Study On The Synthesis And Optical Properties Of Doped Fluorescence Carbon Quantum Dots

Carbon quantum dots(CQDs),also known as carbon dots,have been widely used in bioassays,light-emitting diodes,photocatalysis and other fields due to their better chemical stability,biocompatibility,low toxicity,and ease of chemical modification than traditional quantum dots.However,current CQDs still face several issues that need to be addressed,thereby limiting their application potential.Firstly,there are few reports on CQDs with fluorescent double-emission or multi-fluorescence emission,and the luminescence adjustability of CQDs cannot be achieved.Secondly,few reported CQDs emit light in the red band,which is limited to some extent for the application in living biological cells;finally,the luminescence mechanism of CQDs is still an open topic,and there is not enough experimental evidence to give a clear definition.Studies have found that doping heteroatoms into CQDs is one of important means to solve current problems.In this thesis,CQDs of multiple fluorescence emission of single-element and two-element doping were realized by different experimental methods using green and cheap carbon sources,and their physical properties and luminescence mechanism were explored.The effects of doping element,hydrothermal reaction conditions and solvents on their luminescent properties were investigated,and the mechanism of influence and luminescence mechanism were revealed.The innovative research results obtained are as follows:Firstly,in order to find a cheap,environmentally friendly carbon material,dry plant green leaves are used as a carbon source,and N,N-dimethylformamide(DMF)is used as a solvent to pass a simple green solvothermal method.Bright blue fluorescent CQDs were prepared.Studies have shown that the nitrogen element in the leaves of plants can achieve the doping of CQDs.The N element can introduce the energy level of N between C? and C?*.The existence of this energy level determines the luminescent center of CQDs.In addition,the presence of oxygen-containing functional groups and quantum confinement effects on the surface of CQDs can cause CQDs to exhibit significant excitation wavelength dependence.The use of ethanol modification can reduce surface defects of CQDs and attenuate the excitation wavelength dependence of CQDs.This work provides a new idea for the selection of green carbon sources for the preparation of CQDs and provides new ideas for reducing the excitation wavelength dependence of CQDs.Secondly,in order to extend the application range of CQDs and realize the dual fluorescence emission of CQDs,the CQDs are doped with two elements.A one-step hydrothermal method using a citric acid(CA)as a carbon source and a reactant of 1,2,4-triaminobenzene dihydrochloride(BD)as a nitrogen-chlorine co-doping source was successfully prepared.Ultra-small size nitrogen-chlorine co-doped CQDs with unique properties of blue-green light double emission.The CQDs have good dispersibility,high degree of carbonization,and double fluorescence emission peaks.By adjusting the molar concentration ratio of CA to BD and the reaction time,the particle size and luminescence peak position of the nitrogen-chlorine co-doped CQDs can be changed accordingly.Studies have shown that both chlorine and nitrogen elements form new energy levels between C? and C?*.The blue and green light of nitrogen-chlorine co-doped CQDs are derived from the conjugated ?-domain and Cl correlation states,respectively.The luminescence peak position and particle size are attributed to the synergistic and competitive mechanism of the conjugated ?-domain,the quantum confinement effect,and the correlation state of Cl.This work provides a new method for diatomic doping of CQDs,broadening the thinking for explaining the luminescence mechanism of CQDs.Finally,in order to reduce the experimental cost and simplify the experimental procedure,the 1,2,4-triaminobenzene dihydrochloride(BD)was used as a carbon source and a doping source material,and deionized water was used as a solvent.The idea of using dual element doping in the past to achieve dual emission of fluorescence is broken.The N-doped fluorescent double-emitting CQDs are synthesized quickly and simply by hydrothermal method.Among them,the emission at 581 nm is attributed to the energy level correlation between N and C?*,and the blue fluorescence emission at 473 nm is derived from the eigenstate luminescence of the internal conjugated ? domain of CQDs.In addition,the fluorescence emission of CQDs in the red light band is achieved by changing the reaction solvent from water to DMF.The red fluorescence in the N,N-dimethylformamide(DMF)solvent is attributed to the correlation state in which the O element forms O between C? and C?*.This work can promote the development of elemental doped CQDs with dual fluorescence emission and red fluorescence emission,providing backup materials of CQDs with excellent performance for bioimaging,sensors and optoelectronic devices.