John E. Sanchez

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.

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.

Structural analysis of the epitaxial interface Ag/ZnO in hierarchical nanoantennas

Detectors, photo-emitter, and other high order radiation devices work under the principle of directionality to enhance the power of emission/transmission in a particular direction. In order to understand such directionality, it is important to study their coupling mechanism of their active elements. In this work, we present a crystalline orientation analysis of ZnO nanorods grown epitaxially on the pentagonal faces of silver nanowires. The analysis of the crystalline orientation at the metal-semiconductor interface (ZnO/Ag) is performed with precession electron diffraction under assisted scanning mode. In addition, high resolution X-ray diffraction on a Bragg-Brentano configuration has been used to identify the crystalline phases of the arrangement between ZnO rods and silver nanowires. The work presented herein provides a fundamental knowledge to understand the metal-semiconductor behavior related to the receiving/transmitting mechanisms of ZnO/Ag nanoantennas.

Resonance properties of Ag-ZnO nanostructures at terahertz frequencies.

Nanoantennas have been fabricated by scaling down traditional antenna designs using nanolithographic techniques and testing them at different optical wavelengths, these particular nanoantennas have shown responses in a broad range of frequencies going from visible wavelengths to the range of the terahertz. Some self-assembled nanostructures exist that exhibit similar shapes and properties to those of traditional antenna structures. In this work the emission and absorption properties of self-assembled nanostructures made of zinc oxide nanorods on silver nanowires, which resemble traditional dipole antennas, were measured and simulated in order to test their antenna performance. These structures show resonant properties in the 10-120 THz range, with the main resonance at 60 THz. The radiation pattern of these nanostructures was also obtained by numerical simulations, and it is shown that it can be tailored to increase or decrease its directivity as a function of the location of the energy source of excitation. Experimental measurements were performed by Raman spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR) in order to show existing vibrational frequencies at the resonant frequencies of the nanostructures, measurements were made from ~9 to 103 THz and the results were in agreement with the simulations. These characteristics make these metal-semiconductor Ag/ZnO nanostructures useful as self-assembled nanoantennas in applications such as terahertz spectroscopy and sensing at terahertz frequencies.

Design of a cathodoluminescence image generator using a Raspberry Pi coupled to a scanning electron microscope

In this work, an innovative cathodoluminescence (CL) system is coupled to a scanning electron microscope and synchronized with a Raspberry Pi computer integrated with an innovative processing signal. The post-processing signal is based on a Python algorithm that correlates the CL and secondary electron (SE) images with a precise dwell time correction. For CL imaging, the emission signal is collected through an optical fiber and transduced to an electrical signal via a photomultiplier tube (PMT). CL Images are registered in a panchromatic mode and can be filtered using a monochromator connected between the optical fiber and the PMT to produce monochromatic CL images. The designed system has been employed to study ZnO samples prepared by electrical arc discharge and microwave methods. CL images are compared with SE images and chemical elemental mapping images to correlate the emission regions of the sample.

Electrical Probing of Silver Nanowires in situ Transmission Electron Microscopy

One of the most important aspects of the area of material sciences is the characterization of nanostructured and advanced materials. Silver is one of the most promising materials due to the large thermal and electrical conductivity compared to other metals. However, not all properties of materials scale down in proportion to its one-dimensional nanostructure. In this work we present the electrical behavior of single silver nanowires. By employing an in situ transmission electrical holder, one can measure the resistivity and conductance of individual nanowires while observing the changes of the structure due to the high resolution transmission electron microscope. By applying different voltages we observed the how the nanowire is affected by this increase until failure.

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].

Nano-manipulation of Ag/ZnO Nanoantennas for in-situ TEM Electrical Measurements

Nowadays metal-semiconductor systems have attracted more attention due to their role as an active element in opto-electronic applications, especially in nanoantenna devices where the coupling of a metal with a semiconductor material could lead to the appearing of a resonant system responding to a specific frequency useful to be employed in solar cells, detectors, and nonlinear optical devices [1–2]. Moreover, the in-situ monitoring of physical properties associated with different arrays and configurations will lead to controllable opto-electronic properties. In this context, zinc oxide has been recognized as an excellent semiconductor material that meets most of the features required for those applications due to its easiness to be modifiable, i.e. controlling not only its arrangement distribution, leading to a variety of internal structures, but also its morphological configurations. It is known that using a specific method for the synthesis, different features and morphologies could be achieved.

Precession Electron Diffraction and Orientation Phase Mapping of Assembled Ag/ZnO Nanoantennas

The manipulation of the geometrical and structural arrangement of the constituent’s elements on devices at nanoscale level is highly desirable for a precise monitoring of the opto-electrical properties exhibited for these nanomaterials. In fact, a great effort has being made to understand the coupling mechanisms on metal-semiconductors systems, most precisely at interfaces nanoscale level. For instance, it is well known that the multidirectional radiation pattern generated by the active elements on nanoantenna applications is highly dependent on both the structural and orientation distribution of the receiver elements as well as the passive element on the nanoscale device. For example, Wang, et al [1] have synthesized high order nanostructures in a hierarchical configuration to study the photo-induced optical properties of these systems in function of the ZnO concentration distributed along the silver nanowires. However, few is known about the structural coupling mechanisms between this metal-semiconductor heterojunctions. Thus, to understand the dynamic coupling at the interface level in the Ag/ZnO metalsemiconductor heterojunctions we report the epitaxial growing of zinc oxide nanorods on the pentagonal exposes faces of Ag nanowires resembling a hierarchal nanoantenna. Moreover, the studied of the growth mechanism in the active/contact faces of the metal-semiconductor heterojunction has been done by mapping simultaneously the dynamical electron diffraction pattern under in-situ precession electron diffraction at the heterojunction interface Ag/ ZnO nanosystem. Indeed, by indexing the dynamical diffraction patterns using orientational/phase mapping from the precessed electron diffraction data collected an orientational mapping has been retrieved showing the interfacial growing polar planes (0002) of ZnO nanorods on the pentagonal planes of silver nanowires with a mismatch between planes along the coupling interface. For completeness, grazing angle x-ray diffraction measurements on prepared substrates Ag/ZnO systems shown well-defined peaks associated to the main phases of ZnO nanorods and Ag nanowires respectively. A full understanding of the fit faces mechanism between Ag/ZnO along the mismatch direction undoubtedly will allow elucidating the mechanism through which the contact metal-semiconductor behaves at the heterojunction interface.

Crystalline Phase Mapping Associated to the Magnetic Flux in Cobalt Nanowires

Electron holography is a powerful technique for mapping of the electric, magnetic fields and the crystalline potential within a transmission electron microscope (TEM). Particularly, the possibility to extract the magnetic behavior of a sample at nanoscale, at the remanent and excited states, transforms the microscope in another lab for the measurement of physical properties of materials. Parameters such as the external magnetic field produced by the objective lens, temperature, reversal magnetization, play an important role in the understanding of the magnetic properties of individual nanostructures like nanoparticles and nanowires [1].