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Writing in the Aug. 28, 2000, issue of Physical Review Letters, Uzi
Landman, Robert Barnett and Andrew Scherbakov from the Georgia Institute of
Technology, and Phaedon Avouris from the IBM T.J. Watson Research Center,
report on issues pertaining to the atomic structure, electronic properties and
electrical transport in silicon nanowires. These issues will have to be
considered by designers using devices this small.
"It's a much-discussed expectation that devices of this size will be
different, but in what ways and by how much, remain unknown," says Uzi
Landman, Regents' Professor of Physics and director of the Georgia
Tech Center for Computational Materials Science. "In this study, we have
explored certain unique properties of systems this small through
first-principles quantum mechanical simulations. Such simulations, which are
to the best of our knowledge the largest ones to date, are essential for
gaining reliable and predictive information about these systems."
To boost speed and reduce energy use, engineers are being pushed to make
electronic devices smaller and to pack more of them onto a chip. This pressure
eventually will drive them to use features as small as 1 nanometer
(one-billionth of a meter, or a hundred-thousandth the width of a human hair).
When that happens, he noted, device operation will be dominated by quantum
mechanical effects – and the expectations that have long governed device
design will no longer apply.
The researchers simulated silicon nanowires etched from bulk silicon, or
self-assembled from clusters containing 24 atoms of silicon. In each case, the
silicon was passivated by attaching hydrogen atoms to unused bonds, and the
wires were connected to aluminum leads.
The theoretical simulations produced data on the nanowires' electrical
conductance, the influence of the silicon-metal interface and the role that
doping with aluminum atoms may play in changing materials properties. The work
also suggests new ways of doping ultra-small transistor channels that could
circumvent some current technological issues.
"This work attempts to fill in some of the gaps in our knowledge in this
area," says Avouris, manager of Nanometer Scale Science & Technology at
the Watson Research Center in Yorktown Heights, N.Y. "While the wires on which
we report here are significantly smaller than those likely to be used in the
near future, they are particularly useful because they tell us what to expect
in the fully quantum mechanical limit – the ultimate miniaturization limit.
The calculations have revealed a number of significant changes in important
properties."
Carried out on an IBM SP-2 computer at Georgia Tech, the simulations
revealed that:
• Electronic states formed from a combination of orbitals from the aluminum
leads and the silicon wire atoms penetrate all the way through nanowires of
less than about 1 nanometer in length, giving such silicon bridges a finite
conductance. But in longer structures, these electronic states penetrate only
partially into the nanowire, with the silicon retaining its semiconducting
properties.
• The transfer of electrons from the aluminum to the silicon at the
junction between the two materials creates a localized dipole that forms a
barrier to the flow of electrons. Simulations show the height of such Schottky
barriers at nanoscale metal-to-semiconductor contacts may not be too different
from those found at more familiar-size scales.
• The simulations suggest a way that could overcome some of the anticipated
problems involved in doping the silicon used in devices this small. Doping of
semiconductors is used routinely to tune and optimize device characteristics.
But in nanoscale devices one may expect detrimentally large device-to-device
statistical variations of the dopant concentration. This variability could
cause severe problems at this size scale because electronics designers may not
consistently predict the performance of a collection of such devices. But the
simulations suggest that building nanowires from silicon clusters could offer
a solution. Because the clusters form hollow cages, much like carbon
fullerenes, they could be fabricated around a dopant atom. With each cluster
then containing a dopant atom, device consistency may be achieved.
• The wave-like nature of the electrons could cause interference effects in
the electric conductance through the silicon nanowires used as current
channels. When voltage is applied to open the channel, electrons penetrating
the silicon nanowire from one of the aluminum leads may bounce off the contact
to the other lead and flow back toward the source contact. Upon reaching that
contact, they may bounce off again, and the process may repeat itself.
This behavior results, at certain electron wavelengths and wire
configurations, in interference resonances that cause the channel to appear
transparent, leading to the occurrence of spikes in the current flowing
through the nanoscale channel.
"When building a device, engineers would have to take this into account and
either find ways to use it or avoid it," Landman adds. "In macroscopic
devices, this phenomenon is of no particular consequence, showing again that
small devices are different in ways that go beyond simple scaling with size."
The research was sponsored by the U.S. Department of Energy.
– John Toon
For more information, contact Uzi Landman, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332-0430. (Telephone: 404-894-3368) (E-mail: uzi.landman@physics.gatech.edu)
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But on these smallest-of-size scales,
physical processes are often different than at larger scales, forcing
engineers to reconsider both their expectations of how such nanoscale devices
would perform – and the established physical equations governing them.
Writing in the Aug. 18 issue of the journal Science, Georgia Tech
researchers suggest that jets as small as 6 nanometers in diameter may be
possible to produce. But these tiny devices would require special conditions
to operate and be particularly sensitive to effects not of concern at more
familiar-size scales.
"We are now being driven by fundamental, technological and economical
considerations to explore and evaluate systems that are smaller and smaller,"
Landman says. "We need to understand these systems, because basic physics
issues are especially important to them. There is no point in trying to make
devices of this size scale without knowing what their physical behaviors and
fundamental advantages or limitations are going to be."
To study jets just a few nanometers in diameter, Landman and collaborator
Michael Moseler used molecular dynamics simulations to observe how some
200,000 propane (C3H8) molecules would behave when compressed within a tiny
reservoir and then injected out of a narrow nozzle made of gold. Operating on
an IBM SP2 parallel processing computer, the simulations recorded the dynamics
of the fluid molecules on the femtosecond time scale over periods of several
nanoseconds.
In a second phase of their work, the researchers attempted to model their
observations through the use of the traditional fluid dynamics formulation –
that is, the Navier-Stokes equations. They found, however, these equations did
not account for the effect of thermally induced fluctuations that
significantly influence the stability and dynamic evolution of nanojets. While
these fluctuations are of much less importance at larger-size scales, they
become dominant at the nanoscale regime. Consequently, the researchers derived
a modified form of the equations that modeled their simulation results
remarkably well – thus extending fluid hydrodynamics to the nanoscale domain.
As a next step, the researchers would like to create nanojets
experimentally and use them to apply patterns that could replace current
lithographic processes in the manufacture of nanoscale miniaturized circuits.
They might also be used as "nano gene guns" to insert genetic materials into
cells without causing damage.
The nanojet work is supported by the U.S. Department of Energy, the U.S.
Air Force Office of Scientific Research and in part by the Deutsche
Forshungsgemeinschaft.
– John Toon
For more information, contact Uzi Landman, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332-0430. (Telephone: 404-894-3368) (E-mail: uzi.landman@physics.gatech.edu)
Wrestling with Rendering's Malodors
Researchers develop model regulations, but still face the complicated task of assessing and controlling odors from rendering plants.
Developed in part by researchers at the Georgia Institute of Technology, Georgia's rules for controlling and assessing malodors from food waste rendering plants could become a model for other states facing the same problem.
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Meanwhile malodor control and
assessment remains a complicated task. So Georgia Tech and University of Georgia researchers are working
to improve treatment of odors so they don't waft into communities surrounding
rendering plants.
"The problem of malodors from rendering plants is a longstanding issue. In
fact, the industry dates back to Roman times. Today, it is a nationwide
issue," says Jim Walsh, a researcher at Georgia Tech's Economic Development Institute.
Every year in the United States, 43 billion pounds of food processing waste
materials are sent to rendering plants, where they are converted into pet and
animal feeds. Of that waste, 23 billion pounds is from poultry. "The bottom
line is if we didn't render this material, it would be in landfills," Walsh
says. "Instead, it's turned into usable byproducts."
Dealing with the unwanted byproduct – that is, malodors – is the difficult
part of the equation. Though the problem is being addressed by the rendering
industry, there is not a fix-all solution in sight, Walsh says. "It's just
what we expected," he adds. "Odor is a complicated issue."
In Georgia, the malodor issue reached a boiling point in 1999 when citizen
complaints prompted the state's Department of Agriculture to develop rules to
help manage the problem. Agriculture officials called on Georgia Tech for
assistance, and Walsh helped them develop the state's Malodor Control and
Assessment Program (MalodorCAP), which became mandatory for rendering plants
in August 1999.
The MalodorCAP consists of a control program and a complaint response
program. Rendering facilities determine their critical control points (points
in the operation where odors are controlled), establish critical limits
(operating conditions for control equipment) at each critical control point,
and determine corrective actions to be taken if they exceed critical limits.
The complaint response program requires a facility to respond to any
complaint, record the response on a form approved by the Georgia Department of
Agriculture and keep a log of the complaints. State officials, with Walsh's
help, regularly review and help update facilities' MalodorCAPs.
Though Georgia is taking the lead nationally in monitoring malodors, there
are no state or federal limitations on malodorous chemical compounds emitted
unless these compounds are classified as volatile organic compounds (VOCs) or
hazardous air pollutants (HAPs).
"Odor is a subjective and not a quantitative issue right now, so current
regulations address odor as a nuisance issue," Walsh explains. "But Georgia's
malodor control and assessment rules are unique. They could serve as a model
program for other states."
Now the state-funded Traditional Industries Program for Food Processing
(FoodPAC) and the Agricultural Technology Research Program are supporting
Walsh's efforts as a liaison between industry, researchers and regulators.
Walsh is tracking the latest technologies and research, including projects
ongoing at Georgia Tech and the University of Georgia.
Recently, the two institutions began a collaborative project to enhance wet
scrubber odor treatment technology and improve monitoring for odors and VOCs
in the food processing industry. Wet scrubbers transfer odorous chemicals and
VOCs in the air to the water and neutralize them using oxidizing chemicals.
But wet scrubber operations are often process-specific, and there is limited
data available to effectively improve their performance. University of Georgia
Professors K.C. Das and Jim Kastner hope their ongoing work in developing
methods for chemical characterization of air emissions from rendering
operations will eventually advance wet scrubber design. Characterizing
malodorous chemicals is difficult because they tend to vary and occur in trace
amounts, Walsh explains.
In the current project, UGA researchers are focusing on the mass transfer
of specific odorous compounds into water. Specifically, researchers are
continuing their chemical characterization studies, and also evaluating the
efficiency and appropriateness of water-based treatment technologies. Georgia Tech Research Institute
engineer John Pierson is examining potential improvements to the gas-phase
pre-treatment of total VOCs. Specifically, he is working with the developers
of two different novel chemistries to improve wet scrubber efficiency, and
Pierson is developing a predictive monitoring system to better manage
rendering plant emissions. Both the U.S. Poultry and Egg Association's Protein
and Fats Council and FoodPAC are funding this joint project.
Researchers elsewhere are working on improvements to biofilters, Walsh
adds. Biofilters take airborne emissions from the plant and push them through
a box filled with packing material, such as wood shavings or ceramic balls.
Bacteria grow on the packing material, and then they eat and remove most of
the odorous chemicals. In Georgia, rendering plants in Cumming and Cuthbert
have installed customized biofilters.
Progress in controlling malodors seems slow sometimes, but Walsh says: "I
am constantly hearing of people developing new chemical treatments. A lot of
research and development is going on, not only new chemical treatments, but
new ways of controlling processing operations to eliminate odors."
– By Jane M. Sanders
For more information, contact Jim Walsh, Economic Development Institute, Georgia Institute of Technology, Atlanta, GA 30332-0837. (Telephone: 404-210-5550) (E-mail: jim.walsh@edi.gatech.edu)
Crossing the Digital Divide
Research generates new telecommunications services in rural Georgia.
A Georgia Institute of Technology
research project examining community technology practices has led to the
development of several approaches for building better telecommunications
programs at the local level in Georgia.
Under the name of TechSmart, these services aim to increase information
technology skills, attract technology jobs, encourage capital investment in
information technology and make related products and services more available
and less expensive across rural Georgia – in short, to help bridge "the
digital divide."
The research, funded by Georgia Tech's Economic Development Research Program
and the Georgia Department of Community Affairs, examined the feasibility of
developing an assessment tool for evaluating community readiness in terms of
information infrastructure. The tool measures economic development
professionals' awareness of several components, including:
• types of support for technology professionals;
• hardware, software and Internet access available to businesses and
residents;
• an understanding of needs of major telecommunications customers and
providers;
• an awareness of major community Web sites;
• the role played by those with technical knowledge in economc
development. Researchers later tested the tool in three rural Georgia communities:
Hawksinville/Pulaski County, Jesup/Wayne County and Reidsville/Tatnall County.
Officials there supplied feedback to improve the tool.
"There are many information technology- and telecommunications-related
initiatives under way in rural communities and much potential for more," says
Jan Youtie, the Georgia Tech researcher who headed the study. "Interestingly,
little knowledge of this exists at the local level because community leaders
are not accustomed to paying attention to this area."
Essentially, such assessments determine the gap between the current and
desired state of readiness for e-commerce and economic development. Assessment
results include recommendations for community leadership development and
projects to bridge those gaps.
TechSmart not only provides a mechanism to conduct assessments, it also
offers technology leadership training, awareness-building workshops, and
facilitation of information technology projects related to education,
government and business. To place these services closer to rural communities,
extension centers staffed by Georgia Tech specialists are planned for Albany
and Dublin and proposed for north Georgia outside metro Atlanta.
– Lincoln Bates, EDI
To obtain a benchmark, the analysis drew on experiences
of communities recognized as early adopters of information technology. Those
communities included Newnan, Ga., and Blacksburg, Va.
For more information, contact Jan Youtie, Economic Development Institute, Georgia Institute of Technology, Atlanta, GA 30332-0640. (Telephone: 404-894-6111) (E-mail:mailto:%20jan.youtie@edi.gatech.edu)
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Last updated: Feb. 16, 2001