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QUANTUM DOT AND ITS APPLICATIONS
Presented by: Yashvant Rao
UNDER THE SUPERVISION OF DR. PRAKASH C. JHA
WHAT ARE QUANTUM DOTS?
Quantum dots are semi-conductors that are on the nanometer scale.
Obey quantum mechanical principle of quantum confinement.
Exhibit energy band gap that determines required wavelength of radiation absorption and emission spectra.
Requisite absorption and resultant emission wavelengths dependent on dot size.
Figure- Schematic plot of the single particle energy band gap. The upper parabolic band is the conduction band, the lower the valence.
QUANTUM DOTS (QD)
Semiconductor nanostructuresSize: ~2-10 nm or ~10-50 atoms
in diameter Unique tunability Motion of electrons + holes = excitons Confinement of motion can be created by:
Electrostatic potential e.g. doping, strain, impurities,
external electrodes the presence of an interface between different semiconductor materials
e.g. in the case of self-assembled QDs the presence of the semiconductor surface
e.g. in the case of a semiconductor nanocrystal or by a combination of these
QD – FABRICATION TECHNIQUES
Core shell quantum structures
Self-assembled QDs and Stranski-Krastanov growth MBE (molecular beam
epitaxy) MOVPE
(metalorganics vapor phase epitaxy)
Monolayer fluctuations Gases in remotely doped
heterostructures Schematic representation of different approaches to fabrication of nanostructures: (a) microcrystallites in glass, (b) artificial patterning of thin film structures, (c) self-organized growth of nanostructures
Quantum dot
Quantum dots is a semiconductor whose excitons are confined in all three spatial dimensions. Consequently, such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules
Researching fields: have studied quantum dots in transistors, solar cells, LEDs, and diode lasers. They have also investigated quantum dots as agents for medical imaging and hope to use them as qubits
Colloidal quantum dots irradiated with a UV light. Different sized quantum dots emit different color light due to quantum confinement.
The basic parts of a quantum dot include the core, shell, and surface ligand. The shell usually enhances the emission efficiency and stability of the core quantum dot. In functional uses, such as biological applications, a chemical hook is used to target complimentary materials.
5
Core : “Active Material”1-20 nm Size Spheres,Rods, Disks, etc...
Shells : “Protective orComplementary Layers”
Surface Groups/Ligand: “Passivating, Protective, andChemically Active Layer”
Functional Groups: “Chemical Hooks or otherchemically, electrically, or optically active groups”
Parts of a Quantum Dot Functionalizing a Quantum Dot
Several important quantum confinement
structures, (a)quantum well, (b) quantum wire,
and(c) quantum dot.
Besides confinement in all three dimensions i.e. Quantum
Dot - other quantum confined semiconductors include:
quantum wires, which confine electrons or holes in two
spatial dimensions and allow free propagation in the third.
quantum wells, which confine electrons or holes in one
dimension and allow free propagation in two dimensions.
Quantum Dot , Quantum Wires and Quantum Well
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.
Optical Propertiesquantum dots of the same material, but with different sizes, can emit light of different colors. The physical reason is the quantum confinement effect.
The larger the dot, the redder (lower energy) its fluorescence spectrum. Conversely, smaller dots emit bluer (higher energy) light. The coloration is directly related to the energy levels of the quantum dot.
As with any crystalline semiconductor, a quantum dot's electronic wave functions extend over the crystal lattice. Similar to a molecule, a quantum dot has both a quantized energy spectrum and a quantized density of electronic states near the edge of the band gap.
Figure- The energy band gap associated with semi-conducting materials. In order to produce electric current electrons must exist in the conduction band.
COLORIFIC PROPERTIES
The height,and energy difference,between energy levels increases as the size of the quantum dot decreases.
Smaller Dot=Higher Energy=Smaller Wavelength=Blue Color
COLOR & QUANTUM DOTS
Figure - Solutions of quantum dots of varying size. Note the variation in color of each solution illustrating the particle size dependence of the optical absorption for each sample. Note that the smaller particles are in the blue solution (absorbs blue), and that the larger ones are in the red (absorbs red).
CHARACTERISTICS OF QUANTUM DOT
In addition to such tuning, a main advantage with quantum
dots is that, because of the high level of control possible over
the size of the crystals produced, it is possible to have very
precise control over the conductive properties of the material.
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.
Generally, the smaller size of the crystal, the larger band gap
, the greater 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.
QUANTUM DOTS DESCRIPTION…
The emission and absorption spectra corresponding to the energy band gap of the quantum dot is governed by quantum confinement principles in an infinite square well potential.
The energy band gap increases with a decrease in size of the quantum dot.
QUANTUM CONFINEMENT
Figure As the energy well, or the particle, shrinks the gap in energy levels increases.
Quantum Dots in Photovoltaics
The quantum dots can be engineered to absorb a specific wavelength of light or to absorb a greater portion of sunlight based on the application.
SOLAR CELLS AND PHOTOVOLTAICS Traditional solar cells are made of semi-conductors
and expensive to produce. Theoretical upper limit is 33% efficiency for conversion of sunlight to electricity for these cells.
Utilizing quantum dots allows realization of third-generation solar cells at ~60% efficiency in electricity production while being $100 or less per square meter of paneling necessary.
Effective due to quantum dots’ ability to preferentially absorb and emit radiation that results in optimal generation of electric current and voltage.
HOW QUANTUM DOTS ARE MADE ???......Quantum dots are manufactured in a two step reaction process in a glass flask.
Nucleation: This is initiated by heating a solvent to approximately 500 degrees Fahrenheit and injecting precursors such as cadmium and selenium.
They chemically decompose and recombine as pure CdSe (cadmium selenide) nanoparticles.
Growth: The size of the nanocrystals can be determined based upon varying the length of time of reaction.
SELF-ASSEMBLED QUANTUM DOTS
Each dot is about 20 nanometers wide and 8 nanometers in height
ADDING SHELLS TO QUANTUM DOTScapping a core quantum dot with a shell (several atomic layers of an inorganic wide band semiconductor) reduces nonradiative recombination and results in brighter emission, provided the shell is of a different semiconductor material with a wider bandgap than the core semiconductor material
http://www.youtube.com/watch?v=ohJ0DL2_HGs&feature=related
APPLICATIONS OF QUANTUM DOTS
APPLICATIONSQuantum dots are particularly significant for optical applications due to their high extinction co-efficient , single-electron transistor, implementations of qubits for quantum information process
Computing
Biology
Photovoltaic device
Light emitting device
LEDs (light emitting diodes); solid state white light, lasers, displays, memory, cell phones, and biological markers.
Biological marker applications of quantum dots have been the earliest commercial applications of quantum dots.
In these applications, quantum dots are tagged to a variety of nanoscale agents, like DNA, to allow medical researchers to better understand molecular interactions. (The Next Big Thing is Really Small, Jack Uldrich with Deb Newberry, p. 81)
QUANTUM DOT APPLICATIONS
QUANTUM DOT LEDS
Used to produce inexpensive, industrial quality white light. Marked improvement over traditional LED–phosphor integration
by dot’s ability to absorb and emit at any desired wavelength. Produce white light by intermixing red, green, and blue emitting
dots homogenously within the phosphor difficult to accomplish with the traditional LED-phosphor set up.
http://www.youtube.com/watch?v=SVyC8JW-Q3A&feature=related
Quantum Dot LED
QUANTUM DOT LASER
http://www.youtube.com/watch?v=OaLDF4AJ1hc
0-D confinement in quantum dots allows for higher efficiencies and brighter lasers because you have better control of photon energies.
MEDICAL IMAGING AND DISEASE DETECTION
Can be set to any arbitrary emission spectra to allow labeling and observation of detailed biological processes.
Quantum Dots can be useful tool for monitoring cancerous cells and providing a means to better understand its evolution.
In the future, Qdots could also be armed with tumor-fighting toxic therapies to provide the diagnosis and treatment of cancer.
Qdots are much more resistant to degradation than other optical imaging probes such as organic dyes, allowing them to track cell processes for longer periods of time.
Quantum dots offer a wide broadband absorption spectrum while maintaining a distinct, static emission wavelength.
Nanoparticles of cadmium selenide (quantum dots) glow when exposed to ultraviolet light. When injected, functionalized quantum dots can target cancer tumors. The surgeon can see the glowing tumor, and use it as a guide for more accurate tumor removal.
Antibody-coated QDs within biodegradable polymeric nanospheres.
Upon entering the cytosol, the polymer nanospheres undergo hydrolysis and thereby release the QD bioconjugates.
Live cell imaging with biodegradable Q dot nanocomposites
Quantum dots, visible under UV light, have accumulated in tumors of a mouse.www.whitaker.org/ news/nie2.html
Gao et al. (2004) reported
OTHER FUTURE QUANTUM DOT APPLICATIONS…
Anti-counterfeiting capabilities: inject dots into liquid mixtures, fabrics, polymer matrices, etc. Ability to specifically control absorption and emission spectra to produce unique validation signatures. Almost impossible to mimic with traditional semi-conductors.
Counter-espionage / Defense applications: Integrate quantum dots into dust that tracks enemies. Protection against friendly-fire events.
Research continues. The possibilities seem endless…
CONCLUSIONS Quantum Dots are a new and innovative
perspective on the traditional semiconductor. Quantum Dots can be synthesized to be
essentially any size,and therefore,produce essentially any wavelength of light.
There are many possible applications of Quantum Dots in many different areas of industry/science.
The future looks bright and exciting on all the possible applications of Quantum Dots.
