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The role of self-purification and the electronic structure of magnetically doped semiconductor nanocrystals

The electronic and magnetic properties of Mn-doped Ge, GaAs, and ZnSe nanocrystals and the role of “self-purification” in doping these nanocrystals are briefly reviewed. The ferromagnetic and half-metalicity trends found in the bulk are preserved in the nanocrystals. However, the Mn-related impurity states become much deeper in energy with decreasing nanocrystalline size, causing the ferromagnetic stabilization to be dominated by double exchange via localized holes rather than by a Zener-like mechanism. As such, the size of the nanocrystal can be used to alter the nature of the magnetic interactions within these materials. However, the creation of doped nanocrystals can be difficult, as the distance a defect or impurity must move to reach the surface of a nanocrystal is very small. The spontaneous expulsion of defects or impurities from nanocrystals is sometimes called a “self-purification” process. Self-purification within such systems can be explained through energetic arguments and is an intrinsic property of defects in semiconductor nanocrystals. The formation energies of defects increases as the size of the nanocrystal decreases.