11

By…By…

MEENU CHAUDHARYMEENU CHAUDHARY

C08533C08533

Quantum DotsQuantum Dots

22

IntroductionIntroductionQuantum dots are semiconductors whose excitons are Quantum dots are semiconductors whose excitons are confined in all three dimensions of space.confined in all three dimensions of space.

Quantum dots have properties combined betweenQuantum dots have properties combined between Those of bulk semiconductorsThose of bulk semiconductors

Those of atomsThose of atoms

Different methods to create quantum dots.Different methods to create quantum dots.

Multiple applications.Multiple applications.

33

OutlineOutline

1.1. Quantum Confinement and Quantum DotsQuantum Confinement and Quantum Dots

2.2. Fabrication of Quantum DotsFabrication of Quantum Dots

3.3. Quantum Dot ApplicationsQuantum Dot Applications

44

Bulk SemiconductorsBulk Semiconductors

Electrons in conduction band (and holes in the valence Electrons in conduction band (and holes in the valence band) are free to move in all three dimensions of space.band) are free to move in all three dimensions of space.

B.E.A. Saleh, M.C. Teich. Fundamentals of Photonics. fig. 16.1-10 and 16.1-29.

55

Thin Film SemiconductorsThin Film Semiconductors

Electrons in conduction band (and holes in the valence Electrons in conduction band (and holes in the valence band) are free to move in two dimensions.band) are free to move in two dimensions.

Confined in one dimension by a potential well.Confined in one dimension by a potential well. Potential well created due to a larger bandgap of the Potential well created due to a larger bandgap of the

semiconductors on either side of the thin film.semiconductors on either side of the thin film. Thinner films lead to higher energy levels.Thinner films lead to higher energy levels.

66

Quantum WireQuantum Wire

Thin semiconductor wire surrounded by a material with a Thin semiconductor wire surrounded by a material with a larger bandgap.larger bandgap.

Surrounding material confines electrons and holes in two Surrounding material confines electrons and holes in two dimensions (carriers can only move in one dimension) due to its dimensions (carriers can only move in one dimension) due to its larger bandgap.larger bandgap.

Quantum wire acts as a potential well.Quantum wire acts as a potential well.

77

Quantum DotQuantum Dot

Electrons and holes are confined in all three dimensions Electrons and holes are confined in all three dimensions of space by a surrounding material with a larger of space by a surrounding material with a larger bandgap.bandgap.

Discrete energy levels (artificial atom).Discrete energy levels (artificial atom).

A quantum dot has a larger bandgap.A quantum dot has a larger bandgap.

Like bulk semiconductor, electrons tend to make Like bulk semiconductor, electrons tend to make transitions near the edges of the bandgap in quantum transitions near the edges of the bandgap in quantum dots.dots.

88

Very small semiconductor particles with a size Very small semiconductor particles with a size comparable to the Bohr radius of the excitons comparable to the Bohr radius of the excitons (separation of electron and hole).(separation of electron and hole).

Typical dimensions: 1 – 10 nmTypical dimensions: 1 – 10 nm Can be as large as several Can be as large as several μμm.m. Different shapes (cubes, spheres, pyramids, etc.)Different shapes (cubes, spheres, pyramids, etc.)

Quantum DotQuantum Dot

99

Discrete Energy LevelsDiscrete Energy LevelsThe energy levels depend on the size, and also the The energy levels depend on the size, and also the shape, of the quantum dot.shape, of the quantum dot.

Smaller quantum dot:Smaller quantum dot: Higher energy required to confine excitons to a smaller volume.Higher energy required to confine excitons to a smaller volume.

Energy levels increase in energy and spread out more.Energy levels increase in energy and spread out more.

Higher band gap energy.Higher band gap energy.

.

1010

CdSe Quantum DotCdSe Quantum Dot

5 nm dots: red5 nm dots: red

1.5 nm dots: violet1.5 nm dots: violet

1111

How to Make Quantum DotsHow to Make Quantum Dots

There are three main ways to confine excitons in There are three main ways to confine excitons in semiconductors:semiconductors: LithographyLithography Colloidal synthesisColloidal synthesis Epitaxy:Epitaxy:

Patterned GrowthPatterned Growth

Self-Organized GrowthSelf-Organized Growth

1212

LithographyLithography

Quantum wells are covered with a polymer mask and Quantum wells are covered with a polymer mask and exposed to an electron or ion beam.exposed to an electron or ion beam.The surface is covered with a thin layer of metal, then The surface is covered with a thin layer of metal, then cleaned and only the exposed areas keep the metal layer.cleaned and only the exposed areas keep the metal layer.Pillars are etched into the entire surface.Pillars are etched into the entire surface.

Multiple layers are applied this Multiple layers are applied this way to build up the properties way to build up the properties and size wanted.and size wanted.Disadvantages: slow, Disadvantages: slow, contamination, low density, contamination, low density, defect formation.defect formation.

1313

Colloidal SynthesisColloidal Synthesis

Immersion of semiconductor microcrystals in glass Immersion of semiconductor microcrystals in glass dielectric matrices.dielectric matrices.

Taking a silicate glass with 1% semiconducting phase Taking a silicate glass with 1% semiconducting phase (CdS, CuCl, CdSe, or CuBr).(CdS, CuCl, CdSe, or CuBr).

Heating for several hours at high temperature.Heating for several hours at high temperature. Formation of microcrystals of nearly equal size.Formation of microcrystals of nearly equal size.

Typically group II-VI materials (e.g. CdS, CdSe)Typically group II-VI materials (e.g. CdS, CdSe)

Size variations (“size dispersion”).Size variations (“size dispersion”).

1414

Epitaxy: Patterned GrowthEpitaxy: Patterned Growth

Semiconducting compounds with a smaller bandgap Semiconducting compounds with a smaller bandgap (GaAs) are grown on the surface of a compoundwith a (GaAs) are grown on the surface of a compoundwith a larger bandgap (AlGaAs).larger bandgap (AlGaAs).

Growth is restricted by coating it with a masking Growth is restricted by coating it with a masking compound (SiOcompound (SiO22) and etching that mask with the shape ) and etching that mask with the shape

of the required crystal cell wall shape.of the required crystal cell wall shape.

Disadvantage: density of Disadvantage: density of quantum dots limited by quantum dots limited by mask pattern.mask pattern.

1515

Epitaxy: Self-Organized GrowthEpitaxy: Self-Organized Growth

Uses a large difference in the lattice constants of the Uses a large difference in the lattice constants of the substrate and the crystallizing material.substrate and the crystallizing material.When the crystallized layer is thicker than the critical When the crystallized layer is thicker than the critical thickness, there is a strong strain on the layers.thickness, there is a strong strain on the layers.The breakdown results in randomly distributed islets of The breakdown results in randomly distributed islets of regular shape and size.regular shape and size.

Disadvantages: size and Disadvantages: size and shape fluctuations, ordering.shape fluctuations, ordering.

Cadmium-free quantum dots Cadmium-free quantum dots “CFQD”“CFQD”

In many regions of the world there is now, or soon to be, In many regions of the world there is now, or soon to be, legislation to restrict and in some cases ban heavy legislation to restrict and in some cases ban heavy metals in many household appliances such as IT & metals in many household appliances such as IT & telecommunication equipment, Lighting equipment , telecommunication equipment, Lighting equipment , Electrical & electronic tools, Toys, leisure & sports Electrical & electronic tools, Toys, leisure & sports equipment.equipment.

For QDs to be commercially viable in many applications For QDs to be commercially viable in many applications they MUST NOT CONTAIN cadmium or other restricted they MUST NOT CONTAIN cadmium or other restricted elements LIKE mercury, lead, chromium. elements LIKE mercury, lead, chromium.

So research has been able to create non-toxic quantum So research has been able to create non-toxic quantum dots using dots using silicon..

1616

What is it good for???What is it good for???

Very narrow spectral line width, Very narrow spectral line width, depending on the quantum dot’s size.depending on the quantum dot’s size.

Multiplexed detectionMultiplexed detection

Large absorption coefficients across a Large absorption coefficients across a wide spectral range.wide spectral range.

Small size / high surface-to-volume ratio.Small size / high surface-to-volume ratio.

Very high levels of brightness.Very high levels of brightness.

Blinking. Blinking.

1717

18181818

ApplicationsApplications

Photovoltaic devices: solar cellsBiology : biosensors, imagingLight emitting diodes: LEDsQuantum computationFlat-panel displaysMemory elementsPhotodetectorsLasers

19191919

Solar CellsSolar Cells

Photovoltaic effect: p-n junction. Sunlight excites electrons

and creates electron-hole pairs.

Electrons concentrate on one side of the cell and holes on the other side.

Connecting the 2 sides creates electricity.

20202020

Different Generations of Solar CellsDifferent Generations of Solar Cells

First generation: Single crystal silicon wafer. Advantages: high carrier mobility. Disadvantages: most of photon

energy is wasted as heat, expensive.

Second generation: Thin-film technology. Advantages: less expensive. Disadvantages: efficiency lower

compared with silicon solar cells.

Third generation: Nanocrystal solar cells. Enhance electrical performances of

the second generation while maintaining low production costs.

21212121

Solar Cells EfficiencySolar Cells Efficiency

What limits the efficiency: Photons with lower energy than the band gap are not absorbed. Photons with greater energy than the band gap are absorbed but

the excess energy is lost as heat.

22222222

The quantum dot band gap is tunable and can be used to create intermediate bandgaps. The maximum theoretical efficiency of the solar cell is as high as 63.2% with this method.

How Can Quantum Dots Improve How Can Quantum Dots Improve the Efficiency?the Efficiency?

BIOLOGY: LOCATING BIOLOGY: LOCATING CANCER CELLCANCER CELL

This picture shows This picture shows silicon quantum dots silicon quantum dots fluorescing inside fluorescing inside cancer cells.cancer cells.

2323

..

These quantum dots can be put These quantum dots can be put into single cells, or lots of cells, in into single cells, or lots of cells, in the tissue of living organisms. In the tissue of living organisms. In

future, it is planned to attach future, it is planned to attach specific antibodies to the quantum specific antibodies to the quantum dots – when injected into a body, dots – when injected into a body,

the quantum dots will find and the quantum dots will find and bind to cancer cells, and illuminate bind to cancer cells, and illuminate

them when they fluoresce.them when they fluoresce.

2424

TARGETED DRUG DELIVERYTARGETED DRUG DELIVERY

In this we attach In this we attach drug molecules to the  molecules to the quantum dots, which will then be able to quantum dots, which will then be able to deliver the drug just to the cancer cells deliver the drug just to the cancer cells where it is needed. where it is needed.

Current anti-cancer drugs tend to have a Current anti-cancer drugs tend to have a range of unpleasant side-effects, because range of unpleasant side-effects, because they affect the whole body, not just the they affect the whole body, not just the cancer. cancer.

2525

Light emitting diodes: LEDs

2626

Anti-counterfeitingAnti-counterfeiting

From consumer goods like music and From consumer goods like music and software, to critical products like drug software, to critical products like drug shipments, and even currency itself, shipments, and even currency itself, quantum dots provide a method of creating quantum dots provide a method of creating unique, optical barcodes: the precise unique, optical barcodes: the precise combinations of wavelengths of light combinations of wavelengths of light emitted by complex combinations of emitted by complex combinations of different quantum dot. Embedded in inks, different quantum dot. Embedded in inks, plastic, glass, and polymers, quantum dots plastic, glass, and polymers, quantum dots are invisible to the naked eye and are invisible to the naked eye and impossible to counterfeit.impossible to counterfeit.

2727

SENSORSSENSORS

The properties of quantum dots are such The properties of quantum dots are such that they can be functionalized to emit light that they can be functionalized to emit light on binding to a “target” molecule”, which on binding to a “target” molecule”, which enables them to be used, for example, to enables them to be used, for example, to detect specific airborne pollutants.detect specific airborne pollutants.

2828

2929

ConclusionConclusion

Quantum dot: Semiconductor particle with a size in the order of the Bohr radius

of the excitons. Energy levels depend on the size of the dot.

Different methods for fabricating quantum dots. LithographyLithography Colloidal synthesisColloidal synthesis EpitaxyEpitaxy

Multiple applications.

THANK YOUTHANK YOU3030