Electronic life on the edge: Scientists discover the edge states of graphene nanoribbons

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As far back as the 1990s, long before anyone had actually isolated graphene– a honeycomb lattice of carbon just one atom thick– theorists were predicting extraordinary properties at the edges of graphene nanoribbons. Now physicists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), and their colleagues at the University of California at Berkeley, Stanford University, and other institutions, have made the first precise measurements of the"edge states"of well-ordered nanoribbons.

Agraphenenanoribbon is a strip of graphene that may be only a few nanometers wide (a nanometer is a billionth of a meter). Theorists have envisioned thatnanoribbons, depending on their width and the angle at which they are cut, would have unique electronic, magnetic, and optical features, including band gaps like those in semiconductors, which sheet graphene doesn't have.

"Until now no one has been able to test theoretical predictions regarding nanoribbon edge-states, because no one could figure out how to see the atomic-scale structure at the edge of a well-ordered graphene nanoribbon and how, at the same time, to measure its electronic properties within nanometers of the edge,"says Michael Crommie of Berkeley Lab's Materials Sciences Division (MSD) and UC Berkeley's Physics Division, who led the research."We were able to achieve this by studying specially made nanoribbons with ascanning tunneling microscope."

The team's research not only confirms theoretical predictions but opens the prospect of building quick-acting, energy-efficient nanoscale devices from graphene-nanoribbon switches, spin-valves, and detectors, based on either electron charge or electron spin. Farther down the road, graphene nanoribbon edge states open the possibility of devices with tunable giant magnetoresistance and other magnetic and optical effects.

Crommie and his colleagues have published their research inNature Physics, available May 8, 2011 in advanced online publication.

The well-tempered nanoribbon

"Making flakes and sheets of graphene has become commonplace,"Crommie says,"but until now, nanoribbons produced by different techniques have exhibited, at best, a high degree of inhomogeneity"– typically resulting in disordered ribbon structures with only short stretches of straight edges appearing at random. The essential first step in detecting nanoribbon edge states is access to uniform nanoribbons with straight edges, well-ordered on the atomic scale.

Hongjie Dai of Stanford University's Department of Chemistry and Laboratory for Advanced Materials, a member of the research team, solved this problem with a novel method of"unzipping"carbon nanotubes chemically. Graphene rolled into a cylinder makes a nanotube, and when nanotubes are unzipped in this way the slice runs straight down the length of the tube, leaving well-ordered, straight edges.

Graphene can be wrapped at almost any angle to make a nanotube. The way the nanotube is wrapped determines the pitch, or"chiral vector,"of the nanoribbon edge when the tube is unzipped. A cut straight along the outer atoms of a row of hexagons produces a zigzag edge. A cut made at a 30-degree angle from a zigzag edge goes through the middle of the hexagons and yields scalloped edges, known as"armchair"edges. Between these two extremes are a variety of chiral vectors describing edges stepped on the nanoscale, in which, for example, after every few hexagons a zigzag segment is added at an angle.

These subtle differences in edge structure have been predicted to produce measurably different physical properties, which potentially could be exploited in new graphene applications. Steven Louie of UC Berkeley and Berkeley Lab's MSD was the research team's theorist; with the help of postdoc Oleg Yazyev, Louie calculated the expected outcomes, which were then tested against experiment.

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By"unzipping"carbon nanotubes, regular edges with differing chiralities can be produced between the extremes of the zigzag configuration and, at a 30-degree angle to it, the armchair configuration. Credit: Hongjie Dai, Stanford University, and Michael Crommie et al, Lawrence Berkeley National Laboratory

Chenggang Tao of MSD and UCB led a team of graduate students in performing scanning tunneling microscopy (STM) of the nanoribbons on a gold substrate, which resolved the positions of individual atoms in the graphene nanoribbons. The team looked at more than 150 high-quality nanoribbons with different chiralities, all of which showed an unexpected feature, a regular raised border near their edges forming a hump or bevel. Once this was established as a real edge feature– not the artifact of a folded ribbon or a flattened nanotube– the chirality and electronic properties of well-ordered nanoribbon edges could be measured with confidence, and the edge regions theoretically modeled.

Electronics at the edge

"Two-dimensional graphene sheets are remarkable in how freely electrons move through them, including the fact that there's no band gap,"Crommie says."Nanoribbons are different: electrons can become trapped in narrow channels along the nanoribbon edges. These edge-states are one-dimensional, but the electrons on one edge can still interact with the edge electrons on the other side, which causes an energy gap to open up."

Using an STM in spectroscopy mode (STS), the team measured electronic density changes as an STM tip was moved from a nanoribbon edge inward toward its interior. Nanoribbons of different widths were examined in this way. The researchers discovered that electrons are confined to the edge of the nanoribbons, and that these nanoribbon-edge electrons exhibit a pronounced splitting in their energy levels.

"In the quantum world, electrons can be described as waves in addition to being particles,"Crommie notes. He says one way to picture how different edge states arise is to imagine an electron wave that fills the length of the ribbon and diffracts off the atoms near its edge. The diffraction patterns resemble water waves coming through slits in a barrier.

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Graphene nanoribbons are narrow sheets of carbon atoms only one layer thick. Their width, and the angles at which the edges are cut, produce a variety of electronic states, which have been studied with precision for the first time using scanning tunneling microscopy and scanning tunneling spectroscopy. Credit: Crommie et al, Lawrence Berkeley National Laboratory

For nanoribbons with an armchair edge, the diffraction pattern spans the full width of the nanoribbon; the resulting electron states are quantized in energy and extend spatially throughout the entire nanoribbon. For nanoribbons with a zigzag edge, however, the situation is different. Here diffraction from edge atoms leads to destructive interference, causing the electron states to localize near the nanoribbon edges. Their amplitude is greatly reduced in the interior.

The energy of the electron, the width of the nanoribbon, and the chirality of its edges all naturally affect the nature and strength of these nanoribbon electronic states, an indication of the many ways the electronic properties of nanoribbons can be tuned and modified.

Says Crommie,"The optimist says, 'Wow, look at all the ways we can control these states– this might allow a whole new technology!' The pessimist says, 'Uh-oh, look at all the things that can disturb a nanoribbon's behavior– how are we ever going to achieve reproducibility on the atomic scale?'"

Crommie himself declares that"meeting this challenge is a big reason for why we do research. Nanoribbons have the potential to form exciting new electronic, magnetic, and optical devices at the nanoscale. We might imagine photovoltaic applications, where absorbed light leads to useful charge separation at nanoribbon edges. We might also imagine spintronics applications, where using a side-gate geometry would allow control of the spin polarization of electrons at a nanoribbon's edge."

Although getting there won't be simple --"The edges have to be controlled,"Crommie emphasizes --"what we've shown is that it's possible to make nanoribbons with good edges and that they do, indeed, have characteristic edge states similar to what theorists had expected. This opens a whole new area of future research involving the control and characterization of graphene edges in different nanoscale geometries."

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Experiments settle long-standing debate about mysterious array formations in nanofilms

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(PhysOrg.com) -- Scientists at the California Institute of Technology have conducted experiments confirming which of three possible mechanisms is responsible for the spontaneous formation of three-dimensional (3-D) pillar arrays in nanofilms (polymer films that are billionths of a meter thick). These protrusions appear suddenly when the surface of a molten nanofilm is exposed to an extreme temperature gradient and self-organize into hexagonal, lamellar, square, or spiral patterns.

This unconventional means of patterning films is being developed by Sandra Troian, professor of applied physics, aeronautics, and mechanical engineering at Caltech, who uses modulation of surface forces to shape and mold liquefiable nanofilms into 3-D forms."My ultimate goal is to develop a suite of 3-D lithographic techniques based on remote, digital modulation of thermal, electrical, and magnetic surface forces,"Troian says. Confirmation of the correct mechanism has allowed her to deduce the maximum resolution or minimum feature size ultimately possible with these patterning techniques.

In Troian's method, arbitrary shapes are first sculpted from a molten film by surface forces and then instantly solidified in situ by cooling the sample."These techniques are ideally suited for fabrication of optical or photonic components that exhibit ultrasmooth interfaces,"she explains. The process also introduces some interesting new physics that only become evident at the nanoscale."Even in the land of Lilliputians, these forces are puny at best—but at thenanoscaleor smaller still, they rule the world,"she says.

The experiments leading to this discovery were highlighted on the cover of the April 29 issue of the journalPhysical Review Letters.

The experiments, designed to isolate the physics behind the process, are challenging at best. The setup requires two smooth, flat substrates, which are separated only by a few hundred nanometers, to remain perfectly parallel over distances of a centimeter or more.

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Scanning electron micrograph of solidified protrusions in a 98 nm polystyrene film guided by a remote hexagonal array of cold pins. {Credit: Courtesy of E. McLeod and S. M. Troian, {LIS2T} lab/Caltech.}

Such an experimental setup presents several difficulties, including that"no substrate this size is truly flat,"Troian says,"and even the world's smallest thermocouple is too large to fit inside the gap."In addition, she says,"the thermal gradient in the gap can exceed values of a million degrees per centimeter, so the setup undergoes significant expansion, distortion, and contraction during a typical run."

In fact, all previous studies confronted similar challenges—leading to inaccurate estimates of the thermal gradient and the inability to view the formation and growth of the structures, among other problems."To complicate matters,"Troian says,"all of the previous data in the literature were obtained at very late stages of growth, far beyond the regime of validity of the theoretical models,"Troian says.

The Caltech experiments solved these challenges by reverting to in situ measurements. The researchers replaced the top cold substrate with a transparent window fashioned from a single crystal sapphire, which permitted them to view directly the developing formations. They also used white light interferometry to help maintain parallelism during each run and to record the emerging shape and growth rate of emerging structures. Finite element simulations were also used to obtain much more accurate estimates of the thermal gradient in the tiny gap.

"When all is said and done, our results indicate that this formation process is not driven by electrostatic attraction between the film surface and the nearby substrate—similar to what happens when you run a comb through your hair—or pressure fluctuations inside the film from reflections of acoustic phonons—the collective excitations of molecules—as once believed, Troian explains."The data simply don't fit these models, no matter how hard you try,"she says. The data also did not seem to fit a third model based on film structuring by thermocapillary flow—the flow from warmer to cooler regions that accompanies surface temperature variations.

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(Left) Emergent 3-D protrusions beneath a cold transparent cylindrical mesa (400μm diameter) in a 160 nm polystyrene film subject to a thermal gradient of 240,000 degrees Celsius/cm. (Right) Formations after some have contacted the cold mesa. {Credit: Courtesy of E. McLeod and S. M. Troian, {LIS2T} lab/Caltech}

Troian proposed the thermocapillary model several years ago. Calculations for this"cold-seeking instability"suggest that nanofilms are always unstable in response to the formation of 3-D pillar arrays, regardless of the size of the thermal gradient. Tinyprotrusionsin the film experience a slightly cooler temperature than the surrounding liquid because of their proximity to a cold target. The surface tension of those tips is greater than that of the surrounding film. This imbalance generates a very strong surface force that"pulls"fluid up and"into the third dimension,"she says. This process easily gives rise to large area arrays of dimples, ridges, pillars, and other shapes. A nonlinear version of the model suggests how cold pins can also be used to form more regular arrays.

Troian was initially disappointed that the measurements did not match the theoretical predictions. For example, the prediction for the spacing between protrusions was off by a factor of two or more."It occurred to me that certain properties of thenanofilmto be input into the model might be quite different than those literature values obtained from macroscopic samples,"she notes.

She enlisted the advice of mechanical engineer Ken Goodson at Stanford, an expert on thermal transport in nanofilms, who confirmed that he'd also noticed a significant enhancement in the heat-transfer capability of certain nanofilms. Further investigation revealed that other groups around the world have begun reporting similar enhancement in optical and other characteristics of nanofilms."And voila!… by adjusting one key parameter,"Troian says,"we obtained perfect agreement between experiment and theory. How cool is that!"

Not satisfied by these findings, Troian wants to launch a separate study to find the source of these enhanced properties in nanofilms."Now that our horizon is clear, I guarantee we won't sit still until we can fabricate some unusual components whose shape and optical response can only be formed by such a process."

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Seeing an atomic thickness

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Scientists from NPL, in collaboration with Linkoping University, Sweden, have shown that regions of graphene of different thickness can be easily identified in ambient conditions using Electrostatic Force Microscopy (EFM).

The exciting properties of graphene are usually only applicable to the material that consists of one or two layers of the graphene sheets. Whilstsynthesisof any number of layers is possible, the thicker layers have properties closer to the more common bulkgraphite.

For device applications one- and two-layer graphene needs to be precisely identified apart from thesubstrateand regions of thicker graphene. Exfoliated graphene sheets up to ~100μm in size can be routinely identified by optical microscopy. However, the situation is much more complicated in the case of the epitaxial graphene grown on silicon carbide wafers with a diameter up to 5 inches where the straightforward identification of the graphene thickness is difficult using standard techniques. This research shows that EFM, which is one of the most widely accessible and simplest implementations of scanning probe microscopy, can clearly identify differentgraphenethicknesses. The technique can also be used in ambient environments applicable to industrial requirements.

This work was recently published inNano Letters.

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'Nanowire' measurements could improve computer memory

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(PhysOrg.com) -- A recent study at the National Institute of Standards and Technology may have revealed the optimal characteristics for a new type of computer memory now under development. The work, performed in collaboration with researchers from George Mason University (GMU), aims to optimize nanowire-based charge-trapping memory devices, potentially illuminating the path to creating portable computers and cell phones that can operate for days between charging sessions.

The nascent technology is based on silicon formed into tiny wires, approximately 20nanometersin diameter. These"nanowires"form the basis ofmemorythat is non-volatile, holding its contents even while the power is off—just like the flash memory in USB thumb drives and many mp3 players. Such nanowire devices are being studied extensively as the possible basis for next-generationcomputer memorybecause they hold the promise to store information faster and at lower voltage.

Nanowirememory devicesalso hold an additional advantage over flash memory, which despite its uses is unsuitable for one of the most crucial memory banks in a computer: the local cache memory in the central processor.

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"Cache memory stores the information a microprocessor is using for the task immediately at hand,"says NIST physicist Curt Richter."It has to operate very quickly, and flash memory just isn't fast enough. If we can find a fast, non-volatile form of memory to replace what chips currently use as cache memory, computing devices could gain even more freedom from power outlets—and we think we've found the best way to helpsiliconnanowires do the job."

While the research team is by no means the only lab group in the world working on nanowires, they took advantage of NIST's talents at measurement to determine the best way to design charge-trapping memory devices based on nanowires, which must be surrounded by thin layers of material called dielectrics that store electrical charge. By using a combination of software modeling and electrical device characterization, the NIST and GMU team explored a wide range of structures for the dielectrics. Based on the understanding they gained, Richter says, an optimal device can be designed.

"These findings create a platform for experimenters around the world to further investigate the nanowire-based approach to high-performance non-volatile memory,"says Qiliang Li, assistant professor of Electrical and Computer Engineering at GMU."We are optimistic that nanowire-based memory is now closer to real application."

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'Computer synapse' analyzed at the nanoscale

Researchers at Hewlett Packard and the University of California, Santa Barbara, have analysed in unprecedented detail the physical and chemical properties of an electronic device that computer engineers hope will transform computing.

Memristors, short for memory resistors, are a newly understood circuit element for the development of electronics and have inspired experts to seek ways of mimicking the behaviour of our own brains' activity inside a computer.

Research, published today, Monday, 16 May, in IOP Publishing'sNanotechnology, explains how the researchers have used highly focused x-rays to map out the nanoscale physical and chemical properties of theseelectronic devices.

It is thought memristors, with the ability to 'remember' the total electronic charge that passes through them, will be of greatest benefit when they can act likesynapseswithinelectronic circuits, mimicking thecomplex networkof neurons present in the brain, enabling our own ability to perceive, think and remember.

Mimicking biological synapses - the junctions between two neurons where information is transmitted in our brains– could lead to a wide range of novel applications, including semi-autonomous robots, if complex networks of neurons can be reproduced in an artificial system.

In order for the huge potential of memristors to be utilised, researchers first need to understand the physical processes that occur within the memristors at a very small scale.

Memristors have a very simple structure– often just a thin film made of titanium dioxide between two metal electrodes– and have been extensively studied in terms of their electrical properties.

For the first time, researchers have been able to non-destructively study the physical properties of memristors allowing for a more detailed insight into the chemistry and structure changes that occur when the device is operating.

The researchers were able to study the exact channel where the resistance switching of memristors occurs by using a combination of techniques.

They used highly focusedx-raysto locate and image the approximately one hundred nanometer wide channel where the switching of resistance takes place, which could then be fed into a mathematical model of how the memristor heats up.

John Paul Strachan of the nanoElectronics Research Group, Hewlett-Packard Labs, California, said:"One of the biggest hurdles in using these devices is understanding how they work: the microscopic picture for how they undergo such tremendous and reversible change in resistance.

"We now have a direct picture for the thermal profile that is highly localized around this channel during electrical operation, and is likely to play a large role in accelerating the physics driving the memristive behavior."

This research appears as part of a special issue on non-volatile memory based on nanostructures.

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Sharpening the nanofocus: Researchers use nanoantenna to enhance plasmonic sensing

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(PhysOrg.com) -- Such highly coveted technical capabilities as the observation of single catalytic processes in nanoreactors, or the optical detection of low concentrations of biochemical agents and gases are an important step closer to fruition. Researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab), in collaboration with researchers at the University of Stuttgart in Germany, report the first experimental demonstration of antenna-enhanced gas sensing at the single particle level. By placing a palladium nanoparticle on the focusing tip of a gold nanoantenna, they were able to clearly detect changes in the palladium's optical properties upon exposure to hydrogen.

"We have demonstrated resonant antenna-enhanced single-particle hydrogen sensing in the visible region and presented a fabrication approach to the positioning of a single palladium nanoparticle in the nanofocus of a gold nanoantenna,"says Paul Alivisatos, Berkeley Lab's director and the leader of this research."Our concept provides a general blueprint for amplifying plasmonic sensing signals at the single-particle level and should pave the road for the optical observation of chemical reactions and catalytic activities in nanoreactors, and for local biosensing."

Alivisatos, who is also the Larry and Diane Bock Professor of Nanotechnology at the University of California, Berkeley, is the corresponding author of a paper in the journalNature Materialsdescribing this research. The paper is titled"Nanoantenna-enhanced gas sensing in a single tailored nanofocus."Co-authoring the paper with Alivisatos were Laura Na Liu, Ming Tang, Mario Hentschel and Harald Giessen.

One of the hottest new fields in technology today is plasmonics– the confinement of electromagnetic waves in dimensions smaller than half-the-wavelength of the incident photons in free space. Typically this is done at the interface between metallic nanostructures, usually gold, and a dielectric, usually air. The confinement of the electromagnetic waves in these metallic nanostructures generates electronic surface waves called"plasmons."A matching of the oscillation frequency between plasmons and the incident electromagnetic waves gives rise to a phenomenon known as localized surface plasmon resonance (LSPR), which can concentrate the electromagnetic field into a volume less than a few hundred cubic nanometers. Any object brought into this locally confined field– referred to as the nanofocus - will influence the LSPR in a manner that can be detected via dark-field microscopy.

"Nanofocusing has immediate implications for plasmonic sensing,"says Laura Na Liu, lead author of theNature Materialspaper who was at the time the work was done a member of Alivisatos' research group but is now with Rice University."Metallic nanostructures with sharp corners and edges that form a pointed tip are especially favorable for plasmonic sensing because the field strengths of theelectromagnetic wavesare so strongly enhanced over such an extremely small sensing volume."

Plasmonic sensing is especially promising for the detection of flammable gases such as hydrogen, where the use of sensors that require electrical measurements pose safety issues because of the potential threat from sparking. Hydrogen, for example, can ignite or explode in concentrations of only four-percent. Palladium was seen as a prime candidate for the plasmonic sensing of hydrogen because it readily and rapidly absorbs hydrogen that alters its electrical and dielectric properties. However, the LSPRs of palladiumnanoparticlesyield broad spectral profiles that make detecting changes extremely difficult.

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This is a scanning electron microscopy image showing a palladium nanoparticle with a gold antenna to enhance plasmonic sensing. Image courtesy of Alivisatos group

"In our resonant antenna-enhanced scheme, we use double electron-beam lithography in combination with a double lift-off procedure to precisely position a single palladium nanoparticle in the nanofocus of a gold nanoantenna,"Liu says."The strongly enhanced gold-particle plasmon near-fields can sense the change in the dielectric function of the proximal palladium nanoparticle as it absorbs or releases hydrogen. Light scattered by the system is collected by a dark-field microscope with attached spectrometer and the LSPR change is read out in real time."

Alivisatos, Liu and their co-authors found that the antenna enhancement effect could be controlled by changing the distance between the palladium nanoparticle and the gold antenna, and by changing the shape of the antenna.

"By amplifying sensing signals at the single-particle level, we eliminate the statistical and average characteristics inherent to ensemble measurements,"Liu says."Moreover, our antenna-enhanced plasmonic sensing technique comprises a noninvasive scheme that is biocompatible and can be used in aqueous environments, making it applicable to a variety of physical and biochemical materials."

For example, by replacing the palladium nanoparticle with other nanocatalysts, such as ruthenium, platinum, or magnesium, Liu says their antenna-enhanced plasmonic sensing scheme can be used to monitor the presence of numerous other important gases in addition to hydrogen, including carbon dioxide and the nitrous oxides. This technique also offers a promising plasmonic sensing alternative to the fluorescent detection of catalysis, which depends upon the challenging task of finding appropriate fluorophores. Antenna-enhanced plasmonic sensing also holds potential for the observation of single chemical or biological events.

"We believe our antenna-enhanced sensing technique can serve as a bridge between plasmonics and biochemistry,"Liu says."Plasmonic sensing offers a unique tool for optically probing biochemical processes that are optically inactive in nature. In addition, since plasmonic nanostructures made from gold or silver do not bleach or blink, they allow for continuous observation, an essential capability for in-situ monitoring of biochemical behavior."

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Looking inside nanomaterials in 3 dimensions

On May 13 2011, the journal<i>Science</i>published a paper where scientists from Risoe DTU (Denmark), in collaboration with scientists from China and the USA, report a new method for revealing a 3-D picture of the structure inside a material.

Mostsolid materialsare composed of millions of smallcrystals, packed together to form a fully dense solid. The orientations, shapes, sizes and relative arrangement of these crystals are important in determining many material properties.

Traditionally, it has only been possible to see the crystal structure of a material by looking at a cut surface, giving just 2D information. In recent years, x-ray methods have been developed that can be used to look inside a material and obtain a 3D map of the crystal structure. However, these methods have a resolution limit of around 100nm.

In contrast, the newly developed technique now published inScience, allows 3D mapping of the crystal structure inside a material down to nanometer resolution, and can be carried out using atransmission electron microscope, an instrument found in many research laboratories.

Samples must be thinner than a few hundrednanometers. However, this limitation is not a problem for investigations of crystal structures inside nanomaterials, where the average crystal size is less than 100 nanometers, and such materials are investigated all over the world in a search for materials with new and better properties than the materials we use today.

For example, nanomaterials have an extremely high strength and an excellent wear resistance and applications therefore span from microelectronics to gears for large windmills. The ability to collect a 3D picture of thecrystal structurein these materials is an important step in being able to understand the origins of their special properties.

An example of such a 3D map is given in the figure, showing the arrangement of crystals in a 150nm thick nanometal aluminium film. The crystals have identicallattice structure(arrangement of atoms) but they are orientated in different ways in the 3D sample as illustrated by the labels 1 and 2. The colours represent the orientations of the crystals and each crystal is defined by volumes of the same colour. The individual crystals of various sizes (from a few nm to about 100 nm) and shapes (from elongated to spherical) are clearly seen and mapped with a resolution of 1 nanometer.

An important advantage of such 3D methods is that they allow the changes taking place inside a material to be observed directly. For example, the mapping may be repeated before and after a heat treatment revealing how the structure changes during heating.

This new technique has a resolution 100 times better than existing non-destructive 3D techniques and opens up new opportunities for more precise analysis of the structural parameters innanomaterials.

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Exotic behavior when mechanical devices reach the nanoscale

Most mechanical resonators damp (slow down) in a well-understood linear manner, but ground-breaking work by Prof. Adrian Bachtold and his research group at the Catalan Institute of Nanotechnology has shown that resonators formed from nanoscale graphene and carbon nanotubes exhibit nonlinear damping, opening up exciting possibilities for super-sensitive detectors of force or mass.

In an article to be published inNatureNanotechnology, Prof. Bachtold and his co-researchers describe how they formed nano-scale resonators by suspending tiny graphene sheets or carbon nanotubes and clamping them at each end. These devices, similar to guitar strings, can be set to vibrate at very specific frequencies.

In all mechanical resonators studied to date, from large objects several metres in size down to tiny components just a few tens ofnanometersin length,dampinghas always been observed to occur in a highly predictable, linear manner. However Prof. Bachtold´s research demonstrates that this linear damping paradigm breaks down for resonators with critical dimensions on the atomic scale. Of particular importance they have shown that the damping is strongly nonlinear for resonators based on nanotubes and graphene, a characteristic that facilitates amplification of signals and dramatic improvements in sensitivity.

The finding has profound consequences. Damping is central to the physics of nanoelectromechanical resonators, lying at the core of quantum and sensing experiments. Therefore many predictions that have been made for nanoscale electro-mechanical devices now need to be revisited when considering nanotube and graphene resonators.

This new insight into the dynamics of nano-scale resonators will also enable dramatic improvements in the performance of numerous devices. Already the Prof. Bachtold´s group has achieved a new record in quality factor for graphene resonators and ultra-weak force sensing with a nanotube resonator.

The work is particularly timely because an increasing number of research groups around the world with diverse backgrounds are choosing to study nanotube/grapheneresonators, which have a number of uniquely useful properties.

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Nano-FTIR-nanoscale infrared spectroscopy with a thermal source

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Researchers from the Basque nanoscience research center CIC nanoGUNE and Neaspec GmbH (Germany) have developed an instrument that allows for recording infrared spectra with a thermal source at a resolution that is 100 times better than in conventional infrared spectroscopy. In future, the technique could be applied for analyzing the local chemical composition and structure of nanoscale materials in polymer composites, semiconductor devices, minerals or biological tissue. The work is published in<i>Nature Materials</i>.

The absorption ofinfrared radiationis characteristic for the chemical composition and structure of materials. For this reason, aninfrared spectrumcan be considered as a material's"fingerprint". Infrared spectroscopy has thus become an important tool for characterizing and identifying materials and is widely applied in different sciences and technologies including materials sciences and biomedical diagnostics. However, with conventional optical instruments, such as FTIR (Fourier Transform Infrared) infrared spectrometers, the light cannot be focused to spot sizes below several micrometers. This fundamental limitation prevents infrared-spectroscopic mapping of single nanoparticles, molecules or modernsemiconductor devices.

Researchers at nanoGUNE and Neaspec have now developed aninfrared spectrometerthat allows for nanoscale imaging withthermal radiation. The setup–in short nano-FTIR (see Figure) - is based on a scattering-type near-field microscope (NeaSNOM) that uses a sharp metallic tip to scan the topography of a sample surface. While scanning the surface, the tip is illuminated with the infrared light from a thermal source. Acting like an antenna, the tip converts the incident light into a nanoscale infrared spot (nanofocus) at the tip apex. By analyzing the scattered infrared light with a specially designed FTIR spectrometer, the researchers were able to record infrared spectra from ultra-small sample volumes.

In their experiments, the researchers managed to record infrared images of a semiconductor device from Infineon Technologies AG (Munich)."We achieved a spatial resolution better than 100 nm. This directly shows that thermal radiation can be focused to a spot size that is hundred times smaller than in conventionalinfrared spectroscopy", says FlorianHuth, who performed the experiments. The researcher demonstrated that nano-FTIR can be applied for recognizing differently processed silicon oxides or to measure the local electron density within complex industrial electronic devices."Our technique allows for recording spectra in the near- to far-infrared spectral range. This is an essential feature for analyzing the chemical composition of unknown nanomaterials", explains Rainer Hillenbrand, leader of the Nanooptics group at nanoGUNE.

Nano-FTIR has interesting application potential in widely different sciences and technologies, ranging from semiconductor industry to nanogeochemistry and astrophysics."Based on vibrational fingerprint spectroscopy, it could be applied for nanoscale mapping of chemical composition and structural properties of organic and inorganic nano-systems, including organic semiconductors, solar cells, nanowires, ceramics and minerals", adds FlorianHuth.

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Quantum dots with built-in charge boost solar cell efficiency by 50%

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(PhysOrg.com) -- For the past few years, researchers have been using quantum dots to increase the light absorption and overall efficiency of solar cells. Now, researchers have taken a step further, demonstrating that quantum dots with a built-in electric charge can increase the efficiency of InAs/GaAs quantum dot solar cells by 50% or more.

The researchers, Kimberly Sablon and John W. Little (US Army Research Laboratory in Adelphi, Maryland), Vladimir Mitin, Andrei Sergeev, and Nizami Vagidov (University of Buffalo in Buffalo, New York), and Kitt Reinhardt (AFOSR/NE in Arlington, Virginia) have published their study on the increasedsolar cell efficiencyin a recent issue ofNano Letters.

In their study, the researchers studied heterostructuresolar cellswith InAs/GaAs quantum dots. As photovoltaic materials, the quantum dots allow for harvesting of theinfrared radiationto convert it into electric energy. However, the quantum dots also enhance the recombination of photocarriers and decrease the photocurrent. For this reason, up to now the improvement of photovoltaic efficiency due to quantum dots has been limited by several percent.

Here, the researchers have proposed to charge quantum dots by using selective interdot doping. In their experiments, the researchers compared doping levels of 2, 3, and 6 additional electrons per quantum dot, which resulted in photovoltaic efficiency increases of 4.5%, 30%, and 50%, respectively, compared to an undoped solar cell. For the 6-electron doping level, that 50% increase corresponds to an overall efficiency increase from 9.3% (for undoped solar cells) to 14%.

The researchers attributed this radical improvement of the photovoltaic efficiency to two basic effects. First, the built-in-dot charge induces various transitions of the electrons and enhances harvesting of the infrared radiation. Second, the built-in-dot charge creates potential barriers around dots and these barriers suppress capture processes for electrons and do not allow them to return back into the dots. The effect of potential barriers has beenpreviously used by the researchersto improve the sensitivity of infrared detectors.

In addition, the researchers predict that further increasing the doping level will lead to an even stronger efficiency enhancement, since there was no evidence of saturation. In the future, the researchers plan to further investigate how these effects influence each other at higher doping levels. They predict that further increasing the doping level and radiation intensity will lead to an even stronger efficiency enhancement, since there was no evidence of saturation.

“The methodology and principles developed during this research are applicable to a number of photovoltaic devices with quantum dots and nanocrystals, such as polymer plastic cells and dye-sensitized porous metal oxide Gratzel cells,” Dr. Sergeev toldPhysOrg.com.“Effective harvesting and conversion of infrared radiation due to optimized electron-hole kinetics in structures withquantum dotsand nanocrystals will lead to potential breakthroughs in the area of solar energy conversion.”

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In Brief: Nanodots to the rescue

By applying the magnetic properties of iron nanodots to complex materials, a research team has overcome an obstacle to getting ultra-thin or highly strained films to perform on par with their bulk counterparts.

If the researchers are indeed successful, this work sets the stage for these exotic materials to be used in a wide range of fascinating and potentially technologically revolutionary applications, said Oak Ridge National Laboratory's Zac Ward, lead author of a paper published inPhysical Review Letters.

The problem lies in the fact that at low dimensions or when the material is under strain it loses the characteristics that make it valuable for use in nano-scale electronics.

"What we discovered is a way to activate these materials using themagnetic propertiesof iron nanodots to control the electron spin and tune the behavior,"Ward said.

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Pairing quantum dots with fullerenes for nanoscale photovoltaics

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(PhysOrg.com) -- In a step toward engineering ever-smaller electronic devices, scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have assembled nanoscale pairings of particles that show promise as miniaturized power sources. Composed of light-absorbing, colloidal quantum dots linked to carbon-based fullerene nanoparticles, these tiny two-particle systems can convert light to electricity in a precisely controlled way.

"This is the first demonstration of a hybrid inorganic/organic, dimeric (two-particle) material that acts as an electron donor-bridge-acceptor system for converting light to electrical current,"said Brookhaven physical chemist Mircea Cotlet, lead author of a paper describing thedimersand their assembly method inAngewandte Chemie.

By varying the length of the linker molecules and the size of the quantum dots, the scientists can control the rate and the magnitude of fluctuations in light-induced electron transfer at the level of the individual dimer."This control makes these dimers promising power-generating units for molecular electronics or more efficient photovoltaic solar cells,"said Cotlet, who conducted this research with materials scientist Zhihua Xu at Brookhaven's Center for Functional Nanomaterials.

Scientists seeking to develop molecular electronics have been very interested in organic donor-bridge-acceptor systems because they have a wide range ofcharge transportmechanisms and because their charge-transfer properties can be controlled by varying their chemistry. Recently, quantum dots have been combined with electron-accepting materials such as dyes, fullerenes, andtitanium oxideto produce dye-sensitized and hybrid solar cells in the hope that the light-absorbing and size-dependent emission properties of quantum dots would boost the efficiency of such devices. But so far, thepower conversionrates of these systems have remained quite low.

"Efforts to understand the processes involved so as to engineer improved systems have generally looked at averaged behavior in blended or layer-by-layer structures rather than the response of individual, well-controlled hybrid donor-acceptor architectures,"said Xu.

The precision fabrication method developed by the Brookhaven scientists allows them to carefully control particle size and interparticle distance so they can explore conditions for light-induced electron transfer between individual quantum dots and electron-acceptingfullerenesat the single molecule level.

The entire assembly process takes place on a surface and in a stepwise fashion to limit the interactions of the components (particles), which could otherwise combine in a number of ways if assembled by solution-based methods. This surface-based assembly also achieves controlled, one-to-one nanoparticle pairing.

To identify the optimal architectural arrangement for the particles, the scientists strategically varied the size of thequantum dots- which absorb and emit light at different frequencies according to their size - and the length of the bridge molecules connecting the nanoparticles. For each arrangement, they measured the electron transfer rate using single molecule spectroscopy.

"This method removes ensemble averaging and reveals a system's heterogeneity - for example fluctuating electron transfer rates - which is something that conventional spectroscopic methods cannot always do,"Cotlet said.

The scientists found that reducing quantum dot size and the length of the linker molecules led to enhancements in the electron transfer rate and suppression of electron transfer fluctuations.

"This suppression ofelectron transferfluctuation in dimers with smaller quantum dot size leads to a stable charge generation rate, which can have a positive impact on the application of these dimers inmolecular electronics, including potentially in miniature and large-area photovoltaics,"Cotlet said.

"Studying the charge separation and recombination processes in these simplified and well-controlled dimer structures helps us to understand the more complicated photon-to-electron conversion processes in large-area solar cells, and eventually improve their photovoltaic efficiency,"Xu added.

A U.S. patent application is pending on the method and the materials resulting from using the technique, and the technology is available for licensing.

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Electromechanics also operates at the nanoscale

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What limits the behaviour of a carbon nanotube? This is a question that many scientists are trying to answer. Physicists at University of Gothenburg, Sweden, have now shown that electromechanical principles are valid also at the nanometre scale. In this way, the unique properties of carbon nanotubes can be combined with classical physics– and this may prove useful in the quantum computers of the future.

"We have been studying carbon nanotubes theoretically, in order to see how they behave when they are stimulated to behave according to the laws quantum mechanics. The results provide a completely new platform for scientists to stand on", says Gustav Sonne of the Department of Physics at the University of Gothenburg.

Every day we use a number of different microelectromechanical components for various forms of detection, to determine whether a certain process has taken place or whether a certain substance is present. These cannot be detected without instruments. One example is the detection of rapid accelerations that is used to activate the airbag in a car during an accident. What all of these components have in common is that they combine mechanical and electronic properties in order to react to external stimuli.

Gustav Sonne has taken research down to a whole new dimension– from the micrometer scale to the nanometer scale– and he has studied the younger brothers of these components: nanoelectromechanical systems. The studies have been based on tiny nanotubes suspended between two electrical contacts. He has subsequently calculated how small vibrations in the suspended tubes can be coupled to a current that is led through them.

"Our research has focussed mainly on how these systems, which consist of a tiny, super-light mechanical oscillator (the suspended nanotube), can be described in quantum mechanical terms, and what effects this has on the measurements we can carry out. We have been able to demonstrate a number of new mechanisms for electromechanical coupling that should be possible to observe experimentally. This, in turn, may lead to extremely exotic physical phenomena in these structures, phenomena which may be of interest for research into quantum computers, and other fields."

Interest in nanotubes is based on their outstanding properties: they are among the strongest materials known, weigh next to nothing, and have extremely high conductivity for both electric currents and heat. Carbon nanotubes can be used to manufacture composite materials that are several orders of magnitude stronger than currently available materials.

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Graphene optical modulators could lead to ultrafast communications

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(PhysOrg.com) -- Scientists at the University of California, Berkeley, have demonstrated a new technology for graphene that could break the current speed limits in digital communications.

The team of researchers, led by UC Berkeley engineering professor Xiang Zhang, built a tiny optical device that usesgraphene, a one-atom-thick layer of crystallized carbon, to switch light on and off. This switching ability is the fundamental characteristic of a network modulator, which controls the speed at which data packets are transmitted. The faster the data pulses are sent out, the greater the volume of information that can be sent. Graphene-based modulators could soon allow consumers to stream full-length, high-definition, 3-D movies onto a smartphone in a matter of seconds, the researchers said.

"This is the world's smallest optical modulator, and the modulator in data communications is the heart of speed control,"said Zhang, who directs a National Science Foundation (NSF) Nanoscale Science and Engineering Center at UC Berkeley."Graphene enables us to make modulators that are incredibly compact and that potentially perform at speeds up to ten times faster than current technology allows. This new technology will significantly enhance our capabilities in ultrafastoptical communicationand computing."

In this latest work, described in the May 8 advanced online publication of the journalNature, researchers were able to tune the graphene electrically to absorb light in wavelengths used in data communication. This advance adds yet another advantage to graphene, which has gained a reputation as a wonder material since 2004 when it was first extracted from graphite, the same element in pencil lead. That achievement earned University of Manchester scientists Andre Geim and Konstantin Novoselov the Nobel Prize in Physics last year.

Zhang worked with fellow faculty member Feng Wang, an assistant professor of physics and head of the Ultrafast Nano-Optics Group at UC Berkeley. Both Zhang and Wang are faculty scientists at Lawrence Berkeley National Laboratory's Materials Science Division.

"The impact of this technology will be far-reaching,"said Wang."In addition to high-speed operations, graphene-based modulators could lead to unconventional applications due to graphene's flexibility and ease in integration with different kinds of materials. Graphene can also be used to modulate new frequency ranges, such as mid-infrared light, that are widely used in molecular sensing."

Graphene is the thinnest, strongest crystalline material yet known. It can be stretched like rubber, and it has the added benefit of being an excellent conductor of heat and electricity. This last quality of graphene makes it a particularly attractive material for electronics.

"Graphene is compatible with silicon technology and is very cheap to make,"said Ming Liu, post-doctoral researcher in Zhang's lab and co-lead author of the study."Researchers in Korea last year have already produced 30-inch sheets of it. Moreover, very little graphene is required for use as a modulator. The graphite in a pencil can provide enough graphene to fabricate 1 billion optical modulators."

It is the behavior of photons and electrons in graphene that first caught the attention of the UC Berkeley researchers.

Shown is a scanning electron microscope (SEM) image magnifying the key structures of the graphene-based optical modulator. (Colors were added to enhance the contrast). Gold (Au) and platinum (Pt) electrodes are used to apply electrical charges to the sheet of graphene, shown in blue, placed on top of the silicon (Si) waveguide, shown in red. The voltage can control the graphene's transparency, effectively turning the setup into an optical modulator that can turn light on and off. (Ming Liu image)

The researchers found that the energy of the electrons, referred to as its Fermi level, can be easily altered depending upon the voltage applied to the material. The graphene's Fermi level in turn determines if the light is absorbed or not.

When a sufficient negative voltage is applied, electrons are drawn out of the graphene and are no longer available to absorb photons. The light is"switched on"because the graphene becomes totally transparent as the photons pass through.

Graphene is also transparent at certain positive voltages because, in that situation, the electrons become packed so tightly that they cannot absorb the photons.

The researchers found a sweet spot in the middle where there is just enough voltage applied so the electrons can prevent the photons from passing, effectively switching the light"off."

"If graphene were a hallway, and electrons were people, you could say that, when the hall is empty, there's no one around to stop the photons,"said Xiaobo Yin, co-lead author of the Nature paper and a research scientist in Zhang's lab."In the other extreme, when the hall is too crowded, people can't move and are ineffective in blocking the photons. It's in between these two scenarios that the electrons are allowed to interact with and absorb the photons, and the graphene becomes opaque."

In their experiment, the researchers layered graphene on top of a silicon waveguide to fabricate optical modulators. The researchers were able to achieve a modulation speed of 1 gigahertz, but they noted that the speed could theoretically reach as high as 500 gigahertz for a single modulator.

While components based upon optics have many advantages over those that use electricity, including the ability to carry denser packets of data more quickly, attempts to create optical interconnects that fit neatly onto a computer chip have been hampered by the relatively large amount of space required in photonics.

Light waves are less agile in tight spaces than their electrical counterparts, the researchers noted, so photon-based applications have been primarily confined to large-scale devices, such as fiber optic lines.

"Electrons can easily make an L-shaped turn because the wavelengths in which they operate are small,"said Zhang."Light wavelengths are generally bigger, so they need more space to maneuver. It's like turning a long, stretch limo instead of a motorcycle around a corner. That's why optics require bulky mirrors to control their movements. Scaling down theoptical devicealso makes it faster because the single atomic layer of graphene can significantly reduce the capacitance– the ability to hold an electric charge– which often hinders device speed."

Graphene-based modulators could overcome the space barrier of optical devices, the researchers said. They successfully shrunk a graphene-based optical modulator down to a relatively tiny 25 square microns, a size roughly 400 times smaller than a human hair. The footprint of a typical commercial modulator can be as large as a few square millimeters.

Even at such a small size, graphene packs a punch in bandwidth capability. Graphene can absorb a broad spectrum of light, ranging over thousands of nanometers from ultraviolet to infrared wavelengths. This allows graphene to carry more data than current state-of-the-art modulators, which operate at a bandwidth of up to 10 nanometers, the researchers said.

"Graphene-based modulators not only offer an increase in modulation speed, they can enable greater amounts of data packed into each pulse,"said Zhang."Instead of broadband, we will have 'extremeband.' What we see here and going forward with graphene-based modulators are tremendous improvements, not only in consumer electronics, but in any field that is now limited by data transmission speeds, including bioinformatics and weather forecasting. We hope to see industrial applications of this new device in the next few years."

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Highly sensitive graphene biosensors based on surface plasmon resonance

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Adding a few graphene layers onto the conventional gold-film SPR biosensor will boost up its sensitivity dramatically. The improved sensitivity comes from the graphene layer’s increased adsorption of biomolecules and the graphene layer’s optical modification to the SPR.

Surface plasmon resonance (SPR) biosensors are optical sensors, which use surface plasmon polariton waves to probe the interactions between biomolecules and the sensor surface. In the conventional SPRbiosensorconfiguration, a thin metallic film is coated on one side of the prism, separating the sensing medium and the prism. The metallic film is typically made from noble metals, such as gold and silver, which support the propagation of surface plasmon polariton at visible light frequencies. But, gold is usually preferred because it has good resistance to oxidation and corrosion in different environments.

However, biomolecules adsorb poorly on gold. This drawback limits the sensitivity of the conventional SPR biosensor.

An attractive way to improve the sensitivity of SPR biosensor is to functionalize the gold film with biomolecular recognition elements (BRE) in order to enhance the adsorption of biomolecules on the gold surface.

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Fig. 2 (a) The surface plasmon resonance curves for the conventional biosensor (L = 0) (black thin lines) and the monolayer graphene biosensor (L = 1) (blue thick lines) for He-Ne laser light (λ0 = 633 nm): prism (1.723) | Au (50 nm, 0.1726 + i 3.4218) | graphene (L× 0.34 nm, 3 + i 1.149106) | water (1.33) before (dashed lines) and after (solid lines) the adsorption of biomolecules, assuming the same refractive index change∆n = 0.005. (b) The sensitivity enhancement∆SRIL/SRI0 as a function of the number of graphene layers L.

Here, we propose to use graphene as the BRE, where a sheet of graphene is coated on the gold surface in the conventional SPR biosensor setup. Graphene-on-Au (111) has been proposed and fabricated recently, which is shown to stably adsorb biomolecules with carbon-based ring structures (e.g. ssDNA).

This special property of graphene enables a greater refractive index change near the graphene | sensing medium interface than that of the conventional SPR biosensor. Moreover, the coating of the gold surface withgraphenewill also modify the propagation constant of surface plasmon polariton (SPP); thereby change the sensitivity to refractive index change.

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Pentagonal tiles pave the way towards organic electronics

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(PhysOrg.com) -- New research paves way for the nanoscale self-assembly of organic building blocks, a promising new route towards the next generation of ultra-small electronic devices.

Ring-like molecules with unusual five-fold symmetry bind strongly to a copper surface, due to a substantial transfer of charge, but experience remarkably little difficulty in sideways diffusion, and exhibit surprisingly little interaction between neighbouring molecules. This unprecedented combination of features is ideal for the spontaneous creation of high-density stable thin films, comprising a pavement of these organic pentagonal tiles, with potential applications in computing, solar power and novel display technologies.

Currently, commercial electronics use a top-down approach, with the milling or etching away ofinorganic material, such as silicon, to make a device smaller. For many years the computing power of a given size of computer chip has been doubling every eighteen months (a phenomenon known as Moore's law) but a limit in this growth is soon expected. At the same time, the efficiency of coupling electronic components to incoming or outgoing light (either in the generation of electricity from sunlight, or in the generation of light from electricity in flat-screen displays and lighting) is also fundamentally limited by the development of fabrication techniques at the nanometre scale.

Researchers are therefore looking for ingenious solutions in the creation of ever smaller electronics. The field of nanotechnology is taking a bottom-up approach of creating electronics using naturally self-assembling organic components, such as polymers, which will be capable of spontaneously forming devices with the desired electronic or optical characteristics.

The latest findings are from scientists at the University of Cambridge and Rutgers University who are working on the development of new classes of organic thin films on surfaces. By studying the fundamental forces at play in self-assembling thin films, they are developing the knowledge that will allow them to tailor these films into molecular-scale organic-electronic devices, creating smaller components than would ever be possible with conventional fabrication techniques.

Dr Holly Hedgeland, of the Department of Physics at the University of Cambridge, one of the co-authors of the paper reporting the research, said:"With the semiconductor industry currently worth an estimated $249 billion per year there is a clear motivation towards a molecular scale understanding of innovative technologies that could come to replace those we use today."

It is not simply the electronic properties of a molecule on a surface that will control its potential to form part of a device, but also whether it will move by itself into the required structural configuration and remain stable in that position even if the device becomes heated in use.

Molecules that are strongly bound to the substrate with a high degree of transfer of charge offer a range of new possibilities, though little is currently known of their behaviour. A number of organic molecules, usually featuring carbon rings across which electronic charge can conduct, potentially demonstrate the right electronic properties, but the long-range forces which will govern theirself-assemblyduring the first phases of growth often remain a mystery.

Now the interdisciplinary team based in the Departments of Physics and Chemistry at the University of Cambridge, and the Department of Chemistry and Chemical Biology at Rutgers University, have reported the first dynamical measurements for a new class of organic thin film where cyclopentadienyl molecules (C5H5) receive significant electronic charge from the surface, yet diffuse easily across the surface and show interactions with each other that are much weaker than would typically be expected for the amount of charge transferred.

Hedgeland explained:"By coupling the experimental helium spin echo technique with advanced first-principles calculations, we were able to study the dynamic behaviour of a cyclopentendienyl layer on a copper surface, and to deduce that the charge transfer between the metal and the organic molecule was occurring in a counter-intuitive sense."

Dr Marco Sacchi, of the Department of Chemistry at the University of Cambridge, who carried out the calculations that helped explain the startling new experimental results, said that"the key to the unique behavior of cyclopentadienyl lies in its pentagonal (five-fold) symmetry, which prevents it latching onto any one site within the triangular (three-fold) symmetry of thecopper surfacethrough directional covalent bonds, leaving it free to move easily from site to site; at the same time, its internal electronic structure is just one electron short of an extremely stable 'aromatic' configuration, encouraging a high degree of charge transfer from the surface and creating a strong non-directional ionic bond."

The researchers' findings, reported inPhysical Review Letterstoday, Friday 06 May, highlight the potential of a new category of molecular adsorbate, which could fulfil all the criteria required for useful application.

Hedgeland concluded:"The unusual character of the charge transfer in this case prevents the large repulsive interactions between adjacent molecules that would otherwise have been expected, and hence should enable the formation of unusually high-density films. At the same time, the molecules remain highly mobile and yet strongly bound to the surface, with a large degree of thermal stability. In all, this is a combination of physical properties that offers huge potential benefit to the development of new classes of self-assembled organic films relevant for technological applications."

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Measurement of 'hot' electrons could have solar energy payoff

Basic scientific curiosity paid off in unexpected ways when Rice University researchers investigating the fundamental physics of nanomaterials discovered a new technology that could dramatically improve solar energy panels.

The research is described in a new paper this week in the journalScience.

"We're merging the optics of nanoscale antennas with the electronics of semiconductors,"said lead researcher Naomi Halas, Rice's Stanley C. Moore Professor in Electrical and Computer Engineering."There's no practical way to directly detect infrared light with silicon, but we've shown that it is possible if you marry the semiconductor to a nanoantenna. We expect this technique will be used in new scientific instruments for infrared-light detection and for higher-efficiency solar cells."

More than a third of the solar energy on Earth arrives in the form of infrared light. But silicon -- the material that's used to convert sunlight into electricity in the vast majority of today's solar panels -- cannot capture infrared light's energy. Every semiconductor, including silicon, has a"bandgap"where light below a certain frequency passes directly through the material and is unable to generate an electrical current. By attaching a metal nanoantenna to the silicon, where the tiny antenna is specially tuned to interact with infrared light, the Rice team showed they could extend the frequency range forelectricity generationinto the infrared. When infrared light hits the antenna, it creates a"plasmon,"a wave of energy that sloshes through the antenna's ocean offree electrons. The study of plasmons is one of Halas' specialties, and the new paper resulted from basic research into the physics of plasmons that began in her lab years ago.

It has been known thatplasmonsdecay and give up their energy in two ways; they either emit a photon of light or they convert thelight energyinto heat. The heating process begins when the plasmon transfers its energy to a single electron -- a 'hot' electron. Rice graduate student Mark Knight, lead author on the paper, together with Rice theoretical physicist Peter Nordlander, his graduate student Heidar Sobhani, and Halas set out to design an experiment to directly detect the hot electrons resulting from plasmon decay.

Patterning a metallic nanoantenna directly onto a semiconductor to create a"Schottky barrier,"Knight showed that the infrared light striking the antenna would result in a hot electron that could jump the barrier, which creates an electrical current. This works for infrared light at frequencies that would otherwise pass directly through the device.

"The nanoantenna-diodes we created to detect plasmon-generated hot electrons are already pretty good at harvestinginfrared lightand turning it directly into electricity,"Knight said."We are eager to see whether this expansion of light-harvesting to infrared frequencies will directly result in higher-efficiency solar cells."

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Researchers create novel nanoantennas

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(PhysOrg.com) -- A team of plasmonics researchers has developed a novel type of nanoantenna that could lead to advances in drug and explosives detection.

An international team of plasmonics researchers has developed a novel type of nanoantenna that could one day lead to advances in security applications for the detection of drugs and explosives.

A report of the finding, authored by Swinburne University’s Professor Saulius Juodkazis and Dr Lorenzo Rosa with a collaborator from China, has been published in the scientific journalPhysica Status Solidi: Rapid Research Letters.

Nanoantennas work in much the same way as regular antennas, except they collect light instead of radio waves and are millions of times smaller.

The reason that Professor Juodkazis’ nanoantennas are so unique is that they are fractal– that is they consist of repeating patterns, with the shape of the smallest feature replicated to make identical, yet larger structures.

“Self-replication is an interesting design that is often found in nature. For example, you will see it on some sea shells,” he said.

This fractal approach means that the researchers’ nanoantennas can be scaled down to a very small size, or scaled up to be the width of a human hair– which in nanophotonics terms is extremely large.

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“Once we have the smallest bit fabricated there are no restraints, we can just replicate it and make it larger,” Professor Juodkazis said.“This is something that has been very difficult to achieve up until now. If scientists wanted a larger structure, they would just have to fabricate one.”

“In a sense we have been able to create a customisable nanoantenna that can be used for different applications, making it a very cost effective structure.”

This new type of nanoantenna has many potential applications, such as the development of new types of drug and explosives detection kits.

“The different chemicals found in drugs and explosives are detectable at very specific wavelengths. Nanoantennas are able to recognise these, and in turn identify specific types of drugs and explosives,” Professor Juodkazis said.

While he is pleased with the developments to date, he expects he will be able to extend his nanoantenna research even further when Swinburne’s new plasmonics lab is completed in late 2011.

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