Synthesis And Property Modification Of Room Temperature Phosphorescence Carbon Dots With Boron And Nitrogrn Co-doping

Room temprature phosphorescence（RTP）,as an important photoluminescence phenomenon,recieves wide attention because of its long luminous lifetime,large Stokes shift and high environmental sensitivity.Carbon dots（CDs）,as a new kind of fluorescence nanomaterial,has many excellent properties such as simple preparation,abundant source,low raw material cost,low toxicity,good photostability,adjustable emission and easy functionalization modification.In the past few years,scientists discovered and prepared some phosphorescent carbon dot materials with RTP property,and applied them in the fields of anti-counterfeiting,information encryption,biological imaging,photoelectric devices and so on.However,in order to realize CDs RTP with high performance,there are still some issues to be solved.For example:（1）For most CDs,triplet exciton transition is prohibited at room temperature,corresponding radiative transition cannot be aquired;（2）The synthesis and purification of RTP materials is relatively complex and time-consuming;（3）The reported matrix-free RTP CDs generally mit short wavelength and be excited only by ultraviolet light,which greatly limits its applications in specific fields.Based on the influence of heteroatomic doping on phosphorescence properties,we use different types of organic acid molecules,boric acid and ammonia as precursors to construct two kinds of boron and nitrogen co-doped RTP CDs,which provides new approach and theoretical basis for phosphorescence materials synthesis,performance regulation and application.This paper mainly includes the following contents:1.Preparation of carbon dots using aspartic acid as carbon source and RTP propert studyB and N co-doped RTP CDs were prepared by one-step bottom-up microwave synthesis using L-aspartic acid,boric acid and ammonia as precursors.It is found that the doping of B element is beneficial to increase the emission wavelength and lifetime of phosphor,and the existence of B-C bond effectively reducesΔEST and promotes intersystem crossing（ISC）between S₁and T₁.By controlling the amount of ammonia,the ratio of N element and the content of graphite N in CDs can be adjusted.Thus,the wavelength of phosphorescent emission can be effectively controlled from 500 nm to 552 nm,and the lifetime and quantum yield can be improved.The longest lifetime is up to 754.05ms,and the quantum yield is up to 16.78%.The different colors of the carbon dots under visible light and their long lifetime indicate that the carbon dots have a wide application prospect in anti-counterfeiting and information encryption.2.Preparation of carbon dots with citric acid as carbon source and RTP property studyIn the third chapter,L-aspartic acid was replaced by citric acid as carbon source,and phosphorescent carbon dots were synthesized under the same reaction conditions,so as to explore the influence of different carbon sources on RTP properties.Through the analysis of the structure and optical properties,it is found that the emission wavelength,lifetime and quantum yield of carbon point using citric acid as carbon source are decreased compared with the L-aspartic acid based CDs in Chapter 2.This is because the content of N element and C=O,which play an important role in the process of intersystem crossing,are reduced in the carbon dot structure.Therefore,it is very important to select suitable carbon precursor to prepare RTP CDs with excellent performance.Similarly,effective regulation of phosphorescent emission wavelength（505 nm～545 nm）can also be achieved by controlling the doping amount of N element and graphite N,which further explains the regulation mechanism of N doping on luminescence properties of RTP CDs.In summary,based on the co-doping of B and N atoms,the improvement of RTP lifetime and quantum yield,as well as the regulation of emission wavelength can be achieved by regulating the content and structure of doping elements.At the same time,the excitation wavelength of the obtained RTP CDs is extended to the visible region,which is conducive to the application in specific fields.