Carbon NanoTube(CNT)
Process & Application
OUTLINE
•Introduction
•History
•Structure
•Synthesis
•Properties
•Application
•Challenges
•References
What is CNT?
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.
Can have caps at the ends
making them look like pills
What CNT Really look like!
•Clumps, ropes, Bundles, mats
•Very high tendency to stick
together
History
In 1970s, Morinobu Endo prepared the first carbon
filament of nanometer dimensions
 Richard E. Smalley (Nobel Prize winning in 1996)
discovered the buckyball (C60) and other
fullerenes (1985)
 In 1991, Sumio Iijima had been using TEM
To analyze new type of finite carbon structure, that is
composed of needle-like tubes
Structure
 The structure of CNT can be
represented based on 2D
graphene lattice.
 Properties depending only
on tube diameter and chiral
angle.
 For a random chirality
distribution,1/3 of the
nanotubes are metallic
Cont
What is so unique about CNT is due to the
different electrical and thermal conductivities
they exhibit when their hexagonal structures
are orientated differently. For instance,
armchair, chiral and zig-zag structure allow
the CNT to act like metal, semiconductor and
insulator, respectively.
The difference in orientation can be
determined by a method of measurement
termed chiral vector .
Chiral Vector: Ch = n â1 + m â2,
  Chiral Angle
Cont..
Armchair: n=m
 = 30
Chiral: n!=m
0 < < 30
Zig zag: m=0
 = 0
Where n and m are the number
of unit vector along two
directions in the honeycomb
crystal lattice of graphene.
Cont…
Carbon nanotubes can be metallic or semiconductor
depending on their chirality
Chiral Vector C is defined as the vector from one
open end of the tube to the other after it is rolled
If (n-m) is divisible by 3, the tube is metallic
If (n-m) is not divisible by 3, the tube is
semiconducting.
Why do Carbon Nanotubes form?
Carbon
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.
Eliminates dangling bonds
Nanotube formation
+
Total Energy
Increases Strain Energy
decreases
Types of CNTs
Single Wall CNT (SWCNT)
Consist of just one layer of carbon
average diameter 1.2 nm
Multiple Wall CNT (MWCNT)
Consist of 2 or more layers of carbon
average diameter 20 nm
(Can be metallic or
semiconducting depending on
their geometry)
Synthesis of CNT’s
Evaporation of solid carbon in arc discharge
Laser ablation
Catalytic chemical vapor deposition of carbon
containing gases
Arc Discharge
 Traditional method
 carried out in a medium of a Nobel Gas such as Helium or
Argon.
 requires complex vacuum equipments.
 Such methods are, not only expensive but rather, time
consuming too.
 only MWNT and ropes
Cont..
Two carbon electrodes are kept
with a gap in between. When high
current, about 80 A is passed
through the electrodes where gap
is filled with helium under 300
torr. Cylindrical deposit then
grows at about 2 to 3 mm per
minute. This cathode deposit
contains two portions: the inside
is a black fragile core and the
outside a hard shell.
Laser Abaltion
 A well mixed acetylene-air
mixture is burned inside a tube
furnace
 A laser is used to vaporize a metal
target (either Fe or Ni)
 The post-flame exhaust gas is
mixed with the metallic vapor and
allowed to cool
 During cooling, carbon nanotubes
are formed
 Spongy black deposit
 Diameters, chiralities, metallic or
semiconductor all uncontrolled
Chemical vapor Depostion(CVD)
 Single SWNT for the 1st
time.




Aligned nanotubes
Large scale possible
Relatively cheap
Diameters, chiralities,
metallic or semiconductor
all uncontrolled
Cont…
In CVD, a substrate is prepared with a layer of metal catalyst
particles, most commonly nickel, cobalt, iron, or a combination.
The metal nanoparticles can also be produced by other ways,
including reduction of oxides or oxides solid solutions. The
diameters of the nanotubes that are to be grown are related to the
size of the metal particles. The substrate is heated to approximately
700°C. To initiate the growth of nanotubes, two gases are bled into
the reactor: a process gas (such as ammonia, nitrogen, hydrogen,
etc.) and a carbon-containing gas (such as acetylene, ethylene,
ethanol, methane, etc.). Nanotubes grow at the sites of the metal
catalyst; the carbon-containing gas is broken apart at the surface of
the catalyst particle, and the carbon is transported to the edges of the
particle, where it forms the nanotubes
Properties
Electrical conductivity six orders of magnitude higher than copper
Can be metallic or semiconducting depending on chirality
‘tunable’ bandgap
electronic properties can be tailored through application
of external magnetic field, application of mechanical deformation…
Very high current carrying capacity
Excellent field emitter; high aspect ratio
and small tip radius of curvature are
ideal for field emission
Cont….
 The strongest and most flexible
molecular material because of C-C
covalent bonding and seamless
hexagonal network architecture
 Young’s modulus of over 1 TPa vs 70
GPa for Aluminum
 strength to weight ratio 500 time > for
Al; similar improvements over steel
and 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
Application of CNT
Electronics
•
quantum wire interconnects
• Diodes and transistors for
computing
• Capacitors
• Data Storage
• Field emitters for instrumentation
• Flat panel displays
Carbon Nanotube FET
• First CNT-FET fabricated in 1998
CNT can be used as the conducting
channel of a MOSFET.
• These new devices are very
similar to the CMOS FETs.
• All CNFETs are pFETs by nature.
• nFETs can be made through
• Annealing
• Doping
• Very low current and power
consumption
• Although tubes are 3nm thick
CNFETs are still the size of the
contacts, about 20nm.
Interconnect
Today chips have become “all
wires”
Electro-migration failure
CNT interconnect
• High current density
• Low resistance (6.2k ohm)
• Ballistic behavior
Cont….
SENSORS,NEMS,BIO
•
CNT based microscopy: AFM,
STM…CNT tip is robust ,offers
amazing resolution.
•
Nanotube sensors: force, pressure,
chemical…
Scanning Probe
•
Biosensors
•
Molecular gears, motors, actuators
•
Batteries, Fuel Cells: H2, Li storage
•
Biomedical
-
Lab on a chip
Drug delivery
DNA sequencing
Simulated Mars dust
Challenges
 Selective placement of CNTs
 Control of diameter, chirality
 Doping, contacts
 Novel architectures (not CMOS based!)
 Large scale production
 Development of inexpensive manufacturing processes
Carbon NanoTube(CNT)
Transistor
Stanford Model
Two set of files:
• CNFET.lib - CNFET Models.
• PARAMETERS.lib - Global parameters for the model.
To instantiate the devices in the model, the library must be
included at the beginning of the SPICE file.
.lib ‘CNFET.lib’ CNFET
CNT Model
The model is implemented hierarchically in three levels:
• CNFET_L1: Level 1, models the intrinsic behavior of
MOSFET- like CNFET.
• CNFET_L2: Level 2, models the device non-idealities
(capacitance and resistance of doped S/D CNT region).
• CNFET_L3: Level 3, models the interface between the
CNFET device and CNFET circuits.
CNT
Intrinsic
CNT
channel
Metal
Gate
Substrate
CNTFET_L1
Substrate
CNFET_L2
CNT’s
G/S/D
Metal
Gate
Substrate
G/S/D
CNFET_L3
References
• You can see animations of virtual nanotubes by following these links:
http://www.photon.t.u-tokyo.ac.jp/~maruyama/nanotube.html
http://www.pa.msu.edu/cmp/csc/simindex.html
• You can create your own virtual SWNT at:
http://jcrystal.com/steffenweber/JAVA/jnano/jnano.html
“Chemical Vapor Deposition.”
http://www.sandia.gov/1100/CVDwww/cvdinfo.htm
“Physical Properties of Carbon Nanotubes.”
http://www.pa.msu.edu/cmp/csc/ntproperties/