Chapter 2
Graphene
Graphene is made of a single layer of carbon atoms that are bonded together in a repeating pattern of hexagons.
Graphene is one million times thinner than paper; so thin that it is actually considered two dimensional.
How was it discovered?
• Graphene was first studied theoretically in the 1940s. At the time, scientists thought it was physically
impossible for a two dimensional material to exist due to thermal instability when separated, so they did not
pursue isolating graphene.
• For years, researchers tried to separate graphite into single layers but lacked the right tools. Graphene’s
strong carbon bonds keep it stable and prevent it from breaking. Scientists attempted methods like inserting
molecules and scraping but couldn’t get a single layer. By the early 2000s, they managed to isolate
graphene on other materials, but not as a free-standing layer.
• In 2002, Andre Geim at the University of Manchester became interested in graphene. He challenged a PhD
student to polish a hunk of graphite to as few layers as possible. The student was able to reach 1,000 layers,
but could not hit Geim’s goal of 10 to 100 layers. Geim tried a different approach: tape. He applied it to
graphite and peeled it away to create flakes of layered graphene. By refining this method, his team isolated
a single carbon layer and published their findings in 2004.
• ―Science‖ in October 2004. Geim and his colleague Kostya Novoselov received the Nobel Prize in
physics in 2010 for their work.
• In 2009, researchers were able to create a film of graphene that measured 30 inches across.
Graphene production
The quality of graphene is crucial, as defects, impurities, grain boundaries, and structural disorders can have adverse
impact its electronic and optical properties. For electronic applications, large graphene samples are needed, which
can be achieved through chemical vapor deposition (CVD). However, producing high-quality, single-crystalline
graphene with excellent conductivity and optical transparency remains challenging. Additionally, conventional
synthesis methods often involve toxic chemicals, generating hazardous waste and harmful gases. Therefore,
developing eco-friendly methods for graphene production is essential.
Unusual properties of Graphene
Electrical & Thermal Conductivity
• Graphene exhibits extremely high electron mobility, with reported values over 15,000 cm²·V⁻¹·s⁻¹ and a
theoretical limit of 200,000 cm²·V⁻¹·s⁻¹, limited by scattering from acoustic phonons. Its electrons, acting
like massless photons, can travel sub-micrometer distances without scattering, a phenomena known as
ballistic transport. However, the quality of graphene and its substrate, such as silicon dioxide, can limit
mobility to around 40,000 cm²·V⁻¹·s⁻¹.
• Electrons are the particles that make up electricity. So when graphene allows electrons to move quickly, it
is allowing electricity to move quickly. Graphene allows electrons to move 200 times faster than silicon,
with electrical resistivity at 10 nΩ·m (20 °C), compared to copper's 16.78 nΩ·m(200C).
• It is also an excellent heat conductor and works normally at room temperature. The measured thermal
conductivity of graphene is in the range 3000 – 5000 W/mK at room temperature much higher than metals,
for example for Cu i.e. 401 W/mK.
Strength
• Due to the strength of its 0.142 nm-long carbon bonds, graphene is the strongest material ever discovered,
with a tensile strength of 130 gigapascals, as compared to steel (400 MPa) and Kevlar (375.7 MPa).
• Despite its strength, it is incredibly light, weighing just 0.77 milligrams per square meter—about 1,000
times lighter than paper. A single graphene sheet that can cover a football field, would weigh less than 1
gram.
Flexibility
• Graphene is not only incredibly strong but also highly elastic, retaining its shape after strain. Its carbon
bonds are flexible, allowing it to bend and stretch without breaking. In 2007,AFM tests on graphene sheets
(2-8 nm thick) suspended over silicon dioxide cavities revealed spring constants of 1-5 N/m and a Young's
modulus of 0.5 TPa.
• These exceptional properties are based on flawless graphene, which is currently expensive and challenging
to produce, though advancements in production techniques are reducing costs and complexity.
Optical properties
Graphene’s ability to absorb a rather large 2.3% of white light is also a unique and interesting property, especially
considering that it is only 1 atom thick. Adding another layer of graphene increases the amount of white light
absorbed by approximately the same value (2.3%).Graphene absorbs 2.3 percent of the visible light that hits it,
which means you can see through it without having to deal with any glare.
Applications of Graphene
Solar cells: Solar cells rely on semiconductors to absorb sunlight. Semiconductors are made of an element like
silicon. Using graphene would also allow more efficient solar cells that are hundreds of thousands of times thinner
and lighter than those that rely on silicon.
Energy Storage
Energy storage is a key research focus. Batteries store more energy but charge slowly, while capacitors charge
quickly but hold less energy. The goal is to develop supercapacitors or advanced batteries that combine both benefits
without compromise.
Scientists are working to improve lithium-ion batteries, aiming for higher storage, better lifespan, and faster
charging. Graphene-enhanced batteries could be used in applications like electric vehicles, smartphones, and
laptops, while being lighter and smaller. Supercapacitors, which charge quickly and store a lot of energy, are also
being developed. Graphene-based micro-supercapacitors may be used in low-energy devices like smartphones and
portable computers and could be available commercially in the next 5-10 years.
Composite Materials
Graphene is stronger, stiffer, and lighter than carbon fiber, making it a promising material for aircraft. It could
replace steel in aircraft structures, improving fuel efficiency and range by reducing weight. Its electrical conductivity
might also help protect against lightning strikes and monitor stress on the aircraft. Additionally, graphene could be
used to create strong body armor for military personnel and vehicles.
Ultrafiltration
• Graphene stands out for allowing water to pass through while being nearly impervious to liquids and gases,
including small helium molecules. This makes it ideal for ultrafiltration, acting as a one-atom-thick barrier
that can also measure strain, pressure, and other variables.
• Researchers at Columbia University created graphene filters with pore sizes as small as 5nm, much smaller
than current nanoporous membranes (30-40nm). Despite its tiny pores, graphene’s thinness reduces
filtration pressure while being stronger and less brittle than aluminum oxide. This could revolutionize water
filtration and cost-effective biofuel production.
What are the challenges for Graphene?
• Graphene is still in its early stages compared to materials like silicon and indium tin oxide (ITO). For
widespread use, it must be produced in large quantities at a cost equal to or lower than current materials.
Researchers also need to improve its transparency and conductivity for commercial applications.
• As with monolayer graphene, bilayer graphene also has a zero bandgap and thus behaves like a metal.
Graphene has potential for making transistors, but its metallic nature in pure form prevents it from
functioning as a semiconductor, which is essential for transistors. Researchers are working on ways to
modify graphene. One approach is adding impurities, but this can affect its electrical and other properties.
• Electric car batteries and carbon fiber already use cheaper materials like activated carbon and graphite,
making it unlikely graphene will become affordable enough to replace them soon. Graphene has only been
explored for about a decade, silicon has been studied for nearly 200 years
Carbon Nanotubes
What is a Carbon Nanotube?
A Carbon Nanotube is a tube-shaped material, made of carbon, having a diameter measuring on the nanometer scale.
The graphite layer appears somewhat like a rolled-up chicken wire with a continuous unbroken hexagonal mesh and
carbon molecules at the apexes of the hexagons. Carbon Nanotubes typically have diameters ranging from <1 nm up
to 50 nm. Their lengths are typically several microns.
Carbon Nanotubes can be categorized by the way basic graphene sheet is rolled up
• Zig-zag carbon nanotubes:The rolling up is done in parallel to unit vector a1 of graphene lattice.
• Armchair carbon nanotube:The graphene sheet is turned by 300 before rolling up.
• Chiral carbon nanotubes: If the angle of turning the graphene layer before rolling is between 0 to
300,chiral carbon nanotubes obtained.
• This means that the circumference of any chosen carbon nanotube is unequivocally defined by its
perimetral vector_C
• The naming of carbon nanotubes is based on using a pair of no.s that indicate coiling direction and
perimeter of tube (n,m).The diameter of CNT can be determined by length of vector C = na 1 + ma2 (where
a1 and a2 represent the unit vectors) that represents circumference.
Length of chiral vector = Circumference of nanotube (nm) = a.√(𝑛2 + 𝑚 2 + 𝑚𝑛)
= 𝐚.√(𝑛2+𝑚2 +𝑚𝑛)
Diameter of nano tube= 𝜋
For zig - zag nanotubes m = 0, while armchair tubes are defined by n = m .
Carbon Nanotubes can be categorized by their structures as:
Single-wall Nanotubes (SWNT): it consist of solitary graphene sheet which is rolled up to become a hollow
cylinder that envelops the longitudinal axis of tube.
Multi-wall Nanotubes (MWNT): it consist of concentric arrangement of single walled nanotubes with a usually
constant distance of layer. There are examples with just two nanotubes fit one into another called Double-wall
Nanotubes (DWNT).
Production of Single - Walled Carbon Nanotubes
Arc Discharge Methods
Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during arc discharge. This method creats
CNTs through acr vaporization of two carbon rods placed end to end, 1mm apart , in an enclosure that is usually
filled with inert gas like Ar at low pressure. The carbon from the anode to vaporize and condense into carbon
nanotubes on the cathode. This method is considered one of the earliest techniques for producing carbon nanotubes
and is relatively simple and cost effective, although it can produce a mixture of nanotube types and requires further
purification steps.
Laser Ablation
In 1996 CNTs were 1st synthesis using dual-pulsed laser. The laser ablation method for producing carbon nanotubes
involves using a high-powered laser to vaporize a graphite target containing a metal catalyst (like cobalt or nickel) in
a controlled environment, causing the carbon atoms to condense and form nanotubes on cooler surfaces within the
chamber. This method is often used to produce high-quality single-walled carbon nanotubes with controllable
diameters depending on the laser parameters and catalyst used.
Chemical Vapor Deposition (CVD)
In CVD, a substrate covered with metal catalyst like Ni,Co is heated to approx. 700C.The growth started after two
gase passes through chamber, a carrier gas like H or Ar and hydrocarbon gas like methane or acetylene. Chemical
vapor deposition (CVD) is a method for producing carbon nanotubes (CNTs) by reacting a gas-phase reactant with a
substrate.This can involve oxidizing the catalyst's surface with air and then reducing it with hydrogen. A
hydrocarbon gas is continuously fed through the catalyst. The catalyst breaks down the carbon source at a high
temperature. The carbon atoms are generated and deposited on the catalyst or substrate as CNTs.
What are the Potential Applications for Carbon Nanotubes?
Carbon Nanotube Technology can be used for a wide range of new and existing applications:
Conductive plastics
Structural composite materials
Flat-panel displays
Gas storage
Antifouling paint
Micro- and nano-electronics
Radar-absorbing coating
Technical textiles
Ultra-capacitors
Atomic Force Microscope (AFM) tips
Batteries with improved lifetime
Biosensors for harmful gases
Extra strong fibers
Electronic transport in carbon-nanotubes
A metallic carbon nanotube acts like a very thin wire, but it conducts electricity differently from regular wires. Each
nanotube can carry small but useful currents (~μA) in nanoscale devices. In silicon-based circuits, as components
shrink and copper wires get thinner, they cause heating issues. When copper wires reach 100 nm in thickness, they
face higher resistance and uniformity problems, leading to faults and hot spots. Companies like Intel are exploring
carbon nanotubes to replace copper wires.
Material Thermal Conductivity (W/m.k) Electrical Conductivity
Carbon Nanotubes > 3000 10^6 - 10^7
Copper 400 6 x 10^7
In the automotive industry, carbon nanotubes are used to make plastic parts, like fenders, mirror housings, and door
handles, conductive. This helps during the electrostatic painting process. Carbon nanotubes are also used in
thermoplastics to replace older metal parts, reducing the weight of some components by more than 50%. The unique
properties of carbon nanotubes, combined with modern processing technologies, open up new possibilities for
creating innovative materials for automotive applications.
Mechanical properties
• The carbon nanotubes are the strongest materials known. The strength arises from the strong covalent sp2
bonds among the carbon atoms in a graphene sheet, and it is not much different in nanotubes.
• In the marine industry, they are used in composites to improve the strength, durability, and reduce
weight of sailing vessels. They also help prevent waterborne organisms from sticking to surfaces.
• In sports, carbon nanotubes are used in products like bike frames, hockey sticks, tennis rackets, and
skis, improving strength, toughness, and durability. This has increased demand for carbon
nanotube-based products. They also offer architects and builders cost-effective, flexible solutions
for construction projects.
Thermal properties
Having observed all the other remarkable properties of nanotubes, it should be no surprise that they also display a
huge thermal conductivity. The Aeronautic sector is using carbon nanotubes for flame retardant protection of fuel
tanks and exhaust parts
Optical properties
They are promising candidate in optical applications especially in infrared wavelength region for optical
communication. SWCNT absorbs near infrared which covers the wavelength range that passes through biological
tissues without remarkable scattering , heating or damaging the tissue.Hence optical properties of SWCNT can be
used in photo-thermal properties .
