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