared with LED lighting,the fluorescent materials used for laser diode（LD）lighting not only need to have excellent fluorescent properties,but also need higher thermal stability.At present,the thermal stability of most CDs-based fluorescent materials can not meet the requirements of LD lighting,which greatly limits the application of CDs-based fluorescent materials in LD lighting.This thesis aims to prepare CDs-based fluorescent materials with high thermal stability for meeting the requirements of LD lighting.The fluorescent CDs were prepared by one-step hydrothermal method,and then annealed to improve their thermal stability.And then,three fluorescent films were obtained by integrating the annealed-CDs with excellent properties into three different film-forming agents,respectively.Finally,a LD lighting was achieved by combining a stable fluorescent film with blue LD.Through systematic characterization and analysis of the morphology,structure,optical properties and thermal stability of CDs before and after annealing and their film,the main results are shown,as follows:1.The yellow-green fluorescent CDs,with an emission peak of 532 nm,were prepared by optimizing the concentration of the reactants using citric acid and 2,7-dihydroxynaphthalene as raw materials.The optimal conditions in this research are as follows:the concentration of reactant is 103.5 mg m L^-1,the reaction temperature is 200℃,and the reaction time is 14 h.The fluorescence quantum yield of the obtained CDs is 49%.The prepared CDs were annealed,and the annealed-CDs still have bright fluorescence.Moreover,compared with the CDs before annealing,the annealed-CDs have better thermal stability with their weight loss of only 10%at250℃,and also have good dispersibility,suggesting they can be well combined with the film-forming agents for applying in LD lighting.2.Three fluorescent composite films were prepared by integrating the annealed-CDs into three different film forming agents（polymethyl methacrylate,polyvinyl alcohol and silane coupling agent KH-792）,respectively.The three composite fluorescent films have good light transmittance in the visible light range of 390–780 nm,reaching more than 40%;and for visible light with a wavelength greater than 600 nm,the light transmittance is greater than 50%.The annealed-CDs have no agglomeration in these films and are evenly dispersed,indicating that the CD composite films have the relatively good film-forming performance.The thermal stabilities of the three films are improved compared to CDs,especially the annealed CDs/KH-792 composite film loses less than 10%weight at 400℃,which can meet the thermal stability requirements of fluorescent materials in LD devices.A LD lighting device was fabricated by combining the optimal annealed-CDs/KH-792 film with 405 nm blue light LD,and its color coordinate is（0.47,0.37）,correlated color temperature is 2229 K,and color rendering index is82.In summary,the CDs with better thermal stability were obtained by annealing CDs prepared by the one-step hydrothermal method.And the compact,uniform,flat and thermally stable CDs-based fluorescent film was formed by the cross-linking polymerization of CDs and KH-792.This thesis will open up an advanced way for the application of CD fluorescent materials in LD lighting field.