Quantum Dots/Semiconducting Nanocrystals: Optical Applications, 14-9512

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Principal Investigators
Ralph H. Hill Jr.
Cliff J. Scribner

Inclusive Dates:  10/01/04 – Current

Background - During the last decade or so, great progress has been made in nanotechnology, especially in the field of quantum dots (QDs). Other terms include semiconductor nanocrystals or artificial atoms. Their composition and small size (a few thousand atoms) gives them extraordinary optical properties, which can be customized by changing the size or composition of the dots. These interesting properties are brought about from the quantum confinement of the electrons. Their optical characteristics are different from standard organic dyes and therefore may have new uses. Of particular interest is their strong narrow-band fluorescence and exceptionally broad excitation spectrum. They have been commercially available from several sources in different formats during the last few years.

Approach - The prospects for this rapidly developing area of technology look very promising for the future. The anticipated result of this project is to gain hands-on experience in the field of QDs, specifically in the various types of QDs, both visible fluorescing as well as those in the near-infrared (NIR) region. There are some caveats to be addressed, such as the toxicity of the heavy-metal materials.

Accomplishments - The research team has experimented with commercially available QDs, both visible and NIR fluorescing. These were purchased in several formats, such as suspended in organic solvent, suspended in ultraviolet-curing resin, deposited on a substrate by spin coating, and in a polymer particle format. Most of our work has focused on excitation with a small ultraviolet light-emitting diode (LED) source, thus demonstrating the large excitation cross section of the QDs as compared to standard organic dyes. We have determined that a reasonable amount of short wave ultraviolet or gamma ray exposure does not degrade the fluorescence for the QDs tested. We have studied the robustness of the QDs under various scenarios. For instance, the QDs are resistant to photobleaching while under excitation parameters typical of (or more intense than) standard fluorescent microscopy. We have used Atomic Force Microscopy (AFM) to determine the surface characteristics when the QDs are deposited in polymers. We found that, under some conditions, small QDs clump into larger surface features, which could influence optical performance. See the illustration below for an example using QDs made from cadmium selenide (CdSe) for the core material and zinc sulfide (ZnS) for the shell material, embedded in a (poly)methyl-methacrylate (PMMA) substrate matrix. One invention disclosure was filed related to quantum dots.

Atomic Force Micrograph (AFM) of Quantum Dots (QDs) in a Polymer Matrix (The large bumps are QD clumps, with each individual QD being much smaller, typically on the order of 3.2 nanometers in diameter. Note that the x and y axes are on a different scale from the z axis, which is magnified relative to the x and y axes.)

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