Doping is routinely used to change the electrical properties of bulk semiconductors, like silicon, but efforts to do the same in nanocrystals have met with mixed success. That said, this last year has seen some important progress in this area and Norris’ team’s results are important for several reasons.

First, the researchers unequivocally show that silver dopants can affect the electrical properties of CdSe nanocrystals. Second, because CdSe is the most widely studied colloidal semiconductor material, being able to dope it with electronically active impurities is a major step forward. “Third, we find that adding only a few silver dopants can dramatically affect the properties of the nanocrystals in totally unexpected ways,” explained Norris. “For example, adding just two silver dopant atoms per nanocrystal can increase the fluorescence of the nanocrystals by around 10-fold.”

The team, which includes scientists from the University of Minnesota in the US, Soongsil University in Korea and the University of Duisburg-Essen in Germany, employed a cation-exchange technique in which undoped nanocrystals were simply exposed to silver in solution. Silver diffuses extremely fast in the CdSe lattice (and indeed other semiconductor lattices), so an entire series of doped samples can easily be made by exposing the as-synthesized nanocrystals to solutions containing different concentrations of dopants.

Providing electrons and holes

The doping changes the electrical transport through the nanocrystals by increasing conductivity and changing the Fermi level. The dopants also show more complex behaviour: at low concentrations of silver (below around six silver atoms per nanocrystal), the atoms donate electrons to the nanocrystals (n-type behaviour) but, at higher concentrations of silver, they provide holes (p-type behaviour). Previous studies assumed that the dopants would always behave as either n-type or p-type.

The silver dopants also increase the nanocrystals’ fluorescence (or photoluminescence). “Such a result is unexpected again because until now it was thought that electrically active dopants would destroy the luminescence,” Norris told nanotechweb.org. “This finding will require further work to explain – an exciting prospect for us scientists.”

The doped nanocrystals might be used to make devices like diodes. Conventional silicon diodes are made by placing a layer of silicon doped with impurities that provide extra holes next to a layer doped with impurities that provide extra electrons. Such p-n junctions are central to silicon electronics. “Our work is the first step in doing something similar with nanocrystal films,” said Norris, “and such devices could form the basis of light-emitting diodes, photodiodes (detectors) and solar cells.”

The team now plans to spend some time trying to understand the fundamental physics behind doped nanocrystals. “Our study is only the beginning and at this point we do not really understand much of what the dopant is doing in these materials,” said Norris.

The current results are published in Nano Letters.