| Carbon quantum dots(CQDs)have been widely used in many fields,such as biological imaging,photocatalysis,fluorescence sensing and so on,due to their wide band absorption(absorption extended to visible light),excellent optical stability,low preparation cost,wide source of synthetic materials and good biocompatibility.Current research has been shown that the fluorescence properties of CQDs can be regulated by changing the size and doping.However,the mechanism of regulating the fluorescence properties of CQDs by changing the size and doping is not clear,which seriously restricts the further application of CQDs.In order to solve this problem,this thesis focuses on the effects of size and nitrogen atom doping on the fluorescence properties of CQDs,and reveals the effect mechanism of size and nitrogen atom doping on the fluorescence properties of CQDs.The results can provide a theoretical basis for the controllable adjustment of fluorescence properties by changing size of CQDs and doping.The main research contents are as follows:1.The effect mechanism of size on the fluorescence performance of CQDs was clarified.Using citric acid as the precursor,the CQDs with size distribution of 1?5 nm were synthesized by one?step hydrothermal method and four kinds of CQDs with different mean size were separated.Transmission electron microscopy(TEM)shows that the average size of the four kinds of CQDs were?1.5,2.3,2.8 and 3.4 nm,respectively.The result is further confirmed by electrophoresis,2D excitation-emission contour map,and mass spectrometry analysis.The results of Fourier infrared spectroscopy(FT?IR),Raman spectroscopy,and X?ray photoelectron spectroscopy(XPS)show that the synthesized CQDs have similar oxygen?containing functional groups,C/O ratio and surface defects.The UV?Vis spectrum shows two obvious strong absorption peaks and one weak absorption peak.The position of the three peak are about 220 nm,300 nm and 360 nm,respectively.The absorption peak near 220 nm can be attributed to???*electron transitions of C=C within sp~2 domain;with the increase of the size of CQDs,the position of the absorption peak near 220 nm in UV?Vis spectrum gradually red shift,indicates that the increase of size results in the narrowing of???*band gap;the absorption peak at 300 nm belongs to the n??*electron transitions of the surface functional group,which is almost independent of the size of CQDs,but the strength will increase with the increase of the size;the weak absorption peak near 360 nm is caused by the interaction between particles.The fluorescence excitation spectrum(PLE)shows that there are also n?s*electron transitions in these four different sizes of CQDs.The PLE spectrum also shows that when the size of CQDs are small(less than 2.8 nm),???*electron transitions dominants;when the size of CQDs is larger(more than 2.8 nm),n??*electron transitions dominant.In addition,with the increase of size,the characteristic excitation band originated from???*electron transitions red shift.The PL spectrum study shows that there are two fluorescence emission centers in CQDs,423?443 nm and 463 nm respectively;the fluorescence emission peaks between 423?443 nm can be attributed to the electron transitions of n?s*and???*,the positions red shift with the increase of size;The fluorescence emission peak of 463 nm can be attributed to the n??*electron transitions,and its position hardly changes with the increase of size,indicating that the change of size mainly affects the???*electron transitions.Theoretical calculation and electrochemical analysis further prove that the change of size will result in the change of band structure and???*band gap.2.The effect mechanism of nitrogen atom doping on the fluorescence performance of CQDs was clarified.Using hexamethylenetetramine as nitrogen source and citric acid as carbon source,CQDs and nitrogen doped carbon quantum dots(NCQDs)were synthesized by one?step hydrothermal method.TEM show that both of them have similar size distribution.The mean size is?2.9±0.2 nm;FT?IR,Raman and XPS show that the CQDs and NCQDs have similar oxygen?containing functional groups,carbon oxygen ratio and surface defects.UV?Vis spectrum shows three characteristic absorption peaks.The maximum absorption peak near 218 nm belongs to???*electron transitions in sp~2domain.The absorption peak near 300 nm is attributed to???*electron transitions at the edge of sp~2 domain and n??*electron transitions dominated by surface group.After doping,the absorption peak near 300 nm shows a blue shift of10 nm,and the absorption peak near 360 nm belongs to the electron transitions in the surface region(due to the interaction between particles).After doping,a blue shift of 6nm occurs.The results show that doping has a greater influence on the absorption peak near 300 nm.In addition to the n??*transitions dominated by the surface group,the???*electron transitions at the edge of sp~2 domain also contribute to the peak near 300nm.The blue shift of the excitation band is also shown in the PLE spectrum,and the intensity of the excitation band near 350 nm is the largest,indicating that the surface group and the edge of sp~2 domain are the dominant emission centers.Meantime,the PLE spectrum of NCQDs shows an extra excitation band,which corresponding to the unpaired electron transitions on the nitrogen atom.PL spectrum and fluorescence lifetime reveal that there are two emission centers in CQDs and NCQDs.The emission band near 430 nm corresponding to the fluorescence emission center of sp~2 domain,after doping,location hardly shift;the emission band at 452 nm and 465 nm corresponding to the fluorescence emission center dominated by the surface group,the edge of sp~2 domain and the surface region,after doping,a significant blue shift occurs,indicating that doping has a greater impact on the second fluorescence emission center.In addition,theoretical calculation reveals that the blue shift of PL emission is due to the doping of the pyridine nitrogen.Doping changes the charge distribution which results in the change of the orbital energy and the broadening of band gap.The conclusions are consistent with the results obtained by UV?Vis spectroscopy and electrochemical analysis. |
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