Liquidity crises on different time scales

We present an empirical analysis of the microstructure of financial markets and, in particular, of the static and dynamic properties of liquidity. We find that on relatively large time scales (15 min) large price fluctuations are connected to the failure of the subtle mechanism of compensation between the flows of market and limit orders: in other words, the missed revelation of the latent order book breaks the dynamical equilibrium between the flows, triggering the large price jumps. On smaller time scales (30 s), instead, the static depletion of the limit order book is an indicator of an intrinsic fragility of the system, which is related to a strongly nonlinear enhancement of the response. In order to quantify this phenomenon we introduce a measure of the liquidity imbalance present in the book and we show that it is correlated to both the sign and the magnitude of the next price movement. These findings provide a quantitative definition of the effective liquidity, which proves to be strongly dependent on the considered time scales

Molecular olefin polymerization catalysts: applications of molecular design for properties modulation

Olefin polymerization is an industrial sector of huge economic and social impact. In this context, molecular catalysts have drawn much attention since 1990s, as they can be used for the industrial production of advanced polymeric material which are hardly accessible by heterogeneous Ziegler-Natta technology. The growing commercial demand is pushing research toward the identification of novel active species for the synthesis of innovative (co)polymer architectures. Advances in experimental and especially computational technologies foster the ambition of catalyst development by design, but the passage from the 'classical' trial-and-error to a fully rational approach is hampered by the complex interactions between all species in the catalytic pool and the many complex subtleties that molecular design has to balance. This PhD thesis explores several issues related to rational catalyst development, trying to contribute to a more detailed understanding of the polymerization process by a combined experimental and computational approach. The first part of the work deals with the identification of structure/ properties correlations for the selective synthesis of specific products. In particular, Chapter 2 is dedicated to factors determining reactivity ratios in ethene/α-olefin copolymerization reactions. A suitable computational protocol to accurately reproduce comonomer affinities for a large variety of molecular catalysts is proposed, and utilized to draw tentative conclusions on entropic, electronic and steric effects determining comonomer affinity. Important kinetic considerations on the rate limiting step for chain propagation are provided, potentially explaining the occasionally non-trivial temperature dependence of copolymerization statistics. Due to the rapidly increasing demand, the production of comonomer (e.g. 1-hexene, 1-octene) feedstock has become an issue of growing relevance the polyolefin industry. Chapter 3 summarizes the work carried out during a three months internship at the Imperial College London (UK), working on chromium-catalyzed ethene oligomerization for the selective production of linear α-olefins (LAOs). The case of bis(benzimidazolyl)amine Cr-catalysts was investigated, providing supporting evidence for a mechanistic and kinetic model explaining alternating distribution of products. Along with the production of specific polymer architectures, activity and thermal stability are two key catalyst features to be considered in the design of novel active species, which are examined in the second part of this thesis. Chapter 4 describes a novel chain transfer to solvent process that was recently discovered by our group. This involves CH activation of the toluene solvent by Ti-phosphinimide catalysts, leading to benzyl terminated polypropylenes, and represents a reversible deactivation route being competitive at high temperatures and moderately high pressures. Extensive polymerization and DFT screenings were carried out, aiming at elucidating the mechanism and exploring the scope of this reaction. Finally, cocatalyst influence on polymerization performance is discussed in Chapter 5. Free-trimethylaluminum in commercial MAO solution was effectively trapped by addition of a hindered phenol, allowing to explore the properties of the oligomeric fraction of MAO by NMR spectroscopy. A phosphinimide half-titanocene was used as case study, taking advantage of the presence of a phosphorus atom in the ancillary ligand as spectroscopic probe for 31P NMR. Consequences of the absence of free trialkyl aluminum on the formation of dormant sites are evaluated and connections with the mechanism of chain transfer to solvent are highlighted. This work was carried out in collaboration with the group of Prof. Alceo Macchioni and Prof. Cristiano Zuccaccia at the University of Perugia

Defects in Lead Halide Perovskite Nanocrystals: Analogies and (Many) Differences with the Bulk

Understanding the origin of defects in lead halide perovskite nanocrystals is paramount to attaining long-term structural stability and improved optical efficiency, key features for their successful implementation in optoelectronic devices. Unlike other studies, we explore the possible formation of trap states in explicit, nonperiodic CsPbBr3 nanocrystal models about 3 nm in size. Using density functional theory, we compute the defect formation energies of interstitial, vacancy, and antisite defects in different regions of the nanocrystal (center, surface center, and surface edge), demonstrating that the most stable defect position is found at the surface. We ascribe the high defect tolerance of CsPbBr3 nanocrystals to the fact that vacancies, i.e. the loss of surface ligands as ion pairs, are energetically difficult to form and only excessive stripping of surface ligands might be problematic, as their detachment leaves undercoordinated Br– on the crystal surface that only in this case translates into dee..

Mapping the local dielectric response at the nanoscale by means of plasmonic force spectroscopy

At the present, the local optical properties of nanostructured materials are difficult to be measured by available instrumentation. We investigated the capability of plasmonic force spectroscopy of measuring the optical response at the nanoscale. The proposed technique is based on force measurements performed by combining Atomic Force Microscopy, or optical tweezers, and adiabatic compression of surface plasmon polaritons. We show that the optical forces, caused by the plasmonic field, depend on the local response of the substrates and, in principle, allow probing both the real and the imaginary part of the local permittivity with a spatial resolution of few nanometers

Magnetic hot-spot generation at optical frequencies: from plasmonic metamolecules to all-dielectric nanoclusters

AbstractThe weakness of magnetic effects at optical frequencies is directly related to the lack of symmetry between electric and magnetic charges. Natural materials cease to exhibit appreciable magnetic phenomena at rather low frequencies and become unemployable for practical applications in optics. For this reason, historically important efforts were spent in the development of artificial materials. The first evidence in this direction was provided by split-ring resonators in the microwave range. However, the efficient scaling of these devices towards the optical frequencies has been prevented by the strong ohmic losses suffered by circulating currents. With all of these considerations, artificial optical magnetism has become an active topic of research, and particular attention has been devoted to tailor plasmonic metamolecules generating magnetic hot spots. Several routes have been proposed in these directions, leading, for example, to plasmon hybridization in 3D complex structures or Fano-like magnetic resonances. Concurrently, with the aim of electromagnetic manipulation at the nanoscale and in order to overcome the critical issue of heat dissipation, alternative strategies have been introduced and investigated. All-dielectric nanoparticles made of high-index semiconducting materials have been proposed, as they can support both magnetic and electric Mie resonances. Aside from their important role in fundamental physics, magnetic resonances also provide a new degree of freedom for nanostructured systems, which can trigger unconventional nanophotonic processes, such as nonlinear effects or electromagnetic field localization for enhanced spectroscopy and optical trapping

Terahertz dipole nanoantenna arrays: resonance characteristics

Resonant dipole nanoantennas promise to considerably improve the capabilities of terahertz spectroscopy, offering the possibility of increasing its sensitivity through local field enhancement, while in principle allowing unprecedented spatial resolutions, well below the diffraction limit. Here, we investigate the resonance properties of ordered arrays of terahertz dipole nanoantennas, both experimentally and through numerical simulations. We demonstrate the tunability of this type of structures, in a range (∼1–2 THz) that is particularly interesting and accessible by means of standard zinc telluride sources. We additionally study the near-field resonance properties of the arrays, finding that the resonance shift observed between near-field and far-field spectra is predominantly ascribable to ohmic damping

Robot-Assisted Surveillance in Large Environments

This paper introduces ANSER, a mobile robot designed to perform surveillance in wide indoor and outdoor areas, such as civilian airports, warehouses or other facilities. The paper describes in details the robot subsystems, focusing on its capabilities in autonomous surveillance, localization and navigation

Nonlinear Hall effect as a local probe of plasmonic magnetic hot spots

Recently developed plasmonic nanostructures are able to generate intense and localized magnetic hot spots in a large spectral range from the terahertz to the visible. However, a direct measurement of the magnetic field at the hot spot has not been performed yet, due to the absence of magnetic field detectors that work at those high frequencies and that fit the hot-spot area. We propose to place a graphene ribbon in the hot spot of a plasmonic nanostructure driven by a laser beam, such that a current is generated due to both the magnetic field at the hot spot and the electric field of the laser. We demonstrate that a nonlinear Hall voltage, which can be measured by standard electrical means, builds up across the ribbon, making it possible to directly probe the magnetic field at the hot spot.Comment: 9 pages, 7 figure

The trickle down from environmental innovation to productive complexity

We study the empirical relationship between green technologies and industrial production at very fine-grained levels by employing Economic Complexity techniques. Firstly, we use patent data on green technology domains as a proxy for competitive green innovation and data on exported products as a proxy for competitive industrial production. Secondly, with the aim of observing how green technological development trickles down into industrial production, we build a bipartite directed network linking single green technologies at time t1 to single products at time t2≥t1 on the basis of their time-lagged co-occurrences in the technological and industrial specialization profiles of countries. Thirdly, we filter the links in the network by employing a maximum entropy null-model. Our results emphasize a strong connection between green technologies and the export of products related to the processing of raw materials, notably crucial for the development of climate change mitigation and adaptation technologies. Furthermore, by looking at the evolution of the network over time, we observe a growing presence of more complex green technologies and high-tech products among the significant links, suggesting an increase in their importance in the network