Polarization Analyzed Small Angle Neutron Scattering of Ferrite Nanoparticles

Ferromagnetic nanoparticles offer a range of possible applications in nanotechnology, biomedical practices, and data storage, but important issues exist regarding their true magnetic structure. We have been investigating 9 nm diameter Fe3O4 nanoparticles and 11 nm diameter CoFe2O4 nanoparticles coated with an oleic acid shell. The nanoparticles were synthesized by solution chemistry methods and characterized by X-ray diffraction, SQUID, and vibrating sample magnetometry. Polarization Analyzed Small Angle Neutron Scattering (PASANS) was used under various temperatures and applied magnetic fields to investigate the magnetic structure of the particles. PASANS has revealed the iron oxide particles have a canted magnetic shell in high field that disappears in lower field, while the cobalt iron oxide particles are fully canted. We have developed an energy-based model to explain the origins of this canting and shell formation that agrees with the experimental results

Polarization Analyzed Small Angle Neutron Scattering of Ferrite Nanoparticles

Ferromagnetic nanoparticles offer a range of possible applications in nanotechnology, biomedical practices, and data storage, but important issues exist regarding their true magnetic structure. We have been investigating 9 nm diameter Fe3O4 nanoparticles and 11 nm diameter CoFe2O4 nanoparticles coated with an oleic acid shell. The nanoparticles were synthesized by solution chemistry methods and characterized by X-ray diffraction, SQUID, and vibrating sample magnetometry. Polarization Analyzed Small Angle Neutron Scattering (PASANS) was used under various temperatures and applied magnetic fields to investigate the magnetic structure of the particles. PASANS has revealed the iron oxide particles have a canted magnetic shell in high field that disappears in lower field, while the cobalt iron oxide particles are fully canted. We have developed an energy-based model to explain the origins of this canting and shell formation that agrees with the experimental results

Particle Moment Canting in CoFe2O4 Nanoparticles

Polarization-analyzed small-angle neutron scattering methods are used to determine the spin morphology in high crystalline anisotropy, 11 nm diameter CoFe2O4 nanoparticle assemblies with randomly oriented easy axes. In moderate to high magnetic fields, the nanoparticles adopt a uniformly canted structure, rather than forming domains, shells, or other arrangements. The observed canting angles agree quantitatively with those predicted from an energy model dominated by Zeeman and anisotropy competition, with implications for the technological use of such nanoparticles

Genetic effects on gene expression across human tissues

Characterization of the molecular function of the human genome and its variation across individuals is essential for identifying the cellular mechanisms that underlie human genetic traits and diseases. The Genotype-Tissue Expression (GTEx) project aims to characterize variation in gene expression levels across individuals and diverse tissues of the human body, many of which are not easily accessible. Here we describe genetic effects on gene expression levels across 44 human tissues. We find that local genetic variation affects gene expression levels for the majority of genes, and we further identify inter-chromosomal genetic effects for 93 genes and 112 loci. On the basis of the identified genetic effects, we characterize patterns of tissue specificity, compare local and distal effects, and evaluate the functional properties of the genetic effects. We also demonstrate that multi-tissue, multi-individual data can be used to identify genes and pathways affected by human disease-associated variation, enabling a mechanistic interpretation of gene regulation and the genetic basis of diseas

Genetic effects on gene expression across human tissues

Characterization of the molecular function of the human genome and its variation across individuals is essential for identifying the cellular mechanisms that underlie human genetic traits and diseases. The Genotype-Tissue Expression (GTEx) project aims to characterize variation in gene expression levels across individuals and diverse tissues of the human body, many of which are not easily accessible. Here we describe genetic effects on gene expression levels across 44 human tissues. We find that local genetic variation affects gene expression levels for the majority of genes, and we further identify inter-chromosomal genetic effects for 93 genes and 112 loci. On the basis of the identified genetic effects, we characterize patterns of tissue specificity, compare local and distal effects, and evaluate the functional properties of the genetic effects. We also demonstrate that multi-tissue, multi-individual data can be used to identify genes and pathways affected by human disease-associated variation, enabling a mechanistic interpretation of gene regulation and the genetic basis of disease

An Experimental Study of Atomic Scale Friction and Adhesion for 2D and Layered Materials: the Effects of Interfacial Contact, Compliance, and Commensurability

While frictional behavior has been studied for millennia, trial-and-error approaches to materials selection for sliding systems remain prevalent due to a lack of understanding of the underlying mechanisms of friction. This thesis discusses uses of atomic force microscopy to probe the atomic-scale mechanisms that drive frictional behavior on two-dimensional and layered materials. By independently varying specific physical parameters, a more robust understanding of the intrinsic frictional behavior between materials can be developed. First, it is seen that increasing humidity initially increases friction by creating energetically favorable “pinning” sites on the substrate surface from the adsorption of water molecules. At higher humidities, the large water coverage eliminates the preferential points, thus decreasing friction again. Secondly, it is seen that adhesion will decrease logarithmically with increasing scanning speed. This phenomenon is attributed to a depletion of bonds across the tip/sample interface due to the faster sliding speed. Thirdly, sample deposition methods modify the frictional behavior of MoS2 monolayers, as the stronger adhesion between grown MoS2 and a silicon substrate reduces the monolayer’s ability to conform and create high contact quality compared to exfoliated MoS2. Exfoliated MoS2 shows a friction contrast between monolayer and bilayer regions, while grown MoS2 shows no contrast. Fourthly, the friction force decreases as chalcogen size increases in MoX2 (X=S, Se, Te) such that the friction force follows MoS2\u3eMoSe2\u3eMoTe2. An increase in chalcogen size increases the lattice spacing, creating a wider pathway that allows the tip to detour around high energy sites and thus lower friction. Finally, the friction force on MoS2 shows two behaviors—a strong enhancement of friction at low temperatures or an athermal behavior, with this bifurcation attributed to a change in energy barrier to sliding due to tip changes or advantageous adsorbates. The thermal Prandtl-Tomlinson model does not fit well to the experimental data, highlighting the limitations of its underlying assumptions. Collectively, these results demonstrate the need to examine tip, sample, and substrate interactions and their role in atomic-scale stick-slip friction. The understanding of the nanoscale mechanisms influencing frictional behavior will help advance tribology research toward the goal of predictive, judicious, application-specific materials selection

An Experimental Study Of Atomic Scale Friction And Adhesion For 2d And Layered Materials: The Effects Of Interfacial Contact, Compliance, And Commensurability

While frictional behavior has been studied for millennia, trial-and-error approaches to materials selection for sliding systems remain prevalent due to a lack of understanding of the underlying mechanisms of friction. This thesis discusses uses of atomic force microscopy to probe the atomic-scale mechanisms that drive frictional behavior on two-dimensional and layered materials. By independently varying specific physical parameters, a more robust understanding of the intrinsic frictional behavior between materials can be developed. First, it is seen that increasing humidity initially increases friction by creating energetically favorable “pinning” sites on the substrate surface from the adsorption of water molecules. At higher humidities, the large water coverage eliminates the preferential points, thus decreasing friction again. Secondly, it is seen that adhesion will decrease logarithmically with increasing scanning speed. This phenomenon is attributed to a depletion of bonds across the tip/sample interface due to the faster sliding speed. Thirdly, sample deposition methods modify the frictional behavior of MoS2 monolayers, as the stronger adhesion between grown MoS2 and a silicon substrate reduces the monolayer’s ability to conform and create high contact quality compared to exfoliated MoS$_2$. Exfoliated MoS2 shows a friction contrast between monolayer and bilayer regions, while grown MoS2 shows no contrast. Fourthly, the friction force decreases as chalcogen size increases in MoX2 (X=S, Se, Te) such that the friction force follows MoS2\u3eMoSe2\u3eMoTe2. An increase in chalcogen size increases the lattice spacing, creating a wider pathway that allows the tip to detour around high energy sites and thus lower friction. Finally, the friction force on MoS2 shows two behaviors—a strong enhancement of friction at low temperatures or an athermal behavior, with this bifurcation attributed to a change in energy barrier to sliding due to tip changes or advantageous adsorbates. The thermal Prandtl-Tomlinson model does not fit well to the experimental data, highlighting the limitations of its underlying assumptions. Collectively, these results demonstrate the need to examine tip, sample, and substrate interactions and their role in atomic-scale stick-slip friction. The understanding of the nanoscale mechanisms influencing frictional behavior will help advance tribology research toward the goal of predictive, judicious, application-specific materials selection

Tip-Enhanced Dark Exciton Nanoimaging and Local Strain Control in Monolayer WSe₂

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