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A team of Korean researchers, affiliated with UNIST has recently pioneered in developing a new type of multilayered (Au NPs/TiO2/Au) photoelectrode that boosts the ability of solar water-splitting to produce hydrogen. According to the research team, this special photoelectrode, inspired by the way plants convert sunlight into energy is capable of absorbing visible light from the sun, and then using it to split water molecules (H2O) into hydrogen and oxygen.

This study is a collaboration among scientists, including Prof. Jeong Min Baik (School of Materials Science and Engineering, UNIST), Prof. Jae Sung Lee (School of Energy and Chemical Engineering, UNIST), Prof. Heon Lee (School of Materials Science and Engineering, Korea University), and Prof. Jonghwa Shin (Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology).

This multilayered photoelectrode takes the form of two-dimensional hybrid metal-dielectric structure, which mainly consists of three layers of gold (Au) film, ultrathin TiO2 layer (20 nm), and gold nanoparticles (Au NPs). In a study, reported in the January 21, 2016 issue of Nano Energy, the team reported that this promising photoelectrode shows high light absorption of about 90% in the visible range 380-700nm, as well as significant enhancement in photo-catalytic applications.

Many structural designs, such as hierarchical and branched assemblies of nanoscale materials have been suggested to increase the UV-visible absorption and to enhance water-splitting efficiency. However, through incorporation of plasmonic metal nanoparticles (i.e. Au) to TiO2 structures, their photoelectrodes have shown to enhance the photoactivity in the entire UV-visible region of solar spectrum when compared to the existing ones, the team reports.

Prof. Jeong Min Baik of UNIST (School of Materials Science and Engineering) states, “Several attemps have been made to use UV-based photoelectrodes for hydrogen production, but this is the first time to use the metal-dielectric hybrid-structured film with TiO2 for oxygen production.” Moreover, according to Prof. Baik, this special type of photoelectrode uses approximately 95% of the visible spectrum of sunlight, which makes up a substantial portion (40%) of full sunlight. He adds, “The developed technology is expected to improve hydrogen production efficiency.”

Prof. Heon Lee (Korean University) states, “This metal-dielectric hybrid-structured film is expected to further reduce the overall cost of producing hydrogen, as it doesn’t require complex operation processes.” He continues by saying, “Using nanoimprint lithography, mass production of hydrogen will be soon possible.”

Prof. Baik adds, “This simple system may serve as an efficient platform for solar energy conversion, utilizing the whole UV-visible range of solar spectrum based on two-dimensional plasmonic photoelectrodes.”

This work was supported by the Pioneer Center Program through the National Research Foundation of Korea (NRF) grant, funded by the Korean government (MSIP). It has been also equivalently funded by the 2014 Research Fund of UNIST (Ulsan National Institute of Science and Technology), as well as by the KIST-UNIST partnership program.

Heterostructures formed by different three-dimensional semiconductors form the foundation for modern electronic and photonic devices. Now, University of Washington scientists have successfully combined two different ultrathin semiconductors — each just one layer of atoms thick and roughly 100,000 times thinner than a human hair — to make a new two-dimensional heterostructure with potential uses in clean energy and optically-active electronics. The team, led by Boeing Distinguished Associate Professor Xiaodong Xu, announced its findings in a paper published Feb. 12 in the journal Science.

Senior author Xu and lead authors Kyle Seyler and Pasqual Rivera, both doctoral students in the UW physics department, synthesized and investigated the optical properties of this new type of semiconductor sandwich.

“What we’re seeing here is distinct from heterostructures made of 3-D semiconductors,” said Xu, who has joint appointments in the Department of Physics and the Department of Materials Science and Engineering. “We’ve created a system to study the special properties of these atomically thin layers and their potential to answer basic questions about physics and develop new electronic and photonic technologies.”

When semiconductors absorb light, pairs of positive and negative charges can form and bind together to create so-called excitons. Scientists have long studied how these excitons behave, but when they are squeezed down to the 2-D limit in these atomically thin materials, surprising interactions can occur.

While traditional semiconductors manipulate the flow of electron charge, this device allows excitons to be preserved in “valleys,” a concept from quantum mechanics similar to the spin of electrons. This is a critical step in the development of new nanoscale technologies that integrate light with electronics.

“It was already known that these ultrathin 2-D semiconductor have these unique properties that you cannot find in other 2-D or 3-D arrangements,” said Xu. “But as we show here, when we put these two layers together — one on top of the other — the interface between these sheets becomes the site of even more new physical properties, which you don’t see in each layer on its own or in the 3-D version.”

Xu and his team wanted to create and explore the properties of a 2-D semiconductor heterostructure made up of two different layers of material, a natural expansion of their previous studies on atomically thin junctions, as well as nanoscale lasers based on atomically thin layers of semiconductors. By studying how laser light interacts with this heterostructure, they gathered information about the physical properties at the atomically sharp interface.

“Many groups have studied the optical properties of single 2-D sheets,” said Seyler. “What we do here is carefully stack one material on top of another, and then study the new properties that arise at the interface.”

The team obtained two types of semiconducting crystals, tungsten diselenide (WSe2) and molybdenum diselenide (MoSe2), from collaborators at Oak Ridge National Laboratory. They used facilities developed in-house to precisely arrange two layers, one derived from each crystal, a process that took a few years to fully develop.

“But now that we know how to do it properly, we can make new ones in one or two weeks,” said Xu.

Getting these devices to emit light posed a unique challenge, due to the properties of electrons in each layer.

“Once you have these two sheets of material, an essential question is how to position the two layers together,” said Seyler. The electrons in each layer have unique spin and valley properties, and “how you position them — their twist angle — affects how they interact with light.”

By aligning the crystal lattices, the authors could excite the heterostructure with a laser and create optically active excitons between the two layers.

“These excitons at the interface can store valley information for orders of magnitude longer than either of the layers on their own,” said Rivera. “This long lifetime allows for fascinating effects which may lead to further optical and electronic applications with valley functionality.”

Now that they can efficiently make a semiconductor heterostructure out of 2-D materials, Xu and his team would like to explore a number of fascinating physical properties, including how exciton behavior varies as they change angles between the layers, the quantum properties excitons between layers and electrically driven light emission.

“There’s a whole industry that wants to use these 2-D semiconductors to make new electronic and photonic devices,” said Xu. “So we’re trying to study the fundamental properties of these new heterostructures for things like efficient laser technology, light-emitting diodes and light-harvesting devices. These will hopefully be useful for clean energy and information technology applications. It is quite exciting but there’s a lot work to do.”

Ever smaller, ever faster, ever cheaper – since the start of the computer age the performance of processors has doubled on average every 18 months. 50 years ago already, Intel co-founder Gordon E. Moore prognosticated this astonishing growth in performance. And Moore’s law seems to hold true to this day.

But the miniaturization of electronics is now reaching its physical limits. “Today already, transistors are merely a few nanometers in size. Further reductions are horrendously expensive,” says Professor Jonathan Finley, Director of the Walter Schottky Institute at TUM. “Improving performance is achievable only by replacing electrons with photons, i.e. particles of light.”

Photonics – the silver bullet of miniaturization

Data transmission and processing with light has the potential of breaking the barriers of current electronics. In fact, the first silicon-based photonics chips already exist. However, the sources of light for the transmission of data must be attached to the silicon in complicated and elaborate manufacturing processes. Researchers around the world are thus searching for alternative approaches.

Scientists at the TU Munich have now succeeded in this endeavor: Dr. Gregor Koblmüller at the Department of Semiconductor Quantum-Nanosystems has, in collaboration with Jonathan Finley, developed a process to deposit nanolasers directly onto silicon chips. A patent for the technology is pending.

The candidate Benedikt Mayer and Masters student Lisa Janker in an experiment at the molecular beam epitaxy in the Walter Schottky Institute of the Technische Universitaet Muenchen am teaching Suhl for semiconductor nanostructures and quantum devices, with Prof. Dr. Jonathan Finley; persons depicted (from left): Benedikt Mayer, Lisa Janker; Location: Walter Schottky Institute, Am Coulombwall 4, 85748 Garching, Germany; Date: 02/10/2016; CREDIT: Uli Benz / TU Muenchen

The candidate Benedikt Mayer and Masters student Lisa Janker in an experiment at the molecular beam epitaxy in the Walter Schottky Institute of the Technische Universitaet Muenchen am teaching Suhl for semiconductor nanostructures and quantum devices, with Prof. Dr. Jonathan Finley; persons depicted (from left): Benedikt Mayer, Lisa Janker; Location: Walter Schottky Institute, Am Coulombwall 4, 85748 Garching, Germany; Date: 02/10/2016; CREDIT: Uli Benz / TU Muenchen

Growing a III-V semiconductor onto silicon requires tenacious experimentation. “The two materials have different lattice parameters and different coefficients of thermal expansion. This leads to strain,” explains Koblmüller. “For example, conventional planar growth of gallium arsenide onto a silicon surface results therefore in a large number of defects.”

The TUM team solved this problem in an ingenious way: By depositing nanowires that are freestanding on silicon their footprints are merely a few square nanometers. The scientists could thus preclude the emerging of defects in the GaAs material.

Atom by atom to a nanowire

But how do you turn a nanowire into a vertical-cavity laser? To generate coherent light, photons must be reflected at the top and bottom ends of the wire, thereby amplifying the light until it reaches the desired threshold for lasing.

To fulfil these conditions, the researchers had to develop a simple, yet sophisticated solution: “The interface between gallium arsenide and silicon does not reflect light sufficiently. We thus built in an additional mirror – a 200 nanometer thick silicon oxide layer that we evaporated onto the silicon,” explains Benedikt Mayer, doctoral candidate in the team led by Koblmüller and Finley. “Tiny holes can then be etched into the mirror layer. Using epitaxy, the semiconductor nanowires can then be grown atom for atom out of these holes.”

Only once the wires protrude beyond the mirror surface they may grow laterally – until the semiconductor is thick enough to allow photons to jet back and forth to allow stimulated emission and lasing. “This process is very elegant because it allows us to position the nanowire lasers directly also onto waveguides in the silicon chip,” says Koblmüller.

GaAs nanowires on a silicon surface - Picture: Thomas Stettner / Philipp Zimmermann / TUM

GaAs nanowires on a silicon surface – CREDIT: Thomas Stettner / Philipp Zimmermann / TUM

Basic research on the path to applications

Currently, the new gallium arsenide nanowire lasers produce infrared light at a predefined wavelength and under pulsed excitation. “In the future we want to modify the emission wavelength and other laser parameters to better control temperature stability and light propagation under continuous excitation within the silicon chips,” adds Finley.

The team has just published its first successes in this direction. And they have set their sights firmly on their next goal: “We want to create an electric interface so that we can operate the nanowires under electrical injection instead of relying on external lasers,” explains Koblmüller.

“The work is an important prerequisite for the development of high-performance optical components in future computers,” sums up Finley. “We were able to demonstrate that manufacturing silicon chips with integrated nanowire lasers is possible.”

The research was funded by the German Research Foundation (DFG) through the TUM Institute for Advanced Study, the Excellence Cluster Nanosystems Initiative Munich (NIM) and the International Graduate School of Science and Engineering (IGSSE) of the TUM, as well as by IBM through an international postgraduate program.

The road to more versatile wearable technology is dotted with iron. Specifically, quantum dots of iron arranged on boron nitride nanotubes (BNNTs). The new material is the subject of a study to be published in Scientific Reports later this week, led by Yoke Khin Yap, a professor of physics at Michigan Technological University.

Yap says the iron-studded BNNTs are pushing the boundaries of electronics hardware. The transistors modulating electron flow need an upgrade.

“Look beyond semiconductors,” he says, explaining that materials like silicon semiconductors tend to overheat, can only get so small and leak electric current.

The key to revamping the fundamental base of transistors is creating a series of stepping-stones that use quantum tunneling.

The nanotubes are the mainframe of this new material. BNNTs are great insulators and terrible at conducting electricity. While at first that seems like an odd choice for electronics, the insulating effect of BNNTs is crucial to prevent current leakage and overheating. Additionally, electron flow will only occur across the metal dots on the BNNTs.

In past research, Yap and his team used gold for quantum dots, placed along a BNNT in a tidy line. With enough energy potential, the electrons are repelled by the insulating BNNT and hopscotch from gold dot to gold dot. This electron movement is called quantum tunneling.

“Imagine this as a river, and there’s no bridge; it’s too big to hop over,” Yap says. “Now, picture having stepping stones across the river–you can cross over, but only when you have enough energy to do so.”

Unlike with semiconductors, there is no classical resistance with quantum tunneling. No resistance means no heat. Plus, these materials are very small; the nanomaterials enable the transistors to shrink as well. An added bonus is that BNNTs are also quite flexible, a boon for wearable electronics.

Using bundled strands of DNA to build Tinkertoy-like tetrahedral cages, scientists at the U.S. Department of Energy’s Brookhaven National Laboratory have devised a way to trap and arrange nanoparticles in a way that mimics the crystalline structure of diamond. The achievement of this complex yet elegant arrangement, as described in a paper published February 5, 2016, in Science, may open a path to new materials that take advantage of the optical and mechanical properties of this crystalline structure for applications such as optical transistors, color-changing materials, and lightweight yet tough materials.

This is a schematic illustration of the experimental strategy: Double stranded DNA bundles (gray) form tetrahedral cages. Single stranded DNA strands on the edges (green) and vertices (red) match up with complementary strands on gold nanoparticles. This results in a single gold particle being trapped inside each tetrahedral cage, and the cages binding together by tethered gold nanoparticles at each vertex. The result is a crystalline nanoparticle lattice that mimics the long-range order of crystalline diamond. The images below the schematic are (left to right): a reconstructed cryo-EM density map of the tetrahedron, a caged particle shown in a negative-staining TEM image, and a diamond superlattice shown at high magnification with cryo-STEM. Credit: Brookhaven National Laboratory

This is a schematic illustration of the experimental strategy: Double stranded DNA bundles (gray) form tetrahedral cages. Single stranded DNA strands on the edges (green) and vertices (red) match up with complementary strands on gold nanoparticles. This results in a single gold particle being trapped inside each tetrahedral cage, and the cages binding together by tethered gold nanoparticles at each vertex. The result is a crystalline nanoparticle lattice that mimics the long-range order of crystalline diamond. The images below the schematic are (left to right): a reconstructed cryo-EM density map of the tetrahedron, a caged particle shown in a negative-staining TEM image, and a diamond superlattice shown at high magnification with cryo-STEM. Credit: Brookhaven National Laboratory

“We solved a 25-year challenge in building diamond lattices in a rational way via self-assembly,” said Oleg Gang, a physicist who led this research at the Center for Functional Nanomaterials (CFN) at Brookhaven Lab in collaboration with scientists from Stony Brook University, Wesleyan University, and Nagoya University in Japan.

The scientists employed a technique developed by Gang that uses fabricated DNA as a building material to organize nanoparticles into 3D spatial arrangements. They used ropelike bundles of double-helix DNA to create rigid, three-dimensional frames, and added dangling bits of single-stranded DNA to bind particles coated with complementary DNA strands.

“We’re using precisely shaped DNA constructs made as a scaffold and single-stranded DNA tethers as a programmable glue that matches up particles according to the pairing mechanism of the genetic code-A binds with T, G binds with C,” said Wenyan Liu of the CFN, the lead author on the paper. “These molecular constructs are building blocks for creating crystalline lattices made of nanoparticles.”

The difficulty of diamond

As Liu explained, “Building diamond superlattices from nano- and micro-scale particles by means of self-assembly has proven remarkably difficult. It challenges our ability to manipulate matter on small scales.”

The reasons for this difficulty include structural features such as a low packing fraction-meaning that in a diamond lattice, in contrast to many other crystalline structures, particles occupy only a small part of the lattice volume-and strong sensitivity to the way bonds between particles are oriented. “Everything must fit together in just such a way without any shift or rotation of the particles’ positions,” Gang said. “Since the diamond structure is very open, many things can go wrong, leading to disorder.”

“Even to build such structures one-by-one would be challenging,” Liu added, “and we needed to do so by self-assembly because there is no way to manipulate billions of nanoparticles one-by-one.”

Gang’s previous success using DNA to construct a wide range of nanoparticle arrays suggested that a DNA-based approach might work in this instance.

DNA guides assembly

The team first used the ropelike DNA bundles to build tetrahedral “cages”-a 3D object with four triangular faces. They added single-stranded DNA tethers pointing toward the interior of the cages using T,G,C,A sequences that matched up with complementary tethers attached to gold nanoparticles. When mixed in solution, the complementary tethers paired up to “trap” one gold nanoparticle inside each tetrahedron cage.

The arrangement of gold nanoparticles outside the cages was guided by a different set of DNA tethers attached at the vertices of the tetrahedrons. Each set of vertices bound with complementary DNA tethers attached to a second set of gold nanoparticles.

When mixed and annealed, the tetrahedral arrays formed superlattices with long-range order where the positions of the gold nanoparticles mimics the arrangement of carbon atoms in a lattice of diamond, but at a scale about 100 times larger.

“Although this assembly scenario might seem hopelessly unconstrained, we demonstrate experimentally that our approach leads to the desired diamond lattice, drastically streamlining the assembly of such a complex structure,” Gang said.

The proof is in the images. The scientists used cryogenic transmission electron microscopy (cryo-TEM) to verify the formation of tetrahedral frames by reconstructing their 3D shape from multiple images. Then they used in-situ small-angle x-ray scattering (SAXS) at the National Synchrotron Light Source (NSLS, https://www.bnl.gov/ps/), and cryo scanning transmission electron microscopy (cryo-STEM) at the CFN, to image the arrays of nanoparticles in the fully constructed lattice.

“Our approach relies on the self-organization of the triangularly shaped blunt vertices of the tetrahedra (so called ‘footprints’) on isotropic spherical particles. Those triangular footprints bind to spherical particles coated with complementary DNA, which allows the particles to coordinate their arrangement in space relative to one another. However, the footprints can arrange themselves in a variety of patterns on a sphere. It turns that one particular placement is more favorable, and it corresponds to the unique 3D placement of particles that locks the diamond lattice,” Gang said.

The team supported their interpretation of the experimental results using theoretical modeling that provided insight about the main factors driving the successful formation of diamond lattices.

Sparkling implications

“This work brings to the nanoscale the crystallographic complexity seen in atomic systems,” said Gang, who noted that the method can readily be expanded to organize particles of different material compositions. The group has demonstrated previously that DNA-assembly methods can be applied to optical, magnetic, and catalytic nanoparticles as well, and will likely yield the long-sought novel optical and mechanical materials scientists have envisioned.

“We’ve demonstrated a new paradigm for creating complex 3D-ordered structures via self-assembly. If you can build this challenging lattice, the thinking is you can build potentially a variety of desired lattices,” he said.

Today, computer chips are built by stacking layers of different materials and etching patterns into them.

But in the latest issue of Advanced Materials, MIT researchers and their colleagues report the first chip-fabrication technique that enables significantly different materials to be deposited in the same layer. They also report that, using the technique, they have built chips with working versions of all the circuit components necessary to produce a general-purpose computer.

The layers of material in the researchers’ experimental chip are extremely thin — between one and three atoms thick. Consequently, this work could abet efforts to manufacture thin, flexible, transparent computing devices, which could be laminated onto other materials. “The methodology is universal for many kinds of structures,” says Xi Ling, a postdoc in the Research Laboratory of Electronics and one of the paper’s first authors. “This offers us tremendous potential with numerous candidate materials for ultrathin circuit design.”

The technique also has implications for the development of the ultralow-power, high-speed computing devices known as tunneling transistors and, potentially, for the integration of optical components into computer chips.

“It’s a brand new structure, so we should expect some new physics there,” says Yuxuan Lin, a graduate student in electrical engineering and computer science and the paper’s other first author.

Ling and Lin are joined on the paper by Mildred Dresselhaus, an Institute Professor emerita of physics and electrical engineering; Jing Kong, an ITT Career Development Professor of Electrical Engineering; Tomás Palacios, an associate professor of electrical engineering; and by another 10 MIT researchers and two more from Brookhaven National Laboratory and Taiwan’s National Tsing-Hua University.

Strange bedfellows

Computer chips are built from crystalline solids, materials whose atoms are arranged in a regular geometrical pattern known as a crystal lattice. Previously, only materials with closely matched lattices have been deposited laterally in the same layer of a chip. The researchers’ experimental chip, however, uses two materials with very different lattice sizes: molybdenum disulfide and graphene, which is a single-atom-thick layer of carbon.

Moreover, the researchers’ fabrication technique generalizes to any material that, like molybdenum disulfide, combines elements from group six of the periodic table, such as chromium, molybdenum, and tungsten, and elements from group 16, such as sulfur, selenium, and tellurium. Many of these compounds are semiconductors — the type of material that underlies transistor design — and exhibit useful behavior in extremely thin layers.

Graphene, which the researchers chose as their second material, has many remarkable properties. It’s the strongest known material, but it also has the highest known electron mobility, a measure of how rapidly electrons move through it. As such, it’s an excellent candidate for use in thin-film electronics or, indeed, in any nanoscale electronic devices.

To assemble their laterally integrated circuits, the researchers first deposit a layer of graphene on a silicon substrate. Then they etch it away in the regions where they wish to deposit the molybdenum disulfide.

Next, at one end of the substrate, they place a solid bar of a material known as PTAS.

They heat the PTAS and flow a gas across it and across the substrate. The gas carries PTAS molecules with it, and they stick to the exposed silicon but not to the graphene. Wherever the PTAS molecules stick, they catalyze a reaction with another gas that causes a layer of molybdenum disulfide to form.

In previous work, the researchers characterized a range of materials that promote the formation of crystals of other compounds, any of which could be plugged into the process.

A research team at Umeå University in Sweden has showed, for the first time, that a very efficient vertical charge transport in semiconducting polymers is possible by controlled chain and crystallite orientation. These pioneering results, which enhance charge transport in polymers by more than 1,000 times, have implications for organic opto-electronic devices and were recently published in the journal Advanced Materials.

Conjugated semiconducting polymers (plastic) possess exceptional optical and electronic properties, which make them highly attractive in the production of organic opto-electronic devices, such as for instance photovoltaic solar cells (OPV), light emitting diodes (OLED) and lasers.

Polythiophene polymers, such as poly(3-hexylthiophene), P3HT, have been among the most studied semiconducting polymers due to their strong optical absorbance and ease of processing into a thin film from solution. In both OPVs and OLEDs, charges must be transported in the out of plane (vertical) direction inside the polymer film.

However, until now the vertical charge carrier mobility of organic semiconductors, i.e. the ability of charges to move inside the material, has been too low to produce fast charge transport in electronic devices. Faster charge transport can occur along the polymer chain backbone. However, a method to produce controlled chain orientation and high mobility in the vertical direction has remained elusive until now.

In the present work, a team of chemists and materials scientists, led by Professor David R. Barbero at Umeå University, has found a new method to align chains vertically and to produce efficient transport of electric charges through the chain backbone. In this new study, moreover, high charge transport and high mobility were obtained without any chemical doping, which is often used to artificially enhance charge transport in polymers.

“The transport of electric charge is greatly enhanced solely by controlled chain and crystallite orientation inside the film. The mobility measured was approximately one thousand times higher than previously reported in the same organic semiconductor,” says David Barbero.

In what way will these results affect the field of organic electronics?

“We believe these results will impact the fields of polymer solar cells and organic photodiodes, where the charges are transported vertically in the device. Organic-based devices have traditionally been slower and less efficient than inorganic ones (e.g. made of silicon), in part due to the low mobility of organic (plastic) semiconductors. Typically, plastic semiconductors, which are only semi-crystalline, have hole mobilities about 10,000 times lower than doped silicon, which is used in many electronic devices. Now we show it is possible to obtain much higher mobility, and much closer to that of silicon, by controlled vertical chain alignment, and without doping,” says David Barbero.

The charge transport was measured using nanoscopic electrical measurements, and gave a mobility averaging 3.1 cm2/V.s, which is the highest mobility ever measured in P3HT, and which comes close to a theoretical estimation of the maximum mobility in P3HT. Crystallinity and molecular packing characterisation of the polymer was performed by synchrotron X-ray diffraction at Stanford University’s National Accelerator (SLAC) and confirmed that the high mobilities measured were due to the re-orientation of the polymer chains and crystallites, leading to fast charge transport along the polymer backbones.

These results, published in Advanced Materials, may open up the route to produce more efficient organic electronic devices with vertical charge transport (e.g. OPV, OLED, lasers etc.), by a simple and inexpensive method, and without requiring chemical modification of the polymer.

Researchers from North Carolina State University have demonstrated the transfer of triplet exciton energy from semiconductor nanocrystals to surface-bound molecular acceptors, extending the lifetime of the originally prepared excited state by six orders of magnitude. This finding has implications for fields ranging from solar energy conversion to photochemical synthesis to optoelectronics to light therapy for cancer treatment.

Excitons are the electron/hole pairs formed in semiconductor nanocrystals upon absorption of light, temporarily storing it as chemical energy. In solar cells, for example, the excitons transport energy through the material so that it can be collected and converted into electricity.

In terms of photochemistry, the major drawback to using most semiconductor nanocrystals as photosensitizers lies in their short excited state lifetimes — typically tens of nanoseconds — which renders them inadequate to drive photochemical reactions. NC State chemistry professor Felix Castellano, along with postdoc Cedric Mongin and graduate student Sofia Garakyaraghi, wondered if it would be possible to extend the semiconductor nanocrystal excited state lifetime to time scales long enough to perform chemistry.

“The fundamental question was, ‘Can we take a nanoparticle excited state with a lifetime of tens of nanoseconds and extend it through sensitization,'” says Castellano. “If we take the original nanocrystal excited state and transfer its energy to a triplet acceptor on the surface of the nanomaterial, then the molecular triplet excited state you create should have a long enough lifetime to promote chemical reactions. This would also suggest that semiconductor nanocrystals exhibit molecular-like behavior.”

Castellano’s team used cadmium selenide (CdSe) nanocrystals capped with oleic acid, prepared by Prof. Mikhail Zamkov and his graduate student Natalia Razgoniaeva at Bowling Green State University. Some of the oleic acid is then replaced by the molecular triplet acceptor 9-anthacenecarboxylic acid (ACA). When the CdSe nanocrystal bearing ACA is struck with a green laser pulse, the exciton produced in the CdSe is transferred to the ACA, forming a molecular triplet exciton with a millisecond lifetime. This represents a lifetime extension of six orders of magnitude, enabling subsequent chemical reactivity.

“The other benefit is that by translating the exciton away from the nanoparticle surface, instead of involving the nanoparticle itself in the desired chemical reactions, you won’t degrade the nanoparticle,” says Castellano. “It can keep absorbing light and transferring the energy into the bulk solution.”

Manufacture of longer, thinner, and uncontaminated carbon nanotubes, and successfully isolating them, have been ongoing challenges for researchers. A newly developed method has opened up new possibilities in carbon nanotube development.

As recently reported in an article published online at Scientific Reports, researchers at Kyushu University’s Department of Applied Chemistry have developed a method for obtaining high-quality single-walled carbon nanotubes. The relatively mild process uses an outer stimulus to yield undamaged carbon nanotubes that are purer and longer, and even gives researchers the ability to sort nanotubes according to their structure and length.

Hydrogen bonding allows a fluorene based polymer to grow on specific carbon nanotubes. This changes the solubility of the nanotube allowing it to be separated from other types of nanotubes. Credit: International Institute for Carbon-Neutral Energy Research (I²CNER), Kyushu University

Hydrogen bonding allows a fluorene based polymer to grow on specific carbon nanotubes. This changes the solubility of the nanotube allowing it to be separated from other types of nanotubes. Credit: International Institute for Carbon-Neutral Energy Research (I²CNER), Kyushu University

Previous approaches for isolating or sorting nanotubes have required use of more aggressive techniques. These can contaminate the nanotubes and are difficult to completely remove. They also involve processes that could damage the nanotubes and impair their functionality.

“Our approach involves introducing supramolecular hydrogen-bonding polymers, followed by simply shaking the mixture and changing the polarity of the solvent, rather than applying potentially destructive sonication or chemical modification,” says coauthor Naotoshi Nakashima. “In this way, we can obtain single-walled carbon nanotubes over two microns long that do a fine job maintaining structural integrity.”

The new technique is particularly useful because of the mildness and selectivity of the newly designed hydrogen-bonding polymers used. The presence of fluorene moieties within them enables the specific recognition of and binding to single-walled carbon nanotubes, and specific sorting of tubes with a small diameter. This is particularly beneficial because small-diameter nanotubes are exceedingly useful for optoelectronic devices, such as thin-film transistors and sensors.

“The nanotubes we can obtain using this method can be expected to have superior characteristics to those isolated by previous procedures,” says coauthor Fumiyuki Toshimitsu (Visiting Assistant Professor). “For example, by limiting contamination, their electrical and mechanical properties can be optimized. And by being able to sort nanotubes by length or chirality, we can more precisely customize those used for a particular application.”

The same slip-and-stick mechanism that leads to earthquakes is at work on the molecular level in nanoscale materials, where it determines the shear plasticity of the materials, according to scientists at Rice University and the State University of Campinas, Brazil.

The Rice lab of materials scientist Pulickel Ajayan found that random molecules scattered within layers of otherwise pristine graphene affect how the layers interact with each other under strain.

Plasticity is the ability of a material to permanently deform when strained. The Rice researchers, thinking about future things like flexible electronics, decided to see how graphene oxide “paper” would handle shear strain, in which the sheets are pulled by the ends.

Such deep knowledge is important when making novel advanced materials, said Chandra Sekhar Tiwary, a lead author of the new paper in the American Chemical Society journal Nano Letters and a Rice postdoctoral research associate.

“We want to build three-dimensional structures from two-dimensional materials, so this kind of study is useful,” he said. “These structures could be a thermal substrate for electronic devices, they could be filters, they could be sensors or they could be biomedical devices. But if we’re going to use a material, we need to understand how it behaves.”

The graphene oxide paper they tested was a stack of sheets that lay atop each other like pancakes. Oxygen molecules “functionalized” the surfaces, adding roughness to the otherwise atom-thick sheets.

In experiments and computer models, the team found that with gentle, slow stress, the oxides would indeed catch, causing the paper to take on a corrugated form where layers pulled apart. But a higher strain rate makes the material brittle. “The simulation performed by our collaborators in Brazil provides insight and confirms that if you pull it very fast, the layers don’t interact, and only one layer comes out,” Tiwary said.

“After this study, we now know there are some functional groups that are useful and some that are not. With this understanding we can choose the functional groups to make better structures at the molecular level.”