FAEMCAR · Fundamental and Applied Electromagnetics of Nano-Carbons

Fundamental and Applied Electromagnetics of Nano-Carbons

The quest for larger and larger integration of electronic circuits has raised the question of electromagnetic compatibility: an active element should not affect the working of a neighboring device and should not be perturbed by the latter. In the same time, satellite communications, cellular phones and wireless connections have become so widely spread that electromagnetic radiations in the frequency range 1-100 GHz (microwaves) have widely impregnated our environment. The protection of sensitive devices against microwave perturbations is a real challenge. Protective films and coating materials specially designed to shield microwave radiations are commonly used. The Faemcar project successfully proposed to resort to nanoscopic forms of carbon to design new shielding layers. Carbon is an interesting material for electromagnetic interference shielding, because it is a light element that may be an electrical conductor when it is dominated by sp2 bonds (with this type of bonding, one electron per carbon can hop from atom to atom all across the structure): graphite, carbon blacks, nanotubes, onion-like carbons, some carbon foams, graphite nanoplatelets ... and graphene. Most of these forms of carbon have been used as nanoparticles dispersed in polymers to transform them in shielding materials that conserve the interesting properties of the matrix (ease of shaping, lightness, flexibility, chemically inert ...). Simultaneously to the ability of these new materials to block GHz radiations, other physical and chemical properties of the composites have been investigated. Of special interest was the study of the percolation threshold (the minimum amount of filler particles required to form a interconnected path between two electrodes), the electrical ac conductivity and the dynamical dielectric permittivity of the new composites. Rather then loading the bulk of a polymer matrix with nanoparticles, very thin carbon-based conducting films (pyrolytic carbon, graphene, graphene containing graphitic islands) can be produced and deposited on a substrate or inserted between polymer films in a sandwich-like structure. The graphene/polymer alternation can be repeated several times to produce a multilayer. It has been demonstrated during the Faemcar project that an overall thickness of a few nanometers of conducting sp2 carbon film is sufficient to absorb a great part of incident GHz radiations. These layered composites block most of incoming radiations by absorbing them, thanks to electrical losses, rather then by reflecting the radiations as with usual and thicker shielding layers. The carbon-based heterostructures work like radar absorbing media except that the spirit of the project was to minimize the transmittance of the protective layer rather than minimizing its reflectance. Nanostructures can concentrate a significant fraction of electromagnetic power in small regions. Thanks to that property, some nanoparticles can amplify by several orders of magnitude the probability that an attached molecule absorbs or scatters infra-red and visible light. This is where various types of optical enhanced spectroscopy find their origin, making it possible to detect a very small amount of molecules. A fraction of the research activities of the Faemcar project explored carbon nanostructures from this point of view, either by studying the optical properties of nanocarbons or by looking which ones could become a substrate for enhanced spectroscopy of biomolecules. Graphene oxide came as a good candidate for that purpose. Theoretical work was also carried out to explain electromagnetic measurements obtained from experiment on composites and carbon-based heterostructures. In parallel, other calculations, based on first-principle computer modeling, were developed to predict the electronic properties of nanostructured shapes of graphene, bi-layer graphene and other bi-dimensional materials. This part of the work aimed at guiding the search for new forms of carbon-based systems exhibiting tunable electromagnetic properties. Having generated more than 60 publications in scientific journals, the project has definitively contributed to the field of nano-electromagnetics of carbon nanostructures. An overview of the scientific achievements can be found in the dedicated website http://www.faemcar.be. Being driven by the worrying electromagnetic compatibility issue, the project remained close to practical applications. Researchers of seven countries joined their efforts to reach and even go beyond the assigned objectives, thanks to more than 70 person-months that were spent in exchange visits across the consortium.

Data: CORDIS, © European Union