Module 1
Reference Text Book : Nanoscale science and technology by Robert Kelsall
Nano : The Essentials by T.Pradeep
The study of objects and phenomena at a very small scale, roughly 1-100 nanometers (nm)is called as
Nanoscale science or Nanoscience.
Examples
1. A Red blood cell is approximately 7000nm wide.
2. Water Molecule is almost 0.3nm across.
3. Human hair which is ~ 80,000nm wide.
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Nanotechnology can be defined as the design, characterization, production and application of
structures devices and systems by controlling shape and size at a Nano meter Scale.
•
In Nano science building blocks may consist of anywhere from a few hundred atoms to millions of
atoms.
•
Nanometer scale: The length scale ranging from 1–100 nm where corresponding material properties
are size & shape dependent.
•
The properties of Nano Materials are very much different from those at a larger scale.
Two principal factors cause the properties of Nano Materials to differ significantly from other materials.
1. Increased relative surface area.
2. Quantum confinement effect: electrons can only exist at discrete energy levels. Quantum dots are
nanomaterials that display the effect of quantization of energy.
These factors can charge or enhance properties such as reactivity, strength and electrical characteristics.
Characteristics of Nanoscale materials:
• Fiber that is stronger than spider web
•
Metal 100 times stronger than steel and 1/6 of its weight
•
Catalysts that respond more quickly and to more agents
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Plastics that conduct electricity.
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Coatings that are nearly frictionless –(Shipping Industry)
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Materials that change color and transparency on demand.
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Materials that are self repairing, self cleaning, and never need repainting.
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Nanoscale powders that are five times as light as plastic but provide the same radiation protection
as metal.
What’s so good about the nanoscale ?
o Lots of nanoscale substances behave very differently in the world of atoms /molecules
Example : metal copper is transparent on nanoscale, while gold which is normally unreactive,
becomes chemically very active.
o Carbon , which is quiet soft in its normally occurring form (graphite) becomes incredibly hard
when its tightly packed into microscopic arrangement called nanotube.
o In everyday world, gravity is the most important force we encounter. But on nanoscale, gravity is
much less important than the electromagnetic force between atoms and molecules.
Industry
Aerospace
Materials/Opportunity
Nanomaterials
and
nanocoatings are being
used for the bodies of
aircraft and in aerospace
components.
Automotive
Nanocrystalline
silicon
nitride and silicon carbide
have been used in springs,
ball bearings, and other
automotive components.
Nanocrystalline ceramic Zirconia and alumina liners have been used to
liners for engine cylinders. retain heat in cylinders and improve the efficiency
of combustion.
The latest generation of These nano-engineered plates can store more
batteries
use
nano- energy than conventional plates.
engineered aerogels for
separator plates.
Aerogels for insulation The structure of aerogels makes them excellent
and “smart windows” that insulating materials.
darken when the sun is
bright and get more
transparent in dimmer
light.
Batteries
Building materials
Advantages of Nanomaterials
It has been claimed that the use of nanomaterials
can increase the fatigue strength of aerospace
materials by as much as 300 percent.
Nanomaterials may also be considerably lighter,
reducing the fuel required—a critical issue in
today’s highly unprofitable airline industry.
Nanomaterials may be especially useful for space
vehicles that must meet extreme conditions—
especially with regard to heat.
These
materials
demonstrate
impressive
mechanical and chemical properties that contribute
to both the manufacturability and longevity of
these components.
Machine tools
Televisions and
monitors
Regenerative
Medicine
Nanocrystalline
metal
carbide materials for
cutting
and
drilling.
Nanoparticles
for
improved ceramics.
Nanomaterials used to
improve the resolution of
CRTs. Carbon Nanotubes
used to create CRT-like
field emission displays
(FEDs). Organic polymerbased
flexible displays.
Nanoengineered gels and
other materials are used to
replace lost tissue or to
provide structure for the
regeneration of natural
tissue.
Current
applications include bone
replacement
and
nanostructures that help in
the re-growth of nerves.
Nanocrystalline metal carbide materials provide
harder, longer-lasting materials for drills and
cutting machinery. Conventional ceramics can be
made less brittle and easier to work with through
the addition of nanoparticles.
Various zinc, cadmium, and lead nanomaterials
have been proposed to produce smaller phosphors
/pixels in CRT displays and hence better
resolution. Carbon nanotubes make excellent
emitters and prototypes of FEDs have been built
that combine the visual quality of a CRT, yet may
be only one inch thick.
Nanomaterials are constructed at the size level of
the human cell, which means that they are
incorporated better into the body than other
alternatives. For example, tissues can easily bond
with nanoporous bone substitutes and nerve
healing is improved when grown around
nanostructures. Most nanomaterials used in such
applications are also very strong, which has
obvious advantages. However, there is some worry
that the very fact that nanomaterials integrate well
into natural body structures may cause body
malfunctions, or even new diseases.
Classifications of nanostructured materials
A reduction in the spatial dimension or confinement of particles or quasi particles in a particular
crystallographic direction within a structure generally leads to changes in physical properties of the system
in that direction. Hence one classification of nanostructured materials and systems essentially depends on
the number of dimensions which lie within the nanometre range are
(a) systems confined in three dimensions( 0 Dimensional Nanostructure) – Quantum Dot, nanoparticles,
nanocrystals
Number of nanosized dimensions-3 Number of bulk dimensions- 0
Example – nanocrystals of Gold
(b) systems confined in two dimensions ( 1 Dimensional Nanostructure) – Nanowires, Nanotubes, Quantum
wire, nano rods.
Number of nanosized dimensions-2 Number of bulk dimensions- 1
Example – Carbon nanotube
(c) systems confined in one dimension ( 2 Dimensional Nanostructure)- Quantum well, nanolayers ,nano
films
Number of nanosized dimensions-1 Number of bulk dimensions- 2
Example – SiGe epitaxial layer on Si substrate.
(c) systems confined in zero dimension ( 3 Dimensional Nanostructure)-Bulk materials
Number of nanosized dimensions-0 Number of bulk dimensions- 3
Example – Bulk single crystal of silicon
For various quantum structures, density of states variations with energy is shown in above figure.
The electronic structure of materials is strongly related to the nature of the material. In a three dimensional
object of large size, the electronic structure is not restricted by the dimension of the material. The
wavelength of electrons is much smaller than the typical length of the material.
When the electronic motion is confined in one dimension, and it is free in the other two dimensions, it
results in the creation of ‘quantum wells’ or ‘quantum films’. The quantum well notation implies that the
electrons feel a potential well as they are trapped in the film. Here the density of states shows a step-like
behaviour.
In the case of a one-dimensional system, i.e. when the electrons are free to move only in one direction, we
get a situation wherein the density of states shows a Lorenzian line shape. Such a situation can be seen in
carbon nanotubes.
If the electrons are confined to a point, we get a zero-dimensional system, wherein the electrons are not
free to move at all. Here we get states which are molecular in nature. The situation is schematically depicted
in Fig. 7.1. What is shown in Fig. 7.1 is that while the density of states is smoothly varying in bulk materials,
it shows discontinuities in confined systems. This will lead to steps in two-dimensional confinement,
singularities in one-dimensional confinement and discrete lines in zero-dimensional confinement.
Nano particles:
• Nanoparticles are particles between 1 and 100 nanometers in size.
•
They exhibit three-dimensional confinement. This structure does not permit free particle motion in
any dimension.
•
Nanoparticles may exist as amorphous or crystalline structure; ie., they may have a random
arrangement of the constituent atoms or molecules (amorphous material) or the individual atomic
or molecular units may be ordered into a regular, periodic crystalline structure.
•
If crystalline, each nanoparticle may be either a single crystal or polycrystalline; ie., it is composed
of a number of different crystalline regions or grains of differing crystallographic orientations (i.e.,
polycrystalline) giving rise to the presence of associated grain boundaries within the nanoparticle.
Properties of nanoparticles:
A bulk material should have constant physical properties regardless of its size, but at the nano-scale sizedependent properties are often observed. Thus, the properties of materials change as their size approaches
the nano-scale and as the percentage of atoms at the surface of a material becomes significant.
An example of the change in physical and chemical properties between gold and gold nanoparticles:
Properties
Gold (Au)
Gold Nano
Color
Yellow
Red
Electrical
Conductive
Loses conductivity
Conductivity
at 1-3 nm
Magnetism
Non-magnetic
Becomes magnetic
at 3 nm
Chemical
Chemically inert
Explosive
and
Reactivity
catalytic
Physical and chemical properties of nanoparticles that may change at the nano-scale include:
• Color: Nanoparticles of yellow gold and grey silicon are red in color.
• Melting temperature: Gold nanoparticles melt at much lower temperatures (~300 °C for 2.5 nm
size) than the gold slabs (1064 °C).
• Optical Absorption: Absorption of solar radiation is much higher in materials composed of
nanoparticles than it is in thin films of continuous sheets of material. In both solar PV and solar
thermal applications, controlling the size, shape, and material of the particles, it is possible to
control solar absorption. Zinc oxide particles have been found to have superior UV blocking
properties compared to its bulk substitute. This is one of the reasons why it is often used in the
preparation of sunscreen lotions, and is completely photostable.
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•
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Chemical reactivity: Suspensions of nanoparticles are possible since the interaction of the particle
surface with the solvent is strong enough to overcome density differences, which otherwise usually
result in a material either sinking or floating in a liquid.
Electrical conductivity: Conductivity of bulk Gold disappears when the particle is reduced to nano.
Magnetism: Super-paramagnetism is a form of magnetism, which appears in small ferromagnetic
(or) ferrimagnetic nanoparticles. Ferromagnetic materials smaller than 10 nm can switch their
magnetisation direction using room temperature thermal energy, thus making them unsuitable for
memory storage. (Super Paramagnetic materials are magnetic material with permeability several
times greater than that of ferromagnetic materials).
Mechanical strength: Clay nanoparticles when incorporated into polymer matrices increase
reinforcement, leading to stronger plastics, verifiable by a higher glass transition temperature and
other mechanical property tests. These nanoparticles are hard, and impart their properties to the
polymer (plastic).
Quantum dots:
Quantum dots are extremely small semiconductor structures, usually ranging from 2- 10 nanometers (1050 atoms) in diameter.
• A quantum dot is a structure that is sufficiently small in all directions that electrons contained on it
have no freedom to move in a classical sense and are forced to exhibit quantum characteristics,
occupying discrete energy states just as they would in an atom. Indeed, quantum dots have
sometimes been referred to as artificial atoms.
•
The energy band gap increases with a decrease in size of the quantum dot.
•
QDs obey quantum mechanical principle of quantum confinement.
•
They exhibit energy band gap that determines required wavelength of radiation absorption and
emission spectra.
•
Requisite absorption and resultant emission wavelengths dependent on dot size. The size, shape
and number of electrons can be precisely controlled. In some quantum dots even if one electron
leaves the structure there is a significant change leaves the structure there is a significant change in
the properties.
Applications in Medical imaging and diagnostics:
a. QDs can be used as tool for monitoring cancerous cells and providing a means to better understand its
evolution.
b. QDs are much efficient than other optical imaging probes such as organic dyes, allowing them to track
cell processes for longer periods of time.
c. Quantum dots using its large surface area is well suited for certain kinds of drug delivery.
Quantum dot LEDs:
a. Used to produce inexpensive, industrial quality white light.
b. Produce white light by intermixing red, green and blue emitting dots homogenously than the traditional
LEDs.
c. Quantum dot LED’s are extremely energy efficient. They use only a few watts, while a regular
incandescent lamp uses 30 or more watts for the same amount of light.
Solar cells and photovoltaics:
A cost-effective third-generation solar cell at better power conversion efficiency is possible by utilizing
QDs compared to highly expensive traditional solar cells.
Nano wires:
Systems confined in two dimensions, or quasi-1D systems, include nanowires, nano rods, nanofilaments
and nanotubes: again these could either be amorphous, single-crystalline or polycrystalline (with nanometer
-sized grains). The term ‘nano-ropes’ is often employed to describe bundles of nanowires or nanotubes.
Types of Nanowires:
• Metallic - Made from Nickel, Platinum or Gold
• Semi-conducting - Comprises of Silicon, Indium phosphide or Gallium Nitride
• Insulating - Silicon Dioxide or Titanium dioxide
• Molecular – Involves repeating organic or inorganic molecular units
Ultra-thin films:
Systems confined in one dimension, or quasi-2D systems, include discs or platelets, ultrathin films on a
surface and multilayered materials; the films themselves could be amorphous, single-crystalline or nano
crystalline.
A solvent that contains a molecular material that when applied to a surface, chemically aligns itself to form
the strongest possible bond and appear as a film. If its thickness is in nanoscale, it is called as Ultra-thin
film.
Properties
Thin films are different from bulk materials and they are:
o not fully dense
o under stress
o different defect structures from bulk
o quasi - two dimensional (very thin films)
o strongly influenced by surface and interface effects
o This will change electrical, magnetic, optical, thermal, and mechanical properties.
Nanoclusters
They are systems of bound atoms or molecules, existing as an intermediate form of matter, with properties
that lie between those of atoms (or molecules) and bulk materials. Depending on the kind of constituent
units, they are called either atomic or molecular clusters.
clusters are bound by forces which may be metallic, covalent, ionic, hydrogen bonded or van der Waals in
character and can contain up to a few thousand atoms.
Carbon
Allotropes :Graphite, fullerenes, Diamond
Graphite has a layered structure with each layer, called graphite sheet formed from hexagons of carbon
atom bound together by sp2 hybrid bonds that make 1200 angle with each other. They are soft, electrical
conductor, high anisotropic thermal conductivity and chemical reactive.
Fullerenes – sp2 hybridized contain 12 pentagons – Nanoparticles
Diamond- consists of carbon atoms that are tetrahedrally bonded to each other through sp3 hybrid bonds
that form a 3d network. They are ultra hard, wide band gap distance, has good thermal conductivity,
chemically inert.
Carbon Nanotubes
A single sheet of graphite is called graphene. A carbon nanotube is produced by curling a graphene
sheet. Just like a sheet of paper, planar carbon sheets can also curl in a number of ways. This makes the
carbon sheet helical around the tube axis.
The structure of a cylindrical tube is described in terms of a tubule diameter d and a chiral angle θ as shown
in Fig4.1. The chiral vector C = na1 + ma2 along with the two parameters d and θ define the tube. The unit
vectors a1 and a2 define the graphene sheet. In a planar sheet of graphene (a single sheet of graphite),
carbon atoms are arranged in a hexagonal structure, with each atom being connected to three neighbors. In
Fig. 4.2, each vertex corresponds to a carbon atom. The vector C connects two crystallographically
equivalent points. The angle θ is with respect to the zigzag axis, and it is 30° for the armchair tube. If we
roll over from one end of the tube to the other end, we obtain a cylinder. The rolling can be done in several
ways. The bond angles of the hexagons are not distorted while making the cylinder. The properties of the
tube get modified depending on the chiral angle θ and the diameter d.
Structure of Single walled carbon nanotubes
There are numerous ways in which the tubes can be rolled. While the (n, 0) tubes are called ‘zigzag tubes’
where θ is zero, the (n, n) tubes are called ‘armchair tubes’ where θ is 30°. These two types of tubes have
high symmetry and a plane of symmetry perpendicular to the tube axis. Any other tube (n,m) is a chiral
tube, which can be either left-handed or right-handed. The tubes will be optically active to circularly
polarized light, circulating along the tube axis.
The two important tube parameters, d and θ can be found from n and m.
where rC-C is the C-C distance of the graphene layer (1.421 Å) and C is the length of the chiral vector. Due
to the symmetry of the graphene layer, several tubes, although having different (n,m) notations are indeed
the same. A tube of (0, n) is the same as (n, 0). The tube diameter will increase with an increase in n and
m.
The integers n and m represents the number of unit vectors along 2 directions in honey comb crystal lattice
of graphene.
m = 0 - Zig Zag nano tube (n, o) θ = 0 0
n = m - Arm chair nano tube (n, n) θ = 30 0
n ≠ m – Chiral nanotube
Structure of multi walled carbon nanotubes
Russian Doll model- Sheets of graphite arranged in concentric cylinders.
Parchment model- Single sheet of graphite rolled in around itself resembling a scroll of parchment or rolled
up newspaper.
Multiple Quantum well and Superlattices
A superlattices is a periodic structure of layers, typically thickness of few nanometers of two or more
materials in on direction. Consider a structure consisting of many quantum wells, with each well separated
from neighboring wells by a barrier. If the barriers are sufficiently thick, carriers located in different wells
are essentially isolated and the structure behaves identically to a single well. In this case the structure is
known as a multiple quantum well.
However, if the thickness of the barriers is reduced, carriers in neighboring wells may interact via the part
of their wavefunctions which penetrates into the barriers. Significant interaction will occur if the
wavefunctions (which decay exponentially into the barriers) overlap strongly, requiring relatively thin
barriers. For strong interaction, originally identical states in different wells couple together to form a
miniband of closely spaced states, a system known as a superlattice. Figure 3.12(a) and (b) show the
electronic structures of a multiple quantum well and superlattice, respectively. Figure 3.12(c) demonstrates
how the widths of the minibands increase as the barrier width decreases, allowing an increasing interaction
between states in different wells. The formation of a superlattice allows carrier motion to occur normal to
the plane of the quantum wells, with the resultant structure exhibiting properties intermediate between a
bulk semiconductor (3D) and a true quantum well (2D). By extension it is possible to visualise a structure
consisting of repeated quantum wires (exhibiting quasi 1D to 2D behaviour) or repeated quantum dots
(exhibiting quasi 0D to 1D behaviour).
Nanocrystalline materials
A nanocrystalline (NC) material is a polycrystalline material with a crystallite size of only a few
nanometers. These materials fill the gap between amorphous materials without any long range order and
conventional coarse-grained materials. nanocrystalline material is commonly defined as a crystallite (grain)
size below 100 nm. Grain sizes from 100–500 nm are typically considered "ultrafine" grains.
Nanocrystalline materials are single-phase or multi-phase polycrystals, the crystal
size of which is of the order of a few (typically 1 to 100) nanometers in at least one
dimension. Thus, they can be basically equiaxed in nature and will be termed nanostructure crystallites
(three-dimensional [3-D] nanostructures), or they can consist
of a lamellar structure, and will be termed a layered nanostructure (one-dimensional [l-D] nanostructure),
or they can be filamentary in nature (two-dimensional [2-D]
nanostructure)
Effects of nanometer length scale :Refer Nanoscale science and technology by Robert Kelsall- Page no 19-24