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Complete absorption of electromagnetic waves is paramount in today's applications, ranging from photovoltaics to cross-talk prevention into sensitive devices. In this context, we use a genetic algorithm (GA) strategy to optimize absorption properties of periodic arrays of truncated square-based pyramids made of alternating stacks of metal/dielectric layers. We target ultra-broadband quasi-perfect absorption of normally incident electromagnetic radiations in the visible and near-infrared ranges (wavelength comprised between 420 and 1600 nm). We compare the results one can obtain by considering one, two or three stacks of either Ni, Ti, Al, Cr, Ag, Cu, Au orWfor the metal, and poly(methyl methacrylate) (PMMA) for the dielectric. More than 1017 configurations of geometrical parameters are explored and reduced to a few optimal ones. This extensive study shows that Ni/PMMA, Ti/PMMA, Cr/PMMA and W/PMMA provide high-quality solutions with an integrated absorptance higher than 99% over the considered wavelength range, when considering realistic implementation of these ultra-broadband perfect electromagnetic absorbers. Robustness of optimal solutions with respect to geometrical parameters is investigated and local absorption maps are provided. Moreover, we confirm that these optimal solutions maintain quasi-perfect broadband absorption properties over a broad angular range when changing the inclination of the incident radiation. The study also reveals that noble metals (Au, Ag, Cu) do not provide the highest performance for the present application., Spaceborne Instrumentation

Over the past decades, continued improvement in transistors density, performance and energy efficiency have been sustained by technological innovations. This exponential development predicted by Moore in the 60s transformed the first crude computers from the 70s into the ubiquitous and high-performance sophisticated machines of nowadays. This trend is however coming to end as the size of transistors is reaching the quantum limit. In the last decade, two-dimensional (2D) crystals have emerged as a disruptive materials platform. Those layered crystals are excellent candidates for next-generation technologies due to their ultimate thinness and dangling-bonds-free surface. In addition, they offer better scaling than traditional 2D electron gas based on III-V compounds. In spite of the aforementioned advantages, some technological obstacles still hinder the industrial integration of 2D materials in electronics. The most important ones are the growth and the post-processing of wafer-scale high-quality crystals, robust passivation schemes and device-related aspects such as the contact resistance or doping. In this thesis, first-principles simulations are performed on systems composed of graphene or transition metal dichalcogenides (TMDs). Their transport properties are computed using the OpenMX code based on the Green's functions formalism. First, metal-graphene interfaces are studied in the top-contacted geometry. Charge injection from metal to graphene is understood through the lens of a semi-empirical transport model so that the transport mechanisms impeding the contact resistance are identified. Afterwards, quantum transport is investigated inside the graphene channel. The objective is to benchmark the transport characteristics of polycrystalline samples resembling the ones produced by the industry. Finally, the operation of a TMDs-based atomically thin FET is investigated. Stain-induced current modulation is predicted through nano-scale quantum engineering, a mechanis, (FSA - Sciences de l'ingénieur) -- UCL, 2022

In graphene, charge carriers behave as massless (Dirac) fermions, yielding unprecedented electrical properties. In this thesis, the electronic transport of Dirac fermions is studied in graphene quantum point contacts thanks to low-temperature scanning gate microscopy (SGM), a technique consisting in recording the sample resistance while changing locally the charge carrier density with a biased metallic tip. The focus has been put on backscattering mechanisms and three main directions are explored. First, real space signatures of Klein tunneling, preventing the backscattering of Dirac fermions, are investigated by creating a movable pn junction with the SGM movable top gate. The experimental results are discussed in the light of simulations, revealing that the SGM conductance maps yield an image of the current density around and through the lens, with a direct evidence of Klein tunneling. Second, SGM characterizations are reported under a large magnetic field, in the quantum Hall regime where charge carriers flow in topologically protected quantum Hall edge channels (QHECs). The recorded SGM signatures indicate that counterpropagating QHECs, separated by a few hundreds of nanometers, exist along the same edges in graphene and that charge carriers backscattering is achieved by coupling these QHECs through the localized states of antidots located between them. Third, these antidots are shown to act as nano-sized Fabry-Perot interferometers. A simple model is used to reproduce the experimental results, showing that the signatures associated to two distinct regimes, namely the Aharonov-Bohm and Coulomb dominated regimes, can be explained in a single framework ignoring the Coulomb interactions., (FSA - Sciences de l'ingénieur) -- UCL, 2022

Among the plethora of nano-objects, those conducting electricity attracted a great deal of attention with interesting transport properties such as electrical conductance quantization, electron interferences, single charge effects etc. This thesis aims to study puzzling electron transport signatures emerging as electrons flow through nanoscale electrically-conducting networks patterned within two-dimensional electron systems (2DES), where charge transport occurs in the ballistic and coherent regime at low temperature. These networks, constituted from connected nanowires, exhibit peculiar electrical conductance variations that we managed to associate with electron interferences within individual one-dimensional wires and two-dimensional ballistic electron scattering. Local-scale data on electronic transport were collected with scanning gate microscopy (SGM) - a near-field technique where an electrostatically biased nanoscale conductive tip, scanned in the vicinity of the network, is used to tune the electrostatic potential felt by electrons crossing the network. SGM allowed us to decrypt electron transport mechanisms at the local scale, to demonstrate the presence of an unidimensional electron interferometer in the network and to pinpoint a charge injection mechanism analogous to the two-dimensional Rutherford scattering both experimentally and in simulations. In turn, this work provides useful tools in the perspective of building ‘electron optics’ devices, where a local modulation of the electrostatic potential enables, on one hand, to control Fabry-Pérot electron resonances and, on the other hand, to redirect the electron flow in a similar way as an optical lens curves light rays, allowing to study Rutherford-like ballistic scattering mechanisms., (FSA - Sciences de l'ingénieur) -- UCL, 2018

In the Internet-of-Things vision, billions of wireless sensor nodes will be deployed in our environment to gather and communicate information wirelessly over the internet using a dense number of wireless access points. To implement this vision in an energy-sustainable and cost-effective way, targeting zero maintenance and no batteries, the preferred route to power these wireless sensor nodes is to use ambient radio-frequency energy harvesting or wireless power transfer. Alternative currents (AC) generated on the antenna by the electromagnetic radiation are converted to a direct current (DC) using a rectifier circuit. Designing rectifiers to reach high AC/DC power conversion efficiencies down to the microwatt power range represents a major industrial challenge. Therefore, the first part of this dissertation presents an automated design methodology for passive rectifiers. This methodology has been applied to common rectifier architectures in advanced CMOS technologies at 13.56 MHz, 950 MHz and 2.45 GHz and serves as a benchmarking tool to compare the performance of these circuit topologies for similar target specifications. In addition to simulations, this methodology has been validated through the fabrication and characterization of a chip in an industrial 250 nm CMOS technology. The second part of this dissertation tackles the possible energy harvesting of ambient infrared sources at terahertz frequencies for which several graphene rectification devices have been proposed in the scientific literature. The origin of the rectification properties of graphene transistors are therefore investigated at low frequency through the fabrication, characterization and modeling of these devices. Substrate effects have also been identified and modeled., L’internet des objets est annoncé comme un bouleversement majeur qui va opérer la fusion entre le monde réel et le monde virtuel en connectant notre environnement au réseau internet mondial. De nouvelles applications seront alors rendues possibles, depuis la domotique jusqu’aux implants biomédicaux en passant par l'agriculture intelligente. En pratique, il s’agit du déploiement de milliards de systèmes de capteurs autonomes sans fils dans notre environnement, communiquant grâce à l’ultra-connectivité promise par la téléphonie de cinquième génération (5G). La problématique est alors d’alimenter ces systèmes sans recourir à des batteries, car leur usage aurait un impact écologique significatif et les coûts de maintenance liés à leur remplacement seraient prohibitifs. La récupération de l’énergie des ondes radio émises par des sources artificielles (ex. à 13,56 MHz, 950 MHz ou 2,45 GHz) est la voie la plus prometteuse pour alimenter ces systèmes de capteurs sans fils. Les courants alternatifs (AC) générés au niveau de l’antenne doivent pour cela être convertis vers un courant continu (DC) à l’aide d’un circuit redresseur. Un enjeu industriel majeur est alors le dimensionnement de ce type de circuits pour atteindre de hauts rendements de conversion (AC/DC) pour des puissances de l’ordre de 10 à 100 microwatts. La première partie de cette thèse a permis de développer une méthodologie de conception automatisée qui maximise la puissance DC obtenue par le redresseur par rapport à la puissance AC disponible. Cette méthodologie a été validée à différentes fréquences par des simulations dans des technologies CMOS avancées 65 nm et 28 nm FDSOI, ainsi que par la fabrication et la mesure en laboratoire d’un prototype fabriqué dans une technologie industrielle 250 nm. Outre les sources artificielles, il est envisagé de récupérer l’énergie du rayonnement infrarouge émis naturellement par les corps à température ambiante (28 THz). La deuxième partie de cette thèse a donc consisté à, (FSA - Sciences de l'ingénieur) -- UCL, 2017

During recent years, new types of materials have been discovered with unique properties. One family of such materials is two-dimensional (2D) materials with unusual electronic, mechanical and optical properties, which led to their extensive study for diverse applications. Transition metal carbides, carbonitrides and nitrides (MXenes) are among the latest additions to the 2D world, which are derived from their layered precursors, MAX phases, layered early transition metal ternary carbides and nitrides, composed of M, an early transition metal, A, a group A element and X is C and/or N. In this thesis, as its self-telling title indicated, we present our work on the synthesis, structural characterization and the electron transport in the MAX phases and 2D MXenes. For MAX phases: using high temperature solution growth and slow cooling technique, several MAX phases single crystals have been successfully grown, including Cr2AlC, V2AlC, Ti3SiC2, etc. Structural characterization confirms the single crystalline character of the samples. Experimentally, a set of transport data was obtained from single crystals of V2AlC and Cr2AlC as a function of temperature and magnetic field. Theoretically, a general 2D nearly free electron model was proposed for describing the weak field magneto-transport properties of MAX phases. For MXenes: large-scale V2CTx (T stands for the surface terminations, hydroxyl, oxygen or fluorine)) MXene flakes were successfully synthesized. Mechanical delamination of multilayered V2CTx flakes into few layer flakes and transfer on Si/SiO2 substrate was also achieved. We then detailed the electrical device fabrication and proceeded with electrical measurements down to low temperature, with the aim to extract information on charge carrier behavior. First-hand transport data were obtained on V2CTx MXenes, which brings new understandings to this novel type of 2D material., (FSA - Sciences de l'ingénieur) -- UCL, 2017

In the fields of condensed matter physics and electronics, the advent of new types of materials with unconventional charge carrier dynamics first gives possibilities to explore peculiar physical phenomena, which can then open novel routes for the design of electronic devices. It is the case of graphene, a two-dimensional carbon crystal where charge carriers behave like massless relativistic particles. To benefit from the outstanding properties potentially offered by such a material, the first step is to understand how charge carriers propagate through graphene devices in order to control their motion inside the considered structures. In this thesis, we present experimental results on the electronic transport inside two types of graphene-based devices : (I) nitrogen-incorporated graphene Hall bars and (II) graphene quantum ring (QR) interferometers. A particular attention was brought to the investigation of coherent effects, involving charge carrier interferences. In addition, a scanning probe technique, called scanning gate microscopy (SGM), was used to image and control charge carrier dynamics inside mesoscopic graphene QRs at the local nm-scale. In nitrogen-incorporated graphene, our results evidence the substantial influence of the disorder induced by the presence of nitrogen atoms in the graphene lattice. We then directly observe in real space the effect of disorder in different transport regimes in SGM images of QRs. Beside the influence of disorder, SGM mapping of the charge carrier wave functions inside graphene QRs reveals the formation of scarlike features reminiscent of semi-classical periodic orbits scarring the local density of states along the QRs’ arms. Patterns imaged in SGM are found to be recurrent when varying the charge carrier energy, consistent with theoretical predictions for relativistic quantum scars., (FSA - Sciences de l'ingénieur) -- UCL, 2016

The rapid development of Nanotechnology that tends to progressively replace present systems with novel miniaturized devices based on nano-objects induces an increasing demand for new types of nanomaterials with different structures and improved physical properties. This leads, along with other conceivable candidates, to the production of magnetic nanowires (NWs) and nanotubes (NTs) with unique magnetic properties. These nanomaterials have potential applications in microwave devices, chemical sensors, high density data storage devices, light emitters, etc... They also got an enormous attraction in biological and biomedical applications e.g. for hyperthermia usage to treat cancer cells. When it comes to the application of NW or NT arrays for data storage, there is a basic need of understanding and controlling the magnetization behavior and reversal process. In the present work, magnetic force microscopy (MFM) has been used to characterize low and medium packing density arrays of magnetic NWs and NTs manufactured by electrodeposition in nanoporous polycarbonate membranes. This was realized using a modified MFM setup allowing in-situ and in-field measurements. The influence of the material and geometrical parameters (composition, diameter, shape) on the magnetization reversal process was studied. It is demonstrated that in low density arrays, the dipolar interaction is negligible and that the switching field distribution broadening is mainly due to intrinsic parameters such as the size (diameter) distribution of the NWs. MFM also allows observing the magnetic reversal of individual NWs. This provides detailed information on the local dependence of the dipolar interaction on the inter-wire distance at the nanoscale, which cannot be done by standard magnetometry techniques., (FSA - Sciences de l'ingénieur) -- UCL, 2015