Semiconductor nanocrystals are of great interest due to their small size and unique electronic, optical, and magnetic properties, which can be utilized in a variety of technologies; however, it has proven more difficult to dope impurities into nanocrystals rather than into their bulk crystalline counterparts. This problem led to the widely accepted belief that nanocrystals were intrinsically difficult to dope due to a self-purification process that expels impurities from their interior.
Nanocrystals of zinc selenide can be controllably doped with atoms of manganese, which selectively adsorb on certain crystal facets before being incorporated. The background image shown is a transmission electron micrograph of zinc selenide nanocrystals. Photo courtesy Steve Erwin
In collaboration with scientists at the University of Minnesota (Minneapolis, MN), researchers at the Naval Research Laboratory (NRL; Washington DC) discovered the true doping mechanism in semiconductor nanocrystals and showed that in order to be incorporated, impurities must be able to bind to the nanocrystal surface for a period of time long enough to be incorporated into the nanocrystal. This concept enabled the team to predict conditions favorable for doping in a wide variety of nanocrystal systems.
"This simple idea explains both the general difficulties with doping nanocrystals due to the small relative fraction of 'sticky' surfaces and the specific difficulties with hexagonal structured nanocrystals due to the underlying hexagonal crystal structure, which naturally leads to less sticky surfaces, compared to the more commonly occurring cubic crystal structure," explains NRL materials researcher Steve Erwin.
The group studied zinc selenide (ZnSe) nanocrystals with 25-50 Å diameters. "We focused on manganese (Mn) as a dopant, because its photoluminescence properties enable detailed optical characterization of Mn-doped nanocrystals and because electron paramagnetic resonance can be used to distinguish Mn that is inside the nanocrystal from Mn that is stuck to the surface" Erwin says. However the team's conclu-sions are not limited to either ZnSe nanocrystals or Mn dopants, as the general principle of surface binding applies to many other systems.
This novel concept may pave the way for the development of a variety of new technologies ranging from high-efficiency solar cells and lasers to futuristic "spintronic" and ultra-sensitive biodetection devices.