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EYEING THE CRYSTAL Zu (left) and Norris discuss impurity doping using a nanocrystal model. |
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PHOTO COURTESY OF DAVID NORRIS |
Little children can be stubborn when it comes to including others in their activities. Little crystals can act the same way. But semiconductor nanocrystals, which tend to exclude foreign materials, can be coaxed into being more inclusive, a new study shows.
One of the reasons semiconductors are the workhorse materials of modern electronics is that they can be doped with impurities such as phosphorus that alter conductivity and other electronic properties in a controlled way. Extending doping methods from bulk semiconductors to their nanocrystal counterparts would be a boon to the developing area of nanoelectronics.
Despite some progress toward this goal, scientists have generally been unable to prepare doped semiconductor nanocrystals, and the reasons for the materials' isolationist behavior have largely gone unexplained.
Now, a team of scientists at the Naval Research Lab (NRL), Washington, D.C., and the University of Minnesota has developed a theoretical model based on fundamental parameters and used it as a guide to preparing manganese-doped cadmium selenide nanocrystals, a material that until now was considered undopable (Nature 2005, 436, 91).
Unlike doping of macroscopic materials, which is driven by thermodynamics, doping at the nanoscale is ruled primarily by kinetics, explains Steven C. Erwin, a theoretician who led the NRL group. If an impurity atom can bond to the surface tightly enough and stay long enough, he says, then eventually that atom can be incorporated into the nanocrystal as it grows during synthesis.
According to the model, the binding strength governs the doping process and depends primarily on the crystal's surface morphology; the size and nature of the crystal faces, which have unique binding characteristics; and the properties of the surfactants used in synthesis.
To test the model, the NRL group, which includes Erwin, Michael I. Haftel, Alexander L. Efros, and Thomas A. Kennedy, teamed with Minnesota materials scientists David J. Norris and Lijun Zu. The researchers prepared and analyzed a variety of doped nanocrystals. They found that the concentration of manganese that can be included in zinc selenide as a function of crystal size and shape agrees with the model's predictions. In addition, they used custom synthesis procedures to prepare cadmium selenide crystals with the structure predicted by their model to be most amenable to doping and then demonstrated that CdSe can indeed be doped with manganese.
Giulia Galli, a senior scientist at Lawrence Livermore National Laboratory, notes that the advance could allow properties of nanocrystals to be engineered for numerous applications ranging from solar cells to future "spintronic" memory devices, in which information is carried by electron spin in addition to electrical charge.
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STICKY SURFACE Zinc selenide nanocrystals can be doped with metals such as manganese (dots) by adsorbing the metal selectively on certain crystal faces.
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COURTESY OF STEVEN ERWIN |
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