CARBON NANOTUBES

DIVYA C.S 2ND M.Sc BPSCBPST, KOCHI

• Carbon nanotubes-nanostructures with large application potential

• Carbon nanotube is a sheet of graphite rolled into tube with bonds at the end of sheet forming the bonds that close the tube

• Allotropes of carbon (graphite ， diamond ， Amorphous carbon and Fullerene ) (cylindrical members of the fullerene structural family)

• With a nanostructure. length-to-diameter ratio of up to 132,000,000:1,which is significantly larger than any other material.

• Extraordinary strength and unique electrical properties, efficient thermal conductors. (limited by their potential toxicity)

• They are less than 100 nanometers in diameter and can be as thin as 1 or 2 nm.

INTRODUCTION

• Carbon nanotubes are fullerene-related structures which consist of graphene cylinders closed at either end with caps containing pentagonal rings.

• Discovered in 1991 by the Japanese electron microscopist Sumio Iijima.

• These are large macromolecules that are unique for their size, shape, and remarkable physical properties.

• Carbon nanotubes have been synthesized for a long time as products from the action of a catalyst over the gaseous species originating from the thermal decomposition of Hydrocarbons.

• The worldwide enthusiasm came unexpectedly in 1991, after the catalyst-free formation of nearly perfect concentric multiwall carbon nanotubes (c-MWNTs ） was reported as by-products of the formation of fullerenes by the electric-arc technique

• Economical aspects are leading the game to a greater and greater extent. According to experts, the world market is predicted to be more than 430M$ in 2004 and estimated to grow to several b $ before 2009.

Nanotubes could be produced in bulk quantities by varying the arc-evaporation conditions.

Nanotube hemispheric

• Nanotubes have a very broad range of electronic, thermal, and structural properties that change depending on the different kinds of nanotube (defined by its diameter, length, and chirality, or twist).

• Besides having a single cylindrical wall (SWNTs), Nanotubes can have multiple walls (MWNTs)--cylinders inside the other cylinders.

NANOTUBE GEOMETRY

There are three unique geometries of carbon nanotubes. The three different geometries are also referred to as flavors. The three flavors are armchair, zig-zag, and chiral [e.g. zig-zag (n, 0); armchair (n, n); and chiral (n, m)]. These flavors can be classified by how the carbon sheet is wrapped into a tube

• The (n,m) nanotube naming scheme can be thought of as a vector (Ch) in an infinite graphene sheet that describes how to "roll up" the graphene sheet to make the nanotube. T denotes the tube axis, and a1 and a2 are the unit vectors of graphene in real space

Structure of Carbon NanotubesSingle-Wall Nanotubes SWNTs

Most single-walled nanotubes (SWNT) have a diameter of close to 1 nanometer, with a tube length that can be many millions of times longer. The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a seamless cylinder. The way the graphene sheet is wrapped is represented by a pair of indices (n,m) called the chiral vector. The integers n and m denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. If m = 0, the nanotubes are called "zigzag". If n = m, the nanotubes are called "armchair". Otherwise, they are called "chiral".

Single-Wall Nanotubes SWNTs

Armchair (n,n)The chiral vector is bent, while the translation vector stays straight

Graphene nanoribbon

The chiral vector is bent, while the translation vector stays straight

Zigzag (n,0) Chiral (n,m) n and m can be counted at the end of the tube

Graphene nanoribbon

Single-Wall Nanotubes SWNTs

Fig. 3.3 Image of two neighboring chiral SWNTs within a SWNT bundle as seen by high resolution scanning tunneling microscopy (by courtesy of Prof. Yazdani, University of Illinois at Urbana, USA)

SWNT Rope

Multiwall Nanotubes MWNT

longitudinal view of a concentric multiwall carbon nanotube (c-MWNT) prepared by electric arc.

•Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphite;•In the Russian Doll , sheets of graphite are arranged in concentric cylinders;•In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper. (3.3 Å);

Nanobud

•carbon nanotubes + fullerenes.

•useful properties of both fullerenes and carbon nanotubes.

•In particular, they have been found to be exceptionally good field emitters.

• In composite materials, the attached fullerene molecules may function as molecular anchors preventing slipping of the nanotubes, thus improving the composite’s mechanical properties

Extreme carbon nanotubes

•The longest carbon nanotubes (18.5 cm long) was reported in 2009. These nanotubes were grown on Si substrates using an improved chemical vapor deposition (CVD) method and represent electrically uniform arrays of single-walled carbon nanotubes

•The thinnest carbon nanotube is armchair (2,2) CNT with a diameter of 3 Å

•The thinnest free standing single-walled carbon nanotube is about 4.3 Å in diameter. Researchers suggested that it can be either (5,1) or (4,2) SWCNT, but exact type of carbon nanotube remains questionable.

Synthesis of Carbon Nanotube1 Laser Ablation – Experimental Devices

- graphite pellet containing the catalyst put in an inert gas filled quartz tube; -oven maintained at a temperature of 1,200 ◦C;-energy of the laser beam focused on the pellet;-vaporize and sublime the graphiteSketch of an early laser vaporization apparatus

The carbon species are there after deposited as soot in different regions: water-cooled copper collector, quartz tube walls.

2 Synthesis with CO2 laser

Fig. 3.10 Sketch of a synthesis reactor with a continuous CO2 laser device

Vaporization of a target at a fixed temperature by a continuous CO2 laser beam (λ = 10.6μm). The power can be varied from 100Wto 1,600 W.

The synthesis yield is controlled by three parameters: the cooling rate of the medium where the active, secondary catalyst particles are formed, the residence time, and the temperature (in the 1,000–2,100K range) at which SWNTs nucleate and grow.

3 Electric-Arc Method – Experimental Devices

Sketch of an electric arc reactor. It consists of a cylinder of about 30 cm in diameter and about 1m in height.

After the triggering of the arc between two electrodes, a plasma is formed consisting of the mixture of carbon vapor, the rare inert gas (helium or argon), and the vapors of catalysts. The vaporization is the consequence of the energy transfer from the arc to the anode made of graphite doped with catalysts.

Properties of Carbon Nanotube

•The strongest and stiffest materials .

•1/50,000th the thickness of a human hair.

•In 2000, a MWCN was tested to have a tensile strength of 63 gigapascals (the

ability to endure tension of 6300 kg on a cable with cross-section of 1 mm2.)

•low density for a solid of 1.3 to 1.4 g·cm−3

•Standard single walled carbon nanotubes can withstand a pressure up to 24GPa

without deformation (hardness)

•Extraordinary electrical conductivity, heat conductivity, and mechanical properties.

• They are probably the best electron field-emitter known, largely due to their high

length-to-diameter ratios

Kinetic properties

Multi-walled nanotubes, multiple concentric nanotubes precisely nested within one another, exhibit a striking telescoping property whereby an inner nanotube core may slide, almost without friction, within its outer nanotube shell thus creating an atomically perfect linear or rotational bearing.

Electrical propertiesSemiconductor

Thermal propertiesThe strength of the atomic bonds in carbon nanotubes allows them to withstand high temperatures. Because of this, carbon nanotubes have been shown to be very good thermal conductors.

Application: Efficient Field Emitters

The electrons are taking out from the tips and sent onto an electron sensitive screen layer. Replacing the glass support and protection of the screen by some polymer-based material will even allow the develop of flexible screens.

Fig. 3.28 (a) Principle of a field-emitter-based screen. (b) SEM image of a nanotube-based emitter system (top view). Round dots are MWNT tips seen through the holes corresponding to the extraction grid.

The first commercialflat TV sets and computers using nanotube-based screens are about to be seen in stores. (Motorola, NEC, NKK, Samsung, Thales, Toshiba, etc.)

Fig. 3.29a,b Demonstration of the ability of SWNTs in detecting molecule traces in inert gases.

(a) Increase in a singleSWNT conductance when 20 ppm of NO2 are added to anargon gas flow.

(b) Same with 1% NH3 added to the argongas flow

Application: Chemical Sensors

• AFM probe tips. Single-walled carbon nanotubes have been attached to the tip ofan AFM probe to make the tip "sharper". . Also, the flexibility of the nanotubeprevents damage to the sample surface and the probe tip if the probe tip happens to "crash" into the surface. They attached carbon nanotubes to AFM probes for the purpose of increased resolution as well as decreased wear on sample and probe tip.

• Flat panel display screens. When a nanotube is put into an electric field, it will emit electrons from the end of the nanotube like a small cannon. If thoseelectrons are allowed to bombard a phosphor screen then an image can becreated. When scientists instead use millions of carbon nanotubes as tiny electron guns, the required dimensions change and the creation of a flat panel display becomes possible. Advertising billboards have already been made and are being used.

OTHER APPLICATIONS

• Nanoscale electronics/nanocomputing Scientists have exploited the mechanical and electrical properties of carbon nanotubes to produce molecular electronic devices. When nanotubes are placed in a grid, the intersections of the nanotubes become bits of information that can be stored non-volatilely. Semiconducting nanotubes also can be used as single molecule transistors.

• Nanothermometer. A carbon nanotube can be partially filled with gallium metal.When the temperature is changed, the gallium metal expands or contracts to fillor empty the carbon nanotube. The gallium level in the carbon nanotube variesalmost linearly with temperature. This new device may find use in certainmicroscopies.• Flash photography and carbon nanotubes. Scientists have discovered that as grownsingle-walled carbon nanotubes can be ignited by holding a conventionalcamera flash a few centimeters away and flashing the sample.

• Actuators/Artificial muscles. An actuator is a device that can induce motion. Inthe case of a carbon nanotube actuator, electrical energy is converted tomechanical energy causing the nanotubes to move. Two small pieces of"buckypaper," paper made from carbon nanotubes, are put on either side of apiece of double-sided tape and attached to either a positive or a negativeelectrode. When current is applied and electrons are pumped into one piece ofbuckypaper and the nanotubes on that side expand causing the tape to curl inone direction. This has been called an artificial muscle, and it can produce 50 to100 times the force of a human muscle the same size. Applications include:robotics, prosthetics.

• Microelectro mechanical devices. Dr. Morinobu Endo at Shinshu University mixed nylon with carbon fibers (not nanotubes) 100-200 nm in diameter creating a nanocomposite materials that could be injected into the world’s smallest gear mold. The carbon fibers have good thermal conductivity properties that cause the nanocomposite material to cool more slowly and evenly allowing for better molding characteristics of the nanocomposite. The "improved" properties of the nanocomposite allow it more time to fill the tiny micron-sized mold than nylon would by itself. The tiny gears currently are being made in collaboration with Seiko for use in watches.

• Hydrogen storage. When oxygen and hydrogen react in a fuel cell, electricity is produced and water is formed as a byproduct. If industry wants to make a hydrogen-oxygen fuel cell, scientists and engineers must find a safe way to store hydrogen gas needed for the fuel cell. Carbon nanotubes may be a viable option. Carbon nanotubes are able to store hydrogen and could provide the safe,