Carbon Nanostructures: Fullerenes/Carbon Nanomaterials

1
Carbon NanostructuresFullerenes/Carbon
Nanomaterials

• Nanotechnology ME465, unit 9, 10 and 11
• Peter Filip
• A108, filip_at_siu.edu
• Office hours T/Th 10 12 am

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Lecture Overview

• Forms of Carbon/Bonding in Carbon Materials
• Carbon Nanotubes and their Types
• Formation of Nanotubes
• Properties of Nanotubes
• Uses of Nanotubes

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Carbon and Forms of Carbon

• Sixth element in the periodic table
• Atomic weight 12.011
• Three isotopes
• C12 (99 of the naturally occurring carbon
  -reference for relative atomic mass of 12),
• C13 (has magnetic moment, spin1/2 used as a
  probe in NMR),
• and C14 (radioactive isotope, half life 5730
  years used in dating of artefacts and label
  organic reaction mechanisms)
• Electronic ground state 1s22s22p2
• C exhibits catenation bonding to itself
  limitless number of chains, rings and networks

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Types of Carbon

• Diamond and Graphite alotropes of C with 109?28
  and 120? bonds until 1964
• Other bond angles
• C8H8, 90?, cubane (P. Eaton, University of
  Chicago, 1964)
• C20H20, dodecahedron shape (L. Paquette, Ohio
  State University, 1983)
• Carbon Clusters (3, 11, 15, 19, 23, 40, 50, 60,
  70, 80, 90) C60 fullerene
• Carbon Nanotubes (S. Iijima, 1991, Japan Ref.1)

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Carbon Nanotubes

• Types
• Fabrication
• Structure
• Properties
• Applications

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What is it?

• Sheet of graphite rolled into a tube
• Single-Walled (SWNT) and Multi-Walled (MWNT)
• Large application potential, metallic,
  semiconducting

armchair
SWNT
zigzag
MWNT
chiral
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Types of Carbon nanotubes
Two main types of carbon nanotubes Single-walled
nanotubes (SWNTs) consist of a single graphite
sheet seamlessly wrapped into a cylindrical
tube. Multiwalled nanotubes (MWNTs) comprise an
array of such nanotubes (more than one wall) that
are concentrically nested with in.
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Why Carbon Nanotubes ?

• Small Dimensions

• Chemically Stable

• Mechanically Robust

• High Thermal Conductivity

• High Specific Surface Area (Good Adsorbents)

• Low Resistivity (Ballistic Electron
  Conduction)

Ideal materials for applications in conductive
and high-strength composites energy storage and
energy conversion devices sensors field
emission displays and radiation sources hydrogen
storage media and nanometer-sized semiconductor
devices, probes, and interconnects.
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Fabrication/Nanotube Synthesis

• SWNTs and MWNTs are usually made by
• carbon-arc discharge methods
• C electrodes, 20-25V potential, 1mm, 500 torr, C
  ejected from electrode forms NT on electrode
  (Co, Ni or Fe for SWNTs, ? 1-5nm, 1µm length, no
  catalyst MWNTs,)
• laser ablation of carbon
• 1200?C, pulsed laser, catalysts (Co, Ni),
  condensation (?10-20nm, 100µm length)
• chemical vapor deposition (typically on catalytic
  particles)
• 1100 ?C, decomposition of hydrocarbon gas (e.g.
  CH4), open NTs, catalyst on substrate, industrial
  scale up, ? and length can vary
• Nanotube diameters
• range from 0.4 to gt 3 nm for SWNTs and from 1.4
  to at least 100 nm for MWNTs

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Carbon-arc discharge
The schematic diagram of and arc chamber for CNT
production is shown. After evacuating the
chamber, an appropriate ambient gas is introduced
at the desired pressure, and then a dc arc
voltage is applied between the two graphite rods.
When pure graphite rods are used, the anode
evaporates and the is deposited on the cathode,
which contains CNTs. These CNTs, are MWNTs.
When a graphite rod containing metal catalyst
(Fe, Co, etc.) is used as the anode with a pure
graphite cathode, SWNTs are generated in the form
of soot.
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Typical CVD Furnace Schematics
The CVD method can be used for growing controlled
architectures (aligned as well as patterned) of
carbon nanotubes on various substrates.
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Figure 6.2. Illustration of the molecular and
supramolecular structures associated with
nanotubes at three different length scales. (a)
shows the wrapping of a graphene sheet into a
seamless SWNT cylinder. (b) and (c) show the
aggregation of SWNTs into supramolecular bundles.
The cross-sectional view in (c) shows that the
bundles have triangular symmetry. (d) A MWNT,
another nanotube polymorph composed of
concentric, nested SWNTs. (e) At the
macromolecular scale, bundles of SWNTs are
entangled.
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Structure of Single Walled Carbon Nanotubes

• Structure depends on rolling direction
  (chirality)
• Metallic
• Semi-conducting

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Figure 6.1. Diagram explaining the relationship
of a SWNT to a graphene sheet. The wrapping
vector for an (8,4) nanotube, which is
perpendicular to the tube axis, is shown as an
example. Those tubes which are metallic have
indices shown in red. All other tubes are
semiconducting.
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Three Forms of CNTs

• Chiral
• Zigzag
• Armchair
• Vectors describe the rolling process that occurs
  when a graphite sheet is transformed into a tube

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Orbitals with 60 Carbon Atoms
Figure 5.8. Hückel molecular orbital diagram for
C60 in units of ?. (2? 36 kcal)
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Real and Reciprocal SpaceBrillouin Zone
Figure 6.3. (a) The unit cell of graphene, and
(b) the corresponding reciprocal lattice and
Brillouin zone construction by the perpendicular
bisector method. Dimensions are not to scale,
but orientation between the real and reciprocal
lattice is preserved. Important locations within
the Brillouin zone are ? at the zone center, K at
the zone corner, and M at the midpoint of the
zone edge.
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Real and Reciprocal SpaceBrillouin Zone
Figure 6.4. The dispersion surface of
two-dimensional graphene in proximity to the
Fermi level. The valence and conduction bands
are tangent at each K point. (From Ref. 48 by
permission of the American Physical Society.)
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Figure 6.5. (a) Wrapping vectors and allowed
k-states for (3,0) (zigzag), (4,2), and (3,3)
(armchair) SWNTs. The degeneracy at the K point
is allowed only for the (3,0) and (3,3) tubes,
which behave like metals. The (4,2) tube does
not contain the degeneracy, so it has a band gap.
Note that the lines of allowed k-states are
perpendicular to the wrapping vector for each
tube. (From Ref. 49 by permission of Annual
Reviews.) (b) The band structure of a (6,6)
SWNT. The presence of many overlapping subbands
is typical for SWNTs. (From Ref. 14 by
permission of the Am. Phys. Society.)
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Properties of Nanotubes

• Electrical Properties
• Metallic armchair structure conductive
• Semi-conductors zigzag and chiral
• Depends on diameter (quantum effects)
• Ropes of SWNTs (R10-4?cm-1 at 27?C)
• Combinations transistors
• Bent molecules
• Response to stretching
• Chirality and diameter of nanotubes are important
  parameters!!!

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Figure 6.18. Atomic force microscopy image of an
isolated SWNT deposited onto seven Pt electrodes
by spin-coating from dichloromethane solution.
The substrate is SiO2. An auxiliary electrode is
used for electrostatic gating. (Reproduced with
kind permission of C. Dekker.)
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Properties of Nanotubes

• Mechanical Properties
• Youngs modulus E 1.28 1.8TPa (steel 0.21TPa)
• Strength Rm 45,000 MPa (high strength steel
  2,000 MPa)
• Buckling no fracture change in hybridization
  (from sp2)

Molecular dynamics simulations of a (10,10)
nanotube under axial tension (J. Bernholc, M.
Buongiorno Nardelli and B. Yakobson). Plastic
flow behavior is shown after 2.5 ns at T 3,000
K and 3 strain. The blue area indicates the
migration path (in the direction of the arrow) of
the edge dislocation (green). This sort of
behavior might help make composite materials
that are really tough (as measured by their
ability to absorb energy).
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Some Numbers
Nanotube diameters range from 0.4 to 3 nm for
SWNTs and from 1.4 to at least 100 nm for
MWNTs. Phonons also propagate easily along the
nanotube The measured room temperature thermal
conductivity for an individual MWNT (3000 W/m.K)
is greater than that of natural diamond and the
basal plane of graphite (both 2000
W/m.K). Small-diameter SWNTs are quite stiff and
exceptionally strong, meaning that they have a
high Youngs modulus and high tensile strength.
Youngs modulus for an individual (10, 10)
nanotube is 0.64 TPa, which is consistent with
measurements. 0.64 TPa is about the same as that
of silicon carbide nanofibers (0.66 TPa) but
lower than that of highly oriented pyrolytic
graphite (1.06 TPa). The density-normalized
modulus and strength of this typical SWNT are,
respectively, 19 and 56 times that of steel
wire and, respectively, 2.4 and 1.7 times than
silicon nano rod. Because of the nearly
one-dimensional electronic structure, electronic
transport in metallic SWNTs and MWNTs occurs
ballistically (i.e., without scattering) over
long nanotube lengths, enabling them to carry
high currents with essentially no heating.
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Separation

• Generally a mixture of NTs is produced
• Impurities are removed by chemicals and
  filtration
• Separation between electrodes
• Silicon wafer one electrode
• Carbon nanotubes deposited on wafer
• Metal electrode on top of CNTs
• High current only metallic CNTs conduct
  heating - evaporation

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Derivatization and Functionalization
Figure 6.19. Two common reaction schemes for the
covalent derivatization of SWNTs (I) carboxylic
acid derivatization, and (II) fluorination. Many
variations on these schemes are possible.
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Filling of Nanotubes
Figure 6.20. Transmission electron micrograph of
a MWNT filled with Sm2O3. The interlayer
separation in the MWNT is c.a. 0.34 nm. Lattice
planes in the oxide are clearly seen. (From Ref.
55 by permission of The Royal Society of
Chemistry.)
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Buckyballs in SWNT
Figure 6.21. (a) Transmission electron micrograph
of C60_at_SWNT. The nanotube is surrounded by
vacuum and does not lie on a substrate. The
encapsulated fullerenes form a one-dimensional
chain with a lattice periodicity of c.a. 1.0 nm.
It is possible to obtain diffraction signatures
from these structures. (b) False-color
transmission electron micrograph of La2_at_C80_at_SWNT.
Each C80 cage contains two point scattering
centers which are the individual La atoms
contained within.
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Change of Conductivity
Figure 6.22. Differential conductance spectra of
a C60 peapod. (a) Conductance versus position
(Å) and sample bias (V) for the peapod.
Spatially localized modulations are observed only
for positive sample bias, i.e. in the unoccupied
density of states. The periodicity of these
modulations matches the periodicity of the
encapsulated fullerenes. (b) and (c) show
conductance at constant position and at constant
sample bias. (d) Conductance versus position for
the same location on the SWNT after the C60
molecules have been shuttled into an empty part
of the tube by manipulation with the STM tip. No
periodic modulations are observed. (From Ref. 33
by permission of The Am. Association for the
Advancement of Science.)
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Reinforcement of Composites
critical length interfacial theory (Kelly and
Tyson, 1965 Chawla, 1998)
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Application of Nanotubes

• Variety of Applications
• Cost dependent
• Field Emission and Shielding
• Flat panel displays TV and computer monitors)
• High electrical conductive armchair SWNTs
  shield magnetic fields (protection)
• Computers
• Based on conductivity change (small V change can
  change conductivity 106 times switch on of
  faster than current)
• Fuel Cells
• Storage of charge carriers (Li, H)
• Chemical Sensors
• Sensitivity of vibration modes to the presence of
  other molecules (Raman)
• Catalysts
• hydrogenation
• Mechanical Reinforcement
• 5 (vol) increases strength of Al by factor 2

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CNTs in Electronic Devices
Figure 5.16. Nanoscale electronic device
connected with a nanotube (left). (Reproduced
with kind permission of Ph. Avouris.) La2_at_C80
trapped inside a single walled carbon nanotube.
a.k.a PEAPODS (right). (Reproduced with kind
permission of D. E. Luzzi.)
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Things to Think About

• How many forms (structural modifications) of CNTs
  exist?
• What is chirality and how it influences
  electrical conductivity?
• How are CNTs made? What is the role of Co, Ni,
  Fe? What is the separation process.
• Make a CNT with Rna1ma2 from chicken wire

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Reading

• Obligatory
• Chapters 4 to 6 in required book
• Recommended
• Chapter 5 in recommended book