An Introduction to Carbon Nanotubes

John Sinclair

Outline

History Geometry

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 (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