Nano News: 08/17/15

New functional photonic materials and devices based on magnetism

Digital information technology has great impact on our lives that has strongly motivated scientists to look for faster and more energy-efficient ways to process streams of digital signals. Recently, electrical engineers has proposed using not just electricity but also light to send the signals even inside semiconductor chips, and condensed matter physicists are studying ways of exploiting the spin instead of the charge of electrons for electronic devices. Notably, nothing travels faster than light in our universe and spins generates far less heat during the transmission of magnetic signals. However, there is major problem when both light and spins meet to form a powerful tag team: interaction between light and spins is weaker than that between light and charges. Now, members of the spin-photonics team at Tokyo Tech including KosukeYamamoto, a master's course graduate student at the School of Interdisciplinary Science and Engineering, Yoshitaka Kitamoto, professor at the same school, and Hiro Munekata, professor at the Imaging Science and Engineering Laboratory (ISEL) and the leader of the team, have found through the study of photo-excited precession of magnetization using ultrashort (10-13 sec) weak laser pulses of 1µJ/cm2 or less, that spins in ultra-thin Co/Pd multi-layer films are very susceptible to light; namely, a material that could be a candidate for photo-sensitive magnets ("Low-power photo-induced precession of magnetization in ultra-thin Co/Pd multi-layer films"). These findings were soon followed by the demonstration of polarization modulation of light signals in an optical waveguide with the same class of magnets, where this work was carried out by Kazuhiro Nishibayashi, a lecturer at ISEL and member of spin-photonic team, in collaboration with Hitoki Yoneda, professor at the University of Electro-Communications, and Atsushi Kuga, at the researcher of Science and Technology Research Laboratories, Japan Broadcasting Corporation. In this work ("Demonstration of polarization modulated signals in a multi-mode GdFe-silica hybrid fiber"), Nishibayashi has emphasized the feasibility of the multiplexed transmission of polarization-modulated signals, controlled ultimately by photo-excitation of a class of light-sensitive magnetic layers.

Experimental data of photo-excited precession of magnetization (left), schematic illustration of Co/Pd ultra-thin multi-layers (upper center), and the concept of three-terminal photonic device utilizing photo-magnetic property (lower right). (click on image to enlarge) Munekata and Kitamoto started their work by discussing a class of materials in which interaction between electron orbitals and spin is strong, and later, when Yamamoto joined them, they decided to focus on the interface of Co and Pd, where the spin states are strongly sensitive to the slight charge imbalance at the Co/Pd interface. Illumination with a femto-second laser pulse in the ultra-short time regime, enabled them to successfully modulate the charge imbalance and vary the direction of spin ordering instantaneously. This has given rise to clear observations of oscillatory signals due to precession of ordered spins. It is very common to use a light beam propagating in free space for studying the interactions between light and matter. In an optical waveguide, however, light propagates through strong and weak propagation channels-so called mode-propagation of light- as a consequence of interference induced by light partially reflected at sidewalls of the waveguide. Therefore, it is not straight forward to engineer the interaction between light and spins with light propagating through an optical waveguide. Experimental results obtained by Nishibayashi and colleagues show that specific points of spin in a magnetic film interact with selected modes of light, which is one of the unique points found in a magnet-waveguide system. Inducement of periodic motion of magnetization (ordered spins) with weak laser pulses would enable us to control the polarization plane and group velocity of optical digital signals which propagate in the vicinity of magnetization. Not only multiplexing/de-multiplexing, but various applications such as optical memory (random access type) and signal delay lines (buffer memory), may also be developed by combining photo-sensitive magnetic layers and optical waveguides.

Unusual magnetic behavior observed at a material interface

An exotic kind of magnetic behavior, driven by the mere proximity of two materials, has been analyzed by a team of researchers at MIT and elsewhere using a technique called spin-polarized neutron reflectometry. They say the new finding could be used to probe a variety of exotic physical phenomena, and could ultimately be used to produce key components of future quantum computers. The novel phenomenon occurs at the boundary between a ferromagnet and a type of material called a topological insulator, which blocks electricity from flowing through all of its bulk but whose surface is, by contrast, a very good electrical conductor. In the new work, a layer of topological insulator material is bonded to a ferromagnetic layer. Where the two materials meet, an effect takes place called proximity-driven magnetic order, producing a localized and controllable magnetic pattern at the interface.

This diagram shows the layered structure analyzed for its magnetic properties. Yellow spheres represent tellurium atoms; light blue spheres represent antimony-bismuth; and black spheres represent sulfur. The black sphere with an arrow represents an atom of dopant, and green spheres with arrows show atoms of europium. Different colored arrows show various ways an europium ion can be affected by the interface between the materials: within the plane via Heisenberg interaction (orange), between the planes (green) through super-exchange interaction, or spin-polarized states at the topological insulator surface (blue). (Image courtesy of the researchers) The research is described in a paper appearing this week in the journal ("Proximity-Driven Enhanced Magnetic Order at Ferromagnetic-Insulator–Magnetic-Topological-Insulator Interface"), written by MIT doctoral student Mingda Li, postdoc Cui-Zu Chang, professor of nuclear science and engineering Ju Li, senior scientist Jagadeesh Moodera, and seven others. This “proximity magnetism” effect could create an energy gap, a necessary feature for transistors, in a topological insulator, making it possible to turn a device off and on as a potential building block for spintronics, says Mingda Li, the lead author of the paper. “However, the proximity effect is usually weak,” he says, without this team’s use of a magnetic topological insulator “to enhance it and lock new magnetic order near the interface.” “This could be a building block of quantum computers,” Moodera says. “It also opens up some fundamental new phenomena” for study by physicists. “The interaction at the interface makes this exotic phenomenon possible,” Chang adds. One of the new findings of this research is that the magnetism induced by the proximity of the two materials is not just at the surface, but actually extends into the interior of the topological insulator material. “We were able to show that the magnetism exists inside the topological insulator,” Moodera says. Possible applications of the new findings include the creation of spintronics, transistors based on the spin of particles rather than their charge. These are expected to have low energy dissipation if based on topological insulators, and are a very active area of research. Because the interface produces a channel with virtually no dissipation, it can act as “a perfect quantum wire,” Ju Li says. “It cannot be better than that for a quantum conductance channel. So having this precise control of the magnetic structure could lead to novel quantum spintronics.” He adds that the findings, in addition to near-term practical applications, “from a physics point of view, opens up a huge area of productive work.” This includes the study of predicted physical phenomena such as Majorana fermions, particles predicted in 1937 but not yet observed, which, unlike all known subatomic particles, serve as their own antiparticles. These theorized particles “have yet to be explored. It opens another avenue to explore these things,” he says. “The significance of this work is threefold,” says Qi-Kun Xue, a professor of physics at Tsinghua University in China who was not involved in this work. The three areas, he says, are “to demonstrate proximity magnetism, the enhancement of such magnetism … and the tunability of this interfacial magnetic structure.” He adds that the finding is a significant step toward a device application of magnetic topological insulators. “In particular,” he explains, “the consistent results from independent experimental tools” make the results “robust.”

The world's first electrically powered optical nanoantenna

Using electricity to make a nanoantenna emit light: this is what physicists from the University of Würzburg have accomplished in a world first. They present their antenna measuring just 250 nanometres in ("Electrically driven optical antennas"). optical nanoantenna

A gold particle at its centre: The world's first electrically powered optical antenna. Electrically powered miniature light sources will be useful in the future, for example, in smartphone displays. Given the increased integration of 3D techniques, the required pixel density should soar in these devices. Nano light sources could also be used on computer chips for low-loss data exchange between processor cores at the speed of light. Würzburg physicists have shown a way to implementing such light sources in a pioneering piece of work: In the magazine "Nature Photonics" they describe for the first time how light is generated using an electrically powered nanoantenna made of gold. The antenna was developed by Professor Bert Hecht's team and at the Department of Technical Physics. Applying the laws of antenna technology with light How does an optical antenna work? "In principle, it works in the same way as its big sister, the radio antenna," Bert Hecht explains: An AC voltage is applied that causes electrons in the metal to oscillate. As a result, the antennas emit electromagnetic waves of a defined shape and wavelength specified by the antenna geometry. Transferring the laws of antenna technology to light at nano scale is technically challenging. So the Würzburg physicists had to come up with an idea. In the end, a sophisticated nano structure assured their success: Its optical antenna has two arms each fitted with a contact wire and whose ends almost touch. The tiny space between the arms is prepared with a gold nano particle which touches the one arm and leaves a gap of about one nanometre to the other arm. The gap is so small that electrons can bridge it due to the quantum mechanical tunnel effect when applying voltage, causing oscillations with optical frequencies. The length of the antenna arms determines the colour of the light The antenna thus constructed emits electromagnetic waves in the form of visible light. The colour of the light is determined by the length of the antenna arms. "This has allowed us to build the world's most compact electrically powered light source to date whose properties can moreover be controlled by adjusting the antenna geometry," Hecht further. So in principle, such antennas can be built; however, much work needs yet to be done until they are ready for use. For one thing, the physicists have to further improve the efficiency: Too much electricity is lost as heat when operating the optical antenna. For another, the operational stability has to be increased, as the golden nano structure has only worked for a few hours so far.

In catalysis, a single atom makes all the difference

Fuel cell technology offers a highly efficient and environmentally safe method of electricity generation. However, many of these cells require expensive Platinum catalysts. Therefore, to reduce the costs of fuel cells, more efficient catalysts must be synthesized. Metal clusters containing small numbers of atoms are an exciting class of material that may offer catalytic performance superior to other coarse and ultrafine particles. Highly stable metal particles with a set number of atoms, known as 'magic number' clusters, are relatively easily synthesized. However, it is thought that less stable, 'non-magic-number' clusters may exhibit enhanced catalytic activity. A single atom makes all the difference Unfortunately, investigation of these non-magic-number clusters has been hampered by the lack of a synthesis method that has single-atom precision. Kimihisa Yamamoto and co-workers at Tokyo Institute of Technology have now developed a new method, which allows fine control over the number of atoms in platinum clusters using branching molecules known as dendrimers ("Finding the most catalytically active platinum clusters with low-atomicity"). The effect of the number of atoms per cluster on catalytic performance was also investigated using the oxygen reduction reaction, an important reaction for energy conversion systems such as fuel cells. Yamamoto's group previously accomplished synthesis of Pt12 and Pt13 clusters. Improving on this method, they can now prepare Pt12 through to Pt24 clusters with remarkable precision, by simply adjusting the amounts of dendrimer and metal precursor. This was demonstrated using two different dendrimers via stepwise and random routes.

Schematic of the stepwise dendrimer-templating strategy for synthesizing platinum clusters with a controlled number of atoms. New synthesis method yields improved catalytic materials The catalytic performance varied considerably with the number of atoms in the clusters, with Pt19 exhibiting the best performance. The experimental results and theoretical analysis reveal that larger clusters feature Pt13 cores with excess Pt atoms distributed at the edges. While symmetrical, magic-number Pt13 clusters have low catalytic activity, the addition of edge Pt atoms to this stable core produces clusters with unexpectedly high catalytic activity. The changes in activity are related to changes in the shape of the cluster that depend on the number of atoms. Fueling catalytic performance This work not only presents a synthesis route to prepare platinum clusters with single-atom control, but also sheds new light on structure-property relationships in metal clusters and highlights the great potential of non-magic-number clusters as high-performance catalysts. The new synthetic method presented and the catalytic performance measurements within the study are important steps in improving the performance of fuel cells.

Is graphene hydrophobic or hydrophilic?

The National Physical Laboratory's (NPL) Quantum Detection Group has just published research investigating the hydrophobicity of epitaxial graphene, which could be used in the future to better tailor graphene coatings to applications in medicine, electronics and more. Contrary to widely-held beliefs, the findings indicate that graphene's hydrophobicity is strongly thickness-dependent, with single-layer graphene being significantly more hydrophilic than its thicker counterparts. Graphene is a two-dimensional crystal of carbon, with many potential applications such as flexible electronics, efficient transistors and novel sensors. To encourage the uptake of graphene for electrical applications, the challenges of large-scale production and control of its properties under a variety of environmental conditions need to be addressed. Many graphene-based devices will have to operate in ambient conditions where humidity is non-zero and not monitored. Air humidity can affect graphene's performance through changes in its mechanical and electrical properties - it is therefore critically important to obtain knowledge of graphene's water affinity. The new study, conducted in collaboration with the Naval Research Laboratory, addresses the much-debated question of whether graphene is hydrophobic or hydrophilic. The common assumption is that graphene, as with many other carbon-based materials, is hydrophobic. This work, published in the American Chemical Society journal ("Thickness-Dependent Hydrophobicity of Epitaxial Graphene"), has proved the question to be much more complicated than first thought. The adhesion and friction properties of single- and double-layer graphene were studied using chemical force microscopy with a hydrophobic probe - a variant of atomic force microscopy where a substrate is studied using the forces between a probe and a surface. A larger adhesion force was measured between the probe and double/triple-layer graphene compared to single-layer graphene, showing that double/triple-layer graphene is more hydrophobic. This suggests that the hydrophobicity depends on the thickness of graphene layers. These results were further confirmed by the nanoscale mapping of friction forces: hydrophobic domains showed a lower friction force, a result consistent with the fact that the different levels of hydrophobicity tend to affect the arrangement of surrounding water molecules and, in turn, the sliding motion of the probe tip. The techniques demonstrated by NPL could be used in the future to further our understanding of graphene's wetting behaviour, with a particular focus on the effects of different graphene production methods. In particular, it paves the way to differentiating graphene-based coatings and tailoring them to a specific application. For example, thicker coatings (double-layer graphene or more) are ideal for hydrophobic applications, such as medical equipment and electronic components. On the other hand, single-layer graphene coatings could be used where a hydrophilic surface is required, as for example in anti-fog glass and coatings for buildings.