Transcript Nano-Electronics and Nano- technology A course presented by S. Mohajerzadeh,

Nano-Electronics and Nanotechnology
A course presented by
S. Mohajerzadeh,
Department of Electrical and Computer Eng,
University of Tehran
Carbon structures
Fullerene
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C60, a type of carbon arrangement with 60 carbon atoms placed in 1nm lattice
separation.
Discovery: 1985 by Bukminister Fuller.
12 pentagonal and 20 hexagonal shapes.
Fullerene can be doped (26%) by alkali atoms (sodium) because its empty space
is that much.
‫فولرين‬
1nm in diameter
Discovery: 1985
C60
C70
Fullerene
Total: 10,000 publications!
= 2,000 PhD students?!
Multi-wall and single-wall tubes
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Transmission electron micrograph of single-wall CNT,
(bundles of CNT’s)
Schematic diagram of single-wall tube
Multi-wall tubes
Physical characteristics
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Single wall nanotubes: 1 – 5 nm diameter
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Types of nanotube formation: Armchair, Zigzag, Chiral
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Multi-wall tubes 2-50 nm concentric tubes,
ID : 1.5 – 15 nm, OD : 2.5 – 30
nm
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100 times stronger than steel, r = 1/6 (1.3 – 1.4 g/cm3)
Strong, lightweight materials
kCNT = 2000 (Copper 400) W/m.K
Transmission of heat is better than diamond
Chirality vector
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Although the fabrication of
nanotubes is not by rolling the
graphite sheets, they are
modeled by this phenomenon;
“Ch” or Chirality vector or
circumferential vector is the
translation vector of graphite
plane onto nanotube.
Axis vector is “T” which is
perpendicular to chilarity
vector “Ch” and shows the tube
axis.
Ch= na1 + m a2 where “a1” and
“a2” represent the main
constructing vectors of
graphite sheet.
Chirality vectors
Electrical properties
Semiconductor, metallic
behavior
If n-m=3q then metallic
 Armchair structures,
metallic,
 Chiral and Zigzag
structures,
semiconductor:
 Band gap depends on the
diameter
 Reducing the diameter
leads to higher band
gaps.
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Mechanical properties
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Nanotubes are very strong materials.
If a wire of area A is stressed by a weight “W”, the level of stress is
S=W/A,
Strain is defined as: ε=ΔL/L and S=E ε
ε is called: Young’s module and it is 0.21TPa for nanotubes!!, 10
times more than steel!
1 TPa is equivalent to 10millions atmospheric pressure!!
If we bend the tubes, they act like straws, but come back to their
original status, self-repairing!
When the tube is severely bent, the “sp2” structure converts onto
“sp” orbitals and once the pressure is removed, sp2 orbitals are
reconstructed.
Tensile strength is the measure of how much force is needed to take
apart a material.
For nanotubes, tensile strength is 45 billion Pascal (GPa) whereas
for steel it is only 2GPa!
Characterization methods
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SEM
TEM
Raman (interaction of incoming light with solid
vibrations)
SPM (AFM , STM ,…)
XRD (X-ray diffraction) similar to electron
diffraction
TPO, TGA (temperature programmed oxidation) and
(thermal gravimetric analysis)
Electrical characterization
Applications
Electronics
Hydrogen storage,
Chemical Sensors
Fuel Cells
Nano-transistors, nano-structures
Application in STM
Composite materials,
Catalysts
4.2, 8, 300 (!)wt% of hydrogen in CNT at 25oC
Nano-wires
Single electron behavior
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FET structure at below 1degree Kelvin!
Electron-by-electron transport through the
nanotube, step-wise response
Nano-transistors
Photonic crystals
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Similar to atomic periodicity, a structure with matter periodicity is
created to form a band-gap for optical wavelengths.
Only at certain wavelengths, standing waves can be created and at
some other wavelengths, transmission is prohibited
Field emission devices
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Each sharp tip of nanotube
acts as a field-emitter
device.
The emitted electrons hit
the top electroluminescent material (like
ZnS).
Pixels are clusters of
nanotubes
Standard micro-meter
photo-lithography,
Large area applications
Stable structures are
needed for a reliable
application
Hydrogen storage
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Computer simulations of Adsorption of hydrogen ( ) in
tri-gonal arrays of single-walled carbon nanotubes ( )
Fabrication (growth) Techniques
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Direct current arc-discharge
between carbon electrodes in an
inert-gas environment
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Laser Ablation or Pulsed Laser
Vaporization (PLV)
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Plasma Enhanced CVD
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Catalytic Chemical Vapor
Deposition (CVD)
CCVD
High-pressure CO conversion
(HiPCO)
Carbon Arc-discharge method
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Carbon Atoms are evaporated by a plasma of Helium gas
that is ignited by high currents passed through opposing
carbon anode and cathode
Carbon Arc Discharge
CNT by Carbon Arc Discharge
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Basic Process
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A vacuum chamber is pumped down and back filled with
some buffer gas, typically neon or Ar to 500 torr
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A graphite cathode and anode are placed in close proximity
to each other. The anode may be filled with metal catalyst
particles if growth of single wall nanotubes is required.
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A voltage is placed across the electrodes,
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The anode is evaporated and carbon condenses on the
cathode as CNT
Pulsed Laser Vaporization /Ablation
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Used for the production of
SWNTs
Uses laser pulses to ablate (or
evaporate) a carbon target
Target contains 0.5 atomic
percent nickel and/or cobalt
The target is placed in a tubefurnace
Flow tube is heated to ~1200°C
at 500 Torr
10-200 mg/hr depending on the
laser power density
Plasma CVD
Gas inlet
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Low temperature
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Low Pressure
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DC, RF:13.56MHz
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Microwave:2.47GHz
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Reacting gas
Substrate
Power
suplly
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CH4 ; C2H4 ; C2H6 ; C2H2 ;
CO
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Gas outlet
Catalytic metal (Fe, Ni, Co)
High-pressure CO conversion (HiPCO)
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New method of growing SWNT
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Primary carbon source is carbon monoxide
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Catalytic particles are generated by in-situ thermal
decomposition of iron penta-carbonyl in a reactor heated to 800 1200°C
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Process is done at a high pressure to speed up the growth (~10 atm)
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Promising method for mass production of SWNTs
Chemical Vapor Deposition
Involves heating a catalyst material to high temperatures in
a tube furnace and flowing a hydrocarbon gas through the
tube reactor.
 The materials are grown over the catalyst and are collected
when the system is cooled to room temperature.
 Key parameters are:
simplicity of apparatus
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Catalysts
support
Absolute advantage in
active component
Mass Production
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Source of carbon
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Operational condition
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CVD technique
Catalyst:
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Support:
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Silicon substrates
Quartz substrates
Silica
Zeolites
MgO
Alomina
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Active components :
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 Transition metals i.e.:
Co , Fe, Ni / Mo (or oxides of them)
Nanometric islands
Catalysts effect
Sources of carbon:
 Carbon monoxide
 Hydrocarbons:
 Methane
 Ethylene
 Acetylene
 propylene
 Acetone
 n-pentane
 Methanol
 Ethanol
 Benzene
 Toluene , …
Operational condition:
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Temperature: 600-1100 oC
Pressure: 1-10 atm
Reaction time: 0.5-3 h
Dilutent gas: He, Ar, H2
Resident time of gases:
Volume fraction ( partial pressure)
Flow rate
Carbon products
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Vertical growth, random growth,
Wall thickness in the case of multi-wall growth
Single-wall (shell) nanotube (SWNT)
Multi-wall (shell) nanotube (MWNT)
Graphitic form of carbon
Amorphous form of carbon
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selectivity of SWNT & MWNT
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Carbon Nanotubes, Production by
Catalytic Chemical Vapor Deposition (CCVD)
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SWNT-reinforced composites needs tons of CNT per year
Laser vaporization and arc discharge: g’s/day SWNT
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Carbon source: CO & HC’s: CH4 , C2H2-6 , C6H6
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Conditions: 700-1000 oC, 1-5 atm
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Catalyst formulation: Co/Fe/Ni-Mo on SiO2 , zeolite, …
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Quantification of SWNT: SEM , TEM, AFM, Raman, TPO
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Purification steps:
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Caustic to remove silica
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Acid to remove metals
Carbon Nanotubes
CO deposition on Co-Mo/Silica
Carbon Nanotubes Characterization-Quantification
AFM
Carbon Nanotubes Raman characterization
Graphite
SWNT
Disordered
C
CCVD CNT Cat. & Reaction Eng. Lab.
1mm
20 Kx
Storage of Gases
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Hydrogen storage
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Average storage capacity: at least %8 wt.
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100 km = 1.2 kg H2= 13,500 L(gaseous)
For 500 km : 6 kg H2
( 3.1 kg !?) (DOE)
rCNT  1.2 kg/lit
100 kg CNT
84 lit. CNT