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Scanning tunneling microscopy and spectroscopy of sidewall functionalized singlewalled carbon nanotubes [Elektronische Ressource] / von Andrea Vencelová

149 pages
Scanning tunneling microscopy and spectroscopy of sidewallfunctionalized singlewalled carbon nanotubesDen Naturwissenschaftlichen Fakult atender Friedrich-Alexander-Universit at Erlangen-Nurn? bergzurErlangung des Doktorgradesvorgelegt vonAndrea Vencelov´aaus KoˇsiceAls Dissertation genehmigt von den NaturwissenschaftlichenFakult aten der Universit at Erlangen-Nurn? bergTag der mundlic? hen Prufung:? 29: September 2006Vorsitzender der Promotionskommission Prof. Dr. D.-P. H aderErstberichterstatter: Prof. Dr. L. LeyZweitberichterstatter: Prof. Dr. A. HirschContents1 Introduction 52 Carbon nanotubes 92.1 From graphene to carbon nanotubes. . . . . . . . . . . . . . . . . . . 92.2 Structure of single-walled carbon nanotubes . . . . . . . . . . . . . . 132.3 Electronic properties of SWCNTs . . . . . . . . . . . . . . . . . . . . 172.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.3.2 Band structure of graphene . . . . . . . . . . . . . . . . . . . 182.3.3 Zone-folding approximation . . . . . . . . . . . . . . . . . . . 202.3.4 Effect of curvature on the band structure of SWCNTs . . . . . 232.3.5 Density of states (DOS) . . . . . . . . . . . . . . . . . . . . . 262.4 Synthesis of SWCNTs . . . . . . . . . . . . . . . . . . . . . . . . . . 282.5 Purification of the raw material . . . . . . . . . . . . . . . . . . . . . 293 Experimental methods 303.1 Atomic force microscopy (AFM) . . . . . . . . . . . . . . . . . . . . . 303.
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Scanning tunneling microscopy and spectroscopy of sidewall
functionalized singlewalled carbon nanotubes
Den Naturwissenschaftlichen Fakult aten
der Friedrich-Alexander-Universit at Erlangen-Nurn? berg
zur
Erlangung des Doktorgrades
vorgelegt von
Andrea Vencelov´a
aus KoˇsiceAls Dissertation genehmigt von den Naturwissenschaftlichen
Fakult aten der Universit at Erlangen-Nurn? berg
Tag der mundlic? hen Prufung:? 29: September 2006
Vorsitzender der Promotionskommission Prof. Dr. D.-P. H ader
Erstberichterstatter: Prof. Dr. L. Ley
Zweitberichterstatter: Prof. Dr. A. HirschContents
1 Introduction 5
2 Carbon nanotubes 9
2.1 From graphene to carbon nanotubes. . . . . . . . . . . . . . . . . . . 9
2.2 Structure of single-walled carbon nanotubes . . . . . . . . . . . . . . 13
2.3 Electronic properties of SWCNTs . . . . . . . . . . . . . . . . . . . . 17
2.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.2 Band structure of graphene . . . . . . . . . . . . . . . . . . . 18
2.3.3 Zone-folding approximation . . . . . . . . . . . . . . . . . . . 20
2.3.4 Effect of curvature on the band structure of SWCNTs . . . . . 23
2.3.5 Density of states (DOS) . . . . . . . . . . . . . . . . . . . . . 26
2.4 Synthesis of SWCNTs . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.5 Purification of the raw material . . . . . . . . . . . . . . . . . . . . . 29
3 Experimental methods 30
3.1 Atomic force microscopy (AFM) . . . . . . . . . . . . . . . . . . . . . 30
3.1.1 Principles of measurement . . . . . . . . . . . . . . . . . . . . 30
3.1.2 Operating modes . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.2 Scanning tunneling microscopy (STM) . . . . . . . . . . . . . . . . . 33
3.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2.2 Fundamentals of the tunneling process . . . . . . . . . . . . . 33
3.2.3 Experimental basics of STM . . . . . . . . . . . . . . . . . . . 34
3.2.4 Operating modes . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.2.5 Tunneling current . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.3 Scanning tunneling spectroscopy (STS) . . . . . . . . . . . . . . . . . 40
3.3.1 Tunneling spy . . . . . . . . . . . . . . . . . . . . . . 40
3.3.2 Differential conductivity versus normalized conductivity . . . . 41
3.3.3 Analysis and interpretation of spectroscopic results . . . . . . 43
34 CONTENTS
4 Experimental details and sample preparation 47
4.1 STM equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.2 STM tip preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.3 Sample treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
5 Structural study of SWCNTs 59
5.1 How to determine (n;m) . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.2 Structural imperfections and defects . . . . . . . . . . . . . . . . . . . 67
5.3 Bundles of SWCNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
6 Purification of SWCNTs 86
6.1 Raw material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6.2 Comparison of different purification methods . . . . . . . . . . . . . . 91
6.2.1 Effect of HCl treatment - Sample A . . . . . . . . . . . . . . . 92
6.2.2ofHCltcombinedwithair oxidation-Sample B 94
6.2.3 Effect of 3M HNO - Sample C . . . . . . . . . . . . . . . . . 983
6.2.4 of 10M HNO - D . . . . . . . . . . . . . . . . . 1033
6.2.5 Comparison of the purification methods . . . . . . . . . . . . 105
7 Functionalization of SWCNTs 107
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.2 Topological results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
7.3 Spectroscopic results . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
8 Summary 122
8.1 Structural study of SWCNTs . . . . . . . . . . . . . . . . . . . . . . 122
8.2 Purification of SWCNTs . . . . . . . . . . . . . . . . . . . . . . . . . 123
8.3 Functionalization of SWCNTs . . . . . . . . . . . . . . . . . . . . . . 124
9 Zusammenfassung 126
9.1 Strukturelle Untersuchung von SWCNTs . . . . . . . . . . . . . . . . 126
9.2 Reinigung der SWCNTs . . . . . . . . . . . . . . . . . . . . . . . . . 127
9.3 Funktionalisierung von SWCNTs . . . . . . . . . . . . . . . . . . . . 129
10 Outlook 131
11 Appendix 132
11.1 A.1 Carbon nanotubes - A time line . . . . . . . . . . . . . . . . . . . 132
11.2 A.2 Estimation of the Fermi level shift . . . . . . . . . . . . . . . . . 134
11.3 A.3 CVD for the production of STM tips . . . . . . . . . . . . . . . . 136Chapter 1
Introduction
Quite a number of reports and a considerable literature about carbon nanotubes
(CNTs)havebeenpublisheduptonow. Carbonnanotubesaremoleculesconsisting
of graphenes sheet rolled up into a long cylinder capped at both ends. This simple
structure has sweeping consequences for the properties of carbon nanotubes [1, 2].
The main characteristics of carbon nanotubes are:
2† cylindrical shape of a graphene sheet exhibiting near ideal sp hybridization
of the constituent carbon atoms
† unique aspect ratio (length of tens of „m / diameter of <2 nm) [1]
† characteristic arrangement of hexagons with respect to the main axis, the so
called chirality
† semiconducting or metallic behaviour depending on the chirality
† densityofstateswithtypicalsingularitiesoriginatingfromtheone-dimensional
character of CNTs
† facility to easily build large bundles due to the van der Waals force
† potential for chemical modification
56 CHAPTER 1. INTRODUCTION
The list of physical and mechanical properties of carbon nanotubes shows that
they can beat any potential competition in certain fields of application:
property CNTs comparison
size diameter of 0:6¡1:8 nm photolithography yields lines
(single walled) with diameter >50 nm
3density 1:33¡1:4 g/cm density of aluminium is
32:7 g/cm
9tensile strength 45¢10 Pa tensile strength of
9steel alloy is…2¢10 Pa
7 2max. reachable 10 A=mm cooper wires melt at
4 2current density 10 A=mm
electron emission at 1¡3 V and molybdenum tips require
distance between 50¡100 V
electrods of 1 „m
thermal conductivity 6000 W/Km at RT thermal conductivity of
diamond is 3320 W/Km
–melting point 2800 C in vacuum and aluminium melts
– –750 C in air at 660 C in air
Shortly after the discovery of carbon nanotubes [3] a sort of application boom
started (see appendix A1). The spectrum of their application seems to be not
exhaustible. Insider and interested public know devices based on CNTs like nano-
transistorsaswellastheutopisticspacelift. However,itisonlyalittleknownabout
theprocessingofCNTsbeforetheirapplication. Eventheimpossibilityofthedirect
use of the raw nanotubes material is not apparent.
Allproductionmethodsyieldapartfromcarbonnanotubeshighamountsofaddi-
tionalnon-nanotubemateriallikeamorphouscarbon,carbonnanoparticles,graphite
flakes, fullerenes, and exposed or encapsulated residual metallic catalyst particles.
Fig.1.1 shows an STM image of the raw, unpurified nanotube material on an Au
substrate. The observed CNTs are aligned in bundles and associated with a great
number of small nanoparticles. This fact prevents the direct use of synthesized
material.7
46 nm
0 nm
Figure 1.1: STM image (¡0:4 V,1 nA; RT) of the raw, unpurified carbon nanotube
material deposited on an Au substrate.
Consequently, an effective purification procedure is essential as a pre-stage for
subsequent processing. Chapter 7 discusses the analysis of four purification proce-
dures and gives an overview of their effectiveness.
The knowledge of the structure of carbon nanotubes and their electronic pro-
perties are of great interest. The understanding of the occurrence and the conse-
quences of phenomenon like bending, bundling, pseudogap or opening are relevant
forthetechnicalapplication. WhileChapter2givesanintroductionintotheunique
structure and physical properties of CNTs, Chapter 6 reports on some interesting
anomalous structural and spectroscopic findings observed on carbon nanotubes in
this thesis.
The last part of this study concentrates on a possibility to modify the nature
of carbon nanotubes by the covalent functionalization. Here, a theoretical predic-
tion concerning the coupling of the functional groups on the nanotube wall was
investigated. Chapter 8 discusses the results of covalent sidewall functionalization
by t-Butyllithium. Fig.1.2 shows an STM image of the functional group (t-Butyl
group) observed on a single carbon nanotube marked in the image. This finding
represents the first successful proof of the decoration of carbon nanotubes by the
t-Butyl molecule.8 CHAPTER 1. INTRODUCTION
Figure 1.2: STM current image (¡1 V, 0:3 nA; 4.7K) of the t-Butyl group marked
by the black arrow observed on a single carbon nanotube.
Many experimental results in the field of carbon nanotubes have been recorded
using experimental techniques like transmission electron microscopy (TEM), X-ray
photoelectron spectroscopy (XPS), optical and luminescence measurements, or Ra-
man spectroscopy.
But due to the small dimension of CNTs and the necessity to test the local elec-
tronicproperties,amoredetailedandeffectiveexperimentalmethodwasrequiredin
thisstudy. Here, scanningtunnelingmicroscopy(STM)and, inparticular, scanning
tunneling spectroscopy (STS) offer multilateral advantages. Since STM enables the
scanningofthenanotubestructure,STSmakespossibletoinspectitselectroniccha-
racter and details of the density of states simultaneously. The combination of both
techniques allows a complete investigation. Chapter 4 gives a short introduction
into the used experimental techniques.
The results of this thesis are summarized in the resume and additionally a short
outlook is given showing some aspects of this study which need to be approached in
detail in the future.Chapter 2
Carbon nanotubes
In this Chapter a comprehensive introduction into the physical properties of single-
walled carbon nanotubes (SWCNTs) is given.
It starts with a basic discussion of their structure. Further, the effect of the one-
dimensional nature on the electronic properties is explained. In particular, the
energy dispersion relations of SWCNTs are described together with their characte-
ristic density of states. Additionally, the production techniques and the most used
purification methods are described.
AnexcellentintroductiontotheworldofSWCNTscanbefoundinthepublications
of Saito, Dresselhaus, Dresselhaus [1], and Reich, Thomsen, Maultzsch [2].
2.1 From graphene to carbon nanotubes
Carbon is a versatile element of huge importance. The uniqueness of this chemical
“chameleon” originates from the ability to change its basic valence configuration
2 22s ;2p through a linear combination of the electronic states, called hybridization,
into newly arranged hybrid orbitals. This phenomenon leads to various ways of
bonding, yielding different carbon-based materials.
The most common form of pure carbon on earth is graphite, which is composed
oflayers, called graphene sheets(Fig.2.1). Neighboringcarbonatomswithinasheet
2are bonded due to the sp hybridization (linear combination of one s and two p
2states) giving rise to three sp hybrid orbitals lying in the same plane and one pz
hybrid orbital perpendicularly aligned to them. In this way the well-known honey-
comb lattice is formed, consisting of six-membered carbon rings, hexagons. Layers
are stacked on top of each other as shown in Fig.2.1.
910 CHAPTER 2. CARBON NANOTUBES
Figure2.1: Principle of atomic layer stacking in graphite. The upper (black and blue
atoms) layer is shifted with respect to the lower (gray) layer. Consequently, there
are two different types of atoms: the atoms A with atoms below them, and the atoms
B without adjacent atoms in the lower layer. The interlayer distance amounts to
˚3:35A. The unit cells of graphene are marked by the black lines.
Another form of carbon is diamond, formed by tetrahedrally bonded carbon
3atoms. The sp hybridization (linear combination of one s and three p states)
3allows the formation of four equivalent ? sp hybrid orbitals in this case.
Apart from these long known solids there also exists a group of “small rela-
tives”, which were discovered recently. The fullerenes, which were observed first by
˚Smalley, Curl, and Kroto in 1985 [4], are spherical hollow molecules (7¡ 15Ain
diameter) composed of a shell of carbon atoms arranged in hexagons and pentagons
(five-membered carbon rings). Until 1991 fullerenes represented the last piece in
the chain of known carbon allotropes. During the study of the carbonaceous de-
posit from an arc discharge between two graphite electrodes, Iijima found strange,
highly crystallized carbon filaments, which were only a few nanometers in diameter
and several micrometers long [3]. The subsequent study showed their substructure:
cylinders of carbon atoms arranged as a Russian doll. Herewith the multi-walled
carbon nanotubes (MWCNTs) were first observed.
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