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Institute of Optics, University of Rochester
1
Carbon Nanotubes: theory and applications
Yijing Fu1, Qing Yu2
1 Institute of Optics, University of Rochester
2 Department of ECE, University of Rochester
Institute of Optics, University of Rochester
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Outline
Definition Theory and properties Ultrafast optical spectroscopy Applications Future
Institute of Optics, University of Rochester
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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
Institute of Optics, University of Rochester
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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
Institute of Optics, University of Rochester
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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.
Institute of Optics, University of Rochester
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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
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
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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.
Institute of Optics, University of Rochester
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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.