Photoluminescence Mechanism And Applications Of Multi-Color Fluorescent And Highly Efficient Red-Emitting Carbon Dots

Carbon dots(CDs),as a new type of carbon zero-dimensional nanomaterials,has attracted extensive attention due to its advantages such as stable and adjustable fluorescence(FL)properties,excellent biocompatibility and low toxicity.In recent years,carbon dots have gradually become candidate materials for a large range of promising applications such as electroluminescent devices,biosensing,bioimaging,optical anti-counterfeiting and photocatalysis.Despite more than a decade of development,there are still some key issues in the research on carbon dots that have not been solved,mainly in the following aspects:(1)The photoluminescence(PL)mechanism of carbon dots is so complex that it is still under debate at present,lacking clear and accurate explanation.(2)Long wavelength regional luminescent carbon dots are extensively used in biological and information encryption applications.However,there are few reports on red-emitting carbon dots(R-CDs)with high FL quantum yield.(3)The effect of fluorescence properties of carbon dots on the photocatalytic activity of the composites remains unclear,which limits the application of carbon dots in the photocatalytic direction.(4)The research on red room temperature phosphorescent(RTP)emission material has been reported less,and the full-color information encryption system application is imperfect.Herein,a series of multi-color fluorescent carbon dots and highly efficient R-CDs are prepared by solvent heat treatment.The effects of reaction precursors and synthesis conditions on the fluorescence properties of carbon dots are systematically investigated,and the PL mechanism is further studied.Based on this,fluorescent carbon dots have been applied as precursors to prepare composite materials,which are applied in photocatalysis and anti-counterfeiting encryption fields.1.Synthesis,properties and photocatalysis application of multi-color fluorescent carbon dotsFour types of CDs with various color(blue,green,yellow,and red)emissions are prepared under solvent-free conditions from citric acid and different nitrogen sources(N,N-dimethylformamide,urea,ethanamide,formamide).By detailed characterization and comparison,the PL mechanism of CDs is explored in depth,namely the graphitized sp² conjugated domain and surface functional groups such as C=O and C=N play synergetic roles in adjusting the fluorescence properties.Notably,the size effect is not the dominant mechanism to achieve multi-color fluorescence emissions in this work.The structural configuration of the carbon dots further influences the energy band structure,as demonstrated in simplified energy level diagrams.An absorption peak at nearly 560 nm appears in the visible light region for red-emitting CDs,assigned to an n??*transition of the aromatic structure,thus introducing a new surface state energy level,resulting in a reduction in the energy of electron transition and the expansion into the visible region of the UV-Vis spectrum.Taking advantage of the diverse absorption and emission properties,different CDs/Ti O₂ binary composite materials are obtained for photocatalytic degradation of organic dyes,and it is found that the absorption range in terms of visible light and the band gap of the carbon dots make a difference to the photocatalytic performance of the composites.2.Synthesis,properties and room-temperature phosphorescence application of highly efficient red-emitting carbon dotsTaking amides(formamide,N,N-dimethylformamide),acids(methyl acetate,acetic acid),ketones(propanone),alcohols(methyl alcohol),aldehydes(formaldehyde)and H₂O as the media,five kinds of R-CDs and multi-color carbon dots(yellow-,green-and blue-emitting)are synthesized by solvothermal method by heating citric acid and formamide.Results show that(1)solvent protection is the key factor for the synthesis of red carbon dots with high fluorescence quantum yields;(2)the red emission of R-CDs is due to the synergistic effect between the carbon core state and surface state.The carbon core state accounts for almost 80%,which derive from the large sp² conjugated domain and high content of graphitic nitrogen structure.By establishing nitrogen-doping models and using DFT calculation,it is further demonstrated that the strong p-?conjugation effect caused by nonbonding electrons of graphite N would facilitate HOMO-LUMO splitting of carbon core,thus leading to the significant decrease of HOMO-LUMO gap and realizing intense and independent red emission of carbon dots.Besides,surface groups can also generate red emission by the formation of N,O-related defect state.Finally,a new type of red RTP composite material(R-CDs@PVA)is prepared by combining the synthesized CDs with polyvinyl alcohol(PVA).Energy level splitting in R-CDs structure may lead to a certain degree of energy level crossing,thus reduce energy band gap(?E_ST)between the excited singlet and the excited triplet,which contribute to effective intersystem crossing.Based on this,the multi fluorescence/phosphorescence information encryption systems are constructed.