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Quantum Confinement in Semiconductor Nanocrystals

Semiconductor nanocrystals (quantum dots) are single crystals of semiconductor material typically between 1 and 100 nm. Common examples are cadmium selenide, cadmium sulfide, or zinc sulfide nanocrystals. What make these nanocrystals so interesting are their size-dependent optical and electronic properties. These size tunable properties arise from quantum confinement, which is a result of the nanocrystal being smaller than the bulk semiconductor Bohr exciton diameter (2x the Bohr exciton radius, aB). By forcing the electron and hole to occupy a space smaller than the normal equilibrium distance in the bulk material (dotted circles, above), it requires more energy to promote the electron from the valence band to the conduction band; thus, the smaller the nanocrystal, the larger the band gap of the material and the bluer (higher energy/shorter wavelength) the emission from the nanocrystals.

Z-STEM of CdSe nanocrystals.
There are many 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. Each of these applications has different requirements which can be met by engineering the nanocrystals to exhibit the desired properties. We develop new nanocrystal structures specifically designed to optimize the optical and/or electrical properties for its intended application (the nanocrystals need to behave much differently in a solar cell than in an LED, for example). In addition to designing and synthesizing new nanocrystal architectures, we are able to fully characterize the structural effects on the properties by using a combination of electron microscopy, and both single nanocrystal and ultrafast spectroscopy.
Fourier filtered Z-STEM (color added) of a QDot-655 CdSe/CdS/ZnS nanocrystal.
Nanocrystal structures that we have developed in the Rosenthal lab include: ultrasmall (<2nm) white light emitting CdSe nanocrystals with enhanced quantum yield designed for quantum dot solid state lighting applications, homogeneous alloy CdSxSe1-x nanocrystals and heterogeneous graded alloy CdSxSe1-x nanocrystals as efficient nanoscale emitters for applications in solid state lighting and/or biological probes, and both plasmonic and non-plasmonic CuxInyS2 nanocrystals with tunable surface chemistry for applications in quantum dot photovoltaics.
Selected Publications
Harrison, M. A.; Ng, A.; Hmelo, A. B.; Rosenthal, S. J., CdSSe Nanocrystals with Induced Chemical Composition Gradients. Isr. J. Chem. 2012, 52 (11-12), 1063-1072.

Niezgoda, J. S.; Harrison, M. A.; McBride, J. R.; Rosenthal, S. J., Novel Synthesis of Chalcopyrite CuxInyS2 Quantum Dots with Tunable Localized Surface Plasmon Resonances. Chem. Mater. 2012, 24 (16), 3294-3298.

McBride, J. R.; Dukes, A. D., III; Schreuder, M. A.; Rosenthal, S. J., On ultrasmall nanocrystals. Chem. Phys. Lett. 2010, 498 (1-3), 1-9.

Rosenthal, S. J.; McBride, J.; Pennycook, S. J.; Feldman, L. C., Synthesis, surface studies, composition and structural characterization of CdSe, core/shell and biologically active nanocrystals. Surf. Sci. Rep. 2007, 62 (4), 111-157.

Swafford, L. A.; Weigand, L. A.; Bowers, M. J., II; McBride, J. R.; Rapaport, J. L.; Watt, T. L.; Dixit, S. K.; Feldman, L. C.; Rosenthal, S. J., Homogeneously Alloyed CdSxSe1-x Nanocrystals: Synthesis, Characterization, and Composition/Size-Dependent Band Gap. J. Am. Chem. Soc. 2006, 128 (37), 12299-12306.

Bowers, M. J., II; McBride, J. R.; Rosenthal, S. J., White-Light Emission from Magic-Sized Cadmium Selenide Nanocrystals. J. Am. Chem. Soc. 2005, 127 (44), 15378-15379.