PRECESSION ELECTRON DIFFRACTION-ASSISTED CRYSTAL ORIENTATION MAPPING

PED is a diffraction technique for structure determination, developed by Paul Midgley and Roger Vincent (Vincent and Midgley 1994) to measure the diffracted intensities achieving near kinematical conditions in the DP. In PED. the beam tilt coils are used to get a conical swing of the electron beam around a pivot point in the sample plane. Afterward, the image shift coils are used to de-scan the electron beam to obtain a quasi-kinematic DP in the diffraction plane. The electron beam can be convergent or parallel, and it is inclined away from the optical axis forming a precession angle (

Figure 4.8a shows a schematic representation of the PED-assisted crystal phase. The incident electron beam can be tilted and rotated in a conical hollow' surface around the optical axis using the beam tilt coils. The diffracted intensities are de- scanned w'ith the image shift coils, and then the DP is displayed as a stationary spot pattern. An external ultrafast CCD camera, which is attached to the viewing screen of the microscope, registers every step of the scanned area and records each electron DP to be stored in a computer for a subsequent indexing process. The pattern recognition process is performed by a comparison between calculated cross-correlation values. For each experimental pattern, there is one value for a theoretical DP template computed for every possible orientation and for every expected phase through the crystal lattice parameters from the crystallography open database (COD). Figure 4.8b shows five electron DPs from their respective region in a gold nanoparticle. All these patterns are registered close to the [110] zone axis. The corresponding simulated electron DP is also showm in Figure 4.8c.

(a) Schematic representation of the e-beam in the column of the microscope under a precession geometry,

FIGURE 4.8 (a) Schematic representation of the e-beam in the column of the microscope under a precession geometry, (b) Individual electron DPs taken in five regions along the [110] direction, (c) Example of one simulated electron DP used for matching with the experimental ones.

Reprinted with permission from (Santiago et al. 2016), copyright (2019), Elsevier.

Correlation Contours for Twinned and nontwinned Nanoparticles

In this section, the crystalline orientation mapping applied to polyhedral nanoparticles is employed to multiple twinned metallic nanoparticles. Nanoparticles with different shapes can be obtained as a consequence of the balance of the binding energies of atoms into the nanoparticle and those located at surface planes during the growth stage. The use of surfactant additives during the synthesis of nanoparticles promotes their stabilization and enables the modification of the growth rate and the surface free energy; the latter plays a crucial role in the final shape of the particles (Baletto and Ferando 2005; Barnard 2014). Stability of nanoparticles depends mainly on their size range and by faceting. Smaller clusters of a few atoms to particles of a few nanometers are very unstable if they are not stabilized in a certain media. In contrast, highly faceting structures such as octahedral, decahedral, or icosahedral (Zhou et al. 2008; Carbo-Argibay et al. 20Ю) (Platonic solids) nanoparticles are very stable and over a size range higher than 50 nm maintain a mostly polyhedral morphology based on Wulff-Ringe (Ino 1969; Marks, 1984; Ringe, Van Duyne and Marks 2013) construction formalism, which is applied to multiply twinned particles and oxide nanoparticles. The use of a small and focused electron probe is useful to characterize nanoparticles of this size range (-100 nm) as the point-by-point data acquisition of the PED ACOM-TEM technique allows for correlation and quantification of the crystalline changes during scanning. This permits the visualization and analysis of very complex structures that can be produced by faceting. This type of feature would be easily ignored by standard ТЕМ imaging techniques or volumetric diffraction methods. As an example, the next images investigate the shape of a modified truncated decahedron, which exhibits a barrel-like shape with multiple facets by PED.

Figure 4.9 a-h shows selected gold nanoparticles to highlight the presence of twin boundaries. The set of electron DPs was obtained by scanning the precessed electron beam over the region of interest. The DiGISTAR precession unit from NanoMEGAS was operating at 50 Hz with a precession angle of 0.9°. NBD conditions were used in the JEOL ARM 200F microscope to achieve a probe smaller than 2 nm. The camera length and the distortions introduced by the position of the external CCD camera were corrected during data treatment. Figure 4.9a-d shows regular decahedral particles in which a virtual BF and contour images of several nanoparticles with twin planes are visible. To reconstruct a virtual BF image from the DPs, a virtual aperture is placed over the transmitted beam of the recorded DPs. Then, the average intensity is calculated leaving out the contrast information from the diffraction spots. In this sense, the image formed is a STEM type BF image of the object. Figure 4.9c and d shows the corresponding correlation contour maps exhibiting clearly inner contrasts that correspond to the multiple twins’ boundaries contained within the nanoparticles. Contour maps are built cross-correlating the pixels/DPs from the set of first neighbors. Identical DPs in the vicinity will produce a white pixel to denote the continuity of the same crystalline orientation or phase. In contrast, a difference in the surrounding pixels will be indicated by a grayscale value. As an example, the pinpointed nontwinned particles, denoted by arrows in the larger scan in Figure 4.9g and h, show contours at their perimeter. Because the difference between the crystalline DP and amorphous carbon is high, the corresponding cross-correlated pixel color is black.

Virtual bright-field

FIGURE 4.9 Virtual bright-field (VBF) and contour images obtained by PED ACOM-TEM of decahedral nanoparticles, (a and b) Show the signal derived from the integration of the center beam, which is equivalent to a standard BF image, (c and d) Corresponding contour image obtained cross-correlating the first set of neighbor pixels. The contour map highlights the presence of twin planes and/or changes in crystallinity or phase, (e-h) Larger scans showing the presence of nontwinned nanoparticles (pointed by arrows).

Reprinted with permission from (Santiago 2016), copyright (2019), Elsevier.

Crystal Orientation in Hybrid Nanomaterials

Hybrid materials at nanoscale combines the physical properties of materials to create smart and tunable materials for specific application. For example, in transition metals magnetic, plasmonic, and catalytic properties are well studied in metals such as cobalt, silver, and platinum, respectively. Particularly, the magneto-optical interactions have gained attention due to the potential technological applications in spintronic, electromagnetic shielding, magneto-optical data storage, and others (Maksymov 2015, 2016). Nanoscale magnetic structures, and their ordered arrays, are being considered for use in several advanced technological areas such as micro- electromechanical systems (MEMS) and power devices such as supercapacitors or batteries. Metal-semiconductor materials have attracted great interest in the fabrication of nanoantennas, where the coupling of metal w'ith semiconductors leads to the development of resonant nanostructures responding to a specific frequency bandwidth. In this context, zinc oxide has been recognized as an excellent semiconductor material in w'hich its morphological configurations play an important role in its properties and applications (Sanchez et al. 2016). The crystalline structure plays a determinant role in the properties due to the crystalline anisotropy as well as the heterojunction of the materials. The hybrid material is a ZnO/Ag nanoantenna, where the core is a silver nanow'ire w'ith a pentagonal cross-sectional area. The five surfaces of the ZnO rods are assembled using microwave synthesis (Sanchez et al. 2017). The crystallographic analysis reveals five nucleation sites for the ZnO nanorods, w'hich

Crystalline orientation maps using PED ACOM-TEM

FIGURE 4.10 Crystalline orientation maps using PED ACOM-TEM: orientations (a) x, (b) y, and (c) z. (d) SEM micrograph of the ZnO/Ag assembling and (e) graphical representation of the epitaxial growing.

Reprinted with permission from (Sanchez et al. 2017), copyright (2017), Elsevier.

are oriented along the [001] direction on the (110) planes of the silver. The analysis of the crystalline orientation has been performed by using PED АСОМ (Rauch Veron 2014). The combination of PED+ASTAR gives the full crystallographic information of the hybrid assemble. The electron DPs are collected with an external CCD camera connected to the screen viewing of the microscope. Figure 4.10 shows the crystal orientation map; Figure 4.10a-c corresponds to the orientation map in the x.y.z-directions, respectively. Figure 4.10d and e shows a scanning EM (SEM) image and a schematic representation of the assembled hierarchical nanostructure ZnO/ Ag. The color chart extracted from the pole figures is shown in Figure 4.10d and e. The color code indicates the orientation near to the [110] zone axis (green color) for the orientation in x in silver, which correspond with the parallel direction of the electrons path within the column of the ТЕМ. ZnO nanorods show' distinct orientations because the hexagonal rods are rotated in perpendicular directions to the silver nanow'ires and along their [001] directions.

A detailed analysis of the crystallographic assembling is shown in Figure 4.1 la, w'here an image of a ZnO nanorod w'as recorded using HRTEM. Figure 4.1 lb depicts the crystal orientation map of the Ag/ZnO interface; the yellow' region corresponds to the ZnO nanorod. The HRTEM image shows that the growth direction corresponds to the planes (002) and that the ZnO nanorod is oriented along the [110] zones axis, as show'n in the fast Fourier transform (FFT) inset in Figure 4.1 la. The crystallographic relationship at interface is detailed in Figure 4.1 lb, where three regions of the lateral side of the silver nanowire are projected and show' the colors green, red, and blue. To confirm the orientation of the pentagonal cross-sectional area of silver the crystal orientation map of a decahedral particle is added to the image as well as the index map code referred to the fee area oriented along the fivefold symmetry and its orientation with respect to the ZnO nanorod. The decahedral nanoparticle has five regions, two of them with blue colors oriented along the [111] direction, whereas

(a) HRTEM image of ZnO nanorods. The inset shows the FFT. in which the

FIGURE 4.11 (a) HRTEM image of ZnO nanorods. The inset shows the FFT. in which the

growth direction of the planes (002) along the direction hOOli of ZnO nanorods is indicated, (b) Crystal orientation maps show the most probable orientation of a particular phase for Both ZnO nanorods and Ag nanowires, depicted with the color code.

Reprinted with permission from (Sanchez et al. 2016), copyright (2016), AIP.

the green region corresponds to the [110] direction. Similar reports have demonstrated the fivefold zone axis symmetry measured on pentagonal nanoparticles by PED. Furthermore, after the automated indexation process, the planes (002) of the silver nanowire are oriented with the [110] direction of the ZnO nanorod, as show n in Figure 4.11b.

 
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