Search

Phase-change materials (PCMs) are an important family of alloys employed in non-volatile memories and neuromorphic devices. These devices exploit the ability of PCMs to undergo rapid transitions between amorphous and crystalline phases at moderately high temperature. At ambient conditions, both phases are stable. These properties imply a very strong temperature dependence of the crystallization kinetics, which has been attributed to the high fragility of the supercooled liquid phase. In this work, the fragility index of GeTe, an important PCM, is extracted from a computational exploration of the potential energy landscape of the system by molecular dynamics simulations. For this purpose, a neural-network potential is used to describe the interatomic interactions, which enables the evaluation of the configurational entropy in the deeply supercooled regime. The viscosity and the relaxation times are also calculated in the same temperature range. Finally, the Adam-Gibbs relation is used to extrapolate the data to low temperatures and estimate the glass transition temperature and the fragility index. The latter quantity turns out to be of the order of 135–140, which shows that liquid GeTe is a highly fragile system.

We investigated the photonic properties of topological lattices specifically designed to open a terahertz photonic bandgap in a germanium matrix. An extensive analysis based on three-dimensional finite element method enabled the identification of the requirements in terms of vertical height and lateral dimensions of the Ge motifs to suitably vary the bulk gap between 41 and 65 THz. Furthermore, by interfacing topologically distinct domains, we were able to observe the emergence of non-trivial edge modes that remain stable at a constant frequency despite the varying geometry of the unit cells. These findings provide valuable insights for designing resilient and effective photonic devices based on topological photonic crystals, and open new avenues for the realization of advanced quantum technologies in the THz regime.

CubeSat and small satellites play a very important role in modern space exploration. Their success and diverse capabilities rely on the development of efficient and compact sub-systems. This task is not trivial due to multiple challenges posed by their small size and mass, and the solution calls for conceptually new designs for the fabrication and integration of complex miniaturized satellite components. The importance of additive techniques in small satellite manufacturing is steadily increasing as they enable rapid, large-scale production of sophisticated architectures, such as hollow, webbed parts with a cell-like structure similar to animal bone, with lower mass and improved functionality. Moreover, these architectures feature higher strength, enhanced heat transfer, and efficient thermal and electromagnetic radiation shielding. When compared to traditional subtractive technologies like cutting and milling, additive manufacturing proves to be more versatile and effective in realizing architectures with an increasing intricacy of shapes, structures, and compositions. The perspective explores the suitability of 3D printing in various satellite production tasks, including the propulsion system components and satellite elements. Looking ahead, the challenges and advantages of integrating 3D printing technology into satellite production, emphasizing the need for continuous development through consolidated, proactive collaborative efforts of many devoted teams are outlined.

Fast photoluminescence and scintillation with a Stokes shift larger than 1 eV is achieved in hetero-ligand metal–organic framework (MOF) crystals comprising inorganic linking nodes and fluorescent conjugated ligands. By finely engineering the MOF composition with the use of ligands with strictly complementary emission and absorption properties and highly delocalized molecular electronic orbitals, the singlet excitons diffusion is enhanced through the ligand framework, fully exploiting both Förster and Dexter energy transfer mechanisms. This allows for the sensitization of energy acceptor ligand fluorescence by ultrafast non-radiative energy transfer with a rate up to the THz range. This efficient antenna effect instantly activates the MOF scintillation with a Stokes shift as large as 1.3 eV in the blue spectral range, matching the highest sensitivity spectral window of the best photodetector available. This is obtained using a minimal doping level of the energy acceptor species, with a consequent elimination of emission re-absorption that allows the achievement of a 500% increment of the MOFs scintillation efficiency and the detection of the radioactive krypton isotope 85Kr from the gas phase with an improved sensitivity compared with the reference material.

The chemical vapor deposition of silicon on a patterned silicon substrate leads to the formation of 3D microcrystals, which, due to their inclined top facets and high aspect ratio, produce a light-trapping effect enhancing the optical absorption in the near-infrared (NIR). In this work, it is demonstrated that Si microcrystals can form the building blocks of a new class of NIR sensitive photodetectors operating in a linear or avalanche regime. Microcrystal-based devices are designed by coupling a 2D kinetic-growth model with a Poisson drift-diffusion solver and fabricated by combining electron beam lithography and low-energy plasma-enhanced chemical vapor deposition (LEPECVD). The optoelectronic properties of microcrystal-based p–i–n photodiodes are investigated both theoretically and experimentally by means of finite-difference time-domain (FDTD) simulations and responsivity measurements. At 1000 nm wavelength, the responsivity of microcrystal-based devices is six times higher than that of an equivalent mesa diode. Moreover, the photocurrent gains of Si microcrystals operating as an avalanche photodiode (APD), at the same wavelength, reaches 2 × 104 demonstrating the potentialities of substrate patterning, combined with epitaxial growth, for amplified photodetection applications.

Superlattices made of alternating blocks of the phase change compound Sb2Te3 and of TiTe2 confining layers have been recently proposed for applications in neuromorphic devices. The Sb2Te3 /TiTe2 heterostructure allows for a better control of multiple intermediate resistance states and for a lower drift with time of the electrical resistance of the amorphous phase. However, Sb2Te3 suffers from a low data retention due to a low crystallization temperature T x . Substituting Sb2Te3 with a phase change compound with a higher T x , such as GeTe, seems an interesting option in this respect. Nanoconfinement might, however, alters the crystallization kinetics with respect to the bulk. In this work, we investigated the crystallization process of GeTe nanoconfined in geometries mimicking GeTe/TiTe2 superlattices by means of molecular dynamics simulations with a machine learning potential. The simulations reveal that nanoconfinement induces a mild reduction in the crystal growth velocities which would not hinder the application of GeTe/TiTe2 heterostructures in neuromorphic devices with superior data retention.

The European Spallation Source, which is under construction in Lund, Sweden, aims to be the world's brightest source of neutrons. During the first phase of construction, users will have access to a suite of 15 instruments, which will produce neutrons through a butterfly-shaped moderator system located above the spallation target. The HighNESS project, funded by the European Union Horizon 2020 program, has the goal of providing a new design for the second moderator, which will be situated below the spallation target and will be the most intense neutron source in the world. The new moderator will provide a higher intensity of neutrons in the cold, very cold, and ultracold energy regions, which will advance research in several areas of condensed matter and offer unique opportunities in fundamental physics. Work Package 2 of the HighNESS project is focused on supporting the design of the second moderator by providing nuclear data support for neutronics calculations and improving the accompanying physics software. This paper showcases recent advancements in NJOY + NCrystal, a tool for creating thermal neutron scattering-evaluated nuclear data files and continued work on nanodiamonds as advanced neutron reflectors.

The resurgence of masks due to the COVID-19 pandemic underscores their essential role as a simple yet crucial innovation. The evolution of masks mirrors the dynamic shifts in societal demands over time. Through a comprehensive examination of textile materials employed in medical contexts, this review highlights the enduring significance of filtering and antibacterial attributes of masks since their inception. As living standards advance, there is an escalating emphasis on the breathability of masks. Furthermore, this paper delves into the prospective significance and potential challenges associated with the development of comfortable, biocompatible, self-cleaning masks in the future. Different technologies, including mask detection systems, and advanced materials have been proposed in the framework of a multidisciplinary context. The multifaceted journey of masks encapsulates their pivotal role in safeguarding public health and underscores the ongoing pursuit of improved performance and user comfort.

Energy sustainability and interconnectivity are the two main pillars on which cutting-edge architecture is based and require the realization of energy and intelligent devices that can be fully integrated into buildings, capable of meeting stringent regulatory requirements and operating in real-world conditions. Luminescent solar concentrators, particularly those based on near-infrared emitting reabsorption-free quantum dots, are considered good candidates for the realization of semi-transparent photovoltaic glazing, but despite important advances in optical property engineering strategies, studies of finished devices suitable for real-world operation are still lacking. In this paper, the first example of a fully assembled quantum dot luminescent solar concentrator-based photovoltaic glazing is demonstrated that meets all international standards for photovoltaic and building elements. It is also shown that these devices are capable of functioning as efficient Visible Light Communication (VLC) receivers even under full sunlight, thus combining energy and wireless connectivity functions in a realistic solution for smart, sustainable buildings.

Dall'isolamento del grafene, la nuova classe dei materiali bidimensionali (2D) si è continuamente ampliata, aprendo la strada a molte opportunità nell'evoluzione delle nanotecnologie. In questo gruppo di materiali, gli Xeni sono una classe emergente di materiali simili al grafene, sintetizzati mediante epitassia da fasci molecolari (MBE), che mostrano proprietà eccezionali grazie alla loro natura 2D. Tra questi, il silicene gioca un ruolo cruciale, grazie alla potenziale facilità di integrazione nell'attuale tecnologia al silicio, mentre lo stanene presenta interessanti proprietà, soprattutto legate all'accoppiamento spin-orbita (SOC) e alla sua possibile integrazione come isolante topologico 2D. In questo quadro, lo scopo di questo lavoro di tesi è legato allo studio e all'ottimizzazione della reale manipolazione di Xeni, come il silicene e lo stanene, dal punto di vista del processo, al fine di discutere e risolvere le principali problematiche versola loro implementazione in strutture di dispositivi. In particolare, i campioni considerati sono di tre tipi: silicene cresciuto su Ag(111) su mica, eterostrutture di silicene-stanene cresciute su Ag(111) su mica e campioni di stanene cresciuti su substrato InSb. A differenza del grafene, l'ibridazione mista sp2-sp3 degli Xeni, e in particolare del silicene, comporta un'elevata reattività ambientale che porta alla degradazione del materiale. Lo sviluppo di schemi di incapsulamento affidabili è fondamentale per la scoperta di nuove possibili applicazioni; pertanto, un ruolo chiave in questo lavoro è legato alle eterostrutture di silicene-stanene, in cui il silicene è inserito tra uno strato Al2O3 di protezione sulla superficie superiore e lo stanene su Ag(111) sulla faccia inferiore. L'introduzione di uno strato di stanene ha permesso di sviluppare uno schema di incapsulamento completo per proteggere il silicene dalla degradazione in condizioni ambientali quando viene disassemblato dal substrato di crescita. Co, Since the isolation of graphene, the new class of two-dimensional (2D) materials has been continuously expanded, paving the way for many opportunities in the evolution of nanotechnology. In this materials group, Xenes are an emerging class of graphene-like materials, synthesized by molecular beam epitaxy (MBE), that show exceptional properties due to their 2D nature. Among these, silicene plays a crucial role, as a possible future integration in the current silicon technology, and stanene presents interesting properties, especially related to the spin-orbit coupling (SOC) and its possible integration as a 2D topological insulator. In this framework, the purpose of this work of thesis is related to the study and optimization of the actual manipulation of Xenes, such as silicene and stanene, from a processing point of view, in order to discuss and solve the main issues toward the implementation into device structures. In particular, the considered samples are of three types: silicene grown on Ag(111)-on-mica substrate, heterostructures of silicene-stanene grown on Ag(111)-on-mica and samples of few-layer stanene grown on InSb substrate. Unlike graphene, the mixed sp2-sp3 hybridization of Xenes, and in particular of silicene, results in a high environmental reactivity leading to material degradation. The development of reliable encapsulation schemes is fundamental for the discovery of new possible applications, hence, a key role in this work is related to the vertically stacked silicene-stanene heterostructures, where silicene is sandwiched in between an Al2O3 capping layer on the top surface and the stanene on Ag(111) crystal at the bottom face. The introduction of a stanene buffer layer enabled to develop an all-around encapsulation scheme to protect silicene against degradation under environmental conditions when disassembled from the native substrate. By combining different chemical and structural techniques, such as X-ray photoelectron spectroscopy (XPS) and Raman

The phase change compound Ge2Sb2Te5 (GST225) is exploited in advanced non-volatile electronic memories and in neuromorphic devices which both rely on a fast and reversible transition between the crystalline and amorphous phases induced by Joule heating. The crystallization kinetics of GST225 is a key functional feature for the operation of these devices. We report here on the development of a machine-learned interatomic potential for GST225 that allowed us to perform large scale molecular dynamics simulations (over 10,000 atoms for over 100 ns) to uncover the details of the crystallization kinetics in a wide range of temperatures of interest for the programming of the devices. The potential is obtained by fitting with a deep neural network (NN) scheme a large quantum-mechanical database generated within density functional theory. The availability of a highly efficient and yet highly accurate NN potential opens the possibility to simulate phase change materials at the length and time scales of the real devices.

Questa tesi è una monografia sull'uso del rilevatore a stato solido di carburo di silicio (SiC) come contatore e spettrometro di neutroni veloci per le applicazioni di plasmi da fusione. In essa sono illustrate le attività di ricerca condotte su una serie di prototipi SiC per caratterizzarli, contribuendo al loro sviluppo e alla valutazione del loro potenziale. La rilevazione dei neutroni è il principale modo per ottenere informazioni su un plasma a fusione in un tokamak. Attraverso di essa è possibile determinare la potenza di fusione, individuare la forma e la densità del plasma e stimare la produzione di trizio nel breeding blanket. Misurando lo spettro energetico dei neutroni è inoltre possibile dedurre altre proprietà del plasma, come il rapporto tra deuterio e trizio o la temperatura ionica. Tuttavia, il contesto pone sfide senza precedenti per le diagnostiche: elevati flussi di neutroni, la presenza di intensi campi magnetici, temperature elevate e alti livelli di radiazione. I rilevatori allo stato solido basati sul SiC presentano caratteristiche molto promettenti per affrontare tali sfide. Poiché il SiC è un design di rilevatore a stato solido relativamente inesplorato, le attività di ricerca di questo progetto di dottorato hanno avuto lo scopo di caratterizzare una serie di prototipi SiC, sviluppati in collaborazione con IMM-CNR, individuando i loro punti di forza rispetto allo stato dell'arte dei rilevatori a stato solido, il rilevatore a diamante. Nel corso dell’attività sono stati caratterizzati otto prototipi di SiC mediante irraggiamento di neutroni e particelle cariche. Sono stati anche sviluppati metodi per l'analisi dei dati, la gestione dell'elettronica, la manifattura della componentistica ed altro ancora. Le caratteristiche del rilevatore (efficienza, risoluzione energetica, stabilità ed altro) sono state studiate in diverse configurazioni, individuando metodi per migliorarle e utilizzando i risultati come base per la produzione di n, This thesis is a monography on the use of Silicon Carbide (SiC) Solid State Detector (SSD) as a fast neutron counter and spectrometer for fusion plasma applications. It details the research activities conducted on a series of SiC prototypes to characterize them, contributing to their development and assessment of their potential. Neutron detection is possibly the main way of obtaining information on a fusion plasma in a Tokamak. Through it is possible to determine the fusion power, determine the plasma location and density and estimate the production of tritium in the breeder elements. By measuring the neutron energy spectrum it is also possible to infer other plasma properties, like the fuel ion ratio or the ion temperature. Tokamak environments, though, put unprecedented challenges on the diagnostics: large neutron fluxes, the presence of intense magnetic fields, high temperatures, and intense radiation. SSDs feature very promising characteristics for handling the aforementioned challenges. Since the SiC is a relatively unexplored SSD design, the research activities of this PhD project aimed to characterize a series of SiC prototypes, developed in conjunction with IMM-CNR, finding their strengths compared to the state of art of SSDs, the diamond detector. The activity has been dedicated to characterization of eight prototypes through neutron and charged particles irradiation, developing methods for data analysis, electronics management, hardware manufacturing and more. Detection characteristics (efficiency, energy resolution, stability and more) were studied in different configurations, finding methods to improve them and using the results as a basis for the manufacturing of new, improved prototypes. Stress tests were also conducted under harsh conditions, highlighting the functionality of the detector in harsh environments. Unprecedented methods of operations, like the partial depletion detection, were also experimented, finding new beneficial uses for the detect

I sensori basati su sistemi Micro Elettromeccanici (MEMS) sono oggigiorno la soluzione piu usata per costruire microfoni a bassa potenza, occupando area ridotta e dimostrando performance di alto livello. L’integrazione di diversi microfoni MEMS su dispositive smart portatili é necesaria per ottenere alta qualitá di suono e questo impone di scalare il package del microfono, il che é ottenuto sia minimizzando il sensore, sia scalando l’area dell’elettronica di readout. Le soluzioni circuitali attuali per costruire il readout di sensori audio usano tecniche come il Chopping, per ridurre il rumore Flicker che é fortemente presente in applicazioni a bassa frequenza come i sistemi audio, o amplificatori a transconduttanza, che possono raggiungere alti guadagni e alta linearitá a spese di incrementare area e consumo di potenza. In questa tesi vengono presentate diverse soluzioni di design per costruire amplificatori di circuiti di reaodut per interfacce di microfoni MEMS. I design sfruttano una tecnologia scalata CMOS a 55nm, che rappresenta un buon compromesso tra costi di produzione e riduzione di area ottenibile. Le topologie circuitali presentate nella tesi sono due: il Super Source Follower (SSF), in due diverse versioni, una con dispositivo di ingresso PMOS e una con dispositivo di ingresso NMOS, e il Differential Difference Amplifier (DDA). La prima topologia (SSF) é la naturale evoluzione della topologia piu semplice per implementare un readout a carica costante per microfoni capacitivi. La seconda topologia (DDA) aggiunge complessitá al sistema, ma permette di ottenere corrente ridotta e programmabilitá di guadagno, nonostante la linearitá e il livello di rumore ne risentano e debbano affrontare un’ulteriore ottimizzazione per avere performace migliori dei prodotti attuali. Entrambe le soluzioni circuitali presentate in questo lavoro offrono un considerevole vantaggio in termini di area e potenza e risultano essere competitive in termini di performance ri, Micro Electro-Mechanical Systems (MEMS) sensors are nowadays the main solution to build low-power microphones, occupying reduced area though featuring high-level performances. The integration of multiple MEMS microphones on portable smart devices is needed to obtain high sound quality, and this imposes the scaling of microphone package, achieved through both minimizing the sensor and scaling the readout electronics area. Current circuital solutions for audio sensors readout mainly employ switched circuits, to implement techniques such as Chopping, to reduce low frequency flicker noise strongly affecting low frequency applications such as audio systems, or transconductance amplifiers, which can reach high gain and linearity, at the expenses of increased area and power consumption. In this thesis different design solutions to build up sensor interface amplifiers for MEMS microphone readout circuits are explored. The designs exploit a scaled technology 55nm technology node, which represents a fair trade-off between production costs and achievable area reduction. The here presented topologies are two: the Super Source Follower (SSF), in both PMOS and NMOS input device versions, and the Differential Difference Amplifier (DDA). The first circuits (SSFs) are the natural evolution of the most fundamental circuital topology through which it is possible to implement constant charge readout method for capacitive microphone. The second topology (DDA) adds complexity and elegance to the whole microphone system, achieving low current consumption and versatile gain programmability, although linearity and noise have still to face a thorough optimization to perform beyond current products. However, both circuital solutions reported in this work offer considerable area and power advantages, and results to be competitive in terms of performance with respect to current solutions, although exploiting a deep sub-micron technology node as the 55nm could result disadvantageous for the desi

Closing the gap between experiments and simulations in the investigation of high-pressure silicon phase transitions calls for advanced, new-generation modeling approaches. By exploiting massive parallelization, we here provide molecular dynamics (MD) simulations of Si nanoindentation based on the Gaussian Approximation Potential (GAP). Results are analyzed by exploiting a customized Neural Network Phase Recognition (NN-PR) approach, helping to shed light on the phase transitions occurring during the simulations. Our results show that GAP provides a realistic description of silicon phase transitions. With the support of NN-PR method, the formation mechanism and stability of high-pressure phases are comprehensively studied. Additionally, we also show how simulations based on the less demanding and widely-used Tersoff potential are still useful to investigate the role played by the indenter tip modeling. However, high-pressure phases obtained with GAP are more consistent with observations made in nanoindentation experiments, removing a spurious phase that is shown by Tersoff simulations. This behavior is explained on the base of relative phase stability with comparison with Density Functional Theory (DFT) calculations. This work provides insight into the application of state-of-the-art Machine Learning (ML) methods on nanoindentation simulations, enabling further understanding of the phase transition mechanisms in silicon.

I metodi Machine Learning (ML) in generale e i Neural Network (NN) in particolare rappresentano una delle più attive aree di ricerca degli ultimi anni. La fisica e le Scienze dei Materiali computazionali non sono state immuni a questo sviluppo: approcci NN e ML stanno rapidamente venendo adottati come nuovi strumenti, con la promessa di poter risolvere problemi che fino ad ora sono rimasti intrattabili. In questa tesi verranno presentati alcuni sviluppi di questa direzione di ricerca, concentrandosi sulla possibilità di attaccare problemi tradizionali da nuovi punti di vista. In particolare, la discussione verterà come sia possibile adattare di metodi Machine Learning “puri” per la simulazione di fenomeni mesoscala, come dislocazioni e l’evoluzione morfologica di superfici. Lo scopo è metodologico e la Tesi cerca di essere auto sussistente. In seguito all’introduzione dei concetti generali del Machine Learning e dei Neural Network, si affrontano diversi modelli fisici. I metodi sono presentati contestualmente all’applicazione e sono presentati dal più semplice al più sofisticato. Iniziando con l’approssimazione dell’interazione tra dislocazioni in film eteroepitassiali con semplici feedforward fully connected Neural Netowork, la tesi si sviluppa quindi verso la simulazione dell’evoluzione delle superfici di film coerenti e modelli phase field, utilizzando metodi convoluzionali e ricorrenti rispettivamente. Infine, viene presentata una applicazione di Generative Adversarial Networks ad un problema di meccanica stocastica., Machine Learning (ML) and Neural Networks (NN) specifically represent one of the most active areas of research in the last year. Computational physics and materials science have not been immune to this: Neural Networks and other ML approaches are being adopted as new tools, which promise to solve problems which have been intractable so far. In this Thesis, some advancements in this respect will be shown, focusing on the possibility of tackling traditional problems from new directions. In particular, the discussion will revolve around how to adapt pure Deep Learning approaches to the simulation of mesoscale phenomena such as dislocations and morphological evolution of surfaces. The scope is methodological, and the Thesis tries to be self-contained. After an introduction to basic Machine Learning and Neural Networks concepts, different physical models are tackled. Methods are presented contextually to applications and are introduced from the simplest to the most sophisticated. Starting from the approximation of dislocation interactions in heteroepitaxial films, the Thesis proceeds in showing how to simulate the evolution of coherent films free surfaces and phase field models, using convolutional and recurrent NN respectively. Finally, an application of Generative Adversarial Networks to a stochastic mechanics problem is presented.

The scope of this thesis is the study of some properties of GeSbTe alloys by means of atomistic simulations. These alloys are classified as phase change materials which are employed in data storage applications, such as a novel type of electronic non-volatile memory called Phase Change Memory (PCM). The prototypical alloy used so far in PCM is the Ge2Sb2Te5 (GST225) compound. In recent times, PCM has also gained significant attention for embedded memories for automotive applications. However, the crystallization temperature Tx (420 K) of GST225 is too low for applications that require data retention at higher temperatures. Therefore, other options have been investigated for these applications. Ge-rich GeSbTe alloys have emerged as the most promising materials for embedded memories. The Tx of the Ge-rich GST alloy is higher compared to the stoichiometric alloy GST225, because of phase separation with segregation of Ge atoms during crystallization. However, atomistic details of the phase separation and crystallization processes of Ge-rich GeSbTe alloys are not fully understood. In this respect, atomistic simulations could provide details at the atomistic level of both the crystallization and segregation phenomena. On these premises, in this thesis we addressed by atomistic simulations three different issues on the properties of phase change materials of interest for applications in PCM. Firstly, we calculated the electrical and thermal conductivities and the Seebeck coefficient of the liquid phase of GST225 by molecular dynamics simulations based on density functional theory (DFT), since these coefficients are needed to perform an electrothermal modeling of the PCM. Secondly, we studied the effect of H/N doping on the atomic mobility in Ge5Sb2Te3 and Ge2Te by DFT molecular dynamics in the liquid e supercooled liquid phase. Indeed, in literature it was reported that presence of N could further raise the Tx of Ge-rich GeSbTe alloys probably due to the reduction of Ge mo, The scope of this thesis is the study of some properties of GeSbTe alloys by means of atomistic simulations. These alloys are classified as phase change materials which are employed in data storage applications, such as a novel type of electronic non-volatile memory called Phase Change Memory (PCM). The prototypical alloy used so far in PCM is the Ge2Sb2Te5 (GST225) compound. In recent times, PCM has also gained significant attention for embedded memories for automotive applications. However, the crystallization temperature Tx (420 K) of GST225 is too low for applications that require data retention at higher temperatures. Therefore, other options have been investigated for these applications. Ge-rich GeSbTe alloys have emerged as the most promising materials for embedded memories. The Tx of the Ge-rich GST alloy is higher compared to the stoichiometric alloy GST225, because of phase separation with segregation of Ge atoms during crystallization. However, atomistic details of the phase separation and crystallization processes of Ge-rich GeSbTe alloys are not fully understood. In this respect, atomistic simulations could provide details at the atomistic level of both the crystallization and segregation phenomena. On these premises, in this thesis we addressed by atomistic simulations three different issues on the properties of phase change materials of interest for applications in PCM. Firstly, we calculated the electrical and thermal conductivities and the Seebeck coefficient of the liquid phase of GST225 by molecular dynamics simulations based on density functional theory (DFT), since these coefficients are needed to perform an electrothermal modeling of the PCM. Secondly, we studied the effect of H/N doping on the atomic mobility in Ge5Sb2Te3 and Ge2Te by DFT molecular dynamics in the liquid e supercooled liquid phase. Indeed, in literature it was reported that presence of N could further raise the Tx of Ge-rich GeSbTe alloys probably due to the reduction of Ge mo

In vitro and in vivo stimulation and recording of neuron action potential is currently achieved with microelectrode arrays, either in planar or 3D geometries, adopting different materials and strategies. IrO2 is a conductive oxide known for its excellent biocompatibility, good adhesion on different substrates, and charge injection capabilities higher than noble metals. Atomic layer deposition (ALD) allows excellent conformal growth, which can be exploited on 3D nanoelectrode arrays. In this work, we disclose the growth of nanocrystalline rutile IrO2 at T = 150 °C adopting a new plasma-assisted ALD (PA-ALD) process. The morphological, structural, physical, chemical, and electrochemical properties of the IrO2 thin films are reported. To the best of our knowledge, the electrochemical characterization of the electrode/electrolyte interface in terms of charge injection capacity, charge storage capacity, and double-layer capacitance for IrO2 grown by PA-ALD was not reported yet. IrO2 grown on PtSi reveals a double-layer capacitance (Cdl) above 300 µF∙cm−2, and a charge injection capacity of 0.22 ± 0.01 mC∙cm−2 for an electrode of 1.0 cm2, confirming IrO2 grown by PA-ALD as an excellent material for neuroelectronic applications.

Models of the amorphous phase of the GeTe compound have been generated by depositing individual atoms on a substrate by molecular dynamics simulations. This material is of interest for applications in phase change memories. The kinetic energy of the atoms impinging on the surface is in the range 1.5-10 eV which is typical of magnetron sputtering growth. The simulations with up to 8000 atoms exploit a machine learning potential devised previously by the group. The structural properties of the films depend very little on the energy of the impinging atoms in the range 1.5-10 eV and they are also very similar to those of amorphous models generated by quenching from the melt, but for a larger fraction of Ge-Ge bonds and Ge atoms in tetrahedral geometries in the films. The crystal nucleation and growth are then simulated at 600 K which is close to the temperature of maximal crystal growth velocity exploited in the memory devices. Despite the structural differences mentioned above, the kinetics of crystallization at 600 K is very similar for the as-deposited and melt-quenched amorphous models.

The European Spallation Source (ESS), presently under construction in Lund, Sweden, is a multidisciplinary international laboratory that, once completed at full specifications, will operate the world’s most powerful pulsed neutron source. Supported by a 3 M Euro Research and Innovation Action within the European Union Horizon 2020 program, a design study (HighNESS) is now underway to develop a second neutron source located below the spallation target. Compared to the first source, which is located above the spallation target and designed for high cold and thermal brightness, the new source is being optimized to deliver higher intensity and a shift to longer wavelengths in the spectral regions of cold neutrons (CNs) (2 to 20 Å), very cold neutrons (VCNs) (10 to 120 Å), and ultracold neutrons (UCNs) ((Formula presented.) 500 Å). The second source consists of a large liquid deuterium moderator to deliver CNs and serve secondary VCN and UCN sources, for which different options are under study. These new sources will boost several areas of condensed matter research and will provide unique opportunities in fundamental physics. The HighNESS project is now entering its last year, and we are working toward the Conceptual Design Report of the ESS upgrade. In this paper, results obtained in the first 2 years, ongoing developments, and future perspectives are described.

We present recent developments of improved modelling methods for simulating neutron transport in reflector materials of interest for neutron source applications. These include materials to be used as traditional reflectors around the neutron moderator, such as beryllium, and also novel materials, such as nanodiamond particles, to be used as a reflector for very-cold neutrons in the neutron beam extraction area of a neutron scattering instrument. Of particular interest is the inclusion of physical effects that are not modelled in standard thermal scattering libraries used for Monte-Carlo simulations, such as extinction in beryllium reflectors and effects of small-angle neutron scattering from nanodiamond particles.

While polymorphism is prevalent in crystalline solids, polyamorphism draws increasing interest in various types of amorphous solids. Recent studies suggested that supercooling of liquid phase-change materials (PCMs) induces Peierls-like distortions in their local structures, underlying their liquid-liquid transitions before vitrification. However, the mechanism of how the vitrified phases undergo a possible polyamorphic transition remains elusive. Here, using high-energy synchrotron X-rays, we can access the precise pair distribution functions under high pressure and provide clear evidence that pressure can reverse the Peierls-like distortions, eliciting a polyamorphic transition in GeTe and GeSe. Combined with simulations based on machine-learned-neural-network potential, our structural analysis reveals a high-pressure state characterized by diminished Peierls-like distortion, greater coherence length, reduced compressibility, and a narrowing bandgap. Our finding underscores the crucial role of Peierls-like distortions in amorphous octahedral systems including PCMs. These distortions can be controlled through pressure and composition, offering potentials for designing properties in PCM-based devices.

Predicted topological crystalline insulators such as Pb1−xSnxTe are an interesting candidate for applications in quantum technology, as they can host spin-polarized surface states. Moreover, in the nanowire geometry, a quasi-1D system can be realized with potential applications exploiting Majorana fermions. Herein, the selective area growth of Pb1−xSnxTe islands and nanowires over the full range of x is demonstrated, and their in-depth growth dynamics and faceting are analyzed. By transmission electron microscopy, the single-crystalline and defect-free nature of the grown material and the homogeneous, controllable Pb/Sn ratio in the nanowires is confirmed. With support of phase-field growth simulations, it is shown that the crystal faceting mainly follows the driving force of surface energy minimization, favoring the lowest energy {200} surfaces. A kinetic enhancement of adatom incorporation on {110} facets is recognized to limit their extension with respect to {200} and {111} facets. After inspecting all possible in-plane orientations, we identify the <110> directions as the optimal candidate for the growth of high-quality and perfectly straight Pb1−xSnxTe nanowires, enabling the design of complex networks due to their threefold symmetry. This work opens the way to systematic transport investigation of the carrier density in Pb1−xSnxTe nanowires and can facilitate further optimization of the Pb1−xSnxTe system.

Generative Adversarial Networks (GANs) have shown immense potential in fields such as text and image generation. Only very recently attempts to exploit GANs to statistical-mechanics models have been reported. Here we quantitatively test this approach by applying it to a prototypical stochastic process on a lattice. By suitably adding noise to the original data we succeed in bringing both the Generator and the Discriminator loss functions close to their ideal value. Importantly, the discreteness of the model is retained despite the noise. As typical for adversarial approaches, oscillations around the convergence limit persist also at large epochs. This undermines model selection and the quality of the generated trajectories. We demonstrate that a simple multi-model procedure where stochastic trajectories are advanced at each step upon randomly selecting a Generator leads to a remarkable increase in accuracy. This is illustrated by quantitative analysis of both the predicted equilibrium probability distribution and of the escape-time distribution. Based on the reported findings, we believe that GANs are a promising tool to tackle complex statistical dynamics by machine learning techniques.

The increasing energy demand and the ever more pressing need for clean technologies of energy conversion pose one of the most urgent and complicated issues of our age. Thermoelectricity, namely the direct conversion of waste heat into electricity, is a promising technique based on a long-standing physical phenomenon, which still has not fully developed its potential, mainly due to the low efficiency of the process. In order to improve the thermoelectric performance, a huge effort is being made by physicists, materials scientists and engineers, with the primary aims of better understanding the fundamental issues ruling the improvement of the thermoelectric figure of merit, and finally building the most efficient thermoelectric devices. In this Roadmap an overview is given about the most recent experimental and computational results obtained within the Italian research community on the optimization of composition and morphology of some thermoelectric materials, as well as on the design of thermoelectric and hybrid thermoelectric/photovoltaic devices.

We report the effect of the quantum dot aspect ratio on the sub-gap absorption properties of GaAs/AlGaAs quantum dot intermediate band solar cells. We have grown AlGaAs solar cells containing GaAs quantum dots made by droplet epitaxy. This technique allows the realization of strain-free nanostructures with lattice matched materials, enabling the possibility to tune the size, shape, and aspect ratio to engineer the optical and electrical properties of devices. Intermediate band solar cells have been grown with different dot aspect ratio, thus tuning the energy levels of the intermediate band. Here, we show how it is possible to tune the sub-gap absorption spectrum and the extraction of charge carriers from the intermediate band states by simply changing the aspect ratio of the dots. The tradeoff between thermal and optical extraction is in fact fundamental for the correct functioning of the intermediate band solar cells. The combination of the two effects makes the photonic extraction mechanism from the quantum dots increasingly dominant at room temperature, allowing for a reduction of the open circuit voltage of only 14 mV, compared to the reference cell.

La tesi si pone l'obiettivo di studiare le proprietà ottiche degli Xeni. Gli Xeni, reticoli monoelementali artificiali simili al grafene, hanno mostrato sin dalla loro scoperta grandi potenzialità per applicazioni negli ambiti dell'elettronica e della fotonica. Purtroppo, il loro utilizzo è ancora limitato dall'elevata reattività chimica e dai forti effetti di ibridazione con i substrati nativi. Conoscere la risposta ottica o termo-ottica degli Xeni, escludendo o controllando eventuali effetti del substrato, è quindi funzionale allo sviluppo di nuove tecnologie. La ricerca è stata condotta nell'ambito del progetto X-Fab (ERC COG Grant n. 772261), presso i laboratori CNR-IMM di Agrate Brianza. In questo studio vengono esaminate diverse configurazioni a base di Xene, combinando tecniche di analisi in situ (XPS e LEED) e di spettroscopia ottica (FTIR, Vis-UV e Raman), con lo scopo di determinarne la risposta ottica e valutare l'integrazione degli Xeni in un dispositivo che ne sfrutti le potenzialità. La prima configurazione analizzata prevede l'introduzione di un substrato isolante e otticamente trasparente, come lo zaffiro, a sostituzione del più diffuso argento. Lo zaffiro, nella sua orientazione (0001) presenta un reticolo a simmetria esagonale commensurato a quello di silicene e stanene. La conducibilità ottica di nano-film di stagno cresciuti tramite MBE è stata ricavata da misure di trasmittanza e riflettanza, acquisite tra 0.01 e 6.5 eV. L'analisi spettrale e il successivo confronto con dati teorici già presenti in letteratura hanno permesso di identificare delle caratteristiche non banali, in parte riconducibili allo stanene. L'analisi chimica eseguita in situ durante la crescita ha però evidenziato anche la presenza di una componente dello stagno ossidata dovuta ad effetti del substrato. Il problema emerso è stato affrontato introducendo uno strato di grafene tra i film di stagno e l'Al2O3. L'introduzione dello strato di grafene ha permesso di evi, The aim of this thesis is to investigate the optical properties of Xenes. Since their discovery, Xenes, artificial graphene-like single-element lattices, have shown great potential for applications in electronics and photonics. Unfortunately, their use is still limited by their high chemical reactivity and strong hybridisation effects with native substrates. Understanding the optical or thermo-optical response of Xenes, and thus excluding or controlling possible substrate effects, is therefore crucial for the development of new technologies. The research was carried out as part of the X-Fab (ERC COG Grant n. 772261) project at the CNR-IMM laboratories in Agrate Brianza. In this study, several Xenes-based configurations are investigated using a combination of in situ analysis (XPS and LEED) and optical spectroscopy (FTIR, Vis-UV and Raman) techniques to determine their optical response and evaluate the integration of Xenes into a device that exploits their potential. The first configuration analyzed involved the introduction of an insulating and optically transparent substrate, such as sapphire (Al2O3), to replace the more common silver. Sapphire, in its (0001) orientation, has a hexagonal symmetry lattice commensurate to that of silicene and stanene. The optical conductivity of tin nano-sheets grown by MBE was derived from transmittance and reflectance measurements. The measurements were carried out covering a range from 0.01 to 6.5 eV. Spectral analysis and subsequent comparison with theoretical data from the literature made it possible to identify non-trivial features, some of which can be attributed to stanene. However, in situ chemical analysis during growth also revealed the presence of an oxidised tin component due to substrate effects. This issue was solved by introducing a graphene layer between the tin films and Al2O3. The introduction of the graphene layer prevented oxidation of the tin, as confirmed by XPS analysis. The results of the optical analysis, on

Le attuali tecnologie informatiche stanno raggiungendo i propri limiti fisico-tecnologici e di impatto ambientale, a causa del crescente uso intensivo di dati e dell’alta performance richiesta. Emerge la necessità di rinnovare i concetti di computazione convenzionale, ripartendo dallo studio di dispositivi e materiali alternativi. Nasce quindi un ampio campo di ricerca denominato ‘unconventional computing’, computazione non convenzionale. Di particolare interesse per questo lavoro è il cosiddetto ‘in-materia computing’, che si basa sullo sfruttamento delle proprietà intrinseche dei materiali e sul loro comportamento non lineare. Il lavoro qui presentato si concentra sull'analisi del trasporto di carica e spin dipendente in dispositivi basati su donori poco profondi in silicio. In particolare sono stati studiati due sistemi: i dopant network processing units (DNPU) e dispostivi basati sulla tecnologia silicon-on-insulator (SOI). I DNPU sono dispositivi nanoelettronici a più elettrodi basati su un network di atomi di arsenico in silicio, già utilizzati per in-materia computing. In questo lavoro di tesi, abbiamo approfondito le proprietà fisiche del network, al fine di comprendere i meccanismi funzionali ancora poco chiari e espandere la capacità computazionale del DNPU. È stato analizzato il trasporto di carica, identificando gli intervalli di temperatura in cui il meccanismo di trasporto dominante è variable range hopping, responsabile del comportamento non lineare. In regime di hopping è anche possibile ottenere una resistenza differenziale negativa, necessaria per risolvere problemi di classificazione linearmente inseparabili. La risposta del DNPU è stata studiata anche tramite stimoli esterni, come illuminazione e un campo magnetico statico, mostrando la dipendenza della foto-conduttività dal campo. Infine abbiamo studiato la possibilità di incorporare le proprietà di spin dei droganti, per ottenere nuove funzionalità spin dipendenti. Il trasporto d, The accelerating rise of data-intensive applications is pushing the conventional computing technologies towards physical and environmental limits. A necessity to rethink computing has emerged, starting from the study of alternative device physics, materials, and computational paradigms. A broad research field that is referred to as unconventional computing is emerging. Particularly interesting for this work is the so-called in-materia computing, based on the exploitation of the intrinsic physical properties and non-linear effects of materials to perform computation. The work presented here focuses on the analysis of charge and spin-dependent transport in devices based on networks of shallow donors in silicon. Two different systems have been investigated: the dopant network processing units (DNPU) and silicon-on-insulator (SOI) based devices. The DNPU are multiple electrodes nanoelectronic devices based on a network of arsenic atoms in silicon. They have already been proposed as a promising scalable platform for in material computing. In this work, we investigated the network’s physical properties more in depth, in order to understand the still unclear functional mechanisms and possibly expand the DNPU’s computational capability. Charge transport has been analysed, identifying the temperature ranges in which variable range hopping is the dominant mechanism. Hopping transport is responsible for the high non-linearity required for computation. In the hopping regime, it is also possible to obtain negative differential resistance behaviour, necessary to solve linearly inseparable classification problem. The DNPU’s response was also investigated in response to external stimuli, such as photoexcitation and a static magnetic field, showing field dependent photoconductivity. Finally, we studied the possibility to incorporate the spin physics of the dopants, to obtain novel spin dependent functionalities. Spin dependent transport has been investigated performing electrically

In questa tesi, sono state realizzate tre diverse board: un DC-DC per il supply, un control unit per programmare i segnali d'ingresso del driver, un firing unit che contiene il chip principale(driver e GaN). Abbiamo usato il nitruro di galio perché è meglio adatto del silicio ad alte frequenze. L'obbiettivo principale è di raggiungere 50A di picco di corrente con soli 5ns di pulse. Sono stati usati Altium design per il disegno delle board e Ansys per l'evaluazione delle induttanze parassite. Questo ultimo crea diversi problemi e può limitare il raggiungimento del picco di corrente. Quindi bisogna fare il layout tenendo molta attenzione ai parassiti, In our three years of work, we have achieved the realization of a Firing unit board with the GaN and driver in a system in package. Three different boards were realized: A first board with only the resistor, the second one with the resistor and a shunt resistor and a third board with the laser diode and a shunt resistor. A DC-DC was realized for the supply while a control unit was realized for the control of the input signals of the driver.Unfortunately, no measurements of the firing unit have been done yet as we are still waiting for the chip to be completed. The DC-DC and the control unit board have been measured and tested. LIDAR application is the most attractive and efficient solution for this market. The challenges of LIDAR application consist in the development of the electronics generating a current pulse of 50A that lasts for less than 5ns. The technical area of this activity is fully autonomous self-driving car, and in particular what helps an autonomous vehicle to understand the world around it.

Nel programma di ricerca europeo per la fusione termonucleare controllata sono stati definiti otto differenti obiettivi a lungo termine. Una di queste sfide cruciali riguarda lo smaltimento (exhaust) di particelle ed energia provenienti da un reattore a fusione. Per sviluppare e testare delle strategie alternative atte a risolvere il problema dell'exhaust, una nuova macchina sperimentale è attualmente in costruzione in Italia a Frascati presso il centro di ricerca ENEA: il Divertor Tokamak Test facility (DTT). Per progettare un nuovo tokamak sono richiesti sforzi congiunti di fisici ed ingegneri. Al fine di ridurre i costi e minimizzare i rischi, uno strumento essenziale è la modellizzazione integrata il più completa possibile basata su principi primi. Il presente progetto di dottorato è incentrato sullo sviluppo di simulazioni multi-canale basate sulla fisica dei principali scenari operazionali di riferimento di DTT. Modelli all'avanguardia di trasporto, riscaldamento, fuelling ed equilibrio magnetico vengono integrati in queste simulazioni per predire in modo auto-consistente profili di plasma e parametri di scenario. Vengono anche calcolate tutte le interazioni non lineari tra sistemi di riscaldamento e plasma e tra i diversi canali di trasporto. Durante questo lavoro, le simulazioni di DTT sono state progressivamente migliorate perfezionandone le impostazioni e includendo un crescente numero di aspetti grazie all'aggiunta di codici appositi. Inoltre sono stati inclusi man mano aggiornamenti dei sistemi di riscaldamento, dell'equilibrio magnetico e della configurazione della macchina per seguire l'evoluzione del progetto. Il confronto tra simulazioni analoghe con differenti modelli quasi-lineari di trasporto ci rende fiduciosi dell'affidabilità dei profili di plasma predetti e ci permette di identificare i punti deboli dei modelli nei vari regimi in cui opera DTT. Questi modelli quasi-lineari sono stati inoltre validati mediante simulazioni girocinetiche nel, The European research roadmap towards thermonuclear fusion energy defined eight different missions to guide the long–term programme. One of these crucial challenges is the controlled power and particle exhaust from a fusion reactor. To develop and test alternative strategies to solve the exhaust problem, in Italy a new experimental device is now under construction at the ENEA Research Center in Frascati: the Divertor Tokamak Test facility (DTT). Designing a new tokamak requires concerted efforts of physicists and engineers. To reduce costs and minimise risks, a first–principle based integrated modelling as comprehensive as possible of plasma discharges is an essential tool. The focus of this PhD project was to perform the first physics–based multi–channel simulations of the main baseline operational scenarios of DTT. In these simulations state–of–art modules for transport, heating, fuelling, and magnetic equilibrium are integrated to achieve self–consistent predictions of plasma profiles and scenario parameters. All non–linear interactions between heating and plasma and between the different transport channels are also calculated. During this work, the DTT simulations have been progressively enhanced adding codes to include a growing number of aspects and refining run settings. Moreover, updates of the heating systems, magnetic equilibria, and device configuration have been included to comply with the evolving machine design. The comparison among analogous simulations with different quasi–linear transport models made us confident in the reliability of the predicted plasma profiles and allowed us to identify the weak points of the models in the various DTT operational regimes. A validation of these quasi–linear models against the gyrokinetic simulations in the specific DTT range of parameters was also performed. The prediction accuracy has been improved recursively by matching the core and SOL simulation boundary conditions to guarantee the core–edge–SOL consistency.

Lo scopo di questa tesi, iniziata a novembre 2019, è lo sviluppo di un rivelatore Cherenkov per misurare i raggi gamma da 17 MeV emessi dalla reazione di fusione D-T. Con l'espandersi della pandemia da COVID-19 nel nord Italia, a metà febbraio 2020, è divenuto evidente che il piano iniziale del mio lavoro di tesi dovesse essere fortemente cambiato, a causa della cancellazione delle attività sperimentali che avrebbero dovuto svolgersi nei laboratori UNIMIB/CNR a Milano e al Joint European Torus nel Regno Unito. In accordo con i miei tutor ho iniziato, insieme ad altri ricercatori, a lavorare su base volontaria ad un progetto denominato Mechanical Ventilator Milano (MVM). Il progetto MVM ha coinvolto un gruppo internazionale di più di 150 scienziati e ha prodotto in meno di tre mesi un ventilatore meccanico certificato dalla Food and Drugs Administration per uso su pazienti affetti da COVID-19 in terapia intensiva. L'attività su MVM ha portato, circa un anno dopo, allo sviluppo di un nuovo sensore di ossigeno veloce per applicazioni mediche. Il sensore è in grado di misurare il consumo di ossigeno di un individuo in tempo reale e durante un singolo respiro. La tesi è divisa in tre parti. La prima parte si concentra sullo sviluppo di un contatore di raggi gamma ottimizzato per la misura della potenza di fusione in un reattore a confinamento magnetico. Il gruppo di ricerca in cui mi sono inserito sta sviluppando un metodo innovativo per la misura della potenza prodotta dalle reazioni di fusione basato sulla rivelazione dei raggi gamma da 17 MeV prodotti durante la reazione D+T->5He*. Tipicamente il nucleo di 5He* decade emettendo una particella alfa e un neutrone, ma può anche diseccitarsi sullo stato fondamentale dell'5He, prima che questo si disintegri in una particella alfa e un neutrone, con una probabilità di 10^-5. Questi raggi gamma sono stati misurati al JET nella campagna DT appena conclusa con uno spettrometro gamma basato su un cristallo di LaBr3 e, Aim of this thesis, begun in November 2019, is the development of an innovative Cerenkov detector for measurements of 17 MeV gamma-rays emitted by the D-T fusion reaction in an intense neutron field. With the spread of the COVID-19 pandemics in Northern Italy in February 2020, it became clear that the original program planned for my PhD work had to be significantly changed, since experimental activities to be carried out in the UNIMIB/CNR laboratories in Milan and at the Joint European Torus in the UK had to be cancelled. In agreement with my tutors I volunteered together with other scientists to contribute to a project called Mechanical Ventilator Milan (MVM). The MVM project involved an international team of more than 150 scientists and has produced over the very short period of less than three months a mechanical ventilator approved by the American Food and Drug Administration for use at the intensive care unit of hospitals to treat patients affected by COVID-19. The activities of the MVM project led to the development of a new fast oxygen sensor for medical application, about one year later. The sensor measures the oxygen consumption in real time during a single breath. The thesis is organized in three parts. The first part is focused on the development of a gamma-ray counter optimized for the measurement of the D-T fusion power produced in a magnetic confinement fusion device. The research team I have joined is developing a novel technique for the measurement of DT fusion power in a magnetic confinement device based on the detection of 17 MeV gamma-rays also produced by the D+T->5He* reaction. The 5He* nucleus promptly decays usually emitting an alpha particle and a neutron, but it may de-excite to the ground level emitting a gamma-ray with a probability of the order of 10^-5. These gamma-rays have been detected in the recent DT campaign at JET with a gamma spectrometer based on LaBr3 and a fast digital data acquisition. Since the efficiency of the scintillator

Prototypical phase change compounds, typically based on GeSbTe (GST) alloys, display a crystallization temperature not suitable for embedded Phase Change Memories (ePCM) of interest for applications in the automotive sector. The search for an alternative material is thus a very active research field. Ge-rich GST alloys are emerging as promising materials for ePCM thanks to the higher thermal stability of their amorphous phase. Upon crystallization, Ge-rich GST alloys undergo a phase separation into Ge and other GST alloys. The segregation phenomena enhance the crystallization temperature (Tx), but it comes also with several drawbacks such as a high cell-to-cell variability and a drift of the electrical resistance with time in the set state. The details regarding the decomposition process are largely unknown and are a matter of debate. During my PhD studies, I investigated the phase separation by means of high-throughput Density Functional theory (DFT) calculations based on thermodynamical analysis. We computed the formation free energy of all GST alloys in the central part of the ternary phase diagram modelled in the rocksalt metastable phase, which is the phase relevant to the operation of the memory. Then, we computed all possible decomposition reactions for each GST alloy. We summarized all our thermochemical data in one descriptor called "decomposition propensity", which measures the tendency of an alloy to undergo phase separation. I also studied the structural properties of the amorphous phase of Ge-rich GST alloys as a function of the Ge content. We found that by increasing the Ge content the local structure of the amorphous phase becomes more and more dissimilar from the crystalline phase which might hinder the crystallization kinetic. These results suggest a possible strategy to minimize the phase separation (low decomposition propensity) and still keep high Tx (crystallization might be hindered due to the dissimilarity). Aside the thermodynamic analysis di, Prototypical phase change compounds, typically based on GeSbTe (GST) alloys, display a crystallization temperature not suitable for embedded Phase Change Memories (ePCM) of interest for applications in the automotive sector. The search for an alternative material is thus a very active research field. Ge-rich GST alloys are emerging as promising materials for ePCM thanks to the higher thermal stability of their amorphous phase. Upon crystallization, Ge-rich GST alloys undergo a phase separation into Ge and other GST alloys. The segregation phenomena enhance the crystallization temperature (Tx), but it comes also with several drawbacks such as a high cell-to-cell variability and a drift of the electrical resistance with time in the set state. The details regarding the decomposition process are largely unknown and are a matter of debate. During my PhD studies, I investigated the phase separation by means of high-throughput Density Functional theory (DFT) calculations based on thermodynamical analysis. We computed the formation free energy of all GST alloys in the central part of the ternary phase diagram modelled in the rocksalt metastable phase, which is the phase relevant to the operation of the memory. Then, we computed all possible decomposition reactions for each GST alloy. We summarized all our thermochemical data in one descriptor called "decomposition propensity", which measures the tendency of an alloy to undergo phase separation. I also studied the structural properties of the amorphous phase of Ge-rich GST alloys as a function of the Ge content. We found that by increasing the Ge content the local structure of the amorphous phase becomes more and more dissimilar from the crystalline phase which might hinder the crystallization kinetic. These results suggest a possible strategy to minimize the phase separation (low decomposition propensity) and still keep high Tx (crystallization might be hindered due to the dissimilarity). Aside the thermodynamic analysis di

The recent developments in quantum technologies, as well as advanced detection experiments, have raised the need to detect extremely weak signals in the microwave frequency spectrum. To this aim, the Josephson travelling wave parametric amplifier, a device capable of reaching the quantum noise limit while providing a wide bandwidth, has been proposed as a suitable cryogenic front-end amplifier. This work deals with the numerical study of a Josephson travelling wave parametric amplifier, without approximations regarding the nonlinearity of the key elements. In particular, we focus on the investigation of the system of coupled nonlinear differential equations representing all the cells of the Josephson travelling wave parametric amplifier, with proper input and output signals at the boundaries. The investigation of the output signals generated by the parametric amplification process explores the phase-space and the Fourier spectral analysis of the output voltage, as a function of the parameters describing the pump and signal tones that excite the device. Beside the expected behavior, i.e., the signal amplification, we show that, depending on the system operation, unwanted effects (such as pump tone harmonics, incommensurate frequency generation, and noise rise), which are not accounted for in simple linearized approaches, can be generated in the whole nonlinear system. IEEE

The growing interest in quantum technologies, from fundamental physics experiments to quantum computing, demands for extremely performing electronics only adding the minimum amount of noise admitted by quantum mechanics to the input signal (i.e. quantum-limited electronics). Superconducting microwave amplifiers, due to their dissipationless nature, exhibit outstanding performances in terms of noise (quantum limited), and gain. However, bandwidth and saturation power still show space for substantial improvement. Within the DARTWARS* collaboration, we are developing state-of-the-art microwave superconducting amplifiers based on Josephson junction arrays and on distributed kinetic inductance transmission lines. Here we report the realization of a setup for the characterization of the performances of Josephson Traveling-Wave Parametric Amplifiers (JTWPA) at a temperature of 300 mK. Although in the final experimental setup these amplifiers will operate at a base temperature of about 20 mK, their characterization at 300 mK allows to evidence the main aspects of their performances, but the ultimate noise level. This represents a quick and relatively inexpensive way to test these superconductive devices that can be of help to improve their design and fabrication. IEEE

As famed quantum physicist W. Pauli once said, “The surface was invented by the devil”. The nonequilibrium state of particles forming the surface, and the presence of dangling bonds transform the surfaces into a 2D reactor with high physical and chemical reactivity. When two such active surfaces are matched, their interface becomes even more reactive, giving rise to novel properties or enhanced performance. For this reason, much effort is applied to design nanoengineered interfacial systems for applications spanning all facets of human life. This review article discusses recent, mostly within two years, progress in the design of complex, sophisticated carbon-based interfacial material systems for energy and photonics applications, with the aim to emphasize some of the most interesting and important examples of such systems. Differences in the processes that take place on flat and 3D (curved) surfaces are discussed, with the view of guiding the design and construction of complex functional interfaces, focusing on several points that are of particular importance to the ongoing development of advanced interfacial material systems.

Ge and Si micro-crystals, grown on Si patterned substrates, can be used as absorbing elements for photodetection in the near-infrared. In such microstructures, light confinement effects due to crystal facet, enhance light absorption in the near-infrared as compared to conventional epitaxial layers. Devices based on single micro-crystals and on micro-crystals arrays have been fabricated and characterized. The photocurrent of Si photodetectors based on single micro-crystals have been measured in linear and avalanche regime, demonstrating a state-of-the-gain of ≈ 104. Ge-on-Si photodetectors based on micro-crystal arrays, fabricated using graphene as top contact, have also been fabricated, showing a responsivity in the 1500-1800 nm exceeding that of conventional planar devices.