| Over the past decade, research efforts have produced
revolutionary breakthroughs in both the
synthesis and characterization of quantum dots.
Quantum dots are small nanoscale (order of 10-9
meters) particles that contain between a few
thousand and one million atoms. We now know
that changing the size of materials–going down
to nanosize–can dramatically alter intrinsic
material properties, such as converting a conductor
into an insulator or a nonmagnetic material
into a magnet.
Research Fellow Dr. Alex Zunger heads a team
of researchers from the National Renewable
Energy Laboratory (NREL), Lawrence Berkeley
National Laboratory (LBNL), and University of
Tennessee. Funded by the Office of Science, the
group has been focusing on the physics and computational
challenges of simulating the electronic
properties of quantum dots and quantum dot
architectures. Due to the very large number of
atoms involved, conventional numerical applications
of quantum mechanics are impractical. The
SciDAC team has developed new and improved
algorithms, as well as better preconditioners, to
overcome this obstacle.
A quantum dot confines the electrons to discrete
energy levels that can be controlled by
changing the size and shape of the quantum dot.
These properties make quantum dots potentially
important in a number of areas, such as diode
lasers, advanced quantum computing, and ultrahigh
efficiency solar cells. However, the promise
of such dots depends largely on the ability to connect,
or wire together, different dots. This raises
fundamental quantum-mechanical questions.
Will the electrons in a dot connected to another
dot by a nanowire be confined to a single dot
(become localized)? Or will the electrons be predisposed
to move along the wire (become delocalized)?
What makes the problem involving motion of
electrons from a dot to a wire and then to another
dot difficult is the quantum behavior. For example,
if the dot is sufficiently small, electrons in it
will be squeezed for space–quantum confinement–
and seek to move away into the wire.
However, the existence of another electron in the
wire will repel the electron back into the dot–
quantum correlation. To find out who wins,
quantum confinement or quantum correlation,
powerful new computational techniques are
needed. The Zunger-led team has developed such
methods to handle both quantum confinement
and quantum correlation, and they have applied
the methods to a "nano-dumbbell" consisting of
two dots connected by a wire. They found that,
while electrons could get stuck in the dot, one
could design certain dimensions of wires and dots
that would facilitate electron delocalization. Figure
11 demonstrates this for cadmium telluride
(CdTe) dots. A ten angstrom (Å) difference in size
can delocalize electrons from individual dots.
Such breakthrough calculations enable the design
of three-dimensional quantum architectures–
wired-up dots that can exploit the wonderful
properties of the nanoworld. |
