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Periodic Report Summary - NANOGOLD (Self-organized nanomaterials for tailored optical and electrical properties)

Quality validation date:2013-02-01

Abstract
Composite materials incorporating organic and inorganic components are important in many fields of technology. Within the 'NANOGOLD' project we studied self-organisation, morphology and optical properties of composites to be used as next generation optical materials. The basic idea of our concept was to use electromagnetic resonances on different scales to achieve new electromagnetic properties. Our concept was based on the use of small metallic nanoparticles forming clusters and made in films to serve as three-dimensional metamaterials.

In the first year, we verified the theoretical feasibility of our model. Several techniques were developed to fabricate thin films containing resonant metal nanoparticles. We developed recipes for layer by layer assemblies, surface metal nanoparticle cluster materials and optical device fabrication by templating and with high nanoparticle loads. New self-organising composite materials were synthesised and were planned to be used as base material for further investigations. Having methods for fabricating multidimensional composite structures at hand, we aimed further to design first functional structures, simulate their optical properties, realise them and use advanced spectral characterisation techniques to proof their nonconventional electromagnetic response.

NANOGOLD derived its novelty from the idea of using self-organisation to implement structure at different length scales and combine this with resonance effects. The material class we were looking was mainly formed from amorphous assemblies of nanoparticle clusters. We used nanoparticles that had resonances. Combining them in clusters allowed tuning the resonance frequency and strengths. The final aim was to assemble such clusters to bulk metamaterials. In the first half of the project, foundations were laid to device fabrication. Intensive research on amorphous metamaterial designs based on nanoparticles was started and simulation tools for plasmonic materials structured at different scales were established. A toolbox for plasmonic nanomaterial fabrication was established, based on self assembling materials, physical chemistry techniques for structure formation and templating. Liquid crystal based self-assembled plasmonic materials containing plasmonic entities with sizes larger than 4 nm were synthesised. Structures of highly order liquid crystal phases containing smaller nanoparticles were investigated and successfully described. Unusual structural transitions were found in such materials. Due to a moderate density of plasmonic entities, which weakened the induced dispersion, the optical properties of the actual materials were yet dominated by the organic host material. Important changes of the materials permeability's were expected for larger plasmonic entities of a second generation material. To engineer more actively the density of plasmonic entities meta-atoms and meta-layers built from nanoparticles assemblies were fabricated. Our approach here was electrochemistry that allowed deposition of amorphous multilayer of nanoparticles of nearly any size. We chose effective plasmonic entities and assembled three-dimensional structures starting from a layer built up approach. We showed that, in such structures, magnetic dipole moments could be measured and went on to more complex systems. The minimum distance of plasmonic particles assembled with electrochemistry was given by the electrostatic interaction of wrapping molecules. In order to obtain highest density nanoparticles a particular cluster material was produced based on high density nanoparticle inks. With such a material, clusters of different size and shape could be produced and stabilised without losing the resonance behaviour. Plasmonic resonance and interference as well as scattering were the basic operational principles that, combined in the right manner, could lead to unusual electromagnetic behaviour. We achieved to make a bulk amorphous cluster layer material that showed plasmonic resonances and interference of light, a first step to bulk metamaterial properties.

To assure the use of such materials in devices, certain functionalities needed to be demonstrated. Among others we considered waveguide devices. Technology compatibility of holographic written structures with plasmonic material was approved and would allow us fabrication of first test devices with unusual response based on dense plasmonic cluster materials.

The importance of metamaterial research was shown in the number of initiatives to stimulate the research and was out of question. Despite the impact of the photonic metamaterials for a lot of applications in general, our approach added particular advantages not accessible by using conventional or advanced nanotechnology. The following five impact areas could be reached with our approach of a self-assembled composite metamaterials:

1. physics and technology of metamaterials
2. chemistry of nano-composite material
3. engineering of composite materials for sensing and optics, such as sensing, photonics, resolution, decoration etc.
4. technology of liquid crystal devices with macromolecules
5. management of thermal radiation at the nanoscaled.

Our project was the first bottom up approach for metamaterial fabrication and boosted technology development in the field. A lot of designs today are based on restrictions given by means of nanotechnology fabrication.

Chemistry of nano-composites is a wide field of research. First applications of self organised composite materials with thermotropic properties were anticipated to lead to a rush to this material class. The concept could be widened to allow specific material design for applications like sensing paints. One aspect of materials characterisation like macromolecular assemblies was the development of low-resolution crystallography methods for determination of nanostructures of self-assembled metamaterials. This was important as the structures dimension did not fit well with classical structural analysis means.

By the time of the project elaboration engineering of nano- and microstructure composite materials in the context of optical functionality was rare. Moulding and film processing as well as fibre spinning were used to bring materials into shapes. Bulk materials could be shaped very precisely. If one considered high resolution of spatial modulated materials only very few results were available that showed optical functionality. The main problems were clustering of nanoparticles, de-mixing and wetting problems. Our material concept was explicitly based on such effects and our material designs would largely contribute to applying fabrication concepts available in this area. In a long run, when materials were designed that could be processed at room temperatures, the whole plastic technology process chain including injection moulding and hot embossing would be available for fabrication. Additionally, it was expected that the material would have highly anisotropic electrical conductivity. Such properties could impact device development for organic electronics and solar cells.

One possible short term application of a metamaterial consisting of plasmonic nanoparticles could be on the emission of thermal radiation. As it was predicted theoretically, thick slabs of ordered layers of gold nanoparticles exhibited an emission spectrum which was suppressed in the infrared regime and enhanced above a certain cut-off frequency in the optical regime. Such structures were ideally suited for incandescent light bulbs. The thermal energy provided by heating the metamaterial was transformed into thermal radiation mostly in the visible regime leading to much increased energy efficiency.

Further information on the project could be obtained at http://nanogold.epfl.ch/.

Collaboration sought:N/A

Programme	Project reference	Project title
FP7-NMP	228455	Self-organized nanomaterials for tailored optical and electrical properties

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