荧光碳点的合成、发光机制、...WLEDs中的应用（英文）_岳劲松.pdf

Cite this:NewCarbonMaterials,2023,38(3):478-495DOI:10.1016/S1872-5805(23)60742-5A review of fluorescent carbon dots:synthesis,photoluminescencemechanism,solid-state photoluminescence and applications in whitelight-emitting diodesYUEJing-song1,YUANFang-yu1,QIUHan-xun1,*,LIYing1,LIJing1,XUEYu-hua1,YANGJun-he2（1.School of Materials Science and Engineering,University of Shanghai for Science and Technology,Shanghai 200093,China;2.Shanghai Jian Qiao University,Shanghai 201306,China）Abstract：Carbonnanomaterialswithasizeoflessthan10nm,fluorescentcarbondots(CDs),havebeenextensivelyinvestig-ated,duetotheirexcellentfluorescencetunability,goodbiocompatibility,widerangeofprecursorsandlowcost.Moreover,theirsimplepreparationandexcellentperformanceprovideforawiderangeofapplicationsinthefieldsofopticalsensing,energystorage,biomedicalimaging,andwhitelight-emittingdiodes(WLEDs).Alargenumberofsolid-statephotoluminescentCDshaverecentlybeendevelopedandusedinWLEDs.ThesynthesisstrategiesofCDsarebrieflysummarizedandtheirphotoluminescencemechan-ismsarereviewedaswellastherecentprogressfortheiruseinWLEDs.Finally,prospectsforsolvingthecurrentproblemsandchal-lengesofCDsforWLEDsarebrieflypresentedanddiscussed.Key words:Carbondots；Photoluminescence；Solid-statephotoluminescence；WLEDs1IntroductionWorking as a new type of high-efficient light-emittingsource,whitelight-emittingdiodes(WLEDs)openupatechnicalfieldforthelightingindustryandarefarsuperiortotraditionalincandescentlampsintermsofluminousefficiencyandperformance.Typic-ally,there are two routes currently to fabricateWLEDs:Coatingavarietyofmonochromaticphos-phorsonUVchips12orusingphosphorsbybluelight technology to form white light3.Previously,rareearthphosphorswerewidelyusedasthemain-streamluminescentmaterialinWLEDs4.However,thehighcostandtoxicityofrareearthmaterialsseri-ouslyhinderetheir further development.It is there-foreofgreatsignificancetofindanewgreenlumines-cent material with low cost,low toxicity,and withhighphotoluminescence(PL),sothatitcouldbeex-ploitedonalargescale.Asanovelzero-dimensional(0D)carbonnano-materialsdiscoveredin20045,carbondotsconsistofacarbonizedcoreandavarietyoffunctionalgroupsonsurfaces,whichareoftenaccompaniedbythedop-ingofheteroatomsincludingB,N,PandS610.Com-paredtotraditionalrareearthluminescentphosphor,CDsfeaturebrightluminescence,easeofpreparationand surface-functionalization,good biocompatibilityaswellaslowcostandlowtoxicity.Thesecharacter-isticsmakeCDssignificantintheapplicationfieldsinvolvingbiology1115,chemicalsensing1618andop-toelectronics1921.AlthoughCDshavebeencatchingconsiderableattentionaslightsourcesforWLEDsandgreat progresses have been made so far,solid-stateCDsstillfaceenormouschallengesaslight-emittingmaterials due to the aggregation-caused quenching(ACQ)effect22.Therefore,currently numerous ef-fortshavebeendevotedtoinhibittheACQeffectandenhance the optical properties of light-emittingdevices,asmaygreatlypromotethedevelopmentandapplicationofCDsinWLEDs.Recently,althoughafewreviewshavesummar-ized the preparation,luminescence mechanism andapplicationofCDs2325,therealizationofsolid-statefluorescenceofCDsandtheirapplicationsinWLEDshave seldom demonstrated in detail.Herein,webrieflyoverviewtherecentadvancesinthesolid-stateReceived date：2023-03-20；Revised date：2023-04-24Corresponding author：QIUHan-xun,AssociateProfessor.E-mail:Author introduction：YUEJing-song,MasterStudent.E-mail:第38卷第3期新型炭材料（中英文）Vol.38No.32023年6月NEWCARBONMATERIALSJun.2023photoluminescenceofCDs,andhighlighttheirapplic-ationsinWLEDs,particularlyfocusingonthecurrentproblemsandfutureprospects.Typically,inthefirstsection,abriefintroductiontothesynthesisandpre-parationofCDsisaddressed,secondlyfollowedbythe demonstration of photoluminescence mechanismofCDsandthesolid-statephotoluminescence.Inthethirdsection,theapplicationsinWLEDsweredemon-stratedindetail.Finally,thecurrentchallengesofsol-id-stateCDsinWLEDapplicationsandthefeasibleperspectiveswerepresentedaswell.2SynthesisofCDsThemorphology,sizeanddegreeofcarboniza-tionofCDsareintimatelyinfluencedbypreparationmethods.Accordingly,anydifferenceinstructuremaysignificantlyaffectstheperformanceofCDsinapplic-ations.Therefore,preparationtechniquesofCDsarecriticallyimportant.Since CD was discovered,re-searchers have been pursuing simple,efficient andlarge-scaletechniquetoprepareCDsofhigh-quality.Specifically,two types of strategies have been de-velopedtoobtainCDs,namely“Top-down”and“Bot-tom-up”approachesasshowninFig.1(a-crefersto“Top-down”andd-frefersto“Bottom-up”).Top-down means to cut and destroy largegraphenestructuresofgrapheneoxide(GO),graphenenanosheets,carbon nanotubes(CNTs)etc.by eitherphysical26orchemicaltechniquestoobtainCDs.Themethodsinvolved,suchasarcdischarge5,27,laserab-lation2831,electrochemicalsynthesis3236andsoon,couldefficientlytransformlargecarbonstructuresin-to CDs.However,the commonly used advancedequipment or unique technologies result in highercosts of CDs.CDs synthesized by the“Top-down”strategypossessexcellentgraphenestructure,butthesurfacesarelessfunctionalizedwithchemicalgroups.As a result,the-stacking interaction may occurbetweenthese CDs,which in turn leads to the de-creaseoffluorescenceefficiency.Whilethe“Bottom-up”referstotheapproachtoobtainCDsbypyrolysisorcarbonizationofcarbon-containingprecursorsuponchemicaltreatments.Thecarbonsourcesofthe“Bottom-up”approachareex-tensive,rangingfromsmallorganicmoleculesoroli-PEG1500N(a)(b)(c)(d)(e)a)b)(f)Emission frompassivated surfacePEGsex=365 nmCAH2OoPDEthanolH2SO4180 oC 9 h150 oC 9 h200 oC 9 h180 oC 9 hex=365 nmex=365 nmex=365 nmb-CDsy-CDsyg-CDsr-CDsArcing400500600Ex 325 nmEm 407 nm300315325335350PL Intensity/(a.u.)Wavelength/nmElectrochemical10 V,2 hBoron doped GQDsB dopedgrapheneGlucosamineUreaDI water80604020DI water80604020PyrolysisN-CQDsN-CQDs/ZnO180 oC 5 minPyrolysis180 oC 5 minGlucosamineZinc acetateUreaPEIGAMicrowaveElectrochemicalsynthesisMicrowave-assistedsynthesisPyrolysis methodHydrothermalmethodLaser ablationArc dischargeFig.1ApproachestoprepareCDs:“Top-down”and“Bottom-up”,(a)laserablation28,(b)arcdischarge27,(c)electrochemicalsynthesis33,(d)microwave-assistedsynthesis47,(e)pyrolysisofprecursors54,(f)hydrothermalmethod39.(Reprintedwithpermission)第3期YUEJing-songetal:Areviewoffluorescentcarbondots:synthesis,photoluminescencemechanism479gomersofcitricacid,urea,polyethyleneglycoltocar-bon-enrichedprecursorsoftea,orangepeelandoth-ersbiomassmaterials.Themostlyusedtechniquesin-clude hydrothermal/solvothermal method3744,mi-crowave-assistedsynthesis4549andpyrolysismethod5055,etc.Comparedtothe“Top-down”strate-gy,CDspreparedbythe“Bottom-up”featureahigh-erquantumyield(QY),awidersourceofprecursors,andlowercosts.Inparticular,thehydrothermal/solvo-thermalmethodisregardedasoneofthemostsimpleand inexpensive technique.In these techniques,theprecursorisdissolvedinwaterororganicsolventandplacedinaspeciallydesignedreactionvessel.Thefol-lowingcarbonization of the precursor is accom-plishedunder high temperature and pressure condi-tions.Byoptimizing precursor species,solvent,andthereactionparametersoftemperatureandtime,thecompositionandPLcolorofCDscanbewellregu-lated.TheCDssynthesizedby“Bottom-up”approachpossessmoresurfacefunctionalgroups,soahigherQYcanbeachieved.However,theas-receivedmater-ials are often accompanied by many by-products inthe synthesis process.The subsequent post-purifica-tion procedures become necessary to improve thefluorescenceperformance.3 Photoluminescence mechanism ofCDsExcellentfluorescenceemissionisthemostin-herent and fascinating optical property of CDs.However,the PL mechanism of CDs is not com-pletelyclearsofar.ButwiththeadvancementinCDsresearch,threePLmechanismshavebeenconfirmedby researchers56:the size-dependent emission(de-terminedbythecarboncore)5758,thesurfacedefectstate emission(determined by the carbon backboneandtheattachedchemicalgroups)5962,andthemo-lecularstateemission(determinedbyfluorescentmo-lecules linked on the surface or inside the CDs)63.TheseindicatethatCDsexhibitamorecomplexsys-temthantheyareexpected.3.1 Size-dependent emissionIngeneral,thesizeofCDs(80isexcellent).5.2 Electroluminescent WLEDsAlthough encouraging progresses have beenmade in the study of photoluminescent WLEDs,safetyproblemscausedbytheleakageofultravioletlightcould never been completely solved.Alternat-ively,CDs with an electroluminescence(EL)prop-ertyweredevelopedtoactasalight-emittinglayerinthe WLED structure.CDs-based electroluminescentWLEDshaveasandwichstructuresimilartothatofquantumdot-basedWLEDs,whereCDsactasanin-termediate active light-emitting layer surrounded byaninterfacialtransportlayerandanelectrode.Inthetypical structure of CDs-based electroluminescentWLEDs(Fig.10),it could be divided into 5 parts,namely:anode,hole transport layer(HTL),activelight-emitting layer(ALL),electron transport layer(ETL)andcathode.Amongthem,theALLconsistsofCDsorCDs/polymermaterials.Whenavoltageisap-HNHAcrylamide(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)0.90.80.70.60.50.40.30.20.11.00.80.60.40.20.01.00.80.60.40.20.0400500600Wavelength/nm700400500600Wavelength/nm7008000.00.80.70.60.50.40.30.2Normalized absorbance1.0IIIIIIIViiiiiiivvviviiviiiixxxixii426529 6030.80.60.40.20.01.00.80.60.40.20.0400500600Wavelength/nm700800Normalized PL intensityo/m/p(CDs)=2:4:1(weight)Ex=365 nm400500600Wavelength/nm700Normalized PL intensityNormalized intensityIntensity0.10.0Plasma generatorCarbon dotsWhite LEDsH2NH2NH2NH2NNH2NH2Solvothermal180 oC,12 hSolvothermal180 oC,12 hSolvothermal180 oC,12 ho-CDsm-CDsp-CDso-CDs-PVAm-CDs-PVAp-CDs-PVAex=365 nmOFig.8(a-e)SynthesisofbluefluorescentCDsandtheirapplicationinhighcolorrenderingindexWLEDs92,(f)schematicdiagramofpreparationofo-CDs,m-CDsandp-CDs,(g-j)Photographsoffull-colorCDs/PVAfilmsunderUVlightandtheperformanceofWLEDsfabricatedwiththesefilms94.(Reprintedwithpermission)488新型炭材料（中英文）第38卷plied to WLEDs,due to the applied electric field,holes and electrons are injected into the HTL andETL,respectively.After migration,holes and elec-tronsconvergeintheALLandcombinetoproduceexcitonstotriggerthelightemission109.Thelumines-cence of WLEDs could be modulated by changingCDsintheALL.In 2011,Wang and co-workers104 reportedWLEDs assembled from single-component carbondots,whichachievedaCRIof82atacurrentdensityof5mA/cm2,comparabletothatofsomecommercialWLEDs.Themaximumexternalquantumefficiencyreachedto0.083%,indicatingthegreatpotentialsofCDs as white light electroluminescent devices(Fig.11(a).Later,Jiaetal.110reportedanelectron-donating group passivation strategy to synthesizethree types of red CDs with different emissionwavelengths(Fig.11(d).TheassembledWLEDsex-hibited a maximum brightness of 5248-5909 cd/Aandacurrentefficiencyof3.65-3.85cd/A.Thebright-ness still maintained over 80%of the initial valueafter 50 h of operation,showing excellent stability(Fig.11(e-h).Furthermore,amethodcapableofmod-ulatingWLEDsfromcooltowarmwhitelightwasdeveloped by taking advantage of the luminescenceredshiftsandbroadeningcausedbytheaggregationofCDs,asaccountsfortheWLEDswithtunableCCTsfrom2863to11240K(Fig11(b-c)106.ItisworthnotingthattheseCDs-LEDsachieveamaximumlu-minance of 1414-4917 cd/m2 and high externalquantum efficiencies of 0.08%-0.87%.This workfirstlydemonstratedthatCCT-tunableelectrolumines-centWLEDscouldbeobtainedbycontrollingtheag-gregationofCDs.Recently,ZhousgroupreportedaroutetosynthesizingredCDs(R-CDs)andwhiteCDs(W-CDs)byintroducingfreeradicals.Arecordex-OH(a)(b)(c)OOH2NNH3H2ODaylightUnder UVHO0.5%H2NNH2Single-component white carbon dotsMonochromatic carbon dots382 nm chipWhite LEDsMulticolor LEDsNCoolwhite lightWarmwhite light1%2%4%10%(mass)Aspartic acidMicrowaveheating3 723 K4 344 K3 758 KCRI=91.5(a3)CRI=92.0CRI=87.9400100Spectral composition/%806040200100Spectral composition/%806040200100Spectral composition/%806040200500600Wavelength/nmIntensity/(a.u.)Intensity/(a.u.)Intensity/(a.u.)700800400500600Wavelength/nm700800 400500600Wavelength/nm70080039.7%7.8%6.4%27.9%4.6%13.6%RedOrangeYellowGreenBluePurple622-760 nm597-622 nm577-597 nm492-577 nm435-450 nm400 435 nm27.7%7.7%7.0%40.0%5.0%13.0%RedOrangeYellowGreenBluePurple622-760 nm597-622 nm577-597 nm492-577 nm435-450 nm400 435 nm33.5%7.3%6.2%28.6%3.9%20.4%RedOrangeYellowGreenBluePurple622-760 nm597-622 nm577-597 nm492-577 nm435-450 nm400 435 nm86%87%79.6%Fig.9(a)SynthesisrouteofW-CDs,photosofW-CDsunderdaylightandUV,anddisplaydiagramofWLEDs101;(b)SynthesisrouteofmulticolorCDs103;(c)ConstructionofWLEDswiththeW-CDspreparedinthecitedarticleandperformanceofthedevicesfabricated103.(Reprintedwithpermission)CathodeETLALLHTLSubstratesLight outputAnodeFig.10IllustrationsofthetypicaldevicestructureofCD-basedelectroluminescentLEDs第3期YUEJing-songetal:Areviewoffluorescentcarbondots:synthesis,photoluminescencemechanism489ternalquantumefficiency(0.95%)wasachievedinthefabrication of electroluminescent WLEDs using W-CDsmentionedabove107.AlthoughfruitfulachievementshavebeenmadewithCDsinWLEDs,someofphotophysicalproper-tiesneedtobedeeplyinvestigatedtoenhancethelightstability,colorstability,colorrenderingcapabilityandluminousefficiencyofCDs-WLEDsuntiltheycouldbeusedonalargescale.Inconclusion,CDsshowsgreatpotentialinthefabricationofWLEDs,butmoreeffortsshouldbedevotedtoachievemuchlower-costandhigher-performanceproducts.6SummaryandoutlookSincethediscoveryoffluorescentCDsin2004,therelatedresearchhasbeenextensivelyconducted,duetotheirexcellentemissionfluorescencetunability,lowtoxicity,goodbiocompatibilityandlowcost.Inthisreview,we summarized the main research pro-gressofCDsintheaspectsofpreparation,photolu-minescencemechanism,solid-state photolumines-cence and applications in WLEDs in recent years.TheseprogressesendowCDswithverybroadapplica-tionprospectandpotentials.Despite the great achievements on CDs,manychallengesstillremain.(1)Thecurrentexistingmech-anismscouldonlybeeffectiveforinterpretingagiv-enCDmaterial,whichmakesitdifficulttomaximizetheperformanceofCDs-basedWLEDs.Auniversalandcomprehensiveexplanationofthephotolumines-cencemechanism is strongly desired.(2)The re-search and development of solid-state luminescentCDs,especiallytheself-resistantquenchingCDs,stillfacemanychallenges.Thekeypointliesinthestruc-tural characteristics and quenching mechanism ofCDs.MoretheoreticalstudiesshouldbeconductedtomaketheACQphenomenondeterminedbyregulat-ingcarboncoreandsurfacestateexplicit.Inthisre-gard,auniversalmethodisdesirabletobedevelopedto design the structure of CDs and fundamentallysolvetheACQeffect.(3)Althoughexcitingachieve-(b6)(b5)(b4)(b3)(b2)(b1)(c)(h)(g)(f)(e)(d)(a)LiF/AlVacuum levelITO4.7LiF/Al2.840032:1500 600 700 800Wavelength/nmEL intensity/(a.u.)40016:1500 600 700 800Wavelength/nmEL intensity/(a.u.)4008:1500 600 700 800Wavelength/nmEL intensity/(a.u.)4004:1500 600 700 800Wavelength/nmEL intensity/(a.u.)4002:1500 600 700 800Wavelength/nmEL intensity/(a.u.)4001:1500 600 700 800Wavelength/nmEL intensity/(a.u.)400Voltage/V50040030020001005004003002000100876543210543210100101102103Current efficiency/(cd/A)Current density/(mA/cm2)Current density/(mA/cm2)Luminance/(cd/m2)1040.0W