1. Carbon Nanotubes and Its Devices and Applications
4/27/2017 9:36 PM
Carbon Nanotubes and Its Devices and Applications
By: Joseph Borchardt
Date: 4/15/2016
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2. Outline What are carbon nanotubes? Differences between SWNTs and MWNTs
How are they created?
CNTFET’s
CNT Solar Cell application

3. What are carbon nanotubes?
Carbon Nanotubes (CNTs): Large molecules of pure carbon that are long and thin cylinders, about 1-3nm in diameter, and 100s – 1000s of nanometers long.
Structures:
Single-walled
Multi-walled
Mechanical Properties
Electrical Properties

4. Structure: Single-walled (SWNTs)
Description/Properties:
A single atom layer of graphite wrapped into a seemless cylindrical shape
Diameter ~1nm
Band gap: 0eV - 2eV
Conductivity has metallic or semiconductor behavior
Great candidate for Nano electronics
Popular electrical applications are wire, CNTFETs
Armchair (n, n) structure
Zigzag (n, 0) structure

5. Structure: Multi-walled (MWNTs)
Description/Properties:
Consist of multiple rolled layers (concentric tubes) of graphene (Russian Doll Model), or a single sheet of graphene rolled around itself (Parchment Model)
Distance between layers ~3.4 Angstroms
Usually a zero-gap metal
Unique mechanical Properties
Good for Nano mechanical devices
Triple-walled armchair CNT

6. Electrical Properties
Conductance: metallic, semiconducting with very small band gap, or moderate semiconductor based on structure type.
Electrical Transport: Metallic nanotubes can carry an electric current density of 4 × 109 A/cm2
Doping Characteristics:
Fermi levels
Raman spectroscopy
Defects

7. Mechanical Properties
Tensile Strength:
MWNTs ~63 GPa
Individual CNT shells ~100GPa
Strongest/Stiffest material discovered
Specific Strength:
Density ~1.3 – 1.4 g/cm^3
Best of known materials at 48,000 kN*m*kg^-1
Hardness:
25 GPa without plastic deformation with max (and deformation) around 55 GPa
Has a bulk modulus at 462 – 546 GPa (diamond = 420 GPa)

8. How are they made? Chemical Vapor Deposition: Arc Discharge:
Technique that is achieved by taking a carbon source in a gas phase and using an energy source, such as plasma or a resistively heated coil, to transfer energy to a gaseous carbon molecule. Hydrocarbons then flow through a quartz tube at high temp to break the HC bonds to produce pure Carbon.
Arc Discharge:
First used by Sumio Lijima in 1991.
Uses two graphite rods, one anode and one cathode, placed in an inert gas encased by a low pressure container. The rods act like electrodes kept at different potentials and about 1mm apart (when arc appears). Most nanotubes deposit on cathode.

9. Chemical Vapor Deposition
Properties:
Most widely used
Creates SWNTs and MWNTs
Process lasts ~ 30 minutes
Requires low power input
Low temperature range compared to other methods (~700 Degrees Celsius)
Relatively high purity (91.17%)
Scalable process for potential commercial usage

10. Arc Discharge Properties: Creates SWNTs and MWNTs
Process last ~ 1 minute
Few structural defects due to creation in high temperatures
CNT lengths up to 50 micrometers
30% yield by weight
Not used frequently because it requires temperatures around 1700 Degrees Celsius

11. CNTFET’s Carbon Nanotube Field Effect Transistor:
Is a field-effect transistor that utilizes a single carbon nanotube or an array of carbon nanotubes as the channel material instead of the bulk Si that is used in MOSFETs.
Wrap-around gate CNTFET
Suspended CNT in CNTFET

12. CNTFET’s Properties of CNTFETs:
Theoretically conduct heat equal to that of diamond or sapphire.
Conduction properties and size dimensions will allow for less power consumption than Si as improvements of contacts and impurities are made.
Advantages:
Channel formation control
Lower threshold voltages
High electron mobility, current density, and transconductance
Model of how Top-gate CNTFETs are created (this technique evolved into Wrap-around and Suspended CNTFETs)

13. CNT Solar Cell Application
CNTs and Buckyballs (fullerenes) form snake-like structures to capture electrons (in buckyballs) and pass them through CNTs acting as wires. Electro-hole pairs in SWNT surfaces create a sizeable increase in efficiency up to ~8.5%.
Although this is less than silicon’s 15 – 20% efficiency, it is also much cheaper.
Many improvements are being made by using various sizes and shapes of CNTs to collect larger ranges of photons and increase efficiency.
In September 2015, a working solar energy collector used CNTs to convert optical light to direct current.

14. CNT Solar Cell Application
Pro’s and Con’s:
Less efficient than silicon solar cells
Cheaper than Si
Continuously funded R&D leading to increase in efficiency
Uses chirality to change wavelength absorption
100s currently possible
Hard to accurately control
Working towards MWNTs which can be layered and altered to absorb the entire solar light spectrum.

15. CNT Solar Cell Application

16. Summary Carbon nanotubes can be SWNTs of MWNTs.
Synthesis techniques are the current main focus in R&D as they determine electrical and mechanical properties that the CNTs will have once produced.
CNTFETs appear to be a future replacement for MOSFETs due to the properties of decreased size and efficient conduction over Si.
Have theoretical potential to outperform Si in Solar Cells.
Still a fairly new material, much more testing is needed to solidify measurements and techniques.

17. References

18. Key Concepts and Discussions
CNT basic properties
Differences between SWNTs and MWNTs
How are CNTs made?
What is are CNTFETs
CNT Solar Cell application