Imperfections running through liquid crystals can be used as miniscule tubing, channeling molecules into specific positions to form new materials and nanoscale structures, according to engineers at the University of Wisconsin-Madison. The discovery could have applications in fields as diverse as electronics and medicine. "By controlling the geometry of the system, we can send these channels from any one point to any other point," says Nicholas Abbott, a UW-Madison professor of chemical and biological engineering. "It's quite a versatile approach." So far, Abbott and his collaborators at UW-Madison's Materials Research Science and Engineering Center (MRSEC) have been able to assemble phospholipids — molecules that can organize into layers in the walls of living cells — within liquid crystal defects. Their technique may also be useful for assembling metallic wires and various semiconducting structures vital to electronics. There's also potential for mimicking the selective abilities of a membrane, designing a defect so that one type of molecule can pass through while others can't. "This is an enabling discovery," Abbott says. "We're not looking for a specific application, but we're showing a versatile method of fabrication that can lead to structures you can't make any other way." The researchers — including UW-Madison graduate students Xiaoguang Wang, Daniel S. Miller and Emre Bukusoglu, and Juan J. de Pablo, a former UW-Madison engineering professor now at the University of Chicago — published details of their advance this week in the journal ("Topological defects in liquid crystals as templates for molecular self-assembly"). For about 20 years, Abbott's research has examined the surfaces of soft materials, including liquid crystals — a particular phase of matter in which liquid-like materials also exhibit some of the molecular organization of solids. "We've done a lot of work in the past at the interfaces of liquid crystals, but we're now looking inside the liquid crystal," he says. "We're looking at how to use the internal structure of liquid crystals to direct the organization of molecules. There's no prior example of using a defect in a liquid crystal to template molecular organization." When the researchers manipulate the geometry of a liquid crystalline system, a variety of different defects can result. Abbott's group assembled liquid crystals with defects shaped like ropes or lines they call "disclinations," that formed templates they could fill with amphiphilic (water- and fat-loving) molecules. Then they can link together assemblies of molecules and remove the liquid crystal templates, leaving behind the amphiphilic building blocks in a lasting, nanoscale structure. The research is an example of how liquid crystal research is taking us from the nano to macro world, says Dan Finotello, program director at the National Science Foundation, which funds the MRSEC. "It is also an exquisite demonstration of MRSEC programs' high impact," Finotello says. "MRSECs bring together several researchers of varied experience and complementary expertise who are then able to advance science at a considerably faster rate."
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Organic electronics with an edge
Using sophisticated theoretical tools, A*STAR researchers have identified a way to construct topological insulators — a new class of spin-active materials — out of planar organic-based complexes rather than toxic inorganic crystals ("Topological insulators based on 2D shape-persistent organic ligand complexes"). The unique crystal structure of topological insulators makes them insulating everywhere, except around their edges. Because the conductivity of these materials is localized into quantized surface states, the current passing through topological insulators acquires special characteristics. For example, it can polarize electron spins into a single orientation — a phenomenon that researchers are exploiting to produce ‘spin–orbit couplings’ that generate magnetic fields for spintronics without the need for external magnets. Many topological insulators are made by repeatedly exfoliating inorganic minerals, such as bismuth tellurides or bismuth selenides, with sticky tape until flat, two-dimensional (2D) sheets appear. “This gives superior properties compared to bulk crystals, but mechanical exfoliation has poor reproducibility,” explains Shuo-Wang Yang from the A*STAR Institute of High Performance Computing. “We proposed to investigate topological insulators based on organic coordination complexes, because these structures are more suitable for traditional wet chemical synthesis than inorganic materials.” Coordination complexes are compounds in which organic molecules known as ligands bind symmetrically around a central metal atom. Yang and his team identified novel ‘shape-persistent’ organic ligand complexes as good candidates for their method. These compounds feature ligands made from small, rigid aromatic rings. By using transition metals to link these organic building blocks into larger rings known as ‘macrocycles’, researchers can construct extended 2D lattices that feature high charge carrier mobility. Pinpointing 2D organic lattices with desirable topological insulator properties is difficult when relying only on experiments. To refine this search, Yang and colleagues used a combination of quantum calculations and band structure simulations to screen the electronic activity of various shape-persistent organic complexes. The team looked for two key factors in their simulations: ligands that can delocalize electrons in a 2D plane similar to graphene and strong spin–orbit coupling between central transition metal nodes and ligands. The researchers’ new family of potential organic topological insulators has a 2D honeycomb macrocycles containing tri-phenyl rings, palladium or platinum metals, and amino linking groups. With promising quantum features and high theoretical stability, these complexes may serve as topological insulators in real world applications. “These materials are easy to fabricate, and cheaper than their inorganic counterparts,” says Yang. “They are also suitable for assembling directly onto semiconductor surfaces, which makes nanoelectronic applications more feasible.”
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A magnetic memory bubbling with opportunity
RIKEN researchers have used ultrafast laser pulses to poke, stretch and position tiny magnetic domains on demand ("Photodrive of magnetic bubbles via magnetoelastic waves"). New technology that exploits the spin of electrons, known as spintronics technology, requires rapid and non-invasive methods to manipulate magnetic fields without generating excess heat. Current approaches focus on the interfaces, or ‘domain walls’, between regions with different ferromagnetic spin—applying a spin-polarized electric current, or spin wave, to such areas produces a twisting force that can be used to flip magnetic bits on or off. 
Fig. 1: A three-dimensional image of a photoexcited ‘magnetoelastic wave’—a combination of spin waves and acoustic vibrations—that can optically manipulate magnetic structures. (Image: Naoki Ogawa, RIKEN Center for Emergent Matter Science) The recent discovery of skyrmions—nanoscale, vortex-like magnetic textures that are more stable and information rich than conventional domains—has heightened the demand for improved control over small-scale domain walls. Ultrafast laser pulses are promising for achieving this control since they can generate effective magnetic fields at the desired location with a sub-micrometer spatial resolution. But they usually only excite a fraction of magnetic sites due to the weak interaction between light photons and spin. Naoki Ogawa and his colleagues at the RIKEN Center for Emergent Matter Science explored an optical manipulation strategy of exploiting optically excited ‘magnetoelastic’ waves—hybridized propagations of collective spins and acoustic vibrations of the crystal lattice (Fig. 1) – rather than conventional spin waves. This strong spin excitation was found to exert forces strong enough to attract magnetic domain walls. The researchers tested their concept on films of iron garnet, an insulator known for its long-lived spin waves and distinctively shaped domains, which include narrow stripes and ‘magnetic bubbles’—cylinder-like domains that exhibit similar properties to skyrmions. On spotting a bubble domain with their microscope, the team focused a laser spot close to it and then gradually increased the laser power. Eventually, they generated magnetoelastic waves that were so powerful that the domains followed the laser spot by moving against the collective spin propagation. This technique also worked on stripe domains, causing them to kink in the middle, or, if an endpoint was targeted, to stretch and bend in the film. “These interactions really depend on the curvature of the domain walls,” explains Ogawa. “Smaller structures with steep curvatures such as bubbles can be manipulated easier than larger domains.” The team notes that magnetic bubbles, which were deployed in nonvolatile memory devices in the 1970s, can now help establish the rules for manipulating skyrmions at the nanoscale. “This work is a proof of concept: it shows that magnetoelastic waves have the potential to deliver enough force to magnetic domains,” says Ogawa.
read more "A magnetic memory bubbling with opportunity"
Fig. 1: A three-dimensional image of a photoexcited ‘magnetoelastic wave’—a combination of spin waves and acoustic vibrations—that can optically manipulate magnetic structures. (Image: Naoki Ogawa, RIKEN Center for Emergent Matter Science) The recent discovery of skyrmions—nanoscale, vortex-like magnetic textures that are more stable and information rich than conventional domains—has heightened the demand for improved control over small-scale domain walls. Ultrafast laser pulses are promising for achieving this control since they can generate effective magnetic fields at the desired location with a sub-micrometer spatial resolution. But they usually only excite a fraction of magnetic sites due to the weak interaction between light photons and spin. Naoki Ogawa and his colleagues at the RIKEN Center for Emergent Matter Science explored an optical manipulation strategy of exploiting optically excited ‘magnetoelastic’ waves—hybridized propagations of collective spins and acoustic vibrations of the crystal lattice (Fig. 1) – rather than conventional spin waves. This strong spin excitation was found to exert forces strong enough to attract magnetic domain walls. The researchers tested their concept on films of iron garnet, an insulator known for its long-lived spin waves and distinctively shaped domains, which include narrow stripes and ‘magnetic bubbles’—cylinder-like domains that exhibit similar properties to skyrmions. On spotting a bubble domain with their microscope, the team focused a laser spot close to it and then gradually increased the laser power. Eventually, they generated magnetoelastic waves that were so powerful that the domains followed the laser spot by moving against the collective spin propagation. This technique also worked on stripe domains, causing them to kink in the middle, or, if an endpoint was targeted, to stretch and bend in the film. “These interactions really depend on the curvature of the domain walls,” explains Ogawa. “Smaller structures with steep curvatures such as bubbles can be manipulated easier than larger domains.” The team notes that magnetic bubbles, which were deployed in nonvolatile memory devices in the 1970s, can now help establish the rules for manipulating skyrmions at the nanoscale. “This work is a proof of concept: it shows that magnetoelastic waves have the potential to deliver enough force to magnetic domains,” says Ogawa.
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