Design And Synthesis Of Carbon Quantum Dots And Their Nonlinear Optical Properties

Carbon quantum dots(CQDs)are one of the emerging carbon nanomaterials in recent years.Due to their abundant raw materials,low cost,simple preparation methods,good biocompatibility,and excellent optical properties,CQDs have attracted researchers’ extensive study.CQDs have good application prospects in photoelectrocatalysis,bioimaging and fluorescence detection etc.However,the previous researches are more to explore the optical properties of CQDs under low intensity light.There are few studies on their properties under high intensity light(laser).Such as the nonlinear optical properties and laser properties of CQDs.In this paper,different surface-modified CQDs are prepared by simple electrochemical and hydrothermal methods,and their nonlinear optical properties and laser properties are explored.The main work of this paper is as follows:1.Nitrogen doped carbon quantum dots(N-CDs)are prepared by a simple one-step hydrothermal process and the nonlinear optical properties of N-CDs are studied.When the pulse wavelength is 532 nm and pulse width is in picosecond regime,N-CDs show a switch from saturable absorption to reverse saturable absorption with the increase of pulse energy.At lower pump energy,the nonlinear optical property is dominated by saturable absorption.As the pump energy increases,reverse saturable absorption begins to appear,and eventually plays the dominant role.The saturable absorption is attributed to the ground state photobleaching,and the reverse saturation absorption is mainly attributed to the free carrier absorption/two photon absorption of N-CDs.In nanosecond regime,N-CDs show a typical reverse saturation absorption,which is mainly due to the excited state absorption of N-CDs.We also pointed out the nitrogen doping and the carbonyl groups on the surface of CQDs play a dominant role in its nonlinear optical switching behavior.2.Different surface functionalized carbon quantum dots/rhodamine B(CQD/Rh B)composites are prepared by simple electrochemical methods and hydrothermal process,and their random lasing properties were explored by varying the pump energy.The results show that different surface functionalized CQD/Rh B composites exhibit different laser properties.At a pump energy of 1.86 m J,phosphate groups functionalized carbon quantum dots/rhodamine B(PO4-CQD/Rh B)composite exhibits a linewidth of 3.2 nm at 587 nm.We also demonstrate that the p H value of CQDs solution play an important role on the lasing behavior of CQD/Rh B composite.3.We have prepared a nitrogen and sulfur co-doped carbon quantum dots(N,S-CQDs)by a hydrothermal method.The fluorescence quantum yield can reach about 73 %.The experimental results also show that N,S-CQDs can be used as Fe3+ detection probe with high selectivity and sensitivity.The detection limit is about 0.2 μM.