1. Optical properties of
nanomaterials
PRESENTED BY
ROOPAVATH UDAY KIRAN
M.Tech 1st year
Centre for Nano science and Technology
Code : NST 621
Course instructor : Associate professor Dr. A.Subramania .
2. Brief overview of the talk:
Introduction
Optics
Optical properties
The optical material function
Nanoparticle systems and experimental
optical observables
References
4. About 40 years ago, research on nanoparticles and nanoparticle
matter restarted. The development of new techniques (laser, ESCA,
STM, AFM, SNOM, etc.) and the continuous improvement of
existing techniques (vacuum technology, electron microscopy, etc.)
has allowed new insights into an old subject – the transition from
the atom or molecule to the bulk solid matter
• One famous example is the Lycurgus cup
• Colloidal gold dispersions of Faraday in the nineteenth century.
• Industrial manufacturing of stained glass with colloidal particles
was established in the seventeenth century
• In the nineteenth and twentieth centuries, colloidal nanoparticles
found applications mainly in photography based on silver halide
nanocrystals, as color pigments, and in catalysis
5. Optics
Light
Light is the form of energy detected by the eye, and at
ordinary scales can be treated as a wave. Light waves are part
of the electromagnetic spectrum, ranging continuously from
very long radio waves, with wavelengths of Gm, to high-
energy cosmic rays, with wavelengths of the order of Fm.
A light wave moving to the right can be represented by the
equation:
6. The electromagnetic spectrum; the visible region only occupies a small part of the
whole, from approximately 400–700 nm.
7. The interaction of materials and radiation
The intensity I of an e-m wave, proportional to the
square of its amplitude, is a measure of the energy it
carries. When radiation with intensity Io strikes a
material, a part IR of it is reflected apart IA absorbed,
and a part IT may be transmitted. Conservation of
energy requires that
8.
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10.
11. Metals absorb photons, capturing their
energy by promoting an electron from
the filled part of the conduction band
into a higher, empty level. When the
electron falls back, a photon is
reemitted.
12.
13. The effect of refractive index on the wavelength of light. The
wavelength is compressed in materials with a high refractive index.
14. How does light get through dielectrics?
The reason that radiation of certain wavelengths can enter a
dielectric is that its Fermi level lies at the top of the
valence band, just below a band gap the conduction band
with its vast number of empty levels lies above it. To
excite an electron across the gap requires a photon with an
energy at least as great as the width of the gap, ΔEgap.
Thus radiation with photon energy less than ΔEgap cannot
excite electrons; there are no energy states within the gap
for the electron to be excited into. The radiation sees the
material as transparent, offering no interaction of any sort,
so it goes straight through.
15. Luminescence and phosphors
The emission of radiation by solids at relatively low
temperatures is called luminescence
photoluminescence is light emission brought about by the
absorption of high-energy photons, typically ultraviolet. The
most widely utilised form of photoluminescence is
fluorescence, in which light emission is immediate, taking
place via allowed transitions
Phosphorescence is similar but is typified by the slow
conversion of the exciting energy into light, so that light
emission is delayed, often by considerable lengths of time,
because the light-emitting transitions are forbidden.
17. Reflection from a single
thin film
Monochromatic light travelling
through air falling upon a
homogeneous thin film of an
insulator will be reflected from
the top surface to give a
reflected ray. The light
transmitted into the film will
be repeatedly reflected from
the bottom surface and the
underside of the top surface. At
each reflection, some of the
light will escape to produce
additional reflected and
transmitted rays. Because of
the difference in the paths
taken by the repeatedly
reflected rays, the waves will
interfere with each other.
18. Diffraction
Diffraction occurs when waves interact with objects having a size
similar to the wavelength of the radiation. In general, two regimes
have been explored in most detail: diffraction quite close to the
object which interacts with the light, called Fresnel diffraction,
and diffraction far from the object which interacts with the light,
called Fraunhofer diffraction. The result of diffraction is a set of
bright and dark fringes, due to constructive and destructive
interference, called a diffraction pattern.
Diffraction gratings
Planar diffraction gratings consist of a set of parallel lines with
spacing similar to that of the wavelength of light. A transmission
grating has alternating clear and opaque lines and diffraction
effects are observed in light transmitted by the clear strips. A
reflection grating consists of a set of grooves or blazes and
diffraction effects are observed in the light reflected from the
patterned surface. The effectiveness of a grating is the same
whether light is transmitted through it or reflected from it.
19. Photonic crystals
One common form of a one-dimensional photonic crystal is a
stack of transparent layers of alternating refractive indices
similar to dielectric mirrors. They are called Bragg stacks they
behave like selective mirrors that can pass or reflect specific
wavelengths of the incident light. The simplest model is that of
a transparent material containing regularly- spaced air void.
Two-dimensional photonic band gap crystals can be constructed
from a two-dimensional array of voids or particles in a transparent
medium, and opal is an example of a three-dimensional photonic
band gap crystal. Many animals use natural photonic crystal
structures for the production of vivid colours, including
iridescence in butterfly wings, beetles and feathers.
20. Photoconductivity and photovoltaic solar cells
If radiation of a suitable wavelength falls on a semiconductor, it
will excite electrons across the band gap giving rise to a voltage
and a related increase in conductivity. Solids that behave in this
way are called photovoltaic materials. The magnitude of this
photovoltaic or photoconductive effect is roughly proportional to
the light intensity. It is used in light meters, exposure meters and
automatic shutters in cameras. The first photographic exposure
meters used selenium, cadmium sulphide or silicon. In the case
of selenium, the photovoltage is large enough to be measured
directly and converted to an exposure value. Cadmium sulphide
and silicon need voltage amplification, and these materials require
a power source, usually a battery, to give a reading. In these
materials a voltage is applied across the semiconductor and
illumination levels are measured as an increase in
conductivity.
21. • A p-n junction can act in a similar way to a single piece of
semiconductor. However, the control afforded by the
junction makes the device, a photodiode, far more flexible.
As a result, photodiodes are widely used, especially in solar
cells. A solar cell is a specialist large-area p-n junction with
a depletion region approximately 500 nm thick. (Solar
cells must have a large area, to collect as much sunlight as
possible.) In addition, the normal built-in potential that
exists across the junction, due to the space charge, is
engineered to be high (Figure 13). The junction is not
connected to any external power source. Holes and
electrons produced in the junction region by sunlight are
swept across the depletion region by the high built-in space
charge present, the electrons going from the p to n region
and the holes from the n to p region. This process, called
drift, makes the p region more positive and the n region
more negative, produces a photovoltage and causes a
photocurrent I to flow.
22. Solar cells,
schematic: (a)
sunlight falling on a
p-n junction creates
electron–hole pairs
that are swept into the
external circuit by the
field in the junction
region; (b) operating
cells use a thin
antireflection coating
on the front surface, a
thin n-type layer, a
junction region near
to the front surface
and a reflecting layer
below the cell to
increase efficiency.
23. Dye sensitized solar cells
The method of conversion of sunlight to energy in a conventional
solar cell is quite different to that of most importance on the Earth,
photosynthesis, where the central reactions are oxidation and
reduction. Photoelectrochemical cells, of which dye sensitized
solar cells (DSSCs), also called Gratzel cells, are an important
example, aim to mimic this process. The task of harvesting the
light is left to a sensitizer, which is a dye molecule, and the
carrier transport task is allocated to a semiconductor. Because
the charge separation takes place in the dye, the purity and defect
structure of the semiconductor are not crucial to satisfactory
operation.
24. Dye sensitized solar cell
schematic. (a) Sunlight
absorbed by the dye
liberates an electron into
the semiconductor. The
dye is regenerated by
interaction with an
internal redox couple.
(b) Energy levels in a
cell.
25. The optical properties of quantum wells
• In a quantum well, the electrons and holes occupy electron
and hole sub-bands . When electrons in the upper energy levels
drop to the lower levels in interband transitions, a photon is
emitted. The energy separation of the sub-bands is greater than
the energy gap of bulk material, so the photons will be of
shorter wavelength than those associated with the bulk
semiconductor and are said to be blue-shifted compared with the
bulk.
• Because the dimensions of the quantum well can be changed,
the emission spectrum can be varied or tuned. This feature is
called bandgap engineering. Quantum well structures are widely
used in LEDs and laser diodes to improve device performance.
They do this in a number of ways: by confining electrons and
holes in a limited space, so that recombination is more likely, and
by guiding the output photons by virtue of the differing refractive
indices of the materials. Typical of these device structures is the
single quantum well (SQW) structure used in the first green-
emitting LEDs. A change in the composition of the SQ active
layer allows the colour emission to vary between 450 nm blue to
600 nm yellow.
27. •The optical properties of photoluminescent nanoparticles, which
behave as quantum dots, have been extensively investigated because
they emit fluorescent light that is a precise function of the dimensions
of the quantum dot. For example, CdSe quantum dots of radius 2.9
nm emit at approximately 555 nm, of radius 3.4 nm emit at
approximately 580 nm, and of radius 4.7 nm emit at approximately
625 nm.
•To produce fluorescent light, electrons are excited from the lower
band to the upper band with ultraviolet radiation. The electrons in
higher energy levels subsequently lose energy by non-radiative
transitions to end in the lowest energy level of the upper set. A
photon is then emitted as the electron drops to the topmost energy
level of the lower set
The optical properties of nanoparticles
28. Applications of quantum dots
There are many potential applications for photoluminescent
quantum dots, because they constitute minute but very bright
lamps that can be activated at will by an ultraviolet or blue
light probe. Moreover, the colour output is pure in the sense
that the emission spectrum is narrow. Applications include
• Biological imaging of processes in living cells,
• Production of quantum dot lasers and
• white LEDs.
29. Photoluminescent
colours emitted by
CdS quantum dots.
Quantum dot colours: (a) the change inband structure of a quantum dot as the
diameter falls;(b) fluorescence colours of different diameter dots(schematic).
