Paul Hemphill, Christian Lawler, & Ryan Mansergh Physics 4D 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 zero-dimensional.

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

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. • 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 high- definition 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