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Lesson 11: Carbon Nanotubes - Coggle Diagram
Lesson 11: Carbon Nanotubes
Moore's Law
Industry now uses terminology of “Ultra-Large-Scale Integration” (ULSI), for chips containing more than a million transistors and to emphasize chip complexity.
States that number of transistors on an integrated circuit doubles every 2 years.
Adding transistors with this law to increase function, decrease cost of computing.
Miniaturisation of electronic device generates more heat that might cause malfunction and cause other problems.
Current technology used
Copper Interconnects
Transmit signals within electronic circuit.
Copper is widely used because it is a good conductor.
Additive patterning: material is either added by placing small building blocks in certain locations to build larger structures.
Or creating a mold and filling it to create structures
Silicon Based Field Effect Transistors
Function as a switch or amplifier for electronic signals in electronic circuit devices.
Controls the conductivity of a channel using electric field.
Doped silicon is widely used as it increases the conductivity by introducing impurities.
Limitations due to miniaturisation
1) Increased resistance in interconnects - The smaller the interconnects, the higher the resistivity.
Due to more significant contribution from scattering events at both side wall and grain boundary as length scale decreases.
With increased resistance = amount of heat generated would increase -> cause reliability issues.
2) Electromigration in interconnects - Material in interconnect can get transported away from original site -> cause failure. due to large current density in interconnect.
Electrons colliding with lattice ions dislodges from the original positions.
With an increased temperature, more collisions bound to occur -> higher electromigration risk
Nanomaterials that could be used
Carbon nanotubes, Gold nanoparticle, nanowires.
Carbon Nanotubes
Rolled up versions of graphene sheets where each atom in the sheet is assigned chiral indices by (n,m)
C atom(0,0) joined to C atom (n,m) in rolling action forms C nanotube (n,m)
Types of Carbon Nanotubes & Electrical Properties
The properties is dependent on the way nanotube is being rolled.
Armchair (n,n)
Zigzag (n,0) ; (0,m)
Chiral (n,m)
The diameter of semiconducting nanotube is inversely proportional to band gap.
d= a/ π √ n^2 + nm + m^2
where d: diameter(nm), a: 0.246nm
Electrical Properties
:
Affected by structure of nanotube
If a nanotube (n,m), if n=m or n-m is a multiple of 3 eg. 6, 9, 12: metallic
Thus, all armchair (n=m) nanotubes are metallic
Semiconducting nanotubes : (6,4), (9,1)
Known as one-metallic conductors as the electrons only propagate along tube's axis.
How to propagate further using Moore's Law?
Scattering & Mean Free Path - Electrons scattered on lattice ions and impurities leads to high resistivity in conductors.
Mean free path: average length where electrons can travel freely before hitting.
A large value of mean free path implies that an electrons can travel over a large distance being being scattered away.
Ballistic Conduction
Carbon nanotubes and nanowires can be used as interconnects in miniaturised electronic circuits.
Allowing electrons to propagate without scattering -> negligible heat dissipation in interconnects.
Carbon atoms has covalent bonds that do not promote electromigration.
Single Electron Transistor (SET)
As size of transistor decrease, soon it will be nanosized.
The transistor in future might consist of a single nanoparticle, processing electrons one at a time.
Coulomb Blockade - Electrons need to tunnel from source into "conducting island" and to drain.
But addition of single electrons into the island raises energy level of electrons available -> tunneling impossible.
However, when sufficient voltage is applied at gate electrode, energy levels of energy states lowered.
Thermally Induced Tunneling
U = e^2 / 4 π ε0 d
where U: energy required (Joules , J)
e: charge of single electrons (1.602 x 10^-19 C)
ε0 : electric constant (8.854 x 10^-12 F m^-1)
d: diameter (m)
Thermal energy can supply energy required to charge spherical nanoparticle of diameter with a single electron.
Thermal energy : 4.0 x 10^-21 J per electron.
A sufficient small nanoparticle requires to be fabricated.
Mechanical Properties
Carbon nanotubes are strong and stiff in terms of tensile strength and elastic modulus respectively.
In 2000, a multi-walled made of layers of Graphene carbon nanotube was tested to have 63GPa.
Carbon tubes undergo plastic deformation that is permanent under excessive strain.