n-type colloidal semiconductor nanocrystals

Colloidal semiconductor nanocrystals1,2 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 diodes5,6 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.