Quantum Dots


Nov 1, 2013 (5 years and 4 months ago)


Quantum Dots

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

Dr. Ataiiyan


Image courtesy of Evident Technologies


• 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

What’s So Special About Quantum

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”

First we need some background on semiconductors

Bands and the Bandgap

Image courtesy of Evident Technologies

Bands and the Bandgap


We call the electron
hole pairs

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

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

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


• Traditional incandescent bulbs may be replaced using

QLED technology, since QLEDs can provide a low

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


• Long
term photostability.

• Multiple colors with a single

excitation source.

• Possible uses for tumor

detection in fluorescence


• Possible toxicity issues?

Image courtesy of Invitrogen

Quantum Computing

• Pairs of quantum dots are candidates for qubit


• 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.


• 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


• This “taggant” causes any coated object to become

highly visible when viewed through night
vision goggles.

• Placing the dust in hostile, difficult
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.


• 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



• R. D. Schaller and V. I. Klimove,

Phys. Rev. Lett.
, 186601 (2004)

• Michael J. Bowers II, James R. McBride, and Sandra J. Rosenthal

J. Am. Chem. Soc.;

(44) pp 15378


• 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

• http://www.moo.uklinux.net/kinsler/ircph/maze/quantum

• http://www.chem.ucsb.edu/~strouse_group/learning.html

• http://qt.tn.tudelft.nl/grkouwen/qdotsite.html