Transcript Quantum Dots

Quantum Dots
Image courtesy of Evident Technologies
Paul Hemphill, Christian Lawler, & Ryan Mansergh
Physics 4D
Dr. Ataiiyan
6/16/2006
Introduction:
• What are they?
• How are they made?
Image courtesy of Dr. D. Talapin, University of Hamburg
What are they?
• Quantum dots are semiconductor nanocrystals.
• They are made of many of the same materials
as ordinary semiconductors (mainly combinations
of transition metals and/or metalloids).
• Unlike ordinary bulk semiconductors, which are
generally macroscopic objects, quantum dots are
extremely small, on the order of a few
nanometers. They are very nearly zerodimensional.
What’s So Special About Quantum
Dots?
First we need some background on semiconductors…
• When a wave is confined within a boundary, it has specific
allowed energy levels and other “forbidden” energy levels.
This is true for anything that can be described as a wave
by quantum mechanics.
• In bulk semiconductors, the presence of many atoms
causes splitting of the electronic energy levels, giving
continuous energy bands separated by a “forbidden
zone.” The lower-energy, mostly filled band is called the
valence band and the higher-energy, mostly empty band is
called the conduction band. The energy gap, called the
bandgap, is essentially fixed for a given material.
• Semiconductors can carry a current when some of their
electrons gain enough energy to “jump” the bandgap and
move into the conducting band, leaving a positive “hole”
behind.
Bands and the Bandgap
Image courtesy of Evident Technologies
Bands and the Bandgap
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Excitons
• We call the electron-hole pairs
“excitons.”
• Excitons for a given semiconductor
material have a particular size (the
separation between the electron and the
corresponding hole) called the “exciton
Bohr radius.”
So What?
• In a bulk semiconductor the excitons are only confined to
the large volume of the semiconductor itself (much larger
than the exciton Bohr radius), so the minimum allowed
energy level of the exciton is very small and the energy
levels are close together; this helps make continuous
energy bands.
• In a quantum dot, relatively few atoms are present (which
cuts down on splitting), and the excitons are confined to a
much smaller space, on the order of the material’s exciton
Bohr radius.
• This leads to discrete, quantized energy levels more like
those of an atom than the continuous bands of a bulk
semiconductor. For this reason quantum dots have
sometimes been referred to as “artificial atoms.”
• Small changes to the size or composition of a quantum
dot allow the energy levels, and the bandgap, to be finetuned to specific, desired energies.
How are they made?
• Colloidal Synthesis: This method can be used to create
large numbers of quantum dots all at once. Additionally,
it is the cheapest method and is able to occur at
non-extreme conditions.
• Electron-Beam Lithography: A pattern is etched by an
electron beam device and the semiconducting material
is deposited onto it.
• Molecular Beam Epitaxy: A thin layer of crystals can be
produced by heating the constituent elements separately
until they begin to evaporate; then allowing them to collect
and react on the surface of a wafer.
History & Background:
• A brief history of the development of quantum dots
• The semiconductor properties of quantum dots
Image courtesy of Evident Technologies
A Brief History of QDots
• Research into semiconductor colloids began in the
early 1960s.
• Quantum dot research has been steadily increasing since
then, as evidenced by the growing number of
peer-reviewed papers.
• In the late ‘90s, companies began selling quantum dot
based products, such as Quantum Dot Corporation.
• 2004 - A research group at the Los Alamos Laboratory
found that QDs produce 3 electrons per high energy
photon (from sunlight).
• 2005 - Researchers at Vanderbilt University found that
CdSe quantum dots emit white light when excited by UV
light. A blue LED coated in a mixture of quantum dots
and varnish functioned like a traditional light bulb.
Image courtesy of J. Am. Chem. Soc.
Practical Applications:
• Optical Storage
• LEDs
• Organic Dyes
• Quantum Computing
• Security
• Solar Power
Image courtesy of TDK
Optical Storage
• Quantum dots have been an enabling technology
for the manufacture of blue lasers
• The high energy in a blue laser allows for as much
as 35 times as much data storage than conventional
optical storage media.
• Less affected by temperature fluctuations, which
reduces data errors.
• This technology is currently available in new highdefinition DVD players, and will also be used in the
new Sony Playstation 3.
Light Emitting Diodes
Image courtesy of Sandia National Laboratories
Light Emitting Diodes
• Quantum Light Emitting Diodes (QLEDs) are superior
to standard LEDs in the same ways the quantum dots
are superior to bulk semiconductors.
• The tunability of QDs gives them the ability to emit nearly
any frequency of light - a traditional LED lacks this
ability.
• Quantum dot-based LEDs can be crafted in a wide
range of form factors.
• Traditional incandescent bulbs may be replaced using
QLED technology, since QLEDs can provide a low-heat,
full-spectrum source of light.
Organic Dyes
• In vivo imaging of biological
specimens.
• Long-term photostability.
• Multiple colors with a single
excitation source.
• Possible uses for tumor
detection in fluorescence
spectroscopy.
• Possible toxicity issues?
Image courtesy of Invitrogen
Quantum Computing
• Pairs of quantum dots are candidates for qubit
fabrication.
• The degree of precision with which one can measure
the quantum properties of the dots is very high, so a
quantum computer (which functions by checking the state
and superposition of the quantum numbers in entangled
groups) would be easily constructed.
Security
• Quantum dots can be used in the fabrication of
artificial “dust” set up to emit at a specific frequency
of infrared light.
• This dust could be used in any number of security-related
applications.
• Placing the dust in hostile, difficult-to-monitor terrain would
allow the tracking of forces moving through the area, as it
would stick to their clothing and equipment.
• This “taggant” causes any coated object to become
highly visible when viewed through night-vision goggles.
Solar Power
• The adjustable bandgap of quantum dots allow the
construction of advanced solar cells.
• These new cells would benefit from the adjustability
of the dots, as they would be able to utilize much more
of the sun’s spectrum than before.
• Quantum dots have been found to emit up to three
electrons per photon of sunlight, as opposed to only
one for standard photovoltaic panels.
• Theoretically, this could boost solar power efficiency
from 20-30% to as high as 65%
Conclusion
• A number of additional applications exist or are being
developed that utilize quantum dots.
• Quantum dots provide an example of the possibilities
that research at the nanoscale can provide.
• The future is bright for this new and innovative
technology.
References:
• R. D. Schaller and V. I. Klimove,
Phys. Rev. Lett. 92, 186601 (2004)
• Michael J. Bowers II, James R. McBride, and Sandra J. Rosenthal
J. Am. Chem. Soc.; 2005; 127(44) pp 15378 - 15379
• http://www.ivitrogen.com/
• http://www.evidenttech.com/
• http://www.vanderbilt.edu/exploration/stories/quantumdotled.html
• http://en.wikipedia.org/wiki/Quantum_dots
• http://www.engineering.ucsb.edu/Announce/nakamura.html
• http://www.grc.nasa.gov/WWW/RT2001/5000/5410bailey1.html
• http://www.moo.uklinux.net/kinsler/ircph/maze/quantum-dot.html
• http://www.moo.uklinux.net/kinsler/ircph/maze/quantum-confinement.html
• http://www.chem.ucsb.edu/~strouse_group/learning.html
• http://qt.tn.tudelft.nl/grkouwen/qdotsite.html