Preparation And Tunning Luminescence Mechanism Of Fluorescence Carbon Quantum Dots

Carbon quantum dots(CQDs)are a new kind of carbon nanomaterial which was discovered in 2004.The main structure of CQDs is carbon nanoparticle with the size below 10 nm.Compared to the traditional semiconductor quantum dots,CQDs have attracted great attentions in scientific,industrial and medical applications due to its nontoxicity,bright fluorescence,chemical and optical stabilities,and biocompatibility.At present,the research focus of CQDs is mainly concentrated on metal ion detection,biosensors,fluorescent probes,and energy conversion,etc.Very little research has been reported on the mechanisms of the fluorescence enhancement and the tuning of CQDs.Up to now,the fluorescence intensity and wavelength of CQDs have also not been manual controlled yet.In this paper,the fluorescent CQDs were prepared by electrochemical and pyrolysis methods from citric acid.The morphology and fluorescence properties of CQDs were characterized and analyzed by TEM,XRD,XPS,UV-Vis spectroscopy and PL spectroscopy.To explore the fluorescence enhancement mechanism and the tuning method of CQDs,we try to enhance the fluorescence intensity of CQDs by means such as surface passivation and doping(such as N,S and P)of CQDs to find an effective way to improve the quantum yield of CQDs.The main contents and study results are as follows:1.The surfaces of CQDs prepared by two methods both contain a large amount of oxygenated functional groups such as-O-C=O,-OH,-COOH,etc.These hydrophilic groups make CQDs exhibiting good water solubility and CQDs can stably exist in aqueous solution.These characteristics can facilitate the surface and functional modification of CQDs.Moreover,these surface groups are the main factors that affect the fluorescence properties of CQDs.Therefore,we can tune the fluorescence wavelength by changing the surface modification of CQDs.2.Tuning of CQDs fluorescence wavelength:The particle diameter of water-soluble fluorescent CQDs synthesized by pyrolysis method with the addition of chemical modifier NH3.H2O solution is mainly distributed in the range of 1-5 nm and the average size of them is around 2.5 nm.The particle diameter of water-soluble fluorescent CQDs synthesized by pyrolysis method with the addition of chemical modifier NaOH solution is mainly distributed in the range of 0.8-3.2 nm and the average size of them is around 2 nm.The common characteristics of CQDs prepared by previous ways are well dispersibility,high crystallinity and structure stability.3.Tuning of CQDs fluorescence intensity:The doping of S atoms can enhance the fluorescence intensity of CQDs.When the sulfuric acid solution(H2SO4)with PH = 3 was added in citric acid solution,there have an obvious enhancement of fluorescence in S doped carbon quantum dots(S-CQDs)synthesized by pyrolysis.4.Through the band gap formula,we can calculate that the forbidden bandwidth of CQDs with a blue fluorescence prepared by pyrolysis method is 2.58 e V.Fluorescence quantum yield is an important index of the fluorescence properties of the prepared samples.With the calculation,we can obtain that the fluorescence quantum yields of CQDs synthesized by pyrolysis are 10.5%and 9.7%,respectively.