Institute of Optics, University of

Rochester

1

Carbon Nanotubes:
theory and
applications

Yijing Fu

1

, Qing Yu

2

1 Institute of Optics, University of
Rochester
2 Department of ECE, University of
Rochester

Institute of Optics, University of

Rochester

2

Outline



Definition



Theory and properties



Ultrafast optical spectroscopy



Applications



Future

Institute of Optics, University of

Rochester

3

Definition:
Carbon Nanotube and Carbon
fiber



The history of carbon fiber goes way
back…



The history of carbon nanotube starts
from 1991

Institute of Optics, University of

Rochester

4

Carbon nanotube

CNT: Rolling-up a graphene sheet to form a

tube

Schematic
of a CNT

STM image
of CNT

Institute of Optics, University of

Rochester

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Carbon nanotube

Properties depending on how it is rolled up.

a

1

, a

2

are the graphene

vectors.
OB/AB’ overlaps after rolling
up.
OA is the rolling up vector.

2

1

ma

na

OA





Institute of Optics, University of

Rochester

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Carbon nanotube properties:
Electronic

Electronic 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 R

E

g



R

E

g

/

1



Institute of Optics, University of

Rochester

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Carbon nanotube : band
structure

Band structure
of 2D graphite

(7,7)

(7,0)

Institute of Optics, University of

Rochester

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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.

Institute of Optics, University of

Rochester

9

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

Institute of Optics, University of

Rochester

10

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

Institute of Optics, University of

Rochester

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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

Institute of Optics, University of

Rochester

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Experimental results



Quantized auger recombination in quantum-confined system

is shown here



Τ

2

, Τ

3

~ 4ps, very fast loss of exciton due to auger

recombination. Therefore, optical performance of CNT is

severely limited.

Institute of Optics, University of

Rochester

13

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

Institute of Optics, University of

Rochester

14

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.

Institute of Optics, University of

Rochester

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Applications



Electrical

1.

Field emission in vacuum electronics

2.

Building block for next generation of VLSI

3.

Nano lithography



Energy storage

1.

Lithium batteries

2.

Hydrogen storage



Biological

1.

Bio-sensors

2.

Functional AFM tips

3.

DNA sequencing

Institute of Optics, University of

Rochester

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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.

Institute of Optics, University of

Rochester

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Biological applications: AFM
tips

Carbon 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

Institute of Optics, University of

Rochester

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Biological applications:
Functional AFM tips

Molecular-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

Institute of Optics, University of

Rochester

19

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

Institute of Optics, University of

Rochester

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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…



Challenges

1.

Manufacture: Important parameters are hard to control.

2.

Large quantity fabrication process still missing.

3.

Manipulation of nanotubes.