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An Introduction to Carbon Nanotubes
John Sinclair
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Outline History Geometry Electronic Properties Physical Properties
Rollup Vector Metallicity Electronic Properties Field Effect Transistors Quantum Wires Physical Properties Ropes Separation
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Introduction High Aspect Ratio Carbon nanomaterial
Family inclues Bucky Balls and Graphene Single Wall Carbon Nanotubes (SWCNT) Multiwall Carbon Nanotubes (MWCNT)
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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 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)
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Geometry Rollup Vector Chiral Angle Arm Chair (n,n), θ=30 ○
(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 ○
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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
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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
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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
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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
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Quantum Wires SWCNT Armchair tubes SWCNT deposited over two electrodes
Electrode resistance determined with four point probe and found to be ~ 1 MΩ
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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
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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)
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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
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Summary of Results Typical Values Gdia ~ 478 GPa Ggla ~ 26.2 GPa
Er-dia ~ 1220 GPa Er-gla ~ GPa
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Conclusion On Physical Properties
Shear properties of SWCNT lacking (Even compared to MWCNT ropes) Elastic properties very promising
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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
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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
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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
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Sources M. S. DRESSELHAUS, G. DRESSELHAUS, and R. SAITO. Carbon 33, 7 (1995) R. Martel, T. Schmidt, H. R. Shea, T. Hertel, and Ph. Avourisa. App. Phys. Lett. 73, 17 (1998) Sander J. Tans, Michel H. Devoret et al. Nature 386, (1997) Jean-Paul Salvetat et al. Phys. Rev. Lett. 82, 5 (1999) MICHAEL S. ARNOLD et al. Nature Nanotechnology 1, (2006) academic.pgcc.edu/~ssinex/nanotubes/graphene.gif nano.gtri.gatech.edu/Images/MISC/figure4.gif
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