Carbon Nanotubes and Its Devices and Applications By: Joseph Borchardt Date: 4/15/2016 Outline What are carbon nanotubes? Differences between SWNTs and MWNTs How are they created? CNTFETs CNT Solar Cell application 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
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 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 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 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) How are they made? Chemical Vapor Deposition: 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. 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 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 CNTFETs 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. Suspended CNT in CNTFET Wrap-around gate CNTFET CNTFETs 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)
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. Electrohole pairs in SWNT surfaces create a sizeable increase in efficiency up to ~8.5%. Although this is less than silicons 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. CNT Solar Cell Application Pros and Cons: 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. CNT Solar Cell Application 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. References https://en.wikipedia.org/wiki/Carbon_nanotube#Electrical_properties https:// en.wikipedia.org/wiki/Carbon_nanotube_field-effect_transistor#Key_advantages
https://gigaom.com/2014/04/01/carbon-nanotubes-move-closer-to-competing-withsilicon / http://maeresearch.ucsd.edu/bandaru/Publications_files/JNN_CNT%20review.pdf http://www.nanowerk.com/spotlight/spotid=30661.php https://sites.google.com/site/nanomodern/Home/CNT/syncnt/cvd http://www.sciencedirect.com/science/article/pii/S1872580508600414 http://phys.org/news/2014-09-breakthrough-carbon-nanotube-solar-cells.html http:// www.rsc.org/chemistryworld/2015/09/carbon-nanotube-rectenna-converts-light-elec Key Concepts and Discussions CNT basic properties Differences between SWNTs and MWNTs How are CNTs made? What is are CNTFETs
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