CARBON NANOTUBES

MAHESH

Why Carbon nanotubes so interesting ?

• Technological applications – conductive and high-strength composites – energy storage and conversion devices – sensors, field emission displays – nanometer-sized molecular electronic devices

Usually bulk properties dominate At nanoscale  Surface effects dominate.

 Quantum effects come into play.

 Van der Waals forces become important.

 Gravitational effects can be ignored.

Nanocarbon

• Fullerene • Tubes • Cones • Carbon black • Horns • Rods • Foams • Nanodiamonds Introduction

• Nanocarbon fullerene -

”most symmetrical”

tubes cones ”strongest” ”one of the sharpest” carbon black ”large production” • Properties electrical, mechanical, thermal, storage, caging • Applications antenna, composite, writing, field emission, transistor, yarn, microscopy, storage

Allotropic forms of Carbon

Curl, Kroto, Smalley 1985 graphene Iijima 1991 (From R. Smalley ´s web image gallery)

Properties Graphite – sp 2

Bonding

Diamond – sp 3

What Are Carbon Nanotubes?

• CNT can be described as a sheet of graphite rolled into a cylinder • Constructed from hexagonal rings of carbon • Can have one layer or multiple layers • Can have caps at the ends making them look like pills Information retrieved from: http://www.photon.t.u-tokyo.ac.jp/~maruyama/agallery/agallery.html

Carbon Nanotubes

• Single-wall carbon nanotubes are a new form of carbon made by rolling up a single graphite sheet to a narrow but long tube closed at both sides by fullerene like end caps.. • However, their attraction lies not only in the beauty of their molecular structures: through intentional alteration of their physical and chemical properties fullerenes exhibit an extremely wide range of interesting and potentially useful properties.

Salient features of CNTs

• 100 times stronger than Steel and 1/6 th the weight of steel.(Tensile strength value, 63 GPa, exceeds that of any reported value for any type of material. Applications for very light-weight, high-strength cables and composites, where the carbon nanotubes are the load-carrying element.) • Electrical conductivity as high as copper, thermal conductivity as high as diamond. • Avgerage diameter of 1.2 – 1.4 nm (10000 times smaller than a human hair).

Properties of Carbon nanotubes

•the highest elastic module, and mechanical strength that is approximately 200 times stronger than steel.

•novel electronic properties.

•high thermal conductivity.

•excellent chemical and thermal stability.

•promising electron field emission properties.

•high chemical (such as lithium) storage capacity.

• *Depending on how a nanotube is wrapped up from a single plane of graphite (graphene) it may be semiconducting or metallic.

• Their physical and chemical properties, depend on structural parameters such as their width and helicity.

*J. W. Mintmire, B. I. Dunlap, C. T. White,

Phys. Rev.

Lett. 68 , 631 (1992).

R. Saito, M. Fujita, G. Dresselhaus, M. S. Dresselhaus, Appl. Phys. Lett. 60 , 2204 (1992).

Properties

Nanotube

Electrical conductanse depending on helicity

C h



n



a

1  • Current capacity

m



a

2 If 2

n

3 

m



i

, then metallic else semiconductor Carbon nanotube 1 GAmps / cm 2 Copper wire 1 MAmps / cm 2 • Heat transmission Comparable to pure diamond (3320 W / m .

K) • Temperature stability Carbon nanotube 750 o C (in air) Metal wires in microchips 600 – 1000 o C • Caging May change electrical properties → sensor

• If Cn is the chiral vector then it is defined Cn=

n

â1+

m

â • •

Note:

1) OA vector shown in figure is a vector perpendicular to the nanotube axis, ie equator of the nanotube.

2) OB is vector in the direction of the axis.

So by rolling the honeycomb sheet as shown above such that points O and A coincide and point B and B' coincides we get the nanotube structure. This is how a 2-d analysis for carbon nanotube is done.

•

Depending

armchair.

on the value of the chiral vector, carbon nanotubes are classified as chiral, zigzag,

• (n,0) or (0,m) zigzag nanotube* • (n,n) armchair nanotube*

•

(n,m) chiral nanotube*

Types of Carbon nanotubes

Depending on the way the graphene sheet is rolled up

Single-walled Carbon Nanotube d d = 0.4nm - 10nm

L = ?

L

Lattice of covalently bonded carbon atoms

Nanotube Classification

• MWNT – Consist of 2 or more layers of carbon – Tend to form unordered clumps • SWNT – Consist of just one layer of carbon – Greater tendency to align into ordered bundles – Used to test theory of

Nanotube Classification

(10, 10) (10, 5) Information retrieved from: http://www.photon.t.u-tokyo.ac.jp/~maruyama/agallery/agallery.html

MWNT

Synthesis of CarbonNanotubes

 Arc Discharge  Laser Ablation  Chemical Vapor Deposition

Purification of CarbonNanotubes

 Acid treatment  SEC  Annealing

ARC DISCHARGE

• It was a process that was originally used to produce C60 fullerenes.

• It is the most common and arguably the easiest way to produce carbon nanotubes, however it produces a mixture of items such as “soot” and catalytic metals in the end product

The machine

Carbon arc-discharge apparatus at Penn State University

Synthesis: arc discharge

• MWNTs and SWNTs • Batch process • Relatively cheap • Many side-products

Laser Ablation

• In 1995 at Rice University it was reported that synthesis of carbon nanotubes could be accomplished by laser vaporization • Laser ablation is very similar to arc discharge. This is due to the very similar reaction conditions and the fact that both reactions probably occur with the same mechanism.

• Laser vaporization produces a higher yield of SWNT with better properties and with a narrower size distribution than nanotubes produced by arc discharge.

• Laser ablation produced nanotubes that are much purer (up to 90% purity) than those produced by arc discharge.

Laser ablation method to produce SWNTs

The Process of Laser Ablation

• A pulsed or continuous laser is used to vaporize a graphite target placed in an oven at 1200ºC • The oven is filled with an argon gas which is used to keep the pressure at 500 Torr.

• A very hot vapor plume forms, which then expands and cools rapidly.

• As the vaporized species cools, small carbon molecules and atoms condense to from larger clusters.

• From the initial clusters, tubular molecules grow into SWNT. This stops when the catalyst particles(which also condense) become too large, or when the conditions have cooled enough where carbon can no longer diffuse through or over the surface of the catalyst particles. • The SWNT’s formed in this case are bundled together by Van der Waals forces.

CVD Basics

A typical CVD set-up consists of a target substrate held in a quartz tube placed inside of a furnace.

• • • • Typical Parameters: Pressure: 1atm Temperature: 700 ° - 900°C Substrate: Si, mica, quartz, or alumina. Carbon supply: CH4 or CO gas Common catalysts: Ni, Fe, or Co.

Procedure: Catalyst sputtered, layered, or specifically placed onto the substrate.

Carbon containing gas is passed over the substrate inside the furnace.

Growth usually occurs via the “base-growth” mechanism.

Chemical Vapor Deposition

• Specific Types of CVD • • • • • • • Plasma-Enhanced Thermal CVD Alcohol Catalytic CVD Vapor-Phase Growth Aero-gel Supported CVD Laser-Assisted Thermal CVD CoMoCat

Chemical Vapor Deposition

1.

2.

3.

4.

Gas enters chamber at room temperature (cooler than the reaction temperature) Gas is heated as it approaches the substrate Gases then react with the substrate or undergo chemical reaction in the “Reaction Zone” before reacting with the substrate forming the deposited material Gaseous products are then removed from the reaction chamber Information and photo retrieved from: http://www.sandia.gov/1100/CVDwww/cvdinfo.htm

Synthesis Method of CNT

III. Chemical Vapor Deposition (CVD) MWCNT 600-800 ° C 2 H 2 → 2C + H 2 SWCNT 900-1000 ° 2CO → C + CO 2

Synthesis: CVD

•Gas phase deposition •Large scale possible •Relatively cheap •SWNTs / MWNTs •Aligned nanotubes •Patterned substrates

Advantages and Advances in CVD technology

• Increased Length and Purity • Large-scale Productivity • Increased Control • Lower Temperatures