CuS2‐Passivated Au-Core, Au3Cu-Shell nanoparticles analyzed by Atomistic-Resolution Cs-Corrected STEM

Au-core, Au3Cu-alloyed shell nanoparticles passivated with CuS2 were fabricated by the polyol method, and characterized by Cs-corrected scanning transmission electron microscopy. The analysis of the high-resolution micrographs reveals that these nanoparticles have decahedral structure with shell periodicity, and that each of the particles is composed by Au core and Au3Cu alloyed shell surrounded by CuS2 surface layer. X-ray diffraction measurements and results from numerical simulations confirm these findings. From the atomic resolution micrographs, we identified edge dislocations at the twin boundaries of the particles, as well as evidence of the diffusion of Cu atoms into the Au region, and the reordering of the lattice on the surface, close to the vertices of the particle. These defects will impact the atomic and electronic structures, thereby changing the physical and chemical properties of the nanoparticles. On the other hand, we show for the first time the formation of an ordered superlattice of Au3Cu and a self-capping layer made using one of the alloy metals. This has significant consequences on the physical mechanism that form multicomponent nanoparticles

Trimetallic nanostructures: the case of AgPd–Pt multiply twinned nanoparticles

We report the synthesis, structural characterization, and atomistic simulations of AgPd–Pt trimetallic (TM) nanoparticles. Two types of structure were synthesized using a relatively facile chemical method: multiply twinned core–shell, and hollow particles. The nanoparticles were small in size, with an average diameter of 11 nm and a narrow distribution, and their characterization by aberration corrected scanning transmission electron microscopy allowed us to probe the structure of the particles at an atomistic level. In some nanoparticles, the formation of a hollow structure was also observed, that facilitates the alloying of Ag and Pt in the shell region and the segregation of Ag atoms on the surface, affecting the catalytic activity and stability. We also investigated the growth mechanism of the nanoparticles using grand canonical Monte Carlo simulations, and we have found that Pt regions grow at overpotentials on the AgPd nanoalloys, forming 3D islands at the early stages of the deposition process. We found very good agreement between the simulated structures and those observed experimentally

Experimental evidence of icosahedral and decahedral packing in one-dimensional nanostructures

Abstract: The packing of spheres is a subject that has drawn the attention of mathematicians and philosophers for centuries and that currently attracts the interest of the scientific community in several fields. At the nanoscale, the packing of atoms affects the chemical and structural properties of the material and, hence, its potential applications. This report describes the experimental formation of 5-fold nanostructures by the packing of interpenetrated icosahedral and decahedral units. These nanowires, formed by the reaction of a mixture of metal salts (Au and Ag) in the presence of oleylamine, are obtained when the chemical composition is specifically Ag/Au = 3:1. The experimental images of the icosahedral nanowires have a high likelihood with simulated electron micrographs of structures formed by two or three Boerdijk Coxeter Bernal helices roped on a single structure, whereas for the decahedral wires, simulations using a model of adjacent decahedra match the experimental structures. To our knowledge, this is the first report of the synthesis of nanowires formed by the packing of structures with 5-fold symmetry. These icosahedral nanowire structures are similar to those of quasicrystals, which can only be formed if at least two atomic species are present and in which icosahedral and decahedral packing has been found for bulk crystals

Pure and doped boron nitride nanotubes

"More than ten years ago, it was suggested theoretically that boron nitride (BN) nanotubes could be produced. Soon after, various reports on their synthesis appeared and a new area of nanotube science was born. This review aims to cover the latest advances related to the synthesis of BN nanotubes. We show that these tubes can now be produced in larger amounts and, in particular, that the chemistry of BN tubes appears to be very important to the production of reinforced composites with insulating characteristics. From the theoretical standpoint, we also show that (BN)-C heteronanotubes could have important implications for nanoelectronics. We believe that BN nanotubes (pure and doped) could be used in the fabrication of novel devices in which pure carbon nanotubes do not perform very efficiently.

Extraction of the Muon Signals Recorded with the Surface Detector of the Pierre Auger Observatory Using Recurrent Neural Networks

The Pierre Auger Observatory, at present the largest cosmic-ray observatory ever built, is instrumented with a ground array of 1600 water-Cherenkov detectors, known as the Surface Detector (SD). The SD samples the secondary particle content (mostly photons, electrons, positrons and muons) of extensive air showers initiated by cosmic rays with energies ranging from $10^{17}~$eV up to more than $10^{20}~$eV. Measuring the independent contribution of the muon component to the total registered signal is crucial to enhance the capability of the Observatory to estimate the mass of the cosmic rays on an event-by-event basis. However, with the current design of the SD, it is difficult to straightforwardly separate the contributions of muons to the SD time traces from those of photons, electrons and positrons. In this paper, we present a method aimed at extracting the muon component of the time traces registered with each individual detector of the SD using Recurrent Neural Networks. We derive the performances of the method by training the neural network on simulations, in which the muon and the electromagnetic components of the traces are known. We conclude this work showing the performance of this method on experimental data of the Pierre Auger Observatory. We find that our predictions agree with the parameterizations obtained by the AGASA collaboration to describe the lateral distributions of the electromagnetic and muonic components of extensive air showers.Comment: 23 pages, 15 figures. Version accepted for publication in JINS

Design and implementation of the AMIGA embedded system for data acquisition

The Auger Muon Infill Ground Array (AMIGA) is part of the AugerPrime upgrade of the Pierre Auger Observatory. It consists of particle counters buried 2.3 m underground next to the water-Cherenkov stations that form the 23.5 km$^2$ large infilled array. The reduced distance between detectors in this denser area allows the lowering of the energy threshold for primary cosmic ray reconstruction down to about 10$^{17}$ eV. At the depth of 2.3 m the electromagnetic component of cosmic ray showers is almost entirely absorbed so that the buried scintillators provide an independent and direct measurement of the air showers muon content. This work describes the design and implementation of the AMIGA embedded system, which provides centralized control, data acquisition and environment monitoring to its detectors. The presented system was firstly tested in the engineering array phase ended in 2017, and lately selected as the final design to be installed in all new detectors of the production phase. The system was proven to be robust and reliable and has worked in a stable manner since its first deployment.Comment: Accepted for publication at JINST. Published version, 34 pages, 15 figures, 4 table

Deep-Learning based Reconstruction of the Shower Maximum XmaxX_{\mathrm{max}} using the Water-Cherenkov Detectors of the Pierre Auger Observatory

The atmospheric depth of the air shower maximum $X_{\mathrm{max}}$ is an observable commonly used for the determination of the nuclear mass composition of ultra-high energy cosmic rays. Direct measurements of $X_{\mathrm{max}}$ are performed using observations of the longitudinal shower development with fluorescence telescopes. At the same time, several methods have been proposed for an indirect estimation of $X_{\mathrm{max}}$ from the characteristics of the shower particles registered with surface detector arrays. In this paper, we present a deep neural network (DNN) for the estimation of $X_{\mathrm{max}}$. The reconstruction relies on the signals induced by shower particles in the ground based water-Cherenkov detectors of the Pierre Auger Observatory. The network architecture features recurrent long short-term memory layers to process the temporal structure of signals and hexagonal convolutions to exploit the symmetry of the surface detector array. We evaluate the performance of the network using air showers simulated with three different hadronic interaction models. Thereafter, we account for long-term detector effects and calibrate the reconstructed $X_{\mathrm{max}}$ using fluorescence measurements. Finally, we show that the event-by-event resolution in the reconstruction of the shower maximum improves with increasing shower energy and reaches less than $25~\mathrm{g/cm^{2}}$ at energies above $2\times 10^{19}~\mathrm{eV}$.Comment: Published version, 29 pages, 12 figure

A Catalog of the Highest-energy Cosmic Rays Recorded during Phase I of Operation of the Pierre Auger Observatory

A catalog containing details of the highest-energy cosmic rays recorded through the detection of extensive air-showers at the Pierre Auger Observatory is presented with the aim of opening the data to detailed examination. Descriptions of the 100 showers created by the highest-energy particles recorded between 1 January 2004 and 31 December 2020 are given for cosmic rays that have energies in the range 78 EeV to 166 EeV. Details are also given of a further nine very-energetic events that have been used in the calibration procedure adopted to determine the energy of each primary. A sky plot of the arrival directions of the most energetic particles is shown. No interpretations of the data are offered

Calibration of the underground muon detector of the Pierre Auger Observatory

To obtain direct measurements of the muon content of extensive air showers with energy above 10$^{16.5}$ eV, the Pierre Auger Observatory is currently being equipped with an underground muon detector (UMD), consisting of 219 10 m$^{2}$-modules, each segmented into 64 scintillators coupled to silicon photomultipliers (SiPMs). Direct access to the shower muon content allows for the study of both of the composition of primary cosmic rays and of high-energy hadronic interactions in the forward direction. As the muon density can vary between tens of muons per m2 close to the intersection of the shower axis with the ground to much less than one per m2 when far away, the necessary broad dynamic range is achieved by the simultaneous implementation of two acquisition modes in the read-out electronics: the binary mode, tuned to count single muons, and the ADC mode, suited to measure a high number of them. In this work, we present the end-to-end calibration of the muon detector modules: first, the SiPMs are calibrated by means of the binary channel, and then, the ADC channel is calibrated using atmospheric muons, detected in parallel to the shower data acquisition. The laboratory and field measurements performed to develop the implementation of the full calibration chain of both binary and ADC channels are presented and discussed. The calibration procedure is reliable to work with the high amount of channels in the UMD, which will be operated continuously, in changing environmental conditions, for several years

Searches for Ultra-High-Energy Photons at the Pierre Auger Observatory

The Pierre Auger Observatory, being the largest air-shower experiment in the world, offers an unprecedented exposure to neutral particles at the highest energies. Since the start of data taking more than 18 years ago, various searches for ultra-high-energy (UHE, $E\gtrsim10^{17}\,\text{eV}$) photons have been performed: either for a diffuse flux of UHE photons, for point sources of UHE photons or for UHE photons associated with transient events like gravitational wave events. In the present paper, we summarize these searches and review the current results obtained using the wealth of data collected by the Pierre Auger Observatory.Comment: Review article accepted for publication in Universe (special issue on ultra-high energy photons