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Carbon Nanotubes: theory and applications

Yijing Fu1, Qing Yu2

1 Institute of Optics, University of Rochester

2 Department of ECE, University of Rochester

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Outline

Definition Theory and properties Ultrafast optical spectroscopy Applications Future

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Definition: Carbon Nanotube and Carbon fiber The history of carbon fiber goes way back…

The history of carbon nanotube starts from 1991

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Carbon nanotube

CNT: Rolling-up a graphene sheet to form a tube

Schematic of a CNT

STM image of CNT

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Carbon nanotube

Properties depending on how it is rolled up.

a1, a2 are the graphene vectors. OB/AB’ overlaps after rolling up. OA is the rolling up vector.

21 manaOA

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Carbon nanotube properties: ElectronicElectronic band structure is determined by symmetry: n=m: Metal n-m=3j (j non-zero integer): Tiny band-gap semiconductor Else: Large band-gap semiconductor.

Band-gap is determined by the diameter of the tube: For tiny band-gap tube: For large band-gap tube:

2/1 REg

REg /1

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Carbon nanotube : band structure

Band structure of 2D graphite

(7,7) (7,0)

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Carbon nanotube: Density of state 1D confined system DOS should give spikes

• Experimental results do show some spikes• Also there are some deviations, further study is needed to explain this.

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Carbon nanotube properties: Mechanical Carbon-carbon bonds are one of the strongest bond

in nature Carbon nanotube is composed of perfect

arrangement of these bonds Extremely high Young’s modulus

Material Young’s modulus (GPa)

Steel 190-210

SWNT 1,000+

Diamond 1,050-1,200

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Ultrafast Optical spectroscopy of CNT Pump-probe experiment is used Provides understanding of CNT linear and

nonlinear optical properties Time-domain measurement provides lifetime

measurement 1-D confined exciton can be studied

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Auger recombination of excitons Theoretical results show strong bound excitons in

semiconducting CNTs with binding energy up to 1eV Auger recombination : Nonradiative recombination of

excitons

Auger rates is enhanced in reduced dimension materials compared to bulk materials

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Experimental results

Quantized auger recombination in quantum-confined system is shown here

Τ2 , Τ3 ~ 4ps, very fast loss of exciton due to auger recombination. Therefore, optical performance of CNT is severely limited.

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Confined exciton effect: blue shift Exciton energy levels are stable when bohr

radius is smaller than the exciton-exciton distance

At intense laser excitation, many-body effects renormalize the exciton energy levels

Due to fast auger recombination, exciton energy level shift is only observed in very short time scale

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Confine exciton effect: experiment At zero time-delay, the absorption spectrum for

pumping wavelength of 1250nm and 1323nm are shown as

At low pumping level, this effect disappears. Thus many-body effect is proposed to explain this exciton blue-shift.

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Applications

Electrical1. Field emission in vacuum electronics2. Building block for next generation of VLSI3. Nano lithography Energy storage1. Lithium batteries2. Hydrogen storage Biological1. Bio-sensors2. Functional AFM tips3. DNA sequencing

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Biological applications: Bio-sensing Many spherical nano-particles have been

fabricated for biological applications. Nanotubes offer some advantages relative

to nanoparticles by the following aspects:1. Larger inner volumes – can be filled with chemical or

biological species.

2. Open mouths of nanotubes make the inner surface accessible.

3. Distinct inner and outer surface can be modified separately.

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Biological applications: AFM tipsCarbon nanotubes as AFM probe tips:1. Small diameter – maximum resolution

2. Excellent chemical and mechanical robustness

3. High aspect ratio

Resolution of ~ 12nm is achieved

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Biological applications:Functional AFM tipsMolecular-recognition AFM probe tips: Certain bimolecular is attached to the CNT tip This tip is used to study the chemical forces between

molecules – Chemical force microscopy

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Biological applications: DNA sequencing Nanotube fits into the

major grove of the DNA strand

Apply bias voltage across CNT, different DNA base-pairs give rise to different current signals

With multiple CNT, it is possible to do parallel fast DNA sequencing

Top view and side view of the assembled CNT-DNA system

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Challenges and future

Future applications:1. Already in product: CNT tipped AFM

2. Big hit: CNT field effect transistors based nano electronics.

3. Futuristic: CNT based OLED, artificial muscles…

Challenges1. Manufacture: Important parameters are hard to control.

2. Large quantity fabrication process still missing.

3. Manipulation of nanotubes.