Exciton mobility and localized defects in single carbon nanotubes studied with tip-enhanced near-field optical microscopy [Elektronische Ressource] / von Carsten Georgi
147 pages
English

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Exciton mobility and localized defects in single carbon nanotubes studied with tip-enhanced near-field optical microscopy [Elektronische Ressource] / von Carsten Georgi

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Dissertation zur Erlangung des Doktorgradesder Fakultät für Chemie und Pharmazieder Ludwig-Maximilians-Universität MünchenExciton Mobility and Localized Defectsin Single Carbon Nanotubes Studied withTip-Enhanced Near-Field Optical MicroscopyvonCarsten GeorgiausSchwerin2010ErklärungDiese Dissertation wurde im Sinne von § 13 Abs. 3 der Promotionsordnung vom 29. Januar1998 von Herrn Prof. Dr. Achim Hartschuh betreut.Ehrenwörtliche VersicherungDiese Dissertation wurde selbständig, ohne unerlaubte Hilfe erarbeitet.München, den 09.12.2010..............................................(Unterschrift des Autors)Dissertation eingereicht am 09.12.20101. Gutachter: Prof. Dr. Achim Hartschuh2. Gutachter: Prof. Dr. Tobias HertelMündliche Prüfung am 07.02.2011iAbstractIn this work, single-walled carbon nanotubes (SWNTs) have been studied using tip-enhanced near-field optical microscopy (TENOM). This technique provides a sub-diffraction spatial resolution of 15nm on the basis of strong local signal enhancement,which allows for nanoscale imaging of the photoluminescence (PL) intensity and energyalong single semiconducting SWNTs. Thereby, the mobility of excitons and their interac-tion with defects and spatial exciton energy variations can be directly visualized. Similarly,the local Raman scattering properties of metallic SWNTs have been investigated, revealingthe microscopic relation of localized defects and the resulting Raman D-band intensity.

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Publié par
Publié le 01 janvier 2010
Nombre de lectures 25
Langue English
Poids de l'ouvrage 9 Mo

Extrait

Dissertation zur Erlangung des Doktorgrades
der Fakultät für Chemie und Pharmazie
der Ludwig-Maximilians-Universität München
Exciton Mobility and Localized Defects
in Single Carbon Nanotubes Studied with
Tip-Enhanced Near-Field Optical Microscopy
von
Carsten Georgi
aus
Schwerin
2010Erklärung
Diese Dissertation wurde im Sinne von § 13 Abs. 3 der Promotionsordnung vom 29. Januar
1998 von Herrn Prof. Dr. Achim Hartschuh betreut.
Ehrenwörtliche Versicherung
Diese Dissertation wurde selbständig, ohne unerlaubte Hilfe erarbeitet.
München, den 09.12.2010
..............................................
(Unterschrift des Autors)
Dissertation eingereicht am 09.12.2010
1. Gutachter: Prof. Dr. Achim Hartschuh
2. Gutachter: Prof. Dr. Tobias Hertel
Mündliche Prüfung am 07.02.2011
iAbstract
In this work, single-walled carbon nanotubes (SWNTs) have been studied using tip-
enhanced near-field optical microscopy (TENOM). This technique provides a sub-
diffraction spatial resolution of 15nm on the basis of strong local signal enhancement,
which allows for nanoscale imaging of the photoluminescence (PL) intensity and energy
along single semiconducting SWNTs. Thereby, the mobility of excitons and their interac-
tion with defects and spatial exciton energy variations can be directly visualized. Similarly,
the local Raman scattering properties of metallic SWNTs have been investigated, revealing
the microscopic relation of localized defects and the resulting Raman D-band intensity.
The first part of the thesis presents a newly developed numerical description of exciton
mobility and local quenching at defect sites, accounting also for the TENOM imaging
process. This highly flexible model is used to quantitatively evaluate experimental ob-
servations such as photo-induced PL blinking and strong spatial PL intensity variations
of single semiconducting SWNTs. The main finding is that exciton propagation can be
described as one-dimensional diffusion with a diffusion length of 100nm for the studied
nanotubes, determined independently from both the PL blinking characteristics and the
direct visualization using high-resolution TENOM. The temporal and spatial PL variations
result from efficient exciton quenching at localized defects and the nanotube ends.
The second part reports on the first observation of exciton localization in SWNTs at room
temperature, leading to strongly confined and bright PL emission. Localization results
fromnarrowexcitonenergyminimawithdepthsofmorethan15meV,evidencedbyenergy-
resolved near-field PL imaging. Complementary simulations using a modified numerical
model accounting for energy gradients are in good agreement, predicting a significant
directed diffusion towards energy minima yielding locally enhanced exciton densities. The
energy variations are attributed to inhomogeneous DNA-wrapping of the nanotubes, used
for their separation during sample preparation.
In the last part, the microscopic relation between the defect-induced Raman D-band and
the defect density has been investigated for metallic SWNTs. The length scale of the D-
bandscatteringprocessinthevicinityofdefectswasimagedwithTENOMforthefirsttime
and found to be about 2nm. Furthermore, localized defects have been photo-generated
intentionally by the strong fields at the tip while recording the evolution of the local Raman
spectrum. Based on this data, a quantitative relation could be determined, that is highly
relevant for the characterization of carbon nanotubes via Raman spectroscopy.
iiiContents
1 Introduction 1
2 Single-walled carbon nanotubes 5
2.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 Real space lattice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2 Reciprocal space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Band structure and excitons . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1 Derivation from graphene . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2.2 Metallic SWNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.3 Semiconducting SWNTs . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.4 Excitons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3 Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.4 Raman scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.4.1 Resonance Raman scattering . . . . . . . . . . . . . . . . . . . . . . 23
2.4.2 Raman scattering of graphene . . . . . . . . . . . . . . . . . . . . . . 23
2.4.3 of SWNTs . . . . . . . . . . . . . . . . . . . . . . 27
3 High-resolution optical microscopy 31
3.1 Propagation of light and the diffraction limit . . . . . . . . . . . . . . . . . 32
3.1.1 Angular spectrum representation . . . . . . . . . . . . . . . . . . . . 32
3.1.2 The resolution limit . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.2 Near-field concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.3 Tip-enhanced near-field optical microscopy . . . . . . . . . . . . . . . . . . . 39
3.3.1 Field enhancement at a metal tip . . . . . . . . . . . . . . . . . . . . 40
3.3.2 Signal enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.3.3 Illumination schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.4 Far-field concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4 Experimental details 49
4.1 TENOM setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.1.1 Confocal laser scanning microscope . . . . . . . . . . . . . . . . . . . 49
4.1.2 Shear-force AFM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.3 Near-field imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.3.1 APD based near-field imaging . . . . . . . . . . . . . . . . . . . . . . 56
4.3.2 Spectroscopic imaging . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5 Photo-induced photoluminescence bleaching and blinking 59
5.1 Influence of excitation power and surrounding atmosphere . . . . . . . . . . 59
5.2 Evaluation of step-like PL bleaching and blinking . . . . . . . . . . . . . . . 65
v6 Exciton diffusion - analytical and numerical treatment 69
6.1 Analytical description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
6.2 Numerical simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.3 simulation of the TENOM imaging process . . . . . . . . . . . . 74
6.4 Simulation of exciton diffusion in an inhomogeneous environment . . . . . . 76
6.5 Modeling of time-resolved experiments . . . . . . . . . . . . . . . . . . . . . 78
6.6 Influence of QY and exciton mobility on the near-field PL enhancement . . 80
7 Exciton mobility and quenching studied with TENOM 83
7.1 PL intensity variations along individual SWNTs . . . . . . . . . . . . . . . . 83
7.2 Exciton quenching near ends and defects - experiment and simulation . . . . 84
7.3 quenching near a localized single defect . . . . . . . . . . . . . . . . 86
7.4 Exciton diffusion length and the SWNT environment . . . . . . . . . . . . . 88
8 Exciton localization visualized using near-field spectroscopic imaging 91
8.1 Observation of strongly localized PL . . . . . . . . . . . . . . . . . . . . . . 92
8.2 Experimental evidence for exciton energy gradients at localization sites . . . 92
8.3 Simulation of exciton diffusion directed by energy gradients . . . . . . . . . 95
8.4 Origin of the energy variations . . . . . . . . . . . . . . . . . . . . . 96
9 High-resolution spectroscopic imaging of localized defects 99
9.1 Spatial extension of the D-band scattering process . . . . . . . . . . . . . . 99
9.2 Tip-induced generation of localized defects . . . . . . . . . . . . . . . . . . . 103
9.3 Relation between defect density and D-band intensity . . . . . . . . . . . . 105
10 Summary and outlook 107
Appendix: Tip-enhanced Raman scattering on single-layer graphene 111
Bibliography 117
Abbreviations 131
List of Figures 133
List of Publications 135
List of Conferences 137
Acknowledgments 139
vi1 Introduction
In the last two decades, the newly discovered nanoscale allotropes of carbon - namely
fullerenes, carbon nanotubes and graphene - have attracted enormous interest due to their
uniqueelectronicandopticalproperties. Thehugeattentiondirectedtothisresearchfieldis
exemplified by the two Nobel prizes that have been awarded for the discovery of fullerenes
and graphene. These nanoscale carbon materials hold great promise for applications in
future nano- and optoelectronic devices, as well as chemical sensors and photovoltaics.
Single-walled carbon nanotubes (SWNTs) are quasi one-dimensional objects that can be
regarded as seamless cylinders formed by rolling up a narrow ribbon of graphene. The
wide variety of possible structures, defined by their geometry with respect to the graphene
lattice, results in many specific optical and electronic properties that can be exploited
in future devices [1,2]. SWNTs can be either metallic with a very high current-carrying
9 2capacity up to 10 A/cm , or they can be semiconducting with band gap energies in the
5 2range

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