Atomistic study of CoCrCuFeNi high entropy alloy nanoparticles: Role of chemical complexity.

High entropy alloy nanoparticles are envisaged as one of the most interesting materials compared to monoatomic materials due to their modulated properties in terms of their convenient surface-to-volume ratio. However, studies are still missing to unveil how composition or nanoparticle size can influence nanoparticle morphology. Based on molecular dynamics simulations, we perform a structural characterization as a function of nanoparticle size and the chemical composition of high entropy alloy nanoparticles subject to multiple annealing cycles. After the multiple thermal loads, we observe a substantial migration of copper atoms towards the np surface, consistent with the experimental results of Cu-based high entropy alloys. The resulting high entropy alloy nanoparticle behaves as a core-shell nanostructure with a rich fcc phase on the surface (50% of Cu) and 5% fcc phase in the nanoparticle core. Inspecting the nanoparticle surface, it is observed that high entropy alloy nanoparticles have a lack of surface facets, leading to a more spherical shape, quite different from mono-metallic nanoparticles with a high number of facets. Performing an average atoms simulation, it showed that nanoparticles are prone to form 111 surface facets independent of the nanoparticle size, suggesting that for high entropy alloy nanoparticles, the chemical complexity avoids the formation of surface facets. The latter can be explained in terms of the lattice distortion inducing tensile/compressive stress that drives the surface reconstruction. All in all our results match extremely well with experimental evidence of FeNiCrCoCu nanocrystalline materials, explaining the Cu segregation in terms of surface energy and mixing enthalpy criteria. We believe that our results provide a detailed characterization of high entropy nanoparticles focusing on how chemical complexity induces morphological changes compared to mono-crystalline nanoparticles. Besides, our findings are valuable for experimental works aimed at designing the shape and composition of multicomponent nanoparticles.

Nanoporous Amorphous Carbon with Exceptional Ultra-High Strength.

Nanoporous materials show a promising combination of mechanical properties in terms of their relative density; while there are numerous studies based on metallic nanoporous materials, here we focus on amorphous carbon with a bicontinuous nanoporous structure as an alternative to control the mechanical properties for the function of filament composition.Using atomistic simulations, we study the mechanical response of nanoporous amorphous carbon with 50% porosity, with sp3 content ranging from 10% to 50%. Our results show an unusually high strength between 10 and 20 GPa as a function of the %sp3 content. We present an analytical analysis derived from the Gibson-Ashby model for porous solids, and from the He and Thorpe theory for covalent solids to describe Young's modulus and yield strength scaling laws extremely well, revealing also that the high strength is mainly due to the presence of sp3 bonding. Alternatively, we also find two distinct fracture modes: for low %sp3 samples, we observe a ductile-type behavior, while high %sp3 leads to brittle-type behavior due to high high shear strain clusters driving the carbon bond breaking that finally promotes the filament fracture. All in all, nanoporous amorphous carbon with bicontinuous structure is presented as a lightweight material with a tunable elasto-plastic response in terms of porosity and sp3 bonding, resulting in a material with a broad range of possible combinations of mechanical properties.

Machine learning modeling for the prediction of plastic properties in metallic glasses.

Metallic glasses are one of the most interesting mechanical materials studied in the last years, but as amorphous solids, they differ strongly from their crystalline counterparts. This matter can be addressed with the development and application of predictive techniques capable to describe the plastic regime. Here, machine learning models were employed for the prediction of plastic properties in CuZr metallic glasses. To this aim, 100 different samples were subjected to tensile tests by means of molecular dynamics simulations. A total of 17 materials properties were calculated and explored using statistical analysis. Strong correlations were found for stoichiometry, temperature, structural, and elastic properties with plastic properties. Three regression models were employed for the prediction of six plastic properties. Linear and Ridge regressions delivered the better prediction capability, with coefficients of determination above [Formula: see text]80% for three plastic properties, whereas Lasso regression rendered lower performance, with coefficients of determination above [Formula: see text]60% for two plastic properties. Overall, our work shows that molecular dynamics simulations together with machine learning models can provide a framework for the prediction of plastic behavior of complex materials.

Research on metallic glasses at the atomic scale: a systematic review.

Metallic glasses (MGs) have been long investigated in material science to understand the origin of their remarkable properties. With the help of computational simulations, researchers have delved into structure-property relationships, leading to a large number of reports. To quantify the available literature, we employed systematic review and bibliometric analysis on studies related to MGs and classical molecular dynamics simulations from 2000 to 2021. It was found that the total number of articles has increased remarkably, with China and the USA producing more than half of the reports. However, high-impact articles were mainly conducted in the latter. Collaboration networks revealed that top contributor authors are strongly connected with other researchers, which emphasizes the relevance of scientific cooperation. In regard to the evolution of research topics, according to article keywords, plastic behavior has been a recurrent subject since the early 2000s. Nevertheless, the traditional approach of studying monolithic MGs at the short-range order evolved to complex composites with characterizations at the medium-range order, including topics such as nanoglasses, amorphous/crystalline nanolaminates, rejuvenation, among others. As a whole, these findings provide researchers with an overview of past and current trends of research areas, as well as some of the leading authors, productivity statistics, and collaboration networks.

Enhancing the Thermal Conductivity of Amorphous Carbon with Nanowires and Nanotubes.

The thermal conductivity of nanostructures can be obtained using atomistic classical Molecular Dynamics (MD) simulations, particularly for semiconductors where there is no significant contribution from electrons to thermal conduction. In this work, we obtain and analyze the thermal conductivity of amorphous carbon (aC) nanowires (NW) with a 2 nm radius and aC nanotubes (NT) with 0.5, 1 and 1.3 nm internal radii and a 2 nm external radius. The behavior of thermal conductivity with internal radii, temperature and density (related to different levels of sp3 hybridization), is compared with experimental results from the literature. Reasonable agreement is found between our modeling results and the experiments for aC films. In addition, in our simulations, the bulk conductivity is lower than the NW conductivity, which in turn is lower than the NT conductivity. NTs thermal conductivity can be tailored as a function of the wall thickness, which surprisingly increases when the wall thickness decreases. While the vibrational density of states (VDOS) is similar for bulk, NW and NT, the elastic modulus is sensitive to the geometrical parameters, which can explain the enhanced thermal conductivity observed for the simulated nanostructures.

Probing the Mechanical Properties of Porous Nanoshells by Nanoindentation.

In this contribution, we present a study of the mechanical properties of porous nanoshells measured with a nanoindentation technique. Porous nanoshells with hollow designs can present attractive mechanical properties, as observed in hollow nanoshells, but coupled with the unique mechanical behavior of porous materials. Porous nanoshells display mechanical properties that are dependent on shell porosity. Our results show that, under smaller porosity values, deformation is closely related to the one observed for polycrystalline and single-crystalline nanoshells involving dislocation activity. When porosity in the nanoparticle is increased, plastic deformation was mediated by grain boundary sliding instead of dislocation activity. Additionally, porosity suppresses dislocation activity and decreases nanoparticle strength, but allows for significant strain hardening under strains as high as 0.4. On the other hand, Young's modulus decreases with the increase in nanoshell porosity, in agreement with the established theories of porous materials. However, we found no quantitative agreement between conventional models applied to obtain the Young's modulus of porous materials.

Enhancing the magnetic response on polycrystalline nanoframes through mechanical deformation.

The mechanical and magnetic properties of polycrystalline nanoframes were investigated using atomistic molecular dynamics and micromagnetic simulations. The magneto-mechanical response of Fe hollow-like nanocubes was addressed by uniaxial compression carried out by nanoindentation. Our results show that the deformation of a nanoframe is dominated at lower strains by the compression of the nanostructure due to filament bending. This leads to the nanoframe twisting perpendicular to the indentation direction for larger indentation depths. Bending and twisting reduce stress concentration and, at the same time, increase coercivity. This unexpected increase of the coercivity occurs because the mechanical deformation changes the cubic shape of the nanoframe, which in turn drives the system to more stable magnetic states. A coercivity increase of almost 100 mT is found for strains close to 0.03, which are within the elastic regime of the Fe nanoframe. Coercivity then decreases at larger strains. However, in all cases, the coercivity is higher than for the undeformed nanoframe. These results can help in the design of new magnetic devices where mechanical deformation can be used as a primary tool to tailor the magnetic response on nanoscale solids.

Thermal Sensitivity on Eccentric Gold Hollow Nanoparticles: A Perspective from Atomistic Simulations.

Eccentricity is a common feature consequence of several synthesis protocols of hollow nanoshells. Despite the crescent interest in these nanoparticles, it is still unclear how an irregular layer on the nanoparticle impacts the macroscopic properties. Here, we study the thermal stability of eccentric hollow nanoparticles (hNPs) for different sizes and eccentricity values by means of classical molecular dynamics simulations. Our results reveal that eccentricity displays a significant role in the thermal stability of hNPs. We attribute this behavior to the irregular shell contour, which collapses due to the thermal-activated diffusive process from the nanoparticle shell's most thin region. The mechanism is driven at low temperature by the nucleation of stacking faults until the amorphization for larger temperature values. Besides, for some particular eccentric hNPs, the shell suffers a surface reconstruction process, transforming the eccentric hNP into a concentric hNP. We believe that our study on thermal effects in eccentric hNPs has relevance because of their outstanding applications for plasmonic and sensing.

Tension-compression behavior in gold nanoparticle arrays: a molecular dynamics study.

The mechanical properties of Au nanoparticle arrays are studied by tensile and compressive deformation, using large-scale molecular dynamics simulations which include up to 16 million atoms. Our results show that mechanical response is dominated by nanoparticle size. For compression, strength versus particle size shows similar trends in strength than full-density nanocrystals. For diameters (d) below 10 nm there is an inverse Hall-Petch (HP) regime. Beyond a maximum at 10 nm, strength decreases following a HP d -1/2 dependence. In both regimes, interparticle sliding and dislocation activity play a role. The array with 10 nm nanoparticles showed the same mechanical properties than a polycrystalline bulk with the same grain size. This enhanced strength, for a material nearly 20% lighter, is attributed to the absence of grain boundary junctions, and to the array geometry, which leads to constant flow stress by means of densification, nanoparticle rotation, and dislocation activity. For tension, there is something akin to brittle fracture for large grain sizes, with NPs debonding perpendicular to the traction direction. The Johnson-Kendall-Roberts contact theory was successfully applied to describe the superlattice porosity, predicting also the array strength within 10% of molecular dynamics values. Although this study is focused on Au nanoparticles, our findings could be helpful in future studies of similar arrays with NPs of different kinds of materials.

Understanding contagion dynamics through microscopic processes in active Brownian particles.

Together with the universally recognized SIR model, several approaches have been employed to understand the contagion dynamics of interacting particles. Here, Active Brownian particles (ABP) are introduced to model the contagion dynamics of living agents that perform a horizontal transmission of an infectious disease in space and time. By performing an ensemble average description of the ABP simulations, we statistically describe susceptible, infected, and recovered groups in terms of particle densities, activity, contagious rates, and random recovery times. Our results show that ABP reproduces the time dependence observed in traditional compartmental models such as the Susceptible-Infected-Recovery (SIR) models and allows us to explore the critical densities and the contagious radius that facilitates the virus spread. Furthermore, we derive a first-principles analytical expression for the contagion rate in terms of microscopic parameters, without considering free parameters as the classical SIR-based models. This approach offers a novel alternative to incorporate microscopic processes into analyzing SIR-based models with applications in a wide range of biological systems.

Thermal Stability of Hollow Porous Gold Nanoparticles: A Molecular Dynamics Study.

Hollow nanoparticle structures play a major role in nanotechnology and nanoscience since their surface to volume ratio is significantly larger than that of filled ones. While porous hollow nanoparticles offer a significant improvement of the available surface area, there is a lack of theoretical understanding, and scarce experimental information, on how the porosity controls or dominates the stability. Here we use classical molecular dynamics simulations to shed light on the particular characteristics and properties of gold porous hollow nanoparticles and how they differ from the nonporous ones. Adopting gold as a prototype, we show how, as the temperature increases, the porosity introduces surface stress and minor transitions that lead to various scenarios, from partial shrinkage for small filling factors to abrupt compression and the loss of spherical shape for large filling. Our work provides new insights into the stability limits of porous hollow nanoparticles, with important implications for the design and practical use of these enhanced geometries.

Ion implantation in nanodiamonds: size effect and energy dependence.

Nanoparticles are ubiquitous in nature and are increasingly important for technology. They are subject to bombardment by ionizing radiation in a diverse range of environments. In particular, nanodiamonds represent a variety of nanoparticles of significant fundamental and applied interest. Here we present a combined experimental and computational study of the behaviour of nanodiamonds under irradiation by xenon ions. Unexpectedly, we observed a pronounced size effect on the radiation resistance of the nanodiamonds: particles larger than 8 nm behave similarly to macroscopic diamond (i.e. characterized by high radiation resistance) whereas smaller particles can be completely destroyed by a single impact from an ion in a defined energy range. This latter observation is explained by extreme heating of the nanodiamonds by the penetrating ion. The obtained results are not limited to nanodiamonds, making them of interest for several fields, putting constraints on processes for the controlled modification of nanodiamonds, on the survival of dust in astrophysical environments, and on the behaviour of actinides released from nuclear waste into the environment.

Bending energy of 2D materials: graphene, MoS2 and imogolite.

The bending process of 2D materials, subject to an external force, is investigated, and applied to graphene, molybdenum disulphide (MoS2), and imogolite. For graphene we obtained 3.43 eV Å2 per atom for the bending modulus, which is in good agreement with the literature. We found that MoS2 is ∼11 times harder to bend than graphene, and has a bandgap variation of ∼1 eV as a function of curvature. Finally, we also used this strategy to study aluminosilicate nanotubes (imogolite) which, in contrast to graphene and MoS2, present an energy minimum for a finite curvature radius. Roof tile shaped imogolite precursors turn out to be stable, and thus are expected to be created during imogolite synthesis, as predicted to occur by self-assembly theory.

Oroya Fever, Verruga Peruana, and Other Bartonelloses Incidence Rates in Colombia (2009-2013).

Background Bartonella bacilliformis, the etiological agent of Carrion's disease and presumed to be transmitted by phlebotomine sandflies, is endemic to the high-altitude valleys of the South American Andes, including Colombia. Methods This observational, retrospective study in which the incidence of bartonelloses (International Classification of Diseases, 10th revision (ICD-10) codes A44.0-A44.9) in Colombia, from 2009-2013, was estimated based on data extracted from the personal health records system (Registro Individual Prestación Servicios, RIPS). Using the official population estimates of the National Statistics Department (Departamento Administrativo Nacional de Estadísticas, DANE), crude and adjusted incidence rates were estimated (cases/100,000 population). Results A total of 1,389 cases were reported (median 289/year), for a cumulative national rate of 3.02 cases/100,000 population; 91.2% were female; 66.8% were <40-year-old (3.8% <9.9-year-old). The cases were 2.9% Oroya fever (A44.0), 13.1% verruga peruana (A44.1), and the rest (85.3%) were other forms of bartonelloses (A44.8-A44.9). The highest rates of Oroya fever were reported in Bolivar (2.5 cases/1,000,000 population). For verruga peruana highest number of cases were reported in Antioquia (32; 17.8%; 5.21 cases/1,000,000 population) and the highest rate at Magdalena (11.54 cases/1,000,000 population) (Risaralda, 6.45; Caldas, 5.1). For other forms of bartonelloses, the highest rates were reported at Magdalena (48.65 cases/1,000,000 population), followed by Huila (32.8) and La Guajira (18.9). At Nariño, Putumayo, Amazonas, Cauca, and Valle del Cauca, 11.7% of the cases of the country were reported. Conclusions Lutzomyia columbiana, the potential vector of Bartonella bacilliformis in Colombia, is distributed not only in Nariño, Cauca, and Valle del Cauca but also in the Antioquia, Caldas, Huila, La Guajira, Risaralda, Cordoba, and Caribbean areas. Given this distribution, the transmission would be occurring, as seen in reported cases, in more areas than previously described by classic reports of these diseases in the country.

Relación entre comportamientos sexuales, y uso de drogas y alcohol en estudiantes de algunos colegios públicos de Manizales, Colombia 2008 / Sexual behavior, drug and alcohol use and its relationship among students in some public schools in Manizales, Colombia, 2008

Objetivo: Este estudio pretende conocer los comportamientos sexuales y la frecuencia del consumo de drogas y alcohol, y su relación en adolescentes de secundaria de cuatro instituciones públicas de la ciudad de Manizales, Caldas, Colombia de 9° a 11° grado. Materiales y Métodos: Se realizó un estudio de corte transversal. Participaron 334 estudiantes, seleccionados por muestreo probabilístico estratificado por colegio de una población de 3423 estudiantes. La recolección de datos se hizo mediante una encuesta que incluyó aspectos sociodemográficos, comportamientos sexuales, consumo de alcohol y drogas. Resultados: El inicio de relaciones sexuales en promedio para hombres fue de 13.64 años y para mujeres de 14.83 años, la mayoría de participantes de género femenino 64.4 por ciento ha tenido una pareja sexual en contraste con el género masculino 21 por ciento; también se encontró un índice de consumo de alcohol de 94.9, siendo la cerveza la más consumida en hombres; el consumo de sustancias psicoactivas fue de 45.2 por ciento siendo el popper el de mayor consumo (38.9 por ciento), estos porcentajes incluyen los bajos consumos. Se encontraron claras relaciones entre las variables de comportamiento sexual y de consumo de alcohol y drogas, en particular resalta la relación entre número de parejas sexuales y consumo de drogas. Conclusiones: Se puede concluir que el inicio de relaciones sexuales en esta población es temprano, con un amplio uso del preservativo como método de planificación (88.8 por ciento) y que el consumo de sustancias psicoactivas es frecuente. Se debe reforzar la promoción y prevención de la salud.

Lithium adsorption on graphite from density functional theory calculations.

The structural, energetic, and electronic properties of the Li/graphite system are studied through density functional theory (DFT) calculations using both the local spin density approximation (LSDA), and the gradient-corrected Perdew-Burke-Ernzerhof (PBE) approximation to the exchange-correlation energy. The calculations were performed using plane waves basis, and the electron-core interactions are described using pseudopotentials. We consider a disperse phase of the adsorbate comprising one Li atom for each 16 graphite surface cells, in a slab geometry. The close contact between the Li nucleus and the graphene plane results in a relatively large binding energy (larger than 1.1 eV). A detailed analysis of the electronic charge distribution, density difference distribution, and band structures indicates that one valence electron is entirely transferred from the atom to the surface, which gives rise to a strong interaction between the resulting lithium ion and the cloud of pi electrons in the substrate. We show that it is possible to explain the differences in the binding of Li, Na, and K adatoms on graphite considering the properties of the corresponding cation/aromatic complexes.

Femtosecond laser nanosurgery of defects in carbon nanotubes.

All self-assembled nanostructures, like carbon nanotubes, exhibit structural imperfections that affect their electronic and mechanical properties and constitute a serious problem for the development of novel electronic nanodevices. Very common defects in nanotubes are pentagon-heptagon pairs, in which the replacement of four hexagons by two pentagons and two heptagons disrupts the perfect hexagonal lattice. In this work, we demonstrate that these defects can be eliminated efficiently with the help of femtosecond laser pulses. By performing nonadiabatic molecular dynamics simulations, we show that in the laser-induced electronic nonequilibrium the pentagon-heptagon pair is transformed back into four hexagons without producing any irreversible damage to the rest of the nanotube.