Doping—the intentional introduction of impurities into a material—is fundamental to controlling the properties of bulk semiconductors. This has stimulated similar efforts to dope semiconductor nanocrystals1,2,3,4. Despite some successes5,6,7,8,9,10,11, many of these efforts have failed, for reasons that remain unclear. For example, Mn can be incorporated into nanocrystals of CdS and ZnSe (refs 7–9), but not into CdSe (ref. 12)—despite comparable bulk solubilities of near 50 per cent. These difficulties, which have hindered development of new nanocrystalline materials13,14,15, are often attributed to ‘self-purification’, an allegedly intrinsic mechanism whereby impurities are expelled. Here we show instead that the underlying mechanism that controls doping is the initial adsorption of impurities on the nanocrystal surface during growth. We find that adsorption—and therefore doping efficiency—is determined by three main factors: surface morphology, nanocrystal shape, and surfactants in the growth solution. Calculated Mn adsorption energies and equilibrium shapes for several nanocrystals lead to specific doping predictions. These are confirmed by measuring how the Mn concentration in ZnSe varies with nanocrystal size and shape. Finally, we use our predictions to incorporate Mn into previously undopable CdSe nanocrystals. This success establishes that earlier difficulties with doping are not intrinsic, and suggests that a variety of doped nanocrystals—for applications from solar cells16 to spintronics17—can be anticipated.
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This work was supported by the US Office of Naval Research, the NSF-MRSEC at the University of Minnesota, and NSF-CTS. Computations were performed at the Department of Defense Major Shared Resource Center at ASC. We thank Y. Nesmelov, P. Hasjim and R. Weber for experimental assistance.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
This document lists and discusses the specific surface reconstructions used in the calculations of semiconductor surface energies and of binding energies for Mn adsorbates. (PDF 18 kb)
This document provides a detailed derivation of the optical model used for describing measured photoluminescence intensity ratios. (PDF 48 kb)
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Erwin, S., Zu, L., Haftel, M. et al. Doping semiconductor nanocrystals. Nature 436, 91–94 (2005) doi:10.1038/nature03832
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