Download Introduction to Nanoscience

Introduction to Nanoscience
DSc Irina Hussainova
Department of Materials Engineering
Tallinn University of Technology
Loeng 1
Nanomaterjalid
How small are nanostructures?
The width of a hair
∼ 30 µm = 3 x 10−5 m
The diameter of a carbon
nanotube is yet another 104
times smaller, i.e.
∼ 3 nm=3 x 10−9 m.
How big is nanometer?
NANO
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What is at the bottom?
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Why is nanoscience attracting so
much interest?
• The fundamental properties of matter
change at the nanoscale.
• The properties of atoms and molecules
are not governed by the same physical
laws as larger objects, but by “quantum
mechanics”.
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What’s interesting about
nanoscale?
• The physical and chemical properties of
nanoparticles can be quite different
from those of larger particles of the
same substance.
• Altered properties can include but are
not limited to colour, solubility, material
strength, electrical conductivity,
magnetic behavior, mobility (within the
environment and within the human
body), chemical reactivity and biological
activity.
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What is nanoscale science?
• The study of objects and phenomena at a
very small scale, roughly 1 to 100
nanometers (nm)
• Nanoscience is the study of phenomena and
manipulation of materials at atomic, molecular
and macromolecular scales, where properties
differ significantly from those at a larger
scale;
• and nanotechnologies is the design,
characterisation, production and application of
structures, devices and systems by controlling
shape and size at the nanometer scale.
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Nanoscience
nanoscience
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What is inside?
Matter
Molecule
Molecule
Atom
Atom
Nucleus
Baryon
Quark
(Hadron)
u
10-14m
10-9m
10-10m
10-2m
Condensed
CondensedMatter/Nanoscience/Chemistry
matter/Nano-Science/Chemistry
Atomic Physics
Nuclear
Physics
Nanoscience
10-15m
protons, neutrons,
mesons, etc.
π,Ω,Λ...
<10-19m
top, bottom,
charm, strange,
up, down
Electron
(Lepton)
<10-18m
High Energy Physics
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2001: A Nanotechnology Odyssey
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Historical background
The first planar integrated
circuit invented in 1961.
“No exponential is
forever….but we can
delay “FOREVER””,
G. Moore, 2003
Integrated Circuits:
1970 – 1 000 transistors – 4004 computer;
1975 – 10 000 transistors - 8080
1980 – 100 000 transistors – 286
1985 – 500 000 transistors – 386 Processor
1990 – 1 000 000 transistors – 486
1995 – 10 000 000 transistors – Pentium
2000 - 100 000 000 transistors – Pentium IV
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Where are nanoworld boundaries?
At some point, the laws of physics will
make it impossible to keep downsizing
microelectronics at this rate.
WHY?
Decrease in size results in the particles physical –
chemical properties changing and, consequently,
the properties of nano-materials are changed
dramatically and sometime cordially.
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Size-Dependent Properties
At the nanometer scale, properties become sizedependent.
For example,
(1) Chemical properties – reactivity, catalysis
(2) Thermal properties – melting temperature
(3) Mechanical properties – adhesion, capillary forces
(4) Optical properties – absorption and scattering of light
(5) Electrical properties – tunneling current
(6) Magnetic properties – superparamagnetic effect
New properties enable new applications
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What is a color of gold?
Size-dependent color of gold
Absorption peak broadens and shifts to longer wavelengths.
Reflection, leading to scattering, is weak at small sizes and
increases when > 50 nm.
100 nm gold particles
λabs = 575 nm
Color = purple-pink
20 nm gold particles
λabs = 521 nm
Color = red
1 nm gold particles
λabs = 420 nm
Color = brown-yellow
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In the quantum world, the rules are
different….
The
classical
world
The
quantum
world
Impenetrable
barrier
Tunnel
effect
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Scale Changes Everything
There are enormous scale differences in our
universe!
• At different scales
– Different forces dominate
– Different models better explain phenomena
Four important ways in which nanoscale materials may differ
from macroscale materials
– Gravitational forces become negligible and electromagnetic forces
dominate
– Quantum mechanics is the model used to describe motion and
energy instead of the classical mechanics model
– Greater surface area to volume ratios
– Random molecular motion becomes more important
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Dominance of electromagnetic forces
• Because the mass of nanoscale objects is so small, gravity
becomes negligible
– Gravitational force is a function of
mass and distance and is weak
between (low-mass) nanosized particles
– Electromagnetic force as a function of
charge and distance is not affected by
mass, so it can be very strong even
when we have nanosized particles
– The electromagnetic force between
two protons is 1036 times stronger than
the gravitational force!
Sources: http://www.physics.hku.hk/~nature/CD/regular_e/lectures/images/chap04/newtonlaw.jpg
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http://www.antonine-education.co.uk/Physics_AS/Module_1/Topic_5/em_force.jpg
Extended internal surface
6 x 1m2
Surface to Volume
Ratio Increases
6m2
6 x (1/2m)2x 8
12m2
6 x (1/3m)2x 27
18m2
– A greater amount of a
substance comes in contact
with surrounding material
– This results in better
catalysts, since a greater
proportion of the material is
exposed for potential reaction
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Surface means a lot
Since reactions occur at the interface of
two substances, when a large percentage
of the particles are located on the
surface, we get maximum exposed surface
area, which means maximum reactivity!
So nanosized groups of particles can make
great catalysts.
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Melting temperature
Melting Point (Microscopic Definition)
Temperature at which the atoms, ions, or
molecules in a substance have enough
energy to overcome the intermolecular
forces that hold the them in a “fixed”
position in a solid
At macroscopic length
scales, the melting
temperature of materials is
size-independent.
For example, an ice cube and
a glacier both melt at the
same temperature.
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Thermal properties
Nanocrystal size
decreases
In contact with
3 atoms
surface energy
increases
melting point
decreases
In contact with
7 atoms
Surface atoms require less
energy to move because they are in
contact with fewer atoms of the
substance
Example:
3 nm CdSe nanocrystal melts at 700 K
compared to bulk CdSe at 1678 K
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Quantum Effects
• Large ZnO particles
– Block UV light
– Scatter visible light
– Appear white
• Nanosized ZnO particles
– Block UV light
– So small compared to the
wavelength of visible light
that they don’t scatter it
– Appear clear
The following are among the most important things that quantum
mechanical models can describe (but classical models cannot):
• Discreteness of energy
• The wave-particle duality of light and matter
• Quantum tunneling
• Uncertainty of measurement
Sources: http://www.apt owders.com/images/zno/im_zinc_oxide_particles.jpg
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http://www.abc.net.au/science/news/stories/s1165709.htm ; http://www.4girls.gov/body/sunscreen.jpg
Discreteness of energy
It is the fact that electrons can only exist at discrete energy levels
that prevents them from spiraling into the nucleus, as classical
models predict.
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http://www3.hi.is/~hj/QuantumMechanics/quantum.html#Discreteness
Historical milestones
In 1901, Max Planck published an analysis that succeeded in
reproducing the observed spectrum of light emitted by a glowing
object.
To accomplish this, Planck had to make mathematical assumption
of quantized energy of the oscillators (atoms of the blackbody)
that emit radiation.
Einstein later proposed that it is the electromagnetic radiation
itself that is quantized, and not the energy of radiating atoms.
In 1905, Albert Einstein provided an explanation of the
photoelectric effect, a hitherto troubling experiment that the
wave theory of light seemed incapable of explaining.
He did so by postulating the existence of photons, quanta of
light energy with particulate qualities.
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Basic of quantum mechanics
Einstein postulated that the electrons can receive
energy from electromagnetic field only in discrete
portions (quanta that were called photons):
an amount of energy E related to the frequency, f, of
the light by
E = h f
where h is Planck's constant (6.626 × 10-34 J seconds).
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The Wave-Particle duality of
light and matter
A central concept in quantum physics is the particle-wave duality,
the fact that fundamental objects in the physical world, electrons,
protons, neutrons, photons and others, all have the same dual nature:
they are at the same time
both particles and waves.
In some situations the particle aspect may be the dominant feature,
in other vice versa; but the behavior of any given object can never
be understood fully by ascribing only one of these aspects to it.
Particles have wave-like properties and vice versa:
–Electrons in atoms are standing waves
–Electrons beams can be diffracted
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Schrödinger Equation
The Schrodinger equation is a differential
equation that describes the time evolution of
Kinetic energy of a
free particle
For a particle moving in a potential V(x,t)
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Quantum tunneling
A nanoscopic phenomenon in which a particle
violates the principles of classical mechanics by
penetrating a potential barrier or impedance higher
than the kinetic energy of the particle.
Electron tunneling is attained when a particle with lower
energy is able to exist on the other side of an energy
barrier with higher potential energy.
Go through the wall
Tunneling
is the penetration of an electron into a
classically forbidden region.
A barrier, in terms of quantum tunneling, may
be a form of energy state analogous to a "hill"
or incline in classical mechanics, which
classically suggests that passage through or
over such a barrier would be impossible without
sufficient energy.
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The principal of quantum tunneling
Electrons exhibit wave behavior and
their position is presented by a wave
(probability) function.
The wave function represents a
finite probability of finding an
electron on the other side of the
potential barrier.
Since the electron does not posses
enough kinetic energy to overcome
the potential barrier, the only way
the electron can appear on the
other side is by tunneling through
the barrier.
Uncertainty of measurement
Heisenberg's uncertainty principle
In his work on formulating quantum mechanics,
Werner Heisenberg postulated his uncertainty principle, which
states:
where
∆ here indicates standard deviation, a measure of spread or
uncertainty; x and p are a particle's position and linear
momentum respectively.
ħ is the reduced Planck's constant (Planck's constant/ 2π).
Heisenberg originally explained this as a consequence of the
process of measuring:
Measuring position accurately would disturb momentum and vice32
versa
How can we see nano-objects?
Optical probe characterization techniques
Electron probe characterization techniques
Scanning probe characterization techniques
Spectroscopic characterization methods
Ion – particle characterization techniques
Thermodynamic characterization methods
Bulk engineering characterization methods
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What kinds of interactions?
The electron is
a subatomic
particle
carrying a
negative
electric
charge.
In general, electrons
are able to penetrate
solid materials to a
depth proportional to
E2
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How does electron interact with matter?
Electron – specimen interactions
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The method that is needed is determined by the question to be solved:
Structure
•(High-Resolution) Transmission Electron Microscopy
•Scanning Transmission Electron Microscopy(STEM)
•Electron diffraction (ED)
Composition
•Energy-dispersive X-ray spectroscopy (EDXS)
•Electron Energy Loss Spectroscopy (EELS)
Morphology
•Scanning Electron Microscopy (SEM)
Elemental
mapping
•Electron Spectroscopic Imaging (ESI)
•STEM + X-ray spectroscopy / EELS
•SEM + X-ray spectroscopy
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Scanning probe microscopy
• In 1981, Gerd Binnig and Heinrich Rohrer, two
IBM scientists working in Zurich, Switzerland,
invented the first scanning tunneling
microscope (STM).
• They were awarded the Nobel Prize in physics
for this work, which gave birth to the
development of a new family of microscopes
known as scanning probe microscopes (SPM).
• The STM works best with conducting
materials, but it is also possible to fix organic
molecules on a surface and study their
structures. For example, this technique has
been used in the study of DNA molecules.
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SPM principal
sample
tip
tunneling
electrons
This scanning probe
microscope, of which there
are several varieties, differs
from the optical and electron
microscopes in that
neither light nor electrons is
used to form an image.
The study of surfaces is an important part of physics, with particular
applications in semiconductor physics and microelectronics.
In chemistry, surface reactions also play an important part, for example
in catalysis.
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Principles of AFM
• Monitors the forces of
attraction and repulsion
between a probe and a sample
surface
• The tip is attached to a
cantilever which moves up and
down in response to forces of
attraction or repulsion with the
sample surface
– Movement of the cantilever is
detected by a laser and
photodetector
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STM
AFM
Shading shows
interaction
strength
Carbon
• Carbon is a basic element of life
• Carbon is special because of its
ability to bond to many elements in
many different ways
• It is the sixth most abundant
element in the universe
• The most known types of carbon
materials: diamond; graphite;
fullerenes; and carbon nanotubes
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Carbon materials
2s and 2p electrons available for bonding
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Diamond and
graphite are two
allotropes of
carbon:
pure forms of the
same element
that differ in
structure.
DIAMOND
- chemical bonding is purely covalent
- highly symmetrical unit cell
- extremely hard
- low electrical conductivity
- high thermal conductivity (superior)
- optically transparent
- used as gemstones and industrial
grinding, machining and cutting
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GRAPHITE
• Layered structure with strong
bonding within the planar layers
and weak, van der Waals bonding
between layers
• Easy interplanar cleavage,
applications as a lubricant and for
writing (pencils)
• Good electrical conductor
• Chemically stable even at high
temperatures
• excellent thermal shock
resistance
Applications:
Commonly used as heating elements (in non- oxidizing atmospheres),
metallurgical crucibles, casting molds, electrical contacts, brushes and
resistors, high temperature refractories, welding electrodes, air
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purification systems, etc.
Graphite
Graphite is a layered compound. In each layer,
the carbon atoms are arranged in a hexagonal
lattice with separation of 0.142 nm, and the
distance between planes is 0.335 nm
The acoustic and thermal properties of
graphite are highly anisotropic, since phonons
propagate very quickly along the tightly-bound
planes, but are slower to travel from one plane
to another.
http://en.wikipedia.org/wiki/Graphite
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Graphene
Graphene is a one-atom-thick planar
sheet of sp2-bonded carbon atoms that
are densely packed in a honeycomb
crystal lattice. It can be viewed as an
atomic-scale chicken wire made of
carbon atoms and their bonds
The carbon-carbon bond
length in graphene is about
0.142 nm. Graphene is the
basic structural element of
some carbon allotropes
including graphite, carbon
nanotubes and fullerenes.
Allotropes of carbon
a) diamond
b) graphite
c) lonsdaleite
(hexagonal
diamond)
d) - f) fullerenes
(C60, C540,
C70);
g) amorphous
carbon
h) carbon
nanotube
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Wikipedia
What are fullerenes?
The American pavilion in the
Expo'67 in Montreal was
designed by architect R.
Buckminster Fuller.
Fullerenes were accidentally
discovered 1985 because of
strange results in mass spectra of
evaporated carbon.
Robert Curl, Harry Kroto, and Richard Smalley
discovered C60 and C70. (Nobel prize in 1996.)
Spherical cluster of 60
carbon atoms
Spherical cluster of 70
carbon atoms
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Fullerenes
The fullerene is the reference to a
family of carbon allotropes,
molecules composed entirely of
carbon, in the form of a hollow
sphere, ellipsoid, tube, or plane.
Spherical fullerenes are also called
buckyballs, and
cylindrical ones are called carbon
nanotubes or buckytubes.
Graphene is an example of a planar
fullerene sheet.
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Geometry of fullerenes
• Arranged in pentagons and
hexagons: each molecule
composed of group of carbon
atoms that are bonded to one
another to form both hexagon
(six carbon atoms) and pentagons
(five atoms) geometrical
configuration.
• There are 20 hexagons and 12
pentagons, which are arranged
such that no two pentagons share
a common side =>
• Similar to soccer ball
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Properties of fullerenes
•
Extremely stable molecule
•
Highest tensile strength of any
known structure or element, including
diamonds which have the highest
tensile strength of all known 3D
structures
•
Also has the highest packing density
of all known structures (including
diamonds)
Impenetrable to all elements under
normal circumstances, even to a
helium atom with an energy of 5eV.
•
The diameter of a C60 molecule is
about 1 (nm). The nucleus to nucleus
diameter of a C60 molecule is about
0.7 nm.
•
Resistant to high temperatures.
May possess semiconductor properties.
Superconducting at low temperatures.
Resistant to pressure and reclaim
their original shape even after
experiencing very high pressure.
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Variations of buckyballs
The smallest fullerene C20
26-fullerene graph
dodecahedral graph
The number of fullerenes
C2n grows with increasing
n = 12,13,14..., roughly in
proportion to n9. For
instance, there are 1812
non-isomorphic fullerenes
C60.
60-fullerene
(truncated
icosahedral
graph)
70-fullerene graph
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Fullerene colors
The fullerenes C60 and C70
may be dispersed in water.
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Modification: Endohedral fullerenes
Endohedral fullerenes are fullerenes that have additional atoms,
ions, or clusters enclosed within their inner spheres.
Enclose a metal molecule into the
structure: Fullerenes with
radioactive metals encapsulated
inside the cage.
Example: Radioactive
Holmium inside a
C82 cage.
Ho@C82
Endohedral fullerenes are characterised by the fact that electrons will transfer from
the metal atom to the fullerene cage and that the metal atom takes a position off53
center in the cage.
One of the most fascinating and
unique feature of fullerenes is
that there is spherical empty
space inside the carbon cage.
This hollow space, ranging from
0.4 to 1.0 nm in diameter on going
from C60 to C240 considering the
van der Waals radius of carbon
(0.17 nm), is nanometer-scale void
and the volume may be varied with
the size of fullerene.
Such a characteristic of fullerene
implies intuitively an idea of
stuffing atoms into its empty
space so as to alter the
molecular and solid state
properties of the fullerenes,
resulting in the formation of a
brand-new family — endohedral
fullerenes.
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Carbon nanotubes - discovery
In 1991, Sumio Iijima discovers
multiwalled nanotubes
(MWNT) using
the method of Krätschmer
and Huffman.
In 1993, Donald Bethune
makes single-walled
(SWNT)
nanotubes by adding
transition metals.
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Carbon nanotubes
Carbon nanotubes (CNTs) are allotropes of
carbon with a cylindrical nanostructure.
Most single-walled nanotubes (SWNT)
have a diameter of close to 1 nm, with a
tube length that can be many millions of
times longer.
Structure is a single sheet graphite
rolled into a tube.
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Zigzag SWCN
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Ways to roll a carbon sheet
Sketch of three different SWNT structures as examples for
(a) a zig-zag-type nanotube, (b) an armchair type nanotube,
(c) a helical nanotube
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Structure of SWNT
All carbon atoms are involved
in hexagonal rings only and
are therefore in equivalent
position,
except at the nanotube tips
where 6×5 = 30 atoms at
each tip are involved in
pentagonal rings.
length
> 1000
diameter
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Properties of nanotubes
• Carbon nanotubes are long meshed wires of carbon
• Longest tubes up to 1mm long and few nanometers thick made
by IBM.
Property
Carbon Nanotubes
Comparatively
Size
0.6-1.8 nm in diameter
Si wires at least 50nm
thick
Tensile
Strength
150 GPa
Steel alloys 0.4 GPa
Resilience Bent and straightened without
damage
Modulus
elasticity
1054 GPa
Steel 208 GPa
Epoxy 3.5 GPa
Conductiv Estimated at 109 A/cm2
ity
Cost
Metals fracture when
bent and restraightened
Cu wires burn at 106
A/cm2
$2500/gram by BuckyUSA in
Houston
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Gold is $15/gram
Nanotube key properties
A broad range of electrical, thermal, and structural
properties depending on the tube diameter, length,
and chirality, or twist.
Diameter : 1−30 nm
Length : < 1 mm
Semiconducting like silicon or a 1000 times better
electrical conductors than copper.
Transport heat twice as good as diamond.
Tensile strength 20 times that of steel and still
flexible.
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Overview of NT properties
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Mechanical properties of NT
The tensile strength of SWNT is 20 times that of steel.
It is even higher for MWNT.
http://www.nsbri.org/HumanPhysSpace/focus6/student2.html 63
Scale effect on Young's modulus
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Why nanotubes are the perfect
creations…
• Superior stiffness and strength to all other
materials
• Extraordinary electric properties
• Reported to be thermally stable in a vacuum up to
2800 degrees Centigrade (and we fret over CPU
temps over 50o C)
• Capacity to carry an electric current 1000 times
better than copper wires
• Twice the thermal conductivity of diamonds
• Pressing or stretching nanotubes can change their
electrical properties by changing the quantum
states of the electrons in the carbon bonds
• They are either conducting or semi-conducting
depending on the their structure
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Metal or semiconductor?
Electrons in carbon nanotubes can only
be at the certain energy levels.
A nanotube is metallic if the energy level
that allows delocalized electrons to flow
between atoms throughout nanotube is
right above the energy level used by
electrons attached to atoms.
Analogy: energy bands in atoms.
The differences in conducting properties are caused by the molecular structure that
results in a different band structure and thus a different band gap. The differences in
conductivity can be derived from the graphene sheet properties.
These single crystal structures can exhibit either
semiconducting or metallic behavior depending only on the
diameter and angle of lattice!
66
Perspectives
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