An Introduction to Carbon Nanotubes

1
An Introduction to Carbon Nanotubes

• John Sinclair

2
Outline

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

3
Introduction

• High Aspect Ratio Carbon nanomaterial
• Family inclues Bucky Balls and Graphene
• Single Wall Carbon Nanotubes (SWCNT)
• Multiwall Carbon Nanotubes (MWCNT)

4
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 Whites 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)

5
Geometry

• Rollup Vector
• (n,m)
• n-m3d
• Chiral Angle
• tan(?) v3m/(2v(n2m2nm))
• Arm Chair (n,n), ?30 ?
• Zig-zag (n,0), ?0 ?
• Chiral, 0?lt ?lt30 ?

6
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

7
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
• QCVG,T (VG,T voltage to deplete CNT of holes)
• C calculated from physical parameters of CNT
• pQ/eL

8
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

9
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

10
Quantum Wires

• SWCNT Armchair tubes
• SWCNT deposited over two electrodes
• Electrode resistance determined with four point
  probe and found to be 1 MO

11
Coulomb Charging

• Contact Resistance Lower than Rquantumh/e226 kO
• C very low s.t. ECe2/2C very large
• If EC ltltkT, Current only flows when VbiasgtEC
• Various gate V taken into account
• Step-like conductance

12
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 ?E
• 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)

13
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

14
Summary of Results

• Typical Values
• Gdia 478 GPa
• Ggla 26.2 GPa
• Er-dia 1220 GPa
• Er-gla 65-90 GPa

15
Conclusion On Physical Properties

• Shear properties of SWCNT lacking (Even compared
  to MWCNT ropes)
• Elastic properties very promising

16
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 gt97 CNT within
  0.02 nm diameter

17
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

18
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

19
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