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

3

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.

4

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.

6

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.

7

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.

8

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.

9

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

10

“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.

12

• 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

14

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.

15

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.

16

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.

17

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.

25

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

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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).

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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.