Nanocomposite solid-state lighting

WhiteLightNanocrystal-based devices produce a higher quality white-light than current light sources and present an opportunity for higher efficiencies.


Semiconductor nanocrystals or quantum dots are single crystals of semiconductor materials typically between 1 and 100 nm. Common examples are cadmium selenide, cadmium sulfide, or zinc sulfide nanocrystals. What make these nanocrystals so interesting are the size tunable optical and electronic properties. These size tunable properties arise from quantum confinement. Quantum confinement is a result of the nanocrystal being smaller than the bulk semiconductor Bohr exciton diameter (right). By forcing the electron and hole to occupy a space smaller than the normal equilibrium distance in the bulk material (dotted line), it takes more energy to promote the electron from the valance band to the conduction band, hence the smaller the nanocrystal is the larger the band gap of the material and the bluer the emission from the nanocrystals becomes. This is illustrated by the CdSe nanocrystals (right).

The nanocrystals actually exhibit the same crystal structure as the respective bulk material. In the case of CdSe, the preferred phase is wurtzite; however it is not uncommon to also see zinc blende phases as it is a simple bond rotation difference and very similar in energy to the wurtzite phase. This is shown beautifully in the Z-STEM image below.

There are several potential applications for quantum dots. The main focuses of the Rosenthal group have been photovoltaics and fluorescent labeling in biological systems. More recently we have ventured into solid state lighting (SSL). Each of these applications has different requirements which can be met by engineering the nanocrystals to exploit the desired properties. For example , nanocrystals for fluorescent labeling need to have a high quantum efficiency, maintain that efficiency under biological conditions and a narrow emission band width for multiplexing experiments. In the case of photovoltaics, where the nanocrystals are serving as a light harvesting active layer, fluorescence is extremely undesirable because fluorescence is simply a loss of carriers (electrons or holes) that could be funneled off of the nanocrystals and used to do work. By modifying the surface by adding another semiconductor as a shell or even changing the surface ligand we can engineer the nanocrystals for a specific task.

A special case of tuning the optical properties of CdSe nanocrystals, was recently discovered by Michael Bowers, James McBride and Sandy Rosenthal and reported in the Journal of the American Chemical Society. In the communication, White-Light Emission from Magic-Sized Cadmium Selenide Nanocrystals broad-band, white-light emission was reported from a single size of ultra small CdSe nanocrystals. These nanocrystals are approximately 1.5 nm in diameter. They emit light throughout most of the visible light region making them appear white when illuminated by ultraviolet or near ultraviolet light.

These white-light emitting nanocrystals can be encapsulated into many different polymers. In the examples on the right, the nanocrystals are in polyurethane wood finish. The lower panels depict films of the nanocrystal-polyurethane composite being illuminated by a frequency doubled Ti:sapphire laser. The white light is clearly seen reflecting from the table surface. The upper right panel is a commercial blue (400 nm) light emitting diode (LED) also coated with the nanocrystal-polyurethane composite. The upper left panel is a vial of the magic-sized nanocrystals (in solution) being illuminated by the same laser source. In this case we are demonstrating these nanocrystals as a frequency downconverting material that takes a single near UV light source and converts it to broadband visible light. We are also investigating the use of this material in electroluminescence devices where the nanocrystals would be excited electrically rather than optically.

The core/shell nanocrystals are nanocrystals of one semiconductor wrapped in an epitaxial shell of another semiconductor. The advantages of this are many, but the main advantage is that adding a shell typically improves the quantum yield of the core nanocrystals material by several fold. The increase can in some cases cause the static quantum yield to be near unity. Reasons for this increase in quantum yield include passivation of surface defects and providing electronic confinement of the charges.

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References

Pinned emission from ultrasmall cadmium celenide nanocrystals, Albert D. Dukes III, Michael A. Schreuder, Jessica A. Sammons, James R. McBride, Nathanael J. Smith and Sandra J. Rosenthal. Journal of Chemical Physics, 129, 121102, (2008)

Bowers, M. J., II; McBride, J. R.; Rosenthal, S. J. Journal of the American Chemical Society 127 pp. 15378-15379 (2005)

McBride, J. R.; Kippeny, T. C.; Pennycook, S. J.; Rosenthal, S. J. Nano letters 4 pp. 1279-1283 (2004)

Kippeny, T. C.; Swafford, L. A.; Rosenthal, S. J. Journal of Chemical Education 79 pp. 1094-1100 (2002)