Collective Excitations in Advanced 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 advance-nanostructure-based optoelectronic devices including microwave, terahertz and light generators, detectors and frequency modulators.

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
spintronics.

Dissemination:
more than 40 actions, including newspaper interviews, popular books, and dissemination at schools

Exploitation:
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