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5.Nanoparticles & quantum dots
• fine particles: cover a range 100 - 2500 nm.
• ultrafine particles, 1 and 100 nm.
• Similar to ultrafine particles, nanoparticles 1100nm.
• Nanoparticles may or may not exhibit size-related
properties that differ significantly from those
observed in fine particles or bulk materials.
• Nanoparticle research is currently an area of
intense scientific interest due to a wide variety of
potential applications in biomedical, optical and
electronic fields.
silica nanoparticles
TEM (a, b, and c) images of prepared mesoporous silica nanoparticles with mean
outer diameter: (a) 20nm, (b) 45nm, and (c) 80nm. SEM (d) image corresponding
to (b). The insets are a high magnification of mesoporous silica particle.
Nanostars of vanadium(IV) oxide
At the small end of the size range, nano-particles are often referred
to as clusters(簇，集群). Spheres(球), rods(棒), fibers(光纤), and cups
are just a few of the shapes that have been grown.
Long history of the Nanoparticle
Although nano-particles are generally
considered an invention of modern
science, they actually have a very long
history. Nano-particles were used by
artisans as far back as the 9th century in
Mesopotamia for generating a glittering
effect on the surface of pots.
Deep Dish from Spain, after 1475[1] Tinglazed earthenware with lustred
decoration, Victoria and Albert Museum,
London
Earthenware cup with lustre
decoration, 10th century, from
Susa, Ira
n
Application of Nanoparticles
nanoparticles of usually yellow gold and gray silicon are red in color;
absorption of solar radiation in photovoltaic cells is much higher in
materials composed of nanoparticles than it is in thin films of continuous
sheets of material – the smaller the particles, the greater the solar
absorption.
the presence of titanium dioxide nanoparticles imparts what we call the
self-cleaning effect, and the size being nanorange, the particles can not
be observed. Zinc oxide particles have been found to have superior UV
blocking properties compared to its bulk substitute.
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).
Application of Nanoparticles
Nanoparticles have also been attached to textile fibers in order
to create smart and functional clothing.
Metal, dielectric, and semiconductor nanoparticles have been
formed, as well as hybrid structures (e.g., core-shell
nanoparticles). Nanoparticles made of semiconducting material
may also be labeled quantum dots if they are small enough
(typically sub 10 nm) that quantization of electronic energy levels
occurs. Such nanoscale particles are used in biomedical
applications as drug carriers or imaging agents.
What are Quantum Dots?
• Quantum dots are semiconductor
nanocrystals that are so small they are
considered dimensionless.
• Quantum dots range from 2-10
nanometers (10-50 atoms)in diameter.
Quantum dot
What is quantum dot?
is a semiconductor whose excitons are
confined in all three spatial dimensions.
Consequently, such materials have
electronic
properties
intermediate
between those of bulk semiconductors
and those of discrete molecules
Researching fields:
have
studied
quantum
dots
in
transistors, solar cells, LEDs, and diode
lasers. They have also investigated
quantum dots as agents for medical
imaging and hope to use them as qubits
WHY?
HOW?
Colloidal quantum dots irradiated with a UV
light. Different sized quantum dots emit
different color light due to quantum
confinement.
Several important
quantum
confinement
structures,
(a)quantum well,
(b) quantum wire, and
(c) quantum dot.
Quantum Dot , Quantum Wires and Quantum Well
Besides confinement in all three dimensions i.e. Quantum
Dot - other quantum confined semiconductors include:
quantum wires, which confine electrons or holes in two
spatial dimensions and allow free propagation in the third.
quantum wells, which confine electrons or holes in one
dimension and allow free propagation in two dimensions.
Colorific Properties
• The height,and energy difference,between
energy levels increases as the size of the
quantum dot decreases.
• Smaller Dot=Higher Energy=Smaller
Wavelength=Blue Color
Color & Quantum Dots
Characteristics of Quantum Dot
Generally, the smaller the size of the crystal, the larger the band
gap, the greater the difference in energy between the highest
valence band and the lowest conduction band becomes, therefore
more energy is needed to excite the dot, and concurrently, more
energy is released when the crystal returns to its resting state.
this equates to higher frequencies of light emitted after
excitation of the dot as the crystal size grows smaller, resulting
in a color shift from red to blue in the light emitted.
In addition to such tuning, a main advantage with quantum dots is
that, because of the high level of control possible over the size of
the crystals produced, it is possible to have very precise control
over the conductive properties of the material
Optical Properties
quantum dots of the same material, but
with different sizes, can emit light of
different colors. The physical reason is the
quantum confinement effect.
The larger the dot, the redder (lower
energy)
its
fluorescence
spectrum.
Conversely, smaller dots emit bluer
(higher energy) light. The coloration is
directly related to the energy levels of the
quantum dot.
 As with any crystalline semiconductor,
a quantum dot's electronic wave functions
extend over the crystal lattice. Similar to a
molecule, a quantum dot has both a
quantized energy spectrum and a
quantized density of electronic states near
the edge of the band gap.
Researchers at Los Alamos National Laboratory
have developed a wireless device that efficiently
produces visible light, through energy transfer
from thin layers of quantum wells to crystals
above the layers.
Applications of Quantum Dots
Applications
Quantum dots are particularly significant for optical applications due to their
high extinction co-efficient , single-electron transistor, implementations of
qubits for quantum information process
Computing
Biology
Photovoltaic device
Light emitting device
Conclusions
• Quantum Dots are a new and innovative
perspective on the traditional semiconductor.
• Quantum Dots can be synthesized to be
essentially any size,and therefore,produce
essentially any wavelength of light.
• There are many possible applications of
Quantum Dots in many different areas of
industry/science.
• The future looks bright and exciting on all the
possible applications of Quantum Dots.