1. James Franck Institute, University of Chicago, 5640 S. Ellis Avenue, Chicago, Illinois 60637 , USA

Correspondence to: Moonsub ShimPhilippe Guyot-Sionnest Correspondence and requests for materials should be addressed to M.S. (e-mail: Email: mshim@uchicago.edu) or P.G.S. (e-mail: Email:  pgs@uchicago.edu).

Colloidal semiconductor nanocrystals^{1, 2} combine the physical and chemical properties of molecules with the optoelectronic properties of semiconductors. Their colour is highly controllable, a direct consequence of quantum confinement on the electronic states³. Such nanocrystals are a form of 'artificial atoms' (ref. 4) that may find applications in optoelectronic systems such as light-emitting diodes^{5, 6} and photovoltaic cells⁷, or as components of future nanoelectronic devices. The ability to control the electron occupation (especially in n-type or p-type nanocrystals) is important for tailoring the electrical and optical properties, and should lead to a wider range of practical devices. But conventional doping by introducing impurity atoms has been unsuccessful so far: impurities tend to be expelled from the small crystalline cores (as observed for magnetic impurities⁸), and thermal ionization of the impurities (which provides free carriers) is hindered by strong confinement. Here we report the fabrication of n-type nanocrystals using an electron transfer approach commonly employed in the field of conducting organic polymers⁹. We find that semiconductor nanocrystals prepared as colloids can be made n-type, with electrons in quantum confined states.