Quantum Dots

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Quantum Dots

Paul Hemphill, Christian Lawler, & Ryan Mansergh
Physics 4D

Dr. Ataiiyan

6/16/2006

Image courtesy of Evident Technologies

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 zero
-
dimensional.

What’s So Special About Quantum
Dots?


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.



First we need some background on semiconductors


Bands and the Bandgap

Image courtesy of Evident Technologies

Bands and the Bandgap

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 fine
-
tuned 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.

• 2004
-

A research group at the Los Alamos Laboratory


found that QDs produce 3 electrons per high energy


photon (from sunlight).

• In the late ‘90s, companies began selling quantum dot


based products, such as Quantum Dot Corporation.

• 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.

• This technology is currently available in new high
-


definition DVD players, and will also be used in the


new Sony Playstation 3.

• Less affected by temperature fluctuations, which


reduces data errors.

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.

• Traditional incandescent bulbs may be replaced using


QLED technology, since QLEDs can provide a low
-
heat,


full
-
spectrum source of light.

• Quantum dot
-
based LEDs can be crafted in a wide


range of form factors.

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.

• This “taggant” causes any coated object to become


highly visible when viewed through night
-
vision goggles.

• 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.

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.

• Theoretically, this could boost solar power efficiency


from 20
-
30% to as high as 65%

• Quantum dots have been found to emit up to three


electrons per photon of sunlight, as opposed to only


one for standard photovoltaic panels.

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