Jesus Cantu-Valle

Mapping the magnetic and crystal structure in cobalt nanowires

Using off-axis electron holography under Lorentz microscopy conditions to experimentally determine the magnetization distribution in individual cobalt (Co) nanowires, and scanning precession-electron diffraction to obtain their crystalline orientation phase map, allowed us to directly visualize with high accuracy the effect of crystallographic texture on the magnetization of nanowires. The influence of grain boundaries and disorientations on the magnetic structure is correlated on the basis of micromagnetic analysis in order to establish the detailed relationship between magnetic and crystalline structure. This approach demonstrates the applicability of the method employed and provides further understanding on the effect of crystalline structure on magnetic properties at the nanometric scale.

Precession electron diffraction-assisted crystal phase mapping of metastable c-GaN films grown on (001) GaAs

The control growth of the cubic meta‐stable nitride phase is a challenge because of the crystalline nature of the nitrides to grow in the hexagonal phase, and accurately identifying the phases and crystal orientations in local areas of the nitride semiconductor films is important for device applications. In this study, we obtained phase and orientation maps of a metastable cubic GaN thin film using precession electron diffraction (PED) under scanning mode with a point‐to‐point 1 nm probe size beam. The phase maps revealed a cubic GaN thin film with hexagonal GaN inclusions of columnar shape. The orientation maps showed that the inclusions have nucleation sites at the cubic GaN {111} facets. Different growth orientations of the inclusions were observed due to the possibility of the hexagonal {0001} plane to grow on any different {111} cubic facet. However, the generation of the hexagonal GaN inclusions is not always due to a 60° rotation of a {111} plane. These findings show the advantage of using PED along with phase and orientation mapping, and the analysis can be extended to differently composed semiconductor thin films. Microsc. Res. Tech. 77:980–985, 2014.

Determination of the surface morphology of gold-decahedra nanoparticles using an off-axis electron holography dual-lens imaging system.

The purpose of this paper is to show surface irregularities in gold decahedra nanoparticles extracted by using off-axis electron holography in a JEOL ARM 200F microscope. Electron holography has been used in a dual-lens system within the objective lenses: main objective lens and objective minilens. Parameters such as biprism voltage, fringe spacing (σ), fringe width (W) and optimum fringe contrast have been calibrated. The reliability of the transmission electron microscope performance with these parameters was carried out through a plug-in in the Digital-Micrograph software, which considers the mean inner potential within the particle leading a precise determination of the morphological surface of decahedral nanoparticles obtained from the reconstructed unwrapped phase and image processing. We have also shown that electron holography has the capability to extract information from nanoparticle shape that is currently impossible to obtain with any other electron microscopy technique.

Electric radiation mapping of silver/zinc oxide nanoantennas by using electron holography

In this work, we report the fabrication of self-assembled zinc oxide nanorods grown on pentagonal faces of silver nanowires by using microwaves irradiation. The nanostructures resemble a hierarchal nanoantenna and were used to study the far and near field electrical metal-semiconductor behavior from the electrical radiation pattern resulting from the phase map reconstruction obtained using off-axis electron holography. As a comparison, we use electric numerical approximations methods for a finite number of ZnO nanorods on the Ag nanowires and show that the electric radiation intensities maps match closely the experimental results obtained with electron holography. The time evolution of the radiation pattern as generated from the nanostructure was recorded under in-situ radio frequency signal stimulation, in which the generated electrical source amplitude and frequency were varied from 0 to 5 V and from 1 to 10 MHz, respectively. The phase maps obtained from electron holography show the change in the distribution of the electric radiation pattern for individual nanoantennas. The mapping of this electrical behavior is of the utmost importance to gain a complete understanding for the metal-semiconductor (Ag/ZnO) heterojunction that will help to show the mechanism through which these receiving/transmitting structures behave at nanoscale level.

Calibration for medium resolution off-axis electron holography using a flexible dual-lens imaging system in a JEOL ARM 200F microscope

In this work the calibration of a medium resolution off-axis electron holography using a dual-lens imaging system in a JEOL ARM 200F is shown. The objective dual-lens configuration allows adjusting the field of view from 35nm to 2.5μm. Subsequently, the parameters used in phase shift reconstruction were calibrated considering biprism voltage versus fringe spacing (σ) and versus fringe width (W). The reliability of the transmission electron microscope performance using these parameters was achieved using gold nanoparticles of known size and adjusting the excitation voltage of the lenses.

Nanodomain induced anomalous magnetic and electronic transport properties of LaBaCo2O5.5+δ highly epitaxial thin films.

A giant magnetoresistance effect (∼46% at 20 K under 7 T) and anomalous magnetic properties were found in a highly epitaxial double perovskite LaBaCo2O5.5+δ (LBCO) thin film on (001) MgO. Aberration-corrected Electron Microscopy and related analytical techniques were employed to understand the nature of these unusual physical properties. The as-grown film is epitaxial with the c-axis of the LBCO structure lying in the film plane and with an interface relationship given by (100)LBCO || (001)MgO and [001]LBCO || [100]MgO or [010]MgO. Orderly oxygen vacancies were observed by line profile electron energy loss spectroscopy and by atomic resolution imaging. Especially, oxygen vacancy and nanodomain structures were found to have a crucial effect on the electronic transport and magnetic properties.

Morphology visualization of irregular shape bacteria by electron holography and tomography

In the current work, irregular morphology of Staphylococcus aureus bacteria has been visualized by phase retrieval employing off‐axis electron holography (EH) and 3D reconstruction electron tomography using high‐angle annular dark field scanning transmission electron microscopy (HAADF‐STEM). Bacteria interacting with gold nanoparticles (AuNP) acquired a shrunken or irregular shape due to air dehydration processing. STEM imaging shows the attachment of AuNP on the surface of cells and suggests an irregular 3D morphology of the specimen. The phase reconstruction demonstrates that off‐axis electron holography can reveal with a single hologram the morphology of the specimen and the distribution of the functionalized AuNPs. In addition, EH reduces significantly the acquisition time and the cumulative radiation damage (in three orders of magnitude) over biological samples in comparison with multiple tilted electron expositions intrinsic to electron tomography, as well as the processing time and the reconstruction artifacts that may arise during tomogram reconstruction.

Controlled Magnetization by Electron Holography of Polycrystalline Cobalt Nanowires

Nowadays the comprehensive understanding of nanoscale materials and their physical properties are of great interest to the scientific and technological community. In particular, magnetic nanostructures of different size, shape and composition (e.g. nanoparticles, nanowires or thin films) possess a great potential to improve current technologies in areas such as: magnetic data storage, electromagnetic sensing [1-2]. Lately, soft magnetic nanowires, (Co, Fe & Ni) have been studied for a while experimentally and by simulations, but there still some questions to be address. Soft magnetic nanowires can switch magnetization in two different modes depending on their thickness, these modes are known as the transverse wall mode and the vortex wall mode. In thin ferromagnetic nanowires (diameter less than 40nm) a simple domain wall nucleates and propagates along the nanowire axis, while the reversal of thick nanowires (diameter more than 40 nm) is achieved via localized curling or vortex mode. The magnetization direction of each magnetic domain will be influenced by the magnetocrystalline anisotropy; typically following the easy magnetization axis, which minimize the magnetocrystalline energy. The magnetization behavior in this nanostructures is dominated by the competition between magnetocrystalline anisotropy and shape anisotropy. In many cases this competition between can frustrates the magnetization direction. It is expected that the magnetostatic coupling between nanostructures have a strong influence on their response to an external field [3].

Characterization of Metallic and Bimetallic Nanoparticles by Off-Axis Electron Holography

It is well known that at nanoscale, parameters such as size, shape and surface morphology play a fundamental role on the properties that materials exhibit. In metallic nanoparticles, an increase in reactivity is obtained when size is reduced. In the case of bimetallic nanoparticles, unique physical and chemical properties can be achieved when composition and shape are controlled [1]. Aberration corrected electron microscopy is an ideal technique to study nanoparticles, however the images obtained correspond to two dimensional projections of three dimensional objects, which correspond to a partial characterization.