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are chemically transformed by exchanging or replacing all of the cations in the crystal lattice with another type of cation.


This cation-exchange technique makes it possible to produce new types of core/shell nanocrystals that are inaccessible through conventional synthesis. Core/shell nanocrystals are heterostructures in which one type of semiconductor is enclosed within another, for example, a cadmium selenide (CdSe) core and a cadmium sulphide (CdS) shell.


“While holding promise for the simple and inexpensive fabrication of multicomponent nanocrystals, the cation-exchange technique has yielded quantum dots and nanorods that perform poorly in optical and electronic devices,” says Alivisatos, a world authority on nanocrystal synthesis who holds a joint appointment with the University of California (UC) Berkeley, where he is the Larry and Diane Bock professor of Nanotechnology.


As Jain tells the story, he was in the process of disposing of CdSe/CdS nanocrystals in solution that were six months old when out of habit he tested the nanocrystals under ultraviolet light. To his surprise he observed significant luminescence. Subsequent spectral measurements and comparing the new data to the old showed that the luminescence of the nanocrystals had increased by at least sevenfold.


“It was an accidental finding and very exciting,” Jain says, “but since no one wants to wait six months for their samples to become high quality I decided to heat the nanocrystals to speed up whatever process was causing their luminescence to increase.”


Jain and the team suspected and subsequent study confirmed that impurities – original cations that end up being left behind in the crystal lattice during the exchange process – were the culprit.


“Even a few cation impurities in a nanocrystal 232 www.compoundsemiconductor.net January / February 2012


are enough to be effective at trapping useful, energetic charge-carriers,” Jain says. “In most quantum dots or nanorods, charge-carriers are delocalized over the entire nanocrystal, making it easy for them to find impurities, no matter how few there might be, within the nanocrystal. By heating the solution to remove these impurities and shut off this impurity-mediated trapping, we give the charge-carriers enough time to radiatively combine and thereby boost luminescence.”


Since charge-carriers are also instrumental in electronic transport, photovoltaic performance, and photocatalytic processes, Jain says that shutting off impurity-mediated trapping should also boost these optoelectronic properties in nanocrystals synthesised via the cation- exchange technique.


This research was supported by the DOE Office of Science.


Further details of this work have been published in the paper, “Highly Luminescent Nanocrystals from Removal of Impurity Atoms Residual From Ion Exchange Synthesis, ” by P.K. Jain et al,published online in Angewandte Chemie,124, Issue 5 on 25 Jan 2012. DOI: 10.1002/ange.201107452


One giant leap for IR technology


Removing gallium from III-V type-II superlattice materials delivers a massive hike in minority carrier lifetime. Thanks to this, these superlattice detectors have the potential to start challenging expensive state-of-the-art HgCdTe infrared imagers


Researchers have successfully demonstrated that InAs/InAsSb Type-II superlattice (T2SL) materials could be a better alternative to conventional InAs/InGaSb T2SLs.


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