"These results overturn a common belief that nanocrystals are intrinsically difficult to dope because they somehow 'self-purify' by expelling impurities from their interior," said David Norris of the University of Minnesota. "According to this view, the same mechanisms that made it possible to grow very pure nanocrystals also made it extremely difficult to dope them. We have shown that doping difficulties are not intrinsic, and indeed are amenable to systematic optimization using straightforward methods from physical chemistry."
The researchers used density-functional theory to model the absorption of impurities from the nanocrystal surface during growth.
"The key lies in the nanocrystal's surface," said Steven Erwin of the Naval Research Laboratory. "If an impurity atom can stick, or 'adsorb', to the surface strongly enough, it can eventually be incorporated into the nanocrystal as it grows. If the impurity binds to the nanocrystal surface too weakly, or if the strongly binding surfaces are only a small fraction of the total, then doping will be difficult."
Erwin, Norris and colleagues found that it was much easier to dope nanocrystals with a zinc-blende crystal structure - such as zinc sulphide, zinc selenide and cadmium sulphide - with manganese, particularly if they contained a large proportion of (001) facets. Nanocrystals with a wurtzite structure, such as cadmium selenide, or a rock-salt structure, for example lead sulphide and lead selenide, were much more difficult or even impossible to dope.
To illustrate their findings, the scientists grew manganese-doped zinc selenide nanocrystals. They found that at higher selenium:zinc ratios the relative surface area of (001) facets was larger and the manganese concentration inside the crystals was higher. Nanocrystals of less than about 2 nm in size, however, appeared to have a central core that resisted doping.
The team also grew cadmium selenide with a zinc-blende structure, rather than its normal wurtzite structure, by depositing a shell of the material around a zinc selenide core. During growth the cadmium selenide adsorbed manganese.
Surfactants also appeared to have an effect on the doping process. The researchers believe this is because they can bind to dopant atoms, preventing them from attaching to the nanocrystal surface. Growing cadmium selenide nanocrystals under the same surfactant conditions as zinc selenide - which is relatively easy to dope - resulted in doping of the wurtzite structure cadmium selenide with 0.14% manganese.
The researchers, who reported their work in Nature, believe that doped nanocrystals could find applications in many areas - from solar cells to spintronics.