an introduction to carbon nanotubes
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An Introduction to Carbon Nanotubes
Outline• History• Geometry
– Rollup Vector– Metallicity
• Electronic Properties– Field Effect Transistors– Quantum Wires
• Physical Properties– Ropes
• Separation
What is Nanotechnology?• Switching devices of nanometer (below 100nm, typically
10nm) dimensions define nanotechnology.
DNA strands as Bits
Molecular orientations as Bits
CNFETsSETs
Self assembled CNT using DNA
Quantum Dots
CNT arrays
DNA self assembly
Logic(Our Focus)
Memory
Fabrication
RTDMolecular
Nano CMOS
Molecules in Solution
Emerging Nanotechnology Drivers
Emerging Nanotechnology Solutions
Computing Devices
CMOS Devices
Solid State Devices
Molecular Devices
Nano CMOS
Quantum Dot
RTD
Quantum Devices
CNFET SET
Electro-mechanical
Photoactive
Quantum Electro-chemical
Introduction
• High Aspect Ratio Carbon nanomaterial– Family inclues Bucky Balls
and Graphene• Single Wall Carbon
Nanotubes (SWCNT)• Multiwall Carbon
Nanotubes (MWCNT)
History• 1952 L. V. Radushkevich and V. M. Lukyanovich
– 50 nm MWCNT Published in Soviet Journal of Physical Chemistry– Cold War hurt impact of discovery– Some work done before 1991 but not a “hot” topic
• 1991-1992 The Watershed– Iijima discovers MWCNT in arc burned rods Mintmire, Dunlap, and White‘s
predict amazing electronic and physical properties• 1993 Bethune and Iijima independently discover SWCNT
– Add Transition metal to Arc Discharge method (same method as Bucky Balls)
• Carbon nanotubes are long meshed wires of carbon• Longest tubes up to 1mm long and few nanometers thick made by IBM.
Property Carbon Nanotubes Comparatively
Size 0.6-1.8 nm in diameter Si wires at least 50nm thick
Strength 45 Billion Pascals Steel alloys have 2 Billion P.
Resilience Bent and straightened without damage Metals fracture when bent and restraightened
Conductivity Estimated at 109 A/cm2 Cu wires burn at 106 A/cm2
Cost $2500/gram by BuckyUSA in Houston Gold is $15/gram
Carbon Nanotubes
Geometry
• Rollup Vector– (n,m)– n-m=3d
• Chiral Angle– tan(θ) =
√3m/(2√(n2+m2+nm))• Arm Chair (n,n), θ=30 ○
• Zig-zag (n,0), θ=0 ○
• Chiral, 0○< θ<30 ○
Field Effect Transistors
• FETs work because of applied voltage on gate changes the amount of majority carriers decreasing Source-Drain Current
• SWCNT and MWCNT used– Differences will be discussed
• Gold Electrodes• Holes main carriers
– Positive applied voltage should reduce current
SWCNT Transport Properties
• Current shape consistent with FET
• Bias VSD = 10 mA• G(S) conductance varies by ~5
orders of magnitude• Mobility and Hole
concentration determined to be large– Q=CVG,T (VG,T voltage to
deplete CNT of holes)– C calculated from physical
parameters of CNT– p=Q/eL
MWCNT Transport Properties
• MWCNT performance is poor without defects– See arrow for twists in
collapsed MWCNT• MWCNT has characteristic
shape of FET• Hole density similar to
SWCNT but Mobility determined to be higher – Determined same as
above
FET Conclusions
• Higher carrier density than graphite• Mobility similar to heavily p-doped silicon• Conductance can be modulated by ~5 orders
of magnitude in SWCNT• MWCNT FET only possible after structural
deformation
Quantum Wires
• SWCNT Armchair tubes• SWCNT deposited over
two electrodes– Electrode resistance
determined with four point probe and found to be ~ 1 MΩ
Coulomb Charging
• Contact Resistance Lower than Rquantum=h/e2~26 kΩ
• C very low s.t. EC=e2/2C very large– If EC <<kT, Current only
flows when Vbias>EC
• Various gate V taken into account
• Step-like conductance
Quantum Wire
• Strongly Temperature dependent conduction curve– Occurs when a discrete electron
level tunnels resonantly though Ef of electrode
– If electron levels of SWCNT where continuous peak would be constant
• E levels separated by ΔE• The resonant tunneling implies that
the electrons are being transported phase coherently in a single molecular orbital for at least the distance of the electrodes (140 nm)
Physical Properties of Ropes
• SWCNT rope laid on ultra-filtration membrane
• AFM tip applies force to measure Shear Modulus G and Reduced Elastic Modulus Er– Er = Elastic Modulus when
Searing is negligible • Displacement of tube/Force
was measured and Er and G where calculated
Summary of Results• Typical Values
– Gdia ~ 478 GPa
– Ggla ~ 26.2 GPa
– Er-dia ~ 1220 GPa– Er-gla ~ 65-90 GPa
Conclusion On Physical Properties
• Shear properties of SWCNT lacking (Even compared to MWCNT ropes)
• Elastic properties very promising
Synthesis and Seperation• One major reason CNT devices have been so hard to scale
up to industry uses is due to the inability to efficiently separate different species of CNT– Different types are produced randomly with 1/3 conducting 2/3
semiconducting• It has now been reported that with the use of structure-
discriminating surfactants one can isolate a batch of CNT such that >97% CNT within 0.02 nm diameter
Overview of Technique
• Surfactants change buoyancy properties of CNT
• Ultra-centrifugation techniques (which are scale-able) are used to separate different CNT
• Effective separation is seen– Separation according to metallicity – Separation according to diameter
Conclusion
• CNT devices show promise in molecular electronics both as wires and FET
• Physical properties are very promising being both strong and light
• Separation techniques continue to be developed to allow companies to make CNT devices
CNT-based nanomotor
IC integrated CNT
CNT-based bio-probe
Nanotube oscillator
CNT Devices
23
Molecular Electronics
Nanowire Arrays(Lieber et al., Harvard)
TubeFET (McEuen et al., Berkeley)
Nanotube Logic (Avouris et al., IBM Research)
Nanotube
24
Length Scale 1 mm
1 mm
1 nm
MEMS DevicesSize of a Microprocessor
Nanotube/ Nanowire Diameter
100 nm l (Mean freepath at RT)
1 ÅAtom
lF (Fermi wavelength)
L
W l: boundary scattering
W lF: quantized effectsL l: ballistic transport
- +-W
Thin Film Thickness in ICs
10 nm
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