Investigating Physical Properties of Novel
Carbon-Based Materials
by
Nasser Soliman Demir
Submitted to the Department of Physics
in partial fulfillment of the requirements for the degree of
Bachelor of Science in Physics
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2004
®0Massachusetts Institute of Technology 2004. All rights reserved.
Author ............................
Department of Physics
May 14, 2004
Certified
by..............................--
......---.
Mildred S. Dresselhaus
Institute Professor, Department of Physics and Department of
Electrical Engineering and Computer Science
Thesis Supervisor
Acceptedby..............................
..............
David E. Pritchard
Senior Thesis Coordinator, Department of Physics
MASSACHUSETTS INSTUTE
OF TECHNOLOGY
JUL 0 7 2004
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2
Investigating Physical Properties of Novel Carbon-Based
Materials
by
Nasser Soliman Demir
Submitted to the Department of Physics
on May 14, 2004, in partial fulfillment of the
requirements for the degree of
Bachelor of Science in Physics
Abstract
In this thesis, we present the results of studies of physical properties in three classes of
novel carbon-based materials: carbon aerogels, single-walled carbon nanotubes, and
high thermal conductivity graphitic foams. The experimental technique of Raman
laser spectroscopy yields structural information about all of these materials that we
are investigating, including how the covalent bonds between the carbon atoms in the
base of the material change in the presence of metal doping (in the case of carbon
aerogels) and. electrochemical doping (in the case of carbon nanotubes). In addition,
we present the results of Raman spectroscopy performed on high thermal conductivity
graphitic foarns, which consist of a weblike region containing highly-aligned filaments,
and an interfoam region consisting of disrupted junctions. The expected Raman
spectra for disordered and graphitic regions are then compared with our experimental
results. While the main characterization technique used was Raman spectroscopy, we
also performed magnetic susceptibility and X-ray diffraction measurements on the
doped carbon aerogels to ascertain other physical properties.
Thesis Supervisor: Mildred S. Dresselhaus
Title: Institute Professor, Department of Physics and Department of Electrical Engineering and Computer Science
3
4
Acknowledgments
Words cannot express my most sincere and deepfelt gratitude to Millie Dresselhaus
for all her help. Even in the most hectic times of the semester, her patience and
dedication towards ensuring that I make progress, and consistent advice on how to
pursue the highest quality research has always been there. I will always remember
all that she has done for me, and she has my utmost respect which words cannot
describe.
I also want to thank Joe Satcher and Ted Baumann of Lawrence Livermore National Laboratory for providing us with much background and guidance about carbon
aerogel synthesis, properties and characterization, the samples to conduct our research
on, and the Lawrence Livermore subcontract for providing us with the funding needed
to pursue our experimental research.
I would also like to express how great a pleasure it was to work with Paola Corio
and Ado Jorio on their paper, and all their valuable suggestions towards improving
the paper, which had taught me an unbelievable amount.
I want to give special thanks to Eduardo for all the work he did on the graphitic
foams project, and will never forget all the times he said, "Don't worry about that;
I'll take care of it," when I felt so overwhelmed. I also want to thank Oak Ridge
National Laboratory for providing us with the samples on the carbon foam project,
and Jim Klett for all his helpful papers in guiding Eduardo and me on this project.
I want to express my utmost gratitude to Laura Doughty for all her work in
making the Dresselhaus research group the amazing success it is. She has been the
woman behind the scenes making sure not only that all the group members got
their feedback on work given to Millie during her hectic schedule, but also personally
managed staying on top of things when I asked Millie to write all my letters of
recommendation to graduate school, which I happily can say now that I will be
pursuing.
And of course, there's Grace, Oded, Georgii, Victor, and Ming, who all have made
working with the Dresselhaus group feel like working for a family. Whether it was
5
answering questions I had regarding my research, or personally training me in SEM
or transport measurements (yes Dr. Rabin you know who I am talking about), or
just cheering me up when I was feeling overwhelmed, I will always remember how
much of a pleasure it was to work with all of you.
A major part of this work could not have succeeded without the contributions
Joe Adario made to the carbon aerogel project. His work on the project with X-ray
diffraction and elemental: analysis brought together parts of the project that otherwise
may have not been brought together.
I would like to give a special thanks to my parents and brother, who always
comforted me on the telephone whenever I felt overwhelmed by MIT, and for their
perrenial faith in my survival and success at this Institute.
Last, but certainly not the least, I want to thank God, the Most Gracious, Most
Merciful, for blessing me not only with the most wonderful family at home, but also
the most wonderful community of friends and colleagues at MIT.
6
Contents
1 Introduction
17
2 Structural, Spectroscopic, and Magnetic Properties Characterization of Doped CAs
2.1
2.2
19
Synthesis of Doped CAs .................
.
.
.
.
.
.
.
20
2.1.1
Synthesis of CAs ................
. . . . . . .
20
2.1.2
Ion-Exchange Method .............
. . . . . . .
20
Elemental analysis/X-Ray Diffraction ..........
22
. . . . . . .
2.3
Raman Spectroscopy ...................
30
. . . . . . .
2.3.1
G-band and D-band Analyses ..........
31
. . . . . . .
2.4
Magnetic Susceptibility ..................
37
3 Spectroelectrochemical Study of SWNTs
3.1
Synthesis of SWNTs: The Electric Arc Method
3.2
Previous Study Using H 2SO 4 As an Electrolyte
3.3
Results Using K2SO4 and HNO3 as Electrolytes
4 Structural Investigation of HTCFs
43
............
............
............
43
46
49
61
4.1
Synthesis of HTCFs ............................
62
4.2
Rarnan Spectroscopy on HTCFs .....................
62
5 Conclusions
69
5.1
Carbon Aerogels ............................
5.2
Single-Walled Carbon Nanotubes
69
....................
7
70
5.3
High Thermal Conductivity Graphitic Foams ..............
70
A EDAX Results for Co-doped CA
71
B EDAX Results for Fe-doped CA
75
C EDAX Results for K-doped CA
79
8
List of Figures
2-1 SEM picture of iron-doped CA heat-treated at 800° C. The CA sample
was coated with 10 nm of gold prior to being investigated by SEM to
prevent charge-up. As can be seen, nanoparticles and pores
10 nm
in size were observed ......................................
21
2-2 Scherrer-Debye experiment performed on CA sample doped with iron
ions. Note that only amorphous carbon peaks are observed, indicating
diffuse X-ray scattering but no diffraction peaks for an ordered crystal.
The values on the abscissa are in units of degrees............
23
2-3 Scherrer-Debye experiment performed on CA samples containing iron
nanoparticles. The sample was heat-treated and carbonized at 1050°
C. The values on the abscissa are in units of degrees ............
.
24
2-4 Scherrer-Debye experiment performed on CA samples containing cobalt
nanoparticles. The sample was heat-treated and carbonized at 1050°
C. The values on the abscissa are in units of degrees ............
.
25
2-5 Scherrer-Debye experiment performed on CA samples containing potassium nanoparticles.
The sample was heat-treated and carbonized at
1050° C. The reason only amorphous carbon are observed, indicating
diffuse X-ray scattering, is that potassium was too light an element to
be resolved using the diffractometer used. The values on the abscissa
are in units of degrees...........................
9
26
2-6 Scherrer-Debye experiment performed on CA sample doped with potassium ions. Note that only amorphous carbon peaks are observed, indicating diffuse X-ray scattering but no diffraction. The values on the
abscissa are in inits of degrees .....................
.........
.
27
2-7 Scherrer-Debye experiment performed on CA sample doped with cobalt
ions. The values on the abscissa are in units of degrees ....... ..
.
28
2-8 Schematic depicting the Raman experimental apparatus used to acquire the data for all the Raman results in this thesis. Taken from the
Renishaw
website .
. . . . . . . . . . . . . . . . . . . . . . . . . . .
32
2-9 Raman spectra for 3 CAs lacking in dopants undertaken in a previous study of CAs, with different densities. The D-band and G-band
33
features are labelled. Taken from [1]. .................
2-10 Sample Raman spectrum taken of a Fe-doped CA, pyrolyzed at 800°
C, with all the relevent features labelled .................
34
2-11 Plots of the tiends in the D- and G- bands as a function of heattreatment temperature, juxtaposed with the corresponding modes for
the undoped CAs. Also included is the Raman spectrum of an inhomogeneously doped sample. (Ru was the dopant in that sample).
.
36
2-12 Magnetic susceptibility measurements at a steady field of 1000 gauss
as temperature was varied from 5 K-300 K, for CA samples doped with
39
cobalt ions and cobalt nanoparticles ...................
2-13 Magnetic susceptibility measurements at a steady field of 1000 gauss
as temperature was varied from 5 K-300 K, for CA samples doped with
iron ions and iron nanoparticles
40
.....................
2-14 Magnetic susceptibility measurements at a steady field of 1000 gauss
as temperature was varied from 5 K-300 K, for CA samples doped with
potassium ions and potassium nanoparticles
............. ......
.
41
3-1 Schematic of an electric arc generator that could be used to synthesize
SWNTs. Motivated by an example depicted in [2]............
10
45
3-2 A schematic depicting the density of states as a function of energy
for metallic SWNTs. The spikes in the diagram are van-Hove singularities (vHSs) in the density of states (DOS). A photon with energy
corresponding to that of the energy of the radiation being used as the
excitation source excites electrons from the valence band (v) to the
conduction band (c). An example of the different energy levels for a
nanotube is given. For semiconducting SWNTs, the DOS level is zero
in the energy range corresponding to the band gap between the first
vHS in the conduction and valence bands ......................
.
47
3-3 A sample of a Kataura plot. On the ordinate are the transition energies,
and on the ordinate are the nanotube diameters.
This plot can be
used to determine which nanotubes are accessed through a given laser
excitation energy (i.e. whether the nanotubes that are in resonance
with a given laser energy are metallic or semiconducting, and which
electronic levels as indexed and depicted by Figure 3-2 .) ......
.
48
3-4 Raman spectra obtained from a Raman backscattering experiment performed using a SWNT film cast on a platinum surface in a 0.5 M H2 SO4
aqueous solution obtained at the applied potentials indicated, in the
direction indicated by the arrow. The excitation radiation used was
Eraser = 1.96 eV. An interesting feature to observe is that the Gband lineshape changes from a metallic lineshape (with a noninfinite,
nonzero coupling constant q in the BWF function) at Vap = 0 to a less
metallic lineshape with increasing Vap.........................
.
50
3-5 Raman spectra obtained from a Raman backscattering experiment using a SWNT film cast on a platinum surface using a 0.5 M K2 SO4
aqueous solution obtained at the applied potentials indicated in the
graph above, indicating this was a p-type doping process. The arrow
indicates the direction of voltage application as the experiment proceeded, and the excitation radiation
11
was Elaser = 1.96 eV ........
52
3-6 Raman spectra obtained from a Raman backscattering experiment using a SWNT film cast on a platinum surface using a 0.5 M K2 SO4
aqueous solution obtained at the applied potentials indicated in the
graph above, indicating this was an n-type doping process. The arrow indicates the direction of voltage application as the experiment
proceeded, and the excitation radiation was Elaser = 1.96 eV......
53
3-7 Raman spectra obtained from a Raman backscattering experiment using a SWNT flm cast on a platinum surface using a 0.5 M HNO3
aqueous solution obtained at the applied potentials indicated in the
graph above. The arrow indicates the direction of voltage application,
and the excitation radiation was Elaser = 1.96 eV ............
55
3-8 Wavenumber of the tangential G-band and for a SWNT film in different
electrolyte media as a function of the applied potential for Elaser = 1.96
eV ....................................................
56
3-9 Wavenumber of the second-harmonic G'-band and for a SWNT film
in different electrolyte media as a function of the applied potential for
57
Elaser = 1.96 eV ..............................
3-10 Wavenumber of RBMs for a SWNT film in different electrolyte media
as a function of the applied potential for Elaser = 1.96 eV .......
.
58
3-11 Intensity (b) o RBMs for a SWNT film in different electrolyte media
as a function cf the applied potential for Elaser = 1.96 eV. The RBM
frequencies
WR,3M
occurred at - 185 cm-1 for K2 SO 4, and
-
195 cm-
1
195 cm- '
for HNO3 , and H2 SO4 had two RBM frequencies located at
and - 217 cm-1- ..............................
59
4-1 SEM picture of an HTCF synthesized using Mitsubishi ARA24 pitch
as the base. This sample has a bulk density 0.54 g/cml3 .........
12
63
4-2 A map indicating the direction we conducted the Raman sweeps. We
first began at the left end of the horizontal arrow and swept to the
right, then pursued the direction perpendicular to the first direction
and swept down the plane .........................
65
4-3 Raman microprobe picture taken of a suspected graphitic region of the
same sample .............................................
66
4-4 Raman microprobe picture taken of a suspected disordered region of
the same sample ...........................................
.
67
4-5 Raman spectra taken of two locations in the sample, in suspected
graphitic and disordered regions. Note that the D-band intensity (located at
1300 cm -1 ) is much higher in region 11 (in our map corre-
sponding to a disordered region) than in region 7 (in our map corresponding to a suspectedly graphitic region.)
............. ......
A-1 EDAX results for CA doped with cobalt ions ...................
.
68
.
72
A-2 Continuation of EDAX results for CA doped with cobalt ions...
73
B-1 EDAX results for CA doped with iron ions...............
76
B-2 Continuation of EDAX results for CA doped with iron ions ......
77
C-1 EDAX results for CA doped with potassium ions...........
.
80
C-2 Continuation of EDAX results for CA doped with potassium ions. . .
81
13
....
14
List of Tables
2.1 Results of nanocrystal width calculations ................
29
2.2
38
Parameters for Samples With Simple Curie-like Behavior ........
15
16
Chapter
1
Introduction
A variety of novel carbon-based materials have been synthesized and engineered for a
variety of applications. Many materials can be manufactured with the use of carbon
either as a base or dopant, due to its electronic configuration of 2s22p2. Carbon nanotubes could be used in the design of supercapacitors and batteries, and can be either
semiconducting or metallic.[2] Carbon aerogels (CAs), although poorly conducting if
not nonconducting, have been proposed as a storing matrix for carbon nanotubes.[3]
More importantly, doped CAs could be used to store charge, indicating a potential
for use as electrodes in electrical devices such as supercapacitors and batteries.[3]
Another novel type of carbon material investigated in this thesis is high thermal conductivity graphitic foam (HTCF). These foams have been synthesized and proposed
for use in systems requiring efficient thermal management, such as in power plants
and air conditioners.[4] Hence the structural and physical properties of these novel
carbon materials are of interest not only for academic scientific questions, but also
for practical engineering applications.
The main techniques used for investigating the physical properties of the aforementioned novel carbon materials include Raman spectroscopy, transport measurements using a superconducting quantum interference device (SQUID), and compositional/elemental analysis utilizing x-ray diffraction (XRD) and scanning electron
microscopy (SEM). The metal-loaded CAs were analyzed using Raman spectroscopy,
XRD, and SQUID measurements, while the HTCFs and SWNTs were analyzed using
17
Raman spectroscopy.
Chapter two gives an overview of and presents the results of the characterization
studies on CAs. It begins with a description of the ion-exchange method used to
synthesize the CAs and how the doping process was performed.
It then presents
the results of the characterization of the CAs by Raman experiments and SQUID
measurements.
Chapter three gives an overview of and presents the results of the spectroelectrochemical studies of SWI[Ts placed on an electrode and using different electrolytes. It
presents evidence on shifting of van Hove singularities (vHs) in the electronic density
of states and investigates the effect of both n-type and p-type doping on the different
Raman spectroscopic features observed by applying an external potential.
Chapter four gives an overview of and presents the results of the Raman experiments on HTCFs.
The procedure for synthesizing HTCFs is described, and the
results of Raman experiments conducted on HTCFs is presented. What is special
about HTCFs is that no previous Raman study on HTCFs has yet been conducted.
Chapter five restates the conclusions reached in the studies of each of the classes
of novel-based carbon materials, and ties the different results together.
18
Chapter 2
Structural, Spectroscopic, and
Magnetic Properties
Characterization of Doped CAs
In this chapter, the results of characterizing the structural, spectroscopic, and magnetic properties of doped CAs are reported. We begin by describing the synthesis of
carbon aerogels, along with a description of the ion-exchange process through which
the aerogels are doped with the metals in question. We then report on the Raman
findings, followed by the magnetic susceptibility measurements made using a superconducting quantum interference device (SQUID). My specific contributions to this
project included acquiring and analyzing data regarding the Raman spectroscopy
taken of the CAs, some SEM characterization of the CAs, and making magnetic susceptibility measurements of the CAs. I also analyzed the data acquired from all the
measurements I took, as well as analyzing the data from the x-ray diffraction DebyeScherrer (20) experiments to determine nanocrystal width of the nanoparticles in the
set of CA samples that were carbonized and heat-treated at 1050° C.
CAs belong to a special class of low-density microcellular materials, and have a
number of very interesting properties. CAs are cluster-assembled porous materials,
and their microstructure consists of a conductive network of covalently bonded carbon
nanoclusters, which are grains 3-25 nm in size. This network also gives rise to a
19
substructure containing mesopores 2-50 nm in size. (See Figure 2-1.) As a result,
very low mass densities (0.1-0.6 g/cm 3 ) and extremely high surface areas (600800 g/cm 3 ) are achieved, but also associated with the materials morphology is a
structural disorder, and as a consequence, one finds a high number of dangling bonds
and defect states not oly on the surface, but also inside the grains.[1] Due to this
unique microstructure, inserting metal species into CAs has been of interest not only
to tailor the electrical properties of CAs, but to also possibly use CAs as a host for
inserting SWNTs and other nanostructured forms of carbon.[3] We hence report on
the effects of doping on the structural and magnetic properties of CAs.
2.1
2.1.1
Synthesis of Doped CAs
Synthesis of CAs
Traditionally, inorganic aerogels are formed through a process of hydrolysis and condensation of metal alkoxides such as tetra-methoxysilane.
The resulting aerogel is
then supercritically dried to form a highly porous structure with the properties described above. For aerogels whose base is synthesized via the aqueous polycondensation of resorcinol with formaldehyde (RF aerogels), the gels are supercritically dried
at conditions of (To = 31°C, P = 7.4MPa), the critical point of carbon dioxide.[l]
2.1.2
Ion-Exchange Method
After the CA is supercritically dried, it can be pyrolyzed at high heat-treatment
temperatures and carbonized if nanoparticles of the dopants are to be achieved. The
ion-exchange method begins with an ionic compound such as table salt (NaCl). A
reaction is applied to the compound that ionizes it into anions and cations, and the
desired species is then transferred into the inorganic aerogel after it has condensed
and undergone hydrolysis. The CA is then supercritically dried, resulting in a highly
porous structure. It is then pyrolyzed and carbonized if nanoparticles of the dopants
are to be made.
20
Figure 2-1: SEM picture of iron-doped CA heat-treated at 800° C. The CA sample
was coated with 10 nm of gold prior to being investigated by SEM to prevent chargeup. As can be seen, nanoparticles and pores
10 nm in size were observed.
21
2.2
Elemental analysis/X-Ray Diffraction
X-ray elemental analysis was achieved using EDAX and the Scherrer-Debye (20)
method and the measurements were conducted by Dr. Joe Adario at the X-Ray
Diffraction laboratory of the MIT Center for Materials Science and Engineering
(CMSE). The results of the Schrerrer-Debye experiments are presented in Figures
2-2 through 2-7, and the results of the EDAX findings are tabulated in the appendices.
One set of CA samples were prepared for elemental analysis/X-ray diffraction
measurements, and there was another set of CA samples doped with iron, cobalt,
and potassium that were prepared for use in the SQUID measurements discussed
in section 2.4. There were a total of six samples that were measured. One set was
prepared via the ion-exchange method, and another set was also heat treated at 1050°
C and carbonized, such that the sample then contained nanoparticles of the dopant.
This was confirmed later via X-ray diffraction.
For the samples heat-treated and
then carbonized, diffraction peaks, corresponding to Miller planes, consistent with
the selection rules of te
crystal structure of the dopant were observed, indicating
that there indeed were nlanoparticles of the dopants in the CA matrix. (See Figures
2-3, 2-4, and 2-5). This conclusion is consistent with there being no diffraction peaks
otherwise for the samples containing dopants in the CA material except for the broad
(002) feature for the disordered sp2 carbon structure associated with carbon aerogels.
However, for the samples containing only ions inserted into the CA matrix via the ionexchange method, only broad peaks due to highly disordered carbon were observed.
(See Figures 2-2, 2-6, and 2-7). The reason why sharp diffraction peaks were not
observed in the CA sample containing potassium nanoparticles is that potassium
is too light an element to be detected for X-ray diffraction peaks by the resolution
of the XRD Debye-Scherrer system used in Dr. Joe Adario's lab.[5] The amorphous
carbon peaks correspond to the broad (002) peaks associated with sp2 carbons. These
results are consistent with the porous structure of CAs, where diffuse X-ray scattering
is dominant, and no sharp diffraction peaks should occur.
22
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are in units f degrees.
23
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24
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Figure 2-4: Scherrer-Debye experiment performed on CA samples containing cobalt
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on the abscissa are in units of degrees.
25
10
20
30
40
50
60
70
80
90
100
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Figure 2-5: Scherrer-Debye experiment performed on CA samples containing potassium nanoparticles. The sample was heat-treated and carbonized at 1050° C. The
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The values on the abscissa are in units of degrees.
26
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27
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28
Table 2.1: Results of nanocrystal width calculations.
Nanoparticle
Co
Fe
Lattice Constants a, c (A)
FWHM B ()
3.23 A
2.87 A
0.6 °
20
51.6 °
0.2°
43.6 °
()
Lattice Spacing t (nm)
21.1 nm
54.3 nm
The lattice constant of the nanocrystals of the dopant materials can be determined
using Bragg's law of diffraction[6],
nA = 2 dhkesinO,
(2.1)
where n is the order of diffraction (1 in our findings), A is the wavelength of the
radiation used, and dhke is the interplanar spacing between two adjacent planes with
Miller indices (hkf), and 0 is the diffraction angle. Since dhkl is related to the lattice
edge in a cubic crystal, a, through
a
dhkl
/h 2 + k 2 + £2
(2.2)
For cobalt, which has a hexagonal close packed (hcp) structure, we keep in mind that
the spacing between adjacent hexagonal cell layers c is given by
c=
a,
(2.3)
where a is the lattice constant.[6] The calculation for the lattice constant of Co was
calculated using its atomic volume, obtained from a periodic table.
We can also
determine the nanocrystal size t through the Scherrer equation[4]
0.89A
t t= Bcos(20)'
B-'l)'(2.4)
(2.4)
where A is the X-ray wavelength that was used for the experiment (Ac,(Ko) = 1.5418
A)[5], B is the full-width-at-half-maximum (FWHM) of the diffraction peak, and 20
is the diffraction angle of the peak. The results are summarized in Table 2.1.
29
2.3
Raman Spectroscopy
The Raman experiments were performed using the back-scattered configuration depicted in Figure 2-8. A laser beam with a known wavelength serves as the excitation
radiation source, and is directed and reflected via several mirrors directly onto the
surface where the sample is placed. For optimal results, the surface of the sample to
be analyzed should be as smooth as possible. If the sample is bulky (such as a CA or
HTCF), the laser beam should be directed perpendicular to the sample surface. Incoming photons from the incident excitation radiation then backscatter off the lattice
sites in the sample, and the backscattered laser beam is reflected via a mirror through
a notch filter into an air-cooled charged coupled device detector. A photodiode then
counts the backscattered photons with energies corresponding to wavenumber shifts
in the 0 - 3000 cm - 1 range, and a sample Raman spectrum is depicted in Fig 2-10.
The different features associated with the spectrum are explained below.
CA samples, doped with iron and ruthenium and pyrolyzed at heat treatment
temperatures of 800°C and 1050°C, were placed on a glass slide. The 514.5 nm (2.41
eV) line from an air-cooled Ar+-ion laser was used as the excitation radiation, and
a lOX objective was used to focus the laser beam on the sample. The scattered
light was measured with an air-cooled charged coupled device (CCD) detector. The
observed features included the disorder-induced D-mode (located at
and the tangential stretching G-mode (located at
-
1580 cm-').
-
1350 cm-1 ),
A sample Raman
spectrum is shown in Figure 2-10, with the aforementioned features described. A
standard Raman spectrum has the intensity of the backscattered light in counts on
the ordinate, and the Raman shift is measured in units of cm -1 on the abscissa. As a
result of energy and momentum conservation, signals should be observed at locations
on the abscissa corresponding to phonon vibrational modes corresponding to the
aforementioned wavenumbers. Furthermore, Raman signals occur when the difference
between the incident eergy Elaser and the backscattered light energy is in resonance
with the energy corresponding to the aforementioned vibrational shifts, and hence
Lorentzian lineshapes should be observed. However, there is one exception to this
30
rule, and that is associated with the G-band mode associated with metallic nanotubes.
Since the energy bands of metals are partially filled according to band theory, there
is an interaction with the energy associated with the filling of the electron states,
and hence there is a coupling and resulting asymmetry in the G-band lineshape (see
Chapter 3). The resulting lineshape is described by the Breit-Wigner-Fano function,
given by
I:0
1 + 2(w-wo)]2
q
+ [2(w-WO)]2'
1
(2.5)
(2.5)
where Io is the maximum intensity, w0 is the resonant frequency, w is the probe
frequency, F is the full-width-at-half-maximum (FWHM) intensity, and q is a coupling
constant.[I] Note that in the limit q
-
oo, the BWF form reduces to a Lorentzian,
as should be the case for semiconducting nanotubes and other non-metallic materials.
The Ranman experiments were conducted on a total of 4 samples. Two of them
were doped with ruthenium and were subsequently pyrolyzed at heat-treatment temperatures of 800° C and 1050° C after supercritical drying, and the other two were
doped with iron and pyrolyzed at the same heat-treatment temperatures after supercritical drying. All the samples were prepared via the ion-exchange method described
above.
2.3.1
G-band and D-band Analyses
Previous Ramnan studies on CAs have also observed the ubiquitous G-band and Dband modes. (See Figure 2-9). A juxtaposition of the Raman features observed in past
studies on pure CAs with the corresponding Raman features observed in this study
is presented in Figure 2-11. Appreciable frequency shifts were observed. Notably, the
frequencies of both the D-modes and G-modes in CAs doped with donor species are
less than those found in pure CAs. While no conclusive information about the bonds
can be drawn from the D-band alone, information on the spring constant based on
the spring lattice model for a solid can be gained from the G-band information. We
note that the frequency of the G-band in doped CAs is less than that in undoped
CAs. This illustrates that the charge introduced into the aerogel by the donor metal
31
·
I
U
7
1
Notc h filter
1I
Figure 2-8: Schematic depicting the Raman experimental apparatus used to acquire
the data for all the Ramnan results in this thesis. Taken from the Renishaw website.
32
300.0
i-o3
z
200.0
cd
C)
Z
U
--
100.0
n.
O O. O
1400.0
1600.0
RAMAN SHIFT (CM-')
1800.0
Figure 2-9: Raman spectra for 3 CAs lacking in dopants undertaken in a previous
study of CAs, with different densities. The D-band and G-band features are labelled.
Taken from [1].
33
Raman Spectrum of Fe-doped
CA at THT=8 0 0 C
I UUUU
9000
8000
7000
Cn
C
0
6000
a)
5000
.a)
CL
4000
0
O
3000
2000
1000
n
-500
0
500
1000
1500
2000
Raman Shift (cm-
2500
3000
3500
1)
Figure 2-10: Sample Raman spectrum taken of a Fe-doped CA, pyrolyzed at 800° C,
with all the relevent features labelled.
34
4000
45,
decreases the effective spring constant between the nearest neighbor carbon atoms.
This means that in the presence of a charged dopant species, the covalent bonds
between nearest neighbor carbon atoms in an aerogel will weaken. This behavior is
consistent with donor doping of carbon materials where electrons are transferred from
the dopant to the carbon host.[7]
There is another very noteworthy feature in Figure 2-11. In the Ru-doped CA
sample, the G-band feature seems to have separated into two components, which
I shall denote as G+ and G-, corresponding to the higher and lower frequencies,
respectively. This can be explained by the fact that in an inhomogeneously doped
sample, different amounts of charge transfer can occur between regions in the sample
that are rich in the dopant and regions in the sample that are poor in the dopant.
Hence, we argue that regions that are rich in ruthenium would have frequencies close
to
WG-
since the doping effect is stronger in those regions, while regions that are poor
in ruthenium would have frequencies closer to WG+ since the doping effect is not as
strong in those regions.
The next feature to consider is the effect that the heat-treatment temperature
seems to have on the different features. No modes seem to be appreciably affected
by a higher heat-treatment temperature, with the exception of a trend observed in
the G+ and G- bands for the same dopant. We note that the Raman measurements
themselves are made at room temperature and not while the heat-treatment process
is being carried out. The higher heat-treatment temperature may result in an increase
in the size of the metal nanoparticles. When the nanoparticles increase in size, the
electrons that had been released by the ions are returned to the metal nanoparticles
which will upshift the carbon G-band as the carbon host material loses electrons.
As a result, nearest-neighbor carbon covalent bonds are strengthened by the effect
of higher heat-treatment temperature.
However, this behavior was observed for the
Ru-doped CA but not for the Fe-doped CA.
35
D-band Frequency for Fe-doped CA
'
1360
E
>' 1350
I
v
I
I
I
I
D-mode for
CA w/out
dopant
_
-
ca)
a)
I
cm1
1340
D-mode1.337
1334
1350 cm -1
O
cm
IU
1330
"O
0
E
I
1320
7!50
I
I
800
850
I
I
900 THT ( C) 950
I
I
1000
1050
G-band Freauencv for Fe-dooed CA
I"
G-band
G-band
for CA w/out
dopant
G-mode
E
)
o 1600 _
I'1605 cm -1
O
1588 cm - 1
a)
1E586 cm-'
I
m
I
i
Q)
1100
U 1580
a)
"0
0
E
I
_
E 1560
X7
00
750
850 THc)
800
950c
1000
1050
1100
G+/G- Mode Frenuencies for Ru-doned CA
- 1600
1591 cm -
E
o
G+-mod
>, 1580
C
0
_
6- 1
a)
-I
- 1560 _
U
t554 cm-1
U. -u
1554 cm -
t rAn/
70 0
1
I
750
800
850
HT
C)
950
1000
1050
Figure 2-11: Plots of the trends in the D- and G- bands as a function of heat-treatment
temperature, juxtaposed with the corresponding modes for the undoped CAs. Also
included is the Raman spectrum of an inhomogeneously doped sample. (Ru was the
dopant in that sample).
36
1100
2.4
Magnetic Susceptibility
Magnetic susceptibility measurements using the SQUID are presented in Figures 212 through 2-14. A scan of the effective moment was carried out by measuring the
magnetic moment as a function of temperature
in the range of 5 K-300 K in 5 K
increments, in an environment with a steady field of 1,000 gauss.
For all of the
CA samples prepared that do not contain nanoparticles of the dopants, Curie-like
behavior is observed at low temperatures.
However, for the samples containing nanoparticles of the dopants, the x(T) (magnetic susceptibility) results differ substantially from simple Curie-like behavior. The
CA sample containing potassium nanoparticles demonstrates Curie-like behavior as
well, with the exception that the effective magnetic moment is much weaker than was
the case with the CA sample doped with potassium ions. This can be explained best
by a sharp drop in the magnetization of the materials as the materials crystallize.
However, this was not the case with the other samples. The results for the potassium nanoparticles contain trends that could be a superposition of the trends for the
samples doped with the nanoparticles of the other dopants. A possible explanation
for this is that not all the ions became nanoparticles as the CA was pyrolyzed and
carbonized. A Curie analysis was also applied to the set of data below 50 K, since
the X(T) versus T behavior resembled that of a simple Curie-like magnet. Although
potassium is diamagnetic, the reason why simple Curie-like behavior was observed
at all in the CAs doped with potassium ions and nanoparticles is that the EDAX findings (see the appendix), indicate the presence of both cobalt and iron in the sample.
Since the concentrations of these substances are on the order of
-
f/g/g, the magni-
tude of the effective moment is much less than the corresponding moments of the CA
samples that demonstrate simple Curie-like behavior containing other dopants.
For materials which exhibit Curie-like behavior, the following functional fitting
form for magnetic susceptibility X as a function of temperature applies,
X(T)
Xo + (T
37
C
(2.6)
Table 2.2: Parameters for Samples With Simple Curie-like Behavior.
Dopant
Co ions
Fe ions
K ions
K Nanoparticles
Xo(emu/g)
(-4.78 ± 3.29) x 10 - 3
(3.57 ± 1.09) x 10-3
0.03) x l0 - 4
(-4.58
(3.29 ± 3.39) x 10 - 4
C (emu.K/g)
(2.29 ± 0.10)
(1.04 ± 0.03)
(9.16 ± 0.34) x 0- 4
(9.26 ± 0.82) x 10-2
where Xo is a diamagnetic background susceptibility in enmu/g (which should be negative for sp2 carbon substances. I] 0 is a parameter with units of temperature, a is a
fitting parameter, and C is the Curie constant.[1] Furthermore, at low temperatures
and low magnetic fields, the expression for C is given by
(7~~~(-'
Jo = A
92 2 NvY 2J)
JoA
where g ~ 2 is the Lande g-factor,
kB
= 1.38 x
10 - 1 6
AB
a
00
xl-ae-dx
( kB ) /o (1 + 3e-x)
=
9.23 x
10-21
(2-7--
(2.7)
2'
erg/gauss is the Bohr magneton,
erg/K is the Boltzmann constant, Nv is the carbon atom density
per mole, A is the molar weight for carbon, and y is the number of spins per atom. [1]
However, for the materials that exhibit Curie-like behavior, the above form for the
susceptibility verses absolute temperature reduces to the simple Curie-law in the case
0 = 0 K and
= 1.
In order to relate form conclusive results about how the free carrier concentration
in the doped CAs differs from that of the previous study on pure CAs[1], a better
understanding of the material needs to be attained and further work is needed with
this analysis.
For the CA sample doped with cobalt nanoparticles, the increasing effective moment with increasing temperature can be best explained by ordered magnetization
occurring which was facilitated by a larger crystal, as determined from the nanocrystal
width calculations of Table 2.1.
The behavior associated with the CA sample containing iron nanoparticles can
be explained by iron's ferromagnetism.
The effective moment seems to approach
an asymptotic value, and this is consistent with the quenching effect in ferromag38
CAs Doped with Cobalt Ions
0.03
0.025
E
a)
0.02
4,
E
0.015
0
2
a)
0.01
Q-)
.2
.J
0.005
n1
v
0
50
100
150
Temperature
200
(K)
250
300
35(
250
300
35(
CAs Doped with Cobalt Nanoparticles
U.Z4
E
r
0.22
C
a)
E
0
0.2
.,
,
0. 18
0
0.16
0
50
100
150
200
Temperature
(K)
Figure 2-12: Magnetic susceptibility measurements at a steady field of 1000 gauss as
temperature was varied from 5 K-300 K, for CA samples doped with cobalt ions and
cobalt nanoparticles.
39
CAs doped with Iron Ions
0.01
--
a)
c-)
E
O
0)
0.008
0.006
0.004
0
ci)
rn
0.002
0
0
53
100
150
200
Temperature
(K)
250
300
35(
250
300
35(
CAs doped with Iron Nanoparticles
0.0485
0.048
E
C
a)
-
c
Q)
E
O
Q1)
.>_
0.0475
0.047
0.0465
O
QP)
,.,-
0.046
0.0455
0
50
100
150
200
Temperature (K)
Figure 2-13: Magnetic susceptibility measurements at a steady field of 1000 gauss as
temperature was varied from 5 K-300 K, for CA samples doped with iron ions and
iron nanoparticles
40
CAs Doped with Potassium Ions
x 10 -
-1.6
3 -1.8
E
-2
C
a)
E
o -2.2
a)
,-0
-2.4
a,)
LU -26R 1-2 R
0
50
100
Temperature
1.4
150
250
200
(K)
CAs Doped With Potassium Nanoparticles
x 10 -
1.2
a)
1
E 0.8
0
a)
a,
LU
0.6
0.4
0.2
0
0
50
100
150
200
Temperature (K)
250
300
Figure 2-14: Magnetic susceptibility measurements at a steady field of 1000 gauss as
temperature was varied from 5 K-300 K, for CA samples doped with potassium ions
and potassium nanoparticles
41
350
netic materials. This is because since ferromagnetic materials retain their "history"
of magnetization, so they can become saturated with magnetization after a certain
temperature.
This is consistent with the x-ray diffraction calculations in Table 2.1
indicating that the iron nanoparticles are smaller, so a smaller temperature is needed
to saturate the material.
42
Chapter 3
Spectroelectrochemical Study of
SWNTs
Conducting Raman experiments on SWNTs using electrochemical techniques has a
number of purposes. Investigating charge transfer is one such purpose. In this chapter,
the effect of charge introduction to SWNTs by use of an externally applied potential
is studied on a number of different metal surfaces. Also, evidence is presented for
shifting the van Hove singularities in the density of states through use of an applied
external potential to change the Fermi level (EF). Similarly,this study contrasts the
results conducted using p-type electrochemical doping (removal of electrons), and ntype electrochemical doping (addition of electrons). All experiments were performed
by Professor Paola Corio of the Instituto de Quimica at Universidade de Sao Paulo,
Brazil. I was involved in the analysis of the different Raman features of the resulting
spectra, with a special emphasis on the behavior of the Raman features with respect
to the different electrolytes.
3.1
Synthesis of SWNTs: The Electric Arc Method
The SWNTs used in this spectroelectrochemical analysis were prepared via the electric
arc discharge method. [2] To grow single wall carbon nanotubes (SWNTs), catalytic
particles are needed for the reaction to take place if isolated, but they are not necessary
43
if bundles of SWNTs are grown. Common catalysts include transition metals such as
cobalt, nickel, and iron.[2] A schematic for an apparatus used to prepare SWNTs via
this method is depicted in Figure 3-1.
Typically, a voltage of -20-25 V is applied across the electrodes, with an electric
current of -- 50-120 A DC. Also, the arc is typically kept in a chamber under pressure
conditions of
a diameter of
500 torr He. The carbon rod electrodes initially installed usually have
5-20 mm. As the voltage is applied and the synthesis procedure is
initiated, the carbon rod serving as the anode decreases in length, and the carbon
material constituting the anode goes towards serving as the base for the growth of
SWNTs, which are subsequently deposited on the anode.
As the deposit grows,
the SWNTs form bundles whose primary axes align more or less along the direction
of current flow. The bundles on the anode are surrounded by a shell consisting
of amorphous carbon ad carbon nanoparticles.
To get the most efficient yield of
SWNTs aligned along te current direction, the synthesis chamber needs to be cooled,
and a typical flow rate for cooling is -5-15 mL/s. As stated earlier, a catalyst is
needed to grow the individual SWNTs in. While the lengths of resulting synthesized
SWNTs all are on the order of -- 1 pm, the resulting diameter distribution of the
synthesized SWNTs follow, to best approximation, a Gaussian distribution.
The
reason why it is not a Gaussian distribution as predicted by the central limit theorem
is that not all diameters are possible; the diameters are restricted to the metallicity
and the values indexing the translation vector in the primitive cell of the graphite
honeycomb lattice.
There have been several methods used to calculate the diameter distribution of the
synthesized SWNTs. The SWNTs in this study, as well as those used in a previous
spectroelectrochemical study alluded to in this chapter, were determined to have a
diameter distribution o dt = (1.25 ± 0.20) nm using Pimnenta's method.[ 8]
44
To PunT
Gas Inlet
]1~
rough
Window
Figure 3-1: Schematic of an electric arc generator that could be used to synthesize
SWNTs. Motivated by an example depicted in [2].
45
3.2
Previous Study Using H2 SO4 As an Electrolyte
This spectroelectrochemical study also relied on data and results acquired from a
previous spectroelectro(hemical study on SWNTs described in Paolaet. al..[9]Raman
spectra of an SWNT film cast on a platinum surface in H2 SO4 aqueous solution
obtained at applied potentials in the range of Vap = 0.0-
1.3 V are presented in
Figure 3-4. The spectra were taken with a laser excitation energy of Elaser = 1.96 eV.
The Raman experiments for this study were performed using the back-scattered
configuration described in Chapter 2. The 632.8 nm (1.96 eV) line from an aircooled Ar+-ion laser was used as the excitation radiation, and a 80X objective was
used to focus the laser beam on the sample. This specific laser line was chosen so
that the resonance window for the radial breathing modes (RBMs) could be accessed
for metallic SWNTs. The transition energies Eii, where i = 1,2,3... between the
valence and conduction bands that correspond to energy differences associated with
van Hove singularities in the density of states, can be examined using a Kataura plot
of Eii vs. nanotube diameter, d, an example of which is given in Figure 3-3. This
Kataura plot is based on upon tight-binding calculations applied to transition energies
corresponding to energy differences between van Hove singularities, and a predicted
theoretical result is that the RBM frequency for an isolated SWNT is related to the
nanotube diameter through the relation[2]
RBM = -
dt
where
WRBMA
(3.1)
is the radial breathing mode frequency, a is a constant determined
experimentally to be 248 nm-cm -1 for isolated SWNTs sitting on a Si/SiO 2 substrate.
The scattered light was measured with an air-cooled charged coupling device (CCD).
Raman spectra obtained from a Raman backscattering experiment performed at
an excitation energy of Elaser= 1.96 eV using a SWNT film cast on a platinum surface
in a 0.5 M H2 SO4 aqueous solution are depicted in Figure 3-4, along with an arrow
indicating the direction the applied potential was changed, in increments of 0.1 V. In
addition to the disorder-inducedD-band (-- 1300cm-1 ), the tangential stretching G46
Densit
Of
States
Filled
States
Figure 3-2: A schematic depicting the density of states as a function of energy for
metallic SWNTs. The spikes in the diagram are van-Hove singularities (vHSs) in the
density of states (DOS). A photon with energy corresponding to that of the energy
of the radiation being used as the excitation source excites electrons from the valence
band (v) to the conduction band (c). An example of the different energy levels for a
nanotube is given. For semiconducting SWNTs, the DOS level is zero in the energy
range corresponding to the band gap between the first vHS in the conduction and
valence bands.
47
Kataura Plot
5)
I
dt (nm)
Figure 3-3: A sample of a Kataura plot. On the ordinate are the transition energies, and on the ordinate are the nanotube diameters. This plot can be used to
determine which nanotubes are accessed through a given laser excitation energy (i.e.
whether the nanotubes that are in resonance with a given laser energy are metallic or
semiconducting, and which electronic levels as indexed and depicted by Figure 3-2 .)
48
band (
1580 cm-1), and the G'-band (
of the D-band (
2630 cm- 1 ), which is the second harmonic
2630 cm- 1 ), which were observed in Raman spectra performed on
CAs, we observe radial breathing modes (RBMs) located in the low frequency end
of the spectra.
It should be noted that since the laser excitation line of 1.96 eV
was used for resonance with metallic nanotubes with diameters in the approximate
range (1.20 nm < dt < 1.35 nmni),as indicated by the Kataura plot(see Figure 3-3),
the application of a positive electrochemical potential decreases the SWNT electron
carrier density, since EF is decreased by this process. In the process of increasing Vp
in Figure 3-3, note that there is a substantial increase i the G-band frequency as Vp
is measured from Vap=0.8 V to 1.2 V, indicating that a van Hove singularity (vHS)
in the density of states was hit, thereby depleting electrons from this state. This is
because for metallic nanotubes, the DOS has a non-zero value in the band gap, and
hence there is an emptying of states as Vap is increased. As soon as a vHS is passed,
there is a large increase in the number of states that are emptied. (See Figure 3-2).
3.3
Results Using K2 SO4 and HNO3 as Electrolytes
An important experimental detail that distinguishes the present study from the previous study using H2SO4 as the electrolyte is that in this study, when K2 SO4 was
used as the electrolyte, both p-type doping (anodic polarization) and rn-type doping
(cathodic polarization) phenomena can be studied. This enabled us to detect a shift
in the van Hove singularity (vHS) since the response seen in the spectra between
empty and filled vHSs revealed an asymmetry leading us to this conclusion. The difference can be seen in Figures 3-5 and 3-6, which correspond to anodic polarization
(p-type doping), and cathodic polarization (n-type doping), respectively. This kind of
experiment was particularly important to carry out since one vital fact about electrochemistry is that the electrochemical potential is not equal to EF due to the fact that
simply placing a sample on a metal surface containing an electrolyte offsets the point
in the DOS vs. EF diagram where VQ = 0, which would normally lie midway between
the valence and conduction bands in the absence of the electrochenmicalinteraction
49
I
.,
C
4i
C2
5X
1000
1500
2W
2500
Raman Shift (cm')
Figure 3-4: Raman spectra obtained from a Raman backscattering experiment performed using a SWNT film cast on a platinum surface in a 0.5 M H2 SO4 aqueous
solution obtained at the applied potentials indicated, in the direction indicated by
the arrow. The excitation radiation used was Elaser = 1.96 eV. An interesting feature
to observe is that the G-band lineshape changes from a metallic lineshape (with a
noninfinite, nonzero coupling constant q in the BWF function) at Vap = 0 to a less
metallic lineshape with increasing Vap.
50
between the nanotube and a surface or environment that can transfer charge.
An additional feature observed in the Raman spectra acquired using HNO3 as the
electrolyte is the consistent appearance of a peak at
1043 cm -1 , resulting from the
presence of the nitrate ions in the electrolyte. (See Figure 3-7). The nitrate signal
could also be used as a "normalization factor" if an intensity analysis is desired. The
transition from a more metallic to less metallic lineshape in the tangential G-mode
is observed, as was the case with the Raman spectra from the other electrolytes, and
this is further discussed when comparing the features from the other electrolytes. The
similarity between the spectra in the electrolytes containing sulfate ions is quite clear.
Other interesting features include the appearance of 3 RBM frequencies, whose
trends in intensity are illustrated in Figures 3-10 and 3-11. In the K2 S0 4 media, the
changes in G-band frequency (see Figure 3-8) show a charge transfer of electrons for
negative potentials (below - 0.4V) while for positive potentials, hole charge transfer
above +0.6V occurs. The energy between - 0.4V and + 0.6V may correspond to
the region of constant DOS, while the energy below - 0.4V may correspond to filling
states in the conduction band (since negative potentials are n-type doping) and above
0.6V to emptying of states in the valence band (since positive potentials are p-type
doping). In the nitrate media, a significant upshift of the G-band frequency occurs
for more positive potentials as compared to the sulfate media.
A similar behavior is seen when we analyze the dependence of the G-band frequency on Vap, depicted in Figure 3-7. The upshift of the G-band indicates p-type
doping. The G' behavior of H2 SO4 and K 2 SO4 is quite similar, but it is different for
HNO 3 . While the slopes of WG versus Vap in Figure 3-8 are very similar for H 2 SO4
and K2 SO4 , in the HNO3 media the significant upshift of this band with increasing applied potential only starts for Vp > 0.8 V. For positive applied potentials,
holes are introduced in the density of states of nanotubes, and anions intercalate.
This would account for the similar behavior observed in H2 SO4 and K2SO 4 , as compared to HNO3 : sulfate anions might intercalate easier than nitrate ions into SWNTs
bundles, and a more effective charge transfer takes place in the sulfate media. Intercalation is the process by which ions occupy sites in the media of the electrolyte
51
A IL
3
1%
>1
P
W
E
C:
P
Ramnn 5hift (cmn-1)
Figure 3-5: Raman spectra obtained from a Raman backscattering experiment using
a SWNT film cast on a platinum surface using a 0.5 M K2 SO4 aqueous solution
obtained at the applied potentials indicated in the graph above, indicating this was
a p-type doping process. The arrow indicates the direction of voltage application as
the experiment proceeded, and the excitation radiation was Elaser= 1.96 eV.
52
'E
r-
Raman ShiW (m-i)
Figure 3-6: Raman spectra obtained from a Raman backscattering experiment using
a SWNT film cast on a platinum
surface using a 0.5 M K 2 SO 4 aqueous solution
obtained at the applied potentials indicated in the graph above, indicating this was
an n-type doping process. The arrow indicates the direction of voltage application as
the experiment proceeded, and the excitation radiation was Elaser
1.96 eV.
53
in question as
Vap
is increased. Also, more generally, anions with higher oxidation
numbers might intercala;e easier than ions with lower oxidation numbers into SWNT
bundles, if charge transfer (related to charge per unit time per mole in an oxidation
reaction) is considered in the reaction describing the oxidation of SWNT bundles.
54
I h
aD
e11)
E
13
Raman Shift (cm')
Figure 3-7: Raman spectra obtained from a Raman backscattering experiment using
a SWNT film cast on a platinum surface using a 0.5 M HNO3 aqueous solution
obtained at the applied potentials indicated in the graph above. The arrow indicates
the direction of voltage application, and the excitation radiation was Elaser = 1.96
eV.
55
a - r.1-,- II I El,^ .:
:-P - 1 .beY
I
I
-
AL
e'
e-1I
::1
I
i
M
3
1,
-
-
-1D-D
II II II II II Ir-r-
-
-
-DI .
-
-
-
-
-
-
-
-
-
-
DD D2DA
.sil
-
-
-
-
-
-
-
-
-
DI DZ ID 1
~na
-
1
-
-
-
1
)
Figure 3-8: Wavenumber of the tangential G-band and for a SWNT film in different
electrolyte media as a function of the applied potential for Elaser= 1.96 eV.
56
.
-' n
im
·
E
E
I2~
ci
no ,
-1D
. ,
-i
I
.
I
DPi
I
I
-4D27
'
I
I
I
I
I
I
-
I
D2 DA DS D:
DD
I
I
I
I
I
12
I
1
Ap 0 le d Pde O I (Y)
Figure 3-9: Wavenumber of the second-harmonic G'-band and for a SWNT film in
different electrolyte media as a function of the applied potential for Elaser = 1.96 eV.
57
-
215
-
4.m
-*
R
I
IL
-AMd
71D
IIM
L
2D.
.'
AlI
1s.
"B
,.E
V
U.-
.,-
-L.
I
m
Ii --- ....
- - - - - - - - - - -- - -' -I -' -I -' -I -' -I -I -I - I - -' -I
. I ' I ' I ' I ' I ' I
Db DZ 1D 12 1
D4 02 D.D DDA
-1D .'-DB
1
RnU1
ApNd
Figure 3-10: Wavenumber of RBMs for a SWNT film in different electrolyte media
as a function of the applied potential for Elaser = 1.96 eV.
58
-
1
-
-
v says) _*_ Wc (
Man'
2 i 7 OYP) -41.-
isSan)
a qMd (
'.,
-43-
q 90d (
20d (
96 eV
1
De
In
i
D2
DD-
. I - I
I
-1D -DZ-DB
I
.
I
I
.
I
I
-DD.2
.
I
I
DD
-
a
I
D
-_
I
I
D
.
I
I
D
.
I
I
DZ
.
I
I
ID
.
ff
I
1
.
I
I
1
-
I
I
1D
plled 1rMA M
Figure 3-11: Intensity (b) of RBMs for a SWNT film in different electrolyte media as
a function of the applied potential for Elaser = 1.96 eV. The RBM frequencies WRBAI
occurred at
185 cm-1 for K 2 SO4 , and
195 cm - for HNO3 , and H 2 SO4 had two
RBM frequencies located at - 195 cm ' and
217 cm-1 .
59
60
Chapter 4
Structural Investigation of HTCFs
The material background for high thermal conductivity graphitic foams (HTCFs) was
motivated by pitch-based carbon foams, which in the past have been used as structural
skeletons for strong materials such as plexiglass. However, the main application of
pitch-based carbon foams in recent years has been as a thermal management material,
and this shift towards a thermal management material application for pitch-based
carbon foams has gained in importance. The main properties sought in synthesizing
materials for thermal management systems are high thermal conductivity, low weight,
low coefficient of thermal expansion, and low cost. HTCFs fall into this category for
materials, especially with regard to their high thermal conductivity per unit volume.
Systems which are the best possible candidates for HTCFs applications include air
conditioners and nuclear reactors. [4]
The purpose of investigating HTCFs by using Raman spectroscopy was to gain
a fundamental understanding about why HTCFs are such good thermal conductors
and to verify what TEM and SEM studies had already indicated based on structural
studies. HTCFs appear to have two classes of regions. One region is highly graphitic,
while the intersections of pores in the HTCF samples are highly disordered(see Fig
4-1). The highly graphitic regions are believed to form a percolation netwok allowing
an easy phonon conduction path. Raman spectroscopy performed on the disordered
cm- 1 ) intensity (because the D-band is
sites should yield a high D-band (1300
disorder-induced) and a lower D-band intensity when graphitic regions are examined.
61
Furthermore, this study enables us to better understand the relation between the
D-band and its second-order harmonic, the G'-band (
2650 cm- 1 ).
My specific contribution to this project included assisting graduate student Eduardo Barros in conducting Raman experiments, and discussing the results with him.
4.1
Synthesis of HTCFs
The HTCF samples in this study were prepared by Oak Ridge National Laboratory (ORNL) using two types of mesophase pitches, Mitsubishi ARA24 and Conoco
Dry Mesophase. The synthesis procedure begins with carbonizing and pyrolyzing a
mesophase pitch at 1000°C, and graphitizing the resulting material at 2800°C. The
mesophase pitches could best be described as interconnected networks of graphitic ligaments. The graphitization heating rates for the samples ranged from 0.5°C/min to
15°C/min, and hence the resulting densities of the HTCFs range from 0.2-0.6 g/cm 3 ,
and the average diameters of the pores in the HTCF samples range from 275-350 Pm
for the ARA24-derived foams, and from 60-90 pm for the Conoco-derived foams.[4]
According to findings from x-ray analysis performed by Dr. James Klett of ORNL,
the HTCFs consist of graphitic webs with an average interplanar spacing of (002)
graphitic planes of do00 2
=
3.355A, corresponding to the average interlayer spacing,
stack heights of up to - 80 nm, and crystallite sizes up to
20 nm.[4] Furthermore,
thermal conductivity measurements indicate that the thermal conductivity of the resulting HTCFs ranges from 40-150 W/m.K. To give a sense of scale, it should be
noted that these thermal conductivities were more than six times greater than those
of solid copper.[4]
4.2
Raman Spectroscopy on HTCFs
Raman spectra taken of two regions of a HTCF are depicted in Figure 4-5. The
Raman spectra obtained from a Raman backscattering experiment performed at an
excitation energy of Elaser= 1.58 eV. A 50X objective was used to focus the laser
62
1-2
owl 60
ILm
Figure 4-1: SEM picture of an HTCF synthesized using Mitsubishi ARA24 pitch as
the base. This sample has a bulk density 0.54 g/cm3 .
63
beam on the sample. The scattered light was measured with an air-cooled charged
coupled device (CCD). The observed features included the disorder-induced D-mode
(located at
1320 cm-'), and the tangential stretching G-mode (located at
-
1580
cm-1 ). In addition, the second harmonic of the D-band, the G' band, was observed.
The interesting D-band result observed was that there was a sharp increase in
the D-band intensity when the incident laser beam was incident upon a suspected
disordered region which had previously been observed (see Figure 4-4) in contrast to
when a suspected graphitic region was observed (see Figure 4-3). No frequency shifts
for any of the features were observed, however. A sweep was performed, whose map-
ping is illustrated in Figure 4-2. The position in the sample in the horizontal sweep
shown in the picture begins at position
and terminates at position 9. The sweep
taken perpendicular to the horizontal sweep begins at position 10 and terminates at
position 12.
While there was a sharp increase in D-band intensity as the two different position scans were measured, there was no increase in G'-band intensity. However,
the asymmetrical shape of the G' peak, which was fitted to a superposition of two
Lorentzian lineshapes, has previously been observed in other forms of graphite. Also,
an asymmetry in the G-band is observed in the same location that an asymmetry
in the G'-band is observed. The feature located at - 1611 cm -
1
grows in intensity
as the D-band grows in intensity. The asymmetry in the G'-band in graphite has
been previously been explained as a result of the intraplanar interaction between two
adjacent planes of graphite associated its hcp structure. 10]
64
Figure 4-2: A map indicating the direction we conducted the Raman sweeps. We first
began at the left end of the horizontal arrow and swept to the right, then pursued
the direction perpendicular to the first direction and swept down the plane.
65
Figure 4-3: Raman microprobe picture taken of a suspected graphitic region of the
same sample.
66
Figure 4-4: Raman microprobe picture taken of a suspected disordered region of the
same sample.
67
7
7
157
0D1i1
3
-
I ntensity
Intensity
EM(a.u,)
(au.)
B1319
1611
// N
u-
-
-
-
.
a1
,
11,1)0
-
.
.
.
.
l.TO
166
.
.
176
.
Raman Shft (cm-')
Raman Shift (cm-')
Am
ALU
2O
Intensity
(au.)
Intensity
(au.)
IMD
110
aIZI
TO
20
a;
Zoo
3M
Raman Shft (cm-1)
Raman Shift (m-')
Figure 4-5: Raman spectra taken of two locations in the sample, in suspected graphitic
1300 cm-1 ) is
and disordered regions. Note that the D-band intensity (located at
much higher in region il (in our map corresponding to a disordered region) than in
region 7 (in our map corresponding to a suspectedly graphitic region.)
68
Chapter 5
Conclusions
We have used X-ray scattering, X-ray diffraction, Raman spectroscopy, and SQUID
measurements to investigate the physical properties of three classes of novel carbonbased materials; carbon aerogels (CAs), single-walled carbon nanotubes (SWNTs),
and high thermal conductivity graphitic foams (HTCFs). The conclusions reached in
each of the chapters are summarized below.
5.1
Carbon Aerogels
We have determined from X-ray diffraction that doped CA samples that are pyrolyzed
and carbonized at high heat-treatment temperature containing nanoparticles of the
dopants whose crystal sizes are on the order of angstroms (A) and whose widths are
on the order of - 10 nm. X-ray diffraction measurements also reveal that the base
contains amorphous carbon, as reinforced by our findings of a porous structure in
the CA matrix from SEM measurements. Raman experiments indicate that in the
presence of doping with a donor species, the nearest neighbor carbon (C-C) covalent
bonds are weakened. Ranman measurements also indicate that, in the presence of Ru,
C-C bonds n-maybe strengthened if the CAs are pyrolyzed at higher heat-treatment
temperature.
Magnetic susceptibility analysis indicates that carbon aerogels doped
with ions exhibit a decrease in absolute value of diamagnetic background value.
69
5.2
Single-Walled Carbon Nanotubes
We have determined, using a spectroelectrochemical study of SWNTs, that the the
Fermi level (EF) is shifted through the use of an applied external potential. We also
determined that different electrolytes affect the charge transfer differently in that
anions of electrolytes with higher oxidation numbers intercalate more easily than
anions with lower oxida:ion numbers into SWNT bundles.
5.3
High Thermal Conductivity Graphitic Foams
We found that Raman spectra, acquired from interfoam regions consisting of disrupted
junctions, have a much higher disorder-induced D-band intensity in comparison to
the corresponding D-band intensity from weblike regions consisting of highly-aligned
filaments, as expected. We also observed asymmetry in the tangential stretching Gband and second order G'-band lineshapes in regions of the sample where the D-band
intensity is relatively large. The asymmetry in the G'-band in graphite traditionally
has been explained by a doublet structure obtained from a lineshape analysis where
the additional spectral feature is identified by an intraplanar interaction between
the two adjacent inequ valent AB planes of graphite, associated with its hexagonal
close-packed (hcp) structure.[10]
70
Appendix A
EDAX Results for Co-doped CA
71
Job Numt
SPECTRO X-LAB
Preset Sample Data
Sample Name:
Description:
Method:
Job Number:
Sample State:
Sample Type:
Sample Status:
TBF91-Co
Dilution Material:
Sample Mass (g):
Dilution Mass (g):
Dilution Factor:
Sample rotation:
Date of Receipt:
Date of Evaluation:
Tqpellet
0
Pressed tablet, 32 mm
Pressed tablet
AAAXXX
None
4.0000
0.0000
1.0000
No
03119204
03119/201
The
error ist te statistical1 error with sigma confidence interval
The error ist the statistical error with 1 sigma confidence interval
Symbol
Element
11
Na
12
Mg
Sodium
Magnesium
Aluminum
Silicon
Phosphorus
Sulfur
Chlorine
Potassium
Calcium
Titanium
Vanadium
Chromium
Manganese
Iron
Cobalt
Nickel
Copper
Zinc
Gallium
Germanium
Arsenic
Selenium
Bromine
Rubidium
Strontium
Yttrium
Zirconium
Niobium
Molybdenum
Silver
Cadmium
Indium
Tin
Antimony
Z
13 Al
14
Si
15
16
P
S
17
CI
19
K
20
22
23
24
25
26
27
28
29
30
Ca
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
31
Ga
32
33
34
35
37
38
39
40
Ge
As
Se
Br
Rb
Sr
Y
Zr
41
Nb
42
47
48
49
50
Mo
Ag
Cd
In
Sn
51
Sb
Norm. No. of Impulses
0.01689 F
0.0
F
0.25469 F
10.505
F
0.30666 F
3.4071
F
0.79395 F
0.0
F
0.11825 F
0.0
F
0.00780 F
0.0
1.1332
0.0
562.16
1.8150
F
F
F
F
F
0.24662 F
0.43521 F
0.04674 F
0.0
F
F
0.0
0.0
F
0.08865 F
0.0
F
0.18214 F
0.95263 F
0.0
0.0
0.0
0.0
F
F
F
F
0.01522 F
72
Abs. Error
Concentration
0.0
F
0.0
F
0.0
F
<0.56
<0.078
<0.022
4.531
0.0484
0.2876
0.130
<0.046
%
%
%
%
%
%
%
%
0.0526
<0.0036
<0.0040
<0.0059
0.0483
<0.060
136100
220
%
%
%
%
PJg/g
pglg
pg/g
<4.9 pg/g
32.7 pg/g
2.9 pg/g
Pg1g
<2.4
<1.6 pg/g
<1.5 pg/g
2.0 Pg/g
<0.8 Pg/g
2.2 Pg/g
pg/g
Pg/g
9.1 pglg
Pg/g
Pg/g
<14
<9.2 pg/g
< 5.9 Pg/g
<4.2 Pg/g
3.2 Pg/g
<2.7 pg/g
<5.0 pg/g
<3.0
%
%
%
0.055
%
0.0044 %
0.0056 %
0.014
%
(0.0)
%
0.0077 %
(0.0)
%
(0.0023) %
(0.0)
%
0.0032 %
(0.0)
%
(0.35)
(0.0)
(0.0)
300
32
pg/c
Pg/s
(0.0)
2.9 Pg/s
1.5 pg/.c
Pg/c
(0.0) Pg/c
(0.0) pg/c.
(0.0) Pg/c.
0.6
(0.0) pg/g
pg/s
PA1
0.5
0.7 PA1
pg/c
PA1
(0.0) pg/c
PA1s
(0.0) pg/c
PA1
pglg
(0.0) pg/;
IJg/g
pg/g
(0.0) PA1
PA1
1.5 pglg
(0.0) PA/
pg/g
PA1
(0.0)
PA/
(0.0)
Job Numbe
SPECTRO X-LAB
TBF91-Co
Sample Name:
Description:
Z
52
53
55
56
57
58
74
80
81
82
83
Symbol
Element
Te
I
Cs
Ba
La
Ce
W
Hg
TI
Pb
Bi
Tellurium
Iodine
Cesium
Barium
Lanthanum
Cerium
Tungsten
Mercury
Thallium
Lead
Bismuth
Date of Receipt:
Date of Evaluation:
Norm. No. of Impulses
F
0.0
0.0
F
0.0
F
0.0
F
0.0
F
0.0
F
0.11606 F
0.0
F
0.0
F
0.05158F
F
0.0
Concentration
< 9.6
< 10
< 15
< 27
<35
< 43
24.2
< 4.2
< 1.9
3.9
<3.0
Abs. Error
pg/g
pg/g
pg/g
pg/g
pg/g
pg/g
pg/g
pg/g
pg/g
pg/g
pg/g
18.74 %
Sum of concentration
73
03/19/200~
03/191200'
(0.0)
(0.0)
(0.0)
(0.0)
(0.0)
(0.0)
9.1
(0.0)
(0.0)
2.2
(0.0)
pg/g
pg/g
pg/g
Pg/g
pg/g
pg/g
g/g
pg/g
pg/g
pg/g
Pg/g
74
Appendix B
EDAX Results for Fe-doped CA
75
Sample Name:
Description:
Method:
Job Number:
Sample State:
Sample Type:
Sample Status:
I I-rts
l-e
Tqpellet
0
Pressed tablet, 32 mm
Pressed tablet
AAAXXX
Dilution Material:
Sample Mass (g):
Dilution Mass (g):
Dilution Factor:
Sample rotation:
Date of Receipt:
Date of Evaluation:
None
4.0000
0.0000
1.0000
No
03/19/2004
03/19/2004
Results
The error ist the sl:atistical error with 1 sigma confidence interval
Z
11
12
Symbol
Element
Na
Mg
Sodium
Magnesium
Aluminum
Silicon
Phosphorus
Sulfur
Chlorine
Potassium
Calcium
Titanium
Vanadium
Chromium
Manganese
13 Al
14
15
Si
P
17
Cl
16S
19K
20 Ca
22 Ti
23 V
24 Cr
25 Mn
26 Fe
27 Co
28 Ni
29 Cu
30 Zn
31
Ga
32 Ge
33 As
34 Se
35 Br
37 Rb
38 Sr
39Y
40 Zr
41
Nb
42 Mo
47 Ag
48 Cd
49 In
50 Sn
Norm. No. of Impulses
Iron
Cobalt
Nickel
Copper
Zinc
Gallium
Germanium
Arsenic
Selenium
Bromine
Rubidium
Strontium
Yttrium
Zirconium
Niobium
Molybdenum
Silver
Cadmium
Indium
Tin
·
,.
76
0.0
F
0.0
F
0.31487 F
10.962
F
1.1862 F
5.6133
F
0.74374 F
0.23072 F
0.0
F
0.0
F
0.0
F
0.25580 F
1.1012
F
569.32
F
8.0405
F
1.2518
F
0.44085 F
0.53337 F
0.04535 F
0.0
F
0.21770 F
0.0
F
0.28120 F
0.0
F
0.22496 F
0.88714 F
F
0.0
F
0.0
F
0.0
F
0.0
0.01007 F
F
0.0
0.0
F
Abs. Error
Concentration
<0.30
<0.049
<0.0094
1.960
0.0811
0.1795
0.0500
0.0720
<0.0077
<0.0033
<0.0066
<0.0035
pg/g
%
%
%
%
%
%
%
%
%
%
%
%
%
%
0.0165
8.681
838
75.3 pg/g
<1.5 pg/g
17.7 pg/g
pg/g
1.3
<1.2 pglg
pglg
3.9
<0.8 pg/g
2.8 pg/g
<0.6 pg/g
1.2 pg/g
3.7 pg/g
<3.5 pg/g
<2.4 pg/g
< 2.6 pg/g
<1.3 pg/g
0.9 pg/g
<0.7 pg/g
<2.0 pg/g
pglg
pg/g
%
(0.0)
(0.0)
(0.0)
0.024
0.0032
0.0028
0.0065
0.0081
(0.0)
(0.0)
(0.0)
(0.0)
0.0013
0.023
39
4.1
%
%
%
%
%
%
%
%
%
%
%
pg/g
pg/g
(0.0)
1.4 pg/g
0.7 pg/g
(0.0) pg/g
0.6 pg/g
(0.0) pg/g
0.3 pg/g
(0.0) pg/g
0.3 pg/g
0.4 pg/g
(0.0) pg/g
(0.0) pg/g
(0.0) Pg/g
(0.0) Pg/g
0.4 Pg/g
pg/g
(0.0) Pg/g
(0.0) Pg/g
pglg
Job Numb
SPECTRO X-LAB
TFB91-Fe
Sample Name:
Description:
Z
Date of Receipt:
Date of Evaluation:
Symbol
Element
52
53
55
56
57
74
80
Te
I
Cs
Ba
La
W
Hg
Tellurium
Iodine
Cesium
Barium
Lanthanum
Tungsten
Mercury
0.07718
0.0
0.0
0.0
0.0
0.29208
0.0
F
F
F
F
F
F
F
81
TI
Thallium
0.0
F
Lead
Bismuth
0.00181 F
0.0
F
82 Pb
83 Bi
Norm. No. of Impulses
Sum of concentration
Concentration
< 0.7
< 4.2
< 6.2
<10
< 12
26.9
< 2.2
Abs. Error
pg/g
pg/g
pg/g
pg/g
pg/g
pg/g
pg/g
(0.0) Pg/g
(0.0) pg/g
(0.0) pg/g
(0.0) pg/g
(0.0) pg/g
4.0 pg/g
(0.0) pg/g
< 1.1 pg/g
(0.0) pg/g
<2.5 pg/g
< 1.6 pg/g
(0.0) pg/g
(0.0) pg/g
11.14 %
77
03/19120C
03/19/20C
78
Appendix C
EDAX Results for K-doped CA
79
b.'LU I KU A-LAJ:
Job Number:
Preset Sample Data
---
TFB91 -K
Sample Name:
Description:
Method:
Job Number:
Sample State:
Sample Type:
Sample Status:
Tqpellet
0
Pressed tablet, 32 mm
Pressed tablet
AAAXXX
Results
-..
Dilution Material:
Sample Mass (g):
Dilution Mass (g):
Dilution Factor:
Sample rotation:
Date of Receipt:
Date of Evaluation:
- -
-
-
-
None
4.0000
0.0000
1.0000
No
03/19/2004
03/19/2004
-
- -
-
- -
The error ist the statistical error with I sigma confidence interval
Z
11
12
Symbol
Element
Na
Mg
Sodium
Magnesium
Aluminum
Silicon
Phosphorus
Sulfur
Chlorine
Potassium
Calcium
Titanium
Vanadium
Chromium
Manganese
Iron
Cobalt
Nickel
Copper
13 Al
14
15
Si
P
16 S
17
19
Cl
K
20
22
23
24
25
26
27
28
29
30
Ca
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
31
Ga
32
33
34
35
37
38
39
40
Ge
As
Se
Br
Rb
Sr
Y
Zr
41
Nb
42 Mo
47 Ag
48 Cd
49
In
50
Sn
51
Sb
Zinc
Gallium
Germanium
Arsenic
Selenium
Bromine
Rubidium
Strontium
Yttrium
Zirconium
Niobium
Molybdenum
Silver
Cadmium
Indium
Tin
Antimony
Norm. No. of Impulses
0.00262
0.0
Concentration
< 0.47
< 0.076
F
F
<0.023
0.27080 F
F
10.222
0.42125
F
3.5571
0.0
24.716
0.0
F
F
F
F
0.0
0.0
F
F
0.0
0.61840
1.6207
0.26601
F
F
F
F
1.5426
F
2.982
0.0454
0.2050
<0.019
> 13.46
0.30166 F
0.37022 F
F
0.0
0.0
F
0.0
F
0.0
F
0.04799 F
0.20156 F
0.28246
1.4082
0.0
0.0
0.0
0.0
0.02009
0.0
0.0
0.0
F
F
F
F
F
F
F
F
F
F
< 0.035
< 0.0026
< 0.0060
pg/g
%
%
%
%
%
%
%
%
%
%
<0.0016
%
%
<0.00096 %
0.0191
47.1 pg/g
135.1 pg/g
< 1.9
17.9 IJg/g
< 1.4 pg/g
<1.3 pg/g
< 1.0 pg/g
<0.9 pg/g
0.7 pg/g
1.9 pg/g
pg/g
2.2 pg/g
8.6 pg/g
<5.9 pg/g
Pg/g
<2.5 pg/g
< 3.8 pg/g
<2.2 pg/g
2.8 pglg
<2.0 pg/g
<3.0 pg/g
< 2.6
(0.0)
(0.0)
(0.0)
%
%
%
0.037
%
0.0034 %
0.0040 %
(0.0)
%
(0.0)
(0.0)
(0.0)
(0.0)
%
%
%
%
%
0.10
0.0015 %
(0.0)
%
5.3 pg/g
4.9
p9g/g
(0.0) pg/g
1.6 pg/g
(0.0) pg/g
(0.0) pg/g
(0.0) pg/g
(0.0) pg/g
0.3
0.3
0.3
0.4
pg/g
pg/g
pg/g
pg/g
(0.0) pg/g
(0.0) pg/g
(0.0) pg/g
(0.0) pg/g
0.9 pg/g
(0.0) pg/g
(0.0) pg/g
(0.0) pg/g
Pae
Date: 03/19/2004
Fiollir C-1
Abs. Error
FhDA'
rpiilt,
fnr CA drinpr] wvith nntcqqiliin
innq
1
Job Number:
SPECTRO X-LAB
TFB91 -K
Sample Name:
Description:
Z
Symbol
52 Te
53 I
55 Cs
56 Ba
57 La
58 Ce
74 W
80 Hg
81 TI
82 Pb
83 Bi
Element
Date of Receipt:
Date of Evaluation:
Norm. No. of Impulses
Tellurium
Iodine
Cesium
Barium
Lanthanum
Cerium
Tungsten
Mercury
Thallium
Lead
Bismuth
0.0
0.0
0.0
0.0
0.0
0.0
0.17962
0.0
0.0
0.03839
0.0
Sum of concentration
Concentration
F
F
F
F
F
F
F
F
F
F
F
<6.0
< 5.9
<9.5
< 16
< 19
< 21
24.4
< 2.3
< 1.4
1.9
< 2.0
Abs. Error
pg/g
pg/g
pg/g
pg/g
IJpg/g
pg/g
pg/g
pg/g
pg/g
pg/g
pg/g
16.73 %
81
03/19/2004
03/19/2004
(0.0)
(0.0)
(0.0)
(0.0)
(0.0)
(0.0)
4.6
(0.0)
(0.0)
1.3
(0.0)
g/g
g/g
pgg
pg/g
pg/g
pg/g
g/g
g/g
g/g
pg/g
Pg/g
82
Bibliography
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Systems.
PhD thesis, Massachusetts Institute of Technology, Department of
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[2] R. Saito et. al. Physical Properties of Carbon Nanotubes. Imperial College Press,
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[7] Conversations with Professor Mildred Dresselhaus.
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Potential dependent surface Raman spectroscopy of single
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370:675-682, January 2003.
83
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84