In response to ever increasing energy demands and subsequent costs, a tremendous emphasis is being placed on energy saving to develop solid state lighting devices in the form of light emitting diodes, or LED’s. Specifically, a need exists for pure white-light LED’s as a more efficient replacement for conventional lighting sources. Switching to solid state lighting would reduce global electricity use by 50% and reduce power consumption by 760 GW in the United States alone over a 20 year period. The complications associated with design and fabrication of such devices have generated great interest in developing white-light phosphors that do not depend on complex doping schemes or combinations of materials. One proposed solution is to use a mixture of semiconductor nanocrystals as the intrinsic emitting layer for an LED device. Semiconductor nanocrystals exhibit high fluorescence quantum efficiencies and large molar absorptivities. However, they still suffer from the problem that simply mixing the traditional red, green, and blue colors to achieve white light results in a loss in total device efficiency due to self absorption for a device of more than a few monolayers.
In the Rosenthal research group, we have demonstrated white-light emission from ultra-small cadmium selenide (CdSe) nanocrystals. This raises the intriguing possibility of using these nanocrystals as a white-light phosphor. These ultra-small nanocrystals exhibit broadband emission (420 – 710 nm) throughout most of the visible light spectrum while not suffering from self absorption. This is the direct result of the extremely narrow size distribution and an unusually large (40-50 nm) Stokes shift making them ideal materials for devices currently under development and also an ideal platform to study the molecule-to-nanocrystal transition. More recently, we have developed a post-synthetic treatment to modify the surface chemistry of the nanocrystals that increases the fluorescence quantum yield of ultrasmall white light emitting CdSe nanocrystals from 8% up to 45%. This brightened emission opens the possibility for even further quantum yield improvement and potential for use of these white light nanocrystals in solid state lighting applications.
Rosson, T. E.; Claiborne, S. M.; McBride, J. R.; Stratton, B. S.; Rosenthal, S. J., Bright White Light Emission from Ultrasmall Cadmium Selenide Nanocrystals. J. Am. Chem. Soc. 2012, 134 (19), 8006-8009.
Schreuder, M. A.; Xiao, K.; Ivanov, I. N.; Weiss, S. M.; Rosenthal, S. J., White Light-Emitting Diodes Based on Ultrasmall CdSe Nanocrystal Electroluminescence. Nano Lett. 2010, 10 (2), 573-576.
Schreuder, M. A.; Gosnell, J. D.; Smith, N. J.; Warnement, M. R.; Weiss, S. M.; Rosenthal, S. J., Encapsulated white-light CdSe nanocrystals as nanophosphors for solid-state lighting. J. Mater. Chem. 2008, 18 (9), 970-975.
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.