Nano-Electronics and Nano-technology

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Nano-Electronics and Nano-technology

• A course presented by S. Mohajerzadeh,Department
  of Electrical and Computer Eng,University of
  Tehran

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Carbon structures
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Fullerene

• 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.

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??????1nm in diameter Discovery 1985
C70
C60
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Fullerene
Total 10,000 publications! 2,000 PhD students?!
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Multi-wall and single-wall tubes

• Transmission electron micrograph of single-wall
  CNT, (bundles of CNTs)
• Schematic diagram of single-wall tube

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Multi-wall tubes
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Physical characteristics

• Single wall nanotubes 1 5 nm diameter
• Types of nanotube formation Armchair, Zigzag,
  Chiral
• Multi-wall tubes 2-50 nm concentric tubes, ID
  1.5 15 nm, OD 2.5 30 nm
• 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

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Chirality vector

• 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.

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Chirality vectors
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Electrical properties

• Semiconductor, metallic behavior
• If n-m3q 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

• Nanotubes are very strong materials.
• If a wire of area A is stressed by a weight W,
  the level of stress is SW/A,
• Strain is defined as e?L/L and SE e
• e is called Youngs 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!

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Characterization methods

• 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

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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

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Nano-wires
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Single electron behavior

• FET structure at below 1degree Kelvin!
• Electron-by-electron transport through the
  nanotube, step-wise response

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Nano-transistors
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Photonic crystals

• 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

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Field emission devices

• Each sharp tip of nanotube acts as a
  field-emitter device.
• The emitted electrons hit the top
  electro-luminescent material (like ZnS).
• Pixels are clusters of nanotubes
• Standard micro-meter photo-lithography,
• Large area applications
• Stable structures are needed for a reliable
  application

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Hydrogen storage

• Computer simulations of Adsorption of hydrogen (
  ) in tri-gonal arrays of single-walled carbon
  nanotubes ( )

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Fabrication (growth) Techniques

• Direct current arc-discharge between carbon
  electrodes in an inert-gas environment
• Laser Ablation or Pulsed Laser Vaporization (PLV)
• Plasma Enhanced CVD
• Catalytic Chemical Vapor Deposition (CVD)
• CCVD
• High-pressure CO conversion (HiPCO)

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Carbon Arc-discharge method

• Carbon Atoms are evaporated by a plasma of Helium
  gas that is ignited by high currents passed
  through opposing carbon anode and cathode

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Carbon Arc Discharge
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CNT by Carbon Arc Discharge

• Basic Process
• A vacuum chamber is pumped down and back filled
  with some buffer gas, typically neon or Ar to 500
  torr
• 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.
• A voltage is placed across the electrodes,
• The anode is evaporated and carbon condenses on
  the cathode as CNT

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Pulsed Laser Vaporization /Ablation

• 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 tube-furnace
• Flow tube is heated to 1200C at 500 Torr
• 10-200 mg/hr depending on the laser power
  density

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Plasma CVD
Gas inlet

• Low temperature
• Low Pressure
• DC, RF13.56MHz
• Microwave2.47GHz
• Reacting gas
• CH4 C2H4 C2H6 C2H2 CO
• Catalytic metal (Fe, Ni, Co)

Substrate
Power suplly
Gas outlet
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High-pressure CO conversion (HiPCO)

• New method of growing SWNT
• Primary carbon source is carbon monoxide
• Catalytic particles are generated by in-situ
  thermal
• decomposition of iron penta-carbonyl in a
  reactor heated to 800 - 1200C
• Process is done at a high pressure to speed up
  the growth (10 atm)
• Promising method for mass production of SWNTs

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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
• Catalysts
• support
• active component
• Source of carbon
• Operational condition

simplicity of apparatus Absolute advantage in
Mass Production
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CVD technique
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Catalyst

• Support
• Silicon substrates
• Quartz substrates
• Silica
• Zeolites
• MgO
• Alomina
• Active components
• Transition metals i.e.
• Co , Fe, Ni / Mo (or oxides of them)

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Nanometric islands
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Catalysts effect
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Sources of carbon

• Carbon monoxide
• Hydrocarbons
• Methane
• Ethylene
• Acetylene
• propylene
• Acetone
• n-pentane
• Methanol
• Ethanol
• Benzene
• Toluene ,

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Operational condition

• 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

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Carbon products

• 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
• selectivity of SWNT MWNT

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Carbon Nanotubes, Production by Catalytic
Chemical Vapor Deposition (CCVD)

• SWNT-reinforced composites needs tons of CNT per
  year
• Laser vaporization and arc
  discharge gs/day SWNT
• Carbon source CO HCs CH4 , C2H2-6 , C6H6
• Conditions 700-1000 oC, 1-5 atm
• Catalyst formulation Co/Fe/Ni-Mo on SiO2 ,
  zeolite,
• Quantification of SWNT SEM , TEM, AFM, Raman,
  TPO
• Purification steps
• Caustic to remove silica
• Acid to remove metals

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Carbon Nanotubes
CO deposition on Co-Mo/Silica
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Carbon Nanotubes Characterization-Quantification
AFM
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Carbon Nanotubes Raman characterization
Graphite
SWNT
Disordered C
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CCVD CNT Cat. Reaction Eng. Lab.
1mm
20 Kx
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Storage of Gases

• Hydrogen storage
• Average storage capacity at least 8 wt.
• 100 km 1.2 kg H2 13,500 L(gaseous)
• For 500 km 6 kg H2
  100 kg CNT
• ?CNT ? 1.2 kg/lit
  84 lit. CNT

( 3.1 kg !?) (DOE)