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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 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

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.

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.

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

Integration of the prototypical GeSbTe (GST) ternary alloys, especially on the GeTe-Sb2 Te3 tie-line, into non-volatile memory and nanophotonic devices is a relatively mature field of study. Nevertheless, the search for the next best active material with outstanding properties is still ongoing. This search is relatively crucial for embedded memory applications where the crystallization temperature of the active material has to be higher to surpass the soldering threshold. Increasing the Ge content in the GST alloys seems promising due to the associated higher crystallization temperatures. However, homogeneous Ge-rich GST in the as-deposited condition is thermodynamically unstable, and phase separation upon annealing is unavoidable. This phase separation reduces endurance and is detrimental in fully integrating the alloys into active memory devices. This work investigated the phase separation of Ge-rich GST alloys, specifically Ge5 Sb2 Te3 or GST523, into multiple (meta)stable phases at different length scales in melt-quenched bulk and annealed thin film. Electron microscopy-based techniques were used in our work for chemical mapping and elemental composition analysis to show the formation of multiple phases. Our results show the formation of alloys such as GST213 and GST324 in all length scales. Furthermore, the alloy compositions and the observed phase separation pathways agree to a large extent with theoretical results from density functional theory calculations.

Shaping two-dimensional (2D) materials in arbitrarily complex geometries is a key to designing their unique physical properties in a controlled fashion. This is an elegant solution, taking benefit from the extreme flexibility of the 2D layers but requiring the ability to force their spatial arrangement from flat to curved geometries in a delicate balance among free-energy contributions from strain, slip-and-shear mechanisms, and adhesion to the substrate. Here, we report on a chemical vapor deposition approach, which takes advantage of the surfactant effects of organic molecules, namely the tetrapotassium salt of perylene-3,4,9,10-tetracarboxylic acid (PTAS), to conformally grow atomically thin layers of molybdenum disulphide (MoS2) on arbitrarily nanopatterned substrates. Using atomically resolved transmission electron microscope images and density functional theory calculations, we show that the most energetically favorable condition for the MoS2 layers consists of its adaptation to the local curvature of the patterned substrate through a shear-and-slip mechanism rather than strain accumulation. This conclusion also reveals that the perylene-based molecules have a role in promoting the adhesion of the layers onto the substrate, no matter the local-scale geometry.

Integration of the prototypical GeSbTe (GST) ternary alloys, especially on the GeTe-Sb2 Te3 tie-line, into non-volatile memory and nanophotonic devices is a relatively mature field of study. Nevertheless, the search for the next best active material with outstanding properties is still ongoing. This search is relatively crucial for embedded memory applications where the crystallization temperature of the active material has to be higher to surpass the soldering threshold. Increasing the Ge content in the GST alloys seems promising due to the associated higher crystallization temperatures. However, homogeneous Ge-rich GST in the as-deposited condition is thermodynamically unstable, and phase separation upon annealing is unavoidable. This phase separation reduces endurance and is detrimental in fully integrating the alloys into active memory devices. This work investigated the phase separation of Ge-rich GST alloys, specifically Ge5 Sb2 Te3 or GST523, into multiple (meta)stable phases at different length scales in melt-quenched bulk and annealed thin film. Electron microscopy-based techniques were used in our work for chemical mapping and elemental composition analysis to show the formation of multiple phases. Our results show the formation of alloys such as GST213 and GST324 in all length scales. Furthermore, the alloy compositions and the observed phase separation pathways agree to a large extent with theoretical results from density functional theory calculations.

Elemental antimony has been recently proposed as a promising material for phase change memories with improved performances with respect to the most used ternary chalcogenide alloys. The compositional simplification prevents reliability problems due to demixing of the alloy during memory operation. This is made possible by the dramatic stabilization of the amorphous phase once Sb is confined in an ultrathin film 3-5 nm thick. In this work, we shed light on the microscopic origin of this effect by means of large scale molecular dynamics simulations based on an interatomic potential generated with a machine learning technique. The simulations suggest that the dramatic reduction of the crystal growth velocity in the film with respect to the bulk is due to the effect of nanoconfinement on the fast beta relaxation dynamics while the slow alpha relaxation is essentially unaffected.

Elemental antimony has been recently proposed as a promising material for phase change memories with improved performances with respect to the most used ternary chalcogenide alloys. The compositional simplification prevents reliability problems due to demixing of the alloy during memory operation. This is made possible by the dramatic stabilization of the amorphous phase once Sb is confined in an ultrathin film 3-5 nm thick. In this work, we shed light on the microscopic origin of this effect by means of large scale molecular dynamics simulations based on an interatomic potential generated with a machine learning technique. The simulations suggest that the dramatic reduction of the crystal growth velocity in the film with respect to the bulk is due to the effect of nanoconfinement on the fast beta relaxation dynamics while the slow alpha relaxation is essentially unaffected.

Closed-shell atoms scattered from a metal surface exchange energy and momentum with surface phonons mostly via the interposed surface valence electrons, i.e., via the creation of virtual electron-hole pairs. The latter can then decay into surface phonons via electron-phonon interaction, as well as into acoustic surface plasmons (ASPs). While the first channel is the basis of the current inelastic atom scattering (IAS) surface-phonon spectroscopy, no attempt to observe ASPs with IAS has been made so far. In this study we provide evidence of ASP in Ni(111) with both Ne atom scattering and He atom scattering. While the former measurements confirm and extend so far unexplained data, the latter illustrate the coupling of ASP with phonons inside the surface-projected phonon continuum, leading to a substantial reduction of the ASP velocity and possibly to avoided crossing with the optical surface phonon branches. The analysis is substantiated by a self-consistent calculation of the surface response function to atom collisions and of the first-principle surface-phonon dynamics of Ni(111). It is shown that in Ni(111) ASP originate from the majority-spin Shockley surface state and are therefore collective oscillation of surface electrons with the same spin, i.e. it represents a new kind of collective quasiparticle: a Spin Acoustic Surface Plasmon (SASP).

The last catalogues of the species of the family Scathophagidae for the individual zoogeographical regions were published in the following years: for the Afrotropical Region in 1976; for the Nearctic Region in 1965; for the Neotropical Region in 1984 and, partly, in 2010; for the Oriental Region in 1977 and for Palaearctic Region in 2008. The changes or additions published since the last catalogue form four basic groups: (I) new genera described (5 in Nearctic Region, 1 in Oriental Region, 8 in Palaearctic Region), (II) new species described (4 in Nearctic Region, 4 in Oriental Region, and 51 in Palaearctic Region), (III) new established synonyms (2 in Afrotropical Region, 1 in Nearctic Region, 4 in Neotropical Region, 4 in Oriental Region, 22 in Palaearctic Region), and (IV) new proposed combinations (1 in Afrotropical Region, 6 in Nearctic Region, 3 in Neotropical Region, 14 in Palaearctic Region).

In recent years strain engineering is proposed in chalcogenide superlattices (SLs) to shape in particular the switching functionality for phase change memory applications. This is possible in Sb2Te3/GeTe heterostructures leveraging on the peculiar behavior of Sb2Te3, in between covalently bonded and weakly bonded materials. In the present study, the structural and thermoelectric (TE) properties of epitaxial Sb2+xTe3 films are shown, as they represent an intriguing option to expand the horizon of strain engineering in such SLs. Samples with composition between Sb2Te3 and Sb4Te3 are prepared by molecular beam epitaxy. A combination of X-ray diffraction and Raman spectroscopy, together with dedicated simulations, allows unveiling the structural characteristics of the alloys. A consistent evaluation of the structural disorder characterizing the material is drawn as well as the presence of both Sb2 and Sb4 slabs is detected. A strong link exists among structural and TE properties, the latter having implications also in phase change SLs. A further improvement of the TE performances may be achieved by accurately engineering the intrinsic disorder. The possibility to tune the strain in designed Sb2+xTe3/GeTe SLs by controlling at the nanoscale the 2D character of the Sb2+xTe3 alloys is envisioned

In recent years strain engineering is proposed in chalcogenide superlattices (SLs) to shape in particular the switching functionality for phase change memory applications. This is possible in Sb2Te3/GeTe heterostructures leveraging on the peculiar behavior of Sb2Te3, in between covalently bonded and weakly bonded materials. In the present study, the structural and thermoelectric (TE) properties of epitaxial Sb2+xTe3 films are shown, as they represent an intriguing option to expand the horizon of strain engineering in such SLs. Samples with composition between Sb2Te3 and Sb4Te3 are prepared by molecular beam epitaxy. A combination of X-ray diffraction and Raman spectroscopy, together with dedicated simulations, allows unveiling the structural characteristics of the alloys. A consistent evaluation of the structural disorder characterizing the material is drawn as well as the presence of both Sb2 and Sb4 slabs is detected. A strong link exists among structural and TE properties, the latter having implications also in phase change SLs. A further improvement of the TE performances may be achieved by accurately engineering the intrinsic disorder. The possibility to tune the strain in designed Sb2+xTe3/GeTe SLs by controlling at the nanoscale the 2D character of the Sb2+xTe3 alloys is envisioned

Ricostruire la storia di un processo evolutivo consente di comprenderne a pieno i risultati: questo lo scopo della presente tesi dottorale che intende indagare sulle origini, gli studi e lo sviluppo di due sistemi fiscali, italiano e svizzero. Ripercorrendo scambi epistolari, documenti e testi, talvolta inediti, di studiosi della levatura di Blumenstein e Vanoni, intercorsi nel periodo di fioritura del fondamento etico e scientifico del processo legislativo tributario italiano ed elvetico, compreso tra il 1938 ed il 1950, è stato possibile delineare quel fil rouge, il quale, passando sia per i fondamentali concetti di natura etico-politica sia per quelli strettamente tecnici, conduce alla scoperta della profonda commistione di principi di equità sociale, come la graduazione del prelievo fiscale, capacità contributiva e redistribuzione dei redditi, che assurgono nella formulazione del diritto tributario in Italia e nella Confederazione svizzera quali elementi essenziali nel perseguimento della giustizia fiscale., Travel through history is necessary to deeply understand the results of an evolutional iter: this Doctoral thesis aims to analize origins, studies and the development of two different taxation frameworks, the italian and the swiss one. Indeed, unreleased exchange of letters, documents and texts, elapsed between 1938 and 1950 among renowed academics – with a particular focus on the work of Blumenstein and Vanoni – which represent a concrete supply to the thriving taxation law systems, lead to the identification of the fil rouge which, passing thorught both etichal and politic concepts and technical aspects, underlines the deep commixture of supreme principles of social equity, such as the graduation of tax charging, ability to pay and redistribution of wealth, which arise as benchmarks to reach the scope of fiscal justice both in Italy and Switzerland.

Ricostruire la storia di un processo evolutivo consente di comprenderne a pieno i risultati: questo lo scopo della presente tesi dottorale che intende indagare sulle origini, gli studi e lo sviluppo di due sistemi fiscali, italiano e svizzero. Ripercorrendo scambi epistolari, documenti e testi, talvolta inediti, di studiosi della levatura di Blumenstein e Vanoni, intercorsi nel periodo di fioritura del fondamento etico e scientifico del processo legislativo tributario italiano ed elvetico, compreso tra il 1938 ed il 1950, è stato possibile delineare quel fil rouge, il quale, passando sia per i fondamentali concetti di natura etico-politica sia per quelli strettamente tecnici, conduce alla scoperta della profonda commistione di principi di equità sociale, come la graduazione del prelievo fiscale, capacità contributiva e redistribuzione dei redditi, che assurgono nella formulazione del diritto tributario in Italia e nella Confederazione svizzera quali elementi essenziali nel perseguimento della giustizia fiscale., Travel through history is necessary to deeply understand the results of an evolutional iter: this Doctoral thesis aims to analize origins, studies and the development of two different taxation frameworks, the italian and the swiss one. Indeed, unreleased exchange of letters, documents and texts, elapsed between 1938 and 1950 among renowed academics – with a particular focus on the work of Blumenstein and Vanoni – which represent a concrete supply to the thriving taxation law systems, lead to the identification of the fil rouge which, passing thorught both etichal and politic concepts and technical aspects, underlines the deep commixture of supreme principles of social equity, such as the graduation of tax charging, ability to pay and redistribution of wealth, which arise as benchmarks to reach the scope of fiscal justice both in Italy and Switzerland.

Medetera Fischer von Waldheim is the most speciose genus in the Medeterinae, with a nearly ubiquitous global distribution. Phylogenetic relationships within Medetera and between Medetera and four other medeterine genera were investigated using mitochondrial (COI, 16S) and nuclear (18S) markers to test morphological hypotheses. Our results confirm most of Bickel's hypotheses. Thrypticus Gerstäcker shows a sister-group relationship with Medetera + Dolichophorus Lichtwardt. The Medetera species included here split into two clades. One clade corresponds to the M. diadema L. - veles Loew species group sensu Bickel. The second clade is largely composed of the M. apicalis (Zetterstedt) species group sensu Bickel and the M. aberrans Wheeler species group sensu Bickel + Dolichophorus. Although most Medeterinae are associated with plants (mainly trees), species in at least two separate lineages demonstrate a secondary return to terrestrial habitats. The implication of this evolutionary phenomenon is briefly discussed

A total of 874 Ixodes ricinus ticks were collected in Switzerland to investigate the genetic diversity of the Borrelia population. We integrated to the RT-PCR method the DNA sequence analysis of a 162-bp fragment of the recA gene. Five genospecies were detected: Borrelia afzelii, Borrelia burgdorferi s.s., Borrelia garinii, Borrelia valaisiana, and Borrelia lusitaniae. A heterogeneous distribution was observed within the B. burgdorferi s.l. genospecies. The most prevalent and diverse genospecies found in Switzerland was Borrelia afzelii, which might suggest a rapid evolution of this genospecies

Using reflection high-energy electron diffraction (RHEED), the growth onset of molecular beam epitaxy (MBE) deposited germanium telluride (GeTe) film on Si(111)-(3 ×)R30°-Sb surfaces is investigated, and a larger than expected in-plane lattice spacing is observed during the deposition of the first two molecular layers. High-resolution transmission electron microscopy (HRTEM) confirms that the growth proceeds via closed layers, and that those are stable after growth. The comparison of the experimental Raman spectra with theoretical calculated ones allows assessing the shift of the phonon modes for a quasi-free-standing ultra-thin GeTe layer with larger in-plane lattice spacing. The manifestation of the latter phenomenon is ascribed to the influence of the interface and the confinement of GeTe within the limited volume of material available at growth onset, either preventing the occurrence of Peierls dimerization or their ordered arrangement to occur normally.

The structural defects in two-dimensional transition metal dichalcogenides, including point defects, dislocations and grain boundaries, are scarcely considered regarding their potential to manipulate the electrical and optical properties of this class of materials, notwithstanding the significant advances already made. Indeed, impurities and vacancies may influence the exciton population, create disorder-induced localization, as well as modify the electrical behaviour of the material. Here we report on the experimental evidence, confirmed by ab initio calculations, that sulfur vacancies give rise to a novel near-infrared emission peak around 0.75 eV in exfoliated MoS2 flakes. In addition, we demonstrate an excess of sulfur vacancies at the flake's edges by means of cathodoluminescence mapping, aberration-corrected transmission electron microscopy imaging and electron energy loss analyses. Moreover, we show that ripplocations, extended line defects peculiar to this material, broaden and redshift the MoS2 indirect bandgap emission.

Simulations of prebiotic NH3 synthesis from NO3- and NO2- on pyrite surfaces under hydrothermal conditions are reported. Ab initio metadynamics calculations have successfully explored the full reaction path which explains earlier experimental observations. We have found that the reaction mechanism can be constructed from stepwise single atom transfers which are compatible with the expected reaction time scales. The roles of the hot-pressurized water and of the pyrite surfaces have been addressed. The mechanistic picture that emerged from the simulations strengthens the theory of chemoautotrophic origin of life by providing plausible reaction pathways for the formation of ammonia within the iron-sulfur-world scenario.

Using reflection high-energy electron diffraction (RHEED), the growth onset of molecular beam epitaxy (MBE) deposited germanium telluride (GeTe) film on Si(111)-(3 ×)R30°-Sb surfaces is investigated, and a larger than expected in-plane lattice spacing is observed during the deposition of the first two molecular layers. High-resolution transmission electron microscopy (HRTEM) confirms that the growth proceeds via closed layers, and that those are stable after growth. The comparison of the experimental Raman spectra with theoretical calculated ones allows assessing the shift of the phonon modes for a quasi-free-standing ultra-thin GeTe layer with larger in-plane lattice spacing. The manifestation of the latter phenomenon is ascribed to the influence of the interface and the confinement of GeTe within the limited volume of material available at growth onset, either preventing the occurrence of Peierls dimerization or their ordered arrangement to occur normally.

Phase change materials are a class of compounds employed for data storage applications such as rewritable optical disks (DVD-RW, Blue-Ray disks) and more recently for non-volatile electronic memories of new generation, named phase change memories (PCM)[1]. These applications rely on a fast (50 ns) and reversible transition between a crystalline and the amorphous phases upon heating. The strong optical and electronic contrast between the crystal and the amorphous allows discriminating between the two phases that correspond to the two states of the memory, i.e. the 0 and 1 bits. In this work I have studied, by means of first principle and classical calculations, the structural, vibrational and thermal properties of some of the most promising and widely used phase-change materials such as GeTe, Ge2Sb2Te5 (GST), GeTe-Sb2Te3 superlattices and InSbTe (IST) alloys. The first part of the thesis is focused on the calculation of bulk thermal conductivity and thermal boundary resistances(TBR) between the active media and the surrounding dielectrics and metallic electrodes. Since in PCMs the phase changes corresponding to the memory writing/erasing processes are induced by Joule heating, heat dissipation and transport are key factors that greatly affect the power consumption and the switching speed of the memory cell. Moreover these quantities also influence the thermal cross-talks among the different bits in a memory array which can rise serious reliability issues, especially in ultrascaled devices. Bulk thermal has been computed on the basis density functional calculations [2] for crystalline GeTe, Sb2Te3 and GST. These calculations allowed to identify the origin of the large variability in experimental measurement for GeTe and the origin of the glass-like thermal conductivity in GST. An estimate of IST thermal conductivity was also obtained based on the minimal thermal conductivity model and ab-initio phonons. Thermal boundary resistance at different interface of crystalline

The structural defects in two-dimensional transition metal dichalcogenides, including point defects, dislocations and grain boundaries, are scarcely considered regarding their potential to manipulate the electrical and optical properties of this class of materials, notwithstanding the significant advances already made. Indeed, impurities and vacancies may influence the exciton population, create disorder-induced localization, as well as modify the electrical behaviour of the material. Here we report on the experimental evidence, confirmed by ab initio calculations, that sulfur vacancies give rise to a novel near-infrared emission peak around 0.75 eV in exfoliated MoS2 flakes. In addition, we demonstrate an excess of sulfur vacancies at the flake's edges by means of cathodoluminescence mapping, aberration-corrected transmission electron microscopy imaging and electron energy loss analyses. Moreover, we show that ripplocations, extended line defects peculiar to this material, broaden and redshift the MoS2 indirect bandgap emission.

Simulations of prebiotic NH3 synthesis from NO3- and NO2- on pyrite surfaces under hydrothermal conditions are reported. Ab initio metadynamics calculations have successfully explored the full reaction path which explains earlier experimental observations. We have found that the reaction mechanism can be constructed from stepwise single atom transfers which are compatible with the expected reaction time scales. The roles of the hot-pressurized water and of the pyrite surfaces have been addressed. The mechanistic picture that emerged from the simulations strengthens the theory of chemoautotrophic origin of life by providing plausible reaction pathways for the formation of ammonia within the iron-sulfur-world scenario.

We have investigated the structural, vibrational, and electronic properties of the amorphous phase of InSb and In3SbTe2 compounds of interest for applications in phase change non-volatile memories. Models of the amorphous phase have been generated by quenching from the melt by molecular dynamics simulations based on density functional theory. In particular, we have studied the dependence of the structural properties on the choice of the exchange-correlation functional. It turns out that the use of the Becke-Lee-Yang-Parr functional provides models with a much larger fraction of In atoms in a tetrahedral bonding geometry with respect to previous results obtained with the most commonly used Perdew-Becke-Ernzerhof functional. This outcome is at odd with the properties of Ge2Sb2Te5 phase change compound for which the two exchange-correlation functionals yield very similar results on the structure of the amorphous phase.

In the realm of layered materials beyond graphene, MoS2 gains a primary role due to its semiconducting nature and n-type transport down to the 2D limit that makes it extremely appealing for electronic and optoelectronic applications. The intrinsic presence of defects causes MoS2 to undergo localization effects. In the present work, solid evidence of Cs impurities in bulky MoS2 crystals in a concentration well beyond the sensitivity threshold of independent compositional spectrometry probes is brought. Unlike conventional intercalation of alkali metals in MoS2, on the basis of the measured crystal structure and ab initio calculations, it is proposed that the incorporation of Cs is stabilized by complex where one Cs atom is associated with a double S vacancy therein resulting in an overall n-type doping of the MoS2. The field effect transistor based on this kind of Cs-doped MoS2 multilayer flakes exhibits a variable range hopping transport and a metal–insulator transition.

An improved inverse simulated annealing method is presented to determine the structure of complex disordered systems from first principles in agreement with available experimental data or desired predetermined target properties. The effectiveness of this method is demonstrated by revisiting the structure of amorphous InSb. Beside being more efficient than conventional quenching from the melt, the present approach results in a mostly tetrahedral structure, which is in excellent agreement with available experimental data.

Phase change memories rest on the ability of some chalcogenide alloys to undergo a fast and reversible transition between the crystalline and amorphous phases upon Joule heating. The fast crystallization is due to a high nucleation rate and a large crystal growth velocity which are actually possible thanks to the fragility of the supercooled liquid that allows for the persistence of a high atomic mobility at high supercooling where the thermodynamical driving force for crystallization is also high. Since crystallization in the devices occurs by rapidly heating the amorphous phase, hysteretic effects might arise with a different diffusion coefficient and viscosity on heating than on cooling. In this work, we have quantified these hysteretic effects in the phase change compound GeTe by means of molecular dynamics simulations. The atomic mobility in the overheated amorphous phase is lower than in supercooled liquid at the same temperature and the viscosity is consequently higher. Still, the simulations of the overheated amorphous phase reveal a breakdown of the Stokes-Einstein relation between the diffusion coefficient and the viscosity, similarly to what we found previously in the supercooled liquid. Evidences are provided that the breakdown is due to the emergence of dynamical heterogeneities at high supercooling.

We have investigated the structural, vibrational, and electronic properties of the amorphous phase of InSb and In3SbTe2 compounds of interest for applications in phase change non-volatile memories. Models of the amorphous phase have been generated by quenching from the melt by molecular dynamics simulations based on density functional theory. In particular, we have studied the dependence of the structural properties on the choice of the exchange-correlation functional. It turns out that the use of the Becke-Lee-Yang-Parr functional provides models with a much larger fraction of In atoms in a tetrahedral bonding geometry with respect to previous results obtained with the most commonly used Perdew-Becke-Ernzerhof functional. This outcome is at odd with the properties of Ge2Sb2Te5 phase change compound for which the two exchange-correlation functionals yield very similar results on the structure of the amorphous phase.

Phase change materials are the active compounds in optical disks and in nonvolatile phase change memory devices. These applications rest on the fast and reversible switching between the amorphous and the crystalline phases, which takes place in the nano domain in both the time and the length scales. The fast crystallization is a key feature for the applications of phase change materials. In this work, we have investigated by means of large scale molecular dynamics simulations the crystal growth of the prototypical phase change compound GeTe at the interface between the crystalline and the supercooled liquid reached in the device upon heating the amorphous phase. A neural network interatomic potential, markedly faster with respect to first-principles methods, allowed us to consider high-symmetry crystalline surfaces as well as polycrystalline models that are very close to the actual geometry of the memory devices. We have found that the crystal growth from the interface is dominant at high temperatures while it is competing with homogeneous crystallization in the melt at lower temperatures. The crystal growth velocity markedly depends on the crystallographic plane exposed at the interface, the (100) surface being kinetically dominant with respect to the (111) surface. Polycrystalline interfaces, representative of realistic conditions in phase change memory devices, grow at significantly slower pace because of the presence of grain boundaries.

Aging is a common feature of the glassy state. In the case of phase-change chalcogenide alloys the aging of the amorphous state is responsible for an increase of the electrical resistance with time. This phenomenon called drift is detrimental in the application of these materials in phase-change nonvolatile memories, which are emerging as promising candidates for storage class memories. By means of combined molecular dynamics and electronic structure calculations based on density functional theory, we have unraveled the atomistic origin of the resistance drift in the prototypical phase-change compound GeTe. The drift results from a widening of the band gap and a reduction of Urbach tails due to structural relaxations leading to the removal of chains of Ge-Ge homopolar bonds. The same structural features are actually responsible for the high mobility above the glass transition which boosts the crystallization speed exploited in the device.

I materiali a cambiamento di fase sono calcogenuri a base di tellurio di notevole interesse tecnologico per la realizzazione di memorie ottiche (DVD) e di memorie elettroniche non volatili di nuova concezione, le memorie a cambiamento di fase o PCM. Questi dispositivi si basano su una veloce (50 ns) e reversibile transizione di fase amorfo-cristallo indotta per riscaldamento. Le due fasi corrispondono ai due stati di memoria che possono essere distinti grazie alla grande differenza tra le proprietà ottiche ed elettroniche dell'amorfo e quelle del cristallo. Nonostante il Ge2Sb2Te5 (GST) sia il materiale attualmente usato nelle PCM, si stanno studiando nuovi materiali con una temperatura di cristallizzazione più alta per aumentare la stabilità termica delle PCM. A questo proposito in questa tesi sono state studiate, attraverso simulazioni di dinamica molecolare ab-initio, diverse leghe ad alta temperatura di cristallizzazione con composizione In3Sb1Te2, In13Sb11Te3 e Ga4Sb6Te3. Queste leghe sono state studiate sperimentalmente e proposte come sostituti del GST, ma le proprietà strutturali e l'origine microscopica dell'elevata temperatura di cristallizzazione della fase amorfa di questi composti non è ancora del tutto chiara. Sono stati, quindi, generati modelli di qualche centinaio di atomi della fase amorfa raffreddando dal liquido in centinaia di ps allo scopo di trovare una relazione tra la struttura dell'amorfo e l'alta temperatura di cristallizzazione di queste leghe. La topologia di legame dei modelli amorfi risulta principalmente tetraedrica, molto diversa dalla geometria della fase cristallina che presenta invece intorni ottaedrici. La presenza di strutture tetraedriche nell'amorfo, assenti invece nella fase cristallina, può quindi costituire un ostacolo alla cristallizzazione con l'effetto di innalzare la temperatura di cristallizzazione rispetto al GST che presenta una geometria di legame prevalentemente ottaedrica sia nell'amorfo che nel cristallo. N, Phase change materials based on chalcogenide alloys are of great technological importance because of their use in optical data storage devices (DVDs) and electronic non-volatile memories of new concept, the Phase Change Memory cell (PCM). These applications rely on a fast (50 ns) and reversible change between the crystalline and the amorphous phases upon heating. The two phases correspond to the two states of the memory that can be discriminated thanks to a large difference in their optical and electronic properties. Although Ge2Sb2Te5 (GST) is the compound presently used as active layer in PCMs, alternative materials with a higher crystallization temperature are under scrutiny in order to increase the thermal stability of the PCM devices. In this respect, we analysed, by means of ab-initio molecular dynamics simulations, different high crystallization temperature alloys with composition In3Sb1Te2, In13Sb11Te3 and Ga4Sb6Te3, which have been experimentally proposed as substitute of GST. However, the structural properties and the microscopical reason of the high thermal stability of the amorphous phases of these compounds is still unclear. We, thus, generated models of the amorphous phase of few hundreds of atoms by quenching from the melt in few hundreds of ps aiming at finding out a relation between the structural properties of the amorphous phase and the high crystallization temperature of these alloys. The topology of our amorphous models turned out to be mostly tetrahedral which differs from the octahedral-like geometry of the crystalline phases. The presence of tetrahedral structures in the amorphous which are absent in the crystalline phase, probably hinders the crystallization process resulting in a higher crystallization temperature with respect to GST which display a mostly octahedral-like structures in both amorphous and the crystalline phase. In the second part of this work we addressed the issue of the resistance drift phenomenon, which consists of an inc

Aging is a common feature of the glassy state. In the case of phase-change chalcogenide alloys the aging of the amorphous state is responsible for an increase of the electrical resistance with time. This phenomenon called drift is detrimental in the application of these materials in phase-change nonvolatile memories, which are emerging as promising candidates for storage class memories. By means of combined molecular dynamics and electronic structure calculations based on density functional theory, we have unraveled the atomistic origin of the resistance drift in the prototypical phase-change compound GeTe. The drift results from a widening of the band gap and a reduction of Urbach tails due to structural relaxations leading to the removal of chains of Ge-Ge homopolar bonds. The same structural features are actually responsible for the high mobility above the glass transition which boosts the crystallization speed exploited in the device.

Phase change materials are the active compounds in optical disks and in nonvolatile phase change memory devices. These applications rest on the fast and reversible switching between the amorphous and the crystalline phases, which takes place in the nano domain in both the time and the length scales. The fast crystallization is a key feature for the applications of phase change materials. In this work, we have investigated by means of large scale molecular dynamics simulations the crystal growth of the prototypical phase change compound GeTe at the interface between the crystalline and the supercooled liquid reached in the device upon heating the amorphous phase. A neural network interatomic potential, markedly faster with respect to first-principles methods, allowed us to consider high-symmetry crystalline surfaces as well as polycrystalline models that are very close to the actual geometry of the memory devices. We have found that the crystal growth from the interface is dominant at high temperatures while it is competing with homogeneous crystallization in the melt at lower temperatures. The crystal growth velocity markedly depends on the crystallographic plane exposed at the interface, the (100) surface being kinetically dominant with respect to the (111) surface. Polycrystalline interfaces, representative of realistic conditions in phase change memory devices, grow at significantly slower pace because of the presence of grain boundaries.

The supercritical water-pyrite interface has been studied by ab initio molecular dynamics simulation. Extreme conditions are relevant in the iron-sulfur world (ISW) theory where prebiotic chemical reactions are postulated to occur at the mineral-water interface. We have investigated the properties of this interface under such conditions. We have come to the conclusion that hot-pressurized water on pyrite leads to an interface where a dry pyrite surface is in contact with the nearby SC water without significant chemical interactions. This picture is markedly different from that under ambient conditions where the surface is fully covered with adsorbed water molecules which is of relevance for the surface reactions of the ISW hypothesis.

A combined synthesis, experiment, and theory approach, using elastic and inelastic helium atom scattering along with ab initio density functional perturbation theory, has been used to investigate the vibrational dynamics and band structure of a recently synthesized organic-functionalized semiconductor interface. Specifically, the thermal properties and lattice dynamics of the underlying Ge(111) semiconductor crystal in the presence of a commensurate (1 × 1) methyl adlayer were defined for atomically flat methylated Ge(111) surfaces. The mean-square atomic displacements were evaluated by analysis of the thermal attenuation of the elastic He diffraction intensities using the Debye-Waller model, revealing an interface with hybrid characteristics. The methyl adlayer vibrational modes are coupled with the Ge(111) substrate, resulting in significantly softer in-plane motion relative to rigid motion in the surface normal. Inelastic helium time-of-flight measurements revealed the excitations of the Rayleigh wave across the surface Brillouin zone, and such measurements were in agreement with the dispersion curves that were produced using density functional perturbation theory. The dispersion relations for H-Ge(111) indicated that a deviation in energy and lineshape for the Rayleigh wave was present along the nearest-neighbor direction. The effects of mass loading, as determined by calculations for CD3-Ge(111), as well as by force constants, were less significant than the hybridization between the Rayleigh wave and methyl adlayer librations. The presence of mutually similar hybridization effects for CH3-Ge(111) and CH3-Si(111) surfaces extends the understanding of the relationship between the vibrational dynamics and the band structure of various semiconductor surfaces that have been functionalized with organic overlayers.