Secrets of mysterious metal hotspots uncovered by new single molecule imaging technique

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The secrets behind the mysterious nano-sized electromagnetic"hotspots"that appear on metal surfaces under a light are finally being revealed with the help of a BEAST. Researchers at the DOE's Lawrence Berkeley National Laboratory have developed a single molecule imaging technology, dubbed the Brownian Emitter Adsorption Super-resolution Technique (BEAST), that has made it possible for the first time to directly measure the electromagnetic field inside a hotspot. The results hold promise for a number of technologies including solar energy and chemical sensing.

"With our BEAST method, we were able to map the electromagnetic field profile within a single hotspot as
small as 15 nanometers with an accuracy down to 1.2 nanometers, in just a few minutes,"says Xiang Zhang,
a principal investigator with Berkeley Lab's Materials Sciences Division and the Ernest S. Kuh Endowed Chaired Professor at the University of California (UC), Berkeley."We discovered that the field is highly localized and, unlike a typical electromagnetic field, does not propagate through space. The field also has an exponential shape that rises steeply to a peak and then decays very fast."

Zhang, who directs the Center for Scalable and IntegratedNanoManufacturing(SINAM), a National Science Foundation Nano-scale Science and Engineering Center at UC Berkeley, is the corresponding author of a paper on this research that appears in the journalNatureunder the title"Mapping the Distribution of Electromagnetic Field Inside a 15nm Sized Hotspot by Single Molecule Imaging."Co-authoring the paper with Zhang were Hu Cang, Anna Labno, Changgui Lu, Xiaobo Yin, Ming Liu and Christopher Gladden.

Under optical illumination, rough metallic surfaces will become dotted with microscopic hotspots, where the light is strongly confined in areas measuring tens of nanometers in diameter, and the Raman (inelastic) scattering of the light is enhanced by up to 14 orders of magnitude. First observed more than 30 years ago, such hotspots have been linked to the impact of surface roughness on plasmons (electronic surface waves) and other localized electromagnetic modes.
However, during the past three decades, little has been learned about the origins of these hotspots.

"Amazingly, despite thousands of papers on this problem and various theories, we are the first to experimentally determine the nature of the electromagnetic field inside of such a nano-sized hotspots,"says Hu Cang, lead author on theNaturepaper and a member of Zhang's research group."The 15 nanometer hotspot we measured is about the size of a protein molecule. We believe there are hotspots that may even be smaller than a molecule."

Because the size of these metallic hotspots is far smaller than the wavelength of incident light, a new technique was needed to map the electromagnetic field within a hotspot. The Berkeley researchers developed the BEAST method to capitalize on the fact that individual fluorescent dye molecules can be localized with single nanometer accuracy. The fluorescence intensity of individual molecules adsorbed on the surface provides a direct measure of the electromagnetic field inside a single hotspot. BEAST utilizes the Brownian motion of single dye molecules in a solution to make the dyes scan the inside of single hotspot stochastically, one molecule at a time.

"The exponential shape we found for the electromagnetic field within a hotspot is direct evidence for the existence of a localizedelectromagnetic field, as opposed to the more common form of Gaussian distribution,"Cang says."There are several competing mechanisms proposed for hotspots and we are now working to further examine these fundamental mechanisms."

BEAST starts with the submerging of a sample in a
solution of freely diffusing fluorescent dye. Since the diffusion of the dye is much faster than the image acquisition time (0.1 milliseconds vs. 50-to-100 milliseconds), the fluorescence produces a homogeneous background. When a dye molecule is adsorbed onto the surface of a hotspot, it appears as a bright spot in images, with the intensity of the spot reporting the local field strength.

"By using a maximum likelihood single molecule localization method, the molecule can be localized with single nanometer accuracy,"Zhang says."After the dye molecule is bleached (typically within hundreds of milliseconds), the fluorescence disappears and the hotspot is ready for the next adsorption event."

Choosing the right concentration of the dye molecules enables the adsorption rate on the surface of a hotspot
to be controlled so that only one adsorbed molecule emits photons at a time. Since BEAST uses a camera to record the single molecule adsorption events, multiple hotspots within a field of view of up to one square millimeter can be imaged in parallel.

In their paper, Zhang and his colleagues see hotspots being put to use in a broad range of applications, starting with the making of highly efficient solar cells and devices that can detect weak chemical signals.

"A hotspot is like a lens that can focus light to a small spot with a focusing power well beyond any conventional optics,"Cang says."While a conventional lens can only focus light to a spot about half the wavelength of visible light (about 200-300nanometers), we now confirm that a hotspot can focus light to a nanometer-sized spot."

Through this exceptional focusing power, hotspots could be used to concentrate sun light on the photocatalytic sites of solar devices, thereby helping to maximize light- harvesting and water-splitting efficiencies. For the detection of weak chemical signals, e.g., from a single
molecule, a hotspot could be used to focus incident light so that it only illuminates the molecule of interest, thereby enhancing the signal and minimizing the background.

BEAST also makes it possible to study the behavior of light as it passes through a nanomaterial, a critical factor for the future development of nano-optics and metamaterial devices. Current experimental techniques suffer from limited resolution and are difficult to implement on the truly nanoscale.

"BEAST offers an unprecedented opportunity to measure how a nanomaterial alters the distribution oflight, which will guide the development of advanced nano-optics devices,"says Cang."We will also use BEAST to answer some challenging problems in surface science, such as where and what are the active sites in a catalyst, how the energy or charges transfer between molecules and a nanomaterial, and what determine surface hydrophobicity. These problems require a technique with electron-microscopy level resolution and optical spectroscopy information. BEAST is a perfect tool for these problems."

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Shining light on graphene sensors

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National Physical Laboratory, together with an international team of scientists, have published research showing how light can be used to control graphene's electrical properties. This advance is an important step towards developing highly sensitive graphene-based electronic devices.

Graphene is an extraordinary two-dimensional material made of a single atomic layer of carbon atoms. It is the thinnest material known to man, and yet is one of the strongest ever tested.

It has unique properties which make it a very exciting material for a huge range of applications from high-speed electronics andsolar cells, to super-sensors capable of detecting single molecules of toxic gases.

It is able to act as a sensor because its entire structure is exposed to its surroundings, and it reacts to any molecules that touch its surface. This reaction causes graphene's electrical properties to alter, i.e. it senses the molecules' presence.

In their paper published in theJournal of Advanced Materials, the team show that whengrapheneis coated with light-sensitive polymers its uniqueelectrical propertiescan be precisely controlled and therefore exploited.

The polymers also protect graphene from contamination.

Light-modified graphene chips have already been used at NPL in ultra-precision experiments to measure the quantum of the electrical resistance.

In the future similar polymers could be used to effectively 'translate' information from their surroundings and influence how graphene behaves. This effect could be exploited to develop robust reliable sensors for smoke, poisonous gases, or any targeted molecule.

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Plasmonics: From metallic foils to cancer treatment

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In a timely review paper, scientists from Japan, Germany, and Spain provide a highly relevant overview of the history, physical interpretation and applications of plasmons in metallic nanostructures.

Tadaaki Nagao at the International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS) and colleagues in Germany and Spain present a review on plasmons in metallicnanomaterials. The article is published this week in the journalScience and Technology ofAdvanced Materials.

The authors provide an extensive overview of the properties of plasmons in nanomaterials with emphasis on pioneering work of Ruthemann and Lang on electron energy loss spectroscopy (EELS) ofelectron motionin thin metal foils; recent infrared analysis of nanoscale metallic nanorods and nanoislands produced by‘top-down’ photolithography; and the potential of metallic atomic wires for supporting plasmonic resonating modes. The review includes detailed explanations of plasmons for in vivo biosensing and nanoantennas.

A plasmon can be visualized as a collective oscillation of electronic‘liquid’ in metals, similar to waves in lake, which are collective mode of the water molecules. Furthermore, surface plasmons are such oscillations confined to the surfaces of metals, which display a strong interaction with light, leading to the formation of so-called‘polaritons’. Futuristic applications of plasmons include ideal lenses and even invisibility cloaks.

Research in the 1940s by Ruthemann and Lang on electrons flowing in thin metal foils using EELS yielded the first experimental sign of the presence of the theoretically predicted‘plasma oscillations’ in metals. In 1957 Richie and colleagues predicted the existence of‘surface localized’ plasmons, which was confirmed by Powell and Swan by EELS a few years later. In the 1960s researchers determined optical dispersion curves using optical spectroscopy, thereby opening up the possibility of optical applications of plasmon structures.

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Illustration of multiplex cancer targeting by SERS nanoparticles encoded by Raman molecules and cancer antibodies.

In this review, Nagao and colleagues offer insights into optical applications of localized surface plasmons in structures produced by photolithography. Specific examples include metallic nanoantenna detectors—where resonant excitation of light leads to ultrahigh electromagnetic field enhancement owing to plasmon polaritons localized at the surface of nanostructures; and optical interactions between arrays of nanorods for‘surface enhanced Raman scattering’, which shows potential for in vivo biomolecular sensing. The authors also describe the fabrication of a prototype random-nanogap antenna for enhanced IR spectroscopy and in situ spectral monitoring of surface enhancement of infrared absorption during film growth.

Furthermore, the authors describe new trends in plasmonics research, in particular observation of plasmonic resonant modes in indium nanowires grown in ultrahigh vacuum on stepped silicon substrates. They predict that these nanowires will be used as building blocks for developing plasmonic devices of the future.

This review includes 86 references and 12 figures, providing an invaluable source of up-to-date information for newcomers and experts in this exciting field of research.

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Scaling up: The future of nanoscience

In the late 1950s, Richard Feynman famously imagined a science where researchers and engineers could achieve remarkable feats by manipulating matter and creating structures all the way down to the level of individual atoms.

Now, over fifty years after"There's Plenty of Room at the Bottom,"four prominent researchers -- David Awschalom, Angela Belcher, Donald Eigler, and Michael Roukes -- are sharing their thoughts about the future of nanoscience and nanotechnology. In a special dialogue ahead of a Kavli Futures Symposium on the same topic, the scientists focused on how Feyman's vision may evolve in the next fifty years, beginning with taking nanoscience in an upward direction.

"We've gained some important beachheads in the science, but we've also made very little progress towards translating this toward what we all often speak of as the"full potential"of nanotechnology,"said Michael Roukes, professor of physics, applied physics andbioengineeringat Caltech and co-director of the Kavli Institute of Nanoscience."Going forward, I think the challenge is to breach this chasm and... actually translate this into stuff that affects our everyday lives.…{It will be about} using the building blocks of individualatoms, molecules, individual nanostructures, and assembling them into larger-scale systems with emergent functionality that will be of great use to humankind."

Roukes explained there are many things that are very exciting about being able to control things at the atomic scale and then–- from the bottom–-"build back to the middle to creating complex systems with just incredibly exquisite control about what these complex systems do. ...{O}ne area that's absolutely ripe for incredible advances is the life sciences and medicine, where aggregations of individual nanodevices to create nanosystems will allow us to embrace, rather than run away from, the complexity of biological systems."

These advances, stated Roukes, could"give us the tools, I believe, to understand and engineer biological circuitry… and ultimately, I think, will give a technological foundation for personalized medicine."

Donald Eigler is renowned for his breakthrough work in the precise manipulation of matter at the atomic level. Agreeing with Roukes, Eigler stated the impact of nanoscience in medicine"is going to grow dramatically over the next 10 to 20 years, especially in the field of regenerative medicine."Loosening his imagination, he could also conceive of other innovations, such as one day"hijacking the brilliant mechanisms of biology"to create functional non-biological nanosystems."In my dreams I can imagine some environmentally safe virus, which, by design, manufactures and spits out a 64-bit adder. We then just flow the virus's effluent over our chips and have the adders attach in just the right places.

"That's pretty far-fetched stuff, but I think it less far-fetched than Feynman in '59."

Angela Belcher is widely known for her work on evolving new materials for energy, electronics and the environment. W. M. Keck Professor of Energy, Materials Science&Engineering and Biological Engineering at the Massachusetts Institute of Technology, Belcher believes the big impact ofnanotechnologyand nanoscience will be in manufacturing -– specifically clean manufacturing of materials with new routes to synthesis of materials, less waste and self-assembling materials."It's happening right now, if you look at manufacturing of certain materials for, say, batteries for vehicles, which is based on nanostructuring of materials and getting the right combination of materials together at the nanoscale. Imagine what a big impact that could have in the environment in terms of reducing fossil fuels. So clean manufacturing is one area where I think we will definitely see advances in the next 10 years or so."

David Awschalom is a professor of physics, electrical, and computer engineering at the University of California, Santa Barbara. A pioneer in the field of semiconductor spintronics, in the next decade or two Awchalom would like to see the emergence of a genuine quantum technology."I'm thinking about possible multifunctional systems that combine logic, storage, communication as powerful quantum objects based on single particles in nature. And whether this is rooted in a biological system, or a chemical system, or a solid state system may not matter and may lead to revolutionary applications in technology, medicine, energy, or other areas."Awschalom also discussed how he expects nanoscience to transform other fields."I believe that the broad umbrella ofnanoscienceis rapidly dissolving the traditional barriers {between scientific disciplines}."

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Coiled nanowires may hold key to stretchable electronics

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Researchers at North Carolina State University have created the first coils of silicon nanowire on a substrate that can be stretched to more than double their original length, moving us closer to incorporating stretchable electronic devices into clothing, implantable health-monitoring devices, and a host of other applications.

"In order to create stretchable electronics, you need to put electronics on a stretchablesubstrate, but electronic materials themselves tend to be rigid and fragile,"says Dr. Yong Zhu, one of the researchers who created the new nanowirecoilsand an assistant professor of mechanical and aerospace engineering at NC State."Our idea was to create electronic materials that can be tailored into coils to improve their stretchability without harming the electric functionality of the materials."

Other researchers have experimented with"buckling"electronic materialsinto wavy shapes, which can stretch much like the bellows of an accordion. However, Zhu says, the maximum strains for wavy structures occur at localized positions– the peaks and valleys– on the waves. As soon as the failure strain is reached at one of the localized positions, the entire structure fails.

"An ideal shape to accommodate large deformation would lead to a uniform strain distribution along the entire length of the structure– a coil spring is one such ideal shape,"Zhu says."As a result, the wavy materials cannot come close to the coils' degree of stretchability."Zhu notes that the coil shape is energetically favorable only for one-dimensional structures, such as wires.

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Here you can see multiple images of the silicon nanocoil as it is being stretched. Credit: Yong Zhu, North Carolina State University

Zhu's team put a rubber substrate under strain and used very specific levels of ultraviolet radiation and ozone to change its mechanical properties, and then placedsiliconnanowireson top of the substrate. The nanowires formed coils upon release of the strain. Other researchers have been able to create coils using freestanding nanowires, but have so far been unable to directly integrate those coils on a stretchable substrate.

While the new coils' mechanical properties allow them to be stretched an additional 104 percent beyond their original length, their electric performance cannot hold reliably to such a large range, possibly due to factors like contact resistance change or electrode failure, Zhu says."We are working to improve the reliability of the electrical performance when the coils are stretched to the limit of their mechanical stretchability, which is likely well beyond 100 percent, according to our analysis."

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NEC's 'Earth simulator' succeeds in prediction of photochemical reactions inside carbon nanotubes

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NEC Corporation announced today that it succeeded in the world's first simulation-based prediction of laser-induced photochemical reactions that can efficiently eject a hydrogen atom from a hydrogen chloride molecule trapped inside a carbon nanotube. The simulation was conducted on the"Earth Simulator,"which NEC deployed for the Japan Agency for Marine-Earth Science and Technology, an independent administrative organization.

These results are expected to pave the way for the quantity synthesis of not only hydrogen, but also inexpensive materials through the facilitation of known photochemical reactions due to the laser pulse irradiation, as well as the development of new materials.

Results were published online on December 7 by thePhysical Review Letters, a prestigious academic journal of the American Institute of Physics.

The Earth Simulator topped the HPC Challenge Award for Fast Fourier Transform (FFT) performance at the SC10 supercomputing conference held in the United States in November 2010. The Earth Simulator demonstrated the world's top-level computing efficiency, especially for complicated applications, including nanoscience, quest for new materials and weather prediction.

The FFT dominates almost half of the processing in the application software used for this research. The Earth Simulator significantly reduced the computing time needed for laser pulse irradiation; taking just two days in contrast to the several months required by conventional supercomputers. As a result, it is now possible to determine the optimum laser intensity in a realistic timeframe with a series of simulations for variable laser intensities.

This research was carried out under a collaborative research contract with JAMSTEC titled"Large-scale Simulation of Characteristics of Carbon Nanotubes."

The application fields of the Earth Simulator, with its outstanding sustained performance, span a wide range of areas. For example, it contributes to more accurate climate change projections and the comprehensive understanding of environmental issues, such as the assessment of the effects of global warming for the fifth report of the Intergovernmental Panel on Climate Change (IPCC), as well as prevention and mitigation of natural disasters through high-resolution simulations of earthquakes and seismic surges. Moreover, the Earth Simulator is utilized heavily in tackling energy issues and developing new materials by leveraging cooperative relations with industry partners.

Looking forward, NEC aims to support leading-edge research and development capitalizing on advanced supercomputers, such as the Earth Simulator, with superior HPC technologies.

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