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Collective Excitations in Advanced Nanostructures

Periodic Reporting for period 2 - CoExAN (Collective Excitations in Advanced Nanostructures)

Reporting period: 2017-10-01 to 2019-09-30

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 high performing electronic devices based on carbon nanostructures.
The idea is to develop carbon based nano-circuits which are able to generate, detect and process broadband electromagnetic (EM) signals. The overall final objective is to design novel advanced nanostructure-based optoelectronic devices including microwave, terahertz and light generators, detectors and frequency modulators, with important technological applications ranging from space to everyday life. We have studied excitonic and plasmonic collective effects in CNTs (especially narrow-band quasi-metallic ones, where excitonic effects are largely overlooked) and in few-layer planar Dirac materials such as graphene, silicene and germanene. We have also studied 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 the participating groups in quantum optics.
Our overall objectives include the theoretical microscopic understanding of new graphene-like materials, the search for their new physical properties, their use in new devices, including their design, planning and possibly their production.
All planned secondments have been executed, and the principal objectives of the project reached. More than 100 articles, around 40 of which joint, were published in high-impact international journals. Many other articles are in preparation or submitted.
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:
(vi) We have studied the electronic and optical properties of 2-D group III-Nitrides and shown that InTlN alloys are eligible as emitter and detector for THz radiation.
(vii) The process of silicene deposition onto graphite surface experimentally and using first-principle molecular dynamics (Fig1) has been studied.
It is possible to obtain silicene under the carbon top layer. Moreover, sapphire is a very good substrate for silicene (Fig2)
(viii) We have developed a theory of topological phase transitions in novel 2D crystal systems including graphene, silicene, germanene, phosphorene. 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:
(xii) We have investigated the 3-D analogue of graphene, Dirac and Weyl semimetals CdAs and TaAs. In Weyl systems the particular spin texture make these materials interesting for
more than 40 actions, including newspaper interviews, popular books, and dissemination at schools
the following studies have been identified as under-development innovations with possible industrial/medical/societal/technological applications:
• Carbon nanotube sponges as tunable materials for microwave passive devices
• Single-walled carbon nanotube based novel 2D van der Waals material as ultra-fast terahertz modulator, for midIR and IR optoelectronics, and for efficient light emitters in visible
• Advanced THz tool to separate quasi-metallic and true metallic quasi-one-dimensional carbon nanostructures
• Tunable perfect THz absorber based on a stretchable ultrathin carbon-polymer bilayer
• Graphene based modulator for THz imaging and for space applications
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 2D 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 have established that sapphire is a good candidate as substrate for silicene. On it 2D silicon will be stable thus obtaining such a novel material as silicene, which 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 (Fig3).
-carbon nanotube sponges as tunable materials for microwave passive devices
-Single-walled carbon nanotube based novel 2D van der Waals material as ultra-fast terahertz modulator and or midIR and IR optoelectronics efficient light emitters in visible
-Advanced THz tool to separate quasi-metallic and true metallic quasi-one-
dimensional carbon nanostructures
-Tunable perfect THz absorber based on a stretchable ultrathin
carbon-polymer bilayer
-Graphene based modulator for THz imaging and for space applications
Figure 1
Figure 3
Figure 2