Chemistry

Two Are Better Than One

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Science  26 Nov 2004:
Vol. 306, Issue 5701, pp. 1439
DOI: 10.1126/science.306.5701.1439c

The key properties of quantum dots, namely their emission color and efficiency, and their stability and solubility, are controlled by the basic chemistry, particle size, presence of defects, and choice of capping ligand (the shell in a core-shell particle). For the visible regime, CdSe has shown the most promise because it can be tuned to emit from red to blue. However, the standard capping materials CdS and ZnSe can become excited by ultraviolet light, leading to photo-oxidation, and there also are toxicity concerns with both of these materials. ZnS is a better choice for biological applications, but it has a large lattice mismatch relative to CdSe, and this leads to defects that degrade the photoluminescent properties.

Talapin et al. have resolved this dilemma by capping the CdSe nanocrystals with two shells. The particles are made with a middle layer of either CdS or ZnSe, which leads to much smaller lattice mismatches at the CdSe/CdS and CdS/ZnS interfaces, and are almost defect-free. The luminescent properties were much less sensitive to the thickness of the capping layers, whereas a single capping material would need to be only two monolayers thick. Surprisingly, the process of putting on two capping layers produces particles with a narrower size distribution. The authors believe that the large lattice mismatch between CdSe and ZnS affects the growth kinetics of the ZnS, and thus this problem too is solved by using sequential capping layers. —MSL

J. Phys. Chem. B. 10.1021/jp046481g (2004).

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