Eric Sembrat's Test Bonanza

A combined computational and experimental study of self-assembled silver-based structures known as superlattices has revealed an unusual and unexpected behavior: arrays of gear-like molecular-scale machines that rotate in unison when pressure is applied to them.

Computational and experimental studies show that the superlattice structures, which are self-assembled from smaller clusters of silver nanoparticles and organic protecting molecules, form in layers with the hydrogen bonds between their components serving as “hinges” to facilitate the rotation. Movement of the “gears” is related to another unusual property of the material: increased pressure on the superlattice softens it, allowing subsequent compression to be done with significantly less force.

Materials containing the gear-like nanoparticles – each composed of nearly 500 atoms – might be useful for molecular-scale switching, sensing and even energy absorption. The complex superlattice structure is believed to be among the largest solids ever mapped in detail using a combined X-ray and computational techniques.

“As we squeeze on this material, it gets softer and softer and suddenly experiences a dramatic change,” said Uzi Landman, a Regents’ and F.E. Callaway professor in the School of Physics at the Georgia Institute of Technology. “When we look at the orientation of the microscopic structure of the crystal in the region of this transition, we see that something very unusual happens. The structures start to rotate with respect to one another, creating a molecular machine with some of the smallest moving elements ever observed.”

The gears rotate as much as 23 degrees, and return to their original position when the pressure is released. Gears in alternating layers move in opposite directions, said Landman, who is director of the Center for Computational Materials Science at Georgia Tech.

Supported by the Air Force Office of Scientific Research and the Office of Basic Energy Sciences in the Department of Energy, the research was reported April 6 in the journal Nature Materials. Researchers from Georgia Tech and the University of Toledo collaborated on the project.

The research studied superlattice structures composed of clusters with cores of 44 silver atoms each. The silver clusters are protected by 30 ligand molecules of an organic material – mercaptobenzoic acid (p-MBA) – that includes an acid group. The organic molecules are attached to the silver by sulfur atoms.

“It’s not the individual atoms that form the superlattice,” explained Landman. “You actually make the larger structure from clusters that are already crystallized. You can make an ordered array from those.”

In solution, the clusters assemble themselves into the larger superlattice, guided by the hydrogen bonds, which can only form between the p-MBA molecules at certain angles.

“The self-assembly process is guided by the desire to form hydrogen bonds,” Landman explained. “These bonds are directional and cannot vary significantly, which restricts the orientation that the molecules can have.”

The superlattice was studied first using quantum-mechanical molecular dynamics simulations conducted in Landman’s lab. The system was also studied experimentally by a research group headed by Terry Bigioni, an associate professor in the Department of Chemistry and Biochemistry at the University of Toledo.

The unusual behavior occurred as the superlattice was being compressed using hydrostatic techniques. After the structure had been compressed by about six percent of its volume, the pressure required for additional compression suddenly dropped significantly. The researchers discovered that the drop occurred when the nanocrystal components rotated, layer-by-layer, in opposite directions.

Just as the hydrogen bonds direct how the superlattice structure is formed, so also do they guide how the structure moves under pressure.

“The hydrogen bond likes to have directionality in its orientation,” Landman explained. “When you press on the superlattice, it wants to maintain the hydrogen bonds. In the process of trying to maintain the hydrogen bonds, all the organic ligands bend the silver cores in one layer one way, and those in the next layer bend and rotate the other way.”

When the nanoclusters move, the structure pivots about the hydrogen bonds, which act as “molecular hinges” to allow the rotation. The compression is possible at all, Landman noted, because the crystalline structure has about half of its space open.

The movement of the silver nanocrystallites could allow the superlattice material to serve as an energy-absorbing structure, converting force to mechanical motion. By changing the conductive properties of the silver superlattice, compressing the material could also allow it be used as molecular-scale sensors and switches.  

The combined experimental and computation study makes the silver superlattice one of the most thoroughly studied materials in the world.

“We now have complete control over a unique material that by its composition has a diversity of molecules,” Landman said. “It has metal, it has organic materials and it has a stiff metallic core surrounded by a soft material.”

For the future, the researchers plan additional experiments to learn more about the unique properties of the superlattice system. The unique system shows how unusual properties can arise when nanometer-scale systems are combined with many other small-scale units.

“We make the small particles, and they are different because small is different,” said Landman. “When you put them together, having more of them is different because that allows them to behave collectively, and that collective activity makes the difference.”

In addition to those already mentioned, Georgia Tech co-authors included research scientist Bokwon Yoon – the paper’s first author – and senior research scientists W.David Luedtke, Robert Barnett and Jianping Gao. Co-authors from the University of Toledo include Anil Desireddy and Brian E. Conn.

This research was supported by the Air Force Office of Scientific Research (AFOSR), and by the Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) under Contract FG05-86ER45234. Any conclusions or opinions expressed are those of the authors and do not necessarily represent the official views of the AFOSR or the DOE.

CITATION: Bokwon Yoon, et al., “Hydrogen-bonded structure and mechanical chiral response of a silver nanoparticle superlattice.” (Nature Materials, 2014). http://dx.doi.org/ 10.1038/NMAT3923.

Research News

Georgia Institute of Technology

177 North Avenue

Atlanta, Georgia  30332-0181  USA

Media Relations Contacts: John Toon (jtoon@gatech.edu) (404-894-6986) or Brett Israel (brett.israel@comm.gatech.edu) (404-385-1933).

Writer: John Toon

In this exciting event, lectures on the physics of chocolate will be presented from White House Executive Pastry Chef William "Bill" Yosses and UPenn Physics Professor Arjun Yodh.

Most of what we eat is squishy - behaving as a solid on a plate, or as a liquid when processed in your mouth.  Squishy Physics investigates materials that are soft and easy to deform and, in most cases, are made from mixtures of phases.  The lectures will cover interesting and entertaining physical questions that are critical to cooking and understanding the properties of food.

This year, Squishy Physics will focus on the exciting science of chocolate! Almost everyone loves the silky smooth taste of chocolate melting on their tongue. You might be surprised that a great deal of science is required to produce your favorite chocolate treat. If you have ever cooked with chocolate, you may have even observed that it can solidify into something very different from the starting ingredient. Would you be shocked to learn that chocolate has six different solid phases?

Together, we can explore the exciting and entertaning intersection of science, gastronomy, and chocolate. Awards will also be presented to the top middle and high school student submissions for the Squishy Physics photography contest.  This contest is organized in conjunction with the Fernbank Science Center.

Free tickets are available online at EventBrite.

Time/Date: Saturday, March 22, 2014, 10:00am - 12:00pm

Location: Georgia Institute of Technology, Instructional Center, Room 103, 759 Ferst Drive NW, Atlanta, GA 30318

Parking:  Flat-rate parking is available on a first-come, first-served basis at the W02 deck and Area 2 lot adjacent to the Student Center.

Two recent papers published in Computing in Cardiology by Ilija Uzelac,  Flavio Fenton and collaborators.

Title: "High-Power Current Source with Real-Time Arbitrary Waveforms for
In Vivo and In Vitro Studies of Defibrillation"
Computing in Cardiology  Volume 40:667-670.

and

Title: "Validation of a Computational Model of Cardiac Defibrillation"
Computing in Cardiology  Volume 40:851-854.

Graduate student Daegene Koh has received the EAPSI (East Asian and Pacific Summer Institutes) Fellowship for Summer 2014. Congratulations! He will be working in Chosun University in Korea with Prof. Kyung-jin Ahn. They will be investigating the contribution from the first stars and galaxies to the cosmic near-infrared background, using massively parallel radiative transfer simulations. The product of this research will secure additional observational constraints on the nature of galaxy formation during the first billion years of the Universe, probing the faintest galaxies that are otherwise individually undetectable with current facilities. However, their cumulative luminosity may leave an imprint in the cosmic near-infrared background, which can be disentangled from galaxies that form closer to home, when utilizing theoretical models of galaxy formation.

Using electrons more like photons could provide the foundation for a new type of electronic device that would capitalize on the ability of graphene to carry electrons with almost no resistance even at room temperature – a property known as ballistic transport.

Research reported this week shows that electrical resistance in nanoribbons of epitaxial graphene changes in discrete steps following quantum mechanical principles. The research shows that the graphene nanoribbons act more like optical waveguides or quantum dots, allowing electrons to flow smoothly along the edges of the material. In ordinary conductors such as copper, resistance increases in proportion to the length as electrons encounter more and more impurities while moving through the conductor.

The ballistic transport properties, similar to those observed in cylindrical carbon nanotubes, exceed theoretical conductance predictions for graphene by a factor of 10. The properties were measured in graphene nanoribbons approximately 40 nanometers wide that had been grown on the edges of three-dimensional structures etched into silicon carbide wafers.

“This work shows that we can control graphene electrons in very different ways because the properties are really exceptional,” said Walt de Heer, a Regent’s professor in the School of Physics at the Georgia Institute of Technology. “This could result in a new class of coherent electronic devices based on room temperature ballistic transport in graphene. Such devices would be very different from what we make today in silicon.”

The research, which was supported by the National Science Foundation, the Air Force Office of Scientific Research and the W.M. Keck Foundation, was reported February 5 in the journal Nature. The research was done through a collaboration of scientists from Georgia Tech in the United States, Leibniz Universität Hannover in Germany, the Centre National de la Recherche Scientifique (CNRS) in France and Oak Ridge National Laboratory – supported by the Department of Energy – in the United States.

For nearly a decade, researchers have been trying to use the unique properties of graphene to create electronic devices that operate much like existing silicon semiconductor chips. But those efforts have met with limited success because graphene – a lattice of carbon atoms that can be made as little as one layer thick – cannot be easily given the electronic bandgap that such devices need to operate.

De Heer argues that researchers should stop trying to use graphene like silicon, and instead use its unique electron transport properties to design new types of electronic devices that could allow ultra-fast computing – based on a new approach to switching. Electrons in the graphene nanoribbons can move tens or hundreds of microns without scattering.

“This constant resistance is related to one of the fundamental constants of physics, the conductance quantum,” de Heer said. “The resistance of this channel does not depend on temperature, and it does not depend on the amount of current you are putting through it.”

What does disrupt the flow of electrons, however, is measuring the resistance with an electrical probe. The measurements showed that touching the nanoribbons with a single probe doubles the resistance; touching it with two probes triples the resistance.

“The electrons hit the probe and scatter,” explained de Heer. “It’s a lot like a stream in which water is flowing nicely until you put rocks in the way. We have done systematic studies to show that when you touch the nanoribbons with a probe, you introduce a method for the electrons to scatter, and that changes the resistance.”

The nanoribbons are grown epitaxially on silicon carbon wafers into which patterns have been etched using standard microelectronics fabrication techniques. When the wafers are heated to approximately 1,000 degrees Celsius, silicon is preferentially driven off along the edges, forming graphene nanoribbons whose structure is determined by the pattern of the three-dimensional surface. Once grown, the nanoribbons require no further processing.

The advantage of fabricating graphene nanoribbons this way is that it produces edges that are perfectly smooth, annealed by the fabrication process. The smooth edges allow electrons to flow through the nanoribbons without disruption. If traditional etching techniques are used to cut nanoribbons from graphene sheets, the resulting edges are too rough to allow ballistic transport.

“It seems that the current is primarily flowing on the edges,” de Heer said. “There are other electrons in the bulk portion of the nanoribbons, but they do not interact with the electrons flowing at the edges.”

The electrons on the edge flow more like photons in optical fiber, helping them avoid scattering. “These electrons are really behaving more like light,” he said. “It is like light going through an optical fiber. Because of the way the fiber is made, the light transmits without scattering.”

The researchers measured ballistic conductance in the graphene nanoribbons for up to 16 microns. Electron mobility measurements surpassing one million correspond to a sheet resistance of one ohm per square that is two orders of magnitude lower than what is observed in two-dimensional graphene – and ten times smaller than the best theoretical predictions for graphene.

“This should enable a new way of doing electronics,” de Heer said. “We are already able to steer these electrons and we can switch them using rudimentary means. We can put a roadblock, and then open it up again. New kinds of switches for this material are now on the horizon.”

Theoretical explanations for what the researchers have measured are incomplete. De Heer speculates that the graphene nanoribbons may be producing a new type of electronic transport similar to what is observed in superconductors.  

“There is a lot of fundamental physics that needs to be done to understand what we are seeing,” he added. “We believe this shows that there is a real possibility for a new type of graphene-based electronics.”

Georgia Tech researchers have pioneered graphene-based electronics since 2001, for which they hold a patent, filed in 2003. The technique involves etching patterns into electronics-grade silicon carbide wafers, then heating the wafers to drive off silicon, leaving patterns of graphene.

In addition to de Heer, the paper’s authors included Jens Baringhaus, Frederik Edler and Christoph Tegenkamp from the Institut für Festkörperphysik, Leibniz Universität, Hannover in Germany; Edward Conrad, Ming Ruan and Zhigang Jiang from the School of Physics at Georgia Tech; Claire Berger from Georgia Tech and Institut Néel at the Centre National de la Recherche Scientifique (CNRS) in France; Antonio Tejeda and Muriel Sicot from the Institut Jean Lamour, Universite de Nancy, Centre National de la Recherche Scientifique (CNRS) in France; An-Ping Li from the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory, and Amina Taleb-Ibrahimi from the CNRS Synchotron SOLEIL in France.

This research was supported by the National Science Foundation (NSF) Materials Research Science and Engineering Center (MRSEC) at Georgia Tech through award DMR-0820382; the Air Force Office of Scientific Research (AFOSR); the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy, and the Partner University Fund from the Embassy of France. Any conclusions or recommendations are those of the authors and do not necessarily represent the official views of the NSF, DOE or AFOSR.

CITATION: Jens Baringhaus, et al., “Exceptional ballistic transport in epitaxial graphene nanoribbons,” (Nature 2013). (http://dx.doi.org/10.1038/nature12952).

Research News
Georgia Institute of Technology
177 North Avenue
Atlanta, Georgia  30332-0181  USA

Media Relations Contacts: John Toon (404-894-6986) (jtoon@gatech.edu) or Brett Israel (404-385-1933) (brett.israel@comm.gatech.edu).

Writer: John Toon

Please welcome Dr. Angelo Bongiorno, Assistant Professor, to the School of Physics. Dr. Bongiorno has recently joined the faculty of the school. He will have a split appointment with the School of Physics and the School of Chemistry & Biochemistry. Dr. Bongiorno's research interests focus on investigating the physics and chemistry of complex materials systems with applications in the energy and microelectronics industries.

If you see him, please welcome him to our team.

Join us in congratulating Dr. Flavio Fenton at the recipient of the 2014 Hesburgh Award Teaching Fellow. Every year, Georgia Tech's Center for Enhanced Teaching and Learning (CETL) invites a small, multidisciplinary group of associate professors to become Hesburgh Award Teaching Fellows. These faculty members are nominated for this honor by their college and meet throughout spring semester to discuss innovative ways to improve student learning and to strengthen teaching on the Georgia Tech campus. Teaching Fellows receive a small stipend to implement a project to improve student learning in a course during the following summer or fall semester. Congratulations Dr. Fenton!

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Scientists are using a one cubic kilometer block of ice at the South Pole to help unravel one of the great scientific mysteries of our time.

The block is part of IceCube, an observatory built in one of the most inhospitable parts of the world to study neutrinos zipping through the Earth from outer space. These subatomic particles normally pass through the Earth as easily as light passes through a pane of glass.But a few of them crash into the ultra-clear ice of IceCube. When they do, they produce secondary particles that can create a faint bluish light called Cherenkov radiation.

Scientists like Ignacio Taboada, an assistant professor in the Georgia Tech School of Physics, are using information from that glow to learn more about these neutrinos – including, perhaps, where in the universe they came from. Using data collected between May 2010 and May 2012, IceCube has measured 28 neutrinos that likely originated outside our solar system, the first time such very-high-energy cosmic neutrinos have been observed.

As the IceCube Neutrino Observatory celebrates the third anniversary of the end of construction on Dec. 18th, it is receiving the “Breakthrough of the Year” award from the British journal Physics World for observing these cosmic visitors.

“We now know that neutrinos from outside our planet are there, and we saw roughly as many as we had expected,” said Taboada, who has been part of the IceCube Collaboration team for 14 years and was a co-author of the Nov. 22 paper in the journal Science that reported the neutrino findings. “We have learned some things about neutrinos, but there are still other things that we still don’t understand.”

The centerpiece of IceCube is a matrix of 86 strings of basketball-sized optical detectors placed one and a half kilometers beneath the Antarctic snow. Each string includes 60 optical detectors that are so sensitive they can register a single photon. Dug into the ice by a special hot-water drill, the strings are arranged in a pattern designed to use the Cherenkov radiation to map the path of neutrinos through the ice.

By observing the path of the particles through the ice block, scientists had hoped to learn where the particles originated in space. But from the 28 neutrino events observed, that hasn’t yet been possible.

“So far, we are not seeing individual sources for the neutrinos,” said Taboada. “We are not seeing any place in the sky that we can point to and say that we see even three or four of the neutrinos coming from. It is very diffuse. It may be that the sources are very weak, or that we have a detector that is not large enough.”

Neutrinos are nearly-massless particles that carry no electrical charge and originate from a variety of sources, including radioactive decay, our own Sun, cosmic rays and events such as exploding stars. From May 2010 to May 2012, IceCube recorded 28 neutrinos with very high energy levels – above 30 trillion electron volts – suggesting they were from beyond our own solar system.

One challenge for the nearly 300 scientists involved in the program is separating cosmic neutrinos from those that originate on Earth. Cosmic neutrinos tend to have much higher energy levels, which can be measured by the amount of light they produce in the ice. Their direction also gives a clue; only cosmic neutrinos can pass through the Earth into the ice.

Built with funds from the National Science Foundation and scientific organizations from three European countries, IceCube was also designed to measure cosmic rays, which accompany the formation of neutrinos. The existence of cosmic rays has been known for more than a hundred years, but their sources are also unknown.

“This is essentially the birth of a new branch of astrophysics,” said Taboada. “We’ve observed the universe with photons, and now we can observe using neutrinos.”

IceCube is also facilitating Taboada’s own research on gamma-ray bursts, which is being conducted with Ph.D. students James Casey and Jacob Daughhetee.

“Gamma ray bursts are gigantic explosions that last for only a few seconds,” Taboada said. “Over those few seconds, they are so bright that they can outshine the rest of the universe combined. They are brief, but they are very powerful while they last.”

Scientists believe that there are at least two kinds of gamma ray bursts: those associated with supernovas that eject jets of matter and radiation, and neutron stars that spiral together and eventually merge. Neutrinos from gamma ray bursts can also be detected by IceCube.

IceCube was built at the South Pole to take advantage of the very clear ice available there and the darkness within the ice. “We used Antarctic ice because we needed a highly transparent material to observe the Cherenkov radiation,” explained Taboada. “The ice allows us to see evidence of these secondary particles.”

But that transparency has a cost. The South Pole experiences extremely cold temperatures, and everything used there must be flown in. Taboada has been to the South Pole three times, each time for a month during the Antarctic summer – when temperatures can still be as low as 45 degrees below zero Celsius (minus 49 degrees Fahrenheit).

“It’s a very work-oriented place,” he said. “Because it is so difficult to get there, you work every day, 12 and 15 hours a day. At the end of a month, you are exhausted.”

During the summer – November through February – as many as 150 people work at the South Pole facilities, known officially as the Amundsen-Scott South Pole Station. During the Antarctic winter, only 50 or 60 people work there, and aircraft don’t land except in extreme emergencies.

While space is tight, the station does have a music room, gymnasium and basketball court, along with a greenhouse for growing fresh vegetables. Water is precious because it is obtained by melting ice, so researchers are limited to two two-minute showers per week.

Construction of IceCube began in 2005, and the facility – now managed by the University of Wisconsin – was turned on in December 2010. With participation from 41 institutions in 12 countries, IceCube is expected to operate for at least ten years. Already, however, there is discussion about enhancing its capabilities – and perhaps expanding it to capture more neutrinos.

Basic science done in facilities like IceCube is really all about human curiosity – which often ends up having a more practical benefit.

“Basic science is not always about finding a cure for cancer,” Taboada said. “But if you look at the last 400 years of basic scientific discovery, research and basic science have time and again resulted in a better life for humans. Satisfying that human curiosity often turns out to be a very long-term, very high-risk and very high payoff investment. That’s a basic reason to do science.”

Research News

Georgia Institute of Technology

177 North Avenue

Atlanta, Georgia  30332-0181  USA

Media Relations Contacts: John Toon (jtoon@gatech.edu)(404-894-6986) or Brett Israel (brett.israel@comm.gatech.edu)(404-385-1933).

Writer: John Toon