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PREPARAZIONI, PROPRIETA’,
APPLICAZIONI
di SISTEMI a BASE
di NANOTUBI di CARBONIO
M.L.Terranova
Dip. di Scienze e Tecnologie Chimiche
Interdisc. MIcro- and NAno-Structured Systems –
MINASlab Universita’ di Roma “TOR VERGATA”
A SINGLE GRAPHITE SHEET: GRAPHENE
An example of mono-dimensional system
TWO DIFFERENT CLASSES
Multi-wall (MWNTs)
Single-wall (SWNTs)
Dext.= 20÷200 Å
Dint. = 10÷100 Å
D = 10÷20 Å
S. Iijima Nature 354: 56 (1991)
S. Iijima, T. Ichihashi Nature, 363: 603 (1993)
PROPERTIES
•
•
•
•
ELECTRONIC
MECHANICAL
CHEMICAL
THERMAL
 OPTICAL
 MAGNETIC
 SPINTRONICS
High chemical stability (inertness against oxidation..)
Structural integrity after intercalation and de-intercalation
High performances of gas storage
Feasibility to attach foreign species or chemical groups
Thermal stability (up to 2000 C under vacuum)
Low density (1.33-1.40 g/cm3) (1/6 the weight of steel)
Mechanical Resistance (Young modulus ~1.8 TPa)
Breaking strength : 13-50 GPa (a strain of 6%)
Volume compressibility : 2 x 10 –3 1/kbar
Reversible deformation modes (bending, axial compression, torsion)
continues………..
 The highest thermal conductivity (2000-6000 W/m·K)
Metallic or semiconducting behaviour
Electrical conductivity (1 GA/cm2 )
High efficiency of Field Emission (FE)
ELECTRONIC STRUCTURE of GRAPHITE
In 3D graphite the inter-planar interactions
are weak with respect to the in-plane C-C
interactions
The electronic structure of 2D graphite is
similar (in first approximation) to that of
3D graphite
The  e * bands of graphene “touch” in 6
points (at the corners of hexagonal Brillouin
zone ) and equal the Fermi energy for one
special wavevector
K- POINT
HOW TO ROLL GRAPHENE
Indexing scheme
Chiral vector : O-A = Ch= na1+ma2
ELECTRONIC STRUCTURE of SWNT
Confinement of electrons
along the tube circumference
kc·Ch=2j
A set of 1D energy states,
corresponing to sections of the band
structure of 2D graphite .
Depending on the position of allowed wavevector with respect to k-point
METAL-LIKE
SEMICONDUCTOR
Metal-like Nanotubes (zero-gap)
(n,n)
(n,m) | n-m=3i
The nanotubes (n,n) and (n,m) with n-m=3i behave as 1D metals :
the density of states at the Fermi level has a finite value.
In metal-like SWNTs the conduction is due to discrete 1-D electronic
states (spaced of about 0.4 meV) which extend along the whole tube length
Semiconducting Nanotubes
(n,m) | n-m3i
DOS= 0 at the Fermi level
Dependance of the gap on the diameter
Eg~1/R,
0.2  Eg  1.2 eV
T.W. Odom, J.L. Huang, P. Kim e
C.M. Lieber, Nature 391 (1998): 62
The gap decreases with diameter increasing .
SWNTs represent the ideal 1D QUANTUM WIRE
The transport properties are dependent on the
geometrical characteristics: HELICITY -DIAMETER
Depending on the structure the nanotubes can
have a metal-like or semiconducting behaviour
The electrons are confined along the circumference and
propagate exclusively along the axis of the cylinder .
The conduction is ballistic.
KEY POINTS
Use of synthesis techniques for the control of architecture:
Alignment
Density of bundles
Orientation
Placement
Definition of post-synthesis protocols for control of :
Chemical state
PRODUCTION METHODS
Sublimation or evaporation of carbon targets
LASER ABLATION
ARC DISCHARGE
Decomposition of hydrocarbons , alcools …
PYROLYSIS
VAPOUR DEPOSITION
ALL THE PROCESSES MUST BE CATALYSED
BY USING

TRANSITION METALS
LASER ABLATION
Sources:

pulsed Nd:YAG lasers (  = 532 nm)

pulsed CO2 lasers (  = 10.6 m)

cw CO2 lasers (  = 10.6 m)

double-pulsed laser systems: 532 nm pulse followed
by a coaxial 1064 nm pulse
T ~ 10 4 K
v ~ 10 6 cm s -1
Targets :
 graphite
 metal/graphite
ARC DISCHARGE
Advantages : low-cost process
Disavantages : low yields of SWNTs
bad quality of nanotubes
dispersed material (all round the walls)
V = 20-25 V
I = 50-120 A
Targets :
 graphite
 metal/graphite
PYROLYSIS
Carbon sources : hydrocarbons
Advantages : continuous production at low cost
low process temperatures
easy to scale-up
Disadvantages : large amounts of amorphous C
large amounts of residual catalysts
T = 700-1000 C
P = 1 Atm
CHEMICAL VAPOUR DEPOSITION
Activation of reactants in vapour phase by :
# PLASMAS (RF,MW..) ,
# FLAMES
T = 700-1000 C
P < 1 Atm
# HOT FILAMENTS
Carbon sources:
hydrocarbons , acetone, alcools, ferrocene…
BUT ALSO: C powders (graphite, amorphous
nanoparticles….)
Advantages
Production of aligned and oriented bundles
Growth onto selected areas
Straightforward scaling up
Easy collection of the material from substrate
CVD APPARATUSES at MINASlab
Microwave Plasma Enhanced CVD
Hot Filament CVD
Thermal CVD
T HE GROWTH
Control of density and orientation
10m
Deposition on shaped substrates
Deposition on patterned surfaces
POST-SYNTHESIS TREATMENTS
Purification
-to separate catalyst particles
-to suppress contamination by other C forms
 Opening
- to increase the reactivity at the open edges
- to make easier filling of nanotubes with gases
 Chemical functionalization
- to solubilize nanotubes
- to selective modificate the intrinsic properties
Filling
- with metals , oxides, salts, carbides, semiconductors ,
enzymes…. (wet chemistry, molten materials )
- with a gas
Mixing
-to prepare nanocomposites
FUNCTIONALIZATION
On sidewalles
The scope :
SOLUBILIZATION in POLAR MEDIA
LINKING of COMPLEX STRUCTURES
MODIFICATION of the PROPERTIES
PREPARATION OF NANOCOMPOSITES
At an open end
CNT-BASED COMPOSITES
The techniques:
BLENDING
MIXING
ELECTROPOLYMERIZATION
The matrices:
Mineral oils
Polymers
metals
Glasses-ceramics
Mechanical properties
Charge transport
Energy storage
Optical properties
POLYMER-BASED NANOCOMPOSITES
fibers
» Conducting Polymers:
polythiophene, polyaniline, polypirrole,…
» Thermoplastic Polymers:
polystyrene, polyamides, …
» Thermally Conductive Polymers:
silicones, epoxy resins …
» Liquid crystals
thermotropic, lyotropic polymer …
flexible layers
films
pastes
 A critical issue in nanocomposites: control of distribution
CONTROL of DISTRIBUTION
POLYMER-ENWRAPPED NANOTUBES
CONDUCTING NETWORKS
FILLER-MATRIX DISPERSIONS
The homogeneity and uniformity of the nanotube dispersion inside the
matrices can be checked using the microscopy techniques AFM and
AFAM (Atomic Force Acousitc Microscopy).
.
AFM
AFAM
AFAM map
The maps enable to evaluate the
quality of the nanotube
dispersion inside the host
matrix
HOW TO CHARACTERIZE THE NANOTUBES
MORPHOLOGY
-Scanning Electron Microscopy (SEM)
-Transmission Electron Microscopy (TEM)
-Scanning Tunnelling Microscopy (STM)
-Atomic Force Microscopy (AFM)
STRUCTURE
-Nanodiffraction Techniques :
*Reflection High Energy Electron Diffr. (RHEED)
*X-Ray Diffraction (XRD)
-Raman Spectroscopy
NANOTUBES in ACTION
CHARGE TRANSPORT in CNT SYSTEMS
-Low electrical resistance : in a 1D system the electrons travelling only forward or
backward have few possibilities to scatter
-Energy dissipated is very small
-Carried currents per given cross-sectional areas larger with respect to common
metals (Cu, Al..)
-No electromigration of atoms (covalent bonds vs. metal bonds)
But the properies of nanotube systems depends on their aggregation .
Membranes
Pressed tablets
Ribbons/wires
Oriented arrays
EXHIBIT DIFFERENT ELECTRICAL BEHAVIOUR
MICRO-NANO-WIRES and CIRCUITS
7
2mm
3mm
5mm
6
CNT ribbons
Current (mA)
5
4
3
2
1
0
-1
0
1
2
3
4
Voltage (V)
CNT aligned by electrical fields (multifinger device)
CNT coated by Ni
5
6
ORGANIZATION & CONDUCTIVITY
The orientation of SWNT bundles strongly improves the conductivity of
the material.
An example: bundles aligned between electrodes by dielectroforesys
700
without Electric Field
Electric Field at 1MHz, Vpp=20V
600
Current (A)
500
400
300
200
100
0
-100
0
As deposited nanotubes
Aligned bundles
(AC field 1MHz)
1
2
3
Voltage (Volt)
4
5
6
WORK in PROGRESS
*Integrated nanocircuits
*Inverters
*Interconnections
*Intramolecular junctions
FIELD-EMISSION
A mechanism for electron emission alternative to thermoionic emission
Thermoionic emission
Field emission
F.E. is a quantum tunnelling: the electrons pass through a barrier in the presence
of a high E.F.
F.E. does not require any heat to extract electrons
F.E. advantages: higher efficiency, less scatter, faster turn-on times, building
of robust and compact devices
F.E. disadvantages: dependence on the materials properties and on the shape
of the cathode
THE Fowler-Nordheim LAW
 b 
J   E exp 


 E 
a
3/ 2
2
Density of emitted current
2
•
•
•
•
Emitting area
Current density
Macrosc. electrical fieldc.
Enhancement Factor
•
Work function
•
STRATEGIES to IMPROVE FIELD EMISSION
•
•
•
Increase of A
Decrease of 
Increase of β
A
J = I/A
E


Organized Arrays
Specific materials
High form factors
L.W.Nordheim Proc.Royal Soc.London A121(1928)626
FIELD EMISSION from SWNT
NANOTUBES
Very high emission current densities :
up to 1 A/cm2 at 5 V/m
 Low values of :
turn-on and threshold ( few V/ m )
 Energy spread  0.2 eV
 Long term stability
F.E : GEOMETRICAL REQUIREMENTS
The F.E efficiency depends on the structure : SW,MW,open/closed tips…
…but also on density and organization
of nanotube arrays
COLD CATHODES for …
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•
•
•
•
•
•
Flat panel displays (FPD)
MEMS systems
Light sources (lamps)
Coherent electron sources
AFM tips
X-rays tubes
Vacuum microelectronics (tube amplifier)
CNT-BASED LIGHT SOURCES
J Wei et al. Appl. Phys. Lett. 84 (2004) 4869
BUILDING a CNT-BASED DISPLAY
Diode configuration
C.A.Spindt et al. J.Appl.Phys.47(1976)5248
In a FED, each pixel has its individual electron source (no electron scanning required ).
COLD CATHODES for X-RAY SOURCES
X-ray emission from a metallic anode bombarded by electrons
The use of a triode-type architecture increases the performances
(reduced threshold voltage, improved emission control )
1,5 – 3 W!
CheMin spectrometer 2009 Mars Science Laboratory
The quick response of CNT-based cold cathodes can be used for 3D X-ray
imaging, obtained irradiating the object from different angles , activating
sequentially different e- sources (without moving and precision mechanics)
Sarrazin et al Adv. X-Ray Anal. 48 (2005) 194]
CNT-BASED FIELD EFFECT TRANSISTORS
Transistors are the basic building blocks of integrated circuits.
CNFET
In the generic CNFET a CNT is placed between two electrodes :a
separate gate electrode controls the flow of current in the channel.
This devices can operate at R.T. with efficiency similar to
that of conventional Si transistors, but with extremely
riduced dimensions and shorter commutation times .
THE FABRICATION of a CNFET
The amount of current flowing through the nanotube channel can be
varied by a factor of 100,000 by changing the voltage applied to the gate
(VG).
R. Martel et al APL 73 (1998):2447 IBM Research Division
AFM nanomanipulation
SINGLE-ELECTRON CNT TRANSISTOR
The world’s first single-electron transistor
: two sharp bends (i.e., large potential
barriers) placed in a CNT 20 nm apart to
create a “conducting island” that electrons
must tunnel in to.
Source: Delft University
and IBM
Circuit example : CNTFET inverter
CNFETs have already been used, at research lab level, to
implement basic logic circuits such as the inverter.
Source: Delft University
VACUUM TUBES
1904 ,Sir Flemming discovers the
thermoionic effect and develops the first
vacuum tube.
1904-1930 Different
vacuum tubes:
• Diode
• Triode
• Tetrode
• Pentode
kinds
of
After : the “era” of solid state devices
BUT…..
development of high frequency/high power electronic components require
compact and efficient valves assembled with material with specific
properties :
radiation hardness
possibility to operate over a wide range of temperatures
reduced dimensions
Propagation of electrons in vacuum : with respect to solids
Longer mean-free path
Lower energy loss
CNT-BASED VACUUM TUBES
Integrated gated F.E. devices based on CNT electron emitters brings
together the advantages of vacuum tubes and solid state power transistors
Last generation of vacuum tubes competitive with solid-state devices
Vacuum tubes represent the amplifier of choice for radar,
telecommunications and space-based communications
AMPLIFIERS : Starting from an initial electrical signal,the aim is to obtain
-special gains
-shape modifications
Miniaturized efficient and compact devices
No heating required
Operational extension to higher frequencies (THz region )
MW and THz AMPLIFIERS
THz sources for :
radar
telecommunications
space-based communications
security applications
THz
SOURCES for …..
security
medical applications
communications
PLANNING a TECHNOLOGY FOR A CNT-TRIODE
-PREPARATION BY LITOGRAPHY of LOCATIONS
-DEPOSITION OF THE CATALYST INSIDE THE
PATTERNS
-IN SITU CVD GROWTH OF ORDERED CNT ARRAYS
Measured output characteristic of the trio
OPTHER- FP7 project
OPTOELECTRONICS TECHNOLOGY
CNT-based transistors can be made ambipolar : the current is conducted by
electrons for positive gate voltage
holes for negative gate voltage
Under appropriate bias conditions electrons
and holes can enter
the nanotube channel simultaneously from opposite ends.
When electron/holes meet, they release energy in form of heat or light
Electronically controlled light sources
EXTRA-BRIGHT BEAMS of IR LIGHT
An array of carbon nanotube transistors
partially suspended from a silicon dioxide
substrate
UNIPOLAR TRANSPORT CONDITIONS
Electrons were injected ( gate : -2.1 V)
from the contacting electrodes into the
nanotube and gained enough energy at
the suspended/supported substrate
interface to generate tightly-bound
electron-hole pairs, which subsequently
neutralize each other and emit light.
3 A current generated 107 photons /nm2 s
courtesy of IBM
P.Avouris (IBM) Science
CMOL Architecture : hybrid CMOS/nanowire/nanodevice
CMOS : complementary metal oxide semiconductor
The basic idea of CMOL circuits is to combine the advantages of CMOS
technology (including its flexibility and high fabrication yield) with the
extremely high potential density of molecularscale two-terminal
nanodevices.
The challenge: precise alignment of nanowires
Source: Stony Brook University
CMOL : advantages and applications
The density of active devices in CMOL may be up to 1012 cm2
and could provide unparalleled information processing
performance up to 1020 operations/cm2/s.
 Terabit scale memories
 Reconfigurable digital circuits with multi tera-flops scale
performance
 Mixed signal Neuromorphic networks that may compete
with biological neural systems in area density, exceeding
their speed at acceptable power dissipation
time-to-market > 15-20 years!
Source: Stony Brook University
OPTICAL PROPERTIES
Non-linear optical properties are evidenced in
nanocomposites , suspended–solubilized nanotube systems
or in specific nanotube aggregates .
* HIGH ORDER HARMONIC GENERATION
produced by specific solid aggregates
*STRONG LUMINESCENCE (UV-Vis)
for SWNTs embedded in varius polymeric matrices
*OPTICAL LIMITING BEHAVIOUR
of SWNTs functionalized with selected groups or chains
HIGH-ORDER HARMONIC GENERATION
The generation of 2° and 3°-order harmonics is due to quantum confinement
of the  electrons and is related to the helicity of the SWNT samples.
Centrosymmetric materials do not generate 2° harmonics .
The generation of 2°hamonics indicate partial anysotropy (chiral CNT or
disorientation )
Pressed SWNT tablets: a Q-switched Nd:Yag laser (1064 nm)
laser pulse : 10 Hz, 100-200 mJ (nanosecond time scale)
2nd
Generazione della seconda armonica
armonics
1,2
1,8
Terza armonica nanotubi da SiC
1,6
1
Intensità normalizzata (a. u.)
1,4
i n ten si tà (a. u .)
1,2
nanotubi di sintesi da
SiC
nanotubi Carbolex
1
0,8
0,6
0,4
3nd armonics
Dati sperimentali
nanotubi da SiC
dati sperimentali
particelle SiC
0,8
0,6
0,4
0,2
0,2
0
531
0
354
531,2
531,4
531,6
531,8
532
532,2
lunghezza d'onda (nm)
532,4
532,6
532,8
533
354,2
354,4
354,6
354,8
355
355,2
355,4
355,6
355,8
356
Lunghezza d'onda (nanometri)
S.Botti et al. Appl. Phys.Lett (2004)
OPTICAL LIMITING BEHAVIOUR
Nanotubes functionalized with selected groups or chains
The fluence optical limiting of pulsed
lasers is due to non –linear scattering
of the nanotube dispersions
Promising systems for:
Z.Jin et al, Chem.Phys.Lett 2002
-Manipulation of optical beams
-Optical switchers
-Devices for fast processing of optical signals
MAGNETIC and SPINTRONICS PROPERTIES
For SWNT systems in a magnetic field the changes of electron spin signals
depend on the orientation with respect to the field and on the SWNT
organization (dispersed/aggregates) .
The encapsulation of:
MAGNETIC & FERROMAGNETIC nanoparticles
1-ferromagnetic contacts and coherent
transport of spin-electrons througtht
nanotubes
2-protection of nanoparticles
against oxidation and reduction of
dipolar particle-particle interactions
High-density magnetic record media
Non-volatile magnetic memories
Spin- electronic magnetic sensors
TOWARDS CNT-BASED FLEXIBLE ELECTRONICS
It is possible to integrate the nanotubes in different matrices :
using polymers different plastic/flexible devices can be produced
Electrodes
Transistors
Light-emitting diodes
Plastic solar cells
-ease production
-flexibility
-versatility
-miniaturization
POLYMER/CNTs HYBRID DEVICES
How to fabricate a flexible transistor
Pentacene
PEDOT:PSS
PVA
Polymide
Support
nanocomposite
Polyimid
PEDOT:PSS
Patterned electrode
PHOTOVOLTAIC CELLS
General scheme of a
“DYE SENSITIZED SOLAR CELL”
DSSC
based on donor-acceptor systems
Michael Grätzel , Nature 414 (2001) 338
The interaction of SWNTs with conjugated polymers allows charge
separation of the photo-generated excitons in the polymer and
efficient electron transport to the electrode through the nanotubes.
SWNT-BASED CATHODES for DSSC
SWNT/conjugated polymer nanocomposites represent efficient
cathodes for the assembling of high performance flexible
photovoltaic cells
Cathode: Platinum
1,5
150
1,0
100
50
0,5
0
0,0
0
50
100
150
200
250
Potenziale (mV)
300
350
I(V)
P(V)
2,5
2,0
250
200
150
1,5
1,0
100
0,5
50
0,0
0 30 60 90 120 150 180 210 240 270 300
Potenziale(mV)
0
Potenza(W)
200
2
2,0
Potenza(W)
2
I(V) 250
P(V)
Corrente (mA/cm )
3,0
2,5
Corrente (mA/cm )
Cathode: CNT+polymers
THERMAL MANAGEMENT
THEORY (#) : predicts K = 6000 W/mK ( S.Berber Phys.Rev.Lett. 84 (2000) 4613 )
EXPERIMENTS : values between 1000 and 3000 W/mK
Heat Sink: polymeric
or epoxy matrices
with CNTs
Thermal Interface Materials
The thermal conductivity increases up to 40%
Resina Epossidica + SWCNTs1%p
Resina Epossidica
Potenza (W)
Chip
Die Substrate
Package
NANOTUBES for INTERCONNECTS
Flip-Chip configuration : charge and heath transport
1
3
2
4
CONCLUSIONS?
CNT: multifunctional structural materials which open a series of
technological opportunities for micro- and nano-electronics.
These exciting carbon nanomaterials are providing the scientific
community with many interesting ideas and potential applications,
some of them practical and some simply dreams for the future
But to obtain the expected benefits a lot of
research work is still needed !