DC • 07/07/05) – Novel electronic devices based upon
nanotechnology may soon be realized due to a new
understanding of how impurities, or 'dopants,' can
be intentionally incorporated into semiconductor
nanocrystals. This understanding, announced today
by researchers at the Naval Research Laboratory (NRL)
and the University of Minnesota (UMN), should help
enable a variety of new technologies ranging from
high-efficiency solar-cells and lasers to futuristic
'spintronic' and ultra-sensitive biodetection devices.
The complete findings of the study are published
in the July 7, 2005, issue of the journal Nature .
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.
long-standing mystery has
been why impurities could
not be incorporated into
some types of semiconductor
nanocrystals. The findings by NRL and UMN researchers
establish the underlying reasons for these difficulties,
and provide a rational foundation for resolving them
in a wide variety of nanocrystal systems. "The key
lies in the nanocrystal's surface," said Dr. Steven
Erwin, a physicist at NRL and lead theorist on the
project. "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, the team could predict conditions 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
to Dr. David Norris, an
Associate Professor of
Chemical Engineering and
Materials Science at UMN
and the lead experimentalist
on the team, "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."
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.
The research was conducted by Dr. Steven Erwin,
Dr. Michael Haftel, and Dr. Alexander Efros from
NRL's Materials Science and Technology Division;
Dr. Thomas Kennedy from NRL's Electronics Science
and Technology Division; and Ms. Lijun Zu and Professor
David Norris from the Department of Chemical Engineering
and Materials Science at the University of Minnesota.
The Office of Naval Research and the National Science
Foundation provided funding for the research
Contact: Donna McKinney
Naval Research Laboratory