¾ Arc Discharge

CNT Synthesis

™ General synthesis

¾ Laser Ablation

ase blation

¾ Arc Discharge

¾ Chemical Vapor Deposition

vs Multi-wall CNTs

™ Control on CNT morphologies

¾ Smallest CNTs

¾ Y-shape CNTs

¾ Bending CNTs

¾ Bending CNTs

™ Challenges on CNT synthesis

™ Challenges on CNT synthesis

Crystalline Carbon

The structure of the buckyballs is a spherical polyhedron in which there are 32 faces with 20 hexagonal (6-angled) and 12 pentagonal (5-angled) surfaces.

Carbon bonding _SP

orbital - π bonds

„ If carbon forms bonds with only three other atoms (sp²hybridization), the remaining valence electron forms a double-bond, also known as a

π

π

b d d l li d h i h b h d i l l lik

bonds are delocalized. That is, they can be shared over an entire molecule like the valence electrons of metal can be shared by all atoms.

„ Ethane is an example, benzene, and conjugated polymers.

D l li d

π

„ Delocalized

π

„ Fullerenes and nanotubes have each carbon bonded to three nearest neighbor carbons and thus have pi bonds throughout their structure.

Graphene sheets

Graphene Graphite MW-CNT

„ A "two dimensional" object, inasmuch as it extends over long distances in the sp²plane, but is quantum confined in the third direction. Thus a large number of continuous states exists for the electron to occupy in large number of continuous states exists for the electron to occupy in this plane. However, such continuum of states can not exist in the transverse direction given the large energy separation between states

„ This unique 2D dimensionality offers prospects for the design of various structures of unique physical and electrical properties

„ Broadly studied for future electronics

Rolling-up of Graphene Sheet

Indexing scheme of nanotubes

zig-zag

arm-chair

Chiral

Carbon Nanotubes (CNT) History

™In 1960 scientist Roger Bacon of Union Carbide produced structures of graphitic basal layers (e.g.

graphene sheets) rolled into scrolls, supporting his discovery with both diffraction and microscopy data.

™Nearly two decades later, Peter Wiles, Jon

Abrahamson, and Brian Rhoades of the University of Canterbury reported finding hollow fibers on the

d f b di h Th

anode of a carbon arc-discharge apparatus. These fibers consisted of concentric layers of wrapped graphene, spaced by essentially the usual graphitic interlayer separation (c.a. 0.34 nm). y p ( )

•Actual

by electron microscopist Sumio Iijima at NEC- Japan.

™Carbon nanotubes belong to the fullerene family.

Carbon Nanotubes

„ Carbon nanotubes are fullerene tubules with nanometer scale diameters

„ Carbon nanotubes are fullerene tubules with nanometer-scale diameters with properties similar to ideal graphite fibers.

„ Nanotubes have distinct mechanical (stability, stiffness and elasticity), electrical (electron transport) and surface properties

electrical (electron transport), and surface properties.

„ The helicity in the arrangement of carbon hexagons along the lattice surface (defined by symmetry and tube diameter) affects the electronic density of states and gives the tubes its unique electronic character density of states and gives the tubes its unique electronic character (metallic or semiconducting).

„ The topology (the closed geometry of layers in each tube) is responsible for its physical properties

for its physical properties.

Multi-walled nanotubes (MWNT) consist of several

i l h

Single-wall nanotubes

(SWNTs) consist of a coaxial graphene cylinders separated by 0.34 nm, and with inner and outer

di t 1 8

(SWNTs) consist of a single graphene atomic cylinder with diameters ranging

between 1 2 nm diameters 1-8 nm

and 2-25 nm (40- 50nm), respectively.

between 1-2 nm.

Multi-wall CNTs

6 7 (5) 5 6 (2) 6 5 (7)

Nature 1991

6.7nm(5) 5.6 nm (2) 6.5 nm (7)

Single Wall CNTs

Nature 1993.

Mechanism of CNT Synthesis

¾The way in which nanotubes are formed is not exactly

known. The growth mechanism is still a subject of controversy, and more than one mechanism might be operative during the and more than one mechanism might be operative during the formation of CNTs.

¾One of the mechanisms consists of three steps, based on in-

it TEM b ti

situ TEM observations.

™First a precursor to the formation of nanotubes and fullerenes, C

, is formed on the surface of the metal catalyst y particle.

™From this metastable carbide particle, a rodlike carbon is formed rapidly

formed rapidly.

™Than, there is a slow graphitisation of its wall.

Synthesis of MWCNT vs SWCNT by Arc discharge

„SWNTs were first made by adding a catalyst (e.g. Fe,Co) into the carbon plasma in the electric arc charge plasma in the electric arc charge process.

„A few grams of SWNTs with up to 75% purity can be produced using this method.

this method.

If SWNTs are preferable, the anode has to be doped with metal catalyst, such as Fe, Co, Ni, Y or Mo. A lot of elements and mixtures of elements have been tested by various authors¹⁶and it is noted that the results vary a lot, even though they use the same elements. This is not surprising as experimental conditions differ. The quantity and quality of the nanotubes obtained depend on various parameters such as the metal concentration, inert gas pressure, kind of gas, the current and system geometry. Usually the diameter is in the range of 1.2 to 1.4 nm.

movie

a) Synthesis in liquid nitrogen

™ A first, possibly economical route to highly crystalline MWNTs is the arc- discharge method in liquid nitrogen, with this route mass production is also possible. For this option low pressures

d i i t t

and expensive inert gasses are not needed.

™ The content of the MWNTs can be as

™ The content of the MWNTs can be as high as 70 % of the reaction product.

Analysis with Auger-spectroscopy revealed that no nitrogen was revealed that no nitrogen was

incorporated in the MWNTs. There is a strong possibility that SWNTs can be

produced with the same apparatus Schematic drawings of the arc discharge apparatus in liquid nitrogen.

p pp

under different conditions.

discharge apparatus in liquid nitrogen.

b) Magnetic field synthesis

•Synthesis of MWNTs in a magnetic field gives defect-free and high purity MWNTs that can be applied as nanosized electric wires for device fabrication. In this case, the arc

di h h i

discharge synthesis was controlled by a magnetic field around the arc plasma.

Schematic diagram of the synthesis system for MWNTs in a magnetic field.

•Extremely pure graphite rods (purity > 99.999 %) were used (purity > 99.999 %) were used as electrodes. Highly pure MWNTs (purity > 95 %) were obtained without further purification, which disorders

walls of MWNTs. SEM images of MWNTs synthesized with (a) and without (b) the magnetic field.

c) Plasma rotating arc discharge

¾A second possibly economical route to mass production of MWNTs is

¾A second possibly economical route to mass production of MWNTs is synthesis by plasma rotating arc discharge technique. The centrifugal force caused by the rotation generates turbulence and accelerates the carbon vapour perpendicular to the anode. In addition, the rotation distributes the p p p , micro discharges uniformly and generates a stable plasma. Consequently, it increases the plasma volume and raises the plasma temperature.

¾At a rotation speed of 5000 rpm a yield of 60 % was found at a formation temperature of 1025 °C without the use of a catalyst. The yield increases up to 90% after purification if the rotation speed is increased and the

i 11 0 °C

temperature is enlarged to 1150 °C.

Schematic diagram of plasma rotating electrode system.

Synthesis of CNTs: laser ablation

Reasonable quantities of pure (>90%) can be produced.

ƒDirect laser vaporization of transitional metal (e.g. Co-Ni,1%) graphite composite electrode targets is done in helium atmosphere at high temperatures (1200^oC).

temperatures (1200 C).

TEM of SWNTs material

Synthesis of SWNTs: Chemical Vapor Deposition (CVD)

„ Advantages: Size and length of produced NTs can be controlled prior to g g p p deposition by adjusting the size of particles and the amount of carbon vapor.

„ Used to produce NTs commercially.

„ Well known technique from microelectronics.

Multiwall tubes at 500 800°C

„ Multiwall tubes at 500-800°C.

•Fe Ni Co or an alloy of the three catalytic metals is initially deposited on a substrate

Thermal chemical vapor deposition

•Fe, Ni, Co or an alloy of the three catalytic metals is initially deposited on a substrate.

•After the substrate is etched in a diluted HF solution with distilled water, the specimen is placed in a quartz boat.

•The boat is positioned in a CVD reaction furnace, and nanometre-sized catalytic metal particles are formed after an additional etching of the catalytic metal film using NH₃gas at a temperature of 750 to 1050^o C.

•As carbon nanotubes are grown on these fine catalytic metal particles in CVD synthesis, forming these fine catalytic metal particles is the most important process.

forming these fine catalytic metal particles is the most important process.

When growing carbon nanotubes on a Fe catalytic film by thermal CVD, the diameter range of the carbon nanotubes depends on the thickness of the catalytic film. By using a thickness of 13 nm, the diameter distribution lies between 30 and 40 nm.

When a thickness of 27 nm is used, the diameter range is between 100 and 200 nm.

The carbon nanotubes formed are l i ll d

multiwalled.

Th l h d CVD h d

Plasma enhanced chemical vapour deposition

•The plasma enhanced CVD method generates a glow discharge in a chamber or a reaction furnace by a high frequency voltage applied to high frequency voltage applied to both electrodes.

•The catalyst has a strong effect on the nanotube diameter, growth rate, the nanotube diameter, growth rate, wall thickness, morphology and microstructure.

•Ni seems to be the most suitable pure-metal catalyst for the growth of aligned multi-walled carbon nanotubes (MWNTs).

•The diameter of the MWNTs is approximately 15 nm. The highest yield of carbon nanotubes achieved

b t 50% d bt i d t

was about 50% and was obtained at relatively low temperatures (below 330^o C).

•In laser-assisted thermal CVD (LCVD) a

Laser-assisted thermal chemical vapor deposition

( )

medium power, continuous wave CO₂laser, which was perpendicularly directed onto a substrate, pyrolyses sensitised mixtures of Fe(CO) vapour and acetylene in a flow Fe(CO)₅vapour and acetylene in a flow reactor.

•The carbon nanotubes are formed by the catalysing action of the very small iron particles.

•By using a reactant gas mixture of iron

•By using a reactant gas mixture of iron pentacarbonyl vapour, ethylene and acetylene both single- and multi-walled carbon nanotubes are produced.

•Silica is used as substrate.

•The diameters of the SWNTs range from 0.7 Experimental set-up for laser- i d C

The diameters of the SWNTs range from 0.7 to 2.5 nm. The diameter range of the MWNTs is 30 to 80 nm.

assisted CVD.

Long CNT synthesis (1)

Efficient chemical vapor deposition synthesis of single-walled carbon

ca bo

nanotubes, where the activity and lifetime of the lifetime of the catalysts are enhanced by

SCIENCE 2004

water.

Long CNT synthesis (2)

¾Water-stimulated enhanced catalytic activity results in

i th f

massive growth of superdense and vertically aligned nanotube forests with nanotube forests with heights up to 2.5 millimeters that can be easily separated from the easily separated from the catalysts, providing

¾nanotube material with carbon purity above carbon purity above 99.98%. Moreover,

¾patterned, highly ¾The water-assisted synthesis method

SCIENCE 2004

organized intrinsic nanotube structures were successfully f b i t d

¾The water assisted synthesis method addresses many critical problems that currently plague carbon nanotube synthesis.

Method Arc discharge Chemical vapor deposition Laser ablation (vaporization)

A summary of the major production methods and their efficiency

Who Ebbesen and Ajayan, NEC, Japan 1992

Endo, Shinshu University, Nagano, Japan

Smalley, Rice, 1995

Connect two graphite rods to a Place substrate in oven, heat to Blast graphite with intense laser pulses; use the How

power supply, place them a few millimeters apart, and throw the switch. At 100 amps, carbon vaporizes and forms a hot plasma.

600 ^oC, and slowly add a carbon- bearing gas such as methane. As gas decomposes it frees up carbon atoms, which recombine in the form of NTs

laser pulses rather than electricity to generate carbon gas from which the NTs form; try various conditions until hit on one that produces prodigious amounts of SWNTs

Typical yield

30 to 90% 20 to 100 % Up to 70%

SWNT Short tubes with diameters of 0 6 - 1 4 nm

Long tubes with diameters ranging from 0 6-4 nm

Long bundles of tubes (5-20 microns), with individual diameter from 1-2 nm 0.6 - 1.4 nm ranging from 0.6-4 nm individual diameter from 1-2 nm.

M-WNT

Short tubes with inner diameter of 1-3 nm and outer diameter of approximately 10 nm

Long tubes with diameter ranging from 10-240 nm

Not very much interest in this technique, as it is too expensive, but MWNT synthesis is possible.

Pro

Can easily produce SWNT, MWNTs. SWNTs have few structural defects; MWNTs without catalyst, not too expensive, open air synthesis

ibl

Easiest to scale up to industrial production; long length, simple process, SWNT diameter controllable, quite pure

Primarily SWNTs, with good diameter control and few defects. The reaction product is quite pure.

possible Con

Tubes tend to be short with random sizes and directions;

often needs a lot of purification

NTs are usually MWNTs and often riddled with defects

Costly technique, because it requires expensive lasers and high power requirement, but is improving

Smallest CNTs

Single-walled 4Å carbon nanotube arrays

•Smallest carbon

Uniformly sized

Spacing of CNT bundles: 3.4nmÅ one-dimensional quantum wires

Nature 2000

nanotubes possible

•0.4nm in dimension

•MWCNTs

q

By the pyrolysis of tripropylamine molecules in the channels of porous zeolite AlPO4-5 (AFI) single crystals.

Y-shape CNTs

Multi-step growth of CNTs Catalysts initiated syntheses

Nano Lett 2004

Y-shape CNTs

™Vapor catalyst consisting of Fe, Ti, and C.

™Y-junction formation is initiated by catalyst attachment onto the side of

i MWNT f hi h th b hi t b

growing MWNTs, from which the branching nanotubes grow

™Nanotube branching can be induced at will via catalyst composition control

™Controlled branching of nanotubes

™Controlled branching of nanotubes

™Cascading Y-junction and quadruple junctions can be synthesized

Bending of CNT and zigzag-shape CNTs

¾CNTs with multiple p bends

¾Bending of CNTs during growth by

h i h

changing the direction of the electric field

¾Zigzag

¾Zigzag morphologies consisting of 2-4 very sharp and alternating 90° bends

¾Tube diameter throughout growth maintains the same maintains the same

¾Unique three- dimensional structures

Nano Lett 2004

¾Some amorphous carbon coating