In situ measurements of plasma properties during gas-condensation of Cu nanoparticles

Since the mean, standard deviation, and modality of nanoparticle size distributions can vary greatly between similar input conditions (e.g., power and gas flow rate), plasma diagnostics were carried out in situ using a double-sided, planar Langmuir probe to determine the effect the plasma has on the heating of clusters and their final size distributions. The formation of Cu nanoparticles was analyzed using cluster-plasma physics, which relates the processes of condensation and evaporation to internal plasma properties (e.g., electron temperature and density). Monitoring these plasma properties while depositing Cu nanoparticles with different size distributions revealed a negative correlation between average particle size and electron temperature. Furthermore, the modality of the size distributions also correlated with the modality of the electron energy distributions. It was found that the maximum cluster temperature reached during plasma heating and the material’s evaporation point regulates the growth process inside the plasma. In the case of Cu, size distributions with average sizes of 8.2, 17.3, and 24.9 nm in diameter were monitored with the Langmuir probe, and from the measurements made, the cluster temperatures for each deposition were calculated to be 1028, 1009, and 863 K. These values are then compared with the onset evaporation temperature of particles of this size, which was estimated to be 1059, 1068, and 1071 K. Thus, when the cluster temperature is too close to the evaporation temperature, less particle growth occurs, resulting in the formation of smaller particles

Formation of aggregated nanoparticle spheres through femtosecond laser surface processing

A detailed structural and chemical analysis of a class of self-organized surface structures, termed aggregated nanoparticle spheres (AN-spheres), created using femtosecond laser surface processing (FLSP) on silicon, silicon carbide, and aluminum is reported in this paper. AN-spheres are spherical microstructures that are 20–100 μm in diameter and are composed entirely of nanoparticles produced during femtosecond laser ablation of material. AN-spheres have an onion-like layered morphology resulting from the build-up of nanoparticle layers over multiple passes of the laser beam. The material properties and chemical composition of the AN-spheres are presented in this paper based on scanning electron microscopy (SEM), focused ion beam (FIB) milling, transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDX) analysis. There is a distinct difference in the density of nanoparticles between concentric rings of the onion-like morphology of the AN-sphere. Layers of high-density form when the laser sinters nanoparticles together and low-density layers form when nanoparticles redeposit while the laser ablates areas surrounding the AN-sphere. The dynamic nature of femtosecond laser ablation creates a variety of nanoparticles that make-up the AN-spheres including Si/C core-shell, nanoparticles that directly fragmented from the base material, nanoparticles with carbon shells that retarded oxidation, and amorphous, fully oxidized nanoparticles

Giant Enhancement of Magnetic Anisotropy in Ultrathin Manganite Films via Nanoscale 1D Periodic Depth Modulation

The relatively low magnetocrystalline anisotropy (MCA) in strongly correlated manganites (La,Sr)MnO3 has been a major hurdle for implementing them in spintronic applications. Here we report an unusual, giant enhancement of in-plane MCA in 6 nm La0.67Sr0.33MnO3 (LSMO) films grown on (001) SrTiO3 substrates when the top 2 nm is patterned into periodic stripes of 100 or 200 nm width. Planar Hall effect measurements reveal an emergent uniaxial anisotropy superimposed on one of the original biaxial easy axes for unpatterned LSMO along (110) directions, with a 50-fold enhanced anisotropy energy density of 5.6 × 106 erg/cm3 within the nanostripes, comparable to the value for cobalt. The magnitude and direction of the uniaxial anisotropy exclude shape anisotropy and the step edge effect as its origin. High resolution transmission electron microscopy studies reveal a nonequilibrium strain distribution and drastic suppression in the c-axis lattice constant within the nanostructures, which is the driving mechanism for the enhanced uniaxial MCA, as suggested by first-principles density functional calculations

Size-controlled, magnetic, and core-shell nanoparticles synthesized by inert-gas condensation

Interest in nanoparticles (2 to 100 nm in diameter) and clusters of atoms (0.5 to 2 nm in diameter) has heightened over the past two and a half decades on both fundamental and functional levels. Nanoparticles and clusters of atoms are an exciting branch of materials science because they do not behave like normal bulk matter, nor do they act like molecules. They can have shockingly different physical, chemical, optical, or magnetic properties from the same material at a larger scale. In the case of nanoparticles, the surface-to-volume ratio can change fundamental properties like melting temperature, binding energy, or electron affinity. The definitions of markers used to distinguish between metallic, semiconducting, and insulating bulk condensed matter, such as the band gap and polarizability, can even be blurred or confused on the nanoscale. Similarly, clusters of atoms can form in structures that are only stable at finite sizes, and do not translate to bulk condensed matter. Thermodynamics of finite systems changes dramatically in nanovolumes such as wires, rods, cubes, and spheres, which can lead to complex core-shell and onion-like nanostructures. Consequently, these changes in properties and structure have led to many new possibilities in the field of materials engineering. Inert-gas condensation (IGC) is a well-established method of producing nanoparticles that condense from the gas phase. Its first use dates back to the early 1990s, and it has been used to fabricate nanoparticles both commercially and in research and development for applications in magnetism, biomedicine, and catalysts. In this dissertation, IGC was used to produce a wide variety of nanoparticles. First, control over the size distributions of Cu nanoparticles and how it relates to the plasma properties inside the nucleation chamber was investigated. Next, the formation of phase pure WFe2 nanoparticles revealed that this Laves phase is ferromagnetic instead of non-magnetic. Finally, core-shell nanoparticles were produced using three thermodynamically different systems, which showed that IGC could be used to produce a wide variety of core-shell particles. These three projects are presented in the context of size-dependent structural and magnetic properties

In situ measurements of plasma properties during gas-condensation of Cu nanoparticles

Since the mean, standard deviation, and modality of nanoparticle size distributions can vary greatly between similar input conditions (e.g., power and gas flow rate), plasma diagnostics were carried out in situ using a double-sided, planar Langmuir probe to determine the effect the plasma has on the heating of clusters and their final size distributions. The formation of Cu nanoparticles was analyzed using cluster-plasma physics, which relates the processes of condensation and evaporation to internal plasma properties (e.g., electron temperature and density). Monitoring these plasma properties while depositing Cu nanoparticles with different size distributions revealed a negative correlation between average particle size and electron temperature. Furthermore, the modality of the size distributions also correlated with the modality of the electron energy distributions. It was found that the maximum cluster temperature reached during plasma heating and the material’s evaporation point regulates the growth process inside the plasma. In the case of Cu, size distributions with average sizes of 8.2, 17.3, and 24.9 nm in diameter were monitored with the Langmuir probe, and from the measurements made, the cluster temperatures for each deposition were calculated to be 1028, 1009, and 863 K. These values are then compared with the onset evaporation temperature of particles of this size, which was estimated to be 1059, 1068, and 1071 K. Thus, when the cluster temperature is too close to the evaporation temperature, less particle growth occurs, resulting in the formation of smaller particles

Ferromagnetism in Laves-phase WFe\u3csub\u3e2\u3c/sub\u3e nanoparticles

While rare-earth based Laves phases are known to exhibit large magnetostriction, the magnetic properties of some binary Laves phases containing transition metals alone are not well known. This is because many of these compounds contain refractory elements that complicate melt processing due to high melting temperatures and extensive phase separation. Here, phase-pure WFe2 nanoclusters, with the hexagonal C14 Laves structure, were deposited via inert gas condensation, allowing for the first known measurement of ferromagnetism in this phase, with MS of 26.4 emu/g (346 emu/cm3) and a KU of 286 kerg/cm3, at 10 K, and a TC of 550 K

Effect of strain on ferroelectric field effect in strongly correlated oxide Sm\u3csub\u3e0.5\u3c/sub\u3eNd\u3csub\u3e0.5\u3c/sub\u3eNiO\u3csub\u3e3\u3c/sub\u3e

We report the effect of epitaxial strain on the magnitude and retention of the ferroelectric field effect in high quality PbZr0.3Ti0.7O3 (PZT)/3.8–4.3 nm Sm0.5Nd0.5NiO3 (SNNO) heterostructures grown on (001) LaAlO3 (LAO) and SrTiO3 (STO) substrates. For SNNO on LAO, which exhibits a first-order metal-insulator transition (MIT), switching the polarization of PZT induces a 10K shift in the transition temperature TMI, with a maximum resistance change between the on and off states of ΔR = Ron ~75%. In sharp contrast, only up to 5% resistance change has been induced in SNNO on STO, where the MIT is second-order, with the modulation of TMI negligibly small. We also observe thermally activated retention of the off state resistance Roff in both systems, with the activation energy of 22 meV (28 meV) for devices on LAO (STO). The time dynamics and thermal response of the field effect instability points to phonon-assisted interfacial trapping of charged mobile defects, which are attributed to strain induced oxygen vacancies. At room temperature, Roff stabilizes at ~55% and ~19% of the initial switching levels for SNNO on LAO and STO, respectively, reflecting the significantly different oxygen vacancy densities in these two systems. Our results reveal the critical role of strain in engineering and modeling the complex oxide composite structures for nanoelectronic and spintronic applications

Formation of aggregated nanoparticle spheres through femtosecond laser surface processing

A detailed structural and chemical analysis of a class of self-organized surface structures, termed aggregated nanoparticle spheres (AN-spheres), created using femtosecond laser surface processing (FLSP) on silicon, silicon carbide, and aluminum is reported in this paper. AN-spheres are spherical microstructures that are 20–100 μm in diameter and are composed entirely of nanoparticles produced during femtosecond laser ablation of material. AN-spheres have an onion-like layered morphology resulting from the build-up of nanoparticle layers over multiple passes of the laser beam. The material properties and chemical composition of the AN-spheres are presented in this paper based on scanning electron microscopy (SEM), focused ion beam (FIB) milling, transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDX) analysis. There is a distinct difference in the density of nanoparticles between concentric rings of the onion-like morphology of the AN-sphere. Layers of high-density form when the laser sinters nanoparticles together and low-density layers form when nanoparticles redeposit while the laser ablates areas surrounding the AN-sphere. The dynamic nature of femtosecond laser ablation creates a variety of nanoparticles that make-up the AN-spheres including Si/C core-shell, nanoparticles that directly fragmented from the base material, nanoparticles with carbon shells that retarded oxidation, and amorphous, fully oxidized nanoparticles

The May Revolution and the Intellectuals of the "National Thinking"

El presente trabajo ahonda en las diversas interpretaciones que sobre la Revolución de Mayo acuñaron una serie de intelectuales de lo que podríamos denominar a grandes rasgos el "Pensamiento Nacional". Rodolfo Puiggrós, Juan José Hernández Arregui, Jorge Abelardo Ramos y Norberto Galasso poseen en común ciertos lineamientos ideológicos articulados alrededor del nacionalismo popular, el latinoamericanismo antiimperialista y el ideario socialista. Se indaga en aquellos aspectos que estos autores comparten pero fundamentalmente, se pone el acento en las principales divergencias en sus interpretaciones a través de un estudio comparativo de sus trabajos. El conjunto de estos intelectuales discute abiertamente con la interpretación que de Mayo hace la llamada "Historia Oficial" cuestionando sus principales pilares, pero a pesar de que todos ellos parten desde el materialismo histórico y comparten una identificación política ("el socialismo nacional") las conclusiones que alcanzarán darán cuenta no sólo de puntos de vista divergentes sino también de trabajos realizados al calor de distintos momentos históricos, encontrándose indisociablemente ligada a las discusiones políticas de su época y claramente atadas al proyecto de país que ellos aspiraban contribuir a construir. Estas articulaciones son también aspectos a los cuales intentaremos echar luz en este trabajo. Hoy, en el marco de los Bicentenarios, donde se generan numerosos foros y actividades que buscan repensar nuestros 200 años de historia, no podemos despreciar estas enriquecedoras discusiones, sino que por el contrario, recuperarlas para ponerlas en debate con las nuevas investigaciones es una tarea necesaria.We look into the various interpretations of the May Revolution that several intellectuals put forward in what we might describe roughly as the "National Thinking". Rodolfo Puiggrós, Juan José Hernández Arregui, Jorge Abelardo Ramos y Norberto Galasso have certain ideological perspectives built around popular nationalism, anti-imperialist Latinamericanism and socialist ideology. We discuss the common aspects of these authors, but the main emphasis is on the divergences between their interpretations, via a comparative study of their work. These intellectuals openly argue against the interpretation of May enshrined in the "Official History", questioning its main pillars. But despite the fact that they all begin from historical materialism and share a political identity ("social nationalism"), the conclusions they reach point not only to divergent perspectives but also to work done in the heat of different historical moments, and are inseparably tied to the political discourse of their time and clearly associated with the national project that they aspire to contributing to build. We shall also attempt to throw some light on these aspects. Today, within the framework of the Bicentenaries and the numerous forums and activities that seek to take a new look at our 200 years of history, we should not neglect these enriching discussions but, rather, bring them into the debate together with new research in an essential task.Fil: Lafit, Facundo. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina