Γραφένιο και διδιάστατα υλικά για εφαρμογές στη Νανοηλεκτρονική

Στην παρούσα διατριβή μελετήθηκαν διδιάστατα υλικά και διερευνήθηκε η δυνατότητα ανάπτυξής τους, οι φυσικές τους ιδιότητες καθώς και η πιθανή εφαρμογή τους σε ηλεκτρονικές διατάξεις. Διερευνήθηκε η ανάπτυξή του γραφενίου σε υποστρώματα νικελίου τα οποία προσφέρουν την δυνατότητα για διαδικασίες ανάπτυξης με χημική εναπόθεση ατμών σε θερμοκρασίες αρκετά χαμηλότερες από αυτές που τυπικά χρησιμοποιούνται. Περαιτέρω κατασκευάστηκαν διατάξεις οι οποίες συνδυάζουν την κλασσική CMOS δομή του MOSFET τρανζίστορ με την διδιάστατη φύση του γραφενίου. Το γραφένιο ενσωματώθηκε στην πύλη πυκνωτικών διατάξεων και δομών τρανζίστορ και μελετήθηκε η επίδρασή του από την σκοπιά του φαινομένου της κβαντικής αρνητικής χωρητικότητας. Τέλος, πέραν του γραφενίου διερευνήθηκε και επιτεύχθηκε η ανάπτυξη ενός άλλου διδιάστατου υλικού, του διτελλουριδίου του αφνίου – HfTe2. Μελετήθηκε η ποιότητα του υμενίου, η κρυσταλλική δομή καθώς και για πρώτη φορά η ηλεκτρονιακή δομή του υλικού η οποία υπέδειξε την ύπαρξη κώνου Dirac όπως στο γραφένιο.In this thesis two-dimensional materials were studied. We investigated their growth, their physical properties and their possible applicability in electronic devices. Specifically we investigated the growth of graphene on nickel substrates which offer the potential for growth procedures with chemical vapor deposition in temperatures much lower than those typically used. Furthermore, we created device which combine the classic CMOS structure of MOSFET transistors with the two-dimensional nature of graphene. The graphene films were embedded in the gate of capacitive devices and we studied its effect on the phenomenon of quantum negative capacitance. Finally, we investigated and achieved the growth of two-dimensional films of Hafnium Ditelluride – HfTe2. We studied the quality of the film, its crystalline structure and for the first time its electronic structure, which showed signs for the existence of Dirac cone like in graphene

On the semi-annual variation of relativistic electrons in the outer radiation belt

The nature of the semi-annual variation in the relativistic electron fluxes in the Earth's outer radiation belt is investigated using Van Allen Probes (MagEIS and REPT) and Geostationary Operational Environmental Satellite Energetic Particle Sensor (GOES/EPS) data during solar cycle 24. We perform wavelet and cross-wavelet analysis in a broad energy and spatial range of electron fluxes and examine their phase relationship with the axial, equinoctial and Russell–McPherron mechanisms. It is found that the semi-annual variation in the relativistic electron fluxes exhibits pronounced power in the 0.3–4.2 MeV energy range at L shells higher than 3.5, and, moreover, it exhibits an in-phase relationship with the Russell–McPherron effect, indicating the former is primarily driven by the latter. Furthermore, the analysis of the past three solar cycles with GOES/EPS indicates that the semi-annual variation at geosynchronous orbit is evident during the descending phases and coincides with periods of a higher (lower) high-speed stream (HSS) (interplanetary coronal mass ejection, ICME) occurrence

Surface-Enhanced Raman Spectroscopy of Graphene Integrated in Plasmonic Silicon Platforms with Three-Dimensional Nanotopography

Integrating graphene with plasmonic nanostructures results in multifunctional hybrid systems with enhanced performance for numerous applications. In this work, we take advantage of the remarkable mechanical properties of graphene to combine it with scalable three-dimensional (3D) plasmonic nanostructured silicon substrates, which enhance the interaction of graphene with electromagnetic radiation. Large areas of femtosecond laser-structured arrays of silicon nanopillars, decorated with gold nanoparticles, are integrated with graphene, which conforms to the substrate nanotopography. We obtain Raman spectra at 488, 514, 633, and 785 nm excitation wavelengths, spanning the entire visible range. For all excitation wavelengths, the Raman signal of graphene is enhanced by 2–3 orders of magnitude, similarly to the highest enhancements measured to date, concerning surface-enhanced Raman spectroscopy of graphene on plasmonic substrates. Moreover, in contrast to traditional deposition and lithographic methods, the fabrication method employed here relies on single-step, maskless, cost-effective, rapid laser processing of silicon in water, amenable to large-scale fabrication. Finite-difference time-domain simulations elucidate the advantages of the 3D topography of the substrate. Conformation of graphene to Au-decorated silicon nanopillars enables graphene to sample near fields from an increased number of nanoparticles. Due to synergistic effects with the nanopillars, different nanoparticles become more active for different wavelengths and locations on the pillars, providing broad-band enhancement. Nanostructured plasmonic silicon is a promising platform for integration with graphene and other 2D materials, for next-generation applications of large-area hybrid nanomaterials in the fields of sensing, photonics, optoelectronics, and medical diagnostics

Epitaxial 2D MoSe₂ (HfSe₂) Semiconductor/2D TaSe₂ Metal van der Waals Heterostructures

Molecular beam epitaxy of 2D metal TaSe₂/2D MoSe₂ (HfSe₂) semiconductor heterostructures on epi-AlN(0001)/Si(111) substrates is reported. Electron diffraction reveals an in-plane orientation indicative of van der Waals epitaxy, whereas electronic band imaging supported by first-principles calculations and X-ray photoelectron spectroscopy indicate the presence of a dominant trigonal prismatic 2H-TaSe₂ phase and a minor contribution from octahedrally coordinated TaSe₂, which is present in TaSe₂/AlN and TaSe₂/HfSe₂/AlN but notably absent in the TaSe₂/MoSe₂/AlN, indicating superior structural quality of TaSe₂ grown on MoSe₂. Apart from its structural and chemical compatibility with the selenide semiconductors, TaSe₂ has a workfunction of 5.5 eV as measured by ultraviolet photoelectron spectroscopy, which matches very well with the semiconductor workfunctions, implying that epi-TaSe₂ can be used for low-resistivity contacts to MoSe₂ and HfSe₂

Prediction of solar proton event fluence spectra from their peak flux spectra

Solar Proton Events (SPEs) are of great importance and significance for the study of Space Weather and Heliophysics. These populations of protons are accelerated at high energies ranging from a few MeVs to hundreds of MeVs and can pose a significant hazard both to equipment on board spacecrafts as well as astronauts as they are ionizing radiation. The ongoing study of SPEs can help to understand their characteristics, relative underlying physical mechanisms, and help in the design of forecasting and nowcasting systems which provide warnings and predictions. In this work, we present a study on the relationships between the Peak Flux and Fluence spectra of SPEs. This study builds upon existing work and provides further insights into the characteristics and the relationships of SPE Peak flux and Fluence spectra. Moreover it is shown how these relationships can be quantified in a sound manner and exploited in a simple methodology with which the Fluence spectrum of an SPE can be well predicted from its given Peak spectrum across two orders of magnitude of proton energies, from 5 MeV to 200 MeV. Finally it is discussed how the methodology in this work can be easily applied to forecasting and nowcasting systems

Harmonization of RBSP and Arase Energetic Electron Measurements Utilizing ESA Radiation Monitor Data

Accurate measurements of trapped energetic electron fluxes are of major importance for the studies of the complex nature of radiation belts and the characterization of space radiation environment. The harmonization of measurements between different instruments increases the accuracy of scientific studies and the reliability of data-driven models that treat the specification of space radiation environment. An intercalibration analysis of the energetic electron flux measurements of the Magnetic Electron Ion Spectrometer (MagEIS) and the Relativistic Electron-Proton Telescope (REPT) instruments on-board the Van Allen Probes (VAP) Mission versus the measurements of the Extremely High Energy Electron Experiment (XEP) unit on-board Arase satellite is presented. The performed analysis demonstrates a remarkable agreement between the majority of MagEIS and XEP measurements and suggests the rescaling of MagEIS HIGH unit and of REPT measurements for the treatment of flux spectra discontinuities. The proposed adjustments were validated successfully using measurements from ESA Environmental Monitoring Unit (EMU) on-board GSAT0207 and the Standard Radiation Monitor (SREM) on-board INTEGRAL. The derived results lead to the harmonization of science-class experiments on-board VAP (2012–2019) and Arase (2017–) and propose the use of the data sets as reference in a series of space weather and space radiation environment developments

Review of solar energetic particle models

Solar Energetic Particle (SEP) events are interesting from a scientific perspective as they are the product of a broad set of physical processes from the corona out through the extent of the heliosphere, and provide insight into processes of particle acceleration and transport that are widely applicable in astrophysics. From the operations perspective, SEP events pose a radiation hazard for aviation, electronics in space, and human space exploration, in particular for missions outside of the Earth’s protective magnetosphere including to the Moon and Mars. Thus, it is critical to improve the scientific understanding of SEP events and use this understanding to develop and improve SEP forecasting capabilities to support operations. Many SEP models exist or are in development using a wide variety of approaches and with differing goals. These include computationally intensive physics-based models, fast and light empirical models, machine learning-based models, and mixed-model approaches. The aim of this paper is to summarize all of the SEP models currently developed in the scientific community, including a description of model approach, inputs and outputs, free parameters, and any published validations or comparisons with data.</p

Graphene and two-dimensional materials for applications in nanoelectronics

In this thesis two-dimensional materials were studied. We investigated their growth, their physical properties and their possible applicability in electronic devices. Specifically we investigated the growth of graphene on nickel substrates which offer the potential for growth procedures with chemical vapor deposition in temperatures much lower than those typically used. Furthermore, we created device which combine the classic CMOS structure of MOSFET transistors with the two-dimensional nature of graphene. The graphene films were embedded in the gate of capacitive devices and we studied its effect on the phenomenon of quantum negative capacitance. Finally, we investigated and achieved the growth of two-dimensional films of Hafnium Ditelluride – HfTe2. We studied the quality of the film, its crystalline structure and for the first time its electronic structure, which showed signs for the existence of Dirac cone like in graphene.Στην παρούσα διατριβή μελετήθηκαν διδιάστατα υλικά και διερευνήθηκε η δυνατότητα ανάπτυξής τους, οι φυσικές τους ιδιότητες καθώς και η πιθανή εφαρμογή τους σε ηλεκτρονικές διατάξεις. Διερευνήθηκε η ανάπτυξή του γραφενίου σε υποστρώματα νικελίου τα οποία προσφέρουν την δυνατότητα για διαδικασίες ανάπτυξης με χημική εναπόθεση ατμών σε θερμοκρασίες αρκετά χαμηλότερες από αυτές που τυπικά χρησιμοποιούνται. Περαιτέρω κατασκευάστηκαν διατάξεις οι οποίες συνδυάζουν την κλασσική CMOS δομή του MOSFET τρανζίστορ με την διδιάστατη φύση του γραφενίου. Το γραφένιο ενσωματώθηκε στην πύλη πυκνωτικών διατάξεων και δομών τρανζίστορ και μελετήθηκε η επίδρασή του από την σκοπιά του φαινομένου της κβαντικής αρνητικής χωρητικότητας. Τέλος, πέραν του γραφενίου διερευνήθηκε και επιτεύχθηκε η ανάπτυξη ενός άλλου διδιάστατου υλικού, του διτελλουριδίου του αφνίου – HfTe2. Μελετήθηκε η ποιότητα του υμενίου, η κρυσταλλική δομή καθώς και για πρώτη φορά η ηλεκτρονιακή δομή του υλικού η οποία υπέδειξε την ύπαρξη κώνου Dirac όπως στο γραφένιο