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CoExAN Report Summary

Project ID: 644076
Funded under: H2020-EU.1.3.3.

Periodic Reporting for period 1 - CoExAN (Collective Excitations in Advanced Nanostructures)

Reporting period: 2015-10-01 to 2017-09-30

Summary of the context and overall objectives of the project

CoExAN brings together experimentalists and theoreticians working in synergy to boost the development of new nano-sized sources of electromagnetic radiation which will become major building blocks of future electronic devices based on carbon nanostructures.
This project aims to develop, fabricate, theoretically and experimentally study carbon based nano-circuits which are able to generate, detect and process broadband electromagnetic (EM) signals. The carbon nanoscale EM sources can be based, in particular, on Cherenkov radiation emerging when electrons move inside carbon nanotubes (CNTs) or between spatially separated graphene sheets. The frequency of the Cherenkov radiation depends on the CNT radius and chirality or on the distance between graphene sheets. The performance of carbon EM nano-emitters is determined by the electron momentum relaxation time, which can be determined by measuring the generated THz and microwave fields. The frequency of the emitted EM radiation can be tuned by acoustic waves that provide distributed feedback for the EM wave. As well, the effects originating from strong coupling between material excitations in carbon-based structures and confined optical modes of microcavities will be investigated. The formation of polariton modes and their collective properties will be analyzed theoretically. Another set of problems to be considered in the proposed research is associated with the quantum mechanics and quantum optics of carbon-based nanostructures. We will look at excitonic and plasmonic collective effects in CNTs (especially narrow-band quasi-metallic ones, where excitonic effects are largely overlooked) and in few-layer planar Weyl materials such as graphene, silicene and germanene. We will also study collective photonics phenomena stemming from the quantum nature of light and look at sophisticated arrangements of carbon-based and other nanostructures in arrays or placing them in microcavities, thus utilizing the significant expertise of some of the participating groups in quantum optics aiming eventually at a design and feasibility study of novel advanced nanostructure-based optoelectronic devices including microwave, terahertz and light generators, detectors and frequency modulators.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

More than 103 secondments have been executed during the first 24 months of the project, representing 82% of the planned ones. This has permitted to reach the principal objectives programmed for this period. 63 articles, 20 of which involve cross-participation of the node partners in the frameworks of the Project, were published in high-impact international journals.

Among the Carbon-based nanostructures:
(i) The instability of electron beam propagating over sandwich graphene structures ( composed of 2,3,4,5 and 6 multilayered graphene/graphitic films sandwiched between PMMA dielectric spacer layers) was studied. It was shown that there are symmetric, asymmetric and hybrid modes supported by sandwich structure. Generation frequency can be tuned by varying of layers number or electrostatic doping of layers in the case of generation on symmetric mode. If generation occurs on asymmetric or hybrid mode, frequency tuning can be provided by varying on graphene interlayer distance.
(ii) The ability of thin conductive films, including graphene, made of all these materials separated by polymer slabs to absorb electromagnetic radiation in microwave-THz frequency range is documented. This opens a new avenue towards the development of a scalable protocol for cost-efficient production of ultra-light electromagnetic shields that can be transferred to commercial applications.
(iii) The electromagnetic scattering theory for a finite-length nanowire with an embedded mesoscopic object was developed.
(iv) We developed a theory of electron-hole and electron-electron pairing in ultra-relativistic quasi-one-dimensional systems and applied it to narrow-gap carbon nanotubes.
(v) We studied optical transition in different types of graphene nanoribbons and bi-layer graphene, silicene and phosphorene nanoclusters discovering strong dipole transitions in the THz range.
Novel 2-dimensional materials have also been investigated:
(vi) We have studied the electronic and optical properties of 2-dimensional group III-Nitrides and shown that InTlN alloys with low Indium content are eligible as emitter and detector for THz radiation.
(vii) Great attention was devoted to silicene, the silicon-based analogous of graphene. The process of silicon deposition onto graphite surface experimentally and using the first-principle molecular dynamics (Fig1).
We demonstrated that due to intercalation of graphite by Si atoms, which possibly occurs through the surface defects, such as “steps” or vacation of carbon, it is possible to obtain bi-dimensional silicon – silicene – under the carbon top layer.
(viii) We have developed a theory of topological phase transitions in novel two-dimensional crystal systems including graphene, silicene, germanene, phosphorene, transition metal dichalcogenides. We have predicted the characteristic spikes of entropy per electron at the topological Lifshits transition points in these structures.
(ix) Excitons, and exciton-exciton interaction, have been studied in transition metals dichalocogenides.
(x) We developed a theory of magnetic confinement of massive and massless charged particles in two-dimensional systems
(xi) We studied two-phonon scattering in graphene in the quantum Hall regime and have shown that this scattering provides a major contribution to dissipative conductivity.
Topological materials have been studied:
(xii) We have investigated the 3-dimensional analogue of graphene, the so called Dirac and Weyl semimetals, in particular CdAs and TaAs. In Weyl systems the particular spin texture make these materials interesting for

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

All our results are fruit of experiments and theoretical approaches beyond the state of the art.
The fabricated graphehe/dielectric sandwiches showed record high microwave and THz absorption for structures of submicron thickness.
The ab-initio computer codes, used in part of the projects, had to be adapted and tested for the case of 2D systems. This allows us now to calculate with high precision the electronic and optical properties of a wide spectra of bi-dimensional materials, such as graphene, silicene, MoS2, and so on.
Potential impact of our studies concerns the development of the novel ultrathin flexible THz and microwave absorbers for security, medicine and environmental applications.
Moreover, we expect to establish the conditions, under which the bi-dimensional silicon will be stable both above and under the graphite top layer. It will help to solve the problem of obtaining such a novel material as silicene, what could be a new basis of electronics. Finally, our theoretical results open an avenue towards spintronics. The spintronics is a valuable alternative to the modern micro-electronics. By substitution of electrons
by spins and information bit carriers one would achieve a significant reduction of thermal losses that consume over 5% of the whole energy produced by humans at present. We have developed a concept
of the spintronic transistor where the critical temperature of the ferromagnetic transition can be halved or doubled by a small variation of the external voltage. This theoretical work paves the way to spintronic transistors and diodes (Fig2).

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