Time-Resolved Photoluminescence and Elastic White Light Scattering Studies of Individual Carbon Nanotubes and Optical Characterization of Oxygen Plasma Treated Graphene [Elektronische Ressource] / Tobias Dominik Gokus. Betreuer: Achim Hartschuh

Tobias Gokus

Informations

• Publié par: ludwig-maximilians-universitat_munchen
• Publié le: 01 janvier 2011
• Nombre de lectures: 6
• Langue: English
• Poids de l'ouvrage: 14 Mo

Extrait

Dissertation zurErlangungdesDoktorgrades
¨ ¨derFakultat furChemieundPharmazie
derLudwig-Maximilians-Universitat¨ Munchen¨
Time-ResolvedPhotoluminescenceandElasticWhiteLight
ScatteringStudiesofIndividualCarbonNanotubes
and
Optical CharacterizationofOxygen PlasmaTreated Graphene
von
Tobias Dominik Gokus
aus
Kirchen (Sieg)
2011Erklar¨ ung
DieseDissertationwurdeimSinnevon§13Abs.3bzw.4derPromotionsordnungvom29.
¨Januar1998 (inderFassungdersechstenAnderungssatzungvom16.August2010)von
HerrnProf.Dr.AchimHartschuhbetreut.
Ehrenwor¨ tlicheVersicherung
DieseDissertationwurdeselbststand¨ ig,ohneunerlaubteHilfeerarbeitet.
M¨ unchen,den29. Juli2011
.......................
(UnterschriftdesAutors)
Dissertationeingereichtam: 29. Juli2011
1.Gutachter: Prof.Dr.AchimHartschuh
2.Gutachter: Prof.Dr.AlexanderHog¨ ele
M¨ undlichePruf¨ ungam: 27. Oktober2011
iAbstract
In the course of this work the excited state dynamics of individual single-walled carbon
nanotubes(SWCNTs) were studied by a combination of confocal PLspectroscopyand time
correlated single photon counting (TCSPC) measurements. Nonradiative decay channels
dominate the excited state dynamics of SWCNTs leading to low photoluminescence (PL)
quantum yields and PL decay times on the picosecond timescale. Knowledge about the
microscopic nature of these decay channels is crucial to improve the material properties.
The measurements on the single nanotube level revealed large tube-to-tube variations of
PL decay times, which could be attributed to different defect densities for different tubes.
For the present SWCNT material the PL decay times only depend weakly on the nanotube
length. SWCNT material synthesized by using a cobalt-molybdenum catalyst (CoMoCAT)
systematically display short monoexponential PL decays, while the PL decay dynamics
of SWCNTs produced high pressure decomposition of carbon monoxide process (HiPco)
is either mono or biexponential depending on the respective local environment of the
nanotube. The transition from a bi- to monoexponential PL decay can be explained by
synthesis-dependent differences in defect densities. This defect related nonradiative decay
channels reduce the amplitude of one decay component below the experimental detection
limit. It is further shown, that photo-induced defects and gold atoms adsorbed to the
sidewallsofSWCNTsareshowntoalterthePLpropertiesofindividualSWCNTs.Additional
low-energyPLsatellitebandsarise inthespectra. Theirorigin canbeattributedtoemission
from nominally dark excitons which are ”brightened” due to defect facilitated mixing of
intrinsic stateswith different parity/spin. The role of defects in the brightening processwas
investigated by time-resolved PL measurements and complementary Raman spectroscopy.
BasedonitsenergyseparationandtheunusuallyslowPLdecaydynamicsthelowestenergy
satellitebandcanbeassignedtotheradiativerecombinationofatripletexciton.
In a second project a common-path interference scattering approach (iSCAT) utilizing a
conventionalinvertedlaserscanningconfocalmicroscopecombinedwithaphotoniccrystal
fibre as a supercontinuum white light source is successfully tested for its capabilities for
elasticscatteringimagingandspectroscopyofindividualSWCNTs.
Finally, it is shown that single layer graphene can selectively be turned luminescent upon
exposure to a mild oxygen plasma. The treatment leads to a strong and spatially uniform
PL which is characterized by a single, broad PL band extending from the visible to the
near infrared spectral region. The analysis of the defect related RamanI /I intensityratioD G
2indicates the formation of nanometer sized islands for which the sp conjugated lattice of
graphene is still preserved. Emission of quantum confined states within these islands is
discussedasapossibleoriginofthePL.
iiiContents
Preface ix
I. TheoreticalBackgroundandExperimentalDetails 1
1. PhysicalPropertiesofSWCNTsandGraphene 3
1.1. StructuralPropertiesofSWCNTsandGraphene................. 5
1.2. ElectronicPropertiesofGrapheneandSWCNTs..... 10
1.2.1. ElectronicEnergyDispersionRelationofGraphene.......... 11
1.2.2. Brillouin ZoneandEnergyDispersionRelation of SWCNTs . . . . . 15
1.2.3. OpticalTransitionEnergies:KatauraPlot................ 22
1.3. OpticalTransitionsinSWCNTs.............. 23
1.3.1. SymmetriesofSWCNTs.............. 24
1.3.2. SelectionRulesforInterbandTransitions.... 26
1.3.3. AbsorptionandPhotoluminescence................... 27
1.4. ExcitonsinCarbonNanotubes............... 28
1.4.1. ExcitonicDispersionRelation................ 32
1.4.2. Excitonenergies................... 36
1.4.3. ExcitonMobilityinSWCNTs................ 37
1.5. PhotoluminescenceofSWCNTs.............. 37
1.5.1. ExcitonicdescriptionofthePLprocesinSWCNTs.......... 38
1.5.2. ExcitedStateDynamics,PLDecayTimesandQuantumYield . . . . 38
1.6. RamanScateringofSWCNTsandGraphene................... 41
1.6.1. RamanScateringofGrapheneandSWCNTs.. 43
1.6.2. RamanSpectraofgrapheneandSWCNTs................ 46
1.6.3. DisorderinNanographiticMaterials: TheI /I ratio... 48D G
2. OpticalMicroscopyMethods 51
2.1. ConfocalMicroscopy................................ 52
2.1.1. SpatialResolutionofOpticalMicroscopes.... 54
2.1.2. GausianLaserBeamandGouyPhaseShift............... 58
2.2.InterferenceScatteringMicroscopy............ 60
3. MaterialsandMethods 71
3.1. TheConfocalMicroscopeSetup.......................... 71
v3.2.SamplePreparation.................................. 75
3.2.1. SWCNTSampleMaterial 75
3.2.2. SamplePreparationforConfocalPLMeasurements.......... 77
3.2.3. HiPCOSWCNTsDispersedinAgaroseGel......... 78
3.2.4. SamplePreparationforiSCATMeasurements........ 79
3.3.ConfocalPLImagingandSpectroscopyofIndividualSWCNTs.. 79
3.4.TimeCorrelatedSinglePhotonCounting..................... 82
3.4.1. AcquisitionofPLTransientsofIndividualSWCNTs.... 84
3.4.2. TransientFiting:GeneralAspects.................... 85
3.4.3. TransientFitting: Mono-andBiexponentialPLDecaysofSWCNTs . 86
3.4.4. TransientFiting:WavelengthDependenceoftheIRF......... 89
3.5.InterferenceScateringMicroscopy.................. 91
3.5.1. CharacterizationofthePCFWhiteLightOutput...... 93
3.5.2. AcquisitionofElasticWhiteLightScatteringImagesandSpectra . . . 94
II. Time-resolvedPLStudiesofIndividualSWCNTs 97
4. ExcitedStateDynamicsofIndividualSWCNTsatRoomTemperature 99
4.1. Introduction.....................................100
4.2.PLDecayTimeDistributionsofIndividualCoMoCATSWCNTs.101
4.3. PLDecayTimeDistributions: DefectMediatedNonradiativeRelaxation . . . 105
4.4.LengthDependenceofPLDecayTimes......................108
4.5.Conclusion................... 115
5. Mono-andBiexponentialPLDecaysofIndividualSWCNTs 117
5.1. Introduction..................................... 117
5.2.PLDecayDynamicsofIndividualHiPcoandCoMoCATSWCNTs 118
5.2.1. PLDecayDynamicsofHiPcoandCoMoCAT(6,5)SWCNTs.....119
5.2.2. PLDecayDynamicsofHiPco(6,4)SWCNTs..............120
5.3.KineticModelfortheBiexponentialPLDecayinSWCNTs ....122
5.3.1. ContributionoftheK-momentumExcitons...............126
5.4.Discusion...........................127
5.4.1. TransitionfromMono-toBiexponentialPLDecays..........127
5.4.2. BiexponentialDecaysinHiPco(6,4)SWCNTs.......128
5.5.Conclusion................................130
6. DefectInducedPLofDarkExcitonicStatesinIndividualSWCNTs 133
6.1. Introduction.....................................133
6.2. OpticalCharacterizationofPhotoinducedLow-energyEmissionBands . . . 134
6.3.PLDecayDynamicsoftheDarkandBrightStates................1366.4.InvestigationoftheDarkState”Brightening”Proces..............138
6.4.1. RamanSpectroscopyofPhotoinducedDefects.......138
6.4.2. TheRoleofOxygeninthePhotoinduced”Brightening”Process . . . 142
6.4.3. BrighteningbyTreatmentWithaColoidalGoldSolution.......143
6.5.Discusion.......................................144
6.5.1. PopulationDynamicsoftheBrightandDarkStates....146
6.5.2. AssignmentofthePLSatelliteBands...................147
6.6.Conclusion..........................149
III. ElasticWhiteLightScatteringSpectroscopyofIndividualSWCNTs 151
7. ElasticWhiteLightScatteringMicroscopyofIndividualSWCNTs 153
7.1. Introduction.....................................153
7.2. ReferenceMeasurementsonGoldNanoparticles.....156
7.3. ReferenceMeasurementsonSingleLayerGraphene...............161
7.4. ElasticWhiteLightScateringSpectroscopyofIndividualSWCNTs......162
7.4.1. Discussion..................................164
7.5. DeterminingtheResonantE ExtinctionCrosSection......16622
7.6. ManipulationoftheiSCATsignal.........................168
7.7. CombinedPLandElasticScateringMeasurements...171
7.8. Conclusion......................................174
IV. OpticalCharacterizationofPlasmaTreatedGraphene 177
8. OpticalCharacterizationofOxygenPlasmaTreatedGraphene 179
8.1. Introduction.....................................179
8.2.PreparationofPLG...180
8.3.PLSpectroscopyandTime-resolvedPLMeasurementsofPLG.........180
8.4.RamanSpectroscopyofPLG......................183
8.5.ElasticWhiteLightScateringofPLG..........185
8.6.Discusion.................................189
8.7. Conclusion.............194
V. Summary 195
9. Summary 197VI. Appendix 201
A. ScatteredFieldofaCarbonNanotube 203