Impurity Doping in PbSe Nanocrystals

We recently proposed that impurity doping in colloidally grown nanocrystals is controlled primarily by kinetics, rather than by thermodynamics.\footnote{S.C. Erwin, L. Zu, M.I. Haftel, Al.L. Efros, T.A. Kennedy, and D.J. Norris. Doping semiconductor nanocrystals. Nature 436, 91 (2005).} In this ``trapped dopant'' model, the diffusion of an impurity through a nanocrystal is negligible at colloidal growth temperatures. Therefore, an impurity can only be incorporated into a growing nanocrystal if it first adsorbs on the surface and is then overgrown. But this simple surface adsorption process is complicated by a competing process: the binding of the impurity by surfactant molecules, which are used in the growth solution to passivate the nanocrystal and control its growth. Here we use density-functional theory to study the interplay and outcome of these two processes for the doping of PbSe nanocrystals by a variety of candidate impurities (Mn, Cl, In, Cd, Tl) in the presence of several widely used growth surfactants (oleic acid, trioctylphosphine, hexadecylamine).