Self-organized Nanomaterials for tailored optical and electrical properties
- Responsible : Toralf SCHARF
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Modern technology refers more and more to combination of different materials for device fabrication. Organics and hybrid materials are often used to replace conventional materials such as glass in microstructure applications. Our research contributes to process development to realize multifunctional materials for new optical materials.
Self organized metamaterials that rely on metallic nanoparticles which are suitably integrated into a dielectric host are at the focus of our activities. If self organization by liquid crystal structures is used for material assembly, small metal particles are of special interest. A small particle is thought of disturbing less the self organization process but there are open questions about their effectiveness. ur metamaterial design concept is based on combination of resonances and interferences of electromagnetic fields in nanostructured composite materials suggest in different publications [2,3,5].
Figure 1. Starting with a small metallic nanosphere (a) it is possible to excite at appropriate wavelength a localized plasmon polariton. The material’s intrinsic dispersion is shown for silver in (b). A medium made of densely packed such spheres (c) allows to obtain strong dispersion in the effective permittivity (d). Forming spheres out of such a matter permits for the excitation of Mie resonances in the magnetic mode of the spectral domain with strongly positive permittivity. Arranging spheres in a densely packed array (e) allows ultimately to obtain a medium where the effective permeability is strongly negative.
Figure 1 outlines the general strategy. The metamaterials we propose are composed of periodically arranged nanoinclusions on two different length scales. Whereas on the first length scale the effective permittivity of the medium is altered, the arrangement on the second length scale permits to modify the effective permeability of the medium. The different design stages are shown in Fig. 1. A noble metal, e.g., gold (Au) or silver (Ag), serves as the starting material. Small nanoparticles arranged in a host dielectric show an effective permittivity modified according to a Lorentzian lineshape in close vicinity to the collective surface plasmon. As denser the particles are packed as stronger the induced dispersion is [1, 2]. A further step of structuring has the potential to show negative effective permeability when a second resonance effect is excited.
The most promising way of realizing metamaterials by a bottom-up approach relies on organic chemistry of mesogenic and macromolecules, plasmon resonance effects and physical chemistry principles of self-organization. In order to design organic-inorganic composite materials with tailored properties, several parameters dominating the system’s plasmonic properties have to be controlled: particle size (including size distribution) and shape, particle distance (period) and organizations in one, two or three dimensions statistical distributions, wires, plates, crystals). Mesogenic rod-like, disk-like and tapershaped (dendrimer) molecules form liquid-crystalline phases even with particles included. At the same time the presence of the particle may induce a higher degree of order than exhibited by the native mesogen. The distance between the entities can be designed to be in anything between 0.3 nm to over 10 nm. Therefore, macromolecules like dendrimers, and mesogens of controlled shapes are well suited to organize small metal particles whose size can range from several nanometers to tens of nanometers.
Actual Projects in this field:
NANOGOLD FP7-NMP- NMP-2008-2.2-2 No. 2245
-  S. Riikonen et al., Phys. Rev. B 71(2005) 235104
-  E. Lidorikis et al., J. Appl. Phys. 101 (2007) 054304
-  C. Rockstuhl et al. Phys.Rev. Lett. 99 (2007) 017401
-  C. Rockstuhl T. Scharf, J. Microscopy, 29 (2008) 281
-  Q. Wu , W. Park, Appl. Phys. Lett. 92 (2008) 153114