125 research outputs found
Patterned probes for high precision 4D-STEM bragg measurements.
Nanoscale strain mapping by four-dimensional scanning transmission electron microscopy (4D-STEM) relies on determining the precise locations of Bragg-scattered electrons in a sequence of diffraction patterns, a task which is complicated by dynamical scattering, inelastic scattering, and shot noise. These features hinder accurate automated computational detection and position measurement of the diffracted disks, limiting the precision of measurements of local deformation. Here, we investigate the use of patterned probes to improve the precision of strain mapping. We imprint a "bullseye" pattern onto the probe, by using a binary mask in the probe-forming aperture, to improve the robustness of the peak finding algorithm to intensity modulations inside the diffracted disks. We show that this imprinting leads to substantially improved strain-mapping precision at the expense of a slight decrease in spatial resolution. In experiments on an unstrained silicon reference sample, we observe an improvement in strain measurement precision from 2.7% of the reciprocal lattice vectors with standard probes to 0.3% using bullseye probes for a thin sample, and an improvement from 4.7% to 0.8% for a thick sample. We also use multislice simulations to explore how sample thickness and electron dose limit the attainable accuracy and precision for 4D-STEM strain measurements
Altered Expression of Genes Involved in GABAergic Transmission and Neuromodulation of Granule Cell Activity in the Cerebellum of Schizophrenia Patients
Objective: Deficits in gamma-aminobutyric acid (GABA) signaling have been described in the prefrontal cortex, limbic system, and cerebellum in individuals with schizophrenia. The purpose of the present study was to further investigate cerebellar gene expression alterations as they relate to decreases in GABAergic transmission by examining the expression of GABAergic markers, N-methyl-D-aspartic- acid (NMDA) receptor subunits, and cerebellum neuromodulators in individuals with schizophrenia. Method: Subjects were postmortem men with a diagnosis of schizophrenia (N=13) and a postmortem intervalmatched non-psychiatric male comparison group (N=13). The authors utilized real-time-quantitative polymerase chain reaction (PCR) to measure mRNA levels of the fol lowing GABAergic markers: glutamic acid decarboxylase (GAD) 65 and 67; GABA plasma membrane transporter- 1 (GAT-1); GABA type A (GABAA) receptor subunits α6, β3, and δ; and parvalbumin. In addition, real-time-quantitative PCR was utilized to assess mRNA levels of the NMDA receptor (NR) subunits NR1, NR2-A, NR2-B, NR2-C, and NR2-D as well as the cerebellar neuromodulators glutamate receptor (GluR)-6, kainate-preferring glutamate receptor subunit-2 (KA2), metabotropic glutamate receptor (mGluR)-2 and mGluR3, and neuronal nitric oxide synthase. Measurements for mRNA levels were determined using lateral cerebellar hemisphere tissue from both schizophrenia and comparison subjects. Results: Schizophrenia subjects showed significant decreases in mRNA levels of GAD67, GAD65, GAT-1, mGluR2, and neuronal nitric oxide synthase. Increases in GABAA-α6 and GABAA-δ as well as GluR6 and KA2 were also observed. Medication effects on the expression of the same genes were examined in rats treated with either haloperidol (Sprague-Dawley rats [N=16]) or clozapine (Long-Evans rats [N= 20]). Both haloperidol and clozapine increased the levels of GAD67 in the cerebellum and altered the expression of other cerebellar mRNAs. Conclusions: These findings suggest that GABA transmission is decreased in the cerebellar cortices in individuals with schizophrenia and additional gene expression changes may reflect an attempt to increase GABA neurotransmission at the cerebellar glomerulus
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Direct imaging of short-range order and its impact on deformation in Ti-6Al.
Chemical short-range order (SRO) within a nominally single-phase solid solution is known to affect the mechanical properties of alloys. While SRO has been indirectly related to deformation, direct observation of the SRO domain structure, and its effects on deformation mechanisms at the nanoscale, has remained elusive. Here, we report the direct observation of SRO in relation to deformation using energy-filtered imaging in a transmission electron microscope (TEM). The diffraction contrast is enhanced by reducing the inelastically scattered electrons, revealing subnanometer SRO-enhanced domains. The destruction of these domains by dislocation planar slip is observed after ex situ and in situ TEM mechanical testing. These results confirm the impact of SRO in Ti-Al alloys on the scale of angstroms. The direct confirmation of SRO in relationship to dislocation plasticity in metals can provide insight into how the mechanical behavior of concentrated solid solutions by the material's thermal history
Nanoscale mosaicity revealed in peptide microcrystals by scanning electron nanodiffraction.
Changes in lattice structure across sub-regions of protein crystals are challenging to assess when relying on whole crystal measurements. Because of this difficulty, macromolecular structure determination from protein micro and nanocrystals requires assumptions of bulk crystallinity and domain block substructure. Here we map lattice structure across micron size areas of cryogenically preserved three-dimensional peptide crystals using a nano-focused electron beam. This approach produces diffraction from as few as 1500 molecules in a crystal, is sensitive to crystal thickness and three-dimensional lattice orientation. Real-space maps reconstructed from unsupervised classification of diffraction patterns across a crystal reveal regions of crystal order/disorder and three-dimensional lattice tilts on the sub-100nm scale. The nanoscale lattice reorientation observed in the micron-sized peptide crystal lattices studied here provides a direct view of their plasticity. Knowledge of these features facilitates an improved understanding of peptide assemblies that could aid in the determination of structures from nano- and microcrystals by single or serial crystal electron diffraction
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In Situ TEM Study of the Degradation of PbSe Nanocrystals in Air
PbSe
nanocrystals have attracted widespread attention due to a
variety of potential applications. However, the practical utility
of these nanocrystals has been hindered by their poor air stability,
which induces undesired changes in the optical and electronic properties.
An understanding of the degradation of PbSe nanocrystals when they
are exposed to air is critical for improving the stability and enhancing
their applications. Here, we use in situ transmission electron microscopy
(TEM) with an environmental cell connected to air to study PbSe nanocrystal
degradation triggered by air exposure. We have also conducted a series
of complementary studies, including in situ environmental TEM study
of PbSe nanocrystals exposed to pure oxygen and PbSe nanocrystals
in H2O using a liquid cell, and ex situ experiments, such
as O2 plasma treatment and thermal heating of PbSe nanocrystals
under different air exposure. Our in situ observations reveal that
when PbSe nanocrystals are exposed to air (or oxygen) under electron
beam irradiation, they experience a series of changes, including shape
evolution of individual nanocrystals with the cuboid intermediates,
coalescence between nanocrystals, and formation of PbSe thin films
through drastic solid-state fusion. Further studies show that the
PbSe thin films transform into an amorphous Pb rich phase or eventually
pure Pb, which suggest that Se reacts with oxygen and can be evaporated
under electron beam illumination. These various in situ and ex situ
experimental results indicate that PbSe nanocrystal degradation in
air is initiated by the dissociation and removal of ligands from the
PbSe nanocrystal surface
Multibeam Electron Diffraction
One of the primary uses for transmission electron microscopy (TEM) is to
measure diffraction pattern images in order to determine a crystal structure
and orientation. In nanobeam electron diffraction (NBED) we scan a moderately
converged electron probe over the sample to acquire thousands or even millions
of sequential diffraction images, a technique that is especially appropriate
for polycrystalline samples. However, due to the large Ewald sphere of TEM,
excitation of Bragg peaks can be extremely sensitive to sample tilt, varying
strongly for even a few degrees of sample tilt for crystalline samples. In this
paper, we present multibeam electron diffraction (MBED), where multiple probe
forming apertures are used to create mutiple STEM probes, all of which interact
with the sample simultaneously. We detail designs for MBED experiments, and a
method for using a focused ion beam (FIB) to produce MBED apertures. We show
the efficacy of the MBED technique for crystalline orientation mapping using
both simulations and proof-of-principle experiments. We also show how the
angular information in MBED can be used to perform 3D tomographic
reconstruction of samples without needing to tilt or scan the sample multiple
times. Finally, we also discuss future opportunities for the MBED method.Comment: 14 pages, 6 figure
Propuesta de estructuras de costos para la pequeña empresa PanaderÃa Las Flores, ubicada en el municipio de San Marcos, en el departamento de Carazo, durante el segundo semestre del año 2018
El objetivo del presente trabajo es proponer una estructura de costos por proceso para la
pequeña empresa PanaderÃa Las Flores, la cual fue realizada con la colaboración del propietario
y sus trabajadores, quienes requerÃan contar con una estructura que les permita mejorar su
gestión administrativa, la utilización y optimización del talento humano, los recursos
(financieros, materiales y tecnológicos).
El primer acápite de documento está compuesto por el marco teórico en el cual exponemos
los conceptos de costos, sus elementos y la importancia que tiene su aplicación en las micros,
pequeñas y medianas empresas.
El segundo acápite está conformado por el diseño metodológico, y las técnicas utilizadas
para la recolección de información, la cual consistió en observar y describir el proceso
productivo que se lleva a cabo con las limitantes que posee para el registro de sus operaciones.
El tercer acápite consiste en el análisis de resultados, describe la historia de la panaderÃa,
ubicación, infraestructura; la base legal por la cual está establecida, estructura organizacional, su
capital humano y el proceso productivo que se realiza para la elaboración del producto.
En el cuarto acápite se plantea la propuesta de una estructura de costo a la PanaderÃa Las
Flores, con sus respectivos flujogramas, y el diseño de formatos para control de sus operaciones.
Con la propuesta de estructura de costos por procesos se pretende, que el dueño de la
pequeña empresa conozca y controle los elementos del costo del producto, y que le aporte
información valiosa a la administración para la acertada y oportuna toma de decisiones, lo que le
generará un mayor conocimiento del registro de sus operaciones y el crecimiento de su
panaderÃ
Strong structural and electronic coupling in metavalent PbS moire superlattices
Moire superlattices are twisted bilayer materials, in which the tunable
interlayer quantum confinement offers access to new physics and novel device
functionalities. Previously, moire superlattices were built exclusively using
materials with weak van der Waals interactions and synthesizing moire
superlattices with strong interlayer chemical bonding was considered to be
impractical. Here using lead sulfide (PbS) as an example, we report a strategy
for synthesizing of moire superlattices coupled by strong chemical bonding. We
use water-soluble ligands as a removable template to obtain free-standing
ultra-thin PbS nanosheets and assemble them into direct-contact bilayers with
various twist angles. Atomic-resolution imaging shows the moire periodic
structural reconstruction at superlattice interface, due to the strong
metavalent coupling. Electron energy loss spectroscopy and theoretical
calculations collectively reveal the twist angle26 dependent electronic
structure, especially the emergent separation of flat bands at small twist
angles. The localized states of flat bands are similar to well-arranged quantum
dots, promising an application in devices. This study opens a new door to the
exploration of deep energy modulations within moire superlattices alternative
to van der Waals twistronics
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