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NANOGOLD Project article by EU Research Autumn 2012 Issue

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The building blocks

of metamaterials Exploring a new concept for the fabrication of metamaterials allows researchers to control their effective properties in an unprecedented manner, opening up new potential applications. We spoke to Toralf Scharf of the NANOGOLD project about their work in developing a metamaterial ink to produce materials with specific electromagnetic properties The use of amorphous structures with structural dimensions that are typically much shorter than the wavelength of the operating electro-magnetic radiation could help researchers explore and analyse metamaterials in greater detail than ever before. This area forms the primary research focus of the NANOGOLD project, an EU-funded initiative which aims to fabricate and apply bulk threedimensional metamaterials. “The basic idea is to make a kind of metamaterial ink, like a type of paint, which allows you to produce a material with non-conventional electro-magnetic properties. This is in the form of a liquid which could then be dried or processed to produce a stable and durable material, but with a certain

volume,” explains Dr Toralf Scharf, the project’s scientific coordinator. “The ground-breaking aspect of our material is that it’s not just a single surface – it will be something that has a truly threedimensional volumetric structure. But it doesn’t necessarily have to be included in a container – it could be a thin film with a finite thickness that possesses bulk properties. This allows us to create electromagnetic materials with high efficiency and to observe effects usually associated only with the realms of fantasy.”

Self assembly The project is following an interdisciplinary approach to form these

metamaterials, bringing together elements of organic chemistry, physics and liquid crystal technology. The starting point is the use of resonant entities, such as metal nanoparticles, to develop composite metamaterials with specific electromagnetic properties. “We introduce the metal nanoparticles directly into an organic molecule. And then we let these hybrid molecules self-assemble via intermolecular interaction,” says Dr Scharf. These hybrid molecules should contain both the metal as the active plasmonic entity and organic molecules in the form of mesogens; effectively they will need to be macro-molecules, as they have to be relatively large to carry and wrap the metal nano-particles. “This metal

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nanoparticle is a resonant entity in the hybrid molecule – it has a resonance frequency at which it interacts in an extreme manner with light,” explains Dr Scharf. “You can take these hybrid molecules and assemble them further on, either by self-organisation of the molecules themselves or with other techniques, to create second-level morphology. The second level morphology will also interact with light and can lead to additional interference phenomena. This interference is structure-driven and not molecule-driven.” This structure–driven interference, and the molecular resonance which comes from the metal entity, leads to particular features of the material. Combining resonance and interference effects allows the project to use different sets of parameters, which Dr Scharf says illustrates the strength of the NANOGOLD concept. “The hybrid molecules form an entire material that already has particular electromagnetic properties, on both the nanoparticle and the organic molecular level. In particular this material has a high load of resonant nanoparticles, a feature needed to explore resonances and their coupling. You can use such an entire material and integrate more functionalities; you can put in functionalities like fluorescence or absorption,” he outlines. This work is based on analysis of both the individual components – the hybrid molecules – and the material as a whole. “We have a kind of technology pool, where we use the material made of hybrid molecules to form the structural components and to go to the second level morphology,” says Dr Scharf. “Then we use the theory to simulate the optical properties and guide us in the most effective direction in terms of material realisation.” A variety of techniques can be used to analyse composite materials, such as reverse-engineering, which can help researchers understand how the material properties can be modified and enhanced. However, Dr Scharf says it is extremely challenging to control self-organisation on the molecular level, partly because of the size of the hybrid molecules. “They’re huge, the molecular interaction is complicated, and it’s extremely hard to predict the kind of self-organisation phase that will occur,” he explains. With larger molecules it is difficult to find evidence of self-organisation; the major challenge is that the metallic entities are relatively big

compared to the molecular attachments (the mesogens) that are used for creating the macro-molecules. “The core is a kind of metallic sphere which you attach molecules to,” continues Dr Scharf. “These molecules are usually much smaller than the metallic sphere itself; however, you want to create something where the metallic sphere is almost hidden in the resulting hybrid molecule.”

Inorganic particles Ensuring that the material’s homogeneity is not disturbed by the nano-particle is a real challenge. The nanoparticles are spheres and are typically between 3-5 nanometres (nms) in size, making hiding an inorganic particle very difficult, while there are also other issues to address. “The resulting material very

The basic idea is to make a kind of

metamaterial ink,

like a type of paint, which allows you to produce a material with nonconventional electromagnetic properties. This is in the form of a liquid which could then be dried or processed to produce

a stable and durable material often has features that are usually not favourable for technology processing – for instance the viscosity is too high, or it needs processing temperatures that are relatively high – let’s say 100°C,” says Dr Scharf. However, it is possible to tune the properties of the material by manipulating the molecules in various ways. “We are looking at so-called thermotropic systems. So with the temperature you can influence physical parameters like viscosity and the structure; you can increase and decrease temperature to adjust properties,” continues Dr Scharf. “The effects on nanoparticle resonance and structural

interference are different. The nanoparticle’s properties and the related resonance are stable but the surrounding morphology and the interference is changing. We get the possibility to tune the material properties and freeze the structure to conserve it.” It is often the case that materials with particular electro-magnetic properties can’t be used to create devices with certain functionalities. Researchers are using a hybrid approach which allows them to be more flexible on the material parameters and second level morphologies. “On the resonance level we’re mainly using the hybrid approach. So we use organic molecules and nanoparticles to create functions,” explains Dr Scharf. One important consideration is that the second morphology level of the material should be below the wavelength of light so that it appears homogenous for visible light; the structure size should be less than around 400 nms. “If you put it a little bit larger than the wavelength of light then you would get diffraction effects, which are dispersive, scattering effects and depend on the wavelengths of light. We want to avoid this,” stresses Dr Scharf. “If your structure is smaller than the wavelength of light then you don’t have this diffraction. You only work in what we call a ‘zero order’ phase, where the light doesn’t bend and the material it experiences can be described as homogenous.” This allows researchers to focus on the effective properties of the material with respect to the wavelength. The aim is to develop a meta-material with an amorphous structure which leads to certain nonconventional optical and electro-magnetic properties. “The concept of meta-materials is that you have something that behaves almost like glass – you can describe it from the outside by a single set of criteria parameters, and you can use this single set of criteria parameters for all angles of incidence. There are two main parameters – the refractive index and the absorption,” explains Dr Scharf. With this approach the materials can be described on a single refractive index image; Dr Scharf says this is very beneficial in terms of the potential applications of NANOGOLD’s research. “We have in mind applications to hide objects and to shield particular areas from electromagnetic energy,” he outlines. “There are also applications in the optical domain, such as making materials with features like complete light absorption.”

At a glance Full Project Title Self-organized metamaterials based on spatially arranged nanoparticles (NANOGOLD) Project Objectives The NANOGOLD project aims at the fabrication and application of bulk electro-magnetic meta-materials. A promising new concept for the exploration of meta-materials is the use of periodic structures with periods considerably shorter than the wavelength of the operating electromagnetic radiation Project Funding €3.52 million Project Partners École polytechnique fédérale de Lausanne EPFL • Virtual Institute for Artificial Electromagnetic Materials and Metamaterials - Metamorphose VI AISBL, Belgium • Universite de Geneve, Switzerland • Ruprecht-Karls-Universitaet Heidelberg, Germany • University of Patras, Greece • Friedrich-Schiller-Universitaet Jena, Germany • Universita della Calabria, Italy • University of Hull, United Kingdom • The University of Sheffield, United Kingdom Contact Details Scientific Coordinator Toralf Scharf École polytechnique fédérale de Lausanne EPFL Rue A-.L. Breguet 2 Neuchatel 2000 Switzerland T: +41 327 183286 F: +41 327 183201 E: W:

Energy detection This feature could be used to convert energy into heat, an attribute which holds real relevance to energy detection and light harvesting. For instance, shining a light on a complete absorber can be used to measure radiative energy; the energy is transferred into heat and the heat is detected and measured; Dr Scharf says this method is very efficient. “This method measures all the energy that comes onto the surface of the material,” he stresses. It’s not entirely clear how this research could be applied, but some areas have been identified. “It could be possible to create devices that are transparent for certain wavelengths but not for others,” says Dr Scharf. “In fact, it is already possible to do this today using some complicated multi-interference structures. We are aiming to develop a kind of ink that could be applied to a surface and allow you to get these functionalities and create particular effects in a single layer. Shields against electro-magnetic radiation could feasibly be used for solar cell applications for example.” The design of the material is based on the resonances, which usually have particular spectral features. This means it’s not possible to cover the whole visible light spectrum. “The resonances have a certain spectral width, and usually they are used to create unusual optical and electro-magnetic properties. So automatically these properties are created only in a particular spectral window, because outside this spectral window you

don’t have the resonance effects, and you need the resonance effects to make these refractive indexes. So as long as the material is based on resonance effects, there’s always a limited spectral window,” explains Dr Scharf. The design of metamaterials is largely based on resonances in nano-entities; Dr Scharf says that high precision machine techniques can be used to create very sharp resonances and spectacular large resonance features. “You get these very high resonance peaks, but at the same time the peak is very narrow,” he outlines. The NANOGOLD project is using more amorphous structures, where the resonances are less sharp, so automatically the effects are wider. However, widening the wavelength is not the project’s primary focus; Dr Scharf says the next big challenge is to explore how functional devices can be made out of the material. “The challenge is to use the available materials and to try to make functional structures and demonstrate the functionality of devices,” he outlines. While the NANOGOLD project is nearing the end of its term, Dr Scharf is keen to pursue further research into metamaterials. “We are looking at the pre-commercial exploitation level at the moment, but the speed of advancement depends very much on the level of funding we are able to attract,” he continues. “We will continue to collaborate with our partners across Europe and aim to get further funding from national research bodies.”

Toralf Scharf

Project Coordinator

Toralf Scharf focuses his research activities at the École polytechnique fédérale de Lausanne on interdisciplinary subjects bringing micro-system, material technology and optics together. With a background in surface physics (MSc), physical chemistry (PhD) and extensive experience in optics. He is familiar with all necessary aspects of technology development and application and can communicate with different scientific communities.

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