ECE505
Spring 09
Fullereness and Carbon Nanotubes
Crystalline Forms of Carbon
In solid form, C exists in two possible crystal
forms: Graphite and Diamond. In the diamond
structure, each C is connected to four
neighboring atoms arranged at the tip of a
tetrahedron by hybrid bonds of type sp3. The
tetrahedral symmetry reflects a dense and
isotropic solid with distances between nearest
neighbours of 0.136 nm.
Graphite (from the greek graphein, meaning ‘to
write’) is a layered structure made up of parallel
planes, each of which is a regular tiling of
hexagons, with the C- atoms at the vertices of
the hexagons. Each C atom on the plane is
connected to 3 nearest neighbours by hybrid
bonds of sp2 type. The bonds on the plane are
strong and characterized by atomic distances of
0.142 nm. The atoms are weakly connected
between consecutive planes, with an interatomic
distance of 0.34 nm
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sp3 bonding
sp3 bonding
C- 1s2- 2s2-2p2 (create
bonding)
Sp2 formed by 2s, 2px, 2py
orbitals are called  bonds.
The 2pz bond is called -state
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A covalent chemical bond is formed between two atoms when their orbitals
overlap and share a pair of electrons. When the orbitals overlap along an
axis between the atoms (internuclear axis), they form a sigma bond. In this
type of bonding the electron density is highest in the space between the
atoms.
Overlap between two S orbitals to form a sigma bond (green). Blue are PI
bonds
For P orbitals, sideways overlapping is also
possible. This results in the formation of pi bonds.
The regions of highest electron density for a pi
bond occur in pairs, parallel to the internuclear
axis, above and below or right and left of the
connecting atoms.
A triple bond consists of a sigma bond (green) and
two pi bonds (red).
http://invsee.asu.edu/nmodules/Carbonmod/bonding.html#orb
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Fullnerenes
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Radio astronomers identified a different form of C existed in red giant stars. In the
lab, Kroto, Curl and Smalley (Nobel Prize Chemistry) discovered that when carbon
nanoclusters were formed on a very hot plasma by vaporizing graphite using a
laser, molecules were arranged in the form of a cage structure: fullnerenes.
C60 unlike the other two forms of C, is a closed structure, with well defined number
of atoms arranged in tips of n hexagons and 12 pentagons with a total of (2(10+n))
atoms. The hexagons form a plane plane surface. Euler’s theorem tells us that 12
pentagons are needed to close the shell and arrive to a closed polyhedron. Of all of
the fullnerenes, C60 is the more stable. (Replica of a football)
Carbon nanotubes were discovered in 1991 by Iijima. These materials are formed
as a by-product of C60 using an arc discharge. The nanotubes are tubular objects
of nanometer diameter and micron length, closed at the ends, made of perfect
graphitic carbon.
Nowadays Carbon Nanotube fibers are made using chemical vapour deposition with
or without the help of a metallic catalyst. These structures are however, not perfect
as the small tubes made by Iijima.
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Fullnerenes
C540
C60 –
6-6 bonds have the character of a
double bond while the 5-6 bonds
have the character of a single bond
http://en.wikipedia.org/wiki/Fullerene
These materials are produced in a discharge where graphite is vaporized at about
3000 degrees Celsius. The soot produced contains many typed of C- including
amorphous C, nanotubes, and a mixture of soluble (n<100) Cn and giant Cn
(n>100).
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Fullnerenes
Physico-chemical properties
Solubility: C60 is insoluble in a wide range of organic polar solvents, like
acetone, alcohol. They are weakly solvent in aromatic solvents such as
benzene and tolune.
Optical properties: C60 absorbs visible light only weakly if illuminated by low
intensity light. Instead, the level of absorption increases significantly when
the light intensity increases (non-linear absorption). This behavior is called
INVERSE SATURABLE ABSORPTION. Theory can explain this behavior,
using a five electronic states level model.
Electro-chemical properties: C60 behaves as an electron acceptor. It can
accept up to 6 electrons by six successive single elecron reductions.
Chemical properties: Most chemical modifications are aimed at increasing
the solubility of C60. Also, different molecules have been grafted onto C60 to
yield molecules with novel properties such as salts C60RM by addition of an
organometallic compound.
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Carbon Nanotubes
Go to:
http://en.wikipedia.org/wiki/Carbon_n
anotube to show how graphene is
rolled to form a nanotube
Carbon nanotubes are
typically 1-10 nm diameter,
micron long. Their crystal
structure is composed of
hexagons; it is a roll-up
graphene sheet. The cylinder
is open at the end, so when it
closed the sp2 bonds distort,
and topological defects are
introduced in the lattice to
curve the plane.
http://en.wikipedia.org/wiki/File:Types_of_Carbon_
Nanotubes.png
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Each end of the nanotube
contains 6 pentagons into the
hexagonal lattice – just like in
C60.
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This figure shows the different
types of carbon nanotubes that
can be obtained by rolling the
graphene sheet.
This crystallographic orientation
gives the CN different electronic
configuration. The armchair
configuration gives rise to
metallic behavior. Zigzag are
semiconducting and Chiral could
be either (left: metallic; right:
semiconducting)
The bandgap of the
semiconducting tubes varies with
tube diameter between about
1.18-0.7 eV when the diameter
changes from 0.6 to 1 nm
Nanoscience, Nanotechnologies and Nanophysics
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C. Dupas, P. Houdy, M. Lahmani
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Carbon Nanotubes
SELF ORGANIZATION OF CARBON NANOTUBES
During growth CN can self-organize in mainly two modes: a) tubes nested
inside one another (MULTIWALL CN); or b) single walled CN of similar
diameter can form bundles. Each bundle forming a periodic arrangement with
triangular symmetry. In each case, the distance between neighboring tubes Is
roughly equal to the distance between two sheets in graphite. This indicates the
chemical bonds are not modified.
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Carbon Nanotubes
SYNTHESIS AND CHARACTERIZATION
Different methods are used to make CN
1. Arc discharge – A DC voltage is applied between two closely spaced electrodes in an inert
atmosphere. The voltage is high; causes electrical breakdown in the gas molecules between
the electrodes. A current of about 100 A flows in the form of an arc whose temperature is
approximately 3000 degrees Celsius. C-atoms vaporize form the anode. The evaporated
material consists of a mixture of C-polymorphs. The CN need to be separated.
2. Laser ablation – Uses 500 mJ pulses from NdYAG to ablate a graphite target doped with
transition metals. SWNT are formed and collected downstream
3. Catalyzed de-composition 4. Chemical Vapor Deposition – Uses a catalyst to grow nanotubes on a surface.
Catalyst
can be spin-coated on the surface of the substrate. The metal salts are oxidized. Growth of
CN takes place by flowing hydrogen fror catalyst reduction and a hydrocarbon feedstock. It is
possible to use shadow masks to grown on patterned substrates.
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Carbon Nanotubes
VIBRATIONAL PROPERTIES
Lots of vibrational modes of the C- molecules are possible
SPECIFIC HEAT AND THERMAL CONDUCTIVITY
The availability of large phonon modes shapes the specific heat of the SW CN.
The phono mode contributions to the specific heat dominate even at T=0.
Thermal conductivity of 20-3000 Wm-1K-1 have been measured. Theory predicts
values about 6000 Wm-1K-1
MECHANICAL PROPERTIES
Theory predicts Young Modulus Y to be 1000 GPa!
For comparison, Ti – Y=110 GPa, Al2O3: Y=350 GPa
Indirect experimental evidence from spectroscopy and
transmission electron microscopy shows Y:1-3 TPa.
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CHARACTERIZATION
Diffraction – provides information on crystal structure
• Transmission electron microscopy of tubes and bundles. Also allows to
measure tube diameter
• X-Ray scattering – Difficult because C-atoms are very light and signals are
weak.
• Neutron Scattering – Same for this case
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RAMAN SPECTROSCOPY: Probes intramolecular
vibrational and electronic states. From these the
CNT diameter can be inferred.
SCANNING TUNNELING MICROSCOPY
To prepare a sample for STM, CNT in solution are
spinned coated onto a conducting substrate. An
STM tip is biased and brought close to the sample,
so that a tunneling current is generated. As the tip
is scanned, the height varies producing a
proportional current. The displacement of the tip
maps out the electronic topology of the sample.
dI/dV is obtained from these measurements. This
is proportional to the DOS
Figure 6. G band for highly ordered pyrolytic graphite
(HOPG), MWNT bundles, one isolated semiconducting
SWNT and one isolated metallic SWNT. The multi-peak
G-band feature is not clear for MWNTs due to the large
tube diameters (see section 10).
New J. Phys. 5 (2003) 139
PII: S1367-2630(03)66293-2
Characterizing carbon nanotube samples with resonance
Raman scattering
A Jorio, M A Pimenta, A G Souza Filho, R Saito, G Dresselhaus
and M S Dresselhaus
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Carbon Nanotubes
TRANSPORT MEASUREMENTS
The electrical conductivity, thermal conductivity and thermo-power of SWNT can
be measured if suitable contact leads are used. However, measurements on a
single CNT are not trivial. For electrical conductivity measurements the contact
resistance of pads must be kept small.
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FUNCTIONALIZATION OF CARBON NANOTUBES
Covalent Modification
Modifies the surface to improve solubility. This is done by attaching COOH
groups to the nanotubes
Physisorption of CNT
Surface adsorption and intercalation. These ‘dopants’ could enhance charge
transfer to or from the NT while minimally perturbing the density of states.
Filling CNT
Filling can allow for the creation of new ‘heterostructure like materials’. Multiwall CNT have been filled with Sm2O3 oxide particles and decorated with Pb
particles and with transition metals (Cr, Ni). The latter was done by direct
synthesis using an arc discharge based technique.
Single wall CNT have been also successfully filled with Ru, Au, Ag, Pt and Pd,
as well as with KCl,-UCl4, AgCl, and AgBr compounds. It is best to fill up CNT
with materials in the vapor phase, else surface tension becomes limiting.
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APPLICATIONS
ELECTRONICS
Using CNT various functional and logical components of integrated circuits have
been demonstrated such as field-effect transistors, Schottky barrier diode, p-n
juntion, memory cells. CNT used for these applications are semiconducting and
have shown remarkable performance.
CNTs could also be used to conduct electricity (metallic CNT). By virtue of the
stability of the C-C bond and its thermal properties, a CNT can sustain currents that
would melt a Cu wire of the same dimensions.
The difficulty of using CNTs is mainly due to the fact that, the control of their helicity
during growth has not yet been achieved and it is hard to integrate them. To
address the latter, a promising technique that uses CVD techniques to synthesize
nanotubes locally on Si dots in a circuit on which catalyst particles have been
developed.
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Carbon Nanotubes - APPLICATIONS
Carbon Nanotubes can be grown on
tips of Si cantilivers for increased
spatial resolution in AFM
Single carbon nanotubes connected to Si contacts.
Schottky Barriers and Coulomb Blockade in Self-Assembled
Carbon Nanotube FETs
L. Marty,† V. Bouchiat,*‡ C. Naud,† M. Chaumont,† T. Fournier,‡ and
A. M. Bonnot†
Laboratoire des Propriétés Electroniques des Solides and Centre de
Recherches sur les Très Basses Températures, CNRS, BP166 X, F38042 Grenoble Cedex 9, France
Nano Letters, 2003, 3 (8), pp 1115–1118
DOI: 10.1021/nl0342848
Publication Date (Web): June 24, 2003
Copyright © 2003 American Chemical Society
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FIELD EMISSION
Cheng and Zhou, C.R. Physique 4, 2003, pp. 1021
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