Nanoscience & Nanotechnology-IIWhat is happening at a very, very small length scale?

Plan of the talk

• Fullerenes

• Graphene

• Carbon Nanotubes

Fullerenes

”The most symmetrical large molecule”

• Discovered in 1985

- Nobel prize Chemistry 1996, Curl, Kroto, and Smalley

Properties

Epcot center, Paris

~1 nm

Architect: R. Buckminster Fuller

• C60, also 70, 76 and 84.- 32 facets (12 pentagons and 20 hexagons)- prototype

from sciencedaily.com

Buckminsterfullerene

Molecule consisting of 60 C atoms

sp2 hybridized bonds

Has 20 hexagons, 12 pentagons

Other related structures have 70 or 84 C atoms

from unusualife.com

C60 is named for Buckminster Fuller who designed geodesic domes.

Bucky Ball (C60) C240 colliding with C60 at

300 eV (Kinetic energy)

Bucky Balls

http://www.pa.msu.edu/cmp/csc/simindex.html

Original report of C60

1996 Nobel Prize in Chemistry

Robert Curl, Sir Harold Kroto, Richard Smalley “for their discovery of fullerenes”.

from Nobelprize.org

Buckyball Discovery

• 1985: British chemist Harry Kroto studied molecules with exactly sixty carbon atoms found near red giant stars

• Kroto collaborated with Richard Smalley and Robert Curl to recreate the conditions in the laboratory and form C60

molecules by laser vaporization of graphite

• The scientists hypothesized that the molecules were made of hexagonal carbon rings blasted apart from the graphite structure, and that the molecule must be spheroid to satisfy valence requirements

Determining the C60 Structure

• After considerable work, Kroto, Smalley, and Curl determined that the structure of the C60 buckyball was a combination of 12 pentagonal and 20 hexagonal rings, forming a spheroid shape with 60 vertices for the 60 carbons.

• The pentagonal rings sit at the vertices of an icosahedron such that no 2 pentagonal rings are next to each other

• Curl, Kroto, and Smalley received the Nobel Prize in 1996 for their work.

• The architect R. Buckminster Fuller designed a geodesic dome for the 1967 Montreal World Exhibition with the same structure; the scientists thus named the new molecule Buckminsterfullerene, which was shortened to fullerene when referring to the family of molecules.

Haymet, A. D. J. "Footballene: a Theoretical Prediction for the Stable, Truncated Icosahedral Molecule C60.”Journal of the American Chemical Society 108 (1986): 319-321.

Fullerene

• Symmetric shape→ lubricant

• Large surface area→ catalyst

• High temperature (~500oC)

• High pressure

• Hollow→ caging particles

• Ferromagnet?- polymerized C60

- up to 220oC

Properties

Fullerene

• Chemically stable as graphite- most reactive at pentagons

• Crystal by weak van der Waals force

• Superconductivity

- K3C60: 19.2 K

- RbCs2C60: 33 K

Kittel, Introduction to Solid State Physics, 7the ed. 1996.

Properties

Bucky Ball properties

• Arranged in pentagons and hexagons• A one atom thick seperation of two spaces; inside the ball

and outside• Highest tensile strength of any known 2D structure or

element, including cross-section of diamonds which have the highest tensile strength of all known 3D structures (which is also a formation of carbon atoms)

• Also has the highest packing density of all known structures (including diamonds)

• Impenetrable to all elements under normal circumstances, even a helium atom with an energy of 5eV (electron Volt)

Synthesis The fullerenes were prepared by evaporation of carbon

electrodes in an electric arc discharge process in helium atmosphere.

The main part of the deposition system consists in a stainless steel, double walled, cylindrical chamber.

Between the two chamber walls is flowing the cooling agent, the temperature being automatically controlled.

The two electrodes were horizontally mounted near the bottom of a reaction chamber.

For the anode electrode pure graphite electrodes of a diameter of 6 mm and the length of 140 mm are used.

The second electrode consists in a pure graphite disk, mounted at the bottom of the reaction chamber.

The reaction chamber acts also like a soot collector. The anode is mounted in a guiding system, controlled by a

mechanical system in order to assure a constant distance between the two electrodes during arc discharge.

Synthesis of fullerenes

DC power supply unit was used and operated at the voltage of 10–20 V and the current of 0–250 A.

To obtain the carbon soot, the chamber is evacuated of the air.

After that, the chamber was filled in with helium gas at the pressure between 50 and 200 Torr.

As current flows between electrodes, graphite electrode gets vaporized.

The arc intensity is controlled by the distance between the electrodes.

Fullerene C60 was extracted from the carbon soot collected in the chamber .

The main technological parameters that control the process efficiency are the discharge current, the distance between the electrodes, the pressure and nature of the working gas, the electrodes composition, shape and dimensions.

Species of fullerenes

• Alkali-doped fullerenes

–Alkali atoms doped on fullerenes

• Exohedral Fullerenes

– Atoms, molecules, and complexes are attached to the exterior of the cage

• Endohedral Fullerenes

– Molecules are enclosed within the cage

Species of fullerenes

Alkali-doped fullerenes

• As fullerene molecule is highly electronegative, itreadily forms compounds with electron donatingatoms, the most common examples being alkalimetals forming alkali-doped fullerides, wherein alkalimetal atoms fill in the space between Buckyballs anddonate valence electron to the neighbouring C60molecule.

• If alkali atoms are potassium or rubidium, thecompounds are superconductors, and they conductelectric current without any resistance attemperatures below 20-40 K, e.g., K 3 C 60 , Rb 3 C 60 .

Endohedral fullerenes & Exohedral Fullerenes Exohedral fullerenes or fullerene

derivatives, which are molecules formed by a chemical reaction between fullerenes and other chemical groups.

Fullerene derivatives are also known as functionalized fullerenes .

As fullerenes possess the conjugated p-system of electron, two main types of primary chemical transformations are possible on fullerene surface: addition reactions and redox reactions, which lead to covalent exohedral adducts and salts respectively.

As fullerenes are insoluble in water, numerous derivatives of fullerenes have been synthesized with improved solubility profile.

When the atom trapped inside is ametal, they are known as metallofullerenes.

Most of endohedral materials aremade out of C 82 , C 84 or even higherfullerenes.

The atoms that form stableendohedral compounds includelanthanum, yttrium, scandium, andsome of the noble gases.

As it is very difficult to open upcarbon cage molecules to enclose aforeign atom inside, endohedralmaterial must be synthesized whileformation of the cage itself.

(The accepted notation forendohedral material is to use the @symbol to show that the first materialis inside the second, e.g., La@C 82 andSc 2 @C 84 .)

Fullerenes Applications

• Hydrogen or oxygen storage: hydrogenation of fullerene produces hydrides. The reaction is reversible and can be catalyzed with metals (low pressure).

• Catalyst: fullerene promotes the conversion of methane into higher hydrocarbons and inhibits coking reactions.

• Sensor: fullerene based capacitors can be used to detect ppm of H2S in N2, ppm of water in isopropanol.

• Diamond precursor: fullerene can be transformed to diamond at high pressure (RT) or can be used as a diamond nucleation center during CVD.

• Alloy strengthening/hardening (Ti), improvement of electrical conductivity of Cu alloys.

• Biomedical field: inhibition of human HIV replication and HIV-1 protease. Biological antioxidant (radical sponge).

What is graphene?

21

Q1. How thick is it? a million times thinner than paper

(The interlayer spacing : 0.33~0.36 nm)

Q2. How strong is it? stronger than diamond

(Maximum Young's modulus : ~1.3 TPa)

Q3. How conductive is it? better than copper

(The resistivity : 10−6 Ω·cm)(Mobility: 200,000 cm2 V-1 s-1)

In late 2004, graphene was discovered by Andre Geim and Kostya Novoselov (Univ. of Manchester).- 2010 Nobel Prize in Physics

But, weak bonding between layersSeperated by mechanical exfoliation of 3D graphite crystals.

Graphene is a 2D building block for carbon-based materials

Electronic structure of graphene

K

Ef

KK’

K’

Pz bondingValence band

Pz anti bondingConduction band

2DEG

Fermi energy

Effective mass (related with 2nd derivative of E(k) ) MasslessGraphene charged particle is massless Dirac fermion. Zero gap semiconductor or Semi-metal

Electrical properties of graphene

Graphene is great conductor;electrons are able to flowthrough graphene moreeasily than through evencopper. The electrons travelthrough the graphene sheetas if they carry no mass, asfast as just one hundredththat of the speed of light.Graphene is a semi-metal andis a zero-gap semiconductor .

High electron mobility at room temperature: Electronic device. Si Transistor, HEMT devices are using 2D electron or hole.

μ (mobility) = vavg / E(velocity/electric field)

Jdrift ~ ρ x vavg

Thermal properties of graphene

Graphene is a perfect thermal conductor. Its thermalconductivity is much higher than all the other carbonstructures as carbon nanotubes, graphite anddiamond (> 5000 W/m/K).

The ballistic thermal conductance of graphene isisotropic, i.e. same in all directions. The study of thermalconductivity in graphene may have importantimplications in graphene-based electronic devices. As devicescontinue to shrink and circuit density increases, high thermalconductivity, which is essential for dissipating heat efficientlyto keep electronics cool, plays an increasingly larger role indevice reliability.

Optical properties of graphene

Optical transmittance control: transparent electrodeReduction of single layer: 2.3%

F. Bonaccorso et al. Nat. Photon. 4, 611 (2010)

Graphene is almostcompletely transparent,yet so dense that eventhe smallest atom heliumcannot pass through it.

Mechanical properties of graphene

Young’s modulus=tensile stress/tensile strainDiamond ~ 1200 GPa

Force-displacement measurement

It was found that graphene is harder than diamond and about 300 times harder than steel. The tensile strength of graphene exceeds 1 TPa. Even though graphene is so robust, it is also very stretchable to make use in flexible and stretchable devices

C. Lee et al. Science 321, 385 (2008)

Chemical properties of graphene

Similar to the surface of graphite, graphene can adsorb and desorb various atomsand molecules (for example, NO2, NH3, K, and OH).

Weakly attached adsorbates often act as donors or acceptors and leadto changes in the carrier concentration, so graphene remains highlyconductive. This can be exploited for applications as sensors forchemicals.

Other than weakly attached adsorbates, graphene can befunctionalized by several chemical groups (for instances OH-, F-)forming graphene oxide and fluorinated graphene. It has also beenrevealed that single-layer graphene is much more reactive than 2, 3 orhigher numbers or layers.

Also, the edge of graphene has been shown to be more reactive than the surface.Unless exposed to reasonably harsh reaction conditions, graphene is a fairly inertmaterial, and does not react readily despite every atom being exposed andvulnerable to it's surroundings.

Preparation and characterization graphene

Preparation methods

Top-down approach(From graphite)

Bottom up approach (from carbon precursors)

- By chemical vapour deposition (CVD)

of hydrocarbon

- By epitaxial growth on electrically

insulating surfaces such as SiC

- Total Organic Synthesis

- Micromechanical exfoliation of graphite (Scotch

tape or peel-off method)

- Creation of colloidal suspensions from graphite

oxide or graphite intercalation compounds (GICs)

Ref: Carbon, 4 8, 2 1 2 7 –2 1 5 0 ( 2 0 1 0 )

Graphene synthesis

Mechanical Exfoliation (Scotch-tape method)

Repeated peeling of graphite to get an atom-thick layer of graphite, which is called grapheme.

This simple, low-budget technique has been widely credited for the explosive growth of interest in graphene.

Unfortunately, however, they are usually available at a size of several-microns (or tens of microns at best), have irregular shapes, and their azimuthal orientation is not deterministically controlled.

Graphene sheets ionic-liquid-modified by electrochemistry using graphite electrodes.

Liu, N. et al. One-step ionic-liquid-assisted electrochemical synthesis of ionicliquid-

functionalized graphene sheets directly from graphite. Adv. Funct. Mater. 18, 1518–1525 (2008).

Direct exfoliation of graphite

Graphene synthesisChemical Vapor Deposition

Graphene and few-layer graphene (FLG) have been grown bychemical vapor deposition (CVD) from C-containing gases on catalyticmetal surfaces and/or by surface segregation of C dissolved in thebulk of such metals.

Depending on the solubility of C in the metal, the former or the lattercan be the dominant growth process, or they can coexist.

The electrical properties of CVD graphene cannot be tested in situ onthe conductive metal substrates.

Thus, processes to transfer graphene on an appropriate insulatingsubstrate have been developed.

The ability to select the host substrate independently of the sacrificialgrowth substrate is a major advantage for graphene grown on metals.

At the same time, the transfer process often affects negativelygraphene's integrity, properties, and performance.

Wrinkle formation, impurities, graphene tearing, and other structuraldefects, can occur during transfer

Potential application of graphene

- Single molecule gas detection

- Graphene transistors

- Integrated circuits

- Transparent conducting electrodes for the replacement of ITO

- Ultracapacitors

- Graphene biodevices

- Reinforcement for polymer nanocomposites: Electrical, thermally conductive nanocomposites, antistatic coating, transparent conductive composites..ect

Graphene Nanoribbon (GNR)• Graphene is a zero-gap semiconductor.

• To extend the real applications, an energy gap is needed, which enables thebasic electric logic states: on and off.

• When graphene is etched or patterned along one specific direction, a novelquasi one-dimensional (1D) structure is obtained, which is a strip ofgraphene, referred as graphene nanoribbon (GNR).Depends on thetermination style, normally, GNR can be divided into two kinds: Armchairand Zigzag

• The width of armchair GNRs is classified by the number of dimer lines (Na)across the ribbons. The width of zigzag GNRs is classified by the number ofzigzag chains (Nz) across the ribbons.

• Perpendicular to the direction of defined width, GNRs repeat theirgeometric structures, and form one-dimensional periodic structures.

Carbon Nanotubes

Types

Fabrication

Structure

Properties

Applications

What are Carbon Nanotubes ?

Carbon nanotubes arefullerene-relatedstructures whichconsist of graphenecylinders closed ateither end with capscontaining pentagonalrings

Rotating Carbon Nanotube

Discovery

They were discovered in 1991 bythe Japanese electronmicroscopist Sumio Iijima whowas studying the materialdeposited on the cathode duringthe arc-evaporation synthesis offullerenes. He found that thecentral core of the cathodicdeposit contained a variety ofclosed graphitic structuresincluding nanoparticles andnanotubes, of a type which hadnever previously been observed

What is it?

• Sheet of graphite rolled into a tube

• Single-Walled (SWNT) and Multi-Walled (MWNT)

• Large application potential, metallic, semiconducting

SWNTMWNT

armchair

zigzag

chiral

Small Dimensions

Low Resistivity (Ballistic Electron Conduction)

High Specific Surface Area (Good

Adsorbents)

Chemically Stable

Mechanically Robust

High Thermal Conductivity

Why Carbon Nanotubes ?

Ideal materials for applications in conductive and high-strength composites; energy

storage and energy conversion devices; sensors; field emission displays and

radiation sources; hydrogen storage media; and nanometer-sized semiconductor

devices, probes, and interconnects.

Types of Carbon nanotubes

Two main types of

carbon nanotubes:

Single-walled

nanotubes (SWNTs)

consist of a single

graphite sheet

seamlessly wrapped

into a cylindrical tube.

Multiwalled nanotubes

(MWNTs) comprise an

array of such nanotubes

(more than one wall)

that are concentrically

nested with in.

Institute of Optics, University of Rochester 41

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

Structure of Single Walled Carbon Nanotubes

• Structure depends on rolling direction (chirality)– Metallic

– Semi-conducting

Figure 6.1. Diagram explaining the relationship of a SWNT to a graphene sheet. The wrapping

vector for an (8,4) nanotube, which is perpendicular to the tube axis, is shown as an example.

Those tubes which are metallic have indices shown in red. All other tubes are semiconducting.

Three Forms of CNTs

• Chiral

• Zigzag

• Armchair

• Vectors describe the rolling process that occurs when a graphite sheet is transformed into a tube

http://upload.wikimedia.org/wikipedia/commons/5/53/Types_of_Carbon_Nanotubes.png

SWNTs and MWNTs are usually made by

• carbon-arc discharge methods– C electrodes, 20-25V potential, 1mm, 500 torr, C ejected from + electrode forms NT on –

electrode (Co, Ni or Fe for SWNTs, 1-5nm, 1µm length, no catalyst = MWNTs,)

• laser ablation of carbon– 1200C, pulsed laser, catalysts (Co, Ni), condensation (10-20nm, 100µm length)

• chemical vapor deposition (typically on catalytic particles)– 1100 C, decomposition of hydrocarbon gas (e.g. CH4), open NTs, catalyst on substrate,

industrial scale up, and length can vary

Nanotube diameters

• range from 0.4 to > 3 nm for SWNTs and from ~1.4 to at least 100 nm for MWNTs

Fabrication/Nanotube Synthesis

Figure shows typical setup used for laser ablation of carbon, which

consists of a furnace, a quartz tube with a window, a target carbon

composite doped with catalytic metals, a water-cooled trap, and flow

systems for the buffer gas to maintain constant pressures and flow

rates. A laser beam (typically a YAG or CO2 laser) is introduced

through the window and focused onto the target located in the center

of the furnace. The target is vaporized in high-temperature Ar buffer

gas and forms SWNTs. The SWNTs produced are conveyed by the

b u f f e r g a s t o t h e t r a p , w h e r e t h e y a r e c o l l e c t e d .

The method has several advantages, such as high-quality SWNT

production, diameter control, investigation of growth dynamics, and

the production of new materials. High-quality SWNTs with minimal

defects and contaminants, such as amorphous carbon and catalytic

metals, have been produced using the laser-furnace method.

Pump

Gas Inlet

Laser Target

Rod

Water Inlet

SWNT

Furnace

Laser

ablation of

carbon for

CNT growth

Carbon-arc discharge

Gas Inlet

Water cooled chamber

Graphite

rod

CNT

Deposit

The schematic diagram of and arc chamber

for CNT production is shown. After evacuating

the chamber, an appropriate ambient gas is

introduced at the desired pressure, and then

a dc arc voltage is applied between the two

graphite rods. When pure graphite rods are

used, the anode evaporates and the is

deposited on the cathode, which contains

C N Ts . T h e s e C N Ts , a r e M W N Ts .

When a graphite rod containing metal catalyst

(Fe, Co, etc.) is used as the anode with a

pure graphite cathode, SWNTs are generated

in the form of soot.

Typical CVD Furnace Schematics

Parameters Used:

Ferrocene/Xylene: 1gm in 100 ml

Gas flow rate: 100 sccm

Growth temperature: 770 0C

Figure 1: Schematics of the nanotube growth

apparatus

The CVD method can be

used for growing controlled

architectures (aligned as well

as patterned) of carbon

nanotubes on various

substrates.40 micron

Aligned CNTs

Properties

Chemical Reactivity

Electrical

Optical

Mechanical Strength

Special properties of carbon nanotubesElectronic, molecular and structural properties of carbon nanotubes are determined to a large extent by their nearly one dimensional structure.The most important properties of CNTs and their molecular background are stated below.

Chemical reactivity.The chemical reactivity of a CNT is, compared with a graphene sheet, enhanced as a direct result of the curvature of the CNT surface. Carbon nanotube reactivity is directly related to the pi-orbital mismatch caused by an increased curvature. Therefore, a distinction must be made between the sidewall and the end caps of a nanotube. For the same reason, a smaller nanotube diameter results in increased reactivity. Covalent chemical modification of either sidewalls or end caps has shown to be possible. For example, the solubility of CNTs in different solvents can be controlled this way. Though, direct investigation of chemical modifications on nanotube behaviour is difficult as the crude nanotube samples are still not pure enough.

Electrical Properties

• If the nanotube structure is armchair then the electrical properties are metallic

• If the nanotube structure is chiral then the electrical properties can be either semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor

• In theory, metallic nanotubes can carry an electrical current density of 4×109 A/cm2 which is more than 1,000 times greater than metals such as copper

Electrical Properties

• Electrical Properties

– Metallic – armchair structure – conductive

– Semi-conductors – zigzag and chiral

• Depends on diameter (quantum effects)

– Ropes of SWNTs (R=10-4cm-1 at 27C)

– Combinations – transistors

• Bent molecules

• Response to stretching

• Chirality and diameter of nanotubes are important parameters!!!

from nanotechweb.org

Carbon nanostructures may be used in new electronic devices

Figure 6.18. Atomic force microscopy image of an isolated SWNT deposited onto seven Pt electrodes by spin-coating from

dichloromethane solution. The substrate is SiO2. An auxiliary electrode is used for electrostatic gating. (Reproduced with

kind permission of C. Dekker.)

Devices made with carbon nanotubes

Devices made with carbon nanotubes

Optical activityTheoretical studies have revealed that the optical activity of chiral nanotubes disappears if the nanotubes become larger.Therefore, it is expected that other physical properties areinfluenced by these parameters too.Use of the optical activity might result in optical devices in which CNTs play an important role.

Mechanical Strength

• Carbon nanotubes have the strongest tensile strength of any material known.

• It also has the highest modulus of elasticity.

MaterialYoung's Modulus (TPa)

Tensile Strength (GPa)

Elongation at Break (%)

SWNT ~1 (from 1 to 5) 13-53E 16

Armchair SWNT

0.94T 126.2T 23.1

Zigzag SWNT 0.94T 94.5T 15.6-17.5

Chiral SWNT 0.92

MWNT 0.8-0.9E 150

Stainless Steel ~0.2 ~0.65-1 15-50

Mechanical Strength

• Mechanical Properties

– Young’s modulus E = 1.28 – 1.8TPa (steel 0.21TPa)

– Strength Rm = 45,000 MPa (high strength steel 2,000 MPa)

– Buckling – no fracture – change in hybridization (from sp2)

Molecular dynamics simulations of a (10,10) nanotube under axial tension (J. Bernholc, M. Buongiorno Nardelli

and B. Yakobson). Plastic flow behavior is shown after 2.5 ns at T = 3,000 K and 3% strain. The blue area indicates

the migration path (in the direction of the arrow) of the edge dislocation (green). This sort of behavior might help make

composite materials that are really tough (as measured by their ability to absorb energy).

VFD (Vacuum Fluorescent Display)LCD (Liquid Crystal Display)CRT (Cathode Ray Tube)FED (Field Emission Display)SET (single Electron Transistor)STM (Scanning tunneling Miceoscope)AFM (Atomic force Microscope)

Application of Nanotubes

Energy storage

a) Hydrogen storage

The advantage of hydrogen as energy source is that its combustion product is water.In addition,hydrogen can be easily regenerated.

The two commonly used means to store hydrogen are :1. Gas phase – due to cylindrical and hollow geometry, and nano scale diameters carbon nanotubes can store a liquid or a gas in the inner cores through a capillary effect.2. Electrochemical adsorption-H atom is adsorbed. This is called chemisorption.

Hydrogen storage

Carbon nanotubes

– Hydrogen carriers based on nanostructured carbon (such as carbon buckyballs and

nanotubes) have been proposed. Despite initial claims of greater than 50 wt%

hydrogen storage, it was later accepted that a realistic number is less than 1 wt%.

"Spillover" Mechanism in Carbon Nanotube Hydrogen Storage

June 7, 2011Schematic of the "spillover" mechanism by which platinum nanoparticles (tan) helps make it possible to store hydrogen (purple) in single-walled carbon nanotubes (gray).

https://news.slac.stanford.edu/image/spillover-mechanism-carbon-nanotube-hydrogen-storage

http://www.greener-industry.org.uk/pages/greener_cars/5_greener_cars_PM2.htm

b) Lithium intercalation

The basic principle of rechargeable lithium batteries is electrochemical intercalation and deintercalation of lithium in both electrodes.An ideal battery has a high-energy capacity, fast charging time and a long cycle time determined by determined by the lithium saturation concentration of the electrode materials.For Li, this is the highest in nanotubes if all the interstitial sites (inter-shell van der Waals spaces, inter-tube channels and inner cores) are accessible for Li intercalation. SWNTs have shown to possess both highly reversible and irreversible capacities. Because of the large observed voltage hysteresis.Li-intercalation in nanotubes is still unsuitable for battery application. This feature can potentially be reduced or eliminated by processing, i.e. cutting, the nanotubes to short segments

Lithium intercalation

• Use of MWNTs in Li-ion batteries

– Laptop notebooks

– Mobile phones

• MWNT powder blended with active materials

– Increases electrical connectivity

– Increases mechanical strength

– Enhances cycle life and rate capability

http://metaefficient.com/computers/the-greenest-notebook-computers-of-2008.html

c) Electrochemical supercapacitors

• Supercapacitors have a high capacitance and potentially applicable in electronic devices.

• Typically, they are comprised two electrodes separated by an insulating material that is ionically conducting in electrochemical devices.

• The capacity of an electrochemical supercapacitor inversely depends on the separation between the charge on the electrode and the counter charge in the electrolyte.

• Because this separation is about a nanometre for nanotubes in electrodes, very large capacities result from the high nanotube surface area accessible to the electrolyte.

• In this way, a large amount of charge injection occurs if only a small voltage is applied.

• This charge injection is used for energy storage in nanotube supercapacitors.

Electrochemical supercapacitors

• Study on packaged cells utilizing forest-grown

SWNTs revealed remarkable performance

– 16 Wh kg-1 energy density

– 10 kW kg-1 power density

– 16 year lifetime forecast

• The only drawback is the high cost of SWNTs

http://www.sciencemag.org/content/339/6119/535/F3.large.jpg

Molecular electronics with CNTs

a) Field emitting devices• If a solid is subjected to a sufficiently high electric field,

electrons near the Fermi level can be extracted from the solid by tunneling through the surface potential barrier.

• This emission current depends on the strength of the local electric field at the emission surface and its work function (which denotes the energy necessary to extract an electron from its highest bounded state into the vacuum level).

• The applied electric field must be very high in order to extract an electron.

• This condition is fulfilled for carbon nanotubes, because their elongated shape ensures a very large field amplification.

Molecular electronics

• FEDs

•CNTFETs

•SETs

Flat screen displays

Saito et al., Jpn. J. Appl. Phys. 37 (1998) L346.

Application

• Field emission

Field emission displays,

electrons coming from

millions of tiny microtips

pass through gates and

light up pixels on a

screen.

This principle is similar

to that of cathode-ray

tubes in television sets.

The difference: Instead

of just one "gun"

spraying electrons

against the inside of the

screens face, there are

as many as 500 million

of them (microtips).

FED Principles

Field Emitting Devices

Single Emitter

Film Emitter

Field Emitting Devices

Single Emitter

Film Emitter

Field Emitting Devices

Single Emitter

Film Emitter

Patterned Film Field Emitters

•Etching and lithography•Conventional CVD•Soft lithography

b) Transistors

• The field-effect transistor – a three-terminal switching device – can be constructed of only one semiconducting SWNT.

• By applying a voltage to a gate electrode, the nanotube can be switched from a conducting to an insulating state.

• Such carbon nanotube transistors can be coupled together, working as a logical switch, which is the basic component of computers.

Transistors• Nanotubes hold the promise of creating novel

devices, such as carbon-based single-electron transistors, that significantly smaller than conventional transistors.

c) Nanoprobes and sensors

• Because of their flexibility, nanotubes can also be used in scanning probe instruments.

• Since MWNT tips are conducting, they can be used in STM and AFM instruments.

• Advantages are the improved resolution in comparison with conventional Si or metal tips and the tips do not suffer from crashes with the surfaces because of their high elasticity.

• However, nanotube vibration, due to their large length, will remain an important issue until shorter nanotubes can be grown controllably.

• Nanotube tips can be modified chemically by attachment of functional groups.

• Because of this, nanotubes can be used as molecular probes, with potential applications in chemistry and biology.Other applications are the following:

• A pair of nanotubes can be used as tweezers to move nanoscalestructures on surfaces.

• Sheets of SWNTs can be used as electromechanical actuators, mimicking the actuator mechanism present in natural muscles.

SWNTs may be used as miniaturised chemical sensors. On exposure toenvironments, which contain NO2, NH3 or O2, the electrical resistancechanges.

Biotechnology

• CNTs have been investigated as components of

– Biosensors

– Medical devices

• Appeal due to compatibility with biomolecules

(DNA/proteins) from two aspects:

– Dimensional

– chemical

http://convergence.ucsb.edu/news/ultra-sensitive-electrical-biosensor-unlocks-potential-instant-diagnostic-devices

Biotechnology

• CNTs enable biological functions like

– Fluoroscence

– Photoacoustic imaging

– Localized heating via near-infrared radiation

http://en.wikipedia.org/wiki/File:FluorescentCells.jpg

Biotechnology (SWNT biosensors)

• Adsorption of target molecules on CNT surface allow for

large changes in

• Electrical impedance

• Optical properties

• Application include

• Gas and toxin detection in industry and

military

• Test strips for hormones and biological

markers (NO2,troponin, estrogen,

progesterone)

http://bme240.eng.uci.edu/students/08s/jentel/Diagnose-hereditary-disease.htm

d) Composite materials• Because of the stiffness of

carbon nanotubes, they are ideal candidates for structural applications.

• For example, they may be used as reinforcements in high strength, low weight, and high performance composites.

• Theoretically, SWNTs could have a Young’s Modulus of 1 TPa.

• MWNTs are weaker because the individual cylinders slide with respect to each other.

• Ropes of SWNTs are also less strong.

• The individual tubes can pull out by shearing and at last the whole rope will break. This happens at stresses far below the tensile strength of individual nanotubes. http://cosmiclog.nbcnews.com/_nv/more/section/archive?year=2

011&month=1&ct=a&pc=25&sp=25

• Nanotubes also sustain large strains in tension without showing signs of fracture. In other directions, nanotubes are highly flexible.

• One of the most important applications of nanotubes based on their properties will be as reinforcements in composite materials.

• However, there have not been many successful experiments that show that nanotubes are better fillers than the traditionally used carbon fibres.

• A main advantage of using nanotubes for structural polymer composites is that nanotube reinforcements will increase the toughness of the composites by absorbing energy during their highly flexible elastic behaviour.

• Other advantages are the low density of the nanotubes, an increased electrical conduction and better performance during compressive load.

• Another possibility, which is an example of a non-structural application, is filling of photoactive polymers with nanotubes. PPV (Poly-p-phenylenevinylene) filled with MWNTs and SWNTs is a composite, which has been used for several experiments. These composites show a large increase in conductivity with only a little loss in photoluminescence and electro-luminescence yields.

• Another benefit is that the composite is more robust than the pure polymer.Of course, nanotube-polymer composites could be used also in other areas.

• For instance, they could be used in the biochemical field as membranes for molecular separations or for osteointegration (growth of bone cells). However, these areas are less explored.

• The most important thing we have to know about nanotubes for efficient use of them as reinforcing fibres is knowledge on how to manipulate the surfaces chemically to enhance interfacial behaviour between the individual nanotubes and the matrix material

e) Templates• Because of the small channels, strong capillary forces exist in

nanotubes.

• These forces are strong enough to hold gases and fluids in nanotubes.

• In this way, it may be possible to fill the cavities of the nanotubes to create nanowires.

Pictures from http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html

The Space Elevator

The Space Elevator

• The Solution: Carbon Nanotubes

– 10x the tensile strengh (30GPa)

• 1 atm = 101.325kPA

• 10-30% fracture strain

• Further Obstacles

– Production of Nanofibers

• Record length 4cm

– Investment Capital: $10 billion

Nanotubes’ excellent strength to weight ratio

creates the potential to build an elevator to space.

Health Hazards

• According to scientists at the National Institute of Standards and Technology, carbon nanotubes shorter than about 200 nanometers readily enter into human lung cells similar to the way asbestos does, and may pose an increased risk to health.

• Carbon nanotubes along with the majority of nanotechnology, are an unexplored matter, and many of the possible health hazards are still unknown.

Conclusions

Biotechnology

Environment

Energy storage

Microelectronics

Coating

Films

http://4.bp.blogspot.com/_JT5P-RJi06I/TF-lJcmNGmI/AAAAAAAAAFQ/ulIUwi-8lMc/s1600/CNT.gif

Water filters

Transistors

Biosensors

screens

Thank You !