1. Quantum Effects
Quantum dots are semiconducting single crystals with almost zero defects ranging in size from 1 to 20 nanometers. Quantum dots can be synthesized from a variety of materials, and are primarily made from the III-V and II-VI class of semiconductors. These include gallium arsenide, indium phosphide, indium arsenide, cadmium selenide, and zinc sulfide.
Photo by Felice Frankel, J. Phys. Chem., 101, (1997).

2. Quantum dots are unique class of semiconductor because they are so small, ranging from 2-10 nanometers (10-50 atoms) in diameter. At these small sizes materials behave differently, giving quantum dots unprecedented tunability
What Is A Quantum-Dot?
Quantum dots are nanostructures created from standard semiconductive materials such as InAs/GaAs. These structures can be modeled as 3-dimensional quantum wells. As a result, they exhibit energy quantization effects even at distances several hundred times larger than the material system lattice constant.
A quantum dot can indeed be visualized as a well. Electrons, once trapped inside the dot, do not alone possess the energy required to escape. We can use quantum physics to our advantage because the smaller a quantum dot is physically, the higher the potential energy necessary for an electron to escape.
Small quantum dots, such as colloidal semiconductor nanocrystals, can be as small as 2 to 10 nanometers, corresponding to 10 to 50 atoms in diameter and a total of 100 to 100,000 atoms within the quantum dot volume. Self-assembled quantum dots are typically between 10 and 50 nm in size.

4. Relative size of quantum dots
Quantum dots are semiconducting single crystals with almost zero defects ranging in size from 1 to 20 nanometers

5. Energy Band Diagrams
In solid state physics and related applied fields, the band gap is the energy difference between the top of the valence band and the bottom of the conduction band in insulators and semiconductors.
The ease with which electrons in a semiconductor can be excited from the valence band to the conduction band depends on the band gap between the bands.

6. Intrinsic SemiconductorsIV
Silicon is a group IV element, and has 4 valence electrons per atom.
In pure silicon the valence band is completely filled at absolute zero.
At finite temperatures the only charge carriers are the electrons in the conduction band and the holes in the valence band that arise as a result of the thermal excitation of electrons to the conduction band.
These charge carriers are called intrinsic charge carriers, and necessarily there are equal numbers of electrons and holes.
Pure silicon is therefore an example of an intrinsic semiconductor.

7. Compound Semiconductors
III V
Compound Semiconductors
In addition to group IV elements, compounds of group III and group V elements, and also compounds of group II and group VI elements are often semiconductors.
The common feature to all of these is that they have an average of 4 valence electrons per atom.
One example of a compound semiconductor is gallium arsenide, GaAs. In a compound semiconductor like GaAs, doping can be accomplished by slightly varying the stoichiometry, i.e., the ratio of Ga atoms to As atoms.
A slight increase in the proportion of As produces n-type doping, and a slight increase in the proportion of Ga produces p-type doping.

8. In a traditional semiconductor most electrons stay in a lower energy level, called the valence band.
When energy is introduced to the material, via heat or voltage, some of the electrons are pumped to the conduction band energy level.
While making the transition from the valence band to the conduction band, the electrons pass through what is called the band gap.
Electrons in a semiconductor are not stable in the conduction band, so they naturally fall back to the valence band.
During this return to the valence band, the absorbed energy is released via electromagnetic radiation (light or heat).

9. Fluorescence

11. Density of States (how closely packed energy levels are)
Quantum confinement
Density of electronic states as a function of structure size
Evolution of the density of states as the dimensionality of the structure is reduced from 3D (bulk) to 0D (quantum dot). The density of states of an ideal quantum dot is discrete, like in an atom.
Quantum dots are ordered collections of hundreds to thousands of semiconductor-type atoms.
The electrons associated with a dot are confined to this small set of atoms.
On the nano-scale, when the electron is confined, the change in energy levels becomes distinctly discrete – a condition known as “quantum confinement”.
Therefore the band gap is a function of size.
When there are lots of atoms, such as in a traditional semiconductor, the bandgap is not a function of size.
The significance of this nano property is that different size quantum dots will fluoresce in different colors.

12. Particle in a Box Analogy
Matter Waves
Particle in a Box Analogy
de Broglie wavelength

13. The Schrödinger Equation
The Schrödinger equation is an equation for finding a particle’s wave function (x) along the x-axis.

14. Particle in a box

15. Quantized energy levels are found by solving the Schrödinger equation.
Particle in a box
Quantized energy levels are found by solving the Schrödinger equation.
Wave function:
Allowed Energies:

16. Quantum Dots - A tunable range of energies
Because quantum dots' electron energy levels are discrete rather than continuous, the addition or subtraction of just a few atoms to the quantum dot has the effect of altering the boundaries of the bandgap
Changing the length of the box changes the energy levels
(3:30)

17. With quantum dots, the size of the bandgap is controlled simply by adjusting the size of the dot
When the quantum dot is exposed to UV light it accepts the photon and sends an electron from the valence band into the conduction band.
The electron wants to go back to its normal state so it travels back to the valence band.
During this process there is electron radiation which emits the light you see in the vials.
Depending on the size of the bandgap the color will vary.
By controlling the size and composition of quantum dots, it is possible to precisely choose the wavelength of light emitted during fluorescence.
Scientists can custom design and fabricate different dots for different uses, ranging from medical imaging to light emitting diodes (LED’s).
Quantum dots are size dependent
Larger quantum dots have smaller gaps between energy levels. These quantum dots emit lower energy photons with a higher wavelength. These quantum dots are typically 10nm in size, and will fluoresce red or reddish.
Conversely smaller quantum dots have larger gaps between energy levels. These quantum dots emit higher energy photons with a lower wavelength, and fluoresce blue or bluish. Precise control of nanocrystals size allows for any color in the visible range to be accurately reproduced.

18. Energy of a photon E=h=hc/

19. Absorption and Emission
J. Lee et al Adv. Materials, 12, 1102 (2000)
The figure above charts the absorption and emission with corresponding visible spectrum of light colors based upon nanocrystal (quantum dot) size.

20. Spectral Codes
ZnSe
CdSe
CdTe
InAs
460
564
729
1033
1771
388
335
Wavelength (nm)
O. Dabbousi et al, J. Phys. Chem., 101, 9463 (1997).
Additionally, the spectral codes of nanocrystals may vary depending on the type of material used. For example, ZnSe emits at the ultraviolet wavelength spectrum; CdSe and CdTe are wavelengths that are visible to the human eye; and InAs is at the infrared spectrum. This figure details the varying spectral codes of the materials which are color coded by semiconductor material listed in the legend.

21. 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.
Quantum dots are manufactured in a two step reaction process in a glass flask.
The first step is 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.
Once these nanocrystals form, the next step involves growth in which sizes of the nanocrystals can be determined based upon varying the length of time of reaction.
Quantum dots can then be functionalized for various biological uses.

22. How Quantum Dots are Made
Quantum dots are manufactured in a two step reaction process in a glass flask.
The first step is 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.
Once these nanocrystals form, the next step involves growth in which sizes of the nanocrystals can be determined based upon varying the length of time of reaction.
Quantum dots can then be functionalized for various biological uses.

23. Self-assembled quantum dots
Each dot is about 20 nanometers wide and 8 nanometers in height
Formation
We prepare quantum dots in the Stranski-Krastanow growth mode using strained InGaAs on GaAs and SiGe on Si. The quantum dots are defect-free and can be incorporated coherently into the host material, which renders them interesting candidates for electronic and optoelectronic applications.

24. Adding Shells to Quantum Dots
capping 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

25. Quantum Dot Applications
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)
Applications
There are many potential applications using quantum dots.
These include: LEDs (light emitting diodes); solid state white light, lasers, displays, memory, cell phones, optical nanocodes and biological markers.
To date, 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)

26. Quantum Dots Help Doctors See Cancer Cells
Many forms of malignant cancer cells have a high concentration (when compared to healthy cells) of epidermal growth factor receptor (EGFR) on there exterior. Using a technique called labeling, scientists can coat Q-dots with an antibody for EGFR, making the Q-dots attach to the cancer cells.
These antibody coated Q-dots are injected into a patient’s bloodstream or tissue. The area of suspected cancer is then illuminated with either ultraviolet or white light, and doctors literally look for the Q-dots; if cancer is present a very noticeable brightly-colored region (red, green blue, etc. depending on the Q-dots used) will be seen .
Doctors can look for these regions with either an endoscope or on a lab bench with a microscope.
If no cancer is present, the nano-crystals will disperse evenly through the region and be virtually invisible. However, in the event of cancer, the Q-dots will attach directly to the cells, making a very obvious collection of illuminated Q-dots.
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.

27. Functionalizing a Quantum Dot
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.

28. Live cell imaging with biodegradable Q dot nanocomposites
Antibody-coated QDs within biodegradable polymeric nanospheres.
Upon entering the cytosol, the polymer nanospheres undergo hydrolysis and thereby release the QD bioconjugates.

29. www.whitaker.org/ news/nie2.html
A major strength of quantum dots stems from the ability to label several different species with different colors of quantum dot, and to differentiate all tagged species with one wavelength of excitation light.
Quantum dots, visible under UV light, have accumulated in tumors of a mouse.

30. Quantum dots, visible under UV light, have accumulated in tumors of a mouse.
Gao et al. (2004) reported imaging and cancer targeting based on semiconductor quantum dots in animal studies in vivo. In control studies Gao et al. observed the uptake, retention, and distribution of quantum dots primarily in the liver, spleen, brain, heart, kidney, and lung in decreasing order. In nude mice growing human prostate cancer xenograft, quantum dots accumulated specifically at cancer targets showing bright orange red color.

31. Here, the nucleus is blue, a specific protein within the nucleus is pink, mitochondria look yellow, microtubules are green, and actin filaments are red.
Researchers can create up to 40,000 different labels by mixing quantum dots of different colors and intensities as an artist would mix paint. In addition to coming in a vast array of colors, the dots also are brighter and more versatile than more traditional fluorescent dyes: They can be used to visualize individual molecules or, like the older labeling techniques, to visualize every molecule of a given type.
QUANTUM DOT CORP., HAYWARD, CA

32. 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.
Quantum Dots in Photovoltaics
Quantum dots are now being investigated for use in photovoltaic cells.
Producers of quantum dots feel claim that the dyes only absorb a small portion of the available sunlight.
Quantum dots will greatly enhance the efficiency of solar cells by replacing the dyes with the semiconducting quantum dots.
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.
Quantum dots can even absorb near infrared light and lower wavelengths which means they offer more versatility to photocells.
They would be attached to the TiO2 nanoparticles just like the dye.
Quantum dot solar cells are solar cells based on a silicon substrate with a coating of nanocrystals.
A thin film of nanocrystals is obtained by a process known as “spin-coating”. This involves placing an amount of the quantum dot solution onto a flat substrate, which is then rotated very quickly. The solution spreads out uniformly, and the substrate is spun until the required thickness is achieved.
Quantum dot based photovoltaic cells based around dye-sensitised colloidal TiO2 films were investigated in 1991 and were found to exhibit promising efficiency of converting incident light energy to electrical energy, and were found to be incredibly encouraging due to the low cost of materials in the search for more commercially viable/affordable renewable energy sources.
Recent research in experimenting with lead selenide (PbSe) semiconductor, as well as with cadmium telluride (CdTe), which has already been well established in the production of "classic" solar cells.

33. Quantum Dot Lasers and LEDs
A quantum dot laser is a semiconductor laser that uses quantum dots as the active laser medium in its light emitting region. Due to the tight confinement of charge carriers in quantum dots, they exhibit an electronic structure similar to atoms. Lasers fabricated from such an active media exhibit device performance thats is closer to gas lasers, and avoid some of the negative aspects of device performance associated with traditional semiconductor lasers based on bulk or quantum well active media. The quantum dot active region may also be engineered to operate at different wavelengths by varying dot size and composition. This allows quantum dot lasers to be fabricated to operate at wavelengths previously not possible using semiconductor laser technology.
First prototypes of GaAs-based quantum dot lasers have successfully been fabricated and tested. The main advantages of QD lasers are lower power consumption and higher temperature stability. Other advantages are low threshold current and high differential efficiency.
The advantage of a quantum dot laser over the traditional semiconductor laser is that their emitted wavelength depends on the diameter of the dot. Quantum dot lasers are cheaper and offer a higher beam quality than conventional laser diodes
Quantum Dot Lasers and LEDs .

34. Schematic of a semiconductor laser

35. Quantum Dot Laser
0-D confinement in quantum dots allows for higher efficiencies and brighter lasers because you have better control of photon energies.
Confinement also increases the efficiency of today's electronics.  The laser is based on a 2-D confinement layer that is usually created with some form of epitaxy like Molecular Beam Epitaxy or Chemical Vapor Deposition.  The bulk of modern lasers created with this method are highly functional, but ultimately inefficient in terms of energy consumption and heat dissipation.  Moving to 1-D confinement in wires or 0-D confinement in quantum dots allows for higher efficiencies and brighter lasers.  Quantum dot lasers are currently the best lasers available though their fabrication is still being worked out.

36. Quantum Dot LED