Investigations On Synthesis Of Multimodal Emission Graphene Quantum Dots And On Their Luminescence Mechanism And Optical Applications

Graphene quantum dots(GQDs),as a zero-dimensional derivative of graphene materials,have a graphene-like frame structure,usually formed from 3-5 layers of graphene to form crystalline nanostructures,with transverse dimensions extending to60 nm and edge structures containing a variety of functional groups.GQDs can be obtained from either graphene-based precursor materials or materials with graphite-like structures.Due to the excellent characteristics of quantum confined effect,boundary effect,inertia,good solubility,photoluminescence,biocompatibility,light stability and high electron mobility,GQDs is expected to become an environmentally friendly alternative to traditional quantum dots,which has attracted extensive attention from a large number of researchers.However,rapid synthesis of GQDs with long wavelength emission and high absolute quantum yield(PLQY)and high quality white light emission under long wave excitation remains challenging.In this paper,multi-peak emission(long wave)GQDs with high PLQY were synthesized by one-pot solvothermal method.The photoluminescence(PL)mechanism of GQDS was studied and analyzed,and they were applied to biological cell imaging and warm white light emitting devices respectively.The main research contents and results are as follows:(1)GQDs with high PLQY and red light emission(R-GQDs)were prepared by one-pot solvothermal method using 2,7-dihydroxynaphthalene(2,7-DHN)and anhydrous piperazine(PA)as precursors and heated in ethanol at 180℃and 3 h.2,7-DHN molecules have aromatic rings rich in sp~2 hybridized carbon domains,which lead to nuclear state emission(green light)of R-GQDs,while PA molecules produce surface state emission(red light)after dehydration condensation reaction with 2,7-DHN.In contrast to the previously reported preparation methods of red light emitting GQDs,our work does not require the addition of acids or bases in the reaction,nor does it require complex silica gel column separation for post-reaction processing.The results show that the red emission of R-GQDs is 590～650 nm,and its PLQY is as high as 54.9%in ethanol and 24.9%in aqueous solution.In addition,it shows narrow size distribution,dual emission centers,concentration-dependent fluorescence emission,low toxicity and good biocompatibility,which we have successfully applied to cell imaging(2)Using the same carbon source and reaction solvent,extend the reaction time to 10 h,one-step solvothermal synthesis of GQDs with white light emission(W-GQDs)under ultraviolet light,its high carbonization makes its structure and luminescence properties slightly changed,using its multi-emission center to prepare white light materials for white light-emitting diodes(WLED)assembly,the preparation process is simple and low cost.The prepared WLED has excellent photoelectric performance and belongs to warm white light,which can be used for indoor lighting and help relieve eye fatigue.