Tuesday, August 16, 2011

Designing diamond circuits for extreme environments

There is a new way to design computer chips and electronic circuitry for extreme environments: make them out of diamond.

A team of electrical engineers at Vanderbilt University has developed all the basic components needed to create microelectronic devices out of thin films of nanodiamond. They have created diamond versions of transistors and, most recently, logical gates, which are a key element in computers.

"Diamond-based devices have the potential to operate at higher speeds and require less power than silicon-based devices," Research Professor of Electrical Engineering Jimmy Davidson said. "Diamond is the most inert material known, so our devices are largely immune to radiation damage and can operate at much higher temperatures than those made from silicon."

Their design of a logical gate is described in the Aug. 4 issue of the journal Electronics Letters. Co-authors of the paper are graduate student Nikkon Ghosh, Professor of Electrical Engineering Weng Poo Kang.

Not an engagement ring

Davidson was quick to point out that even though their design uses diamond film, it is not exorbitantly expensive. The devices are so small that about one billion of them can be fabricated from one carat of diamond. The films are made from hydrogen and methane using a method called chemical vapor deposition that is widely used in the microelectronics industry for other purposes. This deposited form of diamond is less than one-thousandth the cost of "jewelry" diamond, which has made it inexpensive enough so that companies are putting diamond coatings on tools, cookware and other industrial products. As a result, the cost of producing nanodiamond devices should be competitive with silicon.

Potential applications include military electronics, circuitry that operates in space, ultra-high speed switches, ultra-low power applications and sensors that operate in high radiation environments, at extremely high temperatures up to 900 degrees Fahrenheit and extremely low temperatures down to minus 300 degrees Fahrenheit.

Hybrid of old and new

The nanodiamond circuits are a hybrid of old fashioned vacuum tubes and modern solid-state microelectronics and combine some of the best qualities of both technologies. Nanodiamond devices consist of a thin film of nanodiamond that is laid down on a layer of silicon dioxide. Much as they do in vacuum tubes, the electrons move through vacuum between the nanodiamond components, instead of flowing through solid material the way they do in normal microelectronic devices. As a result, they require vacuum packaging to operate.

"The reason your laptop gets hot is because the electrons pumping through its transistors bump into the atoms in the semiconductor and heat them up," Davidson said. "Because our devices use electron transport in vacuum they don't produce nearly as much heat."

This transmission efficiency is also one reason why the new devices can run on very small amounts of electrical current. Another is that diamond is the best electron emitter in the world so it doesn't take much energy to produce strong electron beams. "We think we can make devices that use one tenth the power of the most efficient silicon devices," said Davidson.

The design is also largely immune to radiation damage. Radiation disrupts the operation of transistors by inducing unwanted charge in the silicon, causing an effect like tripping the circuit breaker in your home. In the nanodiamond device, on the other hand, the electrons flow through vacuum so there is nothing for energetic particles to disrupt. If the particles strike the nanodiamond anode or cathode, the impact is limited to a small fluctuation in the electron flow, not complete disruption, as is the case with silicon devices.

"When I read about the problems at the Fukushima power plant after the Japanese tsunami, I realized that nanodiamond circuits would be perfect for failsafe circuitry in nuclear reactors," Davidson said. "It wouldn't be affected by high radiation levels or the high temperatures created by the explosions."

Nanodiamond devices can be manufactured by processes that the semiconductor industry currently uses. The one exception is the requirement to operate in vacuum, which would require some modification in the packaging process. Currently, semiconductor chips are sealed in packages filled with an inert gas like argon or simply encapsulated in plastic, protecting them from chemical degradation. Davidson and his colleagues have investigated the packaging process and have found that the metallic seals used in military-grade circuitry are strong enough to hold an adequate vacuum for centuries.

The research was supported by grants from the U.S. Army.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Vanderbilt University.

DNA strands that select nanotubes are first step to a practical 'quantum wire'

DNA, a molecule famous for storing the genetic blueprints for all living things, can do other things as well. In a new paper, researchers at the National Institute of Standards and Technology (NIST) describe how tailored single strands of DNA can be used to purify the highly desired "armchair" form of carbon nanotubes. Armchair-form single wall carbon nanotubes are needed to make "quantum wires" for low-loss, long distance electricity transmission and wiring.

Single-wall carbon nanotubes are usually about a nanometer in diameter, but they can be millions of nanometers in length. It's as if you took a one-atom-thick sheet of carbon atoms, arranged in a hexagonal pattern, and curled it into a cylinder, like rolling up a piece of chicken wire. If you've tried the latter, you know that there are many possibilities, depending on how carefully you match up the edges, from neat, perfectly matched rows of hexagons ringing the cylinder, to rows that wrap in spirals at various angles -- "chiralities" in chemist-speak.

Chirality plays an important role in nanotube properties. Most behave like semiconductors, but a few are metals. One special chiral form -- the so-called "armchair carbon nanotube"* -- behaves like a pure metal and is the ideal quantum wire, according to NIST researcher Xiaomin Tu.

Armchair carbon nanotubes could revolutionize electric power systems, large and small, Tu says. Wires made from them are predicted to conduct electricity 10 times better than copper, with far less loss, at a sixth the weight. But researchers face two obstacles: producing totally pure starting samples of armchair nanotubes, and "cloning" them for mass production. The first challenge, as the authors note, has been "an elusive goal."

Separating one particular chirality of nanotube from all others starts with coating them to get them to disperse in solution, as, left to themselves, they'll clump together in a dark mass. A variety of materials have been used as dispersants, including polymers, proteins and DNA. The NIST trick is to select a DNA strand that has a particular affinity for the desired type of nanotube. In earlier work, team leader Ming Zheng and colleagues demonstrated DNA strands that could select for one of the semiconductor forms of carbon nanotubes, an easier target. In this new paper, the group describes how they methodically stepped through simple mutations of the semiconductor-friendly DNA to "evolve" a pattern that preferred the metallic armchair nanotubes instead.

"We believe that what happens is that, with the right nanotube, the DNA wraps helically around the tube," explains Constantine Khripin, "and the DNA nucleotide bases can connect with each other in a way similar to how they bond in double-stranded DNA." According to Zheng, "The DNA forms this tight barrel around the nanotube. I love this idea because it's kind of a lock and key. The armchair nanotube is a key that fits inside this DNA structure -- you have this kind of molecular recognition."

Once the target nanotubes are enveloped with the DNA, standard chemistry techniques such as chromatography can be used to separate them from the mix with high efficiency.

"Now that we have these pure nanotube samples," says team member Angela Hight Walker, "we can probe the underlying physics of these materials to further understand their unique properties. As an example, some optical features once thought to be indicative of metallic carbon nanotubes are not present in these armchair samples."

* From the distinctive shape of the edge of the cylinder.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by National Institute of Standards and Technology (NIST).

Journal References:

Xiaomin Tu, Angela R. Hight Walker, Constantine Y. Khripin, Ming Zheng. Evolution of DNA Sequences Toward Recognition of Metallic Armchair Carbon Nanotubes. Journal of the American Chemical Society, 2011; : 110728080027017 DOI: 10.1021/ja205407qXiaomin Tu, Suresh Manohar, Anand Jagota, Ming Zheng. DNA sequence motifs for structure-specific recognition and separation of carbon nanotubes. Nature, 2009; 460 (7252): 250 DOI: 10.1038/nature08116

A new catalyst for ethanol made from biomass: Potential renewable path to fuel additives, rubber and solvents

 Researchers in the Pacific Northwest have developed a new catalyst material that could replace chemicals currently derived from petroleum and be the basis for more environmentally friendly products including octane-boosting gas and fuel additives, bio-based rubber for tires and a safer solvent for the chemicals industry.

To make sustainable biofuels, producers want to ferment ethanol from nonfood plant matter such as cornstalks and weeds. Currently, so-called bio-ethanol's main values are as a non-polluting replacement for octane-boosting fuel additives to prevent engine knocking and as a renewable replacement for a certain percentage of gasoline. To turn bio-ethanol into other useful products, researchers at the Department of Energy's Pacific Northwest National Laboratory and at Washington State University have developed a new catalyst material that will convert it into a chemical called isobutene. And it can do so in one production step, which can reduce costs.

Reported by researchers in the Institute for Integrated Catalysis at PNNL and in the Gene and Linda Voiland School of Chemical Engineering and Bioengineering at WSU, the findings appeared July 21 in the Journal of the American Chemical Society.

"Isobutene is a versatile chemical that could expand the applications for sustainably produced bio-ethanol," said chemical engineer Yong Wang, who has a joint appointment at PNNL in Richland, Wash. and at WSU in Pullman, Wash., and leads research efforts at both institutions.

In addition, this catalyst requires the presence of water, allowing producers to use dilute and cheaper bio-ethanol rather than having to purify it first, potentially keeping costs lower and production times faster.

No Z-Z-Z for the Weary

An important key to unlocking renewables to replace fossil fuel products is the catalyst. A catalyst is a substance that promotes chemical reactions of interest. The catalytic converter in a car, for example, speeds up chemical reactions that break down polluting gases, cleaning up a vehicle's exhaust.

The PNNL and WSU researchers were trying to make hydrogen fuel from ethanol. To improve on a conventional catalyst, they had taken zinc oxide and zirconium oxide and combined both into a new material called a mixed oxide -- the zinc and the zirconium atoms woven through a crystal of oxygen atoms. Testing the mixed oxide out, PNNL postdoctoral researcher Junming Sun saw not only hydrogen, but -- unexpectedly -- quite a bit of isobutene (EYE-SO-BEW-TEEN).

Hydrogen is great, but isobutene is better. Chemists can make tire rubber from it or a safer solvent that can replace toxic ones for cleaning or industrial uses. Isobutene can also be readily turned into jet fuel and gasoline additives that up the octane -- that value listed on gas pumps that prevents an engine from knocking -- such as ETBE.

Sun Shines

No one had ever seen a catalyst create isobutene from ethanol in a one-step chemical reaction before, so the researchers realized such a catalyst could be important in reducing the cost of biofuels and renewable chemicals.

Investigating the catalyst in greater depth, the researchers examined what happened when they used different amounts of zinc and zirconium. They showed that a catalyst made from just zinc oxide converted the ethanol mostly to acetone, an ingredient in nail polish remover. If the catalyst only contained zirconium oxide, it converted ethanol mostly to ethylene, a chemical made by plants that ripens fruit.

But the isobutene? That only arose in useful amounts when the catalyst contained both zinc and zirconium. And "useful amounts" means "a lot." With a 1:10 ratio of zinc to zirconium, the mixed oxide catalyst could turn more than 83 percent of the ethanol into isobutene.

"We consistently got 83 percent yield with improved catalyst life," said Wang. "We were happy to see that very high yield."

Reactionary Insight

The researchers analyzed the chemistry to figure out what was happening. In the single metal oxides experiments, the zinc oxide created acetone while the zirconium oxide created ethylene. The easiest way to get to isobutene from there, theoretically speaking, is to convert acetone into isobutene, which zirconium oxide is normally capable of. And the road from ethanol to isobutene could only be as productive as Sun found if zirconium oxide didn't get side-tracked turning ethanol into ethylene along the way.

Something about the mixed oxide, then, prevented zirconium oxide from turning ethanol into the undesired ethylene. The team reasoned the isobutene probably arose from zinc oxide turning ethanol into acetone, then zirconium oxide -- influenced by the nearby zinc oxide -- turning acetone into isobutene. At the same time, the zinc oxide's influence prevented the ethanol-to-ethylene conversion by zirconium oxide. Although that's two reaction steps for the catalyst, it's only one for the chemists, since they only had to put the catalyst in with ethanol and water once.

To get an idea of how close the reactions had to happen to each other for isobutene to show up, the team combined powdered zinc oxide and powdered zirconium oxide. This differed from the mixed oxide in that the zinc and zirconium atoms were not incorporated into the same catalyst particles. These mixed powders turned ethanol primarily into acetone and ethylene, with some amounts of other molecules and less than 3 percent isobutene, indicating the magic of the catalyst came from the microstructure of the mixed oxide material.

Balancing Act

So, the researchers explored the microstructure using instruments and expertise at EMSL, DOE's Environmental Molecular Sciences Laboratory on the PNNL campus. Using high-powered tools called transmission electron microscopes, the team saw that the mixed oxide catalyst was made up of nanometer-sized crystalline particles.

A closer look at the best-performing catalysts revealed zinc oxide distributed evenly over regions of zirconium oxide. The worst performing catalyst -- with a 1:1 zinc to zirconium ratio -- revealed regions of zinc oxide and regions of zirconium oxide. This suggested to the team that the two metals had to be close to each other to quickly flip the acetone into isobutene.

Experimental results from other analytical methods indicated that the team could optimize the type of chemical reactions that lead to isobutene and also prevent the catalyst from deactivating at the same time. The elegant balance of acidic and basic sites on the mixed oxides significantly reduced carbon from building up and gunking up the catalysts, which cuts their lifespan.

Future work will look into optimizations to further improve the yield and catalyst life. Wang and colleagues would also like to see if they can combine this isobutene catalyst with other catalysts to produce different chemicals in one-pot reactions.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by DOE/Pacific Northwest National Laboratory.

Journal Reference:

Junming Sun, Kake Zhu, Feng Gao, Chongmin Wang, Jun Liu, Charles H. F. Peden, Yong Wang. Direct Conversion of Bio-ethanol to Isobutene on Nanosized ZnxZryOzMixed Oxides with Balanced Acid–Base Sites. Journal of the American Chemical Society, 2011; 133 (29): 11096 DOI: 10.1021/ja204235v

When atoms are surfing on optical waves

 Researchers at the University of Tübingen are working on next generation's computer: They made cold atoms interact with miniature gold wires as small as a thousandth of a millimeter. Illuminating the wires with laser light in a special way, the physicists concentrated the light field at the surface of the wires and, by that, generated so-called surface plasmons. These are bound light fields which might enable the construction of devices for optical computing and for quantum information. Circuits based on these devices would be much faster and more efficient than present technologies.

In order to build an optical computing device the surface plasmons, which are useful for data transfer, must be coupled to data storage elements, such as atoms. This is what the research team lead by Dr. Sebastian Slama is working on. The junior scientist developed techniques at the chair of Prof. Claus Zimmermann which are crucial for positioning cold atoms very close to surfaces such that they can interact with bound light waves. For that atomic gases are cooled in a vacuum chamber down to temperatures as low as a few hundred Nanokelvin.

At such low temperature the atoms no longer behave as a classical gas. They form a so-called Bose-Einstein condensate, in which all atoms are in the same quantum state. The condensate can be regarded as a single huge super-atom and can be shifted by external magnetic fields to the surface, where it feels the influence of the plasmon. "We can generate plasmons which attract the atoms and others which repel them. By structuring the surface we can tailor almost arbitrary potential landscapes for the atoms," says Dr. Slama.

Recently, the scientists published their results in Nature Photonics magazine. First author Christian Stehle, who is working on his PhD thesis and has measured the data (together with Helmar Bender, who is now postdoc at the University of Sao Carlos in Brazil) is enthusiastic: "Our results had a great impact. We managed to get on the title page of the August issue, and the magazine values our work in a comment." However, with this success the scientists' work is not terminated. "Our goal is to build hybrid devices for optical computing and quantum information. We were now able to set a milestone, but there is still a lot to do," says Dr. Slama. In his opinion these goals can only be achieved in cooperation with other scientists. Beside already existing cooperations like the one with the nanotechnology group of Prof. Dieter Kern and Dr. Monika Fleischer, who fabricated the gold structures, Slama has made contact to further scientists in Tübingen, Europe and in Brazil.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Universitaet Tübingen.

Journal Reference:

Christian Stehle, Helmar Bender, Claus Zimmermann, Dieter Kern, Monika Fleischer, Sebastian Slama. Plasmonically tailored micropotentials for ultracold atoms. Nature Photonics, 2011; 5 (8): 494 DOI: 10.1038/nphoton.2011.159