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1EPFL, May 25 2005
Carbon Nanotubes (CNT)1. Intro
1. Huge molecule (mm), tiny molecule (nm)2. fabrication
2. Mechanical Properties3. Electronic Properties
1. Field emission2. Ballistic conductor, Interconnect, Power cable3. Coulomb blockade4. Transistor (FET)
2EPFL, May 25 2005
Carbon Nanotubes (CNT)• Carbon nanotubes are cylindrical molecules ~1 nm in
diameter and 1-100 microns in length. – Quasi 1D object– Molecule almost 1mm long
• They are constituted of carbon atoms only, and can essentially be thought of as a layer of graphite rolled-up into a cylinder.
• Most of these tubular fullerene molecules, discovered in 1991, consist of multiple shells, where many tubes are arranged in a coaxial fashion. (MWNT)
• In 1993 single-wall nanotubes (SWNT) were discovered. Because of their simple and well defined structure they serve as the model systems for theoretical calculations and key experiments.
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From: Dresselhaus, Dresselhaus & Eklund. 1996 Science of Fullerenesand Carbon Nanotubes. San Diego, Academic Press.
4EPFL, May 25 2005 http://www.photon.t.u-tokyo.ac.jp/~maruyama/agallery/agallery.html
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STM (scanning tunneling microscope image) showing Atomic resolution topography, and clear twist of graphenesheet.
Delft University
6EPFL, May 25 2005
TEM of MWNT
10 nm
•10 nm core
• 9 shells clearly seen
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How to make NT• Spark between graphite rods. High current
discharge (typically 20V, 50A) inside an enclosure filled with inert gas (helium, argon...) at low pressure (between 50 and 700 mbar). Metallic particles can be added in the initial electrodes to catalyse single wallnanotube. Few defects
• CVD (Chemical Vapor Deposition): Pyrolysis(700 °C) of a hydrocarbon (acetylene, ethylene, benzene...) over a template substrate (metal catalysts). Many defects, mostly MWNT. Cheap, easy to pattern
• Laser Ablation of graphite. Good control over SWNT diameter, expensive
S-B Lee et al, Nanotechnology 14 (2003) 192–195
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Mechanical Properties• Young's Modulus (SWNT)
~ 1 TPa (steel 0.2 TPa, silicon 0.19 TPa)• Young's Modulus (MWNT)
1.28 TPa• Maximum Tensile Strength
~30 GPa (steel 0.5 GPa, silicon 7 GPa)
Only true if defect free… must pre-select NT, but how?Very strong C-C bond (like diamond), tough, energy absorbing (armor, sporting goods…)Chemically resistant: no dangling bonds to attack.
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Molecular mechanics
Atomistic simulations, taking all bonds into account
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Tungsten film (80 Å)
Silicon substrate
SiO2
Carbon nanotube (100 Å)Oxide barrier
I
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Imaging in non-contact mode:vertical load ~ 10pN
Switching to contact:Lateral and verticalforces ~ 10nN
Manipulation
IBM
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AFM fabrication of a nanotubedevice
SiO2
Nanotube
WOx
Tungsten
1) 3)2)
4) 5) 6)1000 nm 500 nm
500 nm 500 nm 500 nmIBM
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NT as AFM tip
CNT tip locations and diameters are defined by e-beam lithography. CNT probe tips are grown at the end of the fabrication process from the defined nanocatalyst spots on cantilever beams. CNT length, orientation, and crystalline quality are controlled by plasma enhanced chemical vapor deposition (PECVD).In PECVD, an electric field is present in the plasma discharge to direct the nanotubes to grow and align parallel to the electric field. Due to the crystalline morphology of our PECVD-grown CNTs, there is no need in our process to conduct post fabrication treatment to remove and/or to shorten the CNT tips.. CNT probe tips with diameters ranging between 40-80 nm and lengths between 2-6 m m have been produced in NASA Ames Lab
•Intrinsic nanometer scale diameter, high aspect ratio, and strong mechanical robustness of CNTs make them ideal for high lateral resolution imaging and deep trench/via critical dimension imaging
•Usually use MWNT or bundle of SWNT because SWNT too soft
•Either assembled ‘by hand’(glue dipping) or grown after patterning catalyst.
Photo: NASA Ames
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CNT Electrical Properties• Electrical Properties
– 1D object (Quantum wire): band structure reflects this, in some cases can have only 2 conduction bands
– Perfect structure, almost no phonons: Ballistic transport
– Nano: Single electronics, particle in a box– High current density, excellent thermal properties– FET, Diode, LEDs…
personal opinion: Very cool physics, little real application (electronics requires growth of one type of NT. Only 1 dimension is nano: how to integrate? might be good for hybrid use as interconnect. Difficult to pattern)
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Field emission• CNTs have excellent materials properties
which make them have attractive field emission characteristics.
• Large aspect ratio(>1000) • Atomically Sharp tips • High temperature and chemical stability • High electrical and thermal conductivity
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MicroTriode-MEMS Vacuum Tube
Anode
Grid
Cathode
30 µm
10 µm
CarbonNanotubes
Power Gain 45, ft = 160 MHzC. Bower et al, Appl. Phys. Lett. 80, 3820 (2002)
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CNT Field emitter
“Motorola Debuts First Ever Nano Emissive Flat Screen Display Prototype
Building Upon Carbon Nanotube Technology, Motorola Prepares to Revolutionize the Flat Panel Display Industry
Motorola Labs today unveiled a working 5-inch color video display prototype based on proprietary Carbon Nanotube (CNT) technology – a breakthrough technique that could create large, flat panel displays with superior quality, longer lifetimes and lower costs than current offerings. Optimized for a large screen High Definition Television (HDTV) that is less than 1-inch thick, this first-of-its kind NED 5-inch prototype harnesses the power of CNTs to fundamentally change the design and fabrication of flat panel displays.”
http://www.physorg.com/printnews.php?newsid=4031
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Metal or Semiconductor?(n, n) nanotubes: metal,(n, n+3i), i = integer; small gap semiconductorOther nanotubes: larger gap semiconductor
•The unique electronic properties of carbon nanotubes are due to the quantum confinement of electrons normal to the nanotube axis. In the radial direction, electrons are confined by the monolayer thickness of the graphenesheet. Around the circumference of the nanotube, periodic boundary conditions come into play. For example, if a zigzag or armchair nanotube has 10 hexagons around its circumference, the 11th hexagonal will coincide with the first. Going around the cylinder once introduces a phase difference of 2π. •Because of this quantum confinement, electrons can only propagate along the nanotube axis, and so their wavevectors point in this direction. The resulting number of one-dimensional conduction and valence bands effectively depends on the standing waves that are set up around the circumference of the nanotube.
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CNT
As NT diameter increases gap decrease as 1/R (from approx 0.6 eV for 1.4 nm diameter tube)
Number of allowed states also increases with diameter
Good (hype): can ‘tune’ gap
Bad: can’t pick the tube without measuring it.
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Ballistic conductor•In theory metallic NT are ballistic conductors: no scattering, or only elastic scattering•In reality it means much longer coherence length than for metals (10s of microns vs. 10s of nm) •Does this mean Resitivity =0?•No, not a superconductor. Conductivity given by Landauer’s equation:
22 Nii
eG Th
= ∑
If no scattering Ti=1. So For metallic tube, N=2 (2 spins), and so R=6.5 kΩ
High current density (102 to 103 better than Al)
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NT as electrical power line!NASA Awards $11 Million Quantum Wire (Carbon Nanotube)
Contract To Rice University
NASA has awarded Rice University's Carbon Nanotechnology Laboratory a four-year, $11 million contract to produce a prototype power cable made entirely of carbon nanotubes.
The project aims to pioneer methods of producing pure nanotubepower cables, known as quantum wires, which may conduct electricity up to 10 times better than copper and weigh about one-sixth as much. Such technologies may advance NASA's plans to return humans to the moon and eventually travel to Mars and beyond.
Thursday, April 21, 2005
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Single electronics: Coulomb blockadeAt low temperatures step-like current-voltage characteristics are obtained that indicate single-electron transport with Coulomb blockade and resonant tunneling through single molecular orbitals. In Coulomb blockade, transport is blocked at low bias since the capacitance of the nanotube is so small that adding a single electron requires a charging energy that is larger than the thermal energy. The zero-bias conductance can be restored by tuning the electrostatic potential of the tube with a gate voltage. Electron-transport spectroscopy was carried out at mK temperatures. The density of states of the molecule appears to consist of well-separated discrete electron states. Their ~0.4 meV energy separation corresponds to estimates for a one-dimensional 'particle-in-a-box' where the 3 micron long nanotube constitutes the electron box. Electrical conduction through these discrete electron states occurs quantum coherently over remarkably long distances. Coulomb blockade
Delft University
e2
2CE =c
lead
gate
lead
eV
n + 1electron states
0
lead island lead
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NT FETSource (Au) Drain (Au)
Gate (Silicon Substrate)
Nanotube
SiO (140 nm)2
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50
40
30
20
10
0
I SD (n
A)
86420-2-4-6VGATE (V)
10-1110
-1010
-910-810-710-6
G (Ω
−1)
6420-2VGATE(V)
V = 100 mVDS
V = 50 mVDS
V = 10 mVDS
IBM Research
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NT FET CMOS
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Can have holes or electrons as carriers depending on doping (not intrinsic to NT)
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NT as light emitting diode: electron-hole recombination
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Applications• Electronic: FETs, interconnect, LED,
detectors• Power lines• Hydrogen storage• Functionalized to act as chemical detector• For tough materials• STM/AFM tips• …
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References• P. Collins and Ph. Avouris, “Nanotubes for Electronics”, Sci. Am., Dec. 200, p. 62• Ph. Avouris, “Supertubes”, IEEE Spectrum, Aug 2004, p.40• M.S Dresselhaus et al, “Carbon Nanotubes: Synthesis, Structure, Properties and Applications”,
Springer-Verlag, 2001