Oct. 4 , 2005
PHILADELPHIA – Researchers at the University of Pennsylvania may not have turned
lead into gold as alchemists once sought to do, but they did turn lead and selenium
nanocrystals into solids with remarkable physical properties. In the October
5 edition of Physical Review Letters, online now, physicists Hugo E. Romero and
Marija Drndic describe how they developed am artificial solid that can be transformed
from an insulator to a semiconductor.
The Penn physicists are among many modern researchers who have been experimenting
with a different way of transforming matter through artificial solids, formed
from closely packed nanoscale crystals, also called "quantum dots."
"Essentially, we're forming artificial solids from artificial atoms – about 10
times larger than real atoms – whose properties we can fine tune on the quantum
level," said Drndic, an assistant professor in Penn's Department of Physics and
Astronomy. "Artificial solids are expected to revolutionize the fabrication of
electronic devices in the near future, but now we are only beginning to understand
their fundamental behavior."
Artificial solids, in general, are constructed by specifically assembling a number
of nanocrystals, each composed of only a few thousand atoms, into a closely packed
and well-ordered lattice. Previous researchers have demonstrated that quantum
dots can be manipulated to change their physical properties, particularly their
optical properties. In fact, the blue laser, which will soon be put into use
into commercial products, was a result of early research in changing the colors
of quantum dots.
"Many of the physical parameters of these crystals, such as their composition,
particle size and interparticle coupling, represent knobs that can be individually
controlled at nanometer scales," Drndic said. "Variation of any of these parameters
translates directly into either subtle or dramatic changes in the collective
electronic, optical and magnetic response of the crystal. In this case were able
to adjust its electrical properties."
In their study, Drndic and her colleagues looked at the ability of artificial
solids to transport electrons. They demonstrated that, by controlling the coupling
of artificial atoms within the crystal, they could increase the electrical conductivity
of the entire crystal.
According to the researchers, this system promises the possibility of designing
artificial solids that can be switched through a variety of electronic phase
transitions, with little influence from the local environment. Their findings
represent a key step towards the fabrication of functional nanocrystal-based
devices and circuits.
Quantum dots are more than simply analogous to individual atoms; they also demonstrate
quantum effects, like atoms, but on a larger scale. As a tool for research, quantum
dots make it possible for physicists to measure, firsthand, some things only
described in theory.
"It is this versatility in both experiment and theory that can potentially turn
these quantum dot solids into model systems for achieving a general understanding
of the electronic structure of solids," Drndic said. "Not only are we making
strides in creating a future generation of electronics, but in doing so we are
also getting a deeper understanding of the fundamental properties of matter."
This research was funded through grants from the National Science Foundation
and the Office of Naval Research.
news officer, science and engineering
University of Pennsylvania
Office of University Communications