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Nano-carbon based components and materials for high frequency electronics

Final Report Summary - CACOMEL (Nano-carbon based components and materials for high frequency electronics)

A strong expansion of the frequency range towards terahertz and infrared is the major trend in the modern electronics and optoelectronics. It relies on the incorporation of modern nanotechnology that has already given the birth to nanoelectronics, a rapidly developing discipline focused on both the dramatic increase of the component integration level and decrease in a power consumption. The project aimed at understanding of fundamentals of the electromagnetic processes in nanocircuits, theoretical and experimental investigation of underlying mechanisms responsible for their fascinating properties, and development of physical basis for use of these properties in novel nanoelectronic devices. The project focuses on electromagnetic effects in different nano-carbon structures, such as onion-like carbon, both single-walled and multi-walled carbon nanotubes (SW- and MW- CNT), pyrolitic carbon (PyC), graphene, etc. The following main objectives were stated: to reveal the effects of spatial irregularities in the performance of CNT-based components and nanocircuits; to develop electromagnetic compatibility (EMC) theory of the circuits with nano-sized components; to perform experimental and theoretical investigation of high frequency and nonlinear optical properties of nano-carbon materials as potential materials for electromagnetic shielding and optical application.
In accordance with the objectives, we report the following main scientific results.
A new concept of nanoEMC for nanoelectronics, based on the synthesis of the classical electrodynamics and quantum transport theory in nanostructures, has been developed. We demonstrate that classical EMC concepts such as coupling, shielding, and impedance matching, should be reconsidered taking into account quantum correlations and tunneling, as well as spin-spin and dipole-dipole interactions. As a result equivalent circuits will contain additional elements of quantum nature, which significantly influence the EMC. The main concept is illustrated by the example of carbon nanotube based interconnects.
The first experimental evidence of localized plasmon resonance in composite materials containing SWCNTs has been obtained: Fourier-transform infrared spectroscopy showed that an optical-density peak, the same as a terahertz conductivity peak, shifts to higher frequencies as the SWCNT lengths are reduced—in agreement with a tendency predicted for the localized plasmon resonance in finite-length SWNTs. To produce calibrated in length short CNTs for experiments with the length-dependent plasmon resonance, a nondestructive cutting approach has been developed based on low temperature intensive ultrasonication of CNT collections in a mixture of sulfuric and nitric acids. Raman and Fourier transform infrared spectroscopy were used to demonstrate that the cut carbon nanotubes have a low extent of sidewall degradation and their electronic properties are close to those of the untreated tubes.
A first-principles numerical method for calculation of the electronic structure of the point impurities in the SWCNTs based on a Green’s function technique has been developed. The impurities are described by the single-site perturbed muffin-tin potentials in otherwise perfect nanotubes with the rotational and helical symmetries. Nitrogen-doped and pure carbon nanotube based composites were fabricated for investigating their dielectric properties in static regime as well as electromagnetic response properties in microwave frequency range to reveal a role of doping. The first experimental observation of the electromagnetic response enhancement in collections of doped CNTs with arbitrary chirality has been performed. This experiment correlated well with the theoretical model describing "metallization" of semiconducting CNTs under effect of doping, both n-type and p-type.
The performance analysis of global-level on-chip interconnects has been carried out. A simple circuit equivalent model has been developed for these interconnects, which can properly account for the geometrical properties of the carbon interconnects and for their temperature. Two possible realization of carbon nano-interconnects have been modeled based on (i) arrays of graphene nanoribbons (GNRs) and (ii) bundles of CNTs. The electronic transport in the CNTs and GNRs is modeled through the kinetic inductance, the quantum capacitance, and the electrical resistance. The analysis gives a theoretical upper limit of effective permittivity and shielding efficiency of a MWCNT-based composite that can be achieved in practice at a given nanotube length and diameter. The models has been used to study some challenging problems in nanopackaging, such as the degradation of electrical performance due to self-heating and the high-frequency current rowding problem because of the skin-effect, referring to the 22nm and 14-nm technology node.
The CVD technique was employed to synthesize nanometrically semitransparent pyrolytic carbon films with high conductivity on dielectric substrates. A study of electromagnetic properties of PyC films with different thicknesses has been performed and demonstrate the remarkably high absorption up to 50% of incident power. Along with chemical stability, this property makes PyC films attractive for electromagnetic interference shielding in space and airspace communication systems, as well as in portable electronic devices occupying microwave frequency slot.
Femtosecond time-resolved measurements of the light-induced transmission change were employed to study the third-order nonlinearities in SWCNTs in the near infrared spectral range. Specifically, the light induced absorption change was obtained as a function of the pump and probe wavelengths and time delay. Pump-probe measurements with tunable pump wavelengths demonstrated the dependence of fast and slow relaxation processes of excited carriers on the excitation wavelength. We have also calculated the magnitude of third order nonlinear susceptibility, which is as high as 10−11 esu. Carrier transport features in single-wall carbon nanotube films under strong electric fields (up to 105 V/cm) has been studied. Application of electrical pulses of nanosecond duration allowed minimization of Joule heating and resolve intrinsic nonlinearities with the electric field. Investigation within a wide range of temperatures — 4.2–300K — indicates that carrier localization as well as tunneling through the insulating barriers between conducting regions takes place in SWCNT films. Transition from semiconducting behavior to metallic behavior in strong electric field, at the field strength around 3 kV/cm, is explained in terms of variation of transparency of the insulating barriers.
A series of composite samples were fabricated, using a Epikote 828 epoxy resin as a host and CNTs, carbon black of high surface area and exfoliated graphite as fillers. All fabricated composites demonstrated good homogeneity, carbon inclusions were reasonably well dispersed. Microwave measurements of different nanocarbon composites have been performed, mainly in the frequency range 26-37 GHz but other ranges have also been studied. The results have been compared for different compositions and strong variation of the absorption and reflection has been revealed and theoretically interpreted.
As a whole, the project contributes essentially into development of nanoelectromagnetics — a new research discipline emerging as a conceptual response to new request of nanoscience. A convergence of three research communities, the condensed matter community, the optical spectroscopy community and the electromagnetic community (which moves inevitably into THz and IR frequency ranges), is evident as a new tendency. Following this tendency, the partnership developed supplies to the “research market” a new scientific language for describing electromagnetic phenomena in nanostructures. From the viewpoint of applied science, the partnership contributes to the extremely important and innovatively attractive field: the development of the component basis of high-frequency integrated circuits and new materials for electromagnetic applications. The results of the project can be used across the European Research Area, including telecommunications, biology and medicine, and the fight against terrorism (by developing novel electromagnetic shielding materials preventing electronic nets against unauthorized access).
A set of events has been organized for dissemination of the project results and intensive knowledge exchange, including: a special sections "Nanoelectromagnetics" at International Conference on Physics, Chemistry and Applications of Nanostructures, Minsk, (May 2011, May 2013, May 2015); NATO Advanced Research Workshop ‘Nanodevices and Nanomaterials for Ecological Security’, June 2011, Riga-Jurmala, Latvia; special sessions on electromagnetic properties of carbon nanostructures at International conference on Electromagnetics in Advanced Applications, Torino, Italia (September 2011, 2013, 2015); 16th IEEE Workshop on Signal Power Integrity SPI 2012, May, 2012, Sorrento, Italy; International conference on Fundamental and Applied NanoElectroMagnetics FANEM'12, Minsk, Belarus (May 2012); 3rd and 4th International Workshop on Nanocarbon Photonics and Optoelectronics, Polvijärvi, Finland (July 2012 and 2014); International conferences Nanoscience and Nanotechnology, Laboratori Nazionali di Frascati, (October 2012, 2013, 2014). Professors A. Maffucci (Italy) and S. Maksimenko (Belarus) are co-directors of the NATO Advanced Research Workshop “Fundamental and Applied NanoElectroMagnetics” FANEM'15, which will be held in Minsk, Belarus, in May 2015. Finland and Belarus teams joined the EU initiative GRAPHENE FLAGSHIP, EU FP7 project FP7-604391.