Misura della raccolta di luce in una striscia di scintillatore plastico letta con fotomoltiplicatori al silicio

In questo lavoro è stata studiata la risposta a particelle al minimo di ionizzazione di un rivelatore, che sarà utilizzato per l'identificazione di muoni in SHIP (Search for HIdden Particles), esperimento proposto al CERN e ancora in fase di approvazione. Il rivelatore è basato su una striscia di scintillatore lunga tre metri accoppiata ad una fibra ottica Wave Lenght Shifter (WLS) e letta da due fotomoltiplicatori al Silicio (SiPM), posti ad entrambi i capi. Lo scopo principale del lavoro è la misura della raccolta di luce del rivelatore e la valutazione della lunghezza di attenuazione della striscia. I risultati mostrano che, nonostante l'attenuazione, la configurazione scelta per il rivelatore è tale da garantire una buona raccolta di luce lungo tutta la striscia

Electrical conductivity of single Be-doped GaAs nanowires

In this thesis the measurement of the current-voltage characteristics of single nanowires in their as-grown geometry is presented. The studied sample is composed by Be-doped GaAs nanowires grown on Si substrate by molecular beam epitaxy. The measurements have been performed using the two terminal and the four terminal geometry, respectively in the laboratories of Universität Siegen and of Leibniz Universität Hannover. For applications of nanowires in optoelectonic applications the knowledge of electronic properties is fundamental for device optimization. The first aim of this work is the investigation of electric properties of individual nanowires onto the same substrate. The electrical characterization has been performed measuring the current-voltage characteristics of single nanowires in the 2-terminal and 4-terminal geometry. The resistance of single nanowires onto the same substrate has been calculated by fitting the obtained characteristics using thermionic emission theory. The obtained values are different from nanowire to nanowire, meaning differences in conductivity of nanowires on the same substrate. Then, the resistance profile along single nanowires has been measured in the 2-terminal geometry. This measurement shows a quasi-exponential decrease in nanowire conductivity from the bottom to the top part of individual nanowires. The experimental results are in good agreement with numerical simulations obtained using Finite Element Method calculations. The correct implantation of nanowires onto real devices also requires the knowledge of the correlation between the mechanical stress applied to single nanowires and their electric properties. The analysis of this correlation has been performed using the 2 terminal configuration, by applying different mechanical stress to the same nanowire and measuring the current-voltage characteristic at each step. The results show an increase in conductivity of the single nanowire with the increase of the applied tension

Sistemi basati su ossidi metallici per l'efficiente conversione di luce visibile

Oggigiorno la ricerca nel campo delle fonti di energia rinnovabile è fondamentale per arginare la crisi climatica e superare la riduzione della disponibilità di combustibili fossili. Grazie all'elevata intensità dell'energia solare, molte ricerche si concentrano su metodi efficienti per convertirla in altre forme di energia (ad esempio, elettrica o chimica). Uno dei metodi più promettenti per convertire l'energia solare in energia chimica è la fotocatalisi a luce visibile. L'obiettivo principale di questa tesi è lo studio di sistemi su scala nanometrica che siano candidati promettenti per la fotocatalisi di luce visibile, ed in particolare di film sottili di ossido di rame e di ossido di cerio combinati con nanoparticelle (NPs) plasmoniche. Nel secondo caso, l'ossido di cerio è stato accoppiato con le NPs dal momento che l’energia di gap dell'ossido nudo è troppo ampia per l’assorbimento della radiazione visibile; mentre lavori precedenti hanno dimostrato che la formazione di un’eterogiunzione, ottenuta accoppiando nanostrutture plasmoniche con semiconduttori, può aumentare notevolmente l'attività di fotocatalizzatori mediante trasferimento di energia plasmonica dalla nanostruttura metallica al semiconduttore. La prima parte della tesi descriverà la crescita e la caratterizzazione di questi sistemi, volti ad estrarre informazioni sulle loro proprietà ottiche, con un focus specifico sulla dinamica ultraveloce e sull'evoluzione temporale degli stati eccitati. A tale scopo, sono stati studiati sistemi composti da Ag, Au e Cu NPs circondati da CeO2 mediante analisi di assorbanza ed emissione statiche e risolte nel tempo. In primo luogo, sono stati studiati sistemi composti da Ag NP con CeO2 con spettroscopia di fotoemissione risolta in tempo e spettroscopia di assorbimento a raggi X risolta in tempo con laser a elettroni liberi. In secondo luogo, la dinamica ultraveloce degli stati eccitati indotti dall'eccitazione della luce ultravioletta e visibile è stata esplorata in sistemi composti da Au NPs combinate con ossido di cerio, volti a comprendere i meccanismi di eccitazione, utilizzando la spettroscopia di assorbimento transitorio ultraveloce. Infine, l'ultima parte della tesi è focalizzata sulle Cu NPs, anch'esse incorporate in film di CeO2, o circondate da ossidi, in particolare Cu2O, che, grazie al suo band gap nella regione del visibile, è un candidato promettente per la catalisi della luce solare. Le Cu NPs sono state studiate in termini di morfologia, proprietà ottiche e stabilità in condizioni atmosferiche ed è stata sviluppata e studiata una procedura per la crescita di NPs con core metallico e shell di Cu2O. Infine, cristalli e film di Cu2O di diverso spessore sono stati cresciuti e analizzati mediante diffrazione elettronica a bassa energia, microscopia a effetto tunnel e spettroscopia di fotoluminescenza in un ampio intervallo di temperature per ottenere informazioni sul comportamento degli eccitoni.Nowadays, the research in the field of renewable energy sources is fundamental, to stem the climate crisis and to overcome the reducing availability of fossil fuels. Thanks to the high magnitude of solar energy a lot of research is focused on efficient methods to convert it into other energy forms (e.g. electric or chemical). One of the most promising methods to convert solar into chemical energy is visible light photocatalysis. The main aim of this thesis is the investigations of systems at the nanoscale that are promising candidates for visible light photocatalysis, and in particular of thin films of cuprous oxide and of cerium oxide combined with plasmonic nanoparticles (NPs). In the latter, cerium oxide has been coupled with NPs because the band gap of the bare oxide is too wide for the absorption of visible radiation, but previous works demonstrated that the formation of heterojunctions by coupling plasmonic nanostructures with semiconductors can greatly enhance the activity of photocatalysts by plasmonic energy transfer from the metal nanostructure to the semiconductor. The first part of the thesis will describe the growth and characterization of these systems, aimed to extract information on their optical properties, with a specific focus on the ultrafast dynamics and temporal evolution of excited states. For this purpose, systems composed by Ag, Au and Cu NPs surrounded by CeO2 have been investigated by means of time-resolved and static absorbance and emission analysis. First, systems composed by Ag NPs with CeO2 have been studied with time-resolved photoemission spectroscopy and free electron laser time-resolved X-ray absorption spectroscopy. Secondly, the ultrafast dynamics of excited states induced by ultra-violet and visible light excitation has been explored in Au NPs combined with cerium oxide, aimed at understanding the excitation pathways, using femtosecond transient absorption spectroscopy. Finally, the last part of the thesis is focused on Cu NPs, also embedded in CeO2 films, or surrounded by oxides, in particular on Cu2O, that, thanks to its band gap in the visible region, is a promising candidate for solar light catalysis. Cu NPs have been investigated in terms of their morphology, optical properties, and stability in air conditions, and a procedure for growing metallic core-Cu2O shell has been developed and investigated. Finally, Cu2O crystals and films of different thickness have been grown and analyzed by means of low energy electron diffraction, scanning tunneling microscopy and photoluminescence spectroscopy in a wide temperature range to obtain information on the behavior of excitons

Morphology and Optical Properties of Gas-Phase-Synthesized Plasmonic Nanoparticles: Cu and Cu/MgO

In this paper, an investigation of the properties of Cu and Cu/MgO nanoparticles (NPs) is presented. The NPs were obtained with gas-phase synthesis, and the MgO shells or matrices were formed via the co-deposition method on inert substrates. SEM and AFM were used to investigate the NP morphology on Si/SiOx, quartz, and HOPG. The Cu NPs revealed flattening of their shape, and when they were deposited on HOPG, diffusion and formation of small chains were observed. The embedding of Cu NPs in MgO was confirmed by TEM and EDX maps. XPS showed that Cu was in its metallic state, regardless of the presence of the surrounding MgO. UV–Vis revealed the presence of an intense localized surface plasmon resonance (LSPR) for Cu/MgO and for “bare” NPs. These results confirmed the role of MgO as a protective transparent medium for Cu, and the wavelength position of the LSPR in the Cu/MgO system was consistent with calculations. The wavelength position of the LSPR observed for “bare” and post-oxidized Cu NPs was probably affected by the formation of copper oxide shells after exposure to air. This study paves the way for the use of Cu/MgO NPs as plasmonic nanomaterials in applications such as photovoltaics and sensor technology

A Derivative of the Thiopeptide GE2270A Highly Selective against Propionibacterium acnes

A chemical derivative of the thiopeptide GE2270A, designated NAI003, was found to possess a substantially reduced antibacterial spectrum in comparison to the parent compound, being active against just a few Gram-positive bacteria. In particular, NAI003 retained low MICs against all tested isolates of Propionibacterium acnes and, to a lesser extent, against Enterococcus faecalis. Furthermore, NAI003 showed a time- and dose-dependent killing of both a clindamycin-resistant and a clindamycinsensitive P. acnes isolate. Gel shift experiments indicated that, like the parent compound, NAI003 retained the ability to bind to elongation factors Tu (EF-Tus) derived from Escherichia coli, E. faecalis, or P. acnes, albeit with reduced efficiency. In contrast, EF-Tus derived from the NAI003-insensitive Staphylococcus aureus or Streptococcus pyogenes did not bind this compound. These results were confirmed by in vitro studies using a hybrid translation system, which indicated that NAI003 can inhibit most efficiently protein synthesis driven by the P. acnes EF-Tu. P. acnes mutants resistant to NAI003 were isolated by direct plating. With one exception, all analyzed strains carried mutations in the tuf gene, encoding EF-Tu. Because of its selective effect on P. acnes in comparison to resident skin flora, NAI003 represents a promising candidate for the topical treatment of acne, which has already completed a phase 1 clinical study

Dynamics of charge transfer from plasmonic nanoparticles to cerium oxide

The combination of semiconducting oxide-based materials with plasmonic nanoparticles (NPs) aims to efficiently convert solar light into chemical or electric energy, exploiting the excitation of localized surface plasmon resonance (LSPR) in the NPs that leads to a significant energy/charge transfer to the oxide. By performing UV-Visible spectrophotometry measurements on systems composed of Cu/Au NPs embedded in a matrix of CeO2, we observed a wide absorption band in the visible range, ascribed to the LSPR excitation in the NPs. Femtosecond transient absorption spectroscopy at different pump energies across the LSPR band of the NPs unveiled a persistent charge transfer from the NPs to CeO2. Efficiency up to 35% for systems with Au NPs has been estimated

Lifetime of Photogenerated Positive Charges in Hybrid Cerium Oxide-Based Materials from Space and Mirror Charge Effects in Time-Resolved Photoemission Spectroscopy

Space and mirror charge effects in time-resolved photoemission spectroscopy can be modeled to obtain relevant information on the recombination dynamics of charge carriers. We successfully extracted from these phenomena the reneutralization characteristic time of positive charges generated by photoexcitation in CeO2-based films. For the above-band-gap excitation, a large fraction of positive carriers with a lifetime that exceeds 100 ps are generated. Otherwise, the sub-band-gap excitation induces the formation of a significantly smaller fraction of charges with lifetimes of tens of picoseconds, ascribed to the excitation of defect sites or to multiphoton absorption. When the oxide is combined with Ag nanoparticles, the sub-band-gap excitation of localized surface plasmon resonances leads to reneutralization times longer than 300 ps. This was interpreted by considering the electronic unbalance at the surface of the nanoparticles generated by the injection of electrons, via localized surface plasmon resonance (LSPR) decay, into CeO2. This study represents an example of how to exploit the space charge effect in gaining access to the surface carrier dynamics in CeO2 within the picosecond range of time, which is fundamental to describe the photocatalytic processes

Modified Gravity and Cosmology: An Update by the CANTATA Network

General Relativity and the $\Lambda$CDM framework are currently the standard lore and constitute the concordance paradigm. Nevertheless, long-standing open theoretical issues, as well as possible new observational ones arising from the explosive development of cosmology the last two decades, offer the motivation and lead a large amount of research to be devoted in constructing various extensions and modifications. All extended theories and scenarios are first examined under the light of theoretical consistency, and then are applied to various geometrical backgrounds, such as the cosmological and the spherical symmetric ones. Their predictions at both the background and perturbation levels, and concerning cosmology at early, intermediate and late times, are then confronted with the huge amount of observational data that astrophysics and cosmology are able to offer recently. Theories, scenarios and models that successfully and efficiently pass the above steps are classified as viable and are candidates for the description of Nature. We list the recent developments in the fields of gravity and cosmology, presenting the state of the art, high-lighting the open problems, and outlining the directions of future research. Its realization is performed in the framework of the COST European Action "Cosmology and Astrophysics Network for Theoretical Advances and Training Actions"

Deep-Sea Bioluminescence Blooms after Dense Water Formation at the Ocean Surface

The deep ocean is the largest and least known ecosystem on Earth. It hosts numerous pelagic organisms, most of which are able to emit light. Here we present a unique data set consisting of a 2.5-year long record of light emission by deep-sea pelagic organisms, measured from December 2007 to June 2010 at the ANTARES underwater neutrino telescope in the deep NW Mediterranean Sea, jointly with synchronous hydrological records. This is the longest continuous time-series of deep-sea bioluminescence ever recorded. Our record reveals several weeks long, seasonal bioluminescence blooms with light intensity up to two orders of magnitude higher than background values, which correlate to changes in the properties of deep waters. Such changes are triggered by the winter cooling and evaporation experienced by the upper ocean layer in the Gulf of Lion that leads to the formation and subsequent sinking of dense water through a process known as “open-sea convection”. It episodically renews the deep water of the study area and conveys fresh organic matter that fuels the deep ecosystems. Luminous bacteria most likely are the main contributors to the observed deep-sea bioluminescence blooms. Our observations demonstrate a consistent and rapid connection between deep open-sea convection and bathypelagic biological activity, as expressed by bioluminescence. In a setting where dense water formation events are likely to decline under global warming scenarios enhancing ocean stratification, in situ observatories become essential as environmental sentinels for the monitoring and understanding of deep-sea ecosystem shifts