Friday, 16 January 2015

New Horizons spacecraft begins first stages of Pluto encounter

NASA’s New Horizons spacecraft recently began its long-awaited, historic encounter with Pluto. The spacecraft is entering the first of several approach phases that culminate July 14 with the first close-up flyby of the dwarf planet, 4.67 billion miles (7.5 billion kilometers) from Earth.

Artist’s concept of NASA’s New Horizons spacecraft as it passes Pluto and Pluto’s largest moon, Charon, in July 2015. Credit: NASA/JHU APL/SwRI/Steve Gribben

“NASA first mission to distant Pluto will also be humankind’s first close up view of this cold, unexplored world in our solar system,” said Jim Green, director of NASA’s Planetary Science Division at the agency’s Headquarters in Washington. “The New Horizons team worked very hard to prepare for this first phase, and they did it flawlessly.”

The fastest spacecraft when it was launched, New Horizons lifted off in January 2006. It awoke from its final hibernation period last month after a voyage of more than 3 billion miles, and will soon pass close to Pluto, inside the orbits of its five known moons. In preparation for the close encounter, the mission’s science, engineering and spacecraft operations teams configured the piano-sized probe for distant observations of the Pluto system that start Sunday, Jan. 25 with a long-range photo shoot.

The images captured by New Horizons’ telescopic Long-Range Reconnaissance Imager (LORRI) will give mission scientists a continually improving look at the dynamics of Pluto’s moons. The images also will play a critical role in navigating the spacecraft as it covers the remaining 135 million miles (220 million kilometers) to Pluto.

“We’ve completed the longest journey any spacecraft has flown from Earth to reach its primary target, and we are ready to begin exploring,” said Alan Stern, New Horizons principal investigator from Southwest Research Institute in Boulder, Colorado.

NASA’s New Horizons is the first mission to Pluto and the Kuiper Belt of icy, rocky mini-worlds on the solar system’s outer frontier. This animation follows the New Horizons spacecraft as it leaves Earth after its January 2006 launch, through a gravity-assist flyby of Jupiter in February 2007, to the encounter with Pluto and its moons in summer 2015. Credit: NASA/JHUAPL

“We need to refine our knowledge of where Pluto will be when New Horizons flies past it,” said Mark Holdridge, New Horizons encounter mission manager at Johns Hopkins University’s Applied Physics Laboratory (APL) in Laurel, Maryland. “The flyby timing also has to be exact, because the computer commands that will orient the spacecraft and point the science instruments are based on precisely knowing the time we pass Pluto – which these images will help us determine.”

LORRI will take hundreds of pictures of Pluto over the next few months to refine current estimates of the distance between the spacecraft and the dwarf planet. Though the Pluto system will resemble little more than bright dots in the camera’s view until May, mission navigators will use the data to design course-correction maneuvers to aim the spacecraft toward its target point this summer. The first such maneuver could occur as early as March.
“We need to refine our knowledge of where Pluto will be when New Horizons flies past it,” said Mark Holdridge, New Horizons encounter mission manager at Johns Hopkins University’s Applied Physics Laboratory (APL) in Laurel, Maryland. “The flyby timing also has to be exact, because the computer commands that will orient the spacecraft and point the science instruments are based on precisely knowing the time we pass Pluto – which these images will help us determine.”

The “optical navigation” campaign that begins this month marks the first time pictures from New Horizons will be used to help pinpoint Pluto’s location.

Throughout the first approach phase, which runs until spring, New Horizons will conduct a significant amount of additional science. Spacecraft instruments will gather continuous data on the interplanetary environment where the planetary system orbits, including measurements of the high-energy particles streaming from the sun and dust-particle concentrations in the inner reaches of the Kuiper Belt. In addition to Pluto, this area, the unexplored outer region of the solar system, potentially includes thousands of similar icy, rocky small planets.

More intensive studies of Pluto begin in the spring, when the cameras and spectrometers aboard New Horizons will be able to provide image resolutions higher than the most powerful telescopes on Earth. Eventually, the spacecraft will obtain images good enough to map Pluto and its moons more accurately than achieved by previous planetary reconnaissance missions.

– Credit and Resource –

NASA

New Horizons spacecraft begins first stages of Pluto encounter

Neuroscientists investigate how 100 billion nerve cells produce a clear thought or an action

We have approximately 100 billion nerve cells in our brains, all of which communicate with one another. Why do they lead to clear thoughts or purposeful actions instead of mere gibberish? The reason lies, among other things, in a small group of inhibitory nerve cells that can use the messenger GABA to curb the activity of other nerve cells. The neuroscientists Dr. Michael Strüber and Prof. Dr. Marlene Bartos from the University of Freiburg and their colleague from Vienna Prof. Dr. Peter Jonas have discovered that the distances between communicating cells play a part in the regulation of brain networks. The team presents this approach in the current issue of the journal Proceedings of the National Academy of Sciences (PNAS).

Intracellular filled hippocampal basket cells (magenta) and granule cells (green) with a schematic illustration of the distance-dependent inhibition. Credit: Michael Strüber

GABA is released at special contact points, synapses, from a projection of the inhibitory cells that serves precisely this purpose, the axon. The messenger causes an electrical inhibitory current in the target cells. A special subtype of the GABA-releasing cells is the so-called basket cell. It is known to have a strongly inhibitory effect on brain circuits. A reason for this is the fact that basket cells have a long and widely branching axon, with which they can control hundreds to thousands of target cells scattered over a broad area. Up to now it was not clear whether all of these target cells are subject to the same inhibitory current or whether target cells that are more distant from the GABA-releasing basket cell are more difficult to keep under precise control.

With the help of the patch-clamp technique for measuring the inhibitory currents of individual cells, the team discovered that the farther away a target cell is, the smaller and longer are their inhibitory currents. In pharmacological and electrophysiological experiments and by means of detailed microscopic studies, the neuroscientists demonstrated that the basket cell axon with more distant target cells forms less synapses and that other proteins capable of sensing GABA are found in these synapses.

What could be the reason for such a complex structure? While this question cannot be answered in its entirety, the scientists investigated the consequences of a distance-dependent inhibition in computer simulations of neuronal networks. Contrary to expectations, the weakening inhibition enables the basket cells to precisely control the activity of a large number of nerve cells and thus to synchronize them. The synchronization of entire brain areas leads to rhythmic brain activities like gamma oscillations, which serve a crucial function in higher mental processes. The new approach of distance-dependent inhibition could be an important component in the regulation of brain networks that enables the brain to orchestrate the activity of 100 billion individual yet connected nerve cells to produce a thought.

– Credit and Resource –

More information: Michael Strüber, Peter Jonas, Marlene Bartos. (2015) “Strength and duration of perisomatic GABAergic inhibition depend on distance between synaptically connected cells.” Proceedings of the National Academy of Sciences . DOI: 10.1073/pnas.1412996112, http://www.pnas.org/content/early/2015/01/08/1412996112

Provided by Albert Ludwigs University of Freiburg

Neuroscientists investigate how 100 billion nerve cells produce a clear thought or an action

Wednesday, 14 January 2015

Asteroid to Fly By Earth Safely on January 26

An asteroid, designated 2004 BL86, will safely pass about three times the distance of Earth to the moon on January 26. From its reflected brightness, astronomers estimate that the asteroid is about a third of a mile (0.5 kilometers) in size. The flyby of 2004 BL86 will be the closest by any known space rock this large until asteroid 1999 AN10 flies past Earth in 2027.

This graphic depicts the passage of asteroid 2004 BL86, which will come no closer than about three times the distance from Earth to the moon on Jan. 26, 2015.
Image Credit:
NASA/JPL-Caltech

At the time of its closest approach on January 26, the asteroid will be approximately 745,000 miles (1.2 million kilometers) from Earth.

“Monday, January 26 will be the closest asteroid 2004 BL86 will get to Earth for at least the next 200 years,” said Don Yeomans, who is retiring as manager of NASA’s Near Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, California, after 16 years in the position. “And while it poses no threat to Earth for the foreseeable future, it’s a relatively close approach by a relatively large asteroid, so it provides us a unique opportunity to observe and learn more.”

One way NASA scientists plan to learn more about 2004 BL86 is to observe it with microwaves (http://www.jpl.nasa.gov/news/news.php?release=2006-00a ). NASA’s Deep Space Network antenna at Goldstone, California, and the Arecibo Observatory in Puerto Rico will attempt to acquire science data and radar-generated images of the asteroid during the days surrounding its closest approach to Earth.

“When we get our radar data back the day after the flyby, we will have the first detailed images,” said radar astronomer Lance Benner of JPL, the principal investigator for the Goldstone radar observations of the asteroid. “At present, we know almost nothing about the asteroid, so there are bound to be surprises.”

Asteroid 2004 BL86 was initially discovered on Jan. 30, 2004 by a telescope of the Lincoln Near-Earth Asteroid Research (LINEAR) survey in White Sands, New Mexico.

The asteroid is expected to be observable to amateur astronomers with small telescopes and strong binoculars.

“I may grab my favorite binoculars and give it a shot myself,” said Yeomans. “Asteroids are something special. Not only did asteroids provide Earth with the building blocks of life and much of its water, but in the future, they will become valuable resources for mineral ores and other vital natural resources. They will also become the fueling stops for humanity as we continue to explore our solar system. There is something about asteroids that makes me want to look up.”

NASA’s Near-Earth Object Program Office is experiencing its first transition in leadership since it was formed almost 17 years ago. On Jan. 9, after a 39-year-long career at JPL, Yeomans retired. Paul Chodas, a long-time member of Yeomans’ team at JPL, has been designated as the new manager.

NASA detects, tracks and characterizes asteroids and comets using both ground- and space-based telescopes. Elements of the Near-Earth Object Program, often referred to as “Spaceguard,” discover these objects, characterize a subset of them and identify their close approaches to determine if any could be potentially hazardous to our planet.

JPL manages the Near-Earth Object Program Office for NASA’s Science Mission Directorate in Washington. JPL is a division of the California Institute of Technology in Pasadena.

Asteroid Detection and Science

They are the celestial equivalent of sonograms. But their hazy outlines and ghostly features do not document the in-vivo development of a future taxpayer. Instead, they chronicle the exo-planetary comings-and-goings of some of Earth’s least known, most nomadic, and at times most impactful neighbors.

They are radar echoes that are bounced off of asteroids. Scientists from NASA’s Jet Propulsion Laboratory and around the world rely on their ethereal images to tell some out-of-this-world tales of near-Earth objects.

“The standard ground-based tools for asteroid science require a night’s sky, and what you come away with in the end is an image of a dot,” said JPL radar astronomer Dr. Steve Ostro. “With radar astronomy, the sky at high noon is just as inviting as that at midnight, and without launching a full-blown space mission we can actually get valuable information about the physical makeup of these objects.”

In some respects, radar astronomy utilizes the same technology as your microwave oven. But do not bother to haul your glorified croissant warmer outside — it will just confuse the neighbors. Radar astronomy employs the world’s most massive dish-shaped antennas, which beam directed microwave signals at their targets, which can be as close as our moon and as far away as the moons of Saturn. These pulses bounce off the target, and the resulting “echo” is collected and precisely collated. The results can be astounding.

“The closer the target, the better the echo,” said Ostro. “From them we can generate detailed three-dimensional models of the object, define its rotation precisely and get a good idea of its internal density distribution. You can even make out surface features. A good echo can give us a spatial resolution finer than 10 meters.”

Radar astronomy has detected echoes from over 190 near-Earth asteroids to date and has found that, like snowflakes, no two are the same. The returning echoes have revealed both stony and metallic objects, some flying through the cold, dark reaches of space alone, while others have their own satellites. The data indicate that some asteroids have a very smooth surface, while others have very coarse terrain. And finally, their shapes are virtually anything that can be imagined.

One thing that does not have to be imagined is radar astronomy’s ability to nail down the location of an object in time and space. This invaluable capability came in handy in the winter of 2004 when JPL’s Near-Earth Object office was looking for a potentially hazardous asteroid called Apophis.

Discovered by astronomers using optical telescopes, Apophis quickly drew the interest of the near-Earth object monitoring community when its initial orbital plots indicated there was a possibility the 1,300-foot-wide chunk of space rock could impact Earth in 2029. The Near-Earth Object office knew what was needed was more detailed information about Apophis’ location, which they could then use to plot out a more accurate orbit.

Under the watchful eye of Ostro and three other radar astronomers, microwaves from the Arecibo Observatory in Puerto Rico reached out and touched asteroid Apophis on Jan. 27, 29, and 30, 2005. The Arecibo data significantly improved the asteroid’s orbital estimate, redefined its Earth approach distance in 2029 and decidedly reduced the probability of an Earth encounter during its next close approach in 2036.

The 1,000-foot diameter Arecibo telescope is one of only two places in the world where radar astronomy is effectively performed. The other is at the 70-meter Goldstone antenna in California’s Mojave Desert. The two instruments are complementary. The Arecibo radar is not fully steerable (while Goldstone is), but it is 30 times more sensitive. Together they make a formidable asteroid reconnaissance team.

The future of radar astronomy may be just as amazing as some of the images and shape models of nearby space objects that its practitioners have already obtained. There is new technology in the pipeline that will allow imaging of surface features with up to four times more detail than what exists today. And then there are proposals on the table for a potential space mission to a near-Earth asteroid. Candidate asteroids for said mission will need to be pre-approved via detail scientific analysis. The kind of scientific analysis you can only get with radar astronomy.

– Credit and Resource –

NASA

Asteroid to Fly By Earth Safely on January 26

Artificial intelligence helps physicists predict dangerous solar flares

Solar flares can release the energy equivalent of many atomic bombs, enough to cut out satellite communications and damage power grids on Earth, 93 million miles away. The flares arise from twisted magnetic fields that occur all over the sun’s surface, and they increase in frequency every 11 years, a cycle that is now at its maximum.

This solar flare was captured Jan. 12 by NASA’s Solar Dynamics Observatory. Stanford physicists are bringing artificial intelligence techniques to the challenge of predicting such flares. Credit: NASA/SDO and the AIA; EVE; and HMI science teams

Using artificial intelligence techniques, Stanford solar physicists Monica Bobra and Sebastien Couvidat have automated the analysis of the largest ever set of solar observations to forecast solar flares using data from the Solar Dynamics Observatory (SDO), which takes more data than any other satellite in NASA history. Their study identifies which features are most useful for predicting solar flares.

Specifically, their study required analyzing vector magnetic field data. Historically, instruments measured the line-of-sight component of the solar magnetic field, an approach that showed only the amplitude of the field. Later, instruments showed the strength and direction of the fields, called vector magnetic fields, but for only a small part of the sun, or part of the time. Now an instrument on a satellite-based system, the Helioseismic Magnetic Imager (HMI) aboard SDO, collects vector magnetic fields and other observations of the entire sun almost continuously.

Adding machine learning

The Stanford Solar Observatories Group, headed by physics Professor Phil Scherrer, processes and stores the SDO data, which takes 1.5 terabytes of data a day. During a recent afternoon tea break, the group members chatted about what they might do with all that data and talked about trying something different.

They recognized the difficulty of forming predictions from many data points when using pure theory and they had heard of the popularity of the online class on machine learning taught by Andrew Ng, a Stanford professor of computer science.

“Machine learning is a sophisticated way to analyze a ton of data and classify it into different groups,” Bobra said.

Machine learning software ascribes information to a set of established categories. The software looks for patterns and tries to see which information is relevant for predicting a particular category.

For example, one could use machine-learning software to predict whether or not people are fast swimmers. First, the software looks at features of swimmers – their heights, weights, dietary habits, sleeping habits, their dogs’ names and their dates of birth.

And then, through a guess and check strategy, the software would try to identify which information is useful in predicting whether or not a swimmer is particularly speedy. It could look at a swimmer’s height and guess whether that particular height lies within the height range of speedy swimmers, yes or no. If it guessed correctly, it would “learn” that the height might be a good predictor of speed.

The software might find that a swimmer’s sleeping habits are good predictors of speed, whereas the name of the swimmer’s dog is not.

The predictions would not be very accurate after analysis of just the first few swimmers. The more information provided, the better machine learning gets at predicting.

Similarly, the researchers wanted to know how successfully machine learning would predict the strength of solar flares from information about sunspots.

“We had never worked with the machine learning algorithm before, but after we took the course we thought it would be a good idea to apply it to solar flare forecasting,” Couvidat said. He applied the algorithms and Bobra characterized the features of the two strongest classes of solar flares, M and X. Though others have used machine learning algorithms to predict solar flares, nobody has done it with such a large set of data and or with vector magnetic field observations.

M-class flares can cause minor radiation storms that might endanger astronauts and cause brief radio blackouts at Earth’s poles. X-class flares are the most powerful.

Better flare prediction

The researchers catalogued flaring and non-flaring regions from a database of more than 2,000 active regions and then characterized those regions by 25 features such as energy, current and field gradient. They then fed the learning machine 70 percent of the data, to train it to identify relevant features. And then they used the machine to analyze the remaining 30 percent of the data to test its accuracy in predicting solar flares.

Machine learning confirmed that the topology of the magnetic field and the energy stored in the magnetic field are very relevant to predicting solar flares. Using just a few of the 25 features, machine learning discriminated between active regions that would flare and those that would not flare. Although others have used different methods to come up with similar results, machine learning provides a significant improvement because automated analysis is faster and could provide earlier warnings of solar flares.

However, this study only used information from the solar surface. That would be like trying to predict Earth’s weather from only surface measurements like temperature, without considering the wind and cloud cover. The next step in solar flare prediction would be to incorporate data from the sun’s atmosphere, Bobra said.

Doing so would allow Bobra to pursue her passion for solar physics. “It’s exciting because we not only have a ton of data, but the images are just so beautiful,” she said. “And it’s truly universal. Creatures from a different galaxy could be learning these same principles.”

– Credit and Resource –

Study paper: iopscience.iop.org/0004-637X/798/2/135

Provided by Stanford University

Artificial intelligence helps physicists predict dangerous solar flares

Carbon nanotube finding could lead to flexible electronics with longer battery life

University of Wisconsin-Madison materials engineers have made a significant leap toward creating higher-performance electronics with improved battery life—and the ability to flex and stretch.

Led by materials science Associate Professor Michael Arnold and Professor Padma Gopalan, the team has reported the highest-performing carbon nanotube transistors ever demonstrated. In addition to paving the way for improved consumer electronics, this technology could also have specific uses in industrial and military applications.

In a paper published recently in the journal ACS Nano, Arnold, Gopalan and their students reported transistors with an on-off ratio that’s 1,000 times better and a conductance that’s 100 times better than previous state-of-the-art carbon nanotube transistors.

“Carbon nanotubes are very strong and very flexible, so they could also be used to make flexible displays and electronics that can stretch and bend, allowing you to integrate electronics into new places like clothing,” says Arnold. “The advance enables new types of electronics that aren’t possible with the more brittle materials manufacturers are currently using.”

Carbon nanotubes are single atomic sheets of carbon rolled up into a tube. As some of the best electrical conductors ever discovered, carbon nanotubes have long been recognized as a promising material for next-generation transistors, which are semiconductor devices that can act like an on-off switch for current or amplify current. This forms the foundation of an electronic device.

However, researchers have struggled to isolate purely semiconducting carbon nanotubes, which are crucial, because metallic nanotube impurities act like copper wires and “short” the device. Researchers have also struggled to control the placement and alignment of nanotubes. Until now, these two challenges have limited the development of high-performance carbon nanotube transistors.

Building on more than two decades of carbon nanotube research in the field, the UW-Madison team drew on cutting-edge technologies that use polymers to selectively sort out the semiconducting nanotubes, achieving a solution of ultra-high-purity semiconducting carbon nanotubes.

Previous techniques to align the nanotubes resulted in less-than-desirable packing density, or how close the nanotubes are to one another when they are assembled in a film. However, the UW-Madison researchers pioneered a new technique, called floating evaporative self-assembly, or FESA, which they described earlier in 2014 in the ACS journal Langmuir. In that technique, researchers exploited a self-assembly phenomenon triggered by rapidly evaporating a carbon nanotube solution.

The team’s most recent advance also brings the field closer to realizing carbon nanotube transistors as a feasible replacement for silicon transistors in computer chips and in high-frequency communication devices, which are rapidly approaching their physical scaling and performance limits.

“This is not an incremental improvement in performance,” Arnold says. “With these results, we’ve really made a leap in carbon nanotube transistors. Our carbon nanotube transistors are an order of magnitude better in conductance than the best thin film transistor technologies currently being used commercially while still switching on and off like a transistor is supposed to function.”

The researchers have patented their technology through the Wisconsin Alumni Research Foundation and have begun working with companies to accelerate the technology transfer to industry.

– Credit and Resource –

ACS Nano, pubs.acs.org/doi/abs/10.1021/nn5048734

Langmuir, pubs.acs.org/doi/abs/10.1021/la500162x

Provided by University of Wisconsin-Madison

Carbon nanotube finding could lead to flexible electronics with longer battery life

Laser-induced graphene 'super' for electronics

Rice University scientists advanced their recent development of laser-induced graphene (LIG) by producing and testing stacked, three-dimensional supercapacitors, energy-storage devices that are important for portable, flexible electronics.

An electron microscope image shows the cross section of laser-induced graphene burned into both sides of a polyimide substrate. The flexible material created at Rice University has the potential for use in electronics or for energy storage. Credit: Tour Group/Rice University

The Rice lab of chemist James Tour discovered last year that firing a laser at an inexpensive polymer burned off other elements and left a film of porous graphene, the much-studied atom-thick lattice of carbon. The researchers viewed the porous, conductive material as a perfect electrode for supercapacitors or electronic circuits.

To prove it, members of the Tour group have since extended their work to make vertically aligned supercapacitors with laser-induced graphene on both sides of a polymer sheet. The sections are then stacked with solid electrolytes in between for a multilayer sandwich with multiple microsupercapacitors.

The flexible stacks show excellent energy-storage capacity and power potential and can be scaled up for commercial applications. LIG can be made in air at ambient temperature, perhaps in industrial quantities through roll-to-roll processes, Tour said.

The research was reported this week in Applied Materials and Interfaces.

Capacitors use an electrostatic charge to store energy they can release quickly, to a camera’s flash, for example. Unlike chemical-based rechargeable batteries, capacitors charge fast and release all their energy at once when triggered. But chemical batteries hold far more energy. Supercapacitors combine useful qualities of both – the fast charge/discharge of capacitors and high-energy capacity of batteries – into one package.

LIG supercapacitors appear able to do all that with the added benefits of flexibility and scalability. The flexibility ensures they can easily conform to varied packages – they can be rolled within a cylinder, for instance – without giving up any of the device’s performance.

A schematic shows the process developed by Rice University scientists to make vertical microsupercapacitors with laser-induced graphene. The flexible devices show potential for use in wearable and next-generation electronics. Credit: Tour Group/Rice University

“What we’ve made are comparable to microsupercapacitors being commercialized now, but our ability to put devices into a 3-D configuration allows us to pack a lot of them into a very small area,” Tour said. “We simply stack them up.

“The other key is that we’re doing this very simply. Nothing about the process requires a clean room. It’s done on a commercial laser system, as found in routine machine shops, in the open air.”

Ripples, wrinkles and sub-10-nanometer pores in the surface and atomic-level imperfections give LIG its ability to store a lot of energy. But the graphene retains its ability to move electrons quickly and gives it the quick charge-and-release characteristics of a supercapacitor. In testing, the researchers charged and discharged the devices for thousands of cycles with almost no loss of capacitance.

To show how well their supercapacitors scale up for applications, the researchers wired pairs of each variety of device in serial and parallel. As expected, they found the serial devices delivered double the working voltage, while the parallels doubled the discharge time at the same current density.

The vertical supercapacitors showed almost no change in electrical performance when flexed, even after 8,000 bending cycles.

Tour said that while thin-film lithium ion batteries are able to store more energy, LIG supercapacitors of the same size offer three times the performance in power (the speed at which energy flows). And the LIG devices can easily scale up for increased capacity.

“We’ve demonstrated that these are going to be excellent components of the flexible electronics that will soon be embedded in clothing and consumer goods,” he said.

Also…

A new step towards using graphene in electronic applications

A) Diagram of molecular precursors, the resulting graphene nanoribbons and the heterostructured ones. (B) Tunnel microscopy images of the heterostructures synthesised on gold surfaces.

A team of the University of Berkeley and the Centre for Materials Physics (CSIC-UPV/EHU) has managed, with atomic precision, to create nanostructures combining graphene ribbons of varying widths. The work is being published in the prestigious journal Nature Nanotechnology.

Few materials have received as much attention from the scientific world or have raised so many hopes with a view to their potential deployment in new applications as graphene has. This is largely due to its superlative properties: it is the thinnest material in existence, almost transparent, the strongest, the stiffest and at the same time the most strechable, the best thermal conductor, the one with the highest intrinsic charge carrier mobility, plus many more fascinating features. Specifically, its electronic properties can vary enormously through its confinement inside nanostructured systems, for example. That is why ribbons or rows of graphene with nanometric widths are emerging as tremendously interesting electronic components. On the other hand, due to the great variability of electronic properties upon minimal changes in the structure of these nanoribbons, exact control on an atomic level is an indispensable requirement to make the most of all their potential.

The lithographic techniques used in conventional nanotechnology do not yet have such resolution and precision. In the year 2010, however, a way was found to synthesise nanoribbons with atomic precision by means of the so-called molecular self-assembly. Molecules designed for this purpose are deposited onto a surface in such a way that they react with each other and give rise to perfectly specified graphene nanoribbons by means of a highly reproducible process and without any other external mediation than heating to the required temperature. In 2013 a team of scientists from the University of Berkeley and the Centre for Materials Physics (CFM), a mixed CSIC (Spanish National Research Council) and UPV/EHU (University of the Basque Country) centre, extended this very concept to new molecules that were forming wider graphene nanoribbons and therefore with new electronic properties. This same group has now managed to go a step further by creating, through this self-assembly, heterostructures that blend segments of graphene nanoribbons of two different widths.

The forming of heterostructures with different materials has been a concept widely used in electronic engineering and has enabled huge advances to be made in conventional electronics. “We have now managed for the first time to form heterostructures of graphene nanoribbons modulating their width on a molecular level with atomic precision. What is more, their subsequent characterisation by means of scanning tunnelling microscopy and spectroscopy, complemented with first principles theoretical calculations, has shown that it gives rise to a system with very interesting electronic properties which include, for example, the creation of what are known as quantum wells,” pointed out the scientist Dimas de Oteyza, who has participated in this project. This work, the results of which are being published this very week in the prestigious journal Nature Nanotechnology, therefore constitutes a significant success towards the desired deployment of graphene in commercial electronic applications.

Dr Dimas G. de Oteyza, who was previously at Berkeley and at the CFM, is currently working at the Donostia International Physics Center (DIPC) as a Fellow Gipuzkoa. The Fellows Gipuzkoa programme, funded by the Chartered Provincial Council of Gipuzkoa, is in fact devoted to bringing back young researchers with solid post-doctoral training in internationally prestigious groups and centres, by offering them a platform for reincorporation through contracts with a duration of up to five years, which enables them to compete in the best of conditions to obtain tenured positions as researchers in our country.

– Credit and Resource –

Applied Materials and Interfaces, pubs.acs.org/doi/abs/10.1021/am509065d

Provided by Rice University

“Bandgap Engineering of Bottom-Up Synthesized Graphene Nanoribbons by Controlled Heterojunctions.” Y.-C. Chen, T. Cao, C. Chen, Z. Pedramrazi, D. Haberer, D. G. de Oteyza, F. Fischer, S. Loiue, M. F. Crommie, Nature Nanotechnology (2015) DOI: 10.1038/nnano.2014.307.

Journal reference: Nature Nanotechnology search and more info website

Provided by University of the Basque Country

Laser-induced graphene 'super' for electronics

3D plasmonic photonic crystals are the first devices prepared by DNA-guided colloidal crystallization

A polaritonic photonic crystal made by DNA-programmable assembly. (A) Three-dimensional illustration of a plasmonic PPC, in the shape of a rhombic dodecahedron, assembled from DNA-modified gold nanoparticles. Red arrows indicate light rays normal to the underlying substrate, impinging on and backscattering through a top facet of the crystal (FPMs). The blue ones represent light rays entering through the slanted side facets and leaving the PPC through the opposite side, not contributing to the FPMs (Fig. S2). The top right inset shows the top view of the crystal with two sets of arrows defining two polarization bases at the top and side facets. The bottom right inset shows an SEM image of a representative single crystal corresponding to the orientation of the top right inset. (Scale bar, 1 μm.) (B) A 2D scheme showing the geometric optics approximation of backscattering consistent with the explanation in A. The hexagon outline is a vertical cross-section through the gray area in the top right inset of A parallel to its long edge. The box enclosed by a dashed line depicts the interaction between localized surface plasmons and photonic modes (red arrows; FPMs) with a typical near-field profile around gold nanoparticles. The contribution of backscattering through the side facets (blue arrows) to FPMs is negligible. (C) Scheme of plasmon polariton formation. The localized surface plasmons (yellow bar) strongly couple to the photonic modes (red bars; FPMs). Credit: Park DJ, et al. (2014) Plasmonic photonic crystals realized through DNA-programmable assembly. Proc Natl Acad Sci USA Published online before print on December 29, 2014.

As biotechnology and nanotechnology continue to merge, DNA-programmable methods have emerged as a way to provide unprecedented control over the assembly of nanoparticles into complex structures, including customizable periodic structures known as superlattices that allow fine tuning the interaction between light and highly organized collections of particles. Lattice structures have historically been two-dimensional because fabricating three-dimensional DNA lattices has been too difficult, while three-dimensional dielectric photonic crystals have well-established enhanced light–matter interactions. However, the dearth of synthetic means of creating plasmonic crystals (those that exploit surface plasmons produced from the interaction of light with metal-dielectric materials) based on arrays of nanoparticles has prevented them from being experimentally studied. At the same time, it has been suggested that polaritonic photonic crystals (PPCs) – plasmonic counterparts of photonic crystals – can prohibit light propagation and open a photonic band gap (also known as a polariton gap) by strong coupling between surface plasmons and photonic modes if the crystal is in a deep subwavelength size regime. (Polaritons are quasiparticles resulting from strong coupling of electromagnetic waves with an electric or magnetic dipole-carrying excitation.)

To that end, scientists at Northwestern University recently reported strong light-plasmon interactions within 3D plasmonic photonic crystals that have lattice constants and nanoparticle diameters that can be independently controlled in the deep subwavelength size regime by using a DNA-programmable assembly technique – the first devices prepared by DNA-guided colloidal crystallization. The researchers have shown that they can tune the interaction between light and the collective electronic modes of gold nanoparticles by independently adjusting lattice constants and gold nanoparticle diameters, adding that their results in tuning interactions between light and highly-organized nanoscale collections of particles suggest the possibility of applications that include lasers, quantum electrodynamics and biosensing.

Prof. George C. Schatz discussed the paper that he, Prof. Chad A. Mirkin, lead author Daniel J. Park and their co-authors published in Proceedings of the National Academy of Sciences by first addressing the main challenges the scientists encountered in tuning the interaction between light and the collective electronic modes of gold nanoparticles by independently adjusting lattice constants and gold nanoparticle diameters. “The wavelength associated with photonic resonance modes” – such as the Fabry-Pérot interactions that occur with interferometers of the same name – “is defined by an interference condition that depends on geometry of the microstructure, as well as on the effective index of refraction of the material in the microstructure,” Schatz tells Phys.org. “At the same time, the wavelength of plasmon resonances in a gold nanoparticle is determined by collective electron excitation in the particle and depends on the size and shape of the nanoparticle as well as on gold’s refraction index.” The researchers addressed this by fabricating superlattice materials that allow for independently tuning these two wavelengths, and therefore to study the interactions between the resonance modes. Moreover, he adds, the researchers found a range of superlattice and nanoparticle parameters where the photonic modes could be observed both below and above the plasmon energy – that is, its resonance wavelength – enabling them to observe a band gap that indicates strong coupling between the modes.

A second key aspect of their research was using DNA-guided colloidal crystallization to independently control strong light-plasmon interactions within 3D plasmonic photonic crystals that have lattice constants and nanoparticle diameters, as well as synthesizing plasmonic PPCs (polaritonic photonic crystals) from gold nanoparticles. “Prior to our paper and work published last year1 by our colleagues at Northwestern in Prof. Mirkin’s group, the DNA-guided crystallization method had been developed for making superlattice materials with variable gold particle size and lattice spacing,” Schatz explains.

Experimental and theoretical backscattering spectra of PPC1–3. (A) SEM image (Top) and optical bright field reflection mode image (Bottom) of PPC1 on a silicon substrate. (Scale bar, 1 μm.) (B) Measured backscattering spectrum (red solid line) of PPC1 from the center red spot in A, Bottom. Calculated backscattering spectra based on two infinite slab models with BCC crystal geometry (blue solid line) and EMT approximation (blue dashed line). FPMs are indicated by markers. (C–F) The same datasets for PPC2 and PPC3 as in A and B. PPC2 and PPC3 are on indium tin oxide (ITO)-coated glass slides. The optical images show bright spots at the center owing to backscattering from the top and bottom facets. Two vertical lines in F indicate spectral positions where FPMs are suppressed. (Scale bars, 1 μm.) Credit: Park DJ, et al. (2014) Plasmonic photonic crystals realized through DNA-programmable assembly. Proc Natl Acad Sci USA Published online before print on December 29, 2014.

“However,” he continues, “the materials were polycrystalline, and therefore did not exhibit well-defined photonic modes that can allow for probing the interaction between light and surface plasmons. A key advance was the discovery1 of a method for making superlattice single crystals with a well-defined crystal habit – that is, a rhombic dodecahedral shape – and variable size on the order of a few microns.” Nevertheless, it was still unclear that there would be optical modes of high enough quality for Fabry-Pérot resonances to be observed and be tuned across the plasmon resonance. “It took several months to theoretically and experimentally probe and confirm the presence of Fabry-Pérot resonances,” Schatz adds.

Schatz and his colleagues addressed these challenges by using measurements of backscattering – the reflection of waves, particles, or signals back to the source direction – to probe Fabry-Pérot modes. “Although backscattering measurements have been used in other contexts, this was the first application of this technology to DNA superlattice crystals, and it wasn’t immediately clear to us that Fabry-Pérot resonances could be observed for this crystal habit and choice of material,” Schatz notes. However, as detailed in their current paper, the scientists developed a realistic theoretical model of this experiment that predicted the existence of Fabry-Pérot modes and the possibility of observing them via backscattering while doing the experiments. “This stimulated us to do the experiments and persist with this work even though the early results were of poor quality. Furthermore, we used the computational model to guide in optimizing the experiment – including the work in which we coated PPCs with silver.”

In their paper, the researchers discussed further photonic studies and possible applications in lasers, cavity quantum electrodynamics, quantum optics, quantum many-body dynamics, biosensing and other areas suggested by tuning nanoscale light-plasmon interactions. “Past work has observed quantum electrodynamics behavior in dielectric optical cavities, including enhanced and suppressed fluorescence from emitters in these cavities. The present experiments suggest that this type of measurement can be extended to cavities where hybrid plasmonic/photonic modes occur.” He emphasizes that while quantum electrodynamics phenomena via 2D hybrid plasmonic/photonic modes have already been observed for the last several years, their system opens a unique opportunity to utilize 3D crystal modes that contain plasmonic properties. “As a possible application, since plasmon-enhanced lasers have been observed with 2D lattices, the successful observation of 3D hybrid photonic-plasmonic modes suggests that such lasers can be prepared for 3D lattices.”

Another interesting finding is the tunability of DNA interconnects and the corresponding volume fraction of the plasmonic elements. “Tunability of the DNA interconnects provides the ability to change the lattice constant,” Schatz explains,” and with a certain size of nanoparticle, by varying the lattice constant we can tune the gold volume.”

A schematic description of the backscattering signal detection setup. The blue arrows indicate the light incident on the sample and the red arrows the reflected light. Only the reflection mode, not the transmission mode, is reflected. Credit: Park DJ, et al. (2014) Plasmonic photonic crystals realized through DNA-programmable assembly. Proc Natl Acad Sci USA Published online before print on December 29, 2014.

When asked if their findings might interact with or contribute to developments in synthetic biology and synthetic genomics, as well as the accelerating integration of biotechnology and nanotechnology in translational medicine, Schatz pointed out that DNA provides a synthetic ‘hook’ that can be connected to synthetic biology. “We can therefore envision using the genetic programmability of DNA as input to the synthesis of fluorescent proteins in precise locations,” adding that the medical applications of DNA-programmed superlattice materials are only at the concept stage. “From earlier work in the Mirkin group, we know how to use gold nanoparticles coated with DNA in medical diagnostics and therapeutics, so one can imagine future applications where these applications are extended to superlattices. A key point is that the superlattices provide a systematic tool for building structures that combine together inorganic components, such as metal or semiconductor nanoparticles with biomolecules.”

Moving forward, Schatz says, the researchers need to generalize the menu of superlattice crystals. “The micron-scale crystal habits exhibit other photonic modes – that is, functionalities – such as whispering gallery resonance and light focusing. In addition, other nanoparticle components such as silver nanoparticles and quantum dots can be incorporated into superlattices.” This means that the scientists can play with a large number of photonic/electronic degrees of freedom within the framework of a DNA superlattice. “Therefore, we need to establish a well-defined set of photonic applications and studies utilizing and combining those physical degrees of freedom – and theory will play an important role in this process.”

PPC silver coating process. (A) A PPC on a glass slide. (B) A silver layer is deposited on the PPC. (C) The uncoated bottom side of the PPC is exposed after sticking the PPC to the top surface of a PDMS pillar. (D) Another layer of silver is deposited on the uncoated side. (E) The top image shows a PPC at step (C), and the bottom step (D). A 100x objective was used and the scalebar is 2 μm. Credit: Park DJ, et al. (2014) Plasmonic photonic crystals realized through DNA-programmable assembly. Proc Natl Acad Sci USA Published online before print on December 29, 2014.

In terms of additional innovations, Schatz tells Phys.org that “now that we know that plasmon-photonic interactions can exhibit strong coupling, we need to expand this research, probably with different nanoparticles and with different types of photonic resonances. For example, we can incorporate anisotropic nanoparticles that exhibit more interesting plasmonic response to polarization of light – and utilizing other available photonic modes that exhibit light focusing features, we can think about developing optical components such as a plasmonic microlens. Finally, synthesizing quantum dot nanoparticle superlattices, we can perform fundamental physics studies related to the collective exciton emission.”

Schatz concludes that other areas of research might also benefit from their study. “We’re excited about the possibility of using superlattice materials not just in photonics, but also in energy-related applications, including photovoltaics, photocatalysis, and batteries.”

– Credit and Resources –

Plasmonic photonic crystals realized through DNA-programmable assembly, Proceedings of the National Academy of Sciences published online before print December 29 2014, doi:10.1073/pnas.1422649112

1DNA-mediated nanoparticle crystallization into Wulff polyhedral, Nature (2014) 505(7481):73–77, doi:10.1038/nature12739

3D plasmonic photonic crystals are the first devices prepared by DNA-guided colloidal crystallization