2. Carbon nanotubes (CNTs) are
an allotrope of carbon.
Allotrope: each of two or more different physical forms in
which an element can exist.
3.
4. ● CNTs are long, thin cylinders of carbon.
● They can be thought of as a sheet of graphite (a hexagonal
lattice of carbon) rolled into a cylinder.
● Nanotubes have been constructed with length-to-diameter ratio
of up to 132,000,000:1.
CNTs were discovered in 1991 by Sumio Iijima
6. The structure of a carbon nanotube is formed by a layer of
carbon atoms that are bonded together in a hexagonal
(honeycomb) mesh. This one-atom thick layer of carbon is
called graphene, and it is wrapped in the shape of a cylinder
and bonded together to form a carbon nanotube.
Fig.: Graphene
7. Nanotubes can have a single outer wall of carbon, or they
can be made of multiple walls (cylinders inside other
cylinders of carbon). Accordingly they are called:
– Single-walled carbon nanotube
– Multi-walled carbon nanotube
8. Single-walled carbon nanotube structure
● Single-walled carbon nanotubes can be formed in three
different designs: Armchair, Chiral, and Zigzag.
● The design depends on the way the graphene is wrapped
into a cylinder.
9. Multi-walled carbon nanotube structure
● There are two structural models of multi-walled nanotubes:
– Russian Doll model
– Parchment model
10. ● In the Russian Doll model, a carbon nanotube contains
another nanotube inside it (the inner nanotube has a
smaller diameter than the outer nanotube).
11. ● In the Parchment model, a single graphene sheet is rolled
around itself multiple times, resembling a rolled up scroll
of paper.
12. SWCNT vs MWCNT
● MWCNTs have similar properties to SWCNTs.
● The outer walls on MWCNTs can protect the inner carbon
nanotubes from chemical interactions with outside
materials.
● MWCNTs also have a higher tensile strength than SCNTs.
15. Arc Discharge method
● First method successfully used to synthesize CNTs in small
quantities
● Opposing anode and cathode terminals made of 6-mm and 9-
mm graphite rods respectively are placed in an inert
environment (He or Ar at ~500 Torr). A strong current,
typically around 100 A (DC or AC), is passed between the
terminals generating arc-induced plasma that evaporates the
carbon atoms in the graphite. The nanotubes grow from the
surface of these terminals.
● A catalyst can be introduced into the graphite terminal.
● Although MWNTs can be formed without a catalyst, it has
been found that SWNTs can only be formed with the use of a
metal catalyst such as iron or cobalt.
17. Laser Ablation
● First developed in 1995.
● Uses a similar principle as Arc Discharge method.
● Carbon is evaporated at high temperatures from a graphite
target using a powerful and focused laser beam.
● In the most basic laser ablation technique, a 1.25-cm
diameter graphite target is placed in a 2.5-cm diameter, 50-
cm long quartz tube in a furnace controlled at 1200°C and
filled with 99.99% pure argon to a pressure of 500 Torr. A
pulsed Nd:Yag laser beam at 250mJ (10 Hz) is focused
using a circular lens and the beam is swept uniformly across
the graphite target surface. The nanotubes, mixed with
undesired amorphous carbon, are collected on a cooled
substrate at the end of the chamber.
19. Both of these methods have limited potential for scale-up.
Solid graphite must be evaporated at >3000°C to source the
carbon needed, the nanotubes produced are in an entangled
form, and extensive purification is required to remove the
amorphous carbon and fullerenes that are naturally produced in
the process.
20. Chemical Vapor Deposition (CVD)
● CVD has the highest potential for mass production of carbon
nanotubes.
● It can produce bulk amounts of defect-free CNTs at relatively
low temperatures.
● Method:
➔ A substrate material (e.g. alumina, quartz), is cleaned in
preparation for the catalyst deposition.
➔ A porous substrate may be desired, so electrochemical
etching with a hydrofluoric acid/methanol solution may be
performed. Nanotubes can grow at a higher rate on a porous
substrate, suggesting that carbon can diffuse through the
porous substrate layer and feed growing nanotubes.
21. ➔ A catalyst (e.g. iron, nickel) is deposited onto the
substrate by thermal evaporation.
➔ The furnace is raised to a temperature between 500-
1200°C and a hydrocarbon gas such as acetylene,
ethylene, or carbon monoxide is slowly pumped into the
reactor. At these high temperatures carbon dissociates
from the feedstock molecules and diffuses onto the
catalyst.
➔ The atoms arrange themselves into a sheet of nanotubes
on the substrate, combined with impurities such as
amorphous carbon, fullerenes, as well as the catalyst
material.
➔ In most cases these impurities must be removed using a
purification step. An acid treatment followed by
sonification is popular.
23. Strength
● Carbon nanotubes have a higher tensile strength than steel
and Kevlar.
● The strength comes from the sp² bonds between the
individual carbon atoms. This bond is even stronger than the
sp³ bond found in diamond.
● Under high pressure, individual nanotubes can bond
together, trading some sp² bonds for sp³ bonds. This gives
the possibility of producing long nanotube wires.
24. ● Carbon nanotubes are not only strong, they are also elastic.
One can press on the tip of a nanotube and cause it to bend
without damaging to the nanotube, and the nanotube will
return to its original shape when the force is removed.
● A nanotube's elasticity does have a limit, and under very
strong forces, it is possible to permanently deform to shape
of a nanotube.
25. ● A nanotube’s strength can be weakened by defects in the
structure of the nanotube.
● Defects occur from atomic vacancies or a rearrangement of
the carbon bonds. Defects in the structure can cause a small
segment of the nanotube to become weaker, which in turn
causes the tensile strength of the entire nanotube to weaken.
● The tensile strength of a nanotube depends on the strength
of the weakest segment in the tube similar to the way the
strength of a cahin depends on the weakest link in the chain.
26. Hardness
● Standard single-walled carbon nanotubes can withstand a
pressure up to 25 GPa without [plastic/permanent]
deformation. They then undergo a transformation to
superhard phase nanotubes.
● Maximum pressures measured using current experimental
techniques are around 55 Gpa.
● However, these new superhard phase nanotubes collapse at
an even higher, albeit unknown, pressure.
● The bulk modulus of superhard phase nanotubes is 462 to
546 GPa, even higher than that of diamond (420 GPa for
single diamond crystal).
27. Electrical properties
● The structure of a carbon nanotube determines how
conductive the nanotube is.
● When the structure of atoms in a carbon nanotube
minimizes the collisions between conduction electrons and
atoms, a carbon nanotube is highly conductive.
● The strong bonds between carbon atoms also allow carbon
nanotubes to withstand higher electric currents than copper.
● Electron transport occurs only along the axis of the tube.
● Single walled nanotubes can route electrical signals at
speeds up to 10 GHz when used as interconnects on semi-
conducting devices.
● Nanotubes also have a constant resistivity.
28. Thermal Properties
● The 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.
● When compared to copper wires, which are commonly used
as thermal conductors, the carbon nanotubes can transmit
over 15 times the amount of watts per meter per Kelvin.
● The thermal conductivity of carbon nanotubes is dependent
on the temperature of the tubes and the outside environment.
29. Wettability
● The surface wettability of CNT is of importance for its
applications in various settings.
● Although the intrinsic contact angle of graphite is around
90°, the contact angles of most as-synthesized CNT arrays
are over 160°, exhibiting a superhydrophobic property.
● By applying a low voltage as low as 1.3V, the extreme water
repellant surface can be switched into superhydrophilic.
30. Field Emission
● Under the application of strong electric field, tunneling of
electrons from metal tip to vacuum results in field emission
phenomenon.
● Field emission results from the high aspect ratio and small
diameter of CNTs.
● The field emitters are suitable for the application in flat-
panel displays.
● For MWNTs, the field emission properties occur due to the
emission of electrons and light.
● Without applied potential, the luminescence and light
emission occurs through the electron field emission and
visible part of the spectrum, respectively.
31. Aspect Ratio
● One of the exciting properties of CNTs is the high aspect
ratio, inferring that a lower CNT load is required compared
to other conductive additives to achieve similar electrical
conductivity.
● The high aspect ratio of CNTs possesses unique electrical
conductivity in comparison to the conventional additive
materials such as chopped carbon fiber, carbon black, or
stainless steel fiber.
32. Absorbent
● Carbon nanotubes and CNT composites have been emerging
as perspective absorbing materials due to their light weight,
larger flexibility, high mechanical strength and superior
electrical properties.
● CNTs emerge out as ideal candidate for use in gas, air and
water filtration.
● The absorption frequency range of SWNT-polyurethane
composites broaden from 6.4–8.2 (1.8 GHz) to 7.5–10.1
(2.6 GHz) and to 12.0–15.1 GHz (3.1 GHz) (Wang et al.
2013).
● A lot of research has already been carried out for replacing
the activated charcoal with CNTs for certain ultrahigh purity
applications.
34. Nano-Electronics
● One of the most significant potential applications of SWNTs
is believed to be in the domain of nano-electronics. This is
as a result of SWNTs being highly-conductive.
● SWNT ropes are the most conductive carbon fibers known.
● Alternative configurations of a carbon nanotube can result
in the resultant material being semi-conductive like silicon.
● Conductivity in nanotubes is based on the degree of
chirality – i.e. the degree of twist and size of the diameter of
the actual nanotube - which results in a nanotube that is
actually extremely conductive (making it suitable as an
interconnect on an integrated circuit) or non-conductive
(making it suitable as the basis for semi-conductors).
35. Waste water treatment
● CNTs have a very large surface area (e.g., 500 m2 per gram
of nanotube) that gives them a high capacity to retain
pollutants such as water soluble drugs.
● A team at the University of Vienna found that at
concentrations likely to occur in the environment, the tubes
removed 13 tested Polycyclic Aromatic Hydrocarbons
(PAHs) from contaminated water. The results were recently
published in the journal Environmental Science and
Technology.
● However, there are still many health and environmental
questions to answer before such filters find their way into
municipal water treatment plants.
36. Solar cells
● Due to their strong UV/Vis-NIR absorption
characteristics, SWNTs are a potential candidates for
use in solar panels.
● Research has shown that they can provide a sizable
increase in efficiency, even at their current
unoptimized state.
37. Hydrogen storage
● By taking advantage of the capillary effects of the
small carbon nanotubes, it is possible to condense
gases in high density inside single-walled nanotubes.
This allows for gases, most notably hydrogen (H2), to
be stored at high densities without being condensed
into a liquid. Potentially, this storage method could be
used on vehicles in place of gas fuel tanks for a
hydrogen-powered car.
38. Drug delivery
● Systems being used currently for drug delivery include
dendrimers, polymers, and liposomes, but carbon nanotubes
present the opportunity to work with effective structures that
have high drug loading capacities and good cell penetration
qualities.
● These nanotubes function with a larger inner volume to be
used as the drug container, large aspect ratios for numerous
functionalization attachments, and the ability to be readily
taken up by the cell.
39. ● Because of their tube structure, carbon nanotubes can be
made with or without end caps, meaning that without end
caps the inside where the drug is held would be more
accessible.
● Right now with carbon nanotube drug delivery systems,
problems arise like the lack of solubility, clumping
occurrences, and half-life. However, these are all issues that
are currently being addressed and altered for further
advancements in the carbon nanotube field.
● The advantages of carbon nanotubes as nanovectors for drug
delivery remain where cell uptake of these structures was
demonstrated efficiently where the effects were prominent,
showing the particular nanotubes can be less harmful as
nenovehicles for drugs.
40. ● Drug encapsulation has been shown to enhance water
dispersibility, better bioavailability, and reduced
toxicity.
● Encapsulation of molecules also provides a material
storage application as well as protection and
controlled release of loaded molecules.
● All of these result in a good drug delivery basis where
further research and understanding could improve
upon numerous other advancements, like increased
water solubility, decreased toxicity, sustained half-life,
increased cell penetration and uptake, all of which are
currently novel but undeveloped ideas.
41. Biosensors
● A biosensor consists of a receptor that interacts with
the biological analyte, and this interaction is detected
by a transducer that transforms the signal (electrical,
optical, etc.) into a form that can be easily measured.
● Biosensors modified with CNTs have the advantages
of high signal-to-volume ratio, miniaturization from the
CNTs’ size being comparable to many biological
species, function at ambient temperatures, low surface
fouling and high specificity.
42. Diagnostic and imaging tools
● Tools for imaging and tracking the location and movement of
biological objects such as proteins, cells and tissues are critical
in understanding their functionalities and activities in the host
biological system.
● Such tools also provide us the ability to diagnose and possibly
cure various diseases.
● The current techniques use organic fluorophores or quantum
dots (QDs, i.e. luminescent semiconducting nanoparticles).
● However, these organic fluorophores and QDs show serious
photobleaching effect.
● They also have a limited lifetime in aqueous solutions
(approximately 1–2 weeks).
43. ● CNTs make excellent optical sensors compared to these
fluorescent probes due to photobleaching-resistance, non-
quenching, long life-time and high robustness in a
biological environment; thus, they can be exploited as non-
invasive imaging tools for both in vitro and in vivo
applications at cellular or sub-cellular levels.
44. Artificial muscles
● Carbon nano tubes have been used to develop artificial
muscles.
● The artificial muscles are yarns constructed from carbon
nanotubes and infused with paraffin wax.
● They can lift more than 100,000 times their own weight and
generate 85 times more mechanical power than the same
size natural muscle.
45. Space elevator
● A space elevator is a proposed type of space transportation
system.
● The main component would be a cable (also called a tether)
anchored to the surface and extending into space.
● The design would permit vehicles to travel along the cable
from a planetary surface, such as the Earth's, directly into
space or orbit, without the use of large rockets.
● To construct a space elevator on Earth the cable material
would need to be both stronger and lighter (have greater
specific strength) than any known material.
46. ● Carbon nanotubes (CNTs) have
been identified as possibly being
able to meet the specific
strength requirements for an
Earth space elevator.
● Other materials considered have
been boron nitride nanotubes,
and diamond nanothreads which
were first constructed in 2014.
