Preparation,Optical Mechanism And Applications Of Tunable Multicolor Carbon Dots

Multicolor carbon dots(MCDs)have become one of the most promising optical materials due to their advantages such as tunable excitation/emission,excellent optical properties,and good biocompatibility.Although the research on MCDs has attracted much attention,there are still the following challenges:1.The simple and efficient synthesis strategy and luminescent mechanism of MCDs.Most MCDs are prepared by means of precursor screening and chromatographic separation,resulting in complex and tedious preparation processes,low product yields and low optical properties.Since the luminescent mechanism of MCDs is not yet clear,it is impossible to theoretically guide the synthesis of MCDs.2.One-step synthesis strategy of white light carbon dots.At present,white light carbon dots are mainly prepared by mixing the ratio of MCDs according to the principle of color complementarity.However,in the actual preparation process,there are often problems such as difficulty in ratio control,poor repeatability,and poor color rendering performance.3.Synthesis of matrix-free solid-state multicolor carbon dots.The addition of the matrix can effectively prevent the fluorescence quenching of carbon dots in the solid state,but it also affects the luminous efficiency due to the limited loading of carbon dots.4.Development of multicolor phosphorescent carbon dots.At present,the research on multicolor phosphorescent carbon dots is still in the initial stage,and there are few related reports,and the synthesis method and performance improvement are not perfect,which limits its wide application.In response to the above problems,this paper first proposed a simple method to realize MCDs only through time regulation,and explained the synthesis mechanism and luminescent mechanism,and then applied the MCDs to the detection of water content in organic matter,multiple anti-counterfeiting and information encryption,LED lighting and other fields.On this basis,white light carbon dots were successfully prepared by one-step method by adjusting the ratio of precursor materials,and the luminescence mechanism was elucidated,and white light LEDs with high color rendering index were prepared.In addition,a one-step solvothermal method was used to prepare matrix-free MCDs,and its formation mechanism and luminescence mechanism were studied,and used for multicolor LEDs illumination and fingerprint identification.Finally,in the case of the same single precursor,the two-color carbon dots with ultra-long phosphorescence lifetime can be obtained by changing the matrix to achieve advanced anti-counterfeiting and information encryption.The specific work is as follows:(1)Using o-phenylenediamine and L-tryptophan as precursors,solvothermal method was used to synthesize green,yellow and orange emitting light with quantum yields as high as 56%,47%and 34%,respectively,only by adjusting the reaction time.Structural characterization and optical studies show that prolonging the reaction time can oxidize amino nitrogen to graphitic nitrogen,increase the degree of graphitization and surface state,increase the sp~2 conjugated structure,thereby reducing the energy level band gap,and finally lead to a red shift of the emission wavelength.The formation mechanism and luminescent mechanism of the MCDs were further verified by theoretical calculations using the density functional method,and the mutual verification of theory and experiment was realized.Due to the discoloration effect of MCDs in different solvents,it shows excellent water detection ability in organic solvents,and realizes multiple anti-counterfeiting and information encryption.In addition,MCDs were combined with PVP(polyvinylpyrrolidone)for monochromatic LEDs illumination,and the results showed that the MCDs had ideal luminous efficiency.(2)White light carbon dots(w-CDs)were successfully prepared by one-step method by adjusting the material ratio of o-phenylenediamine and L-tryptophan.The combination of thin-layer chromatography and fluorescence emission spectrum shows that the white light carbon dots are composed of two luminescent centers that emit blue light and yellow light.After compounding w-CDs and PVP into thin films,it is found that it has good photothermal stability and photobleaching resistance,and is an ideal material for preparing white light LEDs(WLEDs).The composite material was subsequently used to fabricate WLEDs.The results show that the color coordinates of the WLED are(0.32,0.35),close to pure white light(0.33,0.33),the correlated color temperature(CCT)is 4986 K,and the color rendering index(CRI)is 87.2,which is higher than the national minimum standard of 80,Exhibits excellent luminous efficiency.(3)The preparation of solid-state luminescent CDs was achieved by one-step solvothermal method without adding any matrix,and the maximum emission peak of CDs was red-shifted from 535 nm to 615 nm by adjusting the particle size of CDs,the degree of graphitization,and the content of graphite nitrogen.The quantum yield(QY)reached 28%,16%and 12%.Characterized by FT-IR,XPS,fluorescence lifetime and Zeta potential,etc.,it is revealed that the abundant functional groups on the surface of multicolored carbon dots can prevent theπ-πstacking effect of the carbon core in the aggregated state.Based on the excellent luminescent properties of the carbon dot powder,the rapid identification of latent fingerprints and the preparation of multicolor LEDs were successfully realized.The results show that the MCDs have ideal luminous efficiency and high display details for latent fingerprint imaging.(4)Biuret was selected as the precursor,and boric acid and inorganic salt matrix were added respectively to prepare two kinds of phosphorescence emitting carbon dots.The phosphorescence lifetimes were as long as 418.3 ms and 156.6 ms respectively,and the quantum yields were as high as 22%and 18%.Through a variety of characterization methods(such as FT-IR,XPS,XRD,etc.),studies have shown that the carbonyl and nitrogen heterocycles on the biuret are the main cause of phosphorescence emission.In addition,the halogen and covalent bonds between the carbon dots and the substrate also affect the phosphorescent emission color.The interaction between the matrix and the Carbon dots endows the room-temperature phosphorescent Cdots with a dense and rigid environment,resulting in good photostability.Finally,the application prospect of multicolor phosphorescent carbon dots in the field of room temperature phosphorescence anti-counterfeiting is explored by using the room temperature phosphorescence conversion performance,and the results show that it has broad application prospects.These research results provide a new idea for the preparation of efficient and stable room temperature phosphorescent emission materials,and also provide a reference for the development of new phosphorescent anti-counterfeiting materials.