This thesis was dedicated to decorate graphene sheets with metal and metal oxide nanostructures by RF sputtering technique. Two main objectives were focused in this thesis. 1) To decorate graphene sheets uniformly with metal and metal oxide nanostructure without agglomeration. 2) To explore different kinds of application of decorated graphene sheets with metal and metal oxide nanostructures In the first step, we presented the experimental study results about Nb2O5 deposition onto graphite nanoplatelets (GNPs) by the variation of the deposition process parameters. The structural, chemical and electronic properties of the decorated GNPs with Nb2O5 layers were studied. It was found that with deposition of Nb2O5 layers onto GNPs, tensile strain was developed into the planes of the GNPs. The induced tensile strain in and between the planes of GNPs increased with raising the amount of the Nb2O5 concentration. TEM images shows that GNPs decorated with around 5 to10 nm uniform layer of Nb2O5 at 100 W on their surface were successfully fabricated. From the XPS analysis it was confirmed that, by increasing Nb2O5 layer thickness on the GNPs surface with rising RF power values binding energy downshift in C 1s peak suggests a p-type doping of GNPs due to charge transfer at the interface as a consequence of the higher work function difference between the Nb2O5 (4.70 eV) and GNPs (4.33 eV). In the second step, the interface between the graphene sheets and Nb2O5 nanoparticles were studied. It was established that the structural defects were pronounced with increasing amounts of the Nb2O5 concentration. XPS measurement on graphene/Nb2O5 suggests p-type doping of graphene due to charge transfer at the interface as a consequence of the high work function of Nb2O5. The strong p-doping effect was also confirmed by Raman analysis where the positions of the G and 2D peaks of graphene gradually upshifted upon increasing the Nb2O5 concentration. The uniform distribution of decorated Nb2O5 nanoparticles onto graphene was confirmed from TEM analysis. The ferromagnetic behavior was observed for the undecorated graphene and decorated graphene with Nb2O5 nanoparticles. The ferromagnetic behavior of graphene was enhanced with decoration of the Nb2O5 nanoparticles. In the third step, the effect of the Mg concentration on the structural, chemical and morphological properties of the graphene was described. Well dispersed Mg nanoparticles were decorated onto graphene sheets. It was found that from the XRD results, different sizes of the crystalline Mg nanoparticles were obtained onto graphene sheets with variation of the process parameters.. Raman spectra indicated that G and 2D bands of the graphene were shifted to higher wavenumber with deposition of Mg nanoparticles. The well dispersed and small size of Mg nanoparticles in the range of (8-12 nm) onto graphene sheets was decorated by using a high powder vibration frequency. No agglomeration of the sputtered particles was observed with high powder vibration frequency. This observation was confirmed by TEM micrographs. XPS analysis revealed that the decorated Mg nanoparticles onto graphene were oxidized due to exposure to the atmosphere. The well dispersed decorated Mg nanoparticles onto graphene sheets were studied for the hydrogen absorption and desorption at two different temperatures 330 oC and 360 oC at 2 and 8 bars pressure. The hydrogen up taking capacity for the decorated graphene sheets with Mg nanoparticles was 3 wt. % in whole composite. However, the up taking hydrogen storage capacity of the only Mg nanoparticles was 6.6 wt. %. In the last step, the interaction of the graphene sheets with TiO2 nanoparticles was studied. The XRD results indicated that the lattice of the graphene sheets was distorted with increasing amount of the TiO2 concentration. The particle nature of the deposited TiO2 was confirmed by TEM examination and also the TEM analysis shows that TiO2 nanoparticles were uniformly distributed onto graphene sheets. The Raman analysis showed that the G and 2D bands of graphene were shifted to higher wavenumber with increasing TiO2 concentration onto graphene sheet confirming the p doped graphene with TiO2 nanoparticles. The XPS analysis further confirmed the p doping of graphene upon the deposition of the TiO2 nanoparticles. The binding energy downshift the C 1s core level of was observed after charge transfer from graphene to TiO2 nanoparticles due to the larger work function of TiO2 relatively to that of graphene. It was observed that decorated graphene sheets with TiO2 nanoparticles shows reasonably catalytic activity.

Entanglement is a fundamental property of quantum systems and its characterization is a central problem for physics. Moreover, there is an increasing demand for scalable protocols that can certify the presence of entanglement. This is primarily due to the role of entanglement as a crucial resource for quantum technologies. However, systematic entanglement certification is highly challenging, and this is particularly the case for quantum many-body systems. In this dissertation, we tackle this challenge and introduce some techniques that allow the certification of multipartite entanglement in many-body systems. This is demonstrated with an application to a model of interacting fermions that shows the presence of resilient multipartite entanglement at finite temperatures. Moreover, we also discuss some subtleties concerning the definition entanglement in systems of indistinguishable particles and provide a formal characterization of multipartite mode entanglement. This requires us to work with an abstract formalism that can be used to define entanglement in quantum many-body systems without reference to a specific structure of the states. To further showcase this technique, and also motivated by current quantum simulation efforts, we use it to extend the framework of entanglement witnesses to lattice gauge theories. / L'entanglement è una proprietà fondamentale dei sistemi quantistici e la sua caratterizzazione è un problema centrale per la fisica. Inoltre, vi è una crescente richiesta di protocolli scalabili in grado di certificare la presenza di entanglement. Ciò è dovuto principalmente al ruolo dell'entanglement come risorsa cruciale per le tecnologie quantistiche. Tuttavia, la certificazione sistematica dell'entanglement è molto impegnativa, e questo è particolarmente vero per i sistemi quantistici a molti corpi. In questa dissertazione, affrontiamo questa sfida e introduciamo alcune tecniche che consentono la certificazione dell'entanglement multipartito in sistemi a molti corpi. Ciò è dimostrato con un'applicazione a un modello di fermioni interagenti che mostra la presenza di entanglement multipartito resiliente a temperature finite. Inoltre, discutiamo anche alcune sottigliezze riguardanti la definizione di entanglement in sistemi di particelle indistinguibili e forniamo una caratterizzazione formale dell'entanglement multipartito. Ciò ci richiede di lavorare con un formalismo astratto che può essere utilizzato per definire l'entanglement nei sistemi quantistici a molti corpi senza fare riferimento a una struttura specifica degli stati. Per mostrare ulteriormente questa tecnica, e anche motivata dagli attuali sforzi di simulazione quantistica, la usiamo per estendere la struttura dei testimoni di entanglement alle teorie di gauge del reticolo.

Active particle is a general term used to label a large set of different systems, spanning from a flock of birds flying in a coordinated pattern to a school of fish abruptly changing its direction or to a bacterium self-propelling itself while foraging nourishment. The common property shared by these systems is that their constituent agents, e.g. birds, fishes, or bacteria, are capable of harvesting energy from the surrounding environment and converting it into self-propulsion and directed motion. This peculiar feature characterizes them as out-of-equilibrium systems, in fact, the process of energy consumption and dissipation generates microscopically irreversible dynamics and drives them far from thermal equilibrium. Thanks to their intrinsic out-of-equilibrium nature, active particle systems often display characteristic patterns and behaviors that are not observed in equilibrium physics systems, such as collective motion or motility-induced phase separation. These features prompted the development of theories and algorithms to simulate and study active particles, giving rise to paradigmatic models capable of describing these phenomena, such as the Vicsek model for collective motion, the run-and-tumble model, or the active Brownian particle model. At the same time, synthetic agents have been designed to reproduce the behaviors of these natural active particle systems, and their evolution could play a fundamental role in the nanotechnology of the 21st century and the development of novel medical treatments, in particular controlled drug delivery. A specific type of active particle that uses its directed motion to move at the microscale is called a microswimmer. Examples of these agents are bacteria exploring their surroundings while searching for food or escaping external threats, spermatozoa looking for the egg, or artificial Janus particles designed for specific tasks. Active agents at these scales use different swimming mechanisms, such as rotating flagella or phoretic motion along chemical gradients that they can create. The outcome of their efforts is determined by the interplay of the translational diffusion intrinsic to the dynamics at these scales and the persistent motion that characterizes their self-propulsion. The problem of finding a specific target in a complex environment is essential for microswimmers and active agents in general. Target search is employed by animals and microorganisms for a variety of purposes, from foraging nourishment to escaping potential threats, such as in the case of bacterial chemotaxis. The study of this process is therefore fundamental to characterize the behavior of these systems in nature. Its complete description could then be employed in designing synthetic microswimmers for addressing specific problems, such as the aforementioned targeted drug delivery and the environmental cleansing of soil and polluted water. Here, we provide a detailed study of the target search process for microswimmers exploring complex environments. To this end, we generalize Transition Path Theory, the rigorous statistical mechanics description of transition processes, to the target-search problem. The most general way of modeling a complex environment that the microswimmer has to navigate is through an external potential. This potential can be characterized by high barriers separating metastable states in the system or by the presence of confining boundaries. If a high energy barrier is located between the initial position of the microswimmer and its target, the target search becomes a rare event. Rare events have been thoroughly investigated in equilibrium physics, and several algorithms have been designed to cope with the separation of timescales intrinsic to these problems and enable their investigation via efficient computer simulations. Despite the large set of tools developed for studying passive particles performing rare transitions, the characterization of this process for non-equilibrium systems, such as active particles, is still lacking. One of the main results of this thesis is the generalization to non-equilibrium systems of the Transition Path Sampling (TPS) algorithm, which was originally designed to simulate rare transitions in passive systems. This algorithm relies on the generation of productive trajectories, i.e. trajectories linking the initial state of the particle to the target state, via a Monte Carlo procedure, without the need of simulating long thermal oscillations in metastable states. These trajectories are then accepted according to a Metropolis criterion and are subsequently used to obtain the transition path ensemble, i.e. the ensemble of all reactive paths that completely characterizes the process. The TPS algorithm relies on microscopic reversibility to generate the productive trajectories, therefore its generalization to out-of-equilibrium systems lacking detailed balance and microscopic reversibility has remained a major challenge. Within this work, after deriving a path integral representation for active Brownian particles, we provide a new rule for the generation and acceptance of productive non-equilibrium trajectories, which reduces to the usual expression for passive particles when the activity of the microswimmer is set to zero. This new rule allows us to generalize the TPS algorithm to the case of active Brownian particles and to obtain a first insight into the counterintuitive target-search pathways explored by these particles. In fact, while passive particles perform barrier crossing following the minimum energy path linking the initial state to the target state, we found that active particles, thanks to their activity and persistence of motion, can reach the target more often by surfing higher energy regions of the landscape that lie far from the minimum energy path. The second result of this thesis is a systematic characterization of the target-search path ensemble for an active particle exploring an energy landscape. We do so by analyzing the system’s response to changes in the two adimensional parameters that define the parameter space of the model: the Péclet number and the persistence of the active particle. Our findings show that active Brownian particles can increase their target-finding rates by tuning their Péclet number and their persistence according to the shape and characteristics of the external landscape. We perform this analysis in two different landscapes, namely a double-well potential and the Brown-Müller potential, finding robust features in the target-search patterns. In contrast, other observables of the system, e.g. the target-finding rates, are more responsive to the features of the external environment. Interestingly, our results suggest that, differently from what happens for passive particles, the presence of additional metastable states in the system does not hinder the target-search dynamics of active particles. The third original contribution of this Ph.D. thesis is the generalization of the concept of the committor function to target-search problems. The committor function was first introduced in the framework of Transition Path Theory to study reaction processes. If a definition for a reactant and a product state embedded in the configuration space of the system is provided, the committor function quantifies the probability that a trajectory starting in a given configuration reaches the product state before it can enter the reactant. For this reason, it has been proven to be pivotal for a complete characterization of these events and it is often regarded as the optimal reaction coordinate for thermally activated transitions. The target search problem shares many similarities with transition processes since it is characterized by an initial state from which the agent begins its journey and a target state that the particle is aiming to reach, and often some barriers or obstacles separate the two. Exploiting these similarities, we take advantage of the concept of the committor function to fully characterize a target-search process performed by an active agent. First, we derive the Fokker-Planck equation for an active Brownian particle subject to an external potential, and we use its associated probability current to define the committor function for an active agent. Then, we prove that the active committor satisfies the Backward-Kolmogorov equation analogously to the committor for passive particles. We take advantage of this property to efficiently compute the committor function using a finite-difference algorithm, validating it with brute-force simulations. Finally, we further validate our theory with experiments of a camphor self-propelled disk. This self-propelled disk is capable of moving on a water surface and is studied during its exploration of a circular confining environment. We start by analyzing long recorded trajectories of such a disk moving in a Petri dish, and, after defining a reactant and a product region in the system, we proceed to compute the committor function in three different regions contained in the dish. We analyze all the trajectory slices passing through those regions and we measure how many of them hit the product region and how many hit instead the reactant first, and we obtain the committor in the three regions as a function of the angle. Finally, we simulate a long trajectory of an active Brownian particle exploring a circular confining environment, and we compare the committor as an angular function obtained from brute-force simulations with the committor estimated from experimental data.

In conventional silicon solar cell, the collection probability of light generated carries shows a drop in the high energy range 280-400nm. One of the methods to reduce this loss, is to implement nanometre sized semiconductors on top of a solar cell where high energy photons are absorbed and low energy photons are re-emitted. This effect, called luminescence down-shifter (LDS), modifies the incident solar spectrum producing an enhancement of the energy conversion efficiency of a cell. We investigate this innovative effect using silicon nanoparticles dispersed in a silicon dioxide matrix as active material. In particular, I proposed to model these structures using a transfer matrix approach to simulate its optical properties in combination with a 2D device simulator to estimate the electrical performance. Based on the optimized layer sequences, high efficiency cells were produced within the european project LIMA characterized by silicon quantum dots as active layer. Experimental results demonstrate the validity of this approach by showing an enhancement of the short circuit current density with up to 4%. In addition, a new configuration was proposed to improve the solar cell performances. Here the silicon nanoparticles are placed on a cover glass and not directly on the silicon cells. The aim of this study was to separate the silicon nanocrystals (Si-NCs) layer from the cell. In this way, the solar device is not affected by the Si-NCs layer during the fabrication process, i.e. the surface passivation quality of the cell remains unaffected after the application of the LDS layer. Using this approach, the downshifting contribution can be quantified separately from the passivation effect, as compared with the previous method based on the Si-NCs deposition directly on the solar devices. By suitable choice of the dielectric structures, an improvement in short circuit current of up 1% due to the LDS effect is demonstrated and simulated.

Nanodiamonds are a promising platform for quantum technologies due to the combinations of their inherent properties and the properties of the fluorescent color centers hosted in diamond. They can be employed as quantum sensing devices with spatial resolution in the range of the nanometer and capable of withstanding harsh conditions while also being biocompatible, allowing applications with sensitive biological systems; but they also find application in quantum computing and photonics fields. For all these applications the central features are the properties of the photoluminescent color centers employed, the color centers on which this thesis is focused are the Nitrogen-Vacancy (NV) and Silicon-Vacancy (SiV) centers of diamond. Both centers are of high interest due to spin dependent properties of their fluorescent emission which can be accessed at room temperature. The development of quantum technologies based on such fluorescent nanodiamonds is stifled by the the lack of production techniques that can be easily scaled to industrial levels. In fact most of the more prominent techniques found in literature exhibit drawbacks both in terms of control of particle properties and of scalability. This thesis focuses on the synthesis of nanodiamonds by Pulsed Laser Ablation in Liquid, with particular interest in the possibility of producing continuously nanodiamonds containing NV and SiV centers. For the NV center the technique of choice have been Pulsed Laser Ablation in liquid nitrogen focusing on the yield of the process as the technique has already been experimentally validated. For the SiV centers the ablation process was performed in water and the graphite precursor have been substituted for a composite graphite and silicon carbide precursor.

No description available.

Looking at state of the art of optical devices, it is evident that glass-based rare-earth-activated optical structures represent the technological pillar of a huge number of photonic applications covering Health and Biology, Structural Engineering, Environment Monitoring Systems, Lighting, Laser sources and Quantum Technologies. Among different glass-based systems, a strategic place is assigned to transparent glass-ceramics, nanocomposite materials, which offer specific characteristics of capital importance in photonics. Following this strategy, this PhD thesis exploits tin dioxide (SnO2)-based glass-ceramic activated by erbium ions (Er3+) to put the basis for the fabrication of solid state and integrated lasers. The research discussed in my PhD thesis gives a possible solution to two crucial and decisive points in the development of an optically pumped rare-earth-based laser: (i) the low absorption cross section of the rare-earth ions; (ii) the writing of channels and mirrors in the case of waveguide integrated laser, thanks to the demonstration of two innovative and unique characteristics of SnO2-based transparent glass-ceramics, i.e. luminescence sensitizing and photorefractivity. The role of SnO2 nanocrystals as rare-earth ion luminescence sensitizers allows to overcome the low absorption cross section of the Er3+ ion. The photorefractivity in range of 10-3 of SiO2-SnO2:Er3+ glass-ceramics allows applying the robust direct laser photoinscription technique on the systems to fabricate Bragg gratings and channel waveguides for waveguide integrated lasers. Based on an application-oriented approach, a comprehensive study on SiO2-SnO2:Er3+ glass-ceramic planar waveguides and monoliths, has been carried out. The work covers different research stages and aspects from the material preparation to a complete assessment of systems for the applications employing a rich number and variety of experimental techniques. The energy transfer from SnO2 to Er3+ and the efficient pumping scheme exploiting SnO2 as Er3+ luminescence sensitizers were demonstrated. The relaxation dynamic of the electronic states as well as the location of the dopant and density of states are discussed, and a specific modeling has been developed to the proof of concept realization of the considered devices. The obtained photorefractivity in range of 10-3 allowed the inscription of gratings on the fabricated SiO2-SnO2:Er3+ planar waveguides using UV laser direct writing technique. Exploiting the robust femtosecond laser micromachining, the optical waveguides were inscribed in the fabricated SiO2-SnO2:Er3+ monolithic squares. Another important outcome of this research is the design of a solid state laser with lateral pumping scheme and of an integrated waveguide laser in two different distributed feedback structures using all the parameters measured during the experimental activity.

Biological materials such as wood show outstanding properties due to the self assembly of components from molecular to macroscopic size. An emerging nanotechnology-based strategy consists of the isolation of biological components with size in the range from nanometers to micrometers and of the design of human-driven assembly processes to obtain multifunctional materials. The aim of this thesis was to isolate cellulose nanocrystals, with dimensions of around 4-5 nm in width and some hundred nanometers in length, and investigate their assembly processes through weak interactions among them and with small molecules, like water or ions. Knowing their interaction properties and self-assembly is indeed fundamental in order to fully exploit the potential of nanocellulose in its recently emerging applications. In particular, I focused on cellulose nanocrystals supramolecular self-organization both in absence and presence of water, studying cellulose nanocrystals-based films and hydrogels. In dry conditions, the self-assembly of cellulose nanocrystals on a polylactic support was demonstrated to form few micrometers thick films, characterized by a densely packed arrangement of the crystals leaving elongated cavities of about 0.31 nm cross section between them. These cavities provide the pathway for gaseous 2H2 diffusion. Conversely, these films are impermeable barriers for the transport of gaseous molecules such as O2 and CO2. In aqueous solution, instead, cellulose nanocrystals undergo sonication- or cation-assisted entanglement, forming soft hydrogels. Na+, Ca2+ and Al3+ crosslink the nanocrystals and produce stable hydrogels with structurally ordered domains in which water is confined. Since the gelation process is diffusion controlled, small hydrogel objects with different size and shape have been designed by the coordination-driven assembly of supramolecular rod-like cellulose crystallites, using ionotropic gelation as a methodological approach and Ca2+ as a gelling agent. In parallel to material characterization, particular attention was devoted to the possible exploitation of cellulose nanocrystals-based materials in the biomedical field. In this regard, toxicity studies were performed both on the individual nanocrystals and on the films and hydrogels resulting from their assembly. Moreover, a hybrid cellulose-nanocrystals/chitosan material was developed and characterized, which shows some potential to be used as therapeutic delivery system in the gastrointestinal tract. Indeed, though a mould assisted gelation process, composite hydrogels can be produced, which are degraded by human digestive enzymes and release a model protein according to a biphasic kinetic profile.

Negli ultimi decenni si è vista la nascita di un grande interesse verso l'utilizzo dell'idrogeno come vettore energetico. Le ragioni sono da ricercarsi nelle problematiche sorte in tempi recenti in relazione alle emissioni inquinanti ed al rapido esaurimento delle fonti energetiche. Per poter concepire un'economia basata sull'idrogeno è tuttavia necessario risolvere una serie di problematiche connesse alla sua produzione, immagazzinamento e filtraggio. Il presente lavoro di tesi ha riguardato gli ultimi due punti. In relazione allo stoccaggio del gas, uno dei sistemi più promettenti risulta quello dell'utilizzo d'idruri metallici. Tali materiali presentano ancora una serie di problematiche che ne limitano l'utilizzo pratico, sia in ambito stazionario, che per applicazioni al settore della mobilità. Per superare questi ostacoli è necessaria la comprensione dei meccanismi microscopici con cui avviene la formazione e la dissociazione della fase idruro. Il presente lavoro è stato quindi incentrato sull'approfondimento di tale aspetto, concentrandosi in particolare sul ruolo giocato dai difetti di tipo vacanza in sistemi a base magnesio. Per quanto concerne l'aspetto del filtraggio, le tecniche positroniche sono state impiegate, in concomitanza con altre ad esse complementari, per ottenere una caratterizzazione della porosità, in termini di dimensioni, distribuzione ed interconnettività dei pori, in film di silice da utilizzare per la realizzazione di membrane selettive.

The experimental work presented in this thesis is done to develop an innovative procedure to create a protective nanostructured coating inside the X-band radio frequency cavity, a key component in future particle accelerator. The scope of the multilayer coating is to prevent the breakdown due to high electric and magnetic field. In fact the electrical discharges damage, in irreversible way, the internal surface of the cavity and compromise the final operation of the device. The keen interest on the topic is due to decrease the length and the cost of the next generation linear accelerator. To do this it is essential to enhance the performance of X-band Linacs up to 100MV/m accelerating gradient and to maintain, high as possible, the electrical breakdown reliability. Several studies are made on different materials in order to develop these cavities [1] [2], but the use of physical vapor deposition technique (PVD), to obtain nanostructured coating directly on internal wall of these small sized cavities is not reported in literature. The size of the cavities is of order of few millimeters and the iris aperture ranges from 2 to 6mm: for this reason the direct PVD coating is not possible. Hence a mandrel, that is the negative shape of the cavity, is first coated using PVD technique and finally chemically dissolved after copper electroforming[3]. The novel nanostructured coating is a multilayer composed by two high purity and immiscible metals. One is Copper to guarantee electrical conductivity of the cavity and the second is Molybdenum because it is a refractory metal. Moreover the choice of immiscible materials is important, because these materials do not form alloy during the deposition phase. Keeping a well-defined interface is important to guarantee a barrier effect to the motion of the defects inside the cavity’s material[4][5]. The experimental part of the thesis is divided in three different parts: design and setup of the PVD deposition system, plasma discharge analysis and, finally, the characterization of the coatings. This work is a collaboration between Industrial Engineering Department (University of Trento) and the National Laboratory of Legnaro (National Institute of Nuclear Physics LNL-INFN), but this research involves several institutes in different countries: SLAC (USA), KEK (Japan) and UCLA (Los Angeles USA).