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Nanotechnology
Seizing
the Moment: Improving Control of Quantum Dots
A light bulb isn’t very useful without a reliable on/off
switch. The same holds true for quantum dots. These ultra tiny
electronic nanostructures someday may serve as the ones and zeros
used by a superfast quantum computer, but first physicists need
to refine their ability to turn quantum dots “on” and “off.”
In the June 23 on-line issue of Applied Physics Letters,
researchers from the National Institute of Standards and Technology
(NIST) and the National Renewable Energy Laboratory (NREL) take
a step in the right direction. They report a way to measure accurately
the amount of laser light needed to shift the electrons in a particular
type of quantum dot between two discrete states, a low energy, ground
state and a higher energy, excited state.
The strength of the interaction between quantum dots and electromagnetic
waves like laser light is affectionately known in physical
science circles as the “dipole moment.” Loosely translated,
it’s a number that tells you how easy the dots are to excite.
The new NIST/NREL technique measures the dipole moment directly
by enclosing the dots in a cavity where a pulse of laser light
can pass over them repeatedly. With each successive pass, the laser
light gets a little dimmer as the dots absorb some of the energy.
Averaging the changes in energy over many pulses gives an accurate
measurement of the dipole moment.
The
ability to measure accurately the dipole moment for quantum dots
made of different materials should help nanotechnology
researchers optimize these structures for a variety of applications,
including
both quantum computing and quantum communications.
Buildings
Designing
Efficient Cooling Systems for the Dog Days of Summer
New
software developed by NIST can help cooling system manufacturers
meet Department of Energy goals calling for a 20 percent increase
in energy efficiency of residential air conditioners by 2006.
Manufacturing engineers can use the software, called EVAP-COND,
to improve evaporators and condensers, two types of heat exchangers
that are essential components of every air conditioner. Improved
heat exchangers mean increased energy efficiency.
The software simulations depict the performance of evaporators
and condensers working with any one of 10 cooling agents,
including new
generation atmospheric ozone-safe hydrofluorocarbon fluids and “natural
refrigerants,” such as carbon dioxide or propane. The software’s
computer graphics package enables engineers to observe and to understand
refrigerant behavior throughout the simulated heat exchanger. Different
designs can be tested to achieve desired environmental results.
According to software developer Piotr Domanski, “EVAP-COND
can increase design engineer productivity and can reduce laboratory
testing, thus shortening design-to-production time. This software
can save manufacturers time and money, while it is helping to conserve
energy.”
NIST developed the software with funds from the 21st Century
Research Program of the Air-conditioning and Refrigeration
Technology Institute and the U.S. Department of Energy.
The Windows-based program
can be downloaded
from www2.bfrl.nist.gov/software/evap-cond/.
Materials
Computer
Simulations Mimic Growth of ‘Dizzy Dendrites’
Click here
for high resolution jpg versions of graphics.
Left--"Dizzy
dendrite" pattern grown in an 80-nanometer thick
film of two blended polymers with randomly dispersed
clay particles.
Right--Computer
simulation of the crystal structure for a copper-nickel
alloy with randomly dispersed particles. |
Left--Spiraling
dendrites produced when the simulation assumes that
the particles are randomly dispersed and rotating.
Right
--Multiple crystal structures predicted with rotating
particles. Each color represents a separate crystal. |
Crystals
are more than just pretty faces. Many of the useful properties
associated with metal alloys or polymer blends—like strength,
flexibility and clarity—stem from a material’s specific
crystal microstructure. So the more scientists know about how
crystal patterns grow as a material solidifies, the better they’ll
be able to create new materials with specific properties.
In a recent issue of Nature
Materials, National Institute of
Standards and Technology (NIST) researchers described
work with collaborators
in Hungary and France using computer simulations of crystal growth
to advance understanding of how foreign particles—either additives
or impurities—affect crystal growth patterns. They found
that computer simulations developed to predict the crystal growth
of metal alloys matched
up remarkably well with microscope images of actual crystals grown
in
polymer films with thicknesses far below that of a human hair.
Randomly dispersed foreign particles in both the simulation
and the real materials produced what the researchers
dubbed “dizzy
dendrites.” In both cases, the tree-like branches in the crystals
tend to
curve and split, instead of forming the straight, symmetric patterns
typical of pure crystals. Further simulations indicated
that rotating
the particles in concert during the solidification process produced
spiraling dendrites. Alternating strips of particles with first
one and then another orientation produced zig-zagging patterns.
The researchers
suggest that experimentalists also may be able to reproduce the
crystal patterns seen in these more complex simulations. Possible
methods
include imprinting the crystal growing surface with a patterned
roller (like those used to make a patterned pie crust) or using
external
electromagnetic fields or laser pulses to orient particles in
specific directions.
