UpconvertingNanoparticles
FromFundamentalstoApplications
EditedbyVineetK.Rai
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Prof.Dr.VineetK.Rai IIT(ISM)Dhanbad DepartmentofPhysics
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Contents
Preface xv
1IntroductiontoUpconversionandUpconverting Nanoparticles 1 ManishaMondalandVineetKumarRai
1.1Introduction 1
1.2FrequencyConversionandItsVariousProcesses 2
1.2.1StokesEmission 2
1.2.2Anti-StokesEmission 2
1.2.2.1Ground/Excited-StateAbsorption(GSA/ESA) 3
1.2.2.2EnergyTransferUpconversion(ETU) 4
1.2.2.3CooperativeLuminescenceandCooperativeSensitizationUpconversion (CSU) 5
1.2.2.4Cross-relaxation(CR)andPhotonAvalanche(PA) 6
1.3TransitionMetalsandTheirProperties 7
1.4RareEarthsandTheirProperties 8
1.4.1TrivalentRare-EarthIons 9
1.4.1.1ElectronicStructure 9
1.4.1.2InteractionofRare-EarthIons 10
1.4.1.3DiekeDiagram 13
1.4.2DivalentRare-EarthIons 13
1.5ExcitationandDe-excitationProcessesofRareEarthsinSolid Materials 15
1.5.1ExcitationProcesses 15
1.5.1.1f–fTransition 15
1.5.1.2f–dTransition 15
1.5.1.3ChargeTransferTransition 15
1.5.2EmissionProcesses 15
1.5.2.1EmissionviaRadiativeTransitions 15
1.5.2.2EmissionviaNonradiativeTransitions 16
1.5.2.3EnergyTransferProcesses 16
1.6RateEquationsRelevanttoUCMechanism 18
1.6.1RateEquationsinaBasicThree-LevelSystem 18
1.6.2RateEquationRelatedtoPumpPower-DependentUCEmission 19
1.7TheoreticalDescriptionofOpticalCharacteristicsofRare-EarthIons 20
1.7.1Judd–Ofelt(J–O)TheoryandCalculationofRadiativeParameters 21
1.7.2NephelauxeticEffect 22
1.8AnIntroductiontoUpconvertingNanoparticles 22 Acknowledgments 23
References 23
2SynthesisProtocolofUpconversionNanoparticles 31 LakshmiMukhopadhyayandVineetKumarRai
2.1Introduction 31
2.2HostMatrix 32
2.3SyntheticStrategyofUCNanomaterials 33
2.3.1Solid-StateReactionTechnique 34
2.3.2CoprecipitationTechnique 35
2.3.3Sol–GelTechnique 36
2.3.4Hydro(solvo)thermalTechnique 39
2.3.5CombustionTechnique 40
2.3.6ThermolysisTechnique 42
2.3.6.1ThermolysisinOA-BasedMixedSolvents 43
2.3.6.2ThermolysisinOM-BasedMixedSolvents 43
2.3.6.3ThermolysisinTOPO-BasedMixedSolvents 43
2.3.7Microwave-AssistedSynthesisTechnique 44
2.4SynthesisTechniquesforFabricatingCore@shellArchitectures 45
2.4.1Solid-PhaseReaction 45
2.4.2Liquid-PhaseReaction 46
2.4.2.1StöberTechnique 46
2.4.2.2MicroemulsionTechnique 48
2.4.3Gas-PhaseReaction 51
2.4.4MechanicalMixing 52
2.5OtherSynthesisStrategiestoDevelopLanthanide-DopedUCNPs 52
2.6Conclusion 53
References 53
3CharacterizationTechniquesandAnalysis 67 NehaJain,PrinceK.Jain,RajanK.Singh,AmitSrivastava,andJaiSingh
3.1Introduction 67
3.2X-RayDiffraction(XRD) 69
3.3X-rayPhotoelectronSpectroscopy(XPS) 72
3.4FieldEmissionScanningElectronMicroscopy(FESEM) 74
3.5TransmissionElectronMicroscopy(TEM) 76
3.6Energy-DispersiveX-raySpectroscopy(EDS) 79
3.7ThermogravimetricAnalysis(TGA) 81
3.8Ultraviolet–Visible–Near-Infrared(UV–Vis–NIR)Absorption Spectroscopy 82
3.9DynamicLightScattering(DLS) 84
3.10Photoluminescence(PL)Study 85
3.11PumpPower-DependentUC 87
3.12RecognitionofEmissionColorandColorimetricTheory 88 Acknowledgment 89 References 89
4RamanandFTIRSpectroscopicTechniquesandTheir Applications 97 SauravK.OjhaandAnimeshK.Ojha
4.1RamanSpectroscopy 97
4.2FourierTransformInfrared(FTIR)Spectroscopy 99
4.2.1FTIRinTransmissionMode 100
4.2.2AttenuatedTotalReflectance(ATR) 100
4.2.3DiffuseReflectanceInfraredFourierTransformSpectroscopy (DRIFTS) 100
4.3ApplicationsofRamanSpectroscopy 100
4.3.1RamanStudyofMolecularAssociationinHydrogen-Bonded Systems 100
4.3.2Surface-EnhancedRamanSpectroscopy(SERS) 104
4.3.3ResonanceRamanSpectroscopy(RRS) 106
4.3.4RamanSpectroscopyofSemiconducting,Superconducting,and PerovskiteMaterials 107
4.4ApplicationsofFTIRSpectroscopy 108
4.4.1FTIRSpectroscopyofSemiconductor,Superconductor,Hazardous,and PerovskiteMaterials 108
4.5RamanandFTIRSpectroscopyofUpconvertingNanoparticles 109 References 110
5FundamentalAspectsofUpconvertingNanoparticles(UCNPs) BasedonTheirProperties 117 SushilK.Ranjan,SasankPattnaik,VishabKesarwani,andVineetKumarRai
5.1Introduction 117
5.2ElucidationofDynamicsofUCNPsontheBasisofFluorescenceDecay Times 120
5.2.1GeneralUnderstandingofDepopulationProcessesandUCDecay 120
5.2.2DifferentiatingtheESAandETUMechanismBasedontheDecay Profile 121
5.2.3TheoreticalandExperimentalApproachofUnderstandingtheFactors AffectingUpconversionDecay 123
5.3MeasurementofQuantumYieldofUCNPs 131
5.3.1RoleofQuantumYieldinUpconversion 132
5.3.2OpticalMethodsofMeasuringQuantumYieldofUpconverting Nanoparticles(UCNPs) 133
5.3.2.1RelativeMethodofMeasuringQuantumYield 133
5.3.2.2AbsoluteMethodofMeasuringQuantumYield 133
5.3.2.3MeasurementofIntrinsicQuantumYieldofLanthanide-BasedMaterials UsingLifetimes 134
5.3.3SomeOtherMethodsofDeterminingQuantumYield 134
5.3.3.1Photo-acousticSpectroscopy(PAS) 134
5.3.3.2ThermalLensing(TL)Method 135
References 135
6FrequencyUpconversioninUCNPsContainingTransition MetalIons 141
ManishaPrasadandVineetKumarRai
6.1Introduction 141
6.2SynthesisofTransitionMetalIon-ActivatedLuminescent Nanomaterials 143
6.3StructuralandOpticalCharacterizations 143
6.4FrequencyUpconversionandItsVariousMechanisms 144
6.5Applications 144
6.6MechanismofTransitionMetalIonsinCrystalField 145
6.6.1UCMechanismsinMn-BasedSystem 146
6.6.2UCMechanismsinMn4+ -andTi2+ -BasedSystems 151
6.6.3UCMechanismsinCr3+ -BasedSystem 153
6.6.4UCMechanismsintheFe3+ -BasedSystem 155
6.6.5UCMechanismsinCo3+ -andNi2+ -BasedSystem 157
6.6.6UCMechanismsinCu2+ -,Zn2+ -,andZr4+ -BasedSystem 158
6.6.7UCMechanismsinNb5+ -,Mo3+ -,Ru-,andAg+ -BasedSystem 160
6.6.8UCMechanismsinW6+ -andRe4+ -BasedSystem 161
6.6.9UCMechanismsinOs4+ -andAu-BasedSystem 162 References 164
7FrequencyUpconversioninUCNPsContainingRare-Earth Ions 171
SasankPattnaikandVineetKumarRai
7.1Introduction 171
7.2FamiliarizationwiththeSpectroscopicBehaviorofRE3 + Ion-Doped UCNPs 173
7.2.1PhysicsofTrivalentRare-EarthIons 173
7.2.1.1UCMechanismsinYb3+ -andPr3+ -BasedSystems 174
7.2.1.2UCMechanismsinEr-BasedSystems 175
7.2.1.3UCMechanismsinHo-BasedSystems 177
7.2.1.4UCMechanismsinTm-BasedSystems 179
7.2.1.5UCMechanismsinNd-BasedSystems 181
7.2.1.6Tri-DopedSystems 181
7.2.2ColorModulationinUCNPs 184
7.2.2.1RoleofDopantConcentrationandCombinationofRE3 + IonsinColor Modulation 184
7.2.2.2RoleofHost/DopantCombinationinColorModulation 186
7.2.2.3ControllingtheEmissionColorThroughPhononEffects 186
7.2.2.4TuningUCEmissionUsingFRET 188
7.2.3QuenchingMechanismsinUCNPs 190
7.3RoutestoEnhanceUpconversionLuminescenceinNanoparticles 190
7.3.1DyeSensitizationTechniques 191
7.3.2ConcentrationQuenchingMinimization 192
7.3.2.1SuppressionofSurface-RelatedQuenching 192
7.3.2.2RemovalofDetrimentalCross-Relaxation 193
7.3.3ConfinementofEnergyMigration 194
7.3.4OtherTechniquestoEnhanceUpconversionEmission 195
7.3.4.1Crystal-PhaseModification 195
7.3.4.2ConstructinganActiveCore/ActiveShellStrategy 195
7.3.4.3ConjugatingSurfacePlasmonResonanceTechnique 195
7.3.4.4DielectricSuperlensing-MediatedStrategy 196
7.4TechnologicalApplications 197
7.4.1PhotonicApplications 197
7.4.1.1Light-EmittingDiodes(LEDs) 197
7.4.1.2PhotovoltaicApplications 198
7.4.2Bioimaging 199
7.4.3Photo-InducedTherapeuticApplications 200
7.4.3.1PhotodynamicTherapy 201
7.4.3.2PhotothermalTherapy 201
7.4.3.3PhotoactivatedChemotherapy(PACT) 202
7.4.4OtherEmergingApplications 203
7.4.4.1Anticounterfeiting 203
7.4.4.2SensingandDetection 203
7.4.4.3OptogeneticStimulation 205
7.4.4.4NIRImageVisionofMammals 205 References 206
8SmartUpconvertingNanoparticlesandNewTypesof UpconvertingNanoparticles 221 AkhileshK.Singh
8.1Introduction 221
8.2UpconvertingCore–ShellNanostructures 222
8.3HybridUpconvertingNanoparticles 224
8.4MagneticUpconvertingNanoparticles 226
8.5UC-BasedMetal–OrganicFrameworks 228
8.6SmartUCNPsforSecurityApplications 230
8.7SmartUpconvertingNanoparticlesforBiologicalApplications 233
8.8SmartUpconvertingNanoparticlesforSensing 235
8.9Conclusion 236 References 237
x Contents
9SurfaceModificationand(Bio)Functionalizationof UpconvertingNanoparticles 241 YashashchandraDwivedi
9.1Introduction 241
9.2UpconvertingNanomaterials 242
9.3SurfaceModification 245
9.4BiofunctionalizationofUpconvertingMaterialsandApplications 247 References 257
10FrequencyUpconversioninCore@shellNanoparticles 267 RaghumaniS.Ningthoujam,RashmiJoshi,andManasSrivastava
10.1Introduction 267
10.1.1Downconversion 267
10.1.2Upconversion 271
10.2SynthesisofCore@shellandCore@shell@shellUCNPs 272
10.2.1ThermolysisMethod 272
10.2.2HotInjection 276
10.2.3CationExchange 277
10.2.4StructuralCharacterizations 277
10.2.5OpticalCharacterization 281
10.2.5.1NormalConversionProcessinLn-DopedCore@shell Nanoparticles 283
10.2.5.2Loop-TypeandAvalanche-TypeUpconversionProcessesinCore@shell Nanoparticles 289
10.3FrequencyUpconversionandItsVariousMechanisms 291
10.3.1Inorganic-BasedUpconversion 291
10.4Applications 297
10.4.1BioimagingApplications 297
10.4.1.1Luminescence-BasedImaging 297
10.4.1.2OtherImagingProbes(MRI,CT,andSPECT) 299
10.4.2PhotothermalTherapy(PTT) 301
10.4.3PhotodynamicTherapy(PDT) 303
10.4.4TemperatureSensor 306
10.4.5SecurityInk 308
10.5Conclusion 310 Acknowledgment 311 References 311
11UCNPsinSolar,Forensic,SecurityInk,andAnti-counterfeiting Applications 319 KaushalKumar,NeerajKumarMishra,andKumarShwetabh
11.1Introduction 319
11.2UCNPsforSolarCells 320
11.2.1C-SiSolarCells 321
11.2.2AmorphousSiliconSolarCells 323
11.2.3GaAs-BasedSolarCells 324
11.2.4Dye-SensitizedSolarCells(DSSCs) 324
11.3Forensic,SecurityPrinting,andAnti-counterfeitingApplications 325
11.4Biomedicals 331
11.4.1Bioimaging 333
11.4.2Biosensing 336
11.5DisplayandLightingPurposes 339 References 340
12ApplicationofUpconversioninPhotocatalysisand Photodetectors 347 PriyamSingh,SachinSingh,andPrabhakarSingh SunilKumarSingh
12.1Introduction 347
12.2Photocatalysis 349
12.3Photodetector 357
12.4Conclusion 365 References 365
13UCNPsinLightingandDisplays 375 RiyaDey
13.1Introduction 375
13.2MajorFactorsthatAffecttheUCEmissionEfficiency 375
13.3UCMechanismswithRateEquations 378
13.3.1PumpPowerDependenceintheCaseofDominantETU-Assisted UpconversionoverESA 379
13.3.2PumpPowerDependenceintheCaseofDominantESA-Assisted UpconversionoverETU 380
13.4UCNPsinSolid-StateLaser 380
13.5UCNPsinSolid-StateLightingandDisplays 384
13.5.1RequirementsforLEDApplications 384 References 388
14UpconversionNanoparticlesinpHSensingApplications 395 ManojKumarMahata,RanjitDe,andKangTaekLee
14.1Introduction 395
14.2BasicPropertiesofUCNPs 397
14.3WorkingPrincipleofUCNP-BasedpHSensor 400
14.4PhotonUpconversion-BasedpHSensingSystems 401
14.4.1UpconversionNanoparticlesaspHSensors 401
14.4.2Upconversion-BasedpHSensingMembranes 405
14.5Conclusion 410 References 411
15UpconversionNanoparticlesinTemperatureSensingand OpticalHeatingApplications 417 PraveenK.ShahiandShyamB.Rai
15.1Introduction 417
15.2ClassificationofTemperatureSensors:PrimaryandSecondary Thermometers 420
15.3PerformanceofTemperatureSensors 420
15.3.1ThermalSensitivity 421
15.3.2ThermalUncertainty (�� T)421
15.3.3ReproducibilityandRepeatability 422
15.4TemperatureSensingwithLuminescence 423
15.4.1Time-IntegratedSchemes 424
15.4.1.1FluorescenceIntensityRatio(FIR)orBandShape 424
15.4.1.2Bandwidth 426
15.4.2LifetimeTechnique 427
15.5Upconversion(UC)andUC-BasedThermalSensorofLn3+ Ions 427
15.5.1Upconversion(UC)andUpconvertingNanoparticles(UCNPs) 427
15.5.2Single-CenterUCNanothermometersandMulticenterUC Nanothermometers 428
15.5.3ComplexSystems 430
15.6OpticalHeating 433 References 437
16UpconvertingNanoparticlesinPollutantDegradationand HydrogenGeneration 449
WanniWang,ZhaoyouChu,BenjinChen,andHaishengQian
16.1Introduction 449
16.2DegradationofOrganicPollutants 450
16.2.1DegradationofRhB 451
16.2.2DegradationofMB 455
16.2.3DegradationofMO 460
16.2.4DegradationofVariousOrganicPollutants 462
16.2.5Others 467
16.3DegradationofInorganicPollutants 469
16.4PhotocatalyticHydrogenProduction 473
16.5Conclusion 481 References 481
17UpconvertingNanoparticlesintheDetectionofFungicides andPlantViruses 493
VishabKesarwaniandVineetKumarRai
17.1Introduction 493
17.2VisualDetectionofFungicides 495
17.2.1DetectionMechanisms 495
17.2.1.1ForsterResonanceEnergyTransfer(FRET) 495
17.2.1.2InnerFilterEffect(IFE) 496
17.2.1.3PhotoinducedElectronTransfer(PET) 499
17.2.1.4ElectronExchange(EE) 500
17.2.2SignificantWorksonUpconversion-BasedFungicideDetection 500
17.3DetectionofPlantViruses 505
17.3.1PlantVirusDetection/ManagementStrategies 505
17.3.1.1DirectInteractions 505
17.3.1.2IndirectInteractions 505
17.3.1.3NPsasBiosensorsforVirusDetection 507
17.3.1.4RNAiProcessforAntiviralProtection 507
17.3.2SignificantWorksonPlantVirusDetectionBasedonUCNPs 507
17.4FutureChallengesRegardingNP-BasedFungicideandPlantVirus Detection 509
References 510
18UpconversionNanoparticlesinBiologicalApplications 517 PoulamiMukherjeeandSumantaKumarSahu
18.1Introduction 517
18.2UpconversionNanoparticlesinBioimaging 518
18.2.1CellImaging 518
18.2.2MultimodalImaging 520
18.3UpconversionNanoparticlesinDrugDelivery 522
18.3.1DifferentTypesofSurfaceModification 524
18.3.1.1PolymerCoating 524
18.3.1.2SilicaCoating 524
18.3.1.3MetalOxide-CoatedUCNPs 525
18.3.1.4FunctionalizationofUCNPs 525
18.3.1.5Metal–OrganicFrameworkCoating 525
18.3.2DrugRelease 526
18.3.2.1NIR-TriggeredDrugDeliverySystem 526
18.3.2.2pHandThermoresponsiveDrugRelease 526
18.4UpconversioninPhotodynamicTherapy 526
18.4.1SurfaceModificationofUCNPsforPDT 529
18.5PhotothermalTherapy 531
References 533
Index 539
Preface
Theconversionoflow-energyphotonsintohigh-energyphotons,knownas “frequencyupconversion,”usingadvancedopticalmaterialshasbecomeanemergingresearchfieldwithwideconsequenceandimpactinvariousscientificareas rangingfromhealthcaretoenergyandsecurity.Thematerialsshowingfrequency upconversionpropertiesareknownasupconversion(UC)materials.UCmaterials revealvarietyofapplicationsindifferentfields,viz.colordisplay,two-photon imaginginconfocalmicroscopy,WLEDs,high-densityopticaldatastorage,upconvertors,underseacommunications,solid-statelighting,sensors,photovoltaics, photocatalysis,foodindustry,indicators,anti-counterfeiting,bioimaging,cancer therapyandotherbiologicalfields.Itisknownthatincomparisontoultraviolet (UV)andvisiblelightthenear-infrared(NIR)lightisabundantandnon-destructive innature.Ithasdeeppenetrationintheorganismsandlessharmfulquality. UCluminescentmaterialsinnanosizerangeareknownasUCnanomaterials orUCnanoparticles(UCNPs).UCNPsexcitedwithnon-destructiveNIRlight areabetterchoicethantheconventionaldownconversionnanoparticlesbecause theyarefreefromautofluorescence,havelowlightpenetration,andcauseless severephoto-damagetolivingorganisms.Itisnotabletomentionthatthelow efficiencyofUCmaterialsdefinitelybecomesamajorbarrierfortheirapplication inawiderange.Forresearchers,itisatopprioritytoovercomethisproblem. SeveralengineeredUCNPs,e.g.organic,inorganic,hybrids,andthinfilms,have beenexploredwidelytoobtainhighlyefficientUCluminescentmaterials.Usually, organicluminescentmaterialssufferpoorstabilityunderharshconditionsandhave poorlong-termreliability,buthaveagreaterductilitythaninorganicmaterials. Theinorganicluminescentmaterialsaremoredurableandpossesshighthermal stability.So,thehybridmaterialsconsistingofbothinorganicandorganiccomponents,namely,metalorganicframeworks(MOFs),haveattractedresearcherswith enhancedluminescencepropertiesascomparedtothebareorganicandinorganic materials.Toenhancetheupconversionefficiency,sphericalmetalnanoparticles showingplasmonresonanceincloseproximityoftheUCNPsareutilized.The plasmonicnanostructuresarewidelyusedtoevolvetheUCNPswithimproved electronic,metallic,andopticalproperties.Whenthesurfaceplasmonresonance wavelengthofthemetallicnanostructurematcheswiththeexcitationwavelength ofupconversionmechanism,signalenhancementoccurs.Usually,thecoatingof
gold(Au)andsilver(Ag)nanoparticlesisusedtotunetheluminescenceproperties ofUCNPs,thoughthenanoparticlesexhibitplasmonabsorptionin400–600nm range.
Theupconversionemissionefficiencycanbeenhancedbyseveralways,includingdopingwithsensitizer,non-lanthanides,andcoatingwithinorganicshell. Thenon-lanthanideco-dopinginUCNPshasalsobeenusedfrequentlyinorder togetenhancedluminescenceintensityalongwiththeuseofsensitizerion. Theco-dopingofactivatorandsensitizerionswithproperconcentrationinan appropriatehostmatrixisessentialtoachievehighlyefficientUCemissionas theconcentrationquenchinghasaprejudicialeffectontheluminescenceintensity.Thephononfrequency,stability,costeffectiveness,non-hygroscopic,and non-toxicnatureoftheUCmaterialsareofutmostimportance.Thesecurityofany importantdata,currency,etc.hasbecomeverycrucialtopreventcounterfeiting. UCNPswithhighluminescenceintensitycanbevalidatedinanti-counterfeiting applications.Thesematerialsarealsoutilizedforvisualexposureoffungicides, thiram,etc.,whichcanbebroadlyappliedinsoybeans,apples,winefarming, etc.,toavoidcropdiseasesandexcessiveuseofpesticides.Rare-earth-ions-based UCemissionhastremendousadvantagesintermsoflongexcitedlifetime,sharp emissionbandwidth,lowautofluorescence,highphotostability,highresolution, lowtoxicity,etc.Rare-earthionsarefoundtobeverysensitivetoevensmallchanges inchemicalsurroundings.Therefore,itbecomesessentialtogetinformation aboutthesymmetry,bondingoftheprobeion,andhowtheychangetheiroptical propertieswithchemicalcompositionofthehostmaterials.Forgettingthehigh quantumefficiency,concentrationofthedopantsshouldbehigh,butitmaycause concentrationquenchingduetotheinteractionbetweentheexcitedandunexcited neighbors.Therefore,thenano-structuredmaterialscontainingmetallicnanoparticlesareofparticularinterestbecausethelargelocalfieldaroundtherare-earth ionspositionednearthenanoparticlesmayincreasetheluminescenceefficiency. Amongseveralstrategies,thecoatingofupconversionnanoparticleswithinorganic materialsshellisaneffectivemethodtogetenhancedUCluminescence.The core@shellapproachoffersshieldingtothesurfaceparticlesandthusreducesthe surfacedefectsandpossibilityofquenching.Thiscore@shellarchitectureisvery muchbeneficialinbiomoleculeconjugationandthussuitableformanybiological applications.Differentcoatingstrategieshavebeenemployedaccordingtothe requiredapplicationpurposes.UCNPsprobescanfunctionasmultiplecontrast agentsforconcurrentuseinalteredmedicinalimagingmodalitiesbyproviding correspondingdiagnosticinformation(i.e.MRIandCT).Bio-conjugationonthe surfaceoftheUCNPsshowsamuchenhancedimagingperformanceincomparison totheclinicallyusedfluorescentdyes.Innovativebio-imagingmethodsarebeing establishedbycombiningtheconventionalmedicalimagingmodalitiesusing core-shellstructuredUCNPs.
Thebookentitled UpconvertingNanoparticles:FromFundamentalstoApplications iscompletelydifferentfromthepreviouslypublishedbooksinallrespects, includingthebasics,scientificandtechnologicaldemands.Itisdividedinto eighteenchapters.Chapter1,authoredbyMondalandRai,introducesthebasic
Preface xvii conceptsofupconversion,andupconversionofnano-particles.Theintroductionto frequencyupconversionanditsvariousmechanisms,excitationandde-excitation processesinhostscontainingrare-earthionsalongwiththespectroscopicproperties ofrare-earthions/transitionmetalsaredescribedinthischapter.Therateequations relevanttoexcited-stateabsorptionandenergytransferprocesseswithanoverview oftheUCNPshavebeenintroduced.Chapter2,authoredbyMukhopadhyayand Rai,describesthesynthesisprotocolofupconversionnanoparticles.Inthischapter introductiontohostmaterialsandsynthesisstrategiesofUCnanomaterialslike solid-statereaction,co-precipitation,sol–gel,hydrothermal,combustion,thermolysis,microwave-assistedsynthesis,core@shellsynthesistechniques,etc.havebeen described.Chapters3and4,authoredbyJainetal.;OjhaandOjha,refertocharacterizationtechniquesandanalysis;RamanandFTIRspectroscopictechniques andtheirapplications,respectively.Variousstructuralandopticaltechniquesfor thecharacterizationofUCNPs,viz.X-raydiffraction(XRD),X-rayphotoelectron spectroscopy(XPS),fieldemissionscanningelectronmicroscopy(FESEM),transmissionelectronmicroscopy(TEM),energy-dispersiveX-rayspectroscopy(EDS), thermogravimetricanalysis(TGA),ultraviolet–visible–nearinfrared(UV–Vis–NIR) absorptionspectroscopy,dynamiclightscattering(DLS),photoluminescence, Fouriertransforminfrared(FTIR),havebeenreported.Chapters5,6and7,authored byRanjanetal.;PrasadandRai;andPattnaikandRai,summarizethefundamental aspectsofUCNPsbasedontheirproperties,frequencyupconversioninUCNPs containingtransitionmetalions,andfrequencyupconversioninUCNPscontaining rare-earthions,respectively.AlongwithintroductionthedynamicsofUCNPs onthebasisoffluorescencedecaytimes,quantumyieldmeasurementofUCNPs, frequencyupconversionanditsvariousmechanismshavealsobeeninterpreted. Thevariousroutestoenhancetheupconversionluminescencealongwiththe technologicalapplicationsofUCNPshavebeendescribed.
Chapters8,9,and10,authoredbySingh;Dwivedi;andNingthoujametal.,are devotedtothesmartandnewtypeofupconvertingnanoparticles;surfacemodificationand(bio)functionalizationofupconvertingnanoparticles,andfrequency upconversionincore@shellnanoparticles,respectively.Thesechaptersoutline theupconvertingcore@shellnanostructures,hybridupconvertingnanoparticles, magnetic-upconvertingnanoparticles,UC-basedmetal–organicframeworks,surfacemodification,bio-functionalizationofupconvertingmaterials,synthesisof core@shellandcore@shell@shellUCNPs,anduseofUCNPsforsecurity,biological, andsensingapplications.Chapters11,12,13,14,and15,authoredbyKumar,Mishra andShwetabh;Singhetal.;Dey;Mahata,DeandLee;andShahiandRai,dealwith theUCNPsinsolar,forensic,securityink,andanti-counterfeitingapplications; applicationofupconversioninphotocatalysisandphotodetectors;UCNPsin lightinganddisplays;upconversionnanoparticlesinpH-sensingapplicationsand upconversionnanoparticlesintemperature-sensingandopticalheatingapplications,respectively.Chapters16,authoredbyWangetal.,throwsthelightonUCNPs applicationsindegradationoforganicandinorganicpollutantsalongwiththe photocatalytichydrogengeneration.Thevisualdetectionoffungicidesandplant virusesalongwiththefuturechallengeshavebeenexplainedbyKesarwaniand
RaiinChapter17.Chapter18,authoredbyMukherjeeandSahu,involvesthe applicationofUCNPsinbio-imaging,drugdelivery,photodynamictherapy,and photothermaltherapy.
Thepresentbookisoutcomeoftheuntiringeffortsofallthecontributingauthors. Itwillbeverymuchhelpfultotheresearchersaswellastheundergraduateand post-graduatestudentsstudyingphysics,chemistry,materialsscience,biology, engineering,etc.ingainingaproperunderstandingabouttheupconversion luminescence.Itwaspossibletocompletethisbookonlyduetothegreataffection andblessingsofGurudevPt.ShriRamSharmaAcharyaandGurumatajiMataBhagawatiDeviSharma.Specialthankstoallmyfamilymembersandresearchscholars fortheirmotivationandkindsupport.IwouldalsoliketothanktheWileyteam involvedfromthebeginningtillthecompletionofthebookproposal.Asalarge numberoftopicsrelatedtotheUCNPsandtheirapplicationshavebeencoveredin thisbook,therecouldbethepossibilitythatsomeoftheminuteglitcheshavebeen missedout.Therefore,genuinesuggestionsandcommentsfromthereadersare welcome.Overall,theresearchdevelopmentsonUCNPsandtheirusesindifferent fieldsstartingfromverybasicstoadvancedlevelmakethepresentbookunique.
DepartmentofPhysics
IndianInstituteofTechnology (IndianSchoolofMines), Dhanbad,India
Professor(Dr.)VineetK.Rai
IntroductiontoUpconversionandUpconverting Nanoparticles
ManishaMondal 1,2 andVineetKumarRai 1
1 IndianInstituteofTechnology(IndianSchoolofMines),DepartmentofPhysics,LaserandSpectroscopy Laboratory,Dhanbad826004,Jharkhand,India
2 TezpurUniversity(CentralUniversity),DepartmentofPhysics,Napaam,Tezpur,Sonitpur784028,Assam, India
1.1Introduction
Spectroscopyalmostdealswiththeinteractionoflightandmatter.Itprovidesinformationaboutsplittingofelectromagneticradiationintoitsconstituentwavelengths. Thebeginningofspectroscopyliessincetheobservationoflightdispersionthrough prismbySirIsaacNewton.Amongdifferentspectroscopytechniques,opticalspectroscopydeliversanexceptionaltoolbywhichonecanfinddetailedinformation regardingtheabsorbingandemittingatoms,ions,molecules,defects,theirlocal surroundings,etc.Inaterm,opticalspectroscopyallowslighttopenetrateinside materials.Opticalspectroscopycanbecharacterizedintofourparts:absorption, luminescence,reflection,andscattering.Amarvelousdimensionofresearchcarried outinfindingnovelluminescentmaterialsplaysanimportantroleinopticalcommunication,lighting,medicaldiagnosis,etc.(BerthouandJörgensen1990;Cheng etal.2013;Jiangetal.2016;Linetal.2016;Youetal.2016;DeyandRai2017;Mehra etal.2020).Whenanatomicsystemafterabsorbingthephotonsofappropriate frequencytransitsupwardtoahigherstateandthenbythespontaneousemission process,itmayreturntothegroundstate.Thisde-excitationrouteisfamiliarasthe luminescenceprocess.Theoccurrenceofluminescenceduetoexcitationoflightis knownasphotoluminescence.Ontheotherhand,luminescenceduetoexcitation ofanelectronbeamistermedascathodoluminescence,whichhelpstoidentify impurities,latticedefects,andcrystaldistortions.Radioluminescenceoccursdue toexcitationthroughthehighlyenergeticelectromagneticradiations(i.e. α rays, β rays,and γ rays).Thethermoluminescencephenomenaareusedinradiation dosimetry,datingofmineralsandoldceramics,materialscharacterization,biology, forensic,etc.Itoccurswhenamaterialradiateslightasaconsequenceofrelease ofenergykeptintrapsbythermalheating.Electroluminescenceoccursduetothe passageofelectriccurrentoveramaterial.Theemissionoflightduetomechanical disturbanceoriginatestriboluminescence.Conferringtothediversepositionsof UpconvertingNanoparticles:FromFundamentalstoApplications,FirstEdition.EditedbyVineetK.Rai. ©2022WILEY-VCHGmbH.Published2022byWILEY-VCHGmbH.
1IntroductiontoUpconversionandUpconvertingNanoparticles
theexcitationandemissionbands,theluminescentmaterialscanbecategorized intoStokes-andanti-Stokes-typeluminescentmaterials.Theseprocessesare typicallyexemplifiedbytheJablonskidiagram(Jablonski1935;Jablonski1993). Theluminescentmaterialsarecommonlyknownasphosphors,whichmeans “lightbearer,”thatconsistofhostanddopants.Intheseconstituents,lanthanide materialsaremainlyintroducedintothehostmatrix.Lanthanideshavethemost complicatedelectronicstructuresbecauseoftheirlargenumberofincomplete4f energylevels.Thepresentchapterpresentsabriefoutlookonunderstandingthe frequencyconversionmechanisms,electronicenergylevelsofrare-earth(RE)ions, transitionmetalions,theoreticaldescriptionoftheopticalcharacteristicsofRE ions,andUpconvertingnanoparticles(UCNPs).
1.2FrequencyConversionandItsVariousProcesses
Thephotoluminescentmaterialsareabletodisplayvisibleemissionsviasuitable ultraviolet(UV)ornear-infrared(NIR)excitations.Inthemajorityofcases,excitationenergyisgreaterthanemittedphotonenergy;thisemissioniscalledasStokes emission,andthecorrespondingenergylossisknownasStokesshift.Incertain circumstances,emittedenergyishigherthanabsorbedenergy;thisisknownas anti-Stokesemission.
1.2.1StokesEmission
TheStokes-typeemissionprocesspossessestwotypesoffeaturessuchasdownconversionandquantumcutting(Huangetal.2013;Looetal.2019).Inquantumcutting process,twoormorelowerenergyphotonsareemittedforeachincidenthigh-energy photonabsorption.Inthisprocess,two,threeorfourlow-energyphotonsareemitted becauseoftheabsorptionofoneNIR,visible,orultravioletphoton.Inthisprocess,theconversionefficiencyismorethan100%.Incurrentyears,quantumcuttinghasacknowledgedconsiderabledevotionasabuddingmethodtoimprovethe photovoltaicconversionefficiencyofsolarcells.Ontheotherhand,inthedownconversionprocess,emissionofonelowerenergyphotontakesplacebecauseofthe absorptionofonehigherenergyphoton;thus,theconversionefficiencywillnotgo beyond100%.
1.2.2Anti-StokesEmission
Theanti-Stokesemissionprocessoccurviathreeprocesses:two-photonabsorption (TPA),secondharmonicgeneration(SHG),andupconversion(UC)(Figure1.1) (Pollnauetal.2000;GamelinandGudel2000;Suijver2008;Grzybowskiand Pietrzak2013;Chenetal.2015;Nadortetal.2016).TPAisatypeofnonlinear absorptionprocessthatcanbedefinedasthesimultaneousabsorptionoftwo photonsofsameordifferentfrequenciesbyanatom,ion,ormolecule.Inthis process,theelectronispromotedfromlowenergylevel(i.e.groundstate)toexcited
Figure1.1 Basicenergy-leveldiagramsdepictingtypicalanti-Stokesprocesses.
level,andtheenergyoftheemissiontransitionisequaltothesumoftwo-photon energies.Asthisisathird-ordernonlinearprocess,itiseffectiveatprecisehigh intensities.TPAwasinitiallyanticipatedbyMariaGoeppert-Mayarintheyear 1931.Thiswasexperimentallyverifiedbythelaserafteritsdiscovery.Anumberof techniquesareusedtomeasureTPA,suchastwo-photonexcitedfluorescence,zscan,nonlineartransmission,etc.Ontheotherhand,SHG,“anopticalnonlinear process,”occursfromavirtualstateinamediumhavingsecond-ordernonlinear susceptibility.ThiswasrevealedandexperimentallyverifiedbyFrankenetal. (1961).Theydetectedthesecondharmoniclightwhenanintensebeamof6943Å fromtherubylaserwaspassedthroughthequartzcrystal.Inthisprocess,two photonsofthesamefrequencyinteractwithanonlinearmaterial(i.e.medium)and giverisetoanewphotonofdoublethefrequencyorenergyoftheincidentphotons. Furthermore,UCisalsoananti-Stokesprocessthatconvertsthelowerenergy photonsintohigh-energyphotons,e.g.infraredtovisibleorUVlight(Figure1.1). Itisastepwiseabsorptionprocessinvolvingintermediatestates(Auzel1966; OvsyakinandFeofilov1966).Basically,amongthesethreeprocessesofconverting lowerenergyphotonsintohigherenergyphotons,TPAandSHGneedacoherent beamaswellasaveryhighexcitationbeamintensity.IntheUCprocess,coherent pumpingandhighintensityoftheexcitationbeamarenotnecessarilyrequired. Itoccursevenatlowintensityoftheexcitationbeambecauseofthepresenceofreal intermediatestates(generally,ofmetastablenature).
ThematerialsthatexhibittheUCpropertiesareknownasupconvertingmaterials.Inrecentyears,theseupconvertingmaterialsareextensivelyusedinsensing, infraredcounters,solid-statelasers,solarcells,fingerprintdetection,securityink, upconverters,biologicalfields,etc.(Digonnet1993;Wadeetal.2003;Rai2007;Wang andLiu2009;Guetal.2013;Lietal.2013;WangandZhang2014;Chenetal.2014; MondalandRai2020).Generally,theUCphenomenonobservedinthesematerialsisnotassimpleasdepictedinFigure1.1.SeveralprocessesaccountableforUC mechanismsareasfollows.
1.2.2.1Ground/Excited-StateAbsorption(GSA/ESA)
Ground-stateabsorption(GSA)isoneofthesimplestroutesforUCmechanism (Auzel1973,2004;Garlick1976;Raietal.2013;Reddyetal.2018).Theprocessin whichtheground-stateions(i.e.electrons)afterabsorbingtherequisiteenergyfrom thepumpphotonsarepromotedtothefirstintermediatelevelisknownastheGSA
1IntroductiontoUpconversionandUpconvertingNanoparticles
Energy transfer upconversion (different ion)
Cooperative luminescence
Cross-Relaxation (same ions)
Cross-Relaxation (different ions) Photon avalanche
Figure1.2 SchematicrepresentationofpossibleUCmechanisms:(a)GSA/ESA, (bandc)ETU,(d)cooperativeluminescence,(e)cooperativesensitization,(fandg)CR, and(h)PAprocesses.
process.Conversely,sequentialabsorptionoftwolightquantabyaparticularionis knownasESAprocess(Auzel1973,2004;Garlick1976;Raietal.2013).Inthecaseof ESAprocess,theionpresentintheintermediatestateabsorbsthesecondphotonand transitsupwardtothenexthigherstate.Forexample,theenergy-leveldiagramsfor GSAandESAmechanismsarepresentedinFigure1.2a.Hereatfirst,anionabsorbs thepumpphotonofenergy(=h�� ,where“h”isPlanck’sconstantand“�� ”isthefrequencyoftheincidentphoton)andreachestotheintermediatestateE1(exhibit longlifetime)fromthegroundstateGviatheGSAprocessandthenasecondpump photon(ofthesameenergy)excitestheionfromE1statetothenexthigherstate E2.Aradiativedecayoftheionfromtheexcitedstate(E2)tothegroundstate(G) resultsinUCemission.Thus,asingleionisinvolvedinthewholeESAprocess.For gettingproficientUCemissionthroughtheESAprocess,aladder-likeenergy-level arrangementinionsisessential.
1.2.2.2EnergyTransferUpconversion(ETU)
LiketheESAprocess,theenergytransferupconversion(ETU)processalsoinvolves successiveabsorptionoftwoenergyquantabytheionstooccupytheintermediate (i.e.metastable)state(Figure1.2).AsintheESAprocessthereisaninvolvement
1.2FrequencyConversionandItsVariousProcesses 5 ofsingleion,however,ETUoperateswithintwo(similarordifferent)ions.Inthis mechanism,theinvolvedtwodopantionsaretermedassensitizerandactivator (Heeretal.2003;Boyeretal.2007;Shanetal.2007;Sonietal.2015;Mukhopadhyay andRai2020;PattnaikandRai2020).Atfirst,boththe(different)ionsabsorbthe pumpphotonsfromthegroundstateandthenmovestotheirrespectivemetastable states(E1′ andE1,whereE1′ ≅ E1)throughtheGSAprocess(Figure1.2b).After that,thesensitizerion(presentinE1′ state)handoversitsexcitationenergytothe neighboringactivatorion(presentinE1state)andrelaxesbacktothegroundstate. Theactivatorionaftergainingthisexcitationenergyfromthesensitizerreachesto thenexthigherenergystate(E2).
Whenthetwoinvolveddopantionsaresimilar,thesetwoionsareinitially excitedtotheintermediatestate(E1)afterreceivingtheenergyfrompumpphotons (Figure1.2c).ThetwoionspresentintheE1stateexchangetheirenergyinsuch awaythatoneion(i.e.donor),aftertransferringitsexcitationenergytotheother excitedion(i.e.acceptor),decaysnonradiativelytothelowerenergylevel(G). Theotherion(i.e.acceptor)aftergettingexcitationenergyfromthefirstone(i.e. donor)ispromotedtothenexthigherenergystate(E2).Aradiativetransitionfrom stateE2tothegroundstate(G)generatesaphotonofenergy(=h�� 1 ),whichishigher thantheincidentphotonenergy(=h�� )(Figure1.2).ThisETUprocessisthemost efficientUCemissionprocess(Auzel2004;Raietal.2007,2008).Inthisprocess, thedopantionconcentration(whichregulatestheaveragedistanceconcerning adjacentdopantions)playsakeyroleintheUCemissionintensity.
1.2.2.3CooperativeLuminescenceandCooperativeSensitization
Upconversion(CSU)
UCemissionbyacooperativeenergytransferprocessinvolvestwoions(oneactsas adonorandtheotherionasanacceptor).Inthecooperativeluminescenceprocess, twoionsabsorbthepumpphotonssuccessivelyandreachthehigher(intermediate) stateE1(Figure1.2d).Inthisintermediatelevel,thesetwoionstransfertheir energyinsuchawaythatoneion(donor)transfersitsexcitationenergytotheother one(acceptor)andthedonorreturnstothegroundstate(G).Theacceptor,after gainingtheexcitationenergyfromthedonor,transitsupwardtoahigherenergy state,“whichisavirtualstate.”Thisvirtualstateisalsoknownasthecooperative energystate(Leeetal.1984;Macieletal.2000;Diaz-Torresetal.2005).Fromthis virtualstate,itrelaxesradiativelytothegroundstate(G)viaemittingaphotonof energylargerthantheincidentphotonenergy(Figure1.2d).Ontheotherhand,in thecooperativesensitizationprocess,whentheenergyofthetwoexcitedionsare transferredtoathirdion(ion2),thenitgoesfromthegroundstatetoanexcited statehavingenergyequaltothesumoftheenergiesofthetwoindividualions (Martínetal.2001;Salleyetal.2001,2003).InFigure1.2e,theexcitationenergyof thetwoexcitedions(ion1)presentinthestateE1istransferredtoathirdion(ion2). Thethirdion(ion2)presentinthegroundstate(G),afterabsorbingtheexcitation energycorrespondingtothetwoexcitedions(ion1),movestoitshigherstate(E2). Afterthat,thethirdionfromtheexcitedstate(E2)relaxesradiativelytothelower levels(saygroundstate)viaemittingthephotonsofenergyhigherthanthatof
1IntroductiontoUpconversionandUpconvertingNanoparticles theincidentphoton.Thisprocessisknownascooperativesensitization,andthe emittingstate(E2)inthisprocessisarealstate(Figure1.2e).Thus,thecooperative sensitizationismoreeffectivethancooperativeluminescencebecauseitmay compensatethelowUCemissionefficiency(Dwivedietal.2007;Liangetal.2009).
1.2.2.4Cross-relaxation(CR)andPhotonAvalanche(PA)
Thecross-relaxation(CR)processoccursduetoion–ioninteraction(ionsmay besimilarordifferent)(Chenetal.2014;PattnaikandRai2020)(Figure1.2f,g. Thecross-relaxationbetweentwoidenticalions/moleculesisresponsiblefor self-quenching(Figure1.2f).Intheself-quenchingprocess,theintermediatestates ofboththeions(ion1)havethesameenergy(E1).Whenthecross-relaxationoccurs betweentwodifferentions(Figure1.2g),thefirstionsharesapartofitsexcitation energytothesecondionbytheprocessE2(ion1) + G(ion2) → E1(ion1) + E1′ (ion2)(Figure1.2g).Inthisprocess,thefirstion(ion1)initiallypresentinthe excitedstate(E2)interchangesapartofitsexcitationenergytothesecondion(ion2) thatisinitiallyavailableinthegroundstate(G).Bythisway,thedecreaseinthe energyofthefirstion(ion1)isequaltotheincreaseintheenergyofthesecondion. Thisresultsinboththeions/moleculeschangingsimultaneouslytotheexcitedstate (E1andE1′ ).AmongtheotherUCprocesses,themostexcitingprocessisphoton avalanche(PA),whichwasfirstexperimentallyobservedinPr3+ -dopedinfrared quantumcounters(Chivianetal.1979).Generally,thisPAprocessoccurswhenthe excitationenergyexceedsitsthresholdlimit.Whentheexcitationenergyislower thanthethresholdenergy,theemittedintensityisverypoor,butasitexceedsthe limit,theemittedintensitybecomesenormouslygreater(Joubert1999;Singhetal. 2011;Zhuetal.2012;Mondaletal.2016).ForoccurrenceofPAprocess,atfirst, theintermediatelevelandtheupperexcitedlevelarepopulatedbytheGSA,ESA, andETUprocesses.BytheCRprocessbetweentheseupperexcitedlevelandthe groundstateofaneighboringion,twoionsaregeneratedintheintermediatelevel E1(Figure1.2h).Now,twoionsareavailableintheintermediatestatefortheESA process.Thus,withthefeedbackloopingofESAandCRprocessessimultaneously, thenumberofionsintheintermediatelevelincreases,whichgiverisetostrongUC emission.
ThePAprocessisanunusualpumpingprocessbecauseitmayleadtostrong UCemissionfromtheupperexcitedstateE2withoutanyresonantGSAfromthe groundstate(G)totheintermediatestate(E1)ofion2(Figure1.2h).Thefrequency ofincidentphotonisinresonantwithstateE1′ ofion1andtheupperexcitedstate E2ofion2.AnefficientCRprocess,i.e.E2(ion2) + G(ion1) → E1(ion2) + E1 (ion1),occursbetweenion1andion2.Thisresultsinboththeionstooccupythe intermediatestateE1.ThesetwoionsreadilypopulatethelevelE2throughESA tofurtherinitiatethecross-relaxation.Withthefeedbackloopingoftheseefficient cross-relaxationandESAprocesses,thenumberofionsintheintermediatestateE1 increasesrapidly,whichresultsfurtheranenormousincreaseinthepopulationof levelE2.Thus,inthePAprocess,astrongUCemissionfromstateE2totheground stateG(ofion2)hasbeenobserved.
1.3TransitionMetalsandTheirProperties
Theopticalcentersarenecessaryfortheperfectcrystalstoexhibittheoptical spectra.Dependingontheabsorptionandemissionbandsoftheopticalcenters presentinthepurecrystals,theyarepertinentfordiverseapplications,such asopticalamplifiers,solid-statelasers,colordisplays,absorbers,improvingin luminescencebrightness,fibers,opticalswitches,etc.Anyelementintheperiodic tablemayactasaforeignelementinthecrystal.However,essentially,afewnumber ofelementscanbeionized,whichcangenerateenergylevelsandthusyieldoptical features.Forindustrialapplications,thetwoextremelyimportantelementsare transitionmetalsandREsintheperiodictable.Transitionmetalionsareespecially usedasopticallyactivedopantsintunablesolid-statelasers(Soléetal.2005).These ionsbelongtothefourthperiodoftheperiodictablewithelectronicconfiguration 1s2 2s2 2p6 3s2 3p6 3dn ,where“n (variesfrom1to10)”isthenumberof3delectrons presentinthetransitionmetalions.Generally,valenceelectronsareresponsible foropticaltransitions;hence,inthecaseoftransitionmetals,3delectronsare accountable.Becauseofthelargeradiusoftransitionmetalionsascomparedto lanthanidesandnoshieldingofvalenceelectrons,strongfieldeffectoccurs;hence, theyexhibitthebroadbands.
TheSugano–Tanabediagramexplainstheenergy-leveldiagramforthetransition metalions(Figure1.3)(TanabeandSugano1954a,b).Thespectroscopictermsforthe freeionstatesofthetransitionmetalionsduetotheL-Sinteractionaredescribed
Figure1.3 Tanabe–Suganodiagramforthed3 electronconfigurationintheoctahedral crystalfield.Source:Briketal.(2016).ReprintedwithpermissionofTheElectrochemical Society.
1IntroductiontoUpconversionandUpconvertingNanoparticles
as 2S+1 LJ ,where, L, S,and J denotethetotalorbitalangularmomentum,totalspin angularmomentum,andtotalangularmomentum,respectively.Theenergyseparationamongthe 2S+1 L states,i.e.thestrengthoftheelectron–electroninteraction, canbecalculatedwiththehelpofRacahparameters(A,B,andC)(Soléetal.2005). Onthebasisofoctahedralcrystallattice,SuganoandTanabeexplainedtheoccurrenceofenergylevelsinthecaseoftransitionmetalions,butbyusingthisdiagram, onecanalsointerprettheopticalspectraarisingfromthetransitionmetalionsin differenttypesofhostlattices.
Thisdiagramexplainsthesplittingof 2S+1 L freeionenergystateswiththeratio betweenthestrengthofthecrystalfieldandtheelectron–electroninteraction strength(symbolizedasDq/B)versusthefreeionenergylevels(E/Bunits).Inthis diagram,the y-axisisintermsofenergy“E”scaledbyB(oneoftheRacahparameters).Thesplittedtermsfor 2S+1 L energystatesaretermedasA,T,andElevels.This Sugano–Tanabediagramalsoexplainsthenatureoftheopticalbandsfortransition metalions.Inthecaseofstrongcrystalfieldapproximation,thecrystalfieldeffect dominatesovertheelectron–electroninteractionamong3dions.Accordingly,there arethreesingle-electronorbitalsforeachorbital.Furthermore,accordingtothe Sugano–Tanabediagram,forlowcrystalfieldstrength,theemissionbandisshifted towardthelowerenergyside.Forthisspecificnature,theemissionwavelength inthetransitionmetalionsdependsonaparticularhostmaterial.Thus,doping oftransitionmetalionsindifferenthostmaterialsdirectedtotheadvancementof countlessvarietiesoftunablesolid-statelasers.Mostofthetransitionmetalions areincorporatedintheoctahedralcrystalhostmatrix,sotheirenergylevelcan beexplainedonthebasisofSugano–Tanabediagram(TanabeandSugano1956). However,insomecases,suchasNi2+ ,Co2+ ,andCr2+ ions,thesetransitionmetal ionsareincorporatedinthetetrahedralcrystallatticefordifferentapplications; therefore,theKonigandKremerdiagram(KonigandKremer1997)isapplicablein explainingtheenergylevelsoftransitionmetalionsotherthantheoctahedralone.
1.4RareEarthsandTheirProperties
Mostofthelasers,phosphors,amplifiers,etc.,compriseREelements.Surprisingly, theglobalapplicationsofRE-basedmaterialsareincreasingfromindustryapplicationstomedicalapplications.Thereare15lanthanideelementsalongwithtwomore elementsi.e.scandium(Sc)andyttrium(Y).These15lanthanideelementsarecommonlynamedaslanthanum(La),cerium(Ce),praseodymium(Pr),neodymium (Nd),promethium(Pm),samarium(Sm),europium(Eu),gadolinium(Gd),terbium (Tb),dysprosium(Dy),holmium(Ho),erbium(Er),thulium(Tm),ytterbium(Yb), andlutetium(Lu).MostoftheREelementsareentitledasperthenameofthe inventorsorthenameoftheirrevealedplaces.TheseREelementsareincorporated indifferenthostmaterialsintheirionized(eitherdivalentortrivalent)form. ThedivalentREions{Eu(+2),Yb(+2),andSm(+2)}possessonemoreelectron comparedtothetrivalentionsandthusexhibitdifferentopticalfeaturesandtreat differently.
1.4.1TrivalentRare-EarthIons
TheoutermostelectronicconfigurationsofdivalentandtrivalentREionsare5d 4f n 5s2 5p6 and4f n 5s2 5p6 ,respectively,where n (variesfrom n = 0to14)specifies thenumberofelectronsintheunfilled4fshell.These4f n electronsarethevalence electronsthatareaccountableforthespectroscopictransitions.
1.4.1.1ElectronicStructure
Thepresenceofvalenceelectronsinthe4fshellmakestheREionsasluminescent centersofanyphosphormaterial.Thegroupof15elementscomprisingatomic numberstartingfrom57to71inthesixthperiodoftheperiodictabletogetherwith scandium(Sc)andyttrium(Y)areknownasREelements.WhentheseREelements areintroducedintothehosts,theyeasilyconvertintotheireitherdoublyortriply ionizedstatestoacquiretheirstableelectronicconfigurations.Theoutermost electronicconfigurationsoflanthanum(La,atomicnumberZ = 57)andthelast elementlutetium(Lu,Z = 71)intheirtriplyionizedstateare4f 0 5s2 5p6 and4f14 5s2 5p6 ,respectively.Therearefifteenpossibilitiesforfillingthese4forbitalsasthe forbitalcontainssevensuborbitals.Actually,theseunfilled4fvalenceelectronsare incontrolforopticaltransitions.Table1.1presentstheelectronicarrangementsand groundstatesofeachtriplyionizedREelement.Theactualelectronicconfiguration ofthe15REelements(i.e.fromLatoLu)is[Xe]5d1 6s2 4f n (n = 0to14).However,in
Table1.1 ElectronicconfigurationoftrivalentionicstatesofREelements(Shionoyaetal. 1998).
Numberof4f
La3+ 570and[Xe]4f 0 000
Ce3+ 581and[Xe]4f1 1/235/2
Pr3+ 592and[Xe]4f2 154
Nd3+ 603and[Xe]4f3 3/269/2 4 I9/2
Pm3+ 614and[Xe]4f4 264 5 I4
Sm3+ 625and[Xe]4f5 5/255/2 6 H5/2
Eu3+ 636and[Xe]4f6 330 7 F0
Gd3+ 647and[Xe]4f7 7/207/2 8 S7/2
Tb3+ 658and[Xe]4f8 336
Dy3+ 669and[Xe]4f9 5/2515/2 6 H15/2
Ho3+ 6710and[Xe]4f10 268 5 I8
Er3+ 6811and[Xe]4f11 3/2615/2 4 I15/2
Tm3+ 6912and[Xe]4f12 156 3 H6
Yb3+ 7013and[Xe]4f13 1/237/2 2 F7/2
Lu3+ 7114and[Xe]4f14 000 1 S0
