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 WavesParticle in a Box Analogy

de Broglie wavelength

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

Particle in a box

Quantized energy levels are found by solving the Schrödinger equation.

Particle in a box

L

xn

Lxn

sin2

Wave function:

2

22

8mL

hnEn

...3,2,1n

Allowed Energies:

12EnEn

Quantum Dots - A tunable range of energiesBecause 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

http://www.kqed.org/quest/television/nanotechnology-takes-off (3:30)

Changing the length of the box changes the energy levels

http://nanopedia.case.edu/NWPage.php?page=in.jjm8.007

With quantum dots, the size of the bandgap is controlled simply by adjusting the size of the dot

Energy of a photon E=h=hc/

The figure above charts the absorption and emission with corresponding visible spectrum of light colors based upon nanocrystal (quantum dot) size.

J. Lee et al Adv. Materials, 12, 1102 (2000)

Absorption and Emission

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.

ZnSeCdSeCdTeInAs

460 564 729 1033 1771388335Wavelength (nm)

O. Dabbousi et al, J. Phys. Chem., 101, 9463 (1997).

Spectral Codes

How Quantum Dots are MadeQuantum 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.

http://www.youtube.com/watch?v=MLJJkztIWfg

How Quantum Dots are Made

http://www.mrsec.wisc.edu/Edetc/nanolab/CdSe/index.html

Self-assembled quantum dotsEach 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

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.

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

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.

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

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.

http://www.youtube.com/watch?v=SVyC8JW-Q3A&feature=related

Quantum Dot LED