Carbon nanostructures under high pressure studied by infrared spectroscopy [Elektronische Ressource] / vorgelegt von Komalavalli Thirunavukkuarasu

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• Physik

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• Publié par: universitat_augsburg
• Publié le: 01 janvier 2009
• Nombre de lectures: 27
• Poids de l'ouvrage: 8 Mo

Extrait

Carbon Nanostructures Under High
Pressure Studied By Infrared
Spectroscopy
Dissertation zur Erlangung des Doktorgrades
der Mathematisch-Naturwissenschaftlichen
Fakultät der Universität Augsburg
vorgelegt von
Komalavalli Thirunavukkuarasu
April 2009Erstgutachter: Prof. Dr. C.A. Kuntscher
Zweitgutachter: Prof. Dr. A. Wixforth
Tag der mündlichen Prüfung: 25 May 2009Contents
1 Introduction 1
2 Experimental Techniques 5
2.1 Fourier transform infrared spectroscopy . . . . . . . . . . . . . . . . . . 5
2.1.1 Principle of Fourier transform spectroscopy . . . . . . . . . . . . 5
2.1.2 Fourier transform infrared spectrometer . . . . . . . . . . . . . . 8
2.1.3 Infrared microscope . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2 Infrared spectroscopy and optical response functions . . . . . . . . . . . 11
2.2.1 Optical response functions . . . . . . . . . . . . . . . . . . . . . 11
2.2.2 Drude-Lorentz model . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.3 Optical conductivity of inhomogeneous media . . . . . . . . . . 13
2.3 Infrared spectroscopy under high pressure . . . . . . . . . . . . . . . . 16
2.3.1 Diamond Anvil Cell . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3.2 Pressure determination method . . . . . . . . . . . . . . . . . . 21
2.3.3 transmitting media . . . . . . . . . . . . . . . . . . . . 24
2.3.4 IR measurements with a diamond anvil cell . . . . . . . . . . . 28
2.4 Infrared spectroscopy at synchrotron radiation facility . . . . . . . . . . 32
2.5 spy at low temperatures . . . . . . . . . . . . . . . . 36
3 Fullerene-based compounds 39
3.1 Introduction to fullerenes . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.1.1 Properties of C . . . . . . . . . . . . . . . . . . . . . . . . . . 3960
3.2 Pure C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4370
3.2.1 Basic Properties of C . . . . . . . . . . . . . . . . . . . . . . . 4370
3.2.2 Pressure-dependent properties of C : An infrared spectroscopic70
study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.3 Introduction to Cubane . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.4 Novel rotor-stator compounds . . . . . . . . . . . . . . . . . . . . . . . 59
3.4.1 C ¢C H and C ¢C H . . . . . . . . . . . . . . . . . . . . . . 5960 8 8 70 8 8
3.4.2 Pressure-dependent infrared studies on C ¢C H and C ¢C H 6360 8 8 70 8 8
3Contents
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4 Single-walled Carbon nanotubes 77
4.1 Properties of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . 77
4.1.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.1.2 Electronic properties . . . . . . . . . . . . . . . . . . . . . . . . 79
4.1.3 Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.1.4 SWCNTs under extreme conditions . . . . . . . . . . . . . . . . 92
4.2 Investigated nanotubes samples . . . . . . . . . . . . . . . . . . . . . . 95
4.2.1 Synthesis of SWCNTs by laser ablation . . . . . . . . . . . . . . 96
4.2.2 Unoriented SWCNT films . . . . . . . . . . . . . . . . . . . . . 96
4.2.3 Oriented SWCNTs in polyethylene matrix . . . . . . . . . . . . 99
4.2.4 Magnetically-aligned SWCNT film . . . . . . . . . . . . . . . . 100
4.2.5 Effect of purification on carbon nanotubes . . . . . . . . . . . . 101
4.3 Results and analysis: Ambient pressure studies . . . . . . . . . . . . . . 102
4.3.1 Unoriented SWCNT films . . . . . . . . . . . . . . . . . . . . . 102
4.3.2 Oriented SWCNTs in polyethylene matrix . . . . . . . . . . . . 109
4.3.3 Magnetically-aligned SWCNT film . . . . . . . . . . . . . . . . 112
4.4 Results and analysis: High pressure studies . . . . . . . . . . . . . . . . 114
4.4.1 Unoriented SWCNT films . . . . . . . . . . . . . . . . . . . . . 114
4.4.2 Oriented SWCNTs in polyethylene matrix . . . . . . . . . . . . 118
4.4.3 Magnetically-aligned SWCNT film . . . . . . . . . . . . . . . . 120
4.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
4.5.1 Comparison of studied films at ambient pressure . . . . . . . . . 124
4.5.2 Localization of the carriers . . . . . . . . . . . . . . . . . . . . . 126
4.5.3 Optical transition energies at extreme conditions . . . . . . . . . 129
4.5.4 Pressure-induced structural phase transition . . . . . . . . . . . 134
4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
5 Conclusions and outlook 139
Bibliography 143
Acknowledgements 157
Curriculum Vitae 159
List of publications 161
41 Introduction
The study of carbon nanostructures has emerged to be a giant thriving field of research
with the discovery of fullerenes in 1985 by Kroto and coworkers, and subsequently car-
bon nanotubes in 1991 by Iijima and coworkers. Extremely varied properties of the
carbon-based nanomaterials arise due to its exotic form which leads to reduced dimen-
sionality. While diamond and graphene (graphite) are well known three-dimensional
(3D) and two-dimensional (2D) forms of carbon, the fullerenes and carbon nanotubes
form the zero-dimensional (0D) and one-dimensional (1D) forms, respectively. Due
to the reduced dimensionality, the carbon nanostructures exhibit interesting physical
properties induced by strong many-body correlation effects. Some in proper-
ties of these carbon nanostructures are outstanding mechanical, thermal, electronic,
and electrical properties and chemical robustness. Furthermore, fullerenes readily form
derived materials by combining with a large variety of materials like organic molecules,
alkali metals, etc., to form a wide class of materials (for e.g. alkali fullerides, endo-
hedral fullerenes, exohedral fullerenes, host-guest compounds, polymerized fullerenes)
with extremely broad range of properties. Therefore, fullerenes and its derivatives
have endless list of applications in optical limiters, transistors, catalysts, hydrogen
storage, etc. For this reason, enormous efforts have been undertaken for reliable un-
derstanding of the basic properties of these carbon nanostructures. In addition to the
applications, thefullerene-basedmaterialsandthecarbonnanotubesofferalargescope
for studying the various physical phenomena induced in the low-dimensional systems.
The low-dimensional systems have shed new light on the effects of electron-phonon
interactions, disorder (for example, impurities) and electron-electron interaction on a
quantum system.
Within this project, the study of vibrational and the electronic properties of carbon-
based nanostructures, namely fullerene compounds and carbon nanotubes, has been
performed. The main goal of this project is the characterization of phenomena induced
by the application of external pressure such as structural phase transitions, insulator-
to-metal transition and polymerization reactions using infrared spectroscopy in the
far-infrared up to the visible frequency range as a function of pressure.
Infrared spectroscopy together with the low temperature and high pressure tech-
11 Introduction
niques forms a powerful tool to investigate the dynamics of the charge carriers and
provides important information on the fundamental energy scales involved in the var-
ious physical phenomena. It allows study of both electronic and vibrational excita-
tions, providing useful information on the microscopic mechanism that builds up the
electronic properties of the carbon nanostructures. External hydrostatic pressure com-
presses the lattice increasing the bandwidth of the electronic states and also induces
structural phase transitions. Therefore, in general, the application of pressure is con-
sidered a cleaner way to tune the properties of materials under investigation than
chemical doping, in order to understand the electronic properties. The combination of
high pressure with Raman spectroscopy is well established and widely used, but the
coupling of infrared spectroscopy to the high pressure techniques proved to be more
difficult. The main limitations arise from the sample size which causes diffraction arti-
facts and intensity of the sources. Although the limitations could be partly overcome
by use of infrared microscope and synchrotron radiation facilities, the difficulty of the
experiments make high pressure infrared spectroscopy less common and more novel.
The first part of the project is dedicated to the investigations on C ¢C H and60 8 8
C ¢C H which belong to a new class of rotor-stator fullerene compounds, and the70 8 8
fullerite C at high pressures over a broad frequency range.70
The fullerene-based materials are van der Waals crystals where the molecules are
rotating freely at high temperatures. On cooling or with the application of pressure,
the fullerites undergo orientational ordering transitions where the free rotation of the
fullerene molecules are restricted and eventually frozen. Therefore, the vibrational
degrees of freedom play an intrinsic role in these classes of materials. The vibra-
tional spectra are sensitive indicators for symmetry change during phase transitions,
electron-phonon coupling, and other variations in the internal dynamics of a mate-
rial. The crystalline C undergoes a series of orientational ordering transitions up on70
cooling. Generally, high pressure is ex