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Research summaries for 2018

Smart materials: Delivering drops to their destination

Inspired by the air sacs of insects, a team in China has developed a smart surface that can deliver liquid droplets to a desired location. Wettability is a measure of a material's ability to absorb or repulse fluid on its surface. Smart materials whose wettability can be controlled have the potential to manipulate fluids at the micrometer scale. Such control can be achieved by altering the chemical composition of the surface, but this can be slow. Instead, Hong-Bo Sun and colleagues from Jilin University and co-workers have designed and fabricated a smart material in which the surface is changed using pneumatic micro-air-sacs. When a water droplet comes into contact with the inflated surface, it is captured and transferred to the desired location. Deflating the micro-air-sacs releases the droplet within a second.

NPG Asia Mater Research Summary. Published online 16 February 2018

Transition-metal dichalcogenides: Bringing layers into line

A method for investigating the relative alignment of stacks of two-dimensional layers has been developed by researchers in Korea. Two-dimensional materials, those just a single atom thick, have a host of unusual electronic and optical properties. Placing two-dimensional materials on top of each other to create thicker films offers a way to engineer further novel materials, but the properties of these stacks depend crucially on the relative orientation of each layer. Jong-Hyun Ahn from Yonsei University and colleagues synthesized stacks of molybdenum disulfide monolayers with various angular alignments using a technique called atmospheric-pressure chemical vapor deposition. They then characterized the structures optically and measured a gradual spectroscopic evolution induced by the varying the arrangement. This insight will be useful for designing future optoelectronic devices based on molybdenum disulfide monolayers.

NPG Asia Mater Research Summary. Published online 09 February 2018

Interphase boundaries: Mismatched oxides create electric opportunities

Tin oxide semiconductors can turn metallic at customized interfaces between two crystal phases, a theoretical study has found. Inspired by recent discoveries of surprisingly conductive regions formed when different insulators are sandwiched together, Udo Schwingenschlögl and two colleagues from KAUST in Saudi Arabia investigated whether similar properties could emerge at phase boundaries inside tin oxide materials. Density functional calculations of tin monoxide-dioxide interfaces indicate that certain polar arrangements generate desirable, high-mobility quantum gases even though the two sides of the boundary have the same formal charge – typically, abrupt charge changes are needed to create two-dimensional metallic states. The team proposed that this unexpected behavior arises from charge density discontinuities due to the mismatched lattices of tin monoxide and dioxide.

NPG Asia Mater Research Summary. Published online 09 February 2018

Graphene: Everything in its right place

Strict control of temperature and pressure during annealing is shown to be crucial for controllably stacking graphene by a team in Japan. Graphene is a single layer of carbon atoms in a hexagonal arrangement. When two graphene layers are stacked, some carbon atoms in the top layer sit above the center of each hexagon of the lower layer. A third layer either sits directly aligned with the bottom layer (ABA stacking) or is again displaced from the previous two (ABC stacking). These two arrangements exhibit very different properties. Takashi Takahashi from Tohoku University and colleagues deterministically created either ABA or ABC trilayer graphene by first annealing bilayer graphene on a hydrogen-terminated silicon carbide substrate. They found that the alignment of the third layer is determined by the temperature and pressure of this anneal.

NPG Asia Mater Research Summary. Published online 09 February 2018

Spintronics: Semiconductors get into the spin of things

Non-magnetic semiconductors could be used to create spintronic devices with ultralow power consumptions, predicted by Chinese scientists. Spintronics is an emerging technology that processes information encoded in the spin on electrons. Recent years have also seen the development of topological insulators — materials that conduct a current across their surface with little loss. To merge these two concepts, Zhongqin Yang from Fudan University and her teammates simulated the properties of films of the semiconductor copper sulfide on the surface of manganese selenide. They showed that this creates a topological Chern insulator in which the low-loss surface states are fully spin polarized. Most existing spintronic materials are magnetic, but the team's simulation indicates that ordinary semiconductors could also be useful, opening more possibilities of materials that can realize completely spin polarized, high-speed spintronic devices.

NPG Asia Mater Research Summary. Published online 09 February 2018

Hydrogels: Bursting cancer's bubble

A hydrogel that uses physical force rather than drugs or radiation to kill breast and lung cancer cells has been developed. The approach by Siowling Soh at the National University of Singapore and colleagues exploits the temperature responsiveness of soft, watery polymers known as poly(N-isopropylacrylamides). The researchers coated the hydrogel with cell-adhering arginine-glycine-aspartate peptides, using a dopamine and lysine co-polymer to anchor the biomolecules. After seeding cancer cells onto the peptide-coated hydrogel, they dropped sample temperatures from 37°C to 22°C. The hydrogel expanded due to a phase transition, providing a force large enough to rupture the cancer cells attached to the surface — an easy-to-implement strategy effective for particles made of these coated hydrogels with sizes ranging from micrometers to millimeters. Measurements revealed the hydrogels can apply enough force to rupture a variety of cell types.

NPG Asia Mater Research Summary. Published online 02 February 2018

Chiral magnets: Additional asymmetry shown to be chiral in nature

Fast-moving domain walls separating regions with spin-up or spin-down states are desirable for high-performance spintronic memory and logic devices. The Dzyaloshinskii–Moriya interaction gives rise to an asymmetric distribution of domain wall speeds in chiral magnetic materials, providing a convenient way to determine the strength of the interaction. However, the recent discovery of additional asymmetries has complicated the analysis of measurements. Now, Sug-Bong Choe from Seoul National University and co-workers have experimentally determined the chiral nature of the additional symmetry for the first time. This contribution, which could arise from either a form of energy dissipation called chiral damping or from natural variations in domain wall widths, can alter domain wall speeds by a factor of 100.

NPG Asia Mater Research Summary. Published online 19 January 2018

Fuel cell catalysts: Singling out iron for success

Inexpensive carbon nanotubes containing iron and sacrificial zinc atoms may widen the use of fuel cells in alternative energy applications. The long driving ranges of fuel-cell-powered vehicles are tempered by a reliance on pricy platinum catalysts for converting oxygen and hydrogen into water. An international team led by Chang Liu from the Institute of Metal Research in China has now developed an improved way of synthesizing iron–nitrogen–carbon catalysts that promises to eliminate expensive noble metals from fuel cells. The team turned nitrogen-doped carbon nanotubes into materials with uniform micropores that can host high amounts of single iron atoms – a key contributor to catalytic activity – using thermally sensitive, atom-isolating agents such as zinc. This strategy produced flexible carbon films or cloths that outperformed commercial platinum-based catalysts in terms of both cost and energy usage.

NPG Asia Mater Research Summary. Published online 12 January 2018

Plasmonics: Winging a way to improved reproducibility

Inspired by butterfly wings, a team in China has developed by a technique that enables nanotextured gold surfaces to be reproducibly made. Metal nanostructures can enhance the interaction between light and matter, making them useful for the sensitive optical detection of molecules in a technique called surface-enhanced Raman spectroscopy (SERS). But practical application of this concept requires substrates that can be and behave uniformly across their surfaces. Now, Wang Zhang from Shanghai Jiao Tong University and co-workers has fabricated a SERS substrate using a template based on the wing scales of the green hairstreak butterfly. Its surface consists of an array of three-dimensional gyroid structures. The substrates, which were reproducible, uniform and highly sensitive — enhancing the detection signal by up to a factor of one billion — are promising for highly sensitive molecular detection.

NPG Asia Mater Research Summary. Published online 12 January 2018

Biosensing: A brighter combination

A hybrid material with excellent molecule-sensing abilities has been created by scientists in Israel. Nanomaterials are ideal for optically identifying biomolecules. For example, carbon quantum dots emit light in a way that is highly sensitive to their environment. In contrast, nanoarchitectures based on porous silicon oxide offer versatile biosensing platforms based on light reflectivity but lack the sensitivity of quantum dots. Now, Ester Segal from the Technion Israel Institute of Technology and co-workers have developed a simple way to combine both materials. They formed carbon dots by inserting a carbon precursor into a porous silicon oxide matrix and heating it. This allowed the team to synthesize biocompatible nanomaterials for label-free detection using two independent optical signals. They used their material to detect trypsin and adenosine triphosphate with a better performance than either of the two materials on their own.

NPG Asia Mater Research Summary. Published online 12 January 2018

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