 Research Area: Nanostructures |
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Research Area Summary: We investigate the properties of semiconductor nanostructures using both density-functional theory and semi-empirical methods. We are particularly interested in understanding - and eventually controlling - the process by which intentional impurities, or dopants, are introduced into semiconductor nanocrystals. Doping semiconductor nanocrystals: Nanocrystals are tiny semiconductor particles just a few millionths of a millimeter across. Due to their small size, they exhibit unique electronic, optical, and magnetic properties that can be utilized in a variety of technologies. To move toward this end, chemical methods have been optimized over the last 20 years to synthesize extremely pure nanocrystals. More problematic, however, has been the goal of controllably incorporating selected impurities into these particles. Conventional semiconductor devices, such as the transistor, would not operate without such impurities. Moreover, theory predicts that dopants should have even greater impact on semiconductor nanocrystals. Thus, doping is a critical step for tailoring their properties for specific applications.
A long-standing mystery has been why impurities could not be incorporated into some types of semiconductor nanocrystals. Together with researchers in the University of Minnesota's (UMN) Department of Chemical Engineering and Materials Science, we have recently established the underlying reasons for these difficulties, and have provided a rational foundation for resolving them in a wide variety of nanocrystal systems. The key lies in the nanocrystal's surface: If an impurity atom can stick, or 'adsorb,' to the surface strongly enough, it can eventually be incorporated into the nanocrystal as it grows. If the impurity binds to the nanocrystal surface too weakly, or if the strongly binding surfaces are only a small fraction of the total, then doping will be difficult. From calculations based on this central idea, we predicted what conditions would be favorable for doping. Experiments at UMN then confirmed these predictions, including the incorporation of impurities into nanocrystals that were previously believed to be undopable. Thus, a variety of new doped nanocrystals may now be possible, an important advance toward future nanotechnologies. An exciting aspect of these results is that they overturn a common belief that nanocrystals are intrinsically difficult to dope because they somehow 'self-purify' by expelling impurities from their interior. According to this view, the same mechanisms that made it possible to grow very pure nanocrystals also made it extremely difficult to dope them. We have shown that doping difficulties are not intrinsic, and indeed are amenable to systematic optimization using straightforward methods from physical chemistry. Future efforts will focus on incorporating impurities which are chosen for specific applications. For example, solar cells and lasers could benefit from impurities that add an additional electrical charge to the nanocrystal. In addition, impurities will be chosen to explore the use of nanocrystals in spin electronics (or "spintronics"). Spintronic devices utilize the fact that electrons not only possess charge, but also a quantum mechanical spin. The spin provides an additional degree of freedom that can be exploited in devices to realize a host of new spintronic technologies, from nonvolatile "instant-on" computers to so-called "reconfigurable logic" elements whose underlying circuitry can be changed on-the-fly. Further Reading:
Point of contact: Steven.Erwin@nrl.navy.mil (Privacy Advisory)
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Modification date : Aug 9, 2008 Send comments or corrections to the webmaster (Privacy Advisory). |
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