Substitutional doping is a widely used procedure in semiconductor technology and consists of randomly replacing atoms in a crystal with a small portion of different atoms. The doping atoms can be easily ionized and provide extra charges to the semiconductor, enhancing electrical conductivity. Christopher Murray and colleagues at the University of Pennsylvania, Stanford University and the National Institute of Standards and Technology have now shown that a similar process can be applied to materials known as nanocrystal superlattices.
In a semiconductor nanocrystal, the electronic energy levels resemble those of an atom; by assembling nanocrystals of different sizes and compositions in superlattices, it is possible, in principle, to build artificial materials with a desired set of properties. The researchers found that by mixing gold nanoparticles with nanocrystals of either CdSe or PbSe during the assembly of a superlattice, the gold nanoparticles could be integrated in the superlattice, replacing randomly distributed semiconductor nanocrystals. Increasing the portion of gold nanoparticles enhances the electrical conductivity. However, rather than providing extra carriers by ionization, as is the case for standard semiconductor crystals, the gold nanoparticles form some percolative conduction paths, with the net effect of allowing faster charge transport. This allows the electrical conduction to be varied over six orders of magnitude.
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Pulizzi, F. Nanocrystal superlattices: Ad hoc replacement. Nature Nanotech (2015). https://doi.org/10.1038/nnano.2015.234