M.L.Terranova

Dip. di Scienze e Tecnologie Chimiche Interdisc. MIcro- and NAno-Structured Systems – MINASlab Universita’ di Roma “TOR VERGATA”

PREPARAZIONI, PROPRIETA’, APPLICAZIONI di SISTEMI a BASE di NANOTUBI di CARBONIO

A SINGLE GRAPHITE SHEET: GRAPHENE

An example of mono-dimensional system

TWO DIFFERENT CLASSES

S. Iijima Nature 354: 56 (1991)

Dext.= 20÷200 ÅDint. = 10÷100 Å

Multi-wall (MWNTs)Multi-wall (MWNTs)

Single-wall (SWNTs)Single-wall (SWNTs)

D = 10÷20 Å

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)

The highest thermal conductivity (2000-6000 W/m·K)

Metallic or semiconducting behaviour

Electrical conductivity (1 GA/cm2 )

High efficiency of Field Emission (FE)

continues………..

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 onespecial wavevector

K- POINT

HOW TO ROLL GRAPHENE

Indexing scheme

Chiral vector : O-A = Ch= na1+ma2

A set of 1D energy states, corresponing to sections of the band structure of 2D graphite .

kc·Ch=2j

Confinement of electrons along the tube circumference

SEMICONDUCTOR

ELECTRONIC STRUCTURE of SWNT

Depending on the position of allowed wavevector with respect to k-point

METAL-LIKE

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.

(n,n)(n,m) | n-m=3i

Metal-like Nanotubes (zero-gap)

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

Dependance of the gap on the diameter

Eg~1/R,

0.2 Eg 1.2 eV

(n,m) | n-m3i

T.W. Odom, J.L. Huang, P. Kim e C.M. Lieber, Nature 391 (1998): 62

DOS= 0 at the Fermi level

Semiconducting Nanotubes

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 canhave 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

Chemical state

Orientation

PlacementDensity of bundles

Definition of post-synthesis protocols for control of :

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

Targets : graphite

metal/graphite

T ~ 10 4 K v ~ 10 6 cm s -1

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

graphite

metal/graphite

Targets :

PYROLYSISCarbon 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

# HOT FILAMENTS

T = 700-1000 C

P < 1 Atm

Carbon sources:

hydrocarbons , acetone, alcools, ferrocene…

BUT ALSO: C powders (graphite, amorphous nanoparticles….)

Production of aligned and oriented bundles Growth onto selected areas Straightforward scaling up Easy collection of the material from substrate

Advantages

Hot Filament CVDHot Filament CVD

Microwave Plasma Enhanced CVDMicrowave Plasma Enhanced CVD Thermal CVDThermal CVD

CVD APPARATUSES at MINASlab

T HE GROWTH

Control of density and orientation

10m10m

Deposition on shaped substrates

Deposition on patterned surfaces

Filling- with metals , oxides, salts, carbides, semiconductors , enzymes…. (wet chemistry, molten materials )- with a gasMixing -to prepare nanocomposites

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

POST-SYNTHESIS TREATMENTS

FUNCTIONALIZATION

At an open endOn sidewalles

The scope :

SOLUBILIZATION in POLAR MEDIALINKING of COMPLEX STRUCTURES MODIFICATION of the PROPERTIES PREPARATION OF NANOCOMPOSITES

CNT-BASED COMPOSITES

Mineral oils

metals

Glasses-ceramics

The techniques:

Polymers

BLENDINGMIXINGELECTROPOLYMERIZATION

The matrices:

Mechanical properties

Charge transport

Energy storage

Optical properties

flexible layers

fibers

pastes

films

» Conducting Polymers: polythiophene, polyaniline, polypirrole,…

» Thermoplastic Polymers: polystyrene, polyamides, …

» Thermally Conductive Polymers: silicones, epoxy resins … » Liquid crystals thermotropic, lyotropic polymer …

» Conducting Polymers: polythiophene, polyaniline, polypirrole,…

» Thermoplastic Polymers: polystyrene, polyamides, …

» Thermally Conductive Polymers: silicones, epoxy resins … » Liquid crystals thermotropic, lyotropic polymer …

POLYMER-BASED NANOCOMPOSITES

A critical issue in nanocomposites: A critical issue in nanocomposites: control of distribution A critical issue in nanocomposites: A critical issue in nanocomposites: control of distribution

POLYMER-ENWRAPPED NANOTUBES

CONTROL of DISTRIBUTION

CONDUCTING NETWORKS

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).

.

AFAM map AFAM map

AFMAFMAFAMAFAM

FILLER-MATRIX DISPERSIONS

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

But the properies of nanotube systems depends on their aggregation . Membranes Pressed tablets Ribbons/wires Oriented arrays

EXHIBIT DIFFERENT ELECTRICAL BEHAVIOUR

-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)

CNT ribbons CNT ribbons

CNT aligned by electrical fields (multifinger device) CNT aligned by electrical fields (multifinger device)

0 1 2 3 4 5 6-1

0

1

2

3

4

5

6

7 2mm 3mm 5mm

Cur

rent

(mA

)

Voltage (V)

CNT coated by NiCNT coated by Ni

MICRO-NANO-WIRES and CIRCUITS

As deposited nanotubes Aligned bundles

(AC field 1MHz)

0 1 2 3 4 5 6-100

0

100

200

300

400

500

600

700 without Electric Field Electric Field at 1MHz, Vpp=20V

Curre

nt (

A)

Voltage (Volt)

The orientation of SWNT bundles strongly improves the conductivity

of the material.

ORGANIZATION & CONDUCTIVITY

An example: bundles aligned between electrodes by dielectroforesys

*Integrated nanocircuits*Inverters*Interconnections*Intramolecular junctions

WORK in PROGRESS

FIELD-EMISSION

Thermoionic emission Field emission

A mechanism for electron emission alternative to thermoionic emission

F.E. is a quantum tunnelling: the electrons pass through a barrier in the presenceof a high E.F. F.E. does not require any heat to extract electronsF.E. advantages: higher efficiency, less scatter, faster turn-on times, building of robust and compact devicesF.E. disadvantages: dependence on the materials properties and on the shape of the cathode

THE Fowler-Nordheim LAW

E

bE

aJ

2/322 exp

• Emitting area A

• Current density J = I/A• Macrosc. electrical fieldc. E• Enhancement Factor

• Work function

L.W.Nordheim Proc.Royal Soc.London A121(1928)626

Density of emitted current

• STRATEGIES to IMPROVE FIELD EMISSION

• Increase of A Organized Arrays • Decrease of Specific materials• Increase of β High form factors

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

FIELD EMISSION from SWNT

NANOTUBES

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 …

• Flat panel displays (FPD)• MEMS systems• Light sources (lamps)• Coherent electron sources • AFM tips• X-rays tubes • Vacuum microelectronics (tube

amplifier)

J Wei et al. Appl. Phys. Lett. 84 (2004) 4869

CNT-BASED LIGHT SOURCES

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 ).

1,5 – 3 W!1,5 – 3 W!

CheMin spectrometer 2009 Mars Science LaboratoryCheMin spectrometer 2009 Mars Science Laboratory

COLD CATHODES for X-RAY SOURCES

X-ray emission from a metallic anode bombarded by electronsThe use of a triode-type architecture increases the performances(reduced threshold voltage, improved emission control )

Sarrazin et al Adv. X-Ray Anal. 48 (2005) 194] Sarrazin et al Adv. X-Ray Anal. 48 (2005) 194]

The quick response of CNT-based cold cathodes can be used for 3D X-ray imaging, obtained irradiating the object from different angles , activatingsequentially different e- sources (without moving and precision mechanics)

Transistors are the basic building blocks of integrated circuits.

In the generic CNFET a CNT is placed between two electrodes :a In the generic CNFET a CNT is placed between two electrodes :a separate gate electrode controls the flow of current in the separate gate electrode controls the flow of current in the channel.channel.

CNT-BASED FIELD EFFECT TRANSISTORS

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 .

CNFET

R. Martel et al APL 73 (1998):2447 IBM Research Division

THE FABRICATION of a CNFET

The amount of current flowing through the nanotube channel can The amount of current flowing through the nanotube channel can be varied by a factor of 100,000 by changing the voltage applied be varied by a factor of 100,000 by changing the voltage applied to the gate (VG).to the gate (VG).

Source: Delft University and IBM

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.

AF

M n

ano

man

ipu

lati

on

SINGLE-ELECTRON CNT TRANSISTOR

Source: Delft University

CNFETs have already been used, at research lab level, to implement basic logic circuits such as the inverter.

Circuit example : CNTFET inverter

1904 ,Sir Flemming discovers the thermoionic effect and develops the first vacuum tube.

1904-1930 Different kinds of vacuum tubes: • Diode • Triode • Tetrode • Pentode

VACUUM TUBES

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

After : the “era” of solid state devices

Propagation of electrons in vacuum : with respect to solids Longer mean-free path Lower energy loss

CNT-BASED VACUUM TUBES

Miniaturized efficient and compact devices No heating requiredOperational extension to higher frequencies (THz region )

AMPLIFIERS : Starting from an initial electrical signal,the aim is to obtain

-special gains -shape modifications

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

MW and THz AMPLIFIERS

THz sources for :

radar telecommunications space-based communications security applications

security

communications

medical applications

THz SOURCES for …..

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 triode

OPTHER- FP7 project

OPTOELECTRONICS TECHNOLOGY

CNT-based transistors can be made ambipolar : the current is conducted by

Electronically controlled light sources

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

electrons for positive gate voltageholes for negative gate voltage

An array of carbon nanotube An array of carbon nanotube transistors partially suspended from a transistors partially suspended from a silicon dioxide substratesilicon dioxide substrate

UNIPOLAR TRANSPORT CONDITIONSUNIPOLAR TRANSPORT CONDITIONS

Electrons were injected ( gate : -2.1 Electrons were injected ( gate : -2.1 V) from the contacting electrodes into V) from the contacting electrodes into the nanotube and gained enough the nanotube and gained enough energy at the suspended/supported energy at the suspended/supported substrate interface to generate substrate interface to generate tightly-bound electron-hole pairs, tightly-bound electron-hole pairs, which subsequently neutralize each which subsequently neutralize each other and emit lightother and emit light. .

An array of carbon nanotube An array of carbon nanotube transistors partially suspended from a transistors partially suspended from a silicon dioxide substratesilicon dioxide substrate

UNIPOLAR TRANSPORT CONDITIONSUNIPOLAR TRANSPORT CONDITIONS

Electrons were injected ( gate : -2.1 Electrons were injected ( gate : -2.1 V) from the contacting electrodes into V) from the contacting electrodes into the nanotube and gained enough the nanotube and gained enough energy at the suspended/supported energy at the suspended/supported substrate interface to generate substrate interface to generate tightly-bound electron-hole pairs, tightly-bound electron-hole pairs, which subsequently neutralize each which subsequently neutralize each other and emit lightother and emit light. .

EXTRA-BRIGHT BEAMS of IR LIGHT

courtesy of IBM

P.Avouris (IBM) Science

3 A current generated 107 photons /nm2 s

Source: Stony Brook University

The basic idea of CMOL circuits is to combine the advantages of CMOS technology (including its flexibility and high fabrication yield) with theextremely high potential density of molecularscale two-terminal nanodevices.

CMOL Architecture : hybrid CMOS/nanowire/nanodevice

The challenge: precise alignment of nanowires

CMOS : complementary metal oxide semiconductor

Source: Stony Brook University

The density of active devices in CMOL may be up to 10The density of active devices in CMOL may be up to 1012 12

cmcm22 and could provide unparalleled information and could provide unparalleled information processing performance up to 10processing performance up to 102020 operations/cm operations/cm22/s./s.

Terabit scale memoriesTerabit scale memories

Reconfigurable digital circuits with multi tera-flops Reconfigurable digital circuits with multi tera-flops scale performancescale performance

Mixed signal Neuromorphic networks that may Mixed signal Neuromorphic networks that may compete with biological neural systems in area compete with biological neural systems in area density, exceeding their speed at acceptable power density, exceeding their speed at acceptable power dissipationdissipation

time-to-market > 15-20 years!

CMOL : advantages and applications

Non-linear optical properties are evidenced in nanocomposites , suspended–solubilized nanotube systems or in specific nanotube 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 produced by specific solid aggregates

OPTICAL PROPERTIES

2nd armonics

3nd armonics

S.Botti et al. Appl. Phys.Lett (2004)

Pressed SWNT tablets: a Q-switched Nd:Yag laser (1064 nm) laser pulse : 10 Hz, 100-200 mJ (nanosecond time scale)

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 )

HIGH-ORDER HARMONIC GENERATION

Nanotubes functionalized with selected groups or chains

The fluence optical limiting of pulsed lasers is due to non –linear scattering of the nanotube dispersions

Z.Jin et al, Chem.Phys.Lett 2002

OPTICAL LIMITING BEHAVIOUR

Promising systems for:

-Manipulation of optical beams-Optical switchers -Devices for fast processing of optical signals

MAGNETIC and SPINTRONICS PROPERTIES

The encapsulation of: MAGNETIC & FERROMAGNETIC nanoparticles

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) .

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 mediaNon-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

ElectrodesTransistorsLight-emitting diodesPlastic solar cells

-ease production

-flexibility

-versatility

-miniaturization

POLYMER/CNTs HYBRID DEVICES

PolyimidPEDOT:PSS

PentacenePEDOT:PSS PVA

How to fabricate a flexible transistor

nanocompositenanocomposite

Patterned electrodePatterned electrode

Polymide Support

PHOTOVOLTAIC CELLS

General scheme of a “DYE SENSITIZED SOLAR CELL” DSSC based on donor-acceptor systems

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.

Michael Grätzel , Nature 414 (2001) 338

Cathode: PlatinumCathode: Platinum

SWNT/conjugated polymer nanocomposites represent efficient cathodes for the assembling of high performance flexible photovoltaic cells

SWNT-BASED CATHODES for DSSC

Cathode: CNT+polymersCathode: CNT+polymers

The thermal conductivity increases up to 40%The thermal conductivity increases up to 40%

THERMAL MANAGEMENT

Thermal Interface Materials

THEORY (#) : predicts K = 6000 W/mK ( S.Berber Phys.Rev.Lett. 84 (2000) 4613 )EXPERIMENTS : values between 1000 and 3000 W/mK

Die Substrate

Package

Heat Sink: polymeric or epoxy matrices with CNTs

Chip

Resina Epossidica + SWCNTs1%p

Resina Epossidica

Potenza (W)

11

22

33

44

NANOTUBES for INTERCONNECTS

Flip-Chip configuration : charge and heath transport

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 !

But to obtain the expected benefits a lot of research work is still needed !

CONCLUSIONS?