By Neal Singer
Suppose you believe — as many researchers do — that nanowires will be the electrical current-carriers of the future for certain types of solar arrays, batteries, telephones, and tiny portable computers. But perhaps you also suspect — as many researchers do — that strange and unpredictable effects may take place in the currents carried by these nanowires when placed in proximity to each other — the sine qua non for carrying out their business.
THE EYE OF MIKE LILLY observes two individually powered nanowires, embedded one above the other, in a few atomic layers of Sandia-grown crystal. The unique test device already has yielded new information about nanoworld electrical flows. (Photo by Randy Montoya)
So you’d want to measure the possible changes in voltage of one wire as caused by another, to determine how significant the current boost or drag, so you can allow for it in designing your device. But you have a problem. The best test method available involves putting a charged piece of material called a gate between two nanowires on a single shelf. The gate, flooded with electrons, acts as a barrier: It maintains the integrity, in effect, of the wires on either side of it by repelling any electrons attempting to escape across it. But the smallest wire separation allowed by the gate is 80 nanometers. A much smaller gap is necessary for verisimilitude with expected future devices.
Simple but brilliant test design
Now consider instead this simple but brilliant test design: What lead researcher Mike Lilly (1132) and co-workers at McGill University envisioned was to put the nanowires one above the other, rather than side by side, by separating them with a few atomic layers of very purely grown crystal. The result? Nanowires separated vertically by only 15 nanometers. And because each wire sits on its own independent platform, each can be independently fed and controlled by electrical inputs varied by the researchers.
The researchers found, as reported online at DOI: 10.1038/NNANO.2011.182, and in the upcoming December 2011 Nature Nanotechnology, highly significant effects: Positive voltage boosts could be as high as 25 percent on the second wire.
The work required the crystal-growing expertise of John Reno (1132), the fabrication and measurement skills of McGill doctoral student Dominique Laroche, and elements of previous work by Jerry Simmons (1120).
“There are all sorts of people working on nanowires,” says Mike. “They’ve been doing it for 20 years. At first, you study such wires individually or all together, but eventually you want a systematic way of studying the integration of nanowires into nanocircuitry. That’s what’s happening now. It's important to know how one-dimensional (1-D) wires interact with each other rather than regular wires.
A 1-D wire is not the common thick-waisted household (3-D) wire, which allows current to move across, vertically, and forward, nor is it your smaller flattened micron-sized wires (2-D) in typical electronic devices that allow electrons to move forward and across but not up and down. In 1-D wires, the electrons can only move in one direction: forward, like prisoners coming to lunch, one behind the other.
Though the gallium-arsenide structures used by Mike are fragile, nanowires in general have very practical characteristics — they may crack less than their bigger cousins, they’re cheaper to produce, and offer better electronic control. But it’s as a problem in basic science that their characteristics fascinate Lilly.
The Coulomb drag effect
“In the long run, our test device will allow us to probe how 1-D conductors are different from 2-D and 3-D conductors,” says Mike. “They are expected to be very different, but there are relatively few experimental techniques that have been used to study the 1-D ground state.”
One reason for the difference is the Coulomb force, responsible for what is termed the Coulomb drag effect regardless whether the force hastens or retards currents. Operating between wires, the force is inversely proportional to the square of the distance; that is, in ordinary microelectronics, the force is practically unnoticeable, but at nanodistances, the force is large enough that electrons in one wire can “feel” the individual electrons moving in another placed nearby.
The drag means that the first wire needs more energy because the Coulomb force creates, in effect, increased resistance. “The amount is very small,” says Mike, “and we can't measure it. What we can measure is the voltage of the other wire.”
There are no straightforward answers as to why the Coulomb force creates negative or positive drag, but it does.
What’s known, Mike says, is that “enough electrons get knocked along that they provide positive source at one wire end, negative at the other. A voltage then builds up in the opposite direction to keep electrons in place.”
The so-called Fermi sea — a 3-D concept used to predict the average energy of electrons in metal — should totally break down in 1-D wires, which instead should form a Luttinger liquid, says Mike. A Luttinger liquid is a theoretical model describing the interactions of electrons in a 1-D conductor. To better understand the Luttinger liquid is Mike’s underlying reason for the experiment.
Having an interest on many levels proved useful because making the test device “took us a very long time,” says Mike. “It’s not impossible to do in other labs, but Sandia has crystal-growing capabilities, a microfabrication facility, and support for fundamental research from DOE’s office of Basic Energy Sciences (BES). Their core program is interested in new science and new discoveries, like the work we’re doing in trying to understand the fundamental ideas behind what is going on when you’re working with very small systems.”
Device fabrication was conducted under a user project at the Center for Integrated Nanotechnologies, a DOE Office of Science national user facility jointly run by Sandia and Los Alamos national laboratories. The device design and measurement were completed under the DOE Office of Science BES/Division of Materials Science and Engineering research program. - Neal Singer
By Nancy Salem
Research by a team of Sandia chemists could impact worldwide efforts to produce clean, safe nuclear energy and reduce radioactive waste.
Sandia chemist Tina Nenoff heads a team of researchers focused on removal of radioactive iodine from spent nuclear fuel. They identified a metal-organic framework that captures and holds the volatile gas, a discovery that could be used for nuclear fuel reprocessing and other applications. (Photo by Randy Montoya)
The Sandians used metal-organic frameworks (MOFs) to capture and remove volatile radioactive gas from spent nuclear fuel. “This is one of the first attempts to use a MOF [pronounced “MOF,” not as initials] for iodine capture,” says team lead Tina Nenoff (1114) of Sandia’s Surface and Interface Sciences Department.
The discovery could be applied to nuclear fuel reprocessing as well as to cleanup from nuclear reactor accidents. A characteristic of nuclear energy is that used fuel can be reprocessed to recover fissile materials and provide fresh fuel for nuclear power plants. Countries such as France, Russia, and India are reprocessing spent fuel.
The process also reduces the volume of high-level wastes, a key concern of the Sandia researchers. “The goal is to find a methodology, to line things up so less waste is interred,” Tina says.
Part of the challenge of reprocessing is to separate and isolate components that are not burnable fuel but are radioactive. Tina took knowledge from her early Sandia research into materials for cleanup of nuclear waste from the Cold War and applied it to gas separations for nuclear fuel reprocessing.
The Sandia researchers are part of the Off-Gas Sigma Team, which is led by Oak Ridge National Laboratory and studies waste form capture of volatile gases associated with nuclear fuel reprocessing. Tina’s team is focused on removing iodine, whose isotopes have a half-life of 16 million years, from spent fuel. Other volatile gases of interest include krypton, tritium, and carbon, and are being looked at by other members of the Off-Gas Sigma Team — Pacific Northwest, Argonne, and Idaho national laboratories.
The Sandians studied known materials, including silver-loaded zeolite, a crystalline, porous mineral with regular pore openings, high surface area, and high mechanical, thermal, and chemical stability. Various zeolite frameworks can trap and remove iodine from a stream of spent nuclear fuel, but need added silver to work well. “Silver attracts iodine to form silver iodide,” Tina says. “The zeolite holds the silver in its pores and then reacts with iodine to trap silver iodide.”
But silver is expensive and there are environmental issues, so the team set out to engineer materials that would work like zeolites but have higher capacity for the gas molecules, and not need silver. They explored why and how zeolite absorbs iodine, and used the critical components discovered to find the best MOF, in this case named ZIF-8.
“We studied materials that are known, like the zeolite mordenite, and made new materials,” Tina says. “We investigated the structural properties on how they work and translated that into new and improved materials.”
MOFs are crystalline, porous materials in which a metal center is bound to organic molecules by mild self-assembly chemical synthesis. The choice of metal and organic result in a very specific final framework.
‘Making the hole round’
The trick was to find a MOF highly selective for iodine. The Sandians took the best elements of zeolite Mordenite — its pores, high surface area, stability, and chemical absorption — and identified a MOF that can separate one molecule, in this case iodine, from a stream of molecules. The MOF and pore-trapped iodine gas can then be incorporated into a glass waste form for use in long-term storage.
“We’ve shown that MOFs have the capacity to capture and, more importantly, retain many times more iodine than current materials technologies,” says team member Karena Chapman of Argonne National Laboratory. She added that the iodine can also be trapped by pressure treating the MOF to change the dimensions of its entry/exit apertures. “This process could be compared to putting a square peg through a square hole then making the hole round,” Chapman says.
The Sandia team also fabricated MOFs, made of commercially available products, into durable pellets. The as-made MOF is a white powder with a tendency to blow around. The pellets provide a stable form to use without loss of surface area, Tina says.
Sandia has filed for a patent on the pellet technology, which could have commercial applications. “We figured out a binderless process to make industrially relevant pellets,” Tina says.
The project began six years ago and the Sigma Team was formalized in 2009. It is funded by the DOE Office of Nuclear Energy. Tina has been involved from the beginning, tapping a background in nuclear weapons cleanup. She has been at Sandia 18 years and previously worked on removal of radiological ions from liquid tanks.
Seeking capture and removal solutions.
“Over the years, through my career, I’ve gone back to working on materials associated with separations and waste forms for radiological elements,” she says.
The Sigma Team is seeking capture and removal solutions for all the volatile gases involved in reprocessing. Sandia’s iodine and MOFs research was featured in two recent articles in the Journal of the American Chemical Society authored by Tina and team members Dorina Sava (1114), Mark Rodriguez (1822), Jeffery Greathouse (6915), Paul Crozier (1426), Terry Garino (1816), David Rademacher (1114), Ben Cipiti (6223), Haiqing Liu (1114), Greg Halder, Peter Chupas, and Chapman. Chupas, Halder, and Chapman are from Argonne.
“The most important thing we did was introduce a new class of materials to nuclear waste remediation,” says Dorina, postdoctoral appointee on the project. She joined the team 18 months ago from the University of South Florida, where she did graduate work on such materials.
Tina says a third paper was published this year in Industrial & Engineering Chemistry Research that shows the incorporation of MOFs with iodine in a one-step process, low-temperature glass waste form. “We have a volatile off-gas capture using a MOF and we have a durable waste form,” Tina says.
She and her colleagues are continuing their research into new and optimized MOFs for enhanced volatile gas separation and capture. “We are looking at a broad range of materials and learning from them to make new materials,” Tina says.
-- Nancy Salem