An Introduction to Carbon Nanotubes John Sinclair

Outline History Geometry Electronic Properties Physical Properties Rollup Vector Metallicity Electronic Properties Field Effect Transistors Quantum Wires Physical Properties Ropes Separation

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)

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 ○

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

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, 474-477 (1997) Jean-Paul Salvetat et al. Phys. Rev. Lett. 82, 5 (1999) MICHAEL S. ARNOLD et al. Nature Nanotechnology 1, 60-65 (2006) www.noritake-elec.com/.../nano/structu.gif http://en.wikipedia.org/wiki/Carbon_nanotube academic.pgcc.edu/~ssinex/nanotubes/graphene.gif nano.gtri.gatech.edu/Images/MISC/figure4.gif