Introducing flaws into liquid crystals by inserting microspheres and then controlling them with electrical fields: that, in a nutshell, is the rationale behind a method that could be exploited for a new generation of advanced materials, potentially useful for optical technologies, electronic displays and e-readers. A team of scientists (including research fellows at the International School for Advanced Studies, SISSA, in Trieste) has just published a paper in the journal ("Field-controlled columnar and planar patterning of cholesteric colloids") where they describe just how this approach works and provide the results of a computer simulation. 
Colloids in liquid crystals. “Generally, flaws are the last thing you’d want in a liquid crystal”, explains Giuseppe D’Adamo, postdoctoral fellow at SISSA. “However, this new method allows us to exploit the defects in the material to our advantage”. D’Adamo is first author of a paper just published in Physical Review Letters. The study made computer models of colloidal suspensions in liquid crystals subjected to electrical fields modulated over time. Colloids are particles in suspension (i.e., a condition halfway between dispersion and solution) in a liquid. These composite materials have been receiving plenty of attention for their optical properties for some time now, but the use of electrical fields to modify them at will is an absolute novelty. “Our simulations demonstrate that by switching on or off an electrical field of appropriate intensity we can re-order the colloids by arranging them into columns or planes”, comments Cristian Micheletti of SISSA, co-author of the paper. “This easy-to-control plasticity could make the material suitable for optical-electronic devices such as e-readers, for example”. Liquid crystals are particular types of liquids. In a normal liquid, molecules have no systematic arrangement and, viewed from any angle, they always appear the same. The molecules forming liquid crystals, by contrast, are arranged in precise patterns often dictated by their shape. To get an idea of what happens in a liquid crystal, imagine a fluid made up of tiny needles which, instead of being arranged chaotically, all point in the same direction. This also means that if we look at the liquid from different viewpoints it will change in appearance, for example it might appear lighter or darker (have you ever seen this happen in LCD monitors, especially the older models?). “The useful natural tendency of liquid crystal molecules to spontaneously arrange themselves in a certain pattern can be counteracted by introducing colloids in the fluid. In our case, we used microscopic spherical particles, which ‘force’ the molecules coming into contact with their surface to adapt and rotate in a different direction” explains D’Adamo. “This creates ‘defect lines’ in the material, i.e., circumscribed variations in the orientation of molecules which result in a local change in the optical properties of the medium”. More in detail... These defect lines have an important effect: they enable remote interactions among colloidal particles, by holding them together as if they were thin strings. “Liquid crystal molecules tend to align along the electrical field. By switching the field on and off we create competition between the spontaneous order of the liquid crystal, the order dictated by the surface of the colloidal particles and, finally, the order created by the electrical potential”, says Micheletti. “This competition produces many defect lines that act on the colloids by moving them or clustering them”. “It’s a bit like pulling the invisible strings of a puppet: by carefully modulating the electrical fields we can, in principle, make all the particles move and arrange them as we like, by creating defect lines with the shape we want” continues D’Adamo. “An important detail is that the colloidal configurations are metastable, which means that once the electrical field has been switched off the colloids remain in their last position for a very long time”. In brief, this implies that the system only requires energy when it changes configuration, a major saving. “In this respect, the method works like the electronic ink used in digital readers, and it would be interesting to explore its applicability in this sense”, concludes Micheletti. The study, carried out with the collaboration of SISSA, the University of Edinburgh and the University of Padova, has been included as an Editors’ Suggestion among the Highlights of the journal .
read more "Desirable defects in liquid crystals"
Colloids in liquid crystals. “Generally, flaws are the last thing you’d want in a liquid crystal”, explains Giuseppe D’Adamo, postdoctoral fellow at SISSA. “However, this new method allows us to exploit the defects in the material to our advantage”. D’Adamo is first author of a paper just published in Physical Review Letters. The study made computer models of colloidal suspensions in liquid crystals subjected to electrical fields modulated over time. Colloids are particles in suspension (i.e., a condition halfway between dispersion and solution) in a liquid. These composite materials have been receiving plenty of attention for their optical properties for some time now, but the use of electrical fields to modify them at will is an absolute novelty. “Our simulations demonstrate that by switching on or off an electrical field of appropriate intensity we can re-order the colloids by arranging them into columns or planes”, comments Cristian Micheletti of SISSA, co-author of the paper. “This easy-to-control plasticity could make the material suitable for optical-electronic devices such as e-readers, for example”. Liquid crystals are particular types of liquids. In a normal liquid, molecules have no systematic arrangement and, viewed from any angle, they always appear the same. The molecules forming liquid crystals, by contrast, are arranged in precise patterns often dictated by their shape. To get an idea of what happens in a liquid crystal, imagine a fluid made up of tiny needles which, instead of being arranged chaotically, all point in the same direction. This also means that if we look at the liquid from different viewpoints it will change in appearance, for example it might appear lighter or darker (have you ever seen this happen in LCD monitors, especially the older models?). “The useful natural tendency of liquid crystal molecules to spontaneously arrange themselves in a certain pattern can be counteracted by introducing colloids in the fluid. In our case, we used microscopic spherical particles, which ‘force’ the molecules coming into contact with their surface to adapt and rotate in a different direction” explains D’Adamo. “This creates ‘defect lines’ in the material, i.e., circumscribed variations in the orientation of molecules which result in a local change in the optical properties of the medium”. More in detail... These defect lines have an important effect: they enable remote interactions among colloidal particles, by holding them together as if they were thin strings. “Liquid crystal molecules tend to align along the electrical field. By switching the field on and off we create competition between the spontaneous order of the liquid crystal, the order dictated by the surface of the colloidal particles and, finally, the order created by the electrical potential”, says Micheletti. “This competition produces many defect lines that act on the colloids by moving them or clustering them”. “It’s a bit like pulling the invisible strings of a puppet: by carefully modulating the electrical fields we can, in principle, make all the particles move and arrange them as we like, by creating defect lines with the shape we want” continues D’Adamo. “An important detail is that the colloidal configurations are metastable, which means that once the electrical field has been switched off the colloids remain in their last position for a very long time”. In brief, this implies that the system only requires energy when it changes configuration, a major saving. “In this respect, the method works like the electronic ink used in digital readers, and it would be interesting to explore its applicability in this sense”, concludes Micheletti. The study, carried out with the collaboration of SISSA, the University of Edinburgh and the University of Padova, has been included as an Editors’ Suggestion among the Highlights of the journal .
Two nanocylinders produce resonant electric (E) and magnetic (H) fields when excited with visible light. Earlier theoretical work had predicted the existence of such magnetic hot spots, but this is the first time that they have been observed experimentally with visible light in a nanodimer configuration (see image), according to lead author Reuben Bakker from the A*STAR Data Storage Institute. The researchers numerically calculated the expected electric and magnetic resonances and found good agreement with the experimental results. The use of light to carry information, known as photonics, is critical to the continued growth of information technology. Unfortunately, the diffraction limit of light restricts it from being directed at dimensions smaller than half its wavelength, which imposes a limit on the minimum sizes of photonics-based devices. The use of plasmon resonances in metals — resonant collective oscillations of conduction electrons — has been proposed as a way to overcome this limit. However, metals that support plasmons are often ‘lossy’, which means that the distance the light can travel in them is quite limited. “Typically in metal photonics, researchers have been studying the electric field,” says Bakker. “But we are now looking at materials in the subwavelength regime (below the diffraction limit), where we can create and manipulate the magnetic field as well. Essentially, the electric field creates a current loop inside the nanoparticle and this current loop creates the magnetic resonance.” Being able to manipulate the magnetic field close to the nanodimer provides “another lever to pull so that light does what we want it to do,” says Bakker. To exploit this effect, the nanoparticles need to be made of a high-dielectric-constant material, such as silicon. “We’ve taken the silicon direction because it has a high refractive index and doesn’t have the losses that metals do,” says Bakker. “But silicon may not be the final answer. We know how to work with silicon because of the integrated circuit industry and it is good — but is it the best? We’re still figuring that out.” Bakker sees this work as a step toward more complex systems that could potentially end up as being nanoantennas or waveguide systems. “This nanodimer is an intermediary — it’s not the most useful device in itself. We have to build up our understanding of these systems on an incremental basis,” he says.
Analysis of the electric field inside a semiconductor microparticle containing a metal nanoparticle reveals enhanced absorption of sunlight. Photocatalysts accelerate chemical reactions by absorbing light from the sun and using the energy to drive reactions on their surfaces. They are attractive for environmentally friendly applications such as generating hydrogen from water and breaking down pollutants. Experimental studies have shown that microspheres made from metal-oxide semiconductors and embedded with metal nanoparticles are particularly effective photocatalysts, but researchers have been uncertain about why this was the case. Now, Ping Bai and his colleagues at the A*STAR Institute of High Performance Computing in Singapore have performed computer simulations that reveal what makes these structures such effective photocatalysts. Their study also provides scientists with helpful guidelines for designing plasmonic photocatalysts. Bai and his colleagues used a widely employed computational technique known as the finite element method to analyze how light interacts with a semiconductor microparticle containing a single metal nanoparticle. Their analysis revealed that the refractive index difference between the semiconductor and the catalytic medium sets up an interference pattern within the semiconductor microparticle. This interference enhances the light absorption of the embedded metal nanoparticles as a result of plasmon resonance (see image). As a consequence, the microspheres with embedded metal nanoparticles drive chemical reactions by harnessing solar energy much more efficiently than other commonly used photocatalyst structures. “The broadband absorption enhancement exists everywhere inside the microspheres,” explains Bai, “and the maximum enhancement can be hundred times greater than that of metal nanoparticles or small core–shell photocatalysts.” This explains their superior catalytic rates measured in previous experiments. In addition to explaining previous experimental findings, the analysis can also be used to inform the design of photocatalysts. In particular, it suggests that using semiconductors with higher refractive indices will maximize the broadband absorption induced by the interference, while using a mix of different plasmonic nanoparticles will enable flexible energy harvesting and enhanced selectivity. Finally, the findings also imply that locating the metal nanoparticles close to the surfaces of the microspheres will increase the catalytic rate as a consequence of the very short range of the plasmon near field. Bai and his team are now seeking to join forces with others working in the field. “Our next step is to look for end users and experimental collaborators to design, optimize and fabricate particular photocatalysts,” says Bai.