Forms of Carbon:
Carbon Nanotubes
Nanophysics
Carbon Nanotubes are
among the amazing objects
that science sometimes
creates by accident,
without meaning to, but
that will likely revolutionize
the technological landscape
of the century ahead.
&
Nanotechnology
Forms of Carbon
Why do Carbon nanotubes form
Carbon
Curl, Kroto, Smalley 1985
Graphite (Ambient conditions)
sp2 hybridization: planar
Diamond (High temperature and pressure)
sp3 hybridization: cubic
Nanotube/Fullerene (certain growth conditions)
sp2 + sp3 character: cylindrical
Finite size of graphene layer has dangling bonds. These dangling
bonds correspond to high energy states.
graphene
Iijima 1991
(From R. Smalley´s web image gallery)
Nanotube formation
Eliminates dangling bonds
+
Increases Strain Energy
Total Energy
decreases
1
What is a Carbon nanotube
What is a Carbon nanotube
CNT is a tubular form of carbon with diameter as small as 1nm.
Length: few nm to microns.
CNT is configurationally equivalent to a two dimensional
graphene sheet rolled into a tube.
A CNT is characterized by its Chiral Vector: Ch = n â1 + m â2,
θ → Chiral Angle with respect to the zigzag axis.
OA = Ch = na1 + ma2
cos θ =
2n + m
2 n 2 + m 2 + nm
The diameter of the nanotube is:
D=
Ch
π
=
(
aCC 3 n 2 + m 2 + nm
π
)
1.41 Å<aC=C<1.44 Å
(graphite)
C60
The C-C bond is elongated by the curvature
Classification
Armchair (n,n), n=m
Zig zag (n,0), m=0
• SingleSingle-wall Carbon nanotubes (SWNTs,1993)
– one graphite sheet seamlessly wrapped-up to form a
cylinder
– typical radius 1nm, length up to mm
Chiral (n,m), n≠m
2
Classification
• Ropes:
Ropes: bundles of SWNTs
– triangular array of individual SWNTs
– ten to several hundreds tubes
– typically, in a rope tubes of different
diameters and chiralities
Classification
• Multiwall nanotubes (Iijima 1991)
– russian doll structure, several inner shells
– typical radius of outermost shell > 10 nm
(From R. Smalley´s web image gallery)
(Copyright: A. Rochefort,
Nano-CERCA, Univ. Montreal)
High resolution transmission
electron microscopy image
Carbon Nanotubes Properties: Mechanical
• Carbon-carbon bonds are one of the strongest
bond in nature
• Carbon nanotube is composed of perfect
arrangement of these bonds
• Extremely high Young’s modulus
Material
Steel
SWNT
Diamond
Synthesis
• Growth conditions play a significant role in deciding the
electronic and mechanical properties of CNTs.
• Growth Mechanisms yet to be fully established and
matter of debate.
Young’s modulus (GPa)
190-210
1,000+
1,050-1,200
3
Laser Ablation
Laser Ablation results
In the absence of catalysts in the
target, the soot collected contains
mostly MWNTs. Their quantity and
structure quality are dependent on the
oven temperature. The best quality is
obtained at 1,200 C. At lower
temperatures, the structure quality
decreases, and the nanotube starts
presenting many defects.
T. Guo, et al., Self-assembly of tubular
fullerenes, J. Phys. Chem. 99 (1995) 10694–10697
L. M. de la Chapelle, et al., “A continuous
wave CO2 laser reactor for nanotube synthesis”,
Proc. Electronic Properties Novel Materials-XVI Int.
Winterschool
– AIP Conf. Proc., Melville 1999, ed. by
H. Kuzmany, J. Fink, M. Mehring, S. Roth (Springer,
Berlin, Heidelberg 1999) 486 237– 240
Arc Discharge
Y. Saito, et al., Phys. Rev. B 48 (1993) 1907
Low magnification transmission
electron microscopy image
Addition of a few percent of transition
metals (Ni, Co) in the graphite, as
catalyst, yields to significant
modifications and SWNTs are obtained.
Other factors are also important such
as furnace temperature.
Chemical Vapor Deposition (CVD)
Two graphite rods of few millimeters in
diameter constitute the electrodes
between which a potential difference is
applied. By electron collision into the
anodic rod, carbon clusters from the
anodic graphite rod are condensed on
the surface of the cathodic graphite
rod and carbon nanotubes are formed
along with other products.
Hydrocarbon + Fe/Co/Ni catalyst
Indeed, the products contain also also
nanoparticles, fullerene-like structures
including C-60, nearly amorphous carbon
Choice of catalyst material?
550-750°C
CNT
Steps:
• Dissociation of hydrocarbon.
• Dissolution and saturation
of C atoms in metal nanoparticle.
• Precipitation of Carbon.
Base Growth Mode or Tip Growth Mode?
• Metal support interactions
4
Controlled Growth by CVD
Methane + Porous Si + Fe pattern
MWNTs
CVD
Aligned
a)
SEM image of aligned
nanotubes.
a) SEM image of side view
of towers. Self-alignment
due to Van der Walls
interaction.
a) High magnification SEM
image showing aligned
nanotubes.
d) Growth Process: Base
growth mode.
Electronic Structure of SWNTs
For lowlow-dimensional crystals, the motion in certain directions may be
quantized. This results in the quantization of wavelength and energy
energy
levels of the electrons.
Folding up graphene into a
carbon nanotube quantizes
the momentum in certain
directions. Depending on
how the folding is done the
resulting tube can be
either metallic or
semiconducting.
semiconducting.
Band Structure of Graphene
• Tight-binding model
– valence and conduction bands touch at E=0
– at half-filling Fermi energy is zero
(particle-hole symmetry): no Fermi surface,
six isolated points, only two inequivalent
• Graphene: zero gap semiconductor
Electronic Structure of SWNTs
• Periodic boundary conditions → quantization of k⊥
– nanotube metallic if Fermi points allowed wave
vectors,
otherwise semiconducting !
– necessary condition: (2n+m)/3 = integer
armchair
metallic
zigzag
semiconducting
5
Electronic Structure of SWNTs
• Band structure predicts three types:
– semiconductor if (2n+m)/3 not integer; band gap:
∆E =
2hv F
≈ 1 eV
3R
– metal if n=m (armchair nanotubes)
– small-gap semiconductor otherwise (curvature-induced
gap)
• Experimentally observed: STM map plus conductance
measurement on same SWNT
• In practice intrinsic doping, Fermi energy typically 0.2 to
0.5 eV
Reality is more complex
Density of States of SWNTs
• Metallic tube:
– constant DoS around
E=0
– van Hove singularities at
opening of new
subbands
• Semiconducting tube:
– gap around E=0
• Energy scale ~1 eV
– effective field theories
valid for all relevant
temperatures
Ballistic vs. Diffusive Transport
In a typical metallic crystal, there are many defects and impurities.
Electron scattering off these impurities gives the metal its finite
electrical conductance (as opposed to infinite eletrical conductance).
Each tube can be either metallic or semiconducting!
semiconducting!
If a metal has no impurities is its conductance infinite (or is its
resistance zero?
6
Ballistic vs. Diffusive Transport
If a metal has no impurities is its conductance infinite (or is its
resistance zero?
Ballistic vs. Diffusive Transport
If we use four terminals, we can measure zero resistance if
transport is ballistic
It depends on how you measure it!
If we use two terminals we always measure a finite resistance
If electron transport is ballistic, the electrical conductance is
is
quantized in units of 2e2/h= (12.9kΩ
(12.9kΩ)-1
SWNTs as ideal quantum nanowires
• Only one subband contributes to transport
→ two spin-degenerate channels
l ≥ 1µ m
• Long mean free paths
→ ballistic transport in not too long tubes
Single Electron Transistor
The energy levels in a
SWNT can be compared
with a particle in a box.
A third gate electrode is
used to tune energy
levels
• SWNTs remain conducting at very low
temperatures
→ model systems to study
correlations in 1D metals
7
Applications
• Electrical
1.
2.
Field emission in vacuum electronics
Building block for nanoelectronics
• Energy storage
1.
2.
Lithium batteries
Hydrogen storage
• Biological
1.
2.
3.
Bio-sensors
Functional AFM tips
DNA sequencing
Energy storage
In the case of fuel cells CNT have been thought to store
hydrogen, particularly for automotive applications. Hydrogen
should e contained in small volumes and weights, yet enabling
reasonable driving distances (500Km).
Several publications have reported very high storage capabilities
capabilities
in CNTs,
CNTs, from 10wt% to less than 0.1wt%. However, many of
these experiments have been difficult to reproduce.
Several problems: large variation in type and purity of CNTs,
CNTs,
difficulties in the characterization, mechanism of hydrogen
adsorption not completely understood.
Field Emission
The concept of field emission involves the application of an
electric field along the CNT axis to induce the emission of
electrons from the end of the tube.
The research has been focused toward using SWCNTs and
MWCNTs for flat panel displays and lamps (in television and
computer monitors).
An electric field directs the fieldfield-emitted electrons from the
cathode, where the CNT are located, to an anode where the
electrons hit a phosphorus screen and emit light.
Advantages: high brightness, a wide angle of view and lower
power consumption.
Challenges: development of low voltage phosphorus.
Bio-Sensors
Attaching molecules of biological interest to carbon nanotubes is an ideal
way to realize nanometernanometer-size biosensors. The electrical conductivity of
such functionalized nanotubes would depend on modifications of the
interaction of the probe with the studied media.
The use of the internal cavity of nanotubes for drug delivery would be
another application, but little work has been carried out to investigate
investigate the
harmfulness of nanotubes in the human body.
Mattson et al. have used MWCNTs as a substrate for neuronal growth.
M. Mattson et al. “Molecular functionalization of carbon nanotubes and use as substrates for neuronal
growth”
growth”, J. Molec.
Molec. Neurosci.
Neurosci. 14 (2000), 175175-182
Davis et al. et al. have immobilized different proteins in MWCNTs and
checked that these molecules were still catalytically active.
J.J. Davis et al. The immobilization of proteins in carbon nanotubes,
nanotubes, Inorg.
Inorg. Chim.
Chim. Acta 272 (1998) 261261-266
8
Thermoelectric Devices
Energy
Typically solar energy conversion is achieved via
thermoelectric and photovoltaic materials
Thermoelectric materials convert the heat
collected in electricity.
Photovoltaic convert solar radiation into electric
energy.
Hot Side
I
I
I
N
P
Diffusion
Among all alternative energy resources , solar
energy is by far the most abundant. However, at
the moment , capturing and storing solar energy is
not a trivial task.
Cold Side
The infrared spectrum of sunlight
(wavelength=800-3000nm) is collected
and turned into heat. Thermoelectric
materials convert heat into electricity.
A very important parameter is the
conversion efficiency, related to a
quantity called figure of merit:
Power Generation
• Power Generation:
T(hot)=500 C, T (cold)=50 C
ZT=1, Efficiency = 8 %
ZT=3, Efficiency =17 %
ZT=5, Efficiency =22 %
• Critical Challenges:
Figure of Merit:
Electrical
Conductivity
ZT =
Seebeck
Coefficient
σS2T
ke + k p
Electron
Phonon
Thermal Conductivity
Reduce phonon heat conduction while
maintaining or enhancing electron transport
9
State of the Art in Thermoelectrics
PbTe/PbSeTe Nano
FIGURE OF MERIT (ZT)
max
3.0
S2σ (µ
µW/cmK2)
k (W/mK)
ZT (T=300K)
PbSeTe/PbTe
Quantum-dot
Superlattices
(Lincoln Lab)
AgPbmSbTe2+m
(Kanatzadis)
32
0.6
1.6
Bulk
28
2.5
0.3
Harman et al., Science (2003)
2.5
2.0
Bi2Te3/Sb2Te3
Superlattices
(RTI)
1.5
1.0
Bi2Te3 alloy
PbTe alloy
0.5
Skutterudites
(Fleurial)
Si0.8Ge0.2 alloy
Dresselhaus
0.0
1940
1960
1980
2000
2020
Bi2Te3/Sb2Te3 Nano
Bulk
S2σ (µ
µW/cmK2)
k (W/mK)
ZT (T=300K)
50.9
1.45
1.0
40
0.6
2.4
Venkatasubramanian et al.,
Nature, 2002.
Basic of Photovoltaic cells
Currently the production of photovoltaic
Materials is associated with crystalline
silicon. Devices made of this first
generation, are basically diodes limited
to a conversion efficiency of 30%.
The second generation photovoltaic
materials include thin films,
organic photovoltaic.
More recently, solar cells made of
polycrystalline Cu (In Ga)Se2 (GIGS)
and CdTe have been demonstrated to
Lower the costs and increase efficiency.
Multijunctions cells based on GaAs
(solar concentrator) can achieve
theoretically efficiency of 60%, even
though the best efficiency reported are
40%.
YEAR
10
Organic Solar Cells
Organic photovoltaic cell uses organic molecules.
Low cost, large scale production and flexibility of
organic molecules make them appealing.
Main disadvantages are low efficiencies,
low stability and low strength.
Single layer organic photovoltaic is the simplest
example. A layer of organic electronic material is
Sandwiched between two metal electrodes
(indium tin oxide ITO and Al/Mg/Ca)
Dye sensitive Solar Cells
Batteries
Researchers at Penn State University are
focusing on the use of titania nanotubes
and natural dye in an attempt to make
more cost-effective solar energy
11
Nano Possibilities
Microelectronics
Trends observed by Gordon Moore (Moore’
(Moore’s law)
• Longer Battery Life
– Potentially up to 20+ years
• Faster Recharge
– Potential to recharge in minutes
• Higher and Lower Operating
Temperatures
– From -50°C/-60°F to +75°C/165°F
• Higher Power Output
– Potentially 4 times greater than current
lithium ion rechargeable battery capability
Electronics Developments
Strategies
• TopTop-Down
– Continued reduction in size of bulk semiconductor
devices.
• BottomBottom-up (Molecular Scale Electronics)
– Design of molecules with specific electronic function.
– Design of molecules for self assembly into
supramolecular structures with specific electronic
function.
– Connecting molecules to the macroscopic world.
• Transistor densities achievable under the present and foreseeable
silicon format are not sufficient to allow microprocessors to do the
things imagined for them.
• Continued exponential decrease in silicon device size is achieved by
exponential increase in financial investment. $200 billion for a
fabrication facility by 2015.
Bottom Up
• Molecules are small.
– With transistor size at 180 nm on a side, molecules are
some 30,000 times smaller.
• Electrons are confined in molecules.
– Whereas electrons moving in silicon have many possible
energies that will facilitate jumping from device to
device, electron energies in molecules and atoms are
quantized - there is a discrete number of allowable
energies.
• Molecules are identical.
– Can be fabricated defect-free in enormous numbers.
• Some molecules can selfself-assemble.
– Can create large arrays of identical devices.
12
Bottom Up
• Dynamical stereochemistry.
– Many molecules have distinct stable geometric
structures or isomers. The isomers can have different
optical and electronic properties.
Molecular Electronics
Can we perform all the basic functions of conventional electronic
electronic
components (wires, diodes, and transistors) at the nanoscale?
nanoscale?
Single Molecules
• Synthetic tailorability
– By choice of composition and geometry, one can
extensively vary a molecule’s transport, binding, optical,
and structural properties
Self-assembled monolayers
Historical Perspective
1950’
1950’s: Inorganic Semiconductors
• To make p-doped material, one dopes Group IV (14) elements (Silicon,
Germanium) with electron-poor Group III elements (Aluminum, Gallium,
Indium)
• To make n-doped material, one uses electron-rich dopants such as the
Group V elements nitrogen, phosphorus, arsenic.
Historical Perspective
1960’
1960’s: Organic Equivalents.
– Inorganic semiconductors have their organic molecular counterparts.
Molecules can be designed so as to be electron-rich donors (D) or electronpoor acceptors (A).
– Joining micron-thick films of D and A yields an organic rectifier
(unidirectional current) that is equivalent to an inorganic pn rectifier.
– Organic charge-transfer crystals and conducting polymers yielded organic
equivalents of a variety of inorganic electronic systems: semiconductors,
metals, superconductors, batteries, etc.
• BUT: they weren’t as good as the inorganic standards.
– more expensive
– less efficient
13
Historical Perspective
Molecular Rectifiers
1970’
1970’s: Single Molecule Devices?
• In the 1970’s organic
synthetic techniques start to
grow up prompting the idea that
device function can be combined
into a single molecule.
• Aviram and Ratner suggest a
molecular scale rectifier. (Chem.
Phys. Lett. 1974)
•But, no consideration as to how this molecule would be
incorporated into a circuit or device.
Theoretical concept b y Aviram and Ratner (1974)
Historical Perspective
Molecular Electronics: the idea
19801980-1990’
1990’s: Single Molecule
Devices.
• New imaging and manipulation techniques
Science 271, 1705 (1996)
(Scanning Probe Microscopy)
• Advanced synthetic and characterization
techniques
• Advances in Self-Assembly
Macroscopic/Supramolecular Chemistry
Mechanical Break Junction
Reed et al., Science 278, 253 (1997)
14
Properties of Molecules and Solids
Metal-Semiconductor Contacts
Semiconductor-Semiconductor Contacts
p-n junction-diode
15
N
S
S
S
Oligomers
0.5
10
0.4
8
6
0.3
4
I(pA)
S
Current (nA)
Oligomers
0.2
2
0.1
0
0.0
-2
T=77 K
-4
-0.1
-2
-1500 -1000
S
0
1
2
V(V)
6
4
2
Current (nA)
N
S
S
-1
1500
0.2
S
S
1000
0.4
N
S
500
VBias (mV)
N
S
0
0.0
60 nm x 60 nm
-0.2
I(pA)
S
-500
0
-2
-0.4
-4
-0.6
Sample P3
Au
Au
-6
-0.8
-2
-1500 -1000
Iavarone et al.
-500
0
500
1000
1500
Iavarone et al.
-1
0
1
2
V(V)
VBias (mV)
Resonant Tunneling Diodes (RTD)
Molecular Resonant Tunneling Diodes (MRTD)
M. A. Reed et al., IEEE Proc. 87, 652 (1999)
16
Molecular Resonant Tunneling Diodes (MRTD)
Contact with an STM tip
Resistance much higher at the tip-molecule interface
Datta et al., Phys. Rev. B 59, 7852(1999)
Doping Mechanism
Conformational Switching
P.S. Weiss et al., Science 292, 2303 (2001)
17
Conformational Switching
Field Effect Transistor (FET)
•Switching behavior observed for the unsubstituted oligomer
• The mechanism is related to the density of the surrounding medium
•Similar behavior when sweeping the voltage between -1.4 V and 1.4 V
•Consistent with the ease of rotation of the central ring (~
(~kT)
kT)
Single Electron Transistor (SET)
I-V measurements of SETs
18
Molecules Candidates
Single Nanostructure Transistor
Transistor Behavior based on:
• Nanostructures as channels
• Internal nanostructure junctions
• Crossed nanostructures
Carbon Nanotubes Circuits
Silicon Nanowires Transistors
19
Nanofabrication and molecular circuits
Molecular Circuit Fabrication
• Fabrication of multi-electrode nm-sized junctions
• Wiring junctions to interconnects
• Deposition of a single molecule
• Fabrication of input/output hooked up to power
Interconnect wiring
• Nano-imprint lithogtaphy
• Carbon Nanotubes
Molecule Placement
• Scanning probe microscopy
• Stamping of molecules
• Directed self-assembly
20