Tip-enhanced near-field optical spectroscopy on single-walled carbon nanotubes [Elektronische Ressource] / von Huihong Qian

Huihong Qian

Informations

• Publié par: ludwig-maximilians-universitat_munchen
• Publié le: 01 janvier 2008
• Nombre de lectures: 22
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Dissertation zur Erlangung des Doktorgrades
Der Fakultät für Chemie und Pharmazie
Der Ludwig-Maximilians-Universität München







Tip-enhanced Near-Field Optical

Spectroscopy on Single-Walled Carbon

Nanotubes















von
Huihong Qian
aus
Jiangsu, China




Juli, 2008





Erklärung

Diese Dissertation wurde in Sinne von § 13 Abs. 4 der Promotionsordnung vom
29. Januar 1998 von Herrn Prof. Dr. Achim Hartschuh betreut.





Ehrenwörtliche Versicherung

Diese Dissertation wurde selbstständig, ohne unerlaubte Hilfe erarbeitet.



München, am 08.07.2008









-------------------------------
(Unterschrift des Autors)





Dissertation eingereicht am 08.07.2008
1. Gutachter: Prof. Dr. Achim Hartschuh
2. Gutachter: Prof. Dr. Christoph Bräuchle


Mündliche Prüfung am 19.09.2008 This thesis is dedicated to my parents:
Yongqin Qian
and
Linzhen Gui
Acknowledgement
This thesis covers my three and half years of Ph.D work, contains both theoretical understand-
ing and experimental nding focusing on my research eld. Besides, those unsel sh help and
supports are absolultly necessary in the whole work that will be presented. At the rst position,
I would like to acknowledge the following people, without whom I could not proceed my work
successfully.
I thank Prof. Dr. Achim Hartschuh for almost ve years continual support and encour-
agement. I thank him for guiding me in the eld of both near- eld optical microscopy and
single-wall carbon nanotubes, giving me many opportunities to present the work, and helping
me for the movement to Munc hen.
I thank Prof. Dr. Alfred J. Meixner for many good suggestions, support and encouragement
during the rst one year and three months of my Ph.D in university of Tubingen. I thank him
for leading me into the microscopy world at the very beginning.
I thank Prof. Dr. Ado Jorio for the discussion on the Raman properties of carbon nanotubes
and part of the near- led results presented in this work. I also thank him for the support during
my stay in his group in brazil and our successful collaboration.
I would like to thank Carsten Georgi for the experimental support including the fabrication of
high quality gold tips and contribution to some data analysis on peak tting. I would also like to
thank Paulo T. Araujo (UFMG, Brazil) for some near- eld photoluminescence measurements
and valuable discussions regarding the obtained results and carbon nanotubes.
The DNA-wrapped CoMoCAT nanotube samples in my major publications are provided by
Alexander A. Green and Prof. Mark C. Hersam (Northwestern University), therefore
I would like to thank them for the great samples. For the preparation of the DNA-wrappedii
HiPCO nanotubes samples and the histogram of emission energies in Figure 42(a,b) I would
like to thank Tobias Gokus. For the sample deposition on substrates I would like to thank
Nicolai Hartmann.
I would like to thank my former colleagues Antonio Virgilio Failla, Mathias Steiner, Rafal
Korlacki and Hui Qian in university of Tubingen for great discussions on di erent elds and
collaborated publications that helped me build the sense of science. I would also like to thank
Cinzia Casiraghi (Freie Universit at Berlin) for spending time in our lab performing Rayleigh
scattering and Raman measurements on graphene and the very useful discussion on the results.
I thank Andreas Keilbach for spending his time reading my thesis draft, taking care of me and
encouraging me in my life. I couldn’t go through many di culties without his great support.
In addition, I would like to thank Elke Nadler (Uni Tubingen) and Ste en Schmidt (LMU)
for the SEM measurements on all gold tips, providing easy selection of good tips throughout my
work.
I gratefully acknowledge the nancial support provided by the Deutsche Forschungsgemein-
schaft (DFG-HA4405/3-1 and the Nanosystems Initiative Munich (NIM)) and the Deutscher
Akademischer Austausch Dienst (DAAD Probral).
At the end, I would like to thank my parents for the endless love and the encouragement on
both of my life and studies abroad.Abstract
High resolution optical methods overcome the di raction limit, a step essential for understand-
ing the physical and chemical properties of nanostructures. In this work, I applied tip-enhanced
near- eld optical microscopy (TENOM) to study the optical properties of single-wall carbon nan-
otubes (SWNTs) with nanoscale spatial resolution. Simultaneously obtained near- eld Raman
scattering and photoluminescence (PL) data is shown to provide information with unprecedented
detail on the nanotube structure and the resulting phonon and exciton properties. Near- eld PL
is found to be more localized along single nanotubes than Raman scattering in most cases due
to defects and environmental perturbations. By detecting near- eld PL spectra, my work has
shown exciton energy variations along the same nanotubes induced by the environment. The
local PL energy response to DNA-wrapping reveals large DNA-induced redshifts of the exciton
energy that are two times higher than indicated by spatially averaging confocal microscopy. Ex-
citon energy transfer between two semiconducting nanotubes is observed for the rst time limited
to small distances because of competing fast non-radiative relaxation. The transfer mechanism
is explained by F orster-type electromagnetic near- eld coupling. In addition, towards the end
of a nanotube, PL decay is observed on a length scale of 15-40 nm which is attributed to exciton
propagation followed by additional non-radiative relaxation at the nanotube end. The di erent
enhancement mechanisms of Raman scattering and PL lead to di erent enhancement factors of
the two signals. The PL enhancement can be stronger than the Ramant because of
the very low initial quantum yield of nanotubes. The signal enhancement of Raman scattering
and PL is also found to exhibit di erent tip-sample distance dependencies because of the PL
quenching e ects from the gold tip. The results achieved in my thesis highlight the enormous
capabilities of TENOM for the investigation of nanoscale surfaces.iiCONTENTS iii
Contents
1 Introduction 5
2 Introduction to single-wall carbon nanotubes 9
2.1 Formation of SWNTs and their structures . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Electronic properties of SWNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3 Phonon structures of SWNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4 Raman scattering of SWNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.5 Photoluminescence of SWNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.6 DNA-wrapped SWNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3 Tip enhanced near- eld optical microscopy 31
3.1 High-resolution microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.1.1 Angular spectrum representation . . . . . . . . . . . . . . . . . . . . . . . 31
3.1.2 Resolution limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.1.3 Sub-wavelength Imaging Concepts . . . . . . . . . . . . . . . . . . . . . . 35
3.2 Field-enhancement at a metal tip . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.3 Signal enhancement for Raman scattering and photoluminescence . . . . . . . . . 40
4 Experimental 43
4.1 Tip-enhanced near- eld optical microscopy . . . . . . . . . . . . . . . . . . . . . 43
4.2 Gold tip fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.3 SWNTs sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50