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HI FRIENDS, MYSELF AMIT MAJUMDAR STUDENT OF FINAL YEAR ELECTRONICS & COMMUNICATION ENGG. I HAVE RESEARCHED ABOUT THE QUANTUM DOTS & ITS APPLICATIONS & I THINK THIS MATERIAL WILL BE BENIFICAL TO YOU GAYS. THANKS.
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CONTENTS
1. Introduction………………………………………………………………………2
2. Quantum confinement in semiconductors………………………………………..6
3. Making quantum dots ……………………………………………………………6
o 3.1 Colloidal synthesis…………………………………………………..9
o 3.2 Fabrication…………………………………………………………..10
o 3.3 Viral assembly………………………………………………………11
o 3.4 Electrochemical assembly…………………………………………..12
o 3.5 Bulk-manufacturing of quantum dots………………………………12
o 3.6 Cadmium-free quantum dots - “CFQD”……………………………13
4. Optical properties………………………………………………………………..13
5. Applications …………………………………………………………………….14
o 5.1 Computing………………………………………………………….16
o 5.2 Biology……………………………………………………………..18
o 5.3 Photovoltaic devices………………………………………………..21
o 5.4 Light emitting devices……………………………………………...25
o 5.5 Quantum dot laser …………………………………………………26
o 5.6 Life sciences……………………………………………………….28
o 5.7 quantum dot switches ……………………………………………..33
o 5.8 other applications …………………………………………………34
6. Quantum computer …………………………………………………………...35
7. Conclusion…………………………………………………………………….39
8. Reference……………………………………………………………………...40
2
LIST OF FIGURE
1. Figure 1……………………..………………………..3
2. Figure 2……………………..………………………..4
3. Figure 3……………………………………………..6
4. Figure 4……………………………………………..7
5. Figure 5……………………………………………..8
6. Figure 6…………………………………………….16
7. Figure 7…………………………………………….17
8. Figure 8…………………………………………….18
9. Figure 9…………………………………………….21
10. Figure 10…………………………………………..27
11. Figure 11…………………………………………..29
12. Figure 12…………………………………………..30
13. Figure 13…………………………………………..32
14. Figure 14…………………………………………..33
15. Figure 15…………………………………………..34
16. Figure 16…………………………………………..35
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1. INTRODUCTION:-
Modern electronics, as well as many other fields of science, rely on the use of
semi-conductors. Quantum dots (QDs) are particles that hold a droplet of free electrons
which simulates “the ultimate miniaturized semiconductor.” Any material that can
conduct electricity better than an insulator but not as well as a conductor is considered a
semi-conductor. What makes semi-conductors so important is that their unique structure
allows different semi-conductors to carry current under different circumstances. This
gives the user more control over the flow of current. Most semi-conductors are crystalline
substances such as germanium and silicon. We can see its use from the basis of electronic
parts such as diodes and transistors to biomedical processes.
Conventional semi-conductors are used often in electrical circuits. However, they
have limited ranges of tolerance for the frequency of the current they carry. The low
tolerance of traditional semi-conductors often poses a problem to circuits, and many of its
other applications. This is what makes the use of quantum dots so important. As they are
fabricated artificially, different quantum dots can be made to tolerate different current
frequencies through a much larger range than conventional ones (Figure 1). The use of
quantum dots as semi-conductors offers more freedom to just about everything involving
the use of semi-conductors (Quantum Dots Explained, 2005).
Figure 1: White light is shined on vials containing a solution that holds quantum dots engineered for
different frequencies. The alteration of the band-gap makes each vial absorb and re-emit a different
wavelength of light or in other words each vial of quantum is engineered to show a different color of light.
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Quantum dots can best be described as false atoms. The primary material that a quantum
dot is made out of is called a “hole”, or a substance that is missing an electron from its
valence band giving it a positive charge. The primary material is extremely small, which
is why it is called a dot, and at that size, electrons start to orbit it. Since quantum dots do
not have protons or neutrons in the center, their mass is much smaller. Since the mass at
the center is smaller than that of an atom, quantum dots exert a smaller force on the
orbiting electrons causing an orbit larger than that of a regular atom (figure 2) (K.
Daneshvar, personal communication, Jul 15, 2005). With a mass that small, scientists are
able to precisely calculate and change the size of the band-gap of the quantum dot by
adding or taking electrons. The band-gap of a quantum dot is what determines which
frequencies it will respond to, so being able to change the band-gap is what gives
scientists more control and more flexibility when dealing with its applications (Quantum
Dots Explained, 2005).
Figure 2: The above image compares the orbit of a hydrogen atom to that of a quantum dot. Since the
artificial atom has almost no mass compared to the hydrogen atom, the orbit is much larger.
5
A quantum dot is a semiconductor whose excitations are confined in all three spatial
dimensions. As a result, they have properties that are between those of bulk
semiconductors and those of discrete molecules. They were discovered by Louis E. Brus,
who was then at Bell Labs. The term "Quantum Dot" was coined by Mark Reed.
Researchers 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.
In layman's terms, quantum dots are semiconductors whose conducting characteristics are
closely related to the size and shape of the individual crystal. 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. For example, in fluorescent dye applications, 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. The main
advantages in using 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.
Quantum dots are nano technology crystals that emit light. The wave length with which
they emit light depends on the size of the crystals. Quantum dots are made of various
materials, such as lead sulfide, cadmium silinate etc[1]. Quantum dots are important
because of their power to emit a particular wave length and color depending on their
composition and size. The main aim of this work is to find application of the quantum
dots work to detect biological entities such as unicell organism (bacterial, micro
organisms), single cell genes.
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Figure3. Quantum dots colors as per nano scale.
2. Quantum confinement in semiconductors:-
In an unconfined (bulk) semiconductor, an electron-hole pair is typically bound within a
characteristic length, which is called the exciton Bohr radius and is estimated by
replacing the positively charged atomic core with the hole in the Bohr formula. If the
electron and hole are constrained further, then properties of the semiconductor change.
This effect is a form of quantum confinement, and it is a key feature in many emerging
electronic structures.
Besides confinement in all three dimensions i.e. Quantum Dot - other quantum confined
semiconductors include:
1. Quantum wires, which confine electrons or holes in two spatial dimensions and
allow free propagation in the third.
2. Quantum wells, which confine electrons or holes in one dimension and allow free
propagation in two dimensions.
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3.Making quantum dots:-
One way to synthesize quantum dots is through molecular beam epitaxy . In this
process, certain chemicals are evaporated and then sprayed to condense into small
objects on a substrate (Molecular Beam Epitaxy, 2005). The condensation of the
chemical on the substrate is similar to water on glass. If someone drops water on glass
the water condenses into many balls (figure 3). As more layers are sprayed onto the
substrate the size of the balls starts to build up into pyramid-shaped objects.
Eventually, the balls build up to a specific size and they’re quantum dots. This
process has some downsides though. It is much harder to use quantum dots while
they are still attached to the substrate. While they are all attached together on the
substrate they act as one solid which almost defeats the purpose of creating the
quantum dots (K. Daneshvar, personal communication, Jul 15, 2005).
Figure 4: The different chemicals, once sprayed onto the substrate, acts almost like water
and form together in “balls”, or quantum dots.
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Another way to form quantum dots is through electron beam lithography. This
process is a little like etching a chip. A mask is created with an electron beam that has
many tiny holes in it. Then evaporated chemicals, similar to the ones used in epitaxy, are
sprayed through the mask onto a substrate, creating many little balls (figure 4). This
process has some of the same shortcomings as epitaxy, mainly that the quantum dots are
still connected to the substrate after synthesis. Additionally, scientists have found it
difficult to create such small masks that need to have holes just nanometers in diameter.
Lithography was originally a very popular process for creating quantum dots; however,
this process creates many defects and is slow compared to the other processes (Electron
Beam Lithography, 2005).
Figure 5 : The illustration shows x-rays being shined through a mask. The x-ray light
reacts with the photo-resist on the wafer to create quantum dots.
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Colloidal synthesis is a process that involves creating quantum dots in a liquid. This is
by far the best technique for the formation of quantum dots because the process can occur
under “benchtop conditions,” or in a normal laboratory setting. When certain materials,
primarily those from periodic groups two through four, are dissolved in a certain type of
polymer solute the solution can enter into a phase where particles can come together to
form quantum dots. Since the size is dependent on time, the longer the dots are left in the
solution the bigger they get. This is in part what makes colloidal synthesis the most
popular method. Scientists can use time to change the properties of the quantum dot,
engineering it for certain light frequencies. This process, unlike lithography and epitaxy,
synthesizes the quantum dots in such a way that they are suspended individually, making
it easier for use in applications (Colloidal Particles, 2005)
There are several ways to confine excitons in semiconductors, resulting in different
methods to produce quantum dots. In general, quantum wires, wells and dots are grown
by advanced epitaxial techniques in nanocrystals produced by chemical methods or by
ion implantation, or in nanodevices made by state-of-the-art lithographic techniques.
3.1 Colloidal synthesis
Colloidal semiconductor nanocrystals are synthesized from precursor compounds
dissolved in solutions, much like traditional chemical processes. The synthesis of
colloidal quantum dots is based on a three-component system composed of: precursors,
organic surfactants, and solvents. When heating a reaction medium to a sufficiently high
temperature, the precursors chemically transform into monomers. Once the monomers
reach a high enough super saturation level, the nanocrystal growth starts with a
nucleation process. The temperature during the growth process is one of the critical
factors in determining optimal conditions for the nanocrystal growth. It must be high
enough to allow for rearrangement and annealing of atoms during the synthesis process
while being low enough to promote crystal growth. Another critical factor that has to be
stringently controlled during nanocrystal growth is the monomer concentration. The
growth process of nanocrystals can occur in two different regimes, “focusing” and
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“defocusing”. At high monomer concentrations, the critical size (the size where
nanocrystals neither grow nor shrink) is relatively small, resulting in growth of nearly all
particles. In this regime, smaller particles grow faster than large ones (since larger
crystals need more atoms to grow than small crystals) resulting in “focusing” of the size
distribution to yield nearly monodisperse particles. The size focusing is optimal when the
monomer concentration is kept such that the average nanocrystal size present is always
slightly larger than the critical size.
When the monomer concentration is depleted
during growth, the critical size becomes larger than the average size present, and the
distribution “defocuses” as a result of Ostwald ripening.
There are colloidal methods to produce many different semiconductors, including
cadmium selenide, cadmium sulfide, indium arsenide, and indium phosphide. These
quantum dots can contain as few as 100 to 100,000 atoms within the quantum dot
volume, with a diameter of 10 to 50 atoms. This corresponds to about 2 to 10 nanometers,
and at 10 nm in diameter, nearly 3 million quantum dots could be lined up end to end and
fit within the width of a human thumb.
Large batches of quantum dots may be synthesized via colloidal synthesis. Due to this
scalability and the convenience of bench top conditions, colloidal synthetic methods are
promising for commercial applications. It is acknowledged to be the least toxic of all the
different forms of synthesis.
11
3.2 Fabrication:-
• Self-assembled quantum dots are typically between 5 and 50 nm in size. Quantum
dots defined by lithographically patterned gate electrodes, or by etching on two-
dimensional electron gases in semiconductor heterostructures can have lateral
dimensions exceeding 100 nm.
• Some quantum dots are small regions of one material buried in another with a
larger band gap. These can be so-called core-shell structures, e.g., with CdSe in
the core and ZnS in the shell or from special forms of silica called ormosil.
• Quantum dots sometimes occur spontaneously in quantum well structures due to
monolayer fluctuations in the well's thickness.
• Self-assembled quantum dots nucleate spontaneously under certain conditions
during molecular beam epitaxy (MBE) and metallorganic vapor phase epitaxy
(MOVPE), when a material is grown on a substrate to which it is not lattice matched.
The resulting strain produces coherently strained islands on top of a two-dimensional
"wetting-layer." This growth mode is known as Stranski-Krastanov growth. The
islands can be subsequently buried to form the quantum dot. This fabrication method
has potential for applications in quantum cryptography (i.e. single photon sources)
and quantum computation. The main limitations of this method are the cost of
fabrication and the lack of control over positioning of individual dots.
• Individual quantum dots can be created from two-dimensional electron or hole
gases present in remotely doped quantum wells or semiconductor heterostructures
called lateral quantum dots. The sample surface is coated with a thin layer of
resist. A lateral pattern is then defined in the resist by electron beam lithography.
This pattern can then be transferred to the electron or hole gas by etching, or by
depositing metal electrodes (lift-off process) that allow the application of external
voltages between the electron gas and the electrodes. Such quantum dots are
mainly of interest for experiments and applications involving electron or hole
transport, i.e., an electrical current.
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• The energy spectrum of a quantum dot can be engineered by controlling the
geometrical size, shape, and the strength of the confinement potential. Also, in
contrast to atoms, it is relatively easy to connect quantum dots by tunnel barriers
to conducting leads, which allows the application of the techniques of tunneling
spectroscopy for their investigation.
• Confinement in quantum dots can also arise from electrostatic potentials
(generated by external electrodes, doping, strain, or impurities).
3.3 Viral assembly:-
Lee et al. (2002) reported using genetically engineered M13 bacteriophage viruses to
create quantum dot biocomposite structures.[9] As a background to this work, it has
previously been shown that genetically engineered viruses can recognize specific
semiconductor surfaces through the method of selection by combinatorial phage
display.[10] Additionally, it is known that liquid crystalline structures of wild-type viruses
(Fd, M13, and TMV) are adjustable by controlling the solution concentrations, solution
ionic strength, and the external magnetic field applied to the solutions. Consequently, the
specific recognition properties of the virus can be used to organize inorganic
nanocrystals, forming ordered arrays over the length scale defined by liquid crystal
formation. Using this information, Lee et al. (2000) were able to create self-assembled,
highly oriented, self-supporting films from a phage and ZnS precursor solution. This
system allowed them to vary both the length of bacteriophage and the type of inorganic
material through genetic modification and selection.
13
3.4 Electrochemical assembly :-
Highly ordered arrays of quantum dots may also be self-assembled by electrochemical
techniques. A template is created by causing an ionic reaction at an electrolyte-metal
interface which results in the spontaneous assembly of nanostructures, including quantum
dots, onto the metal which is then used as a mask for mesa-etching these nanostructures
on a chosen substrate
3.5 Bulk-manufacturing of quantum dots :-
Conventional, small-scale quantum dot manufacturing relies on a process called “high
temperature dual injection” which is impractical for most commercial applications that
require large quantities of quantum dots. A reproducible method for creating larger
quantities of consistent, high-quality quantum dots involves producing nanoparticles from
chemical precursors in the presence of a molecular cluster compound under conditions
whereby the integrity of the molecular cluster is maintained and acts as a prefabricated
seed template. Individual molecules of a cluster compound act as a seed or nucleation
point upon which nanoparticle growth can be initiated. In this way, a high temperature
nucleation step is not necessary to initiate nanoparticle growth because suitable
nucleation sites are already provided in the system by the molecular clusters. A
significant advantage of this method is that it is highly scalable.
3.6 Cadmium-free quantum dots - “CFQD”:-
In many regions of the world there is now a restriction or ban on the use of heavy metals
in many household goods which means that most cadmium based quantum dots are
unusable for consumer-goods applications. For commercial viability, a range of
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restricted, heavy metal-free quantum dots has been developed showing bright emissions
in the visible and near infra-red region of the spectrum and have similar optical properties
to those of CdSe quantum dots.
Cadmium and other restricted heavy metals used in conventional quantum dots is of a
major concern in commercial applications. For Quantum Dots to be commercially viable
in many applications they must not contain cadmium or other restricted metal elements.
4. Optical properties:-
An immediate optical feature of colloidal quantum dots is their coloration. While the
material which makes up a quantum dot defines its intrinsic energy signature, the
nanocrystal's quantum confined size is more significant at energies near the band gap.
Thus 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. Quantitatively speaking, the bandgap energy that
determines the energy (and hence color) of the fluorescent light is inversely proportional
to the size of the quantum dot. Larger quantum dots have more energy levels which are
also more closely spaced. This allows the quantum dot to absorb photons containing less
energy, i.e., those closer to the red end of the spectrum. Recent articles in nanotechnology
and in other journals have begun to suggest that the shape of the quantum dot may be a
factor in the coloration as well, but as yet not enough information is available.
Furthermore, it was shown [12] that the lifetime of fluorescence is determined by the size
of the quantum dot. Larger dots have more closely spaced energy levels in which the
electron-hole pair can be trapped. Therefore, electron-hole pairs in larger dots live longer
causing larger dots to show a longer lifetime.
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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.
Qdots can be synthesized with larger(thicker) shells (CdSe qdots with CdS shells). The
shell thickness has shown direct correlation to the lifetime and emission intensity.
5. Applications:-
Quantum dots are particularly significant for optical applications due to their theoretically
high quantum yield. In electronic applications they have been proven to operate like a
single-electron transistor and show the Coulomb blockade effect. Quantum dots have also
been suggested as implementations of qubits for quantum information processing.
The ability to tune the size of quantum dots is advantageous for many applications. For
instance, larger quantum dots have a greater spectrum-shift towards red compared to
smaller dots, and exhibit less pronounced quantum properties. Conversely, the smaller
particles allow one to take advantage of more subtle quantum effects.
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Figure 6 :- 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.
Being zero dimensional, quantum dots have a sharper density of states than higher-
dimensional structures. As a result, they have superior transport and optical properties,
and are being researched for use in diode lasers, amplifiers, and biological sensors.
Quantum dots may be excited within the locally enhanced electromagnetic field produced
by the gold nanoparticles, which can then be observed from the surface Plasma resonance
in the photo luminescent excitation spectrum of (CdSe)ZnS nanocrystals. High-quality
quantum dots are well suited for optical encoding and multiplexing applications due to
their broad excitation profiles and narrow/symmetric emission spectra. The new
generations of quantum dots have far-reaching potential for the study of intracellular
processes at the single-molecule level, high-resolution cellular imaging, long-term in vivo
observation of cell trafficking, tumor targeting, and diagnostics.
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5.1 Computing:- Prior to the introduction of the QD, microelectronic technology has
focused on reducing the size of transistors to produce increasingly smaller, faster and
more efficient computers (Shrinking Information Storage to the Molecular Level).
However, this method is reaching its physical limit due to the restrictions placed by the
laws of physics that do not allow these devices to operate below a certain size. With this
advantageous feature of QDs, information storage can be brought down to the molecular
level. Since no flow of electrons to transmit a signal is needed, electric current does not
need to be produced and heat problems are avoided. Also, the quantum dot devices are
sensitive enough to and can make a usage of the charges of single electrons. With
improvements in quantum-dot ordering and positioning, it is possible for us to hope in the
near future to address and store information optically in a single quantum dot, thus
opening the possibility of ultrahigh-density memory devices.
Figure 7: Nanocomputers might have a completely new type of structure made up of
‘cells’. One way of building this structure would be using quantum-dots (Towards
Quantum Information Technology, 2002).
18
Quantum dot technology is one of the most promising candidates for use in solid-state
quantum computation. By applying small voltages to the leads, the flow of electrons
through the quantum dot can be controlled and thereby precise measurements of the spin
and other properties therein can be made. With several entangled quantum dots, or qubits,
plus a way of performing operations, quantum calculations and the computers that would
perform them might be possible.
Figure 7: Quantum of Computer
19
5.2 Biology:-
In modern biological analysis, various kinds of organic dyes are used. However, with
each passing year, more flexibility is being required of these dyes, and the traditional
dyes are often unable to meet the expectations.[13] To this end, quantum dots have quickly
filled in the role, being found to be superior to traditional organic dyes on several counts,
one of the most immediately obvious being brightness (owing to the high quantum yield)
as well as their stability (allowing much less photo bleaching). It has been estimated that
quantum dots are 20 times brighter and 100 times more stable than traditional fluorescent
reporters.[13] For single-particle tracking, the irregular blinking of quantum dots is a
minor drawback.
The usage of quantum dots for highly sensitive cellular imaging has seen major advances
over the past decade. The improved photo stability of quantum dots, for example, allows
the acquisition of many consecutive focal-plane images that can be reconstructed into a
high-resolution three-dimensional image[14]. Another application that takes advantage of
the extraordinary photo stability of quantum dot probes is the real-time tracking of
molecules and cells over extended periods of time [15]. Researchers were able to observe
quantum dots in lymph nodes of mice for more than 4 months [16].
Semiconductor quantum dots have also been employed for in vitro imaging of pre-labeled
cells. The ability to image single-cell migration in real time is expected to be important to
several research areas such as embryogenesis, cancer metastasis, stem-cell therapeutics,
and lymphocyte immunology.
Scientists have proven that quantum dots are dramatically better than existing methods
for delivering a gene-silencing tool, known as si RNA, into cells.
First attempts have been made to use quantum dots for tumor targeting
under in vivo conditions. There exist two basic targeting schemes: active targeting and
passive targeting. In the case of active targeting, quantum dots are functionalized with
20
tumor-specific binding sites to selectively bind to tumor cells. Passive targeting utilizes
the enhanced permeation and retention of tumor cells for the delivery of quantum dot
probes. Fast-growing tumor cells typically have more permeable membranes than healthy
cells, allowing the leakage of small nanoparticles into the cell body. Moreover, tumor
cells lack an effective lymphatic drainage system, which leads to subsequent
nanoparticle-accumulation.
One of the remaining issues with quantum dot probes is their in vivo toxicity. For
example, CdSe nanocrystals are highly toxic to cultured cells under UV illumination. The
energy of UV irradiation is close to that of the covalent chemical bond energy of CdSe
nanocrystals. As a result, semiconductor particles can be dissolved, in a process known as
photolysis, to release toxic cadmium ions into the culture medium. In the absence of UV
irradiation, however, quantum dots with a stable polymer coating have been found to be
essentially nontoxic. Then again, only little is known about the excretion process of
polymer-protected quantum dots from living organisms. These and other questions must
be carefully examined before quantum dot applications in tumor or vascular imaging can
be approved for human clinical use.
Another potential cutting-edge application of quantum dots is being researched, with
quantum dots acting as the inorganic fluorophore for intra-operative detection of tumors
using fluorescence spectroscopy.
In the biological field of science, QDs have become known to be very useful.
Recent studies of QDs have resulted in developing new fluorescence
immunocytochemical probes (Development and application of quantum dots for
immunocytochemistry of human erythrocytes, 2002). A probe is a substance that is
radioactively labeled or otherwise marked and used to detect or identify another
substance in a sample. A fluorescence immunocytochemical probe is usually used to
detect antigens in tissues (figure 5). In contrast to organic fluorophores, which are not
photostable, QDs have properties of high brightness, photostability, narrow emission
spectra and an apparent large Stokes’ shift, thus they can replace the usage of organic
21
fluorophores. The current mode of detecting the antigens which takes from two to six
days can speed up to a matter of hours using quantum dots.
Figure 8: Immunocytochemical probes are used in the dead rodent. The probes in
the body have circulated and now show up under florescent light, creating a much safer
alternative to the x-ray.
22
5.3 Photovoltaic devices:-
Quantum dots may be able to increase the efficiency and reduce the cost of today's
typical silicon photovoltaic cells. According to an experimental proof from 2006
(controversial results), quantum dots of lead selenide can produce as many as seven
excitons from one high energy photon of sunlight (7.8 times the band gap energy).[19]
This compares favorably to today's photovoltaic cells which can only manage one exciton
per high-energy photon, with high kinetic energy carriers losing their energy as heat. This
would not result in a 7-fold increase in final output however, but could boost the
maximum theoretical efficiency from 31% to 42%. Quantum dot photovoltaic would
theoretically be cheaper to manufacture, as they can be made "using simple chemical
reactions."[19] The generation of more than one exciton by a single photon is called
multiple exciton generation (MEG) or carrier multiplication.
The efficiency of solar cells is the electrical power it puts out as percentage of the power
in incident sunlight. One of the most fundamental limitations on the efficiency of a solar
cell is the ‘band gap’ of the semi-conducting material used in conventional solar cells: the
energy required to boost an electron from the bound valence band into the mobile
conduction band. When an electron is knocked loose from the valence band, it goes into
the conduction band as a negative charge, leaving behind a ‘hole’ of positive charge.
Both electron and hole can migrate through the semi-conducting material.
In a solar cell, negatively doped (n-type) material with extra electrons in its otherwise
empty conduction band forms a junction with positively doped (p-type) material, with
extra holes in the band otherwise filled with valence electrons. When a photon with
energy matching the band gap strikes the semiconductor, it is absorbed by an electron,
which jumps to the conduction band, leaving a hole. Both electron and hole migrate in
the junction’s electric field, but in opposite directions. If the solar cell is connected to an
external circuit, an electric current is generated. If the circuit is open, then an electrical
potential or voltage is built up across the electrodes.
23
Photons with less energy than the band gap slip right through without being absorbed,
while photons with energy higher than the band gap are absorbed, but their excess energy
is wasted, and dissipated as heat. The maximum efficiency that a solar cell made from a
single material can theoretically achieve is about 30 percent. In practice, the best
achievable is about 25 percent.
It is possible to improve on the efficiency by stacking materials with different band gaps
together in multi-junction cells. Stacking dozens of different layers together can increase
efficiency theoretically to greater than 70 percent. But this results in technical problems
such as strain damages to the crystal layers. The most efficient multi-junction solar cell is
one that has three layers: gallium indium phosphide/gallium arsenide/germanium
(GaInP/GaAs/Ge) made by the National Center for Photovoltaics in the US, which
achieved an efficiency of 34 percent in 2001.
Recently, entirely new possibilities for improving the efficiency of photovoltaics have
opened up.
5.3.1 Quantum dot possibilities
Quantum dots or nanoparticles are semi-conducting crystals of nanometre (a billionth of a
metre) dimensions. They have quantum optical properties that are absent in the bulk
material due to the confinement of electron-hole pairs (called excitons) on the particle, in
a region of a few nanometres.
The first advantage of quantum dots is their tunable bandgap. It means that the
wavelength at which they will absorb or emit radiation can be adjusted at will: the larger
the size, the longer the wavelength of light absorbed and emitted. The greater the
bandgap of a solar cell semiconductor, the more energetic the photons absorbed, and the
greater the output voltage. On the other hand, a lower bandgap results in the capture of
more photons including those in the red end of the solar spectrum, resulting in a higher
output of current but at a lower output voltage. Thus, there is an optimum bandgap that
corresponds to the highest possible solar-electric energy conversion, and this can also be
24
achieved by using a mixture of quantum dots of different sizes for harvesting the
maximum proportion of the incident light.
Another advantage of quantum dots is that in contrast to traditional semiconductor
materials that are crystalline or rigid, quantum dots can be molded into a variety of
different form, in sheets or three-dimensional arrays. They can easily be combined with
organic polymers, dyes, or made into porous films (“Organic solar power”, this series). In
the colloidal form suspended in solution, they can be processed to create junctions on
inexpensive substrates such as plastics, glass or metal sheets.
When quantum dots are formed into an ordered three-dimensional array, there will be
strong electronic coupling between them so that excitons will have a longer life,
facilitating the collection and transport of ‘hot carriers’ to generate electricity at high
voltage. In addition, such an array makes it possible to generate multiple excitons from
the absorption of a single photon (see later).
Quantum dots are offering the possibilities for improving the efficiency of solar cells in at
least two respects, by extending the band gap of solar cells for harvesting more of the
light in the solar spectrum, and by generating more charges from a single photon.
5.3.2 Extending the solar cell band gap into infrar ed
Infrared photovoltaic cells – which transform infrared light into electricity - are attracting
much attention, as nearly half of the approximately 1000Wm3 of the intensity of sunlight
is within the invisible infrared region. So it is possible to use the visible half for direct
lighting while harvesting the invisible for generating electricity . Photovoltaic cells that
respond to infrared – ‘thermovoltaics’ - can even capture radiation from a fuel-fire
emitter; and co-generation of electricity and heat are said to be quiet, reliable, clean and
efficient. A 1 cm2 silicon cell in direct sunlight will generate about 0.01W, but an
efficient infrared photovoltaic cell of equal size can produce theoretically 1W in a fuel-
fired system.
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One development that has made infrared photovoltaics attractive is the availability of
light-sensitive conjugated polymers - polymers with alternating single and double carbon-
carbon (sometimes carbon-nitrogen) bonds. It was discovered in the 1970s that chemical
doping of conjugated polymers increased electronic conductivity several orders of
magnitude. Since then, electronically conducting materials based on conjugated polymers
have found many applications including sensors, light-emitting diodes, and solar cells .
Conjugated polymers provide ease of processing, low cost, physical flexibility and large
area coverage. They now work reasonably well within the visible spectrum.
In order to make conjugated polymers work in the infrared range, researchers at the
University of Toronto wrapped the polymers around lead sulphide quantum dots tuned
(by size) to respond to infrared . The polymer poly(2-methoxy-5-(2’-ethylhexyloxy-p-
phenylenevinylene)] (MEH-PPV) on its own absorbs between ~400 and ~600 nm.
Quantum dots of lead sulphide (PbS) have absorption peaks that can be tuned from ~800
to ~2000 nm. Wrapping MEH-PPV around the quantum dots shifted the polymer’s
absorption into the infrared.
The researchers demonstrated a convincing, albeit very small photovoltaic effect, giving
a power-conversion efficiency of 0.001 percent. Professor Ted Sargent, the lead scientist,
is optimistic however, emphasizing that their device is simply a prototype of how to
capture infrared energy , and predicts commercial implementation within 3-5 years.
5.3.3 Multiple excitons from one photon
Researchers led by Arthur Nozik at the National Renewable Energy Laboratory Golden,
Colorado in the United States really grabbed the headline when they demonstrated that
the absorption of a single photon by their quantum dots yielded - not one exciton as
usually the case - but three of them .
The formation of multiple excitons per absorbed photon happens when the energy of the
photon absorbed is far greater than the semiconductor band gap. This phenomenon does
not readily occur in bulk semiconductors where the excess energy simply dissipates away
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as heat before it can cause other electron-hole pairs to form. But in semi-conducting
quantum dots, the rate of energy dissipation is significantly reduced, and the charge
carriers are confined within a minute volume, thereby increasing their interactions and
enhancing the probability for multiple excitons to form.
The researchers report a quantum yield of 300 percent for 2.9nm diameter PbSe (lead
selenide) quantum dots when the energy of the photon absorbed is four times that of the
band gap. But multiple excitons start to form as soon as the photon energy reaches twice
the band gap. Quantum dots made of lead sulphide (PbS) also showed the same
phenomenon.
The findings are further confirmation of Nozik’s theoretical prediction in 2000 that
quantum dots could increase the efficiency of solar cells through multiple exciton
generation. In 2004, researchers Richard Shaller and Victor Klimov at Los Alamos
National Laboratory New Mexico were the first to demonstrate this phenomenon
experimentally using quantum dots made of lead selenide.
“We have shown that solar cells based on quantum dots theoretically could convert more
than 65 percent of the sun’s energy into electricity, approximately doubling the efficiency
of solar cells”, said Nozik
5.4 Light emitting devices:-
There are several inquiries into using quantum dots as light-emitting diodes to make
displays and other light sources, such as "QD-LED" displays, and "QD-WLED" (White
LED). In June, 2006, QD Vision announced technical success in making a proof-of-
concept quantum dot display and show a bright emission in the visible and near infra-red
region of the spectrum. Quantum dots are valued for displays, because they emit light in
very specific gaussian distributions. This can result in a display that more accurately
renders the colors that the human eye can perceive. Quantum dots also require very little
power since they are not color filtered. Additionally, since the discovery of "white-light
emitting" QD, general solid-state lighting applications appear closer than ever.
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A color liquid crystal display (LCD), for example, is usually powered by a single
fluorescent lamp (or occasionally, conventional white LEDs) that is color filtered to
produce red, green, and blue pixels. Displays that intrinsically produce monochromatic
light can be more efficient, since more of the light produced reaches the eye
5.5 Quantum dot laser :-
QDs also have other applications like quantum dot lasers which promises far more great
advantages than quantum well lasers. Because QD lasers are less temperature-dependent
and less likely to degrade under elevated temperature, it allows more flexibility for lasers
to operate more efficiently (Chapter 5: quantum dot lasers, 1999). Other beneficial
features of QD lasers include low threshold currents, higher power, and great stability
compared to the restrained performance of the conventional lasers. Respectively, the QD
laser will play a significant role in optical data communications and optical networks
Figure 9:- quantum dot laser
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Expected advantages of quantum dot laser
1. Quantum dot lasers should be able to emit light at wavelengths determined by
the energy levels of the dots, rather than the band gap energy. Thus, they offer
the possibility of improved device performance and increased flexibility to
adjust the wavelength.
2. Quantum dot lasers have the maximum material gain and differential gain,
at least 2-3 orders higher than quantum-well lasers ].
3. The small active volume translates to multiple benefits, such as low power
high frequency operation, large modulation bandwidth, small dynamic chirp,
small linewidth enhancement factor, and low threshold current.
4. Quantum dot lasers also show superior temperature stability of the
threshold current. The threshold current is given by the relation,
I threshold(T) = Ithreshold(Tref). exp((T-Tref)/To),
where T is the active region temperature, Tref is the reference temperature, and T0
is an emperically-determined "characteristic temperature", which is itself a
function of temperature and device length. In quantum dot lasers T0 can be high,
because one can effectively decouple electron-phonon interaction by increasing
the intersubband separation. This leads to undiminished room-temperature
performance without external thermal stabilization.
5. In addition, quantum dot lasers suppress the diffusion of non-equilibrium
carriers, resulting in reduced leakage from the active region.
6. More novel structures such as distributed feedback lasers and single-dot
VCSELs promise ultra-stable single mode operation.
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5.6 Life Sciences :-
Quantum dots have applications in the biological world as flurescencent tags
Quantum dots are nanometer-scale Nano crystals composed of a few hundred to a few
thousand semiconductors atoms out of bio-inert materials – meaning they are non
intrusive and nontoxic to the body.
Additionally, unlike fluorescent dyes (which tend to decompose and lose their ability to
fluoresce), quantum dots maintain their integrity with standing more cycles of excitation
and light emission before they start to fade. Changing their size or composition allows us
to cater their optical properties- which means they are fluoresce in a multitude of color .
Interestingly enough, quantum dots can even be tuned to fluoresce in different colors with
the same wavelength of light i.e. we can choose quantum dots size where the frequency
of light required to make one group of dots fluoresce is an even multiple of the frequency
required to make another group of dots fluorescet; both dots then fluoresces with the
same wavelength of light. This allows multiple tags to be tracked while using a single
light source.
Quantum dots are insoluble in water soluble. This is the main reason they are restricted in
biological uses. To overcome this problem the quantum dots are coated with polymer
layer.
This enables quantum dot to mix with water
Figure 10: Quantum dots coating.
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Quantum dots are also used to detection the behavior of the cells which cause breast
cancer a burning problem in the present day world. Using this technology scientist are
planning to find the properties of the cancer cells so that they can make a nano drug that
can cure the infected part of the cell.
Figure 11: . Quantum dots attached to breast cancer cells.
Another application of quantum dots is in deoxyribonucleic acid (DNA). DNA is the
nucleic acid carrying the genetic blueprint of all forms of cellular life .
The double helix structure of DNA is shown in figure. We can map our DNA using
Quantum dots. DNA can be attached to gold or silver nanodots (14nmwide) that are
suspended in a liquid . Each gold particle has the same base pair- but when a linker (such
as Anthrax DNA) is introduced, the gold particles form larger clusters, which change
their optical properties .
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Here particle size indicates the wave length of the light, hence color. Imagine with out the
linker, the liquid looks purple; with the linker however looks red- providing a quick
macroscopic analysis. Because of this color change, these are called colorimetric sensors.
The type of DNA damage is not repaired by a single protein. “In these types of processes
there are likely multiple proteins that come into play. So, one of the current challenges is
observing this complex process happening in a live cell in real time. This can be done
using quantum dots technique.
Application of Quantum dots flour dyes
These are used in
• Oligonucleotides can be successfully couples to molecular beacons which can
serve as basis for DNA, RNA assay [3]
• Flowcytometry application as recorders by emitting multiple laser sources of
conventional flow cytometers so that we can reduce cost of cytometer system [3].
• High throughput screening assay (due to the ability to conjugate to small
molecules) [3].
• Quantum dot flour dyes have 15 -20nm fluorescence lifetime which will enable
them to study the signal noise ratio effectively [4].
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Quantum dots can also be used in the study of antibiotic release into the
Figure 12 . Quantum dots in anti biotic applications
Figure shows two different wave length of quantum dots (red and green) that are
attached to the antibiotic which are used to cure the effected cells.
Quantum dots can also be used in live and fixed fluorescence cell labeling such as
cellular tracking, stem cell differentiation tracking, genetic instability monitoring,
molecular location tracking. Figure 6 below shows us the tracking of cells in mice which
is presently carried out by Evidenttech
33
Figure 13 . mice cell detection using quantum dots
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5.7 Optical switches:-
Optical switches have been a major research objective in the scientific community. The
use of optical switches would increase the rate
at which data can be transferred. With regular
switches, data can only travel as fast as the
electrical current can, but optical switches can
travel almost as fast as the speed of light. The
principle of optical switches is that semi-
conductors will only allow certain levels of
energy to pass through it. So if we place a
quantum dot semi-conductor in a circuit, but
supply a voltage below the acceptable range,
current will not flow. However, if we shine a
Figure 14: Optical Switch light on the quantum dot semi-conductor, it
would put enough energy into the semi-conductor that it will allow current to flow (figure
14). This idea is mainly for powering electronic devices, but using quantum-dots as
receivers for electrical data is just a step up (Quantum Dots: How they Work, 2005).
35
5.8 Other applications of quantum dots :-
Quantum dots have applications outside of biology and engineering. An idea that may be
instituted in the future would be against counterfeiting money. The treasury could
engineer quantum dots to be responsive to a specific frequency of light and suspend them
in ink that they would print onto money. Shining light with the same frequency that the
ink solution has been engineered for would reveal whether or not the money is real or
counterfeit. This idea can be used for just about any substance that could be illegally
duplicated (Harnessing the Power of Quantum Dots, 2005).
Figure 15: Currency with Quantum Dots
Another security application involves attaching quantum dots to dust. QDs can be
engineered so that they have the same properties as dust and give off infrared radiation.
In hostile areas, this “quantum dust” can be used to track wanted criminals or the
movement of hostile activity. In urban areas, “quantum dust” can be used as a security
device to set off alarms if the infrared radiation is detected (Harnessing the Power of
Quantum Dots, 2005).
Though QDs are still under research for other possible applications and need more
technological advancement in order to be put into use, the features introduced will grant
far better optical communication, significant change in electronic devices, and even
detection of antigens in the body tissues.
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6. Quantum computer:-
A quantum computer is any device for computation that makes direct use of
distinctively quantum mechanical phenomena, such as superposition and entanglement ,
to perform operations on data.
The basic principle of quantum computer:
The quantum properties of particles can be used to represent and structure data, and
that quantum mechanisms can be devised and built to perform operations with these data.
6.1 What is Qubits:-
� The device computes by manipulating those bits with the help of logic gates.
� A qubit can hold a one, a zero, or, crucially, a superposition of these.
� Manipulating those qubits with the help of quantum logic gates.
� A classical computer has a memory made up of bits , where each bit holds either a
one or a zero.
� The qubits can be in a superposition of all the classically allowed states.
� the register is described by a wave function.
� the phases of the numbers can constructively and destructively interfere with one
another; this is an important feature for quantum algorithms
� For an n qubit quantum register, recording the state of the register requires 2n
complex numbers
� (the 3-qubit register requires 23 = 8 numbers).
� Consequently, the number of classical states encoded in a quantum register grows
exponentially with the number of qubits.
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� For n=300, this is roughly 1090, more states than there are atoms in the
observable universe.
6.2 Quantum superposition:-
� Quantum superposition is the application of the superposition principle to
quantum mechanics.
� The superposition principle is the addition of the amplitudes of wave
functions , or state vectors
� . It occurs when an object simultaneously "possesses" two or more values for
an observable quantity
� (e.g. the position or energy of a particle).
6.3 Quantum entanglement:-
� is a quantum mechanical phenomenon in which the quantum states of two or more
objects have to be described with reference to each other, even though the
individual objects may be spatially separated .
� leads to correlations between observable physical properties of the systems.
� For example, it is possible to prepare two particles in a single quantum state such
that when one is observed to be spin-up, the other one will always be observed to
be spin-down and vice versa.
� It is impossible to predict, according to quantum mechanics, which set of
measurements will be observed. As a result, measurements performed on one
system seem to be instantaneously influencing other systems entangled with it.
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6.4 Coloration:-
� The larger the dot, the redder.
� The smaller the dot, the bluer.
� The coloration is directly related to the energy levels of the quantum dot.
6.5 Blue shift:-
� The bandgap energy inversely proportional to the square of the size of the
quantum dot.
� Larger quantum dots have more energy levels which are more closely spaced.
� This allows the quantum dot to absorb photons containing less energy, i.e.
those closer to the red end of the spectrum.
6.6 Comparison to atom:-
� Both have a discrete energy spectrum and bind a small number of electrons.
� In contrast to atoms, the confinement potential in quantum dots does not
necessarily show spherical symmetry.
� In addition, the confined electrons do not move in free space but in the
semiconductor host crystal.
� Play an important role for all quantum dot properties.
6.7 Advantages of Quantum dots:-
� Sharper density of states
� Superior transport and optical properties, and are being researched for use in
diode lasers, amplifiers, and biological sensors.
� use in solid-state quantum computation . By applying small voltages to the
leads, one can control the flow of electrons through the quantum dot and
thereby make precise measurements of the spin and other properties
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� Another cutting edge application of quantum dots is also being researched as
potential artificial fluorophore for intra-operative detection of tumors using
fluorescence spectroscopy .
� Quantum dots may have the potential to increase the efficiency and reduce the
cost of todays typical silicon photovoltaic cells.
� 7-fold increase in final output
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Conclusion
The whole concentration of this seminar was to provide introduction, construction &
applications of quantum dots in various fields . Quantum dots application in study of
breast cancer cells, antibiotic drugs is still at research level. Research is to be carried out
in the areas like DNA moments, labeling of proteins, tagging of nucleic acid and so on.
It if definite that using quantum dots many of the dark spots of life science can be
studied.
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References
1. Evident Technologies, (2005). Quantum dots explained. Retrieved Jul. 14,
2005, from Evident Technologies Web site:
http://www.evidenttech.com/qdot-definition/quantum-dot-about.php.
2. Molecular beam epitaxy. (2005). Retrieved Jul. 14, 2005, from Wikipedia: the
Free Encyclopedia Web site:
http://en.wikipedia.org/wiki/Molecular_beam_epitaxy.
3. Electron beam lithography. (2005). Retrieved Jul. 14, 2005, from Wikipedia:
the Free Encyclopedia Web site:
http://en.wikipedia.org/wiki/Electron_beam_lithography.
4. Colloidal particles. (2005). Retrieved Jul. 14, 2005, from Semiconductor
Nanospheres and Quantum Dots Web site:
http://www.its.caltech.edu/~mankei/ee150sp03/qdots.ppt#267,11,Colloidal
Particles.
5. Harnessing the power of quantum dots. (2005). Retrieved Jul. 14, 2005, from
Quantum Dots Explained Web site: http://www.evidenttech.com/qdot-
definition/quantum-dot-use.php.
6. F.Tokumasu & J. Dvorak. (2002). Retrieved September 3, 2003, from
Development and application of quantum dots for immunocytochemistry of
human erythrocytes Web site:
http://72.14.207.104/search?q=cache:AGz07tWobZEJ:www.qdots.com/live/u
pload_documents/tokumasu-dvorak.pdf+quantum+dot+application&hl=en.