Doping PbSe nanocrystals: Predictions based on a trapped-dopant model

We recently proposed that impurity doping in colloidally grown semiconductor nanocrystals is often controlled primarily by kinetics rather than by thermodynamics. In this “trapped-dopant” model the diffusion of an impurity through a nanocrystal is negligible at colloidal growth temperatures. Consequently, an impurity can only be incorporated as a dopant into a growing nanocrystal if it first adsorbs on the surface and is then overgrown. This surface adsorption can be complicated by a competing process: the binding of the impurity by surfactant molecules and other agents added to 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 processes for the doping of PbSe nanocrystals by a number of candidate dopants (Mn, Co, Cl, In, Cd, Tl, etc.) in the presence of two widely used growth additives (oleic acid and hexadecylamine). The results suggest that successful doping requires making a trade-off between surface adsorption (which favors small dopants) and interior trapping (which favors large dopants). Moreover, the widely used growth agent oleic acid binds strongly to almost all dopants, suggesting that the standard growth procedure may require modification for successful doping to be realized.