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Chapter 5 (Overview of NanoMaterials (Definition (Materials with at least…
Chapter 5
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Carbon Nanotubes
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Synthesis of CNTs
Physical:
- Arc Discharge
- Laser Ablation (Cu or Ni as catalyst)
Comparison Table Click Here
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Chemical:
- Chemical Vapor Deposition (Cu or Ni as catalyst)
++ CVD is amenable to be scaled up
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++ Growth Process is more controlled than Physical --> Create an array of aligned CNTs
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Application of CNTs
Near-field Microscope Probe:
- High mechanical strength of CNT
- Small diameter of SWNT
Field Emission:
- Basically used as electron sources
- Higher Conductivity of CNTs than Si or W
- Lower Energy Consumption due to lower heating temperature
- Chemically Inert to poisoning
- Physically Inert to sputtering
Chemical / Gas Sensors:
- CNTs highly sensitive to surrounding atmosphere chemical changes
- Concentration of Gas vs CNT electrical property is linear
- Adsorption of gas is reversible
- Simple to use since it can be placed in small systems and can operate at room temperature
Gas Adsorption and Hydrogen Storage:
- Nano-sized channels facilitate adsorption of liquids and gases
Traditional Alternatives:
- Liquid H2 (expensive due to temp required)
- Compressed H2 (Bulky)
Nanowires and Nanorods
Synthesis Process
Spontaneous growth:
- Driven by reduction of Gibbs free energy):
- Anisotropic Growth is required, where crystal growing in an orientation is faster than other directions
Evaporation (or dissolution) - condensation growth:
- Process found here
- Growth orientation can affect properties of nanowire / nanorod
- Presence of imperfection in specific crystal directions such as screw dislocation
- Preferential accumulaion of or posoning by impurites on specific facets
Vapor (or solution)-liquid-solid (VLS or SLS) growth:
- No Screw Dislocations
- Impurities always required
- Catalyst is used to i) acts as a a trap of growth species and ii) Direct and confine the crystal growth onto a specific orientation and within a confined area
- Trivia Found here (read wetting especially)
Electrospinning:
- 1) Electrical Forces at the surface of a polymer solution overcomes the surface tension
- 2) An electrifically charged jet is ejected
- 3) When the jet solidifies, an electrically charged fiber remains
- 4) The charged fiber is directed or accelerated by electrical forces and collected (in sheets or other forms)
- Process parameters are i) Solution Concentration, ii) Applied Electric Field Strength, Feeding Rate of the Precursor Solution
Template Based Synthesis
Process:
- Based on the template production process, the channel size, morphology, size distribution and density of the pores are defined.
- 1) template material must match processing conditions. Eg. electrical insulator required for a template used in electrochemical deposition
- 2) Depositing materials or solution must wet the internal pore walls
- 3) For synthesis of nanorods . nanowire deposition starts form one end of template channel and move the other
- 4) For nanowries, deposition start from the pore wall and proceed inwards
Type of Template Material:
- Anodized Alumina Membrane, Made by anodic oxidisation of aluminum sheets in sulfuric, oxalic or phosphorous acid
- Radiation track-etched polymer, Made by bombarding a non-porous poly-carbonate sheet with nuclear fission to create tracks. Tracks are then chemically etched into pores
Lithography:
1) Wafer is prepared
2) Wafer is evenly coated with photoresist (via spinning)
3) Wafer with photorsist is prebaked to adhere photoresist
4) Patterned phase shift mask (transparent elastomer) covers wafer, and UV light shines through the mask and onto the wafer
5) Depending on positive or negative tone photoresist, parts of the photoresist is dissolved and the wafer is developed
6) Patters are transferred onto substrate via reactive ion etching or wet etching
7) Photoresist is stripped