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Quantum Effects
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
Relative size of quantum dots
http://www.qdots.com
Energy Band Diagrams
Intrinsic Semiconductors
IV
Compound Semiconductors
III
V
Fluorescence
Density of States
(how closely packed energy levels are)
Quantum confinement
Matter Waves
Particle in a Box Analogy
de Broglie wavelength
The Schrödinger Equation
The Schrödinger equation is an equation for
finding a particle’s wave function (x) along the
x-axis.
Particle in a box
Particle in a box
Quantized energy levels are found by solving the Schrödinger equation.
Wave function:
 n x  
2
 nx 
sin 

L  L 
Allowed Energies:
2
h
En  n 2
8mL2
En  n 2 E1
n  1,2,3...
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
http://www.kqed.org/quest/television/nanotechnology-takes-off (3:30)
With quantum dots, the size of the bandgap is controlled simply by adjusting the
size of the dot
http://nanopedia.case.edu/NWPage.php?page=in.jjm8.007
Energy of a photon E=h=hc/
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.
Spectral Codes
ZnSe
CdSe
CdTe
InAs
335
388
460
564
729
1033
1771
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.
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.
How Quantum Dots are Made
http://www.youtube.com/watch?v=MLJJkztIWfg
http://www.mrsec.wisc.edu/Edetc/nanolab/CdSe/index.html
Self-assembled quantum dots
Each dot is about 20 nanometers wide
and 8 nanometers in height
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
http://www.youtube.com/watch?v=ohJ0DL2_HGs&feature=related
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)
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.
Parts of a Quantum
Dot
Functionalizing
a Quantum
Dot
Core : “Active Material”
1-20 nm Size Spheres,
Rods, Disks, etc...
Shells : “Protective or
Complementary Layers”
Functional Groups:
“Chemical Hooks or other
chemically, electrically, or
optically active groups”
Surface Groups/Ligand: “Passivating, Protective, and
Chemically Active Layer”
The basic parts of a quantum dot include the core, shell, and surface ligand. The
5
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.
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.
Quantum dots, visible under UV light, have accumulated in tumors of a mouse.
Here, the nucleus is blue, a
specific protein within the nucleus
is pink, mitochondria look yellow,
microtubules are green, and actin
filaments are red.
QUANTUM DOT CORP., HAYWARD, CA
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 Dot Lasers and LEDs
Schematic of a semiconductor laser
Quantum Dot Laser
0-D confinement in quantum dots allows for higher efficiencies and brighter lasers
because you have better control of photon energies.
http://www.youtube.com/watch?v=OaLDF4AJ1hc
Quantum Dot LED
http://www.youtube.com/watch?v=SVyC8JW-Q3A&feature=related