Synthesis And Applicatio Ns Of Carbon Dots And Their Composites

Carbon dots(CDs)as a novel carbon-based material discovered in recent years,has attracted extensive research due to their excellent luminescent properties.Compared with traditional semiconductor quantum dots and organic dyes,CDs not only maintain the advantages of low toxicity and good biocompatibility of carbon-based materials,but also have incomparable advantages such as adjustable luminescence range,good light stability,no light scintillation,easy functionalization,low cost and easy large-scale synthesis.Although the development of CDs has made great progress over the past decade,there are still many key problems unsolved,mainly in the following aspects:(1)Due to the variety of precursors and synthesis methods,there is no clear explanation for the luminescence mechanism of CDs;(2)Aggregation-induced luminescence quenching resulting in weak or almost no fluorescence of carbon dot in solid state,which greatly limits their applications.Therefore,this paper focuses on the synthesis,luminescence properties and applications of CDs and carry out the following work:1.Exploring the luminescence mechanism of CDs.According to the theory of band engineering,the luminescence of CDs can be regulated by controlling the degree of dehydration and carbonization of precursors.In this work,the green fluorescence emission CDs were synthesized by solvothermal synthesis using glycerol as solvent and citric acid and urea as precursors for the first time.Water,glycerol and DMF as solvents increase the degree of dehydration and carbonization of precursors in turn.Therefore,as prepared CDs can be directly affected by adjusting the mixing ratio of the three solvents.According to the theory of energy band regulation,the size of sp~2 conjugate domain increases with degree of carbonization lead to the fluorescence emission of CDs redshifted.The preparation of full-color emissive CDs provides a method to control the fluorescent color of CDs,which is of great significance to the development of carbon dot.2.Achieving solid-state luminescence of CDs.Sodium silicate was used as the dispersing matrix for the first time.CDs were dispersed in the matrix materials by microwave curing,and full-color emissive CD-based phosphors were successfully prepared.By microwave irradiation of the mixture of CDs and sodium silicate,sodium silicate dehydrates rapidly to form a stable silica network structure,and CDs in sodium silicate are fixed in situ in the silica network structure.In addition,the sodium ions in sodium silicate can form metal cations-surface-functionalization and enhance the photoluminescence quantum yield of CDs.Furthermore,the blue,green and red fluorescent CDs were mixed with sodium silicate simultaneously,and the homogeneous white emissive CD-based phosphors with different color temperatures were successfully prepared by adjusting the mixing ratio of CDs.The preparation of full-color and white light emissive carbon dot-based phosphors will promote the application and development of CDs in solid-state lighting and visible light communication.3.Exploring the application of carbon dot-based phosphors in solid-state lighting and visible light communications.Phosphors have important application value in the field of solid-state lighting.After the preparation of full-color and white light emissive carbon dot-based phosphors,we further applied them in the field of solid-state lighting as the light converter of phosphor conversion LED.Full-color emissive CD-based LED were achieved by combine full-color emissive carbon dot-based phosphors with a 395nm InGaN chip.A double converter WLED with a color rendering index of 82.4 was fabricated by combine red and green emissive carbon dot-based phosphors with a 395nm InGaN chip.WLEDs with color rendering index of more than 85 were fabricated by combine white light emissive CD-based phosphors with a 375 nm InGaN chip.In the field of visible light communication,short luminescent lifetime of the light converter phosphors is necessary to have a high response speed.The fluorescence lifetime of the white emissive CD-based phosphors is about 8 ns,which have a great potential of applicating in visible light communication.Using 405 nm laser diode as excitation source,the visible light communication based on white light emissive CD-based phosphors is realized,with a modulation bandwidth of 75 MHz and a data transmission rate of 224 Mbps for the first time.By filtering the excessive excitation light with 420 nm filter,the blue light hazard can be effectively avoided and healthy lighting is realized.4.The phosphorescent properties,luminescence mechanism and application of CDs@PVA composites were studied.In addition to excellent fluorescence properties,CDs also have phosphorescent emission.In this work,thermal-treatment controlled room temperature phosphorescence of CDs@PVA composites was realize for multilevel data encryption.The thermal-treatment controlled phosphorescence has been attributed to the transfer of photoexcitation from the excited singlet state to the triplet state through intersystem crossing,followed by radiative transition to the ground state,which is due to decrease of quenchers(oxygen)in the CDs and suppression of the vibrational dissipations through the chemical bonding of CDs in the PVA matrix.A concept of thermal treatment controlled multilevel fluorescence/phosphorescence data encryption is realized using CDs@PVA composites,which can be utilized for design and development of novel composite materials for anti-counterfeiting and data encryption.