USBAS , G. G. S. I. P. UNIVERSITY, DWARKA NEW DELHI-110075

Carbon Nanotubes

Submitted by:-Kadis prasad

Mtech (Engg. Physics)00740809710

Contents:-

• Introduction• History• The carbon age• Types of Nanotubes• Synthesis and Characterization of nanotube• Properties of Nanotubes• Application• References

Introduction

• Carbon, a group IV element, has two crystalline forms: diamond and graphite. Carbon nanotubes (CNTs) are allotropes of carbon.

• CNTs are members of the fullerene structural family, which also includes the spherical buckyballs.

• Carbon nanotubes are one of the most commonly mentioned building blocks of nanotechnology.

History :-

• 1970: Morinobu Endo-- First carbon filaments of nanometer dimensions, as part of his PhD studies at the University of Orleans in France. He grew carbon fibers about 7 nm in diameter using a vapor-growth technique. Filaments were not recognized as nanotubes and were not studied.

• 1991:Sumio Iijima-- NEC Laboratory in Tsukuba-- used high-resolution transmission electron microscopy to observe carbon nanotubes.

In order to visualize how nanotubes are built up, we start with graphite, which is the most stable form of crystalline carbon.

The carbon age:-

• Carbon-based products – since the invention of synthetic graphite over 100 years ago – are an integral part of our lives. “We don’t make things, we make things better.”

• Atomic carbon is a very short-lived species and, therefore, carbon is stabilized in various multi-atomic structures with different molecular configurations called allotropes. The three relatively well-known allotropes of carbon are graphite and diamond.

Graphite:-

Hexagonal graphite:

I. Graphite has a structure containing layers of atoms arranged at the corners of contiguous hexagons.

II. (not to be confused with hexagonal close packed).

III. The ease with which layers slide against each other is consistent with the much larger distance between carbon atoms in different layers (.335 nm) than between carbon atoms in the same layer (.142 nm).

IV. The lattice constant a = .246 nm

Graphene:-

• The carbon-carbon bond length in graphene is about 0.142 nanometers. Graphene sheets stack to form graphite with an interplanar spacing of 0.335 nm, which means that a stack of three million sheets would be only one millimetre thick. Graphene is the basic structural element of some carbon allotropes including graphite, charcoal, carbon nanotubes and fullerenes.

• The Nobel Prize in Physics for 2010 was awarded to Andre Geim and Konstantin Novoselov "for groundbreaking experiments regarding the two-dimensional material graphene“.

• Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice, the term graphene was coined as a combination of graphite and the suffix -ene

Carbon nanotubes

• CNTs are named on the basis of derived from their size, since the diameter of a nanotube is on the order of a few nanometers, while they can be up to several millimeters in length.

• The nanotubes stem in their quasi-one dimensional (1D) structure and the graphite-like arrangement of the carbon atoms in the shells.

• CNT: Rolling-up a graphene sheet to form a tube

Nanotubes are formed by rolling up a graphene sheet into a cylinder and capping each end with half of a fullerene molecule.

Types of Nanotubes:-

• There are two types of carbon nanotubes:-– Single wall nanotubes (SWNT’s)• Consist of just one layer of carbon• Greater tendency to align into ordered bundles

– Multi wall nanotubes (MWNT’s)• Consist of 2 or more layers of carbon

Types of Nanotubes:-

Rolling of graphene gives 3 basic types of nanotubes:-– Armchair– Zigzag– Chiral• possibilities to form a cylinder with a graphene sheet: the

simplest way of visualizing this is to use a "de Heer abacus“

a) “de Heer abacus”: to realize a (n,m) tube, move n times a1

and m times a2 from the origin to get to point (n,m) and rollup the sheet so that the two points coincide...

•Chirality - twist of the nanotube•Described as the vector R (n, m) R = na1 + ma2

•Armchair vector, R vector, angle • Φ = 0 , armchair nanotube ̊�•0 < ̊� Φ < 30 , chiral ̊�nanotube• Φ > 30 , zigzag nanotube ̊�

Synthesis :-

• Synthesis of carbon nanotubes can be done by different methods:-

1) Arc discharge method2) Laser ablation method3) Chemical vapour deposition method

a) Plasma enhanced chemical vapour depositionb) Thermal chemical vapour depositionc) Vapour phase growth

ARC DISCHARGE METHOD

• CNTs through arc-vaporization of two carbon rods placed end to end, separated by approximately 1mm, in an enclosure that is usually filled with inert gas at low pressure.

• The discharge vaporizes the surface of one of the carbon electrodes, and forms a small rod-shaped deposit on the other electrode.

LASER ABLATION PROCESS• A pulsed laser vaporizes a graphite target in a high temperature reactor while an inert gas is bleed into the chamber. The nanotubes develop on the cooler surfaces of the reactor, as the vaporized carbon condenses.

-AdvantageProduction of high quality carbon nanotubes

•Disadvantages-High temperature process-Grow carbon nanotubes in highly tangled forms with unwanted

carbon and metal impurities.-need to purify-Hard to control

CHEMICAL VAPOUR DEPOSITIONThe silicon wafer placed inside a tube furnace. The furnace is slowly heated until it reaches about 900° C while argon gas flows at 600 ccm through the tube. At this point, the sample is annealed for 10-to15 minutes under an additional l flow of 400 ccm of hydrogen. The argon and hydrogen flows are then replaced by methane (99.99%) at a flow rate of 1,000-to 6,000-cm3 min-1 for some 3 to 5 minutes, and finally, the furnace is cooled under argon. This technique yields some single-walled carbon nanotubes (SWCN) with diameters in the range from 0.8 nm to 3.0 nm, and length of up to tens of micrometers.

Catalytic Chemical Vapour Deposition

Advantages: No purification needed, Direct on substrate growth vertical floating

technique can run continuously

Disadvantages: expensive compared with arc 550-900°CFe/Ni/Co catalyst on support or substrate

Further CVD can classify in different methods:-

Plasma enhanced chemical vapour deposition

• Carbon nanotubes will be grown on the metal particles on the substrate by glow discharge generated from high frequency power.

• A substrate is placed on the grounded electrode. In order to form a uniform film, the reaction gas is supplied from the opposite plate. Catalytic metal, such as Fe, Ni and Co

Thermal chemical vapour deposition

• The substrate is etched in diluted HF solution with distilled water, the specimen is placed in a quartz boat. The boat is positioned in a CVD reaction furnace, and nanometer sized ‐catalytic metal particles are formed after an additional etching of the catalytic metal film using NH3 gas at a temperature of 750 to 1050o C. As carbon nanotubes are grown on these fine catalytic metal particles in CVD synthesis .

Vapour phase growth

• Vapour phase growth is a synthesis method of carbon nanotubes, directly supplying reaction gas and catalytic metal

in the chamber without a substrate.

• Two furnaces are placed in the reaction chamber. Ferrocene is used as catalyst. In the first furnace, vaporization of catalytic carbon is maintained at a relatively low temperature. Fine catalytic particles are formed and when they reach the second furnace, decomposed carbons are absorbed and diffused to the catalytic metal particles. Here, they are synthesized as carbon nanotubes.

• The diameter of the carbon nanotubes by using vapour phase growth are in the range of 2 - 4 nm for SWNTs and between 70 and 100 nm for MWNTs.

Characterization :-

• The experimental techniques used for growth and characterization of carbon nanotubes are :-– Scanning electron microscopy (SEM), – Transmission electron microscopy (TEM), – Energy dispersive X-ray spectroscopy (EDS),– Raman spectroscopy and– X-ray diffraction

Scanning electron microscope

• In SEM, images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern.

• The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition, and other properties such as electrical conductivity.

Transmission electron microscopy

• In TEM, by a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through.

• An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen.

X-ray Diffraction

• In an X-ray diffraction measurement, a crystal is mounted on a goniometer and gradually rotated while being bombarded with X-rays, producing a diffraction pattern of regularly spaced spots known as reflections.

• The two-dimensional images taken at different rotations are converted into a three-dimensional model of the density of electrons within the crystal and using mathematical method like Fourier transform etc…….

Properties :-

• Strength and elasticity-CNTs are the strongest and stiffest materials on earth,

in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp² bonds formed between individual carbon atoms.

• Thermal conductivity and expansion-The thermal conductivity of carbon nanotubes is

dependent on the temperature and the large phonon mean free paths.

• High aspect ratio-High aspect ratio means that a lower loading of CNTs

is needed compared to other conductive additives to achieve the same electrical conductivity.

• Electrical Conductivity-CNTs can be highly conducting, and hence can be said

to be metallic. Their conductivity has been shown to be a function of their chirality, the degree of twist as well as their diameter.

• Electronic properties-The narrow diameter of SWNTs has a strong influence

on its electronic excitations due to its small size compared to the characteristic length scale of low energy electronic excitations. Combined with the particular shape of the electronic band structure of graphene, carbon nanotubes are ideal quantum wires.

-This is due to the very peculiar band structure of graphene and is absent in systems that can be described with usual free electron theory.

• Mechanical properties-While tubular nano-morphology is also observed for

many two-dimensional solids, carbon nanotubes are unique through the particularly strong three folded bonding of the curved graphene sheet, which is stronger than in diamond as revealed by their difference in C−C bond length (0.142 vs. 0.154 nm for graphene and diamond respectively).

-This makes carbon nanotubes – SWNTs or c-MWNTs – particularly stable against deformations. The tensile strength of SWNTs can be 20 times that of steel and has actually been measured equal to 45 GPa.∼

Reactivity

• The chemical reactivity of graphite, fullerenes, and carbon nanotubes exhibits some common features. Like any small object, carbon nanotubes have a large surface with which they can interact with their environment.

• SWNTs are stable up to 750 ◦C in air and up to 1∼ ,500–1,800 ◦C in inert atmosphere beyond which they transform into regular, polyaromatic solids

Applications :-