M' Meyyappan

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Carbon Nanotubes
M. Meyyappan NASA Ames Research Center Moffett
Field, CA 94035 email mmeyyappan_at_mail.arc.nasa.g
ov
Source Carbon Nanotubes Science and
Applications, Editor M. Meyyappan, CRC Press,
2004
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Outline
What are carbon nanotubes? Properties and
applications summary Growth and
characterization - CVD - PECVD Nanoelectr
onics Applications in AFM Field
Emission Biosensors Chemical
sensors Other applications EMI Shielding,
composites, storage Summary
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Carbon Nanotube
CNT is a tubular form of carbon with diameter as
small as 1 nm. Length few nm to microns. CNT
is configurationally equivalent to a two
dimensional graphene sheet rolled into a tube
(single wall vs. multiwalled).
See textbook on Carbon Nanotubes Science and
Applications, M. Meyyappan, CRC Press, 2004.
CNT exhibits extraordinary mechanical properties
Youngs modulus over 1 Tera Pascal, as stiff as
diamond, and tensile strength 200 GPa. CNT can
be metallic or semiconducting, depending on
(m-n)/3 is an integer (metallic) or not (semicon).
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CNT Properties
The strongest and most flexible molecular
material because of C-C covalent bonding and
seamless hexagonal network architecture Strengt
h to weight ratio 500 times greater than Al,
steel, titanium one order of magnitude
improvement over graphite/epoxy Maximum
strain 10 much higher than any
material Thermal conductivity 3000 W/mK in
the axial direction with small values in the
radial direction Very high current carrying
capacity (107 - 109 A/cm2) Excellent field
emitter high aspect ratio and small tip radius
of curvature are ideal for field
emission Other chemical groups can be attached
to the tip or sidewall (called
functionalization)
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Potential CNT Applications
Sensors, Bio, NEMS CNT based microscopy
AFM Nanotube sensors bio, chemical Batter
ies (Li storage), Fuel Cells, H2 storage
(?) Nanoscale reactors, ion channels Biomedi
cal - Nanoelectrodes for implantation - Lab on
a chip - DNA sequencing through AFM
imaging - Artificial muscles - Vision chip for
macular degeneration, retinal cell
transplantation Molecular gears, motors,
actuators
Electronics CNT quantum wire
interconnects Diodes and transistors for
computing Data Storage Capacitors Field
emission based flat panel displays Field
emitters for instrumentation
Challenges
Challenges
Controlled growth Functionalization
with probe molecules, robustness Integration,
signal processing Fabrication techniques
Control of diameter, chirality Doping,
contacts Novel architectures (not CMOS
based!) Development of inexpensive
manufacturing processes
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Potential CNT Applications Structural, Mechanical
High strength composites Cables, tethers,
beams Multifunctional materials Functionaliz
e and use as polymer back bone - plastics with
enhanced properties like blow molded
steel Heat exchangers, radiators, thermal
barriers, cryotanks Radiation
shielding Filter membranes, supports Body
armor, space suits
Challenges
- Control of properties, characterization - Disper
sion of CNT homogeneously in host
materials - Large scale production - Application
development
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CNT Synthesis
CNT has been grown by laser ablation (pioneered
at Rice University) and carbon arc process
(NEC, Japan) - early 90s. - SWNT, high
purity, purification methods
CVD is ideal for patterned growth
(electronics, sensor applications) - Well
known technique from microelectronics - Hydr
ocarbon feedstock - Growth needs catalyst
(transition metal Fe, Ni, Co) - 500-900
deg. C. - Numerous parameters influence CNT
growth (temperature, choice of feedstock,
H2 and other diluents, choice of
catalyst and preparation)
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Carbon Nanotubes Some Examples
SWNT
MWNT Tower
Close view of MWNT Tower
MWNT Structures
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Ion Beam Sputtering of Multilayer Catalysts for
Nanotube Growth
Solution based techniques are time consuming
also hard to confine catalysts in
ultrasmall patterns. Physical techniques
(sputtering, laser deposition) are
much quicker, amenable for patterning.
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Catalyst Characterization
Catalyst surface characterized by AFM (with
SWNT tip) and STM. AFM image of as-sputtered 10
nm iron catalyst (area shown is 150 nm x 150
nm). Also, the same surface after heating to
750 C (and cooled) showing Fe particles
rearranging into clusters.
Delzeit et al. J. Phys. Chem. B, 2002, Vol.
106, p. 5629.
STM image of a nickel catalyst showing
nanoscale particles These results are
consistent with high resolution TEM
showing particles as small as 2 nm.
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Role of the Underlayer
Alloying a catalyst with a non-catalytic metal
increases the number of reactive sites through
surface clusters, as shown in a thermodynamic and
kinetic study (Nolan et al, J. Phys. Chem. B,
1998). No need for any other chemical
pre-treatment or preparation by ion bombardment
to create particles. Allows tuning the final
electrical conductivity of the structure
The underlayer can play the role of a barrier
between an incompatible catalyst and the
substrate an example of this is Fe on
HOPG In some cases, excess and unused catalyst
lifts off and catalyses a second layer of
nanotubes - sort of an anomalous layered growth
pattern. Use of an underlayer prevents this
from happening.
Delzeit et al., J. Phys. Chem. B, 2002, Vol. 106,
p. 5629.
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CVD Growth Mechanisms For Carbon Nanotubes
Adsorption and decomposition of feedstock on
the surface of the catalyst particle Diffusion
of carbon atoms into the particle Once
supersaturation is reached, carbon precipitates
into a crystalline tubular form Base growth or
tip growth Growth stops when graphitic overcoat
occurs on the growth front - catalyst poisoning
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SWNTs on Patterned Substrates

• Surface masked by a 400 mesh TEM grid
• - Methane, 900 C, 10 nm Al/1.0 nm Fe 10 minutes

TEM image
SWNTs inside a grid
Catalyst particles
Delzeit et. Al, Chem. Phys. Lett., 2001, Vol.
348, p. 368.
Growth on a patterned grid
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Raman Analysis of SWNTs
Delzeit et. al, Chem. Phys. Lett., 2001, Vol.
348, p. 368.
2 mw laser power, 1 µm focus spot Characterist
ic narrow band at 1590 cm-1 Signature band at
1730 cm-1 at SWNTs Diameter distribution 1.14
nm to 2 nm consistent with TEM results High
metallic of NTs
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Multiwall Nanotube Towers
- Surface masked by a 400 mesh TEM grid 20 nm
Al/ 10 nm Fe nanotubes grown for 10 minutes
Grown using ethylene at 750o C
Delzeit et al. J. Phys. Chem. B, Vol. 106, 5629
(2002).
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MWNTs by Thermal CVD Summary
MWNT towers have been grown on a wide variety
of substrates Si, quartz, mica, HOPG Very
strong attachment of the towers to the silicon
substrate (scotch tape test) HOPG surface
exhibited the weakest attachment Catalyst/under
layer formulations are similar for SWNT and MWNT
growth but the growth conditions are
different - 900 C for SWNT vs. 750 C for
MWNT - Methane for SWNT vs. ethylene for
MWNT Theoretical analysis by Karzow and Ding
(Phys. Rev. B, 1999) shows that higher
temperatures (900 C) and low supply of carbon
favor SWNTs the opposite favors MWNTs
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Why Plasma in Nanotube Growth?
Plasma CVD is a variation of the
well-established thermal CVD, offers additional
control parameters such as power, substrate bias
to exert influence on growth if
possible Conventional wisdom of plasma
processing does NOT seem to apply
here - Typically PECVD enables a lower
temperature operation than thermal CVD since
electrons are energetic and lead to the
production of reactive species without the high
thermal energy to breakup the precursor. - CNT
growth is catalyst promoted and requires 500 C
for catalyst activation. So, traditional
cold plasma with very low substrate
temperatures may not be possible. So, is
plasma just another way to produce
species? - May be - If anything, excessive
production of C-bearing species may commonly
lead to only MWNTs and filaments, not SWNTs
(unless C-source is somehow controlled). - One
recognized advantage is vertical alignment of
nanotubes due to the electric field.
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Plasma Reactor for CNT Growth
Certain applications such as nanoelectrodes,
biosensors would ideally require individual,
freestanding, vertical (as opposed to towers or
spaghetti-like) nanostructures The high
electric field within the sheath near the
substrate in a plasma reactor helps to grow such
vertical structures dc, rf, microwave,
inductive plasmas (with a biased
substrate) have been used in PECVD of such
nanostructures
Cassell et al., Nanotechnology, 15 (1), 2004
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MWNTs vs. MWNFs
Filaments consist of stacked-cone arrangement
of graphite basal plane sheets grow with
particles at the tip hydrogen is believed to
satisfy the valences at cone edges in
filaments The orientation angle ? (between
graphite basal planes and tube axis) increases
with increasing hydrogen concentration When ?
0, MWNTs. These are filaments with no graphite
edges, requiring no valence-satisfying species
such as hydrogen Nolan et al provide evidence
(all thermal CVD) that material produced with
CO disproportionation (without any H2) was
MWNTs addition of H2 produced filaments as
of H2 , ? up to 30 Better to call
these structures as MWNFs (filaments) instead of
graphitic carbon fibers (GCFs) or vapor grown
carbon fibers (VGCFs) both of which denote solid
cylinders alternatively, called simply carbon
nanofibers (CNFs)
Nolan et al, JPC, B, 1998
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TEM Images of MWNTs and MWNFs
SEM
TEM
MWNF
MWNT
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Raman Spectra at 633 nm Excitation Plasma CVD
Samples
MWNF
D band centered at 1350 cm-1 Tangential G
band at 1590 cm-1 Shoulder peaked around 1616
cm-1 only in fibers
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High Volume Production of CNTs
Needed for composites, hydrogen storage, other
applications which need bulk material Floatin
g catalyst CVD (instead of supported
catalysts) - Carbon source (CO,
hydrocarbons) - Floating catalyst source (Iron
pentacarbonyl, ferrocene) - Products
Nanotubes, catalyst particles, impurities - K
gs/day only now need very high scaleup for price
to come down Arc synthesis
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Purification of CNTs
Purpose To remove all unwanted material and
obtain the highest yield with no damage to the
nanotubes Several processes have been reported
in the literature a typical process involves
the following steps - A sample (50 mg) is
transferred to a 50 ml flask, with 25 ml
concentrated HCl and 10 ml of concentrated
HNO3 - Solution heated for 3 hrs, constantly
stirred with a magnetic stirrer in a reflux
apparatus equipped with a water-cooled condenser
? removes metal and graphite
particles - Suspension transferred into
centrifuge tubes and spun at 3220 g for 30
min. - After pouring of the supernatant, solids
are resuspended, spun three times in
deionized water - The solid is treated with
NaOH (0.01 µ) and centrifuged for 30 minutes
? nanotube bundles with tube ends capped with
half fullerenes - Sample dried overnight in a
vacuum oven at 60 C
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Functionalization
Attaching chemical groups to the sidewall of
CNTs to modify the properties as needed for
applications - Chemical modification of the
sidewall may improve the adhesion
characteristics of CNTs in a host matrix to
make composites - Chemical or
biosensors Saturation of 2 the C atoms in
SWNTs with C-Cl sufficient to change electronic
band structures dramatically done with
reacting SWNTs with dichlorocarbene (Chen et al,
Science, 282, 95 (1998). Fluorination of SWNTs
with F2 gas flow at 250-600C for 5
hrs. (Michelson et al, CPL, 296, 188 (1998) has
been shown to attach F covalently to the
sidewall Cold plasma approach to
functionalization (Khare et al, Appl. Phys.
Lett., Vol. 81, 5237, 2002)
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Functionalization Using a Glow Discharge
Microwave reactor setup for Functionalization
Just as the difference between wet etching and
dry etching, Dry approach to
functionalization using a plasma minimizes
chemical use and waste, is efficient and
rapid. The key is to find the source gas that
produces the atom or radical that needs to be
attached to CNTs.
To date, functionalization of H (from H2), N
(from N2), F (from CF4 or F2) has been
demonstrated.
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Atomic H Functionalization Using a Plasma FTIR
Results
Functionalization is complete in a minute,
evidenced by the fact that various features in
the FTIR spectra stop changing.
(Khare et al, Appl. Phys. Lett., Vol. 81, 5237,
2002)
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Applications to be Discussed
Application
Property
Strength, flexibility, aspect
ratio Electrical conductivity Thermal
conductivity Electrical conductivity,
density High strength
AFM probes Interconnects Chip
cooling EMI shielding Composites
Nanoelectronics and sensors, field emission
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Nanotube Electronic Devices FETs

• Semiconducting nanotube as conducting channel in
  a field effect transistor showed large on/off
  ratio and relatively high mobility.
• Uncontrolled synthesis of metallic and
  semiconductor nanotubes places significant
  contraints on assembly of device arrays.

Courtesy - C. Lieber
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CNT-based Logic and Memory Devices
First Single-Walled Nanotube Logic
Device Inverter Demonstration (Liu et al., Appl.
Phys. Lett., Vol. 79, 3329, 2001)
NOR Gate, SRAM, Ring Oscillators, Memory Devices
Demonstrated
CNT vs. Silicon
CNT Si Gate Length 260
nm 15 nm Gate Oxide Thickness
15 1.4 nm Transconductance µS/µm
2321 975 IBM
Bin Yu
(IEDM)
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What is Expected from Emerging Alternative
Technologies?
(From SRC/NASA Ames Workshop on Nanotubes and
Nanoelectronics, 1998)
Must be easier and cheaper to manufacture than
CMOS High current drive ability to drive
capacitances of interconnects of any
length High level of integration (gt1010
transistors/circuit) High reproducibility
(better than ? 5) Reliability (operating time
gt 10 years) Very low cost ( lt 1
µcent/transistor) Better heat dissipation
characteristics and cooling solutions Novel
architectures Fault tolerant? Evolvable?
Neural? Novel state variables
Spin? Everything about the new technology must
be compelling and simultaneously further CMOS
scaling must become difficult and not
cost-effective. Until these two happen
together, the enormous infrastructure built
around silicon will keep the silicon engine
humming.
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Multi-terminal Nanotube Junctions
M. Menon and D. Srivastava, J. Mat. Res. Vol. 13,
2357 (1998) D. Srivastava, S. Saini and M. Menon,
Molecular Electronics, Ed. Aviram and Ratner, 178
(1998)
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Multi-wall Y-junction Carbon Nanotubes
Experimental Synthesis of Multi-wall Carbon
Nanotube Y-Junctions (2000)
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Four-level CNT Dentritic Neural Tree
Neural tree with 14 symmetric
Y-junctions Branching and switching of signals
at each junction similar to what happens in
biological neural network Neural tree can be
trained to perform complex switching and
computing functions Not restricted to only
electronic signals possible to use acoustic,
chemical or thermal signals
Cassell et al., Appl. Phys. Lett., 2003, Vol. 82,
p. 817.
500 nm
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Field Emission
When subjected to high E field, electrons near
the Fermi level can overcome the energy barrier
to escape to the vacuum level Compare to
thermionic emission, where the source is heated
to 1000º C to give the electrons the energy
required to overcome the surface potential
barrier Common tips Mo, Si,
diamond Applications - Cathode ray lighting
elements - Flat panel displays - Gas
discharge tubes in telecom networks - Electron
guns in electron microscopy - Microwave
amplifiers
Courtesy P. Sarrazin
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Field Emission Model
Fowler - Nordheim equation ? is work
function, ? is field enhancement factor Plot
of ln (I/V2) vs. (1/V) should be linear At low
emission levels, linearity seen in the high
field region, current saturates Critical
low threshold E field, high current density,
high emission site density (for high resolution
displays)
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Field Emission (cont.)
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Field Emission Test Apparatus
Cathode and anode enclosed in an evacuated cell
at a vacuum of 10-9 - 10-8 Torr Cathode
glass or polytetrafluoroethylene substrate with
metal- patterned lines - nanotube film
transferred to substrate or grown directly on
it Anode located 20-500 µm from
cathode Turn-on field electric-field required
to generate 1 nA - should be
small Threshold field electric field required
to yield 10 mA/cm2
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Structure of CNT Field Emitters
Nature of nanotubes (SWNTs, MWNTs,
CNFs) Clean emitting sites vs. adsorbates
(water vapor, oxygen) Microstructure Screen
ing effect Diode vs. triode
Current is controlled by gate voltage,
independent of acceleration voltage
High voltage required and/or gap needs to be
adjusted
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Flat Panel Displays Using Nanotubes
Working full color flat panel displays and
CRT-lighting elements have been demonstrated in
Japan and Korea Display - Working anode, a
glass substrate with phosphor coated ITO
stripes - Anode and cathode perpendicular to
each other to form pixels at the intersection
- Phosphors such as Y2O2S Eu (red), Zns Cu, Al
(green), ZnS Ag, Cl (blue) - 40 display
showing a uniform and stable image Lighting
Element - Phosphor screen printed on the inner
surface of the glass and backed by a thin Al
film (100 nm) to give electrical
conductivity - Lifetime testing of the lighting
element shows a lifespan over 10000 hrs.
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CNT in Microscopy
Atomic Force Microscopy is a powerful technique
for imaging also CD metrology, nanomanipulation,
as platform for sensor work, nanolithography... C
onventional silicon and other tips wear out
quickly. CNT tip is robust, offers amazing
resolution.
2 nm thick Au on Mica imaged with SWNT
Simulated Mars dust
Written using multiwall tube
Nguyen et al., Nanotechnology, 12, 363 (2001)
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AFM Imaging with Single Wall Nanotube Tips
2 nm thick Au on Mica
Si3N4 on Silicon substrate
- Remarkable nanoscale resolution is
evident - More importantly, the same high
resolution is maintained with continued use of
the probe for a long time probe doesnt wear
out easily.
Nguyen et al., Nanotechnology, 2001, Vol. 12, p.
363.
5 nm thick Ir on Mica
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MWNT Scanning Probe
Profilometry in Semiconductor Manufacturing
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High Resolution Imaging of Biological Materials
DNA
PROTEIN
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Imaging in Aqueous Environments
The hydrophobic nature of the CNT graphitic
sidewall is chemically incompatible with
aqueous solutions. Probes are unstable when
submerged in solution. The CNT probe is
treated with a ethylene diamine coating,
rendering it hydrophilic.
DNA on mica in 20 mM Tris HCl and 10 mM magnesium
chloride solution (near physiological conditions)
R.M. Stevens et al., IEEE Trans.
Nanobioscience, Vol. 3, pp. 56-60 (2004).
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CNT Interconnects
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Motivation for Using Carbon Nanofibers (CNFs)

• Physical limits of copper interconnects and vias
  will soon be reached if scaling trends continue
• CNFs, with their vertical orientation, have the
  capability to fulfill both size and performance
  requirements for next generation ICs
• In contrast, SWNTs, in spite of their better
  conductivity than CNFs, may be difficult to work
  with since filling a via with spaghetti-like
  structures will lead to unreliable interconnects.

Chen et al., IEEE Elec. Dev. Lett., 19, 508(1998)
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MWNT Interconnects ?

• CNT advantages
• Small diameter, high aspect ratio
• High current carrying capacity
• Highly conductive along the axis
• High mechanical strength

Infineon
Question How to integrate this into
device processing?
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Cu Damascene Interconnects
G. Steinlesberger, et al., Microelectronic
Engineering, 64, 409 (2002).
H. H. Hwang, M. Meyyappan, G.S.Mathad, and
R.Ranade, J. Vac. Sci. Technol., B 20(6), 2199
(2002).

• Challenges
• Etching high aspect ratio features
• Void-free filling
• Surface and grain boundary scattering
• Electromigration

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MWNT in Vias and Contact Holes
F. Kreupl et al., Microelectronic Engineering,
64, 399 (2002).

• Problems
• Difficult to grow in narrow, high aspect ratio
  holes
• Poor mechanical stability
• Poor contact
• Will easily unravel during the chemical
  mechanical polishing step

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Bottom-up Approachfor CNT Interconnects
Ti, Mo, Cr, Pt
Ni
At 400 to 800 C
J. Li, Q. Ye, A. Cassell, H. T. Ng, R. Stevens,
J. Han, M. Meyyappan, Appl. Phys. Lett., 82(15),
2491 (2003)
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Reliability Measurement of CNF Via
No degradation of CNF via was observed over
several days of high current density stress
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Chip Cooling
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Thermal Resistance Measurement Apparatus
Material MWNTs intercalated with Cu
(electrochemical approach)
Courtesy Quoc Ngo, Brett Cruden, and Jun Li
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Thermal Resistance Measurement

• Material is placed between two Cu blocks
• Upper block
• Apply 6.8 psi pressure
• Designed contact area up to 1 in2
• Heated with a resistive heater
• Lower block is maintained at a constant
  temperature (lt room temperature)

• Apparatus in the ambient environment
• Temperature gradient is measured through material
  at different power levels
• Thermal resistance is calculated including that
  through the materials as well as two interfaces

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Determining Contact Resistance
RuFaze RuFaze Si - Rsi - RCu RCNT/Cu
RCNT/Cu- RuFaze - RCu
Sample 1 CNT/Cu
Sample 2 CNT/Cu
Sample 3 CNT (as-grown)
RCNT/Cu0.93 K.cm2/W STDEV 0.12
RCNT/Cu0.84 K.cm2/W STDEV 0.22
RCNT2.30 K.cm2/W STDEV 0.33
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Comparison to Real Thermal Budget
Normalizing to Area
RCNT/Cu0.404 K/W
Sample 1
RCNT/Cu0.358 K/W
Sample 2
Sample 3
RCNT0.42 K/W
R. Viswanath et. Al, Intel Tech. Jour., Q3 (2000)
Best recent result RCNT/Cu 0.098 cm2.K/W
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Mechanical Stability of CNT/Cu Film
Before compressive stress
After compressive stress
Fiber integrity is maintained up to 60 psi
(normal pressure values for packaging)
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CNT Based Biosensors
Probe molecules for a given target can be
attached to CNT tips for biosensor
development Electrochemical approach
requires nanoelectrode development using
PECVD grown vertical nanotubes The signal can
be amplified with metal ion mediator
oxidation catalyzed by Guanine.
High specificity Direct, fast
response High sensitivity Single molecule
and cell signal capture and detection
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Commonly Used Carbon Electrodes
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Nanoelectrode for Sensors
Nanoscale electrodes create a dramatic
improvement in signal detection over traditional
electrodes
Traditional Macro- or Micro- Electrode
Nanoelectrode Array
Electrode
Insulator

• CNT tips are at the scale close to molecules
• Dramatically reduced background noise

Nano- Electrode
Scale difference between macro-/micro-
electrodes and molecules is tremendous Backgroun
d noise on electrode surface is therefore
significant Significant amount of target
molecules required
Multiple electrodes results in magnified signal
and desired redundance for statistical
reliability. Can be combined with other
electrocatalytic mechanism for magnified signals.
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Macroelectrode vs. Nanoelectrode
I total Iplanar Iradial
glassy carbon
Semi-infinite planar linear diffusion
Semi-infinite hemispherical diffusion
When reducing the size (r), orders of magnitude
improvements are found in (1) Spatial
resolution defined by r (3) Temporal
resolution Cell time constant t RuCd r
Cd0/4k (2) Sensitivity signal-to-noise ratio
is/in µ nFC0D0/r
Nanoelectrode Array
High sensitivity Easily measurable signal
Fast detection
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Nanoelectrode Array Fabrication
Embedded CNT Array after planarization
30 dies on a 4 Si wafer
63
Carbon Nanotube Electrodes at Different Densities
CNT coverage 20 (3.0x109
CNTs/cm2) Average nearest-neighbor
distance 300 nm
CNT coverage lt 1 (1x108 CNTs/cm2) Average
nearest-neighbor distance gt 1500 nm
CV in 1mM K4Fe(CN)6 in 1M KCl at 20 mV/s
J. Li, H. T. Ng, A. Cassell, W. Fan, H. Chen, Q.
Ye, J. Koehne, J. Han, M. Meyyappan, Nano.
Lett., 2003, 3 (5), 597.
64
Functionalization of DNA
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Electrochemical Detection by AC Voltametry
J. Li, H. T. Ng, A. Cassell, W. Fan, H. Chen, J.
Koehne, J. Han, M. Meyyappan, Nano. Lett., 2003,
3, 597.
66
Fabrication of Genechip

• Potential applications
• Lab-on-a-chip applications
• Early cancer detection
• Infectious disease detection
• Environmental monitoring
• Pathogen detection

30 dies on a 4 Si wafer
This biochip needs to be integrated with
microfluidics capability to produce a functional
Lab-on-a-chip system.
67
CNTs in Bio-Medical Applications
Approach 1 Vision Chip Development of an
implantable device consisting of an array of
carbon nanotubes on a silicon chip for
restoration of vision in patients with macular
degeneration and other retinal disorders Approac
h 2 Carbon Nanotube Bucky Paper for Retinal
Cell Transplantation A meshwork of carbon
nantotubes as a substrate for retinal cell growth
and as a carrier to facilitate surgical
transplantation of retinal cells into the retina
of patients with macular degeneration
What is macular degeneration?
Courtesy David Loftus
68
Age Related Macular Degeneration

• Loss of central vision, due to the death of
  retinal pigment epithelium (death of
  photoreceptor cells)

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Vision Chip
Why Carbon Nanotubes? Mechanical
Properties Electrical Properties Chemical
Properties Engineer-ability Biocompatibilit
y
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Vision Chip Challenges
The work to date has focused exclusively on
issues related to the interface with the retinal
tissue, and does not involve work with CCD
chips.   Hurdle 1 To demonstrate
biocompatibility of carbon nanotubes in intact
retinal tissue.   Hurdle 2 To demonstrate that
carbon nanotube towers have sufficient mechanical
strength to penetrate retinal tissue. Hurdle 3
To demonstrate the ability of carbon nanotubes
to convey electrical signals to the retinal
ganglion cell layer.
73
Preliminary Biocompatibility Data
74
Carbon Nanotube Tower on a Silicon Chip
Tower consists of a bundle of multi-walled carbon
nanotubes
Hurdle 1 Tissue Biocompatibility Hurdle 2
Mechanical Strength
75
Hurdle 3 Electrophysiology Testing
Electrophysiology testing will consist of retinal
tissue stimulation by the Quad Chip, with
recording of electrical activity in the ganglion
cell layer adjacent to the CNT towers.
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Retinal Cell Transplantation

• In the early stage of macular degeneration,
  retinal pigment epithelial (RPE) cells die, which
  leads to loss of photoreceptors.
  Solution?replace the cells that are lost.
• RPE cells and iris pigment epithelial (IPE) cells
  can be harvested from the eye, grown in culture,
  then put back into the eye (autologous
  transplantation).

77
Problems with Retinal Cell Transplantation

• Transplantation of suspensions of epithelial
  cells into the sub-retinal space fails to
  re-establish the proper architecture of the RPE
  layer. Instead of a sheet of uniformly oriented
  cells, you get a jumble of cells.
• Solution
• Establish the proper orientation of the
  epithelial cells prior to transplantation, by
  growing them in culture on a physical support

78
Current Status, Problems and a Possible Solution
The Obvious Strategy Natural Substrates for
Retinal Transplantation

• Anterior Lens Capsule (basal lamina)
• Descemets Membrane (posterior cornea)

Excellent growth of retinal epithelial cells,
assembly of true epithelial architecture. Probl
em! Membranes with attached epithelial cells
cannot be easily implanted into the eye, because
the membranes are flimsy and tend to curl up.
They lack the mechanical properties necessary for
surgical handling.
Solution Carbon Nanotube Bucky Paper A
meshwork of carbon nanotubes formed into a
paper-like structure
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RPE cells grown on Carbon Nanotube Bucky Paper
As-prepared bucky paper
SEM Image after growth of RPE results
Light micrograph/histological staining of RPE
grown on bucky paper
Confluent monolayer, with uniform orientation
of cells Excellent attachment of RPE cells to
the Bucky Paper surface confirmation of correct
apical/basolateral orientation
80
Implantation of Carbon Nanotube Bucky Paper into
the Sub-Retinal Space of an Albino Rabbit
Result Bucky paper is easily manipulated during
surgery (does not tear and stays flat), and is
immunologically well-tolerated by the eye.
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Carbon Nanotube Biocompatibility
NEEDS

• Long Duration Space Flight How to deliver
  medical therapy?
• Acute injury
• Hemostatic Bucly Paper
• Bucky Paper for Wound Healing
• Cancer Therapy
• Adoptive Immunotherapy Delivered by Encapsulated
  Cells
• Immune Shielded Delivery of Chemotherapy
• Therapy for diabetes
• Transplantation of Islet Cells

• Implantable Physiological Sensors
• Remote sensing
• Early medical intervention
• Novel medical countermeasures
• Cardiovascular physiology

82
Chemical Sensors Why Use Nanotubes?
Compared to existing systems, potential exists
to improve sensitivity limits, and certainly
size and power needs Why? Nanomaterials have
a large surface area. Example SWCNTs have a
surface area 1600 m2/gm which translates to
the size of a football field for only 4
gm. Large surface area large
adsorption rates for gases and vapors
changes some measurable properties of the
nanomaterial basis for
sensing - Dielectric - Capacitance - Condu
ctance - Deflection of a cantilever - -
4 grams
83
Conductivity Change of CNTs Upon Gas/Vapor
Adsorption
Early chemical sensors were of the CHEMFET type
with SnO2 and other oxide conducting
channels Similar CNT-FETs have been tested in
the literature, exposing to NH3, NO2, etc.
change in conductivity has been observed (Kong
et al., Science, Vol. 287, 2000) Limitations
of CNT-FET - Single SWCNT is hard to transfer
or grow in situ - Even a film of SWCNTs by
controlled deposition in the channel is
complex - 3-terminal device is complex to
fabricate - Commercial sensor market is very
cost sensitive
84
SWCNT Chemiresistor
Conventional thin film transistor approach is
complex and expensive Two terminal
chemiresistor is cheaper, easier to
fabricate 1. Interdigited electrode device
fabrication 2. Disperse purified nanotubes in
DMF (dimethyl formamide) 3. Solution casting of
CNTs across the electrodes
Jing Li et al., Nano Lett., 3, 929 (2003)
85
SWCNT Sensor Performance
Sensor tested for NO2, NH3, CH4, Cl2, HCl,
acetone, benzene, nitrotoulene Test
condition Flow rate 400 ml/min Temperature
23o C Purge carrier gas N2 or
Air Sensitivity in the ppm to ppb
range Selectivity through (1) doping, (2)
coating CNTs with polymers, (3) multiplexing
with signal processing Need more work to
speedup recovery to baseline
Detection limit for NO2 is 44 ppb.
86
Sensing Mechanisms
NO2
Nitrotoluene
ONO
.
e
P-type
E0
Intratube Modulation
Intertube Modulation
87
Scalable Array Approach (Multi-channel Sensing
Chip)
12 to 36 sensing elements are on a chip (1cm x
1cm) now with heaters and thermistors. Number
of sensing elements can be increased on a
chip. Number of chips can be increased on a 4
wafer. Wafer size can be increased to 6, 8,
or 12. SWCNT solution-casting by ink jetting
or using microarrays
88
CNT-Based Composites
Carbon nanotubes viewed as the ultimate
nanofibers ever made Carbon fibers have been
already used as reinforcement in high strength,
light weight, high performace
composites - Expensive tennis rackets,
air-craft body parts Nanotubes are expected to
be even better reinforcement - C-C covalent
bonds are one of the strongest in
nature - Youngs modulus 1 TPa ? the in-plane
value for defect-free graphite Problems - Crea
ting good interface between CNTs and polymer
matrix necessary for effective load
transfer ? CNTs are atomically smooth h/d
same as for polymer chains ? CNTs are largely
in aggregates ? behave differently from
individuals Solutions - Breakup aggregates,
disperse or cross-link to avoid
slippage - Chemical modification of the surface
to obtain strong interface with surrounding
polymer chains
WHY?
89
General Issues in Making CNT Composites
Polymer matrix composites - Nanotube
dispersion - Untangling - Alignment - Bonding
- Molecular Distribution - Retention of neat-CNT
properties Metal and Ceramic Matrix
Composites - High temperature stability - Reacti
vity - Suitable processing techniques - Choice
of chemistries to provide stabilization and
bonding to the matrix.
90
Conducting Polymers Based on Carbon Nanotubes
High aspect ratio allows percolation at lower
compositions than spherical fillers (less than
1 by weight) Neat polymer properties such as
elongation to failure and optical transparency
are not decreased. ESD Materials Surface
resistivity should be 1012 - 105
?/sq - Carpeting, floor mats, wrist straps,
electronics packaging EMI Applications
Resistivity should be lt 105 ?/sq - Cellular
phone parts - Frequency shielding coatings for
electronics High Conducting Materials Weight
saving replacement for metals - Automotive
industry body panels, bumpers (ease of painting
without a conducting primer) - Interconnects
in various systems where weight saving is critical
91
Smart Materials, Special Coatings
Carbon nanotubes can be embedded in high
performance composites as reinforcing agents
and strain sensors allowing for nondestructive
monitoring and distributed sensing of large
structures SWNT/PVOH fibers with 60 wt
SWNT ? tensile strength similar to spider
silk - These fibers can be woven into textiles
to create garments with sensing and EMI
shielding capabilities. Thermally conductive
coatings (with nanotubes incorporated into
polymers) - Deicing aircrafts in cold weather
by applying current to the coatings
92
Nanotubes EMI Shielding
More more components are packaged in smaller
spaces where electromagnetic interference
becomes a problem - Ex Digital electronics
coupled with high power transmitters as in
many microwave systems or even cellular phone
systems Growing need for thin coatings which
can help to isolate critical components
from other components of the system and the
external world Carbon nanofibers have been
tested for EMI shielding nanotubes have even
more potential - Act as absorber/scatterer of
radar and microwave radiation - High aspect
ratio is advantageous - Efficiency is boosted
by small diameter. Large d will have material
too deep inside to affect the process. At
sub-100 nm, all of the material participate in
the absorption - Carbon fibers and nanotubes (lt
2 g/cc) have better specific conductivity than
metal fillers, sometimes used as radar absorbing
materials.
93
Advantages of Nanomaterials in Purification,
Waste Remediation
Large surface area ( 1600 m2/g for CNTs)?
increased adsorption capacity Carbon
nanotubes as catalyst support Possible lower
temperature conversion reactions Compact by
weight and volume Improved efficiency, low
power Carbon nanotube membranes as filters
(aerosol filtration, HEPA ULPA filters,
removing contaminants and viruses)
Or SWNTs NO
N2 CO2
94
Hydrogen Storage in CNTs
Impediments to commercialization of fuel cells
safe storage and delivery of hydrogen
fuel Potential solution adsorption of H2 in
a solid support ? storage at relatively low
pressures and high T DOE Target 6.5 wt, 62
kg H2/m3 Carbon nanotubes may be attractive
for H2 storage - porous structure - low
density Storage mechanisms physisorption? T
o date, several groups have confirmed 1 uptake
easily Higher claims (5-8) are not
verifiable or reproducible
95
Lithium Storage in CNTs
Rechargeable lithium batteries work by
intercalation and de-intercalation of lithium
between two electrodes - Transition metal oxide
cathode and graphite anode Production
improvement high energy capacity, fast charging
time, long cycle time How do you get high
energy capacity? - Determined by the saturation
Li concentration of the electrode
material For graphite, this concentration
is LiC6 ? yields a capacity of 372 mA h/g For
nanotubes ? inner cores, inter-tube channels,
interstitial sites (inter-shell van der Waals
spaces) all are available for Li
intercalation To date, a reversible capacity
of 1000 mA h/g has been demonstrated Exact
locations of Li ions unknown and need
investigation
96
Summary
CNTs have very interesting, unique and useful
mechanical and electronic properties. Potenti
al applications include electronics devices,
composites, sensors, batteries. Numerous
challenges are ahead of us - Control of
properties - Large scale production - Robust
functionalization - Application development -
97
Acknowledgement
M. Anantram Alan Cassell Lance Delzeit Jing
Li Jun Li Quoc Ngo Cattien Nguyen Deepak
Srivastava Ramsey Stevens