Researchers at the University of Pennsylvania may not have turned leadinto gold as alchemists once sought to do, but they did turn lead andselenium 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 developedam artificial solid that can be transformed from an insulator to asemiconductor.
The Penn physicists are among many modern researchers who have beenexperimenting with a different way of transforming matter throughartificial solids, formed from closely packed nanoscale crystals, alsocalled "quantum dots."
"Essentially, we're forming artificial solids from artificial atoms --about 10 times larger than real atoms -- whose properties we can finetune on the quantum level," said Drndic, an assistant professor inPenn's Department of Physics and Astronomy. "Artificial solids areexpected to revolutionize the fabrication of electronic devices in thenear future, but now we are only beginning to understand theirfundamental behavior."
Artificial solids, in general, are constructed by specificallyassembling a number of nanocrystals, each composed of only a fewthousand atoms, into a closely packed and well-ordered lattice.Previous researchers have demonstrated that quantum dots can bemanipulated to change their physical properties, particularly theiroptical properties. In fact, the blue laser, which will soon be putinto use into commercial products, was a result of early research inchanging the colors of quantum dots.
"Many of the physical parameters of these crystals, such as theircomposition, particle size and interparticle coupling, represent knobsthat can be individually controlled at nanometer scales," Drndic said."Variation of any of these parameters translates directly into eithersubtle or dramatic changes in the collective electronic, optical andmagnetic response of the crystal. In this case were able to adjust itselectrical properties."
In their study, Drndic and her colleagues looked at the ability ofartificial solids to transport electrons. They demonstrated that, bycontrolling the coupling of artificial atoms within the crystal, theycould increase the electrical conductivity of the entire crystal.According to the researchers, this system promises the possibility ofdesigning artificial solids that can be switched through a variety ofelectronic phase transitions, with little influence from the localenvironment. Their findings represent a key step towards thefabrication of functional nanocrystal-based devices and circuits.
Quantum dots are more than simply analogous to individual atoms; theyalso demonstrate quantum effects, like atoms, but on a larger scale. Asa tool for research, quantum dots make it possible for physicists tomeasure, firsthand, some things only described in theory.
"It is this versatility in both experiment and theory that canpotentially turn these quantum dot solids into model systems forachieving a general understanding of the electronic structure ofsolids," Drndic said. "Not only are we making strides in creating afuture generation of electronics, but in doing so we are also getting adeeper understanding of the fundamental properties of matter."
This research was funded through grants from the National Science Foundation and the Office of Naval Research.
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