Physicists conduct most precise measurement yet of interaction between atoms and carbon surfaces

An illustration of atoms sticking to a carbon nanotube, 
affecting the electrons in its surface.David Cobden and students

Physicists at the University of Washington have conducted the most precise and controlled measurements yet of the interaction between the atoms and molecules that comprise air and the type of carbon surface used in battery electrodes and air filters — key information for improving those technologies.

A team led by David Cobden, UW professor of physics, used a carbon nanotube — a seamless, hollow graphite structure a million times thinner than a drinking straw — acting as a transistor to study what happens when gas atoms come into contact with the nanotube’s surface. Their findings were published in May in the journal Nature Physics.

Cobden said he and co-authors found that when an atom or molecule sticks to the nanotube a tiny fraction of the charge of one electron is transferred to its surface, resulting in a measurable change in electrical resistance.

“This aspect of atoms interacting with surfaces has never been detected unambiguously before,” Cobden said. “When many atoms are stuck to the minuscule tube at the same time, the measurements reveal their collective dances, including big fluctuations that occur on warming analogous to the boiling of water.”

Lithium batteries involve lithium atoms sticking and transferring charges to carbon electrodes, and in activated charcoal filters, molecules stick to the carbon surface to be removed, Cobden explained.

“Various forms of carbon, including nanotubes, are considered for hydrogen or other fuel storage because they have a huge internal surface area for the fuel molecules to stick to. However, these technological situations are extremely complex and difficult to do precise, clear-cut measurements on.”

This work, he said, resulted in the most precise and controlled measurements of these interactions ever made, “and will allow scientists to learn new things about the interplay of atoms and molecules with a carbon surface,” important for improving technologies including batteries, electrodes and air filters.

Co-authors were Oscar Vilches, professor emeritus of physics, doctoral students Hao-Chun Lee and research associate Boris Dzyubenko, all of the UW. The research was funded by the National Science Foundation.

Source: http://www.washington.edu/news/2015/05/28/physicists-conduct-most-precise-measurement-yet-of-interaction-between-atoms-and-carbon-surfaces/

Stanford Scientists Use DNA to Assemble a Transistor From Graphene

Graphene, a sheet of carbon atoms arrayed in a honeycomb pattern, could be a better semiconductor than silicon.

DNA is the blueprint for life. Could it also become the template for making a new generation of computer chips based not on silicon, but on an experimental material known as graphene?

That’s the theory behind a process that Stanford chemical engineering professor Zhenan Bao reveals in Nature Communications.

Bao and her co-authors, former post-doctoral fellows Anatoliy Sokolov and Fung Ling Yap, hope to solve a problem clouding the future of electronics: consumers expect silicon chips to continue getting smaller, faster and cheaper, but engineers fear that this virtuous cycle could grind to a halt.

Why has to do with how silicon chips work.

Everything starts with the notion of the semiconductor, a type of material that can be induced to either conduct or stop the flow of electricity. Silicon has long been the most popular semiconductor material used to make chips.

The basic working unit on a chip is the transistor. Transistors are tiny gates that switch electricity on or off, creating the zeroes and ones that run software.

To build more powerful chips, designers have done two things at the same time: they’ve shrunk transistors in size and also swung those gates open and shut faster and faster.

The net result of these actions has been to concentrate more electricity in a diminishing space. So far that has produced small, faster, cheaper chips. But at a certain point, heat and other forms of interference could disrupt the inner workings of silicon chips.

"We need a material that will let us build smaller transistors that operate faster using less power," Bao said.

Graphene has the physical and electrical properties to become a next-generation semiconductor material – if researchers can figure out how to mass-produce it.

To the right is a honeycomb of graphene atoms. To the left is a double strand of DNA. The white spheres represent copper ions integral to the chemical assembly process. The fire represents the heat that is an essential ingredient in the technique. (Anatoliy Sokolov) 

Graphene is a single layer of carbon atoms arranged in a honeycomb pattern. Visually it resembles chicken wire. Electrically this lattice of carbon atoms is an extremely efficient conductor.

Bao and other researchers believe that ribbons of graphene, laid side-by-side, could create semiconductor circuits. Given the material’s tiny dimensions and favorable electrical properties, graphene nano ribbons could create very fast chips that run on very low power, she said.

"However, as one might imagine, making something that is only one atom thick and 20 to 50 atoms wide is a significant challenge," said co-author Sokolov.

To handle this challenge, the Stanford team came up with the idea of using DNA as an assembly mechanism.

Physically, DNA strands are long and thin, and exist in roughly the same dimensions as the graphene ribbons that researchers wanted to assemble.

Chemically, DNA molecules contain carbon atoms, the material that forms graphene.

The real trick is how Bao and her team put DNA’s physical and chemical properties to work.

The researchers started with a tiny platter of silicon to provide a support (substrate) for their experimental transistor. They dipped the silicon platter into a solution of DNA derived from bacteria and used a known technique to comb the DNA strands into relatively straight lines.

Next, the DNA on the platter was exposed to a copper salt solution. The chemical properties of the solution allowed the copper ions to be absorbed into the DNA.

Next the platter was heated and bathed in methane gas, which contains carbon atoms. Once again chemical forces came into play to aid in the assembly process. The heat sparked a chemical reaction that freed some of the carbon atoms in the DNA and methane. These free carbon atoms quickly joined together to form stable honeycombs of graphene.

"The loose carbon atoms stayed close to where they broke free from the DNA strands, and so they formed ribbons that followed the structure of the DNA," Yap said.

So part one of the invention involved using DNA to assemble ribbons of carbon. But the researchers also wanted to show that these carbon ribbons could perform electronic tasks. So they made transistors on the ribbons.

"We demonstrated for the first time that you can use DNA to grow narrow ribbons and then make working transistors," Sokolov said.

The paper drew praise from UC Berkeley associate professor Ali Javey, an expert in the use of advanced materials and next-generation electronics.

"This technique is very unique and takes advantage of the use of DNA as an effective template for controlled growth of electronic materials,” Javey said. “In this regard the project addresses an important research need for the field."

Bao said the assembly process needs a lot of refinement. For instance, not all of the carbon atoms formed honeycombed ribbons a single atom thick. In some places they bunched up in irregular patterns, leading the researchers to label the material graphitic instead of graphene.

Even so, the process, about two years in the making, points toward a strategy for turning this carbon-based material from a curiosity into a serious contender to succeed silicon.

"Our DNA-based fabrication method is highly scalable, offers high resolution and low manufacturing cost," said co-author Yap. "All these advantages make the method very attractive for industrial adoption."

The experiment was supported in part by the National Science Foundation and the Stanford Global Climate and Energy Program.

Tom Abate is associate director of communications in the Stanford School of Engineering.

Source: http://engineering.stanford.edu/news/stanford-scientists-use-dna-assemble-transistor-graphene

Airbrushing Could Facilitate Large-Scale Manufacture of Carbon Nanofibers

Researchers from North Carolina State University used airbrushing techniques to grow vertically aligned carbon nanofibers on several different metal substrates, opening the door for incorporating these nanofibers into gene delivery devices, sensors, batteries and other technologies.

“Because we’re using an airbrush, this technique could easily be incorporated into large-scale, high-throughput manufacturing processes,” says Dr. Anatoli Melechko, an adjunct associate professor of materials science and engineering at NC State and co-author of a paper describing the work. “In principle, you could cover an entire building with it.”

“It’s common to use nickel nanoparticles as catalysts to grow carbon nanofibers, and we were able to coat metal substrates with nickel nanoparticles using an airbrush,” says Dr. Joseph Tracy, an associate professor of materials science and engineering at NC State and senior author of the paper. “Airbrushing gives us a fairly uniform coating of the substrate and it can be applied to a large area at room temperature in a short period of time.”

After applying the nickel nanoparticles, the researchers airbrushed the substrate with a layer of silicon powder and heated the coated substrate to 600 degrees Celsius in a reactor filled with acetylene and ammonia gas. In the reactor, carbon nanofibers formed under the nickel nanoparticles and were held upright by a silicon-enriched coating. The finished product resembles a forest of nanofibers running perpendicular to the substrate. The researchers tested this technique successfully on aluminum, copper and titanium substrates.

“Growing carbon nanofibers on a metal substrate means the interface between the two materials is highly conductive, which makes the product more useful as an electrode material for use in a range of potential applications,” says Mehmet Sarac, a Ph.D. student at NC State and lead author of the paper.

The paper, “Airbrushed Nickel Nanoparticles for Large-Area Growth of Vertically Aligned Carbon Nanofibers on Metal (Al, Cu, Ti) Surfaces,” was published online Sept. 9 in ACS Applied Materials & Interfaces. The paper was co-authored by NC State Ph.D. students Bryan Anderson, and Adedapo Oni; former NC State graduate students Dr. Ryan Pearce and Justin Railsback; former NC State postdoctoral researcher Dr. Ryan White; Dr. James LeBeau, an assistant professor of materials science and engineering at NC State; and Dale Hensley of Oak Ridge National Laboratory. The work was supported by the National Science Foundation, the Defense Threat Reduction Agency, the U.S. Department of Energy and the Republic of Turkey’s Ministry of National Education.

Source http://news.ncsu.edu/releases/wms-tracy-airbrush-2013/

Advancing Graphene for Post-Silicon Computer Logic

Team of UC Riverside researchers pioneer new approach for graphene logic circuits

A team of researchers from the University of California, Riverside’s Bourns College of Engineering have solved a problem that previously presented a serious hurdle for the use of graphene in electronic devices.

Scanning electron microscopy image of graphene device used in the study. The scale bar is one micrometer. The UCR logo next to it is implemented with etched graphene.

Graphene is a single-atom thick carbon crystal with unique properties beneficial for electronics including extremely high electron mobility and phonon thermal conductivity. However, graphene does not have an energy band gap, which is a specific property of semiconductor materials that separate electrons from holes and allows a transistor implemented with a given material to be completely switched off.

A transistor implemented with graphene will be very fast but will suffer from leakage currents and power dissipation while in the off state because of the absence of the energy band gap. Efforts to induce a band-gap in graphene via quantum confinement or surface functionalization have not resulted in a breakthrough. That left scientists wondering whether graphene applications in electronic circuits for information processing were feasible.

The UC Riverside team – Alexander Balandin andRoger Lake, both electrical engineering professors, Alexander Khitun, an adjunct professor of electrical engineering, and Guanxiong Liu and Sonia Ahsan, both of whom earned their Ph.Ds from UC Riverside while working on this research – has eliminated that doubt.

“Most researchers have tried to change graphene to make it more like conventional semiconductors for applications in logic circuits,” Balandin said. “This usually results in degradation of graphene properties. For example, attempts to induce an energy band gap commonly result in decreasing electron mobility while still not leading to sufficiently large band gap.”

Alexander Balandin, a professor of Electrical Engineering

“We decided to take alternative approach,” Balandin said. “Instead of trying to change graphene, we changed the way the information is processed in the circuits.”

The UCR team demonstrated that the negative differential resistance experimentally observed in graphene field-effect transistors allows for construction of viable non-Boolean computational architectures with the gap-less graphene. The negative differential resistance – observed under certain biasing schemes – is an intrinsic property of graphene resulting from its symmetric band structure. The advanced version of the paper with UCR findings can be accessed at http://arxiv.org/abs/1308.2931.

Modern digital logic, which is used in computers and cell phones, is based on Boolean algebra implemented in semiconductor switch-based circuits. It uses zeroes and ones for encoding and processing the information. However, the Boolean logic is not the only way to process information. The UC Riverside team proposed to use specific current-voltage characteristics of graphene for constructing the non-Boolean logic architecture, which utilizes the principles of the non-linear networks.

Roger Lake, a professor of electrical engineering

The graphene transistors for this study were built and tested by Liu at Balandin’s Nano-Device Laboratory at UC Riverside. The physical processes leading to unusual electrical characteristics were simulated using atomistic models by Ahsan, who was working under Lake. Khitun provided expertise on non-Boolean logic architectures.

The atomistic modeling conducted in Lake’s group shows that the negative differential resistance appears not only in microscopic-size graphene devices but also at the nanometer-scale, which would allow for fabrication of extremely small and low power circuits.

The proposed approach for graphene circuits presents a conceptual change in graphene research and indicates an alternative route for graphene’s applications in information processing according to the UC Riverside team.

http://ucrtoday.ucr.edu/17286

Process devised for ultrathin carbon membranes

Physicists from Bielefeld University have developed a new method of fabrication

In the future, carbon nanomembranes are expected to be able to filter out very fine materials. These separating layers are ultrathin, consisting of just one layer of molecules. In the long term, they could allow to separate gases from one another, for example, filtering toxins from the air. At present, the basic research is concerned with the production of nanomembranes. A research team working with Professor Dr. Armin Gölzhäuser of Bielefeld University has succeeded in developing a new path to produce such membranes. The advantage of this procedure is that it allows a variety of different carbon nanomembranes to be generated which are much thinner than conventional membranes. The upcoming issue of the renowned research journal ‘ACS Nano’ reports on this research success.  

More than ten years ago, Professor Gölzhäuser and his then team created the groundwork for the current development, producing a carbon nanomembrane from biphenyl molecules. In the new study, the process was altered so as to allow the use of other starting materials. The prerequisite is that these molecules are also equipped with several so-called phenyl rings. For their new method, the researchers use the starting material in powder form. They dissolve the powder to pure alcohol and immerse very thin metal layer in this solution. After a short time the dissolved molecules settle themselves on the metal layer to form a monolayer of molecules. After being exposed to electron irradiation, the monolayer becomes a cross-linked nanomembrane. Subsequently the researchers ensure that the metal layer disintegrates, leav-ing only the nanomembrane remaining. ‘Up until now, we have produced small samples which are are a few centimetres square’, says Gölzhäuser. ‘However, with this process it is possible to make nanomembranes that are as big as square metres.’  

This new method is so special because the researchers can produce made-to-measure nanomembranes. ‘Every starting material has a different property, from thickness or trans-parency to elasticity. By using our process, these characteristics are transferred onto the nanomembrane.’ In this way, carbon nanomembranes can be produced to address many dif-ferent needs. ‘That was not possible before now’, says Gölzhäuser. 

Furthermore, graphene can be made from nanomembranes. Researchers worldwide are expecting graphene to have technically revolutionising properties, as it has an extremely high tensile strength and can conduct electricity and heat very well.  The conversion from nanomembranes into graphene is simple for the Bielefeld researchers: The membranes have to be heated in a vacuum at a temperature of about 700 degrees Celsius.   Gölzhäuser’s team is working on the project with physicists from Ulm University, Frankfurt University and the Max Planck Institute for Polymer Research. The study has been funded by the Federal Ministry of Education and Research (BMBF) and the German Research Foundation (DFG).

http://ekvv.uni-bielefeld.de/blog/uninews/entry/process_devised_for_ultrathin_carbon