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8/2/2019 The Materials Science of Nano Electronics
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The Materials Science of Nanoelectronics
MAT 791Q, Fall Semester 2003
Oct. 31, 2003
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CNT structure
CNT synthesis
CNT electronic properties
Carbon Nanotubes (CNT)
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Carbon Nananotubes: What are they?
Carbon nanotubes (NT) are hollowelongated cylindrical arrangements of
atoms, closed at the ends withhemispherical caps
They can be a single-wall or a multi-wall variety, when several cylinders
(and the corresponding caps) form anested Russian doll structure.
A perfect wall of an NT is essentially
a rolled up monoatomic layer ofgraphite, that is a hexagonalhoneycomb lattice of covalent C-Cbonds.
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CNT is a tubular form of carbon with diameter as small as 1 nm.
Length: few nm to microns.
CNT is configurationally equivalent to a two dimensional graphene
sheet rolled into a tube.
CNT can be metallic orsemiconducting, depending on
chirality.
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Single-walled and Multi-walled CNT
There are two main types of Carbon Nanotubes, Multi-Walled Nanotubes and Single-
Walled Nanotubes. MWNTs contain overlapping cylindrical tubes, like a coaxial cable.
Their diameters range from a few nanometers to around 40nm, depending on thenumber of concentric tubes. SWNTs, on the other hand, consist of one tube with a
diameter of approximately 1.4nm.
Multi-Walled CNT Single-Walled CNT
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Multi-Walled CNT
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What are the major structural characteristics?
NT diameter ranges from 1 nm to 20-30 nm, and is much less than itslength
Interlayer separation in a multi-wall NT is close to 0.34 nm (like ingraphite).
A crucial property is the corkscrew symmetry of NT depending on theorientation of the hexagonal pattern of the wall with respect to the
molecular axis, often called helicity and specified by the angle
between the NT circumference and the zigzag atomic motif of thehexagonal wall. Another conventional way to identify the helicity is bythe vector (c1,c2) on the graphene lattice corresponding to the NT
perimeter. NTs with c2 = 0 ( = 0) are called zigzag, and those with c2
= c1 ( = /6) are armchair. These types are achiral, while any tubulewith 0 < < /6 is chiral.
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Fabrication of Carbon Nanotube Materials:
Arc-discharge method
I
He
Arc-
discharge
Fullerenes:
P500Torr
SWNT: Catalyst required
The process involves striking a plasma between two graphite electrodes in a vacuumchamber filled with He. Relatively large quantities (102 -103gram/day) of MWNTs can be
produced when the system is optimized. Materials produced typically consist of 50-70%MWNTs with large variations in outer (5-50nm) and inner (2-10nm) shell diameters and
length. The impurities are multi-walled nanotube particles and amorphous carbonstructures.
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Fabrication of Carbon Nanotube
Materials: Laser-ablation
Laser ablation
Oven T~ 1200oC
SWNT:Catalyst required
Yield in soot ~90% (with
two lasers)
SWNTs with relatively high purity (60-70%) and narrow diameter distribution(1.2-1.8nm) can be fabricated by ablating a graphite target mixed with metalcatalysts at high temperature. Both metallic and semiconducting nanotubes are
produced. The laser ablation method affords more experimental control but at aslower production rate compared to that of the arc-discharge method.
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Fabrication of Carbon Nanotube
Materials: Chemical Vapor Deposition
CnHm
CVD
Oven T~ 500-1000oC
catalyst
MWNT: low quality
Catalyst required
~60% yield of all carbon feed
SWNT
Catalyst required
Yield 200% in excess of
Catalyst weight
CVD provides a low temperature alternative to the two high temperature
processes discussed above and can potentially lead to economical highvolume production.
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CNT has been grown by laser ablation
(pioneering at Rice) and carbon arc process
(NEC, Japan) - early 90s.- SWNT, high purity, purification methods
CVD is ideal for patterned growth
(electronics, sensor applications)
- Well known technique frommicroelectronics
- Hydrocarbon feedstock
- Growth needs catalyst
(transition metal)
- Multiwall tubes at500-800 deg. C.
- Numerous parameters
influence CNT growth
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12CNT T and Y Junctions
Large scale computer simulations based on ab initio
methods enable understanding nanotube characteristics
and serve as design tool- Evaluation of mechanical properties
- Evaluation of electronic properties
- Electron transport in CNT devices
- Functionalization of the nanotubes- Design of electrical and mechanical devices
- Evaluation of storage potential (H2, Li)CNT Molecular Network
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Representative images of CNT bundles
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Carbon nanothermometer containing gallium
YIHUA GAO AND YOSHIO BANDO
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Energy band structure of CNT
CNT can be either metallic or semiconducting, depending on the 1st
Brillouin Zone. The 1st Brillouin Zone is the primitive cell of the lattice
in reciprical space, and it exhibits the wavevectors of the lattice. From
this, a simple relationship stating that if:
n-m is divisible by three, metallic
n-m not divisible by three, semiconducting, with a gap of~0.5 eV.
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How do NT conduct?
for the same elemental composition, the answer dependson helicity.
Armchair NT are metallic, with zero gap and an estimatedcarrier density ~1022 cm-3 for equivalent bulk material;
chiral tubes with c2 - c1 multiple of 3 have a very small(meV) curvature-induced gap ~1/d2, and other types are
semiconductors with a gap ~1/d in the range of 1 eV.
Conductivity can be vulnerable to minor deformations.Besides these elastic perturbations, any atomic disorder or
doping with other elements can change the electronicssignificantly.
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High-resolution STM image of an individual nanotube,showing the chiral winding of the hexagon lattice along
the tube axis (Dekker)
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Band gap measurements
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Energy band in metallic CNT (Lieber)
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Electrical Characterization of CNT
Individual single-wall nanotube
deposited on 2 Pt electrodes (C.Dekker)
4-probe measurements(Schoenenberger)
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Electrical characteristics of CNT
Author SampleMethod
Min. effective
resistivity at
300 K, cm
Current-
carrying
capacity,
A/cm2
Song et
al.
Bundles of
MWNT2-t 6.5x10-3
De Heer
et al.
Oriented film
of MWNT2-t 20x10-3
Frank et
al.
Individual
MWNT 2-t >10
7
Dai et
al.
Individual
MWNT2-t(STM)
8x10-4
De Pablo
et al.
Individual
MWNT
2-t(planar)
1.5x10-5 2.4x108
Ebbesson
et al.
Individual
MWNT4-t(planar)
5.1x10-6 6x106
Bachtold
et al.
Individual
MWNT 4-t(planar) 3x10-5
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Continuous Quantum Wire?
=== ke
h
e
hR 5.6
422
122min
h
e
h
eG
22
42
2 ==
L=200 nm
Liang et al.
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CNT/metal Schottky diodes
Dekker
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Bandgap modulation of CNT by
encapsulated metallofullerenesLee et al
Gd@C82
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H. Dai: Chemical Doping of CNT
Esaki tunnel diode
pn
junctiondiodes
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CNT p-n-p structure
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Oxygen desorption/adsorption doping
(Avouris)
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P. Avouris: Engineering CNT and CNT circuits by
electrical breakdown
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Architecture
Non-classical
CMOS
Memory
Logic
Time
Emerging Technology Sequence
StrainedSi
VerticalTransistor
FinFET Planardouble gate
Phase Change
Nano FG SET Molecular
Magnetic RAM
SETRSFQ QCA Molecular
RTD-FET
Quantum
computing
CNNDefect
Tolerant
QCA
3DIntegration
FD SOI
Molecular
Emerging
TechnologyVectors
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Summary
Carbon nanotube potentially could be used as nanoelectronic
components
Naturally structured
Electronic properties can be controlled by geometry and surface
adsorbates
Ballistic conductance demonstrated at length up to 0.2 m
BUT There are no methods allowing for reproducible synthesis of
CNTs with desired properties (e.g. geometry)
No methods for large scale integration of CNTs in useful devices
and circuits