Carbon Nanotubes

CARBON NANOTUBES• Discovered in 1991 by Sumiyo Iijima, a Japanese

scientist working at the NEC Corporation. • Is a tubular form of carbon with a diameter as small

as 0.4 nm and length from a few nanometers up to a millimeter. 

• Length-to-diameter ratio of a carbon nanotube can be as large as 28,000,000:1, which is unequalled by any other material.

• Carbon exists in several forms; graphite and diamond are the most familiar.

• A CNT is characterized by its Chiral Vector: Ch = n â1 + m â2,

Formation of CNT• Process which is used to form these

Nanotubes, is called Chemical Vapor Deposition.

• Can be made in a standard chemistry laboratory.

• A quartz tube about 1 inch in diameter serves as the growth reactor and is inserted inside a tube furnace.

Scanning electron microscopy of nanofibers covered with nanotubes

• Tube furnace is a standard

heating device for conducting, syntheses and purifications.

• Nanotube grows on a silicon wafer that is placed at a central location inside the quartz tube.

• A thin layer of iron or nickel or cobalt is applied to the silicon wafer to serve as a catalyst to grow the nanotubes.

• In a few minutes, the silicon wafer appears black, indicating that it is covered with nanotubes.

Types of CNTs

1. Single Wall CNT (SWCNT)2. Multiple Wall CNT (MWCNT)

Can be metallic or semiconducting depending on their geometry.

Single Walled CNT’s

• Made by  a single layer of a graphite sheet, cutting it into a small piece of any size, and rolling it.

Single-walled carbon nanotubes

SWCNT….

• Characterized by a set of two integers (n, m) called the Chirality vector.

• When (n-m)/3 is an integer (for example when n is 8 and m is 2), then the nanotube has metallic properties.

• If (n-m)/3 is not an integer, the corresponding nanotube behaves like it is a semiconductor.

• Ability to create tubes of either metallic or semiconductor nature is of great practical importance.

Conti..• Single wall carbon nanotubes exhibit

extraordinary mechanical properties.• Hundred times stronger than steel at one-sixth

of its weight.• Ability to carry current and heat along the

axial direction is extraordinary.• Has the potential to replace copper wires as

conductors. • Scientists and engineers envision all carbon-

based electronics using semiconducting and metallic carbon nanotubes of different values of n and m.

Types of Single-Walled Carbon Nanotubes

n and m can be counted at the end of the tube

Zigzag (n,0)

Types of Single-Walled Carbon Nanotubes

Chiral (n,m)

Armchair (n,n)

Armchair (n,m) = (5,5) = 30

Zig Zag (n,m) = (9,0) = 0

Chiral (n,m) = (10,5)0 < < 30

MWCNT

A tower of multiwalled carbon Nanotubes

• Take multiple layers of a graphite sheet and roll them in the form of a cylinder.

Fig. 2. (a). Band structure of a graphene sheet (top) and the first Brillouin zone (bottom).(b) Band structure of a metallic (3,3) CNT. (c) Band structure of a (4,2) semiconducting CNT. The allowed states in the nanotubes are cuts of the graphene bands indicated by the white lines. If the cut passes through a K point, the CNT is metallic; otherwise, the CNT is semiconducting.

CNT Properties

CNT Properties (cont.)

Nanotubes Growth Methodsa) Arc Discharge b) Laser

Abalation• Involves condensation of C-

atoms generated from evaporation of solid carbon sources. Temperature ~ 3000-4000K, close to melting point of graphite.

• Both produce high-quality SWNTs and MWNTs.

• MWNT: 10’s of m long, very straight & have 5-30nm diameter.

• SWNT: needs metal catalyst (Ni,Co etc.). Produced in form of ropes consisting of 10’s of individual nanotubes close packed in hexagonal crystals.

Nanotubes Growth Methods

c) Chemical Vapor Deposition:

Hydrocarbon + Fe/Co/Ni catalyst 550-750°C CNT

Steps:• Dissociation of hydrocarbon.• Dissolution and saturation

of C atoms in metal nanoparticle.• Precipitation of Carbon.

Choice of catalyst material?

Base Growth Mode or Tip Growth Mode?

• Metal support interactions

Application Electrical1. Field emission in vacuum electronics2. Building block for next generation of

VLSI3. Nano lithography

Energy storage4. Lithium batteries5. Hydrogen storage

Biological6. Bio-sensors7. Functional AFM tips8. DNA sequencing

Challenges & Future..Future applications:• Already in product: CNT tipped AFM• Big hit: CNT field effect transistors based

nano electronics.• Futuristic: CNT based OLED, artificial

muscles…

Challenges:• Manufacture: Important parameters are

hard to control.• Large quantity fabrication process still

missing.• Manipulation of nanotubes.

Conclusion• Their phenomenal mechanical properties,

and unique electronic properties make them both interesting as well as potentially useful in future technologies.

• Significant improvement over current state of electronics can be achieved if controllable growth is achieved.

• Growth conditions play a significant role in deciding the electronic and mechanical properties of CNTs.

• Growth Mechanisms yet to be fully established.

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