Study of the Growth Mechanism of Solution-Synthesized Symmetric Tellurium Nanoflakes at Atomic Resolution.

As a new member of 2D materials, 2D tellurium (Te) has recently attracted much attention due to its intriguing properties. Through hydrothermal processing, 2D Te with tunable thickness and size has been realized, and its growth mechanism has also been studied. However, the tailored growth of 2D Te nanoflakes with symmetrical morphologies and interfacial moiré fringes has never been reported. Here, 2D Te nanoflakes have been prepared using the hydrothermal method, and mirror-symmetrical shapes (including "V-shape," "heart-shape," and "paper airplane-shape") with obvious moiré fringes in the middle of the nanoflakes are observed. Comprehensive transmission electron microscopy (TEM) techniques are utilized for structural characterization of these nanoflakes, especially the moiré fringes in the symmetry axis region of the nanoflakes. The systematic analyses of the moiré fringes and the observation of obvious overlapping edges of the composing nanoflakes from the cross-sectional samples reveal the possible mechanism of morphological evolution for these symmetrical nanoflakes. These details may fill the research gap in the controllable growth of 2D Te nanomaterials, pave the way for the fabrication of 2D Te moiré superlattices and in-plane homojunctions, and promote their future versatile applications.

STEM multiplication nano-moiré method with large field of view and high sensitivity.

Strain is one of the important factors that determine the photoelectric and mechanical properties of semiconductor materials and devices. In this paper, the scanning transmission electron microscopy multiplication nano-moiré method is proposed to increase the measurement range and sensitivity for strain field. The formation principle, condition, and measurement range of positive and negative multiplication moiré fringes (PMMFs and NMMFs) are analysed in detail here. PMMF generally refers to the multiplication of field of view, NMMF generally refers to the multiplication of displacement measurement sensitivity. Based on the principle of multiplication nano-moiré, Theoretical formulas of the fringe spacing and strain field are derived. Compared with geometric phase analysis of deformation measurements based on high-resolution atom images, both the range of field of view and the sensitivity of displacement measurements of the multiplication moiré method are significantly improved. Most importantly, the area of field of view of the PMMF method is increased by about two orders of magnitude, which is close to micrometre-scale with strain measurement sensitivity of 2 × 10-5. In addition, In order to improve the quality of moiré fringe and the accuracy of strain measurement, the secondary moiré method is developed.The strain laws at the interface of the InP/InGaAs superlattice materials are characterised using the developed method.

High-resolution imaging of organic pharmaceutical crystals by transmission electron microscopy and scanning moiré fringes.

Formulation processing of organic crystalline compounds can have a significant effect on drug properties, such as dissolution rate or tablet strength/hardness. Transmission electron microscopy (TEM) has the potential to resolve the atomic lattice of these crystalline compounds and, for example, identify the defect density on a particular crystal face, provided that the sensitivity of these crystals to irradiation by high-energy electrons can be overcome. Here, we acquire high-resolution (HR) lattice images of the compound furosemide using two different methods: low-dose HRTEM and bright-field (BF) scanning TEM (STEM) scanning moiré fringes (SMFs). Before acquiring HRTEM images of furosemide, a model system of crocidolite (asbestos) was used to determine the electron flux/fluence limits of low-dose HR imaging for our scintillator-based, complementary metal-oxide semiconductor (CMOS) electron camera by testing a variety of electron flux and total electron fluence regimes. An electron flux of 10 e- /(Å2 s) and total fluence of 10 e- /Å2 was shown to provide sufficient contrast and signal-to-noise ratio to resolve 0.30 nm lattice spacings in crocidolite at 300 kV. These parameters were then used to image furosemide which has a critical electron fluence for damage of ≥10 e- /Å2 at 300 kV. The resulting HRTEM image of a furosemide crystal shows only a small portion of the total crystal exhibiting lattice fringes, likely due to irradiation damage during acquisition close to the compound's critical fluence. BF-STEM SMF images of furosemide were acquired at a lower electron fluence (1.8 e- /Å2 ), while still indirectly resolving HR details of the (001) lattice. Several different SMFs were observed with minor variations in the size and angle, suggesting strain due to defects within the crystal. Overall BF-STEM SMFs appear to be more useful than BF-STEM or HRTEM (with a CMOS camera) for imaging the crystal lattice of very beam-sensitive materials since a lower electron fluence is required to reveal the lattice. BF-STEM SMFs may thus prove useful in improving the understanding of crystallization pathways in organic compounds, degradation in pharmaceutical formulations and the effect of defects on the dissolution rate of different crystal faces. Further work is, however, required to quantitatively determine properties such as the defect density or the amount of relative strain from a BF-STEM SMF image.

Spot moiré fringes: determination of domain boundaries and structural parameters in ordered nanoporous structures.

Spot moiré fringes are generated by the superposition between a nanoporous structure and a digital three-way grating. The spot moiré fringes are useful for the characterization of the domain boundaries and structural parameters in ordered nanoporous materials. The pitches and the orientations of the nanopore arrays in three directions can be simultaneously determined in a large view field.

Moiré fringe imaging of heterostructures by scanning transmission electron microscopy.

A heterostructured crystalline bilayer specimen is known to produce moiré fringes (MFs) in the conventional transmission electron microscopy (TEM). However, the understanding of how these patterns form in scanning transmission electron microscopy (STEM) remains limited. Here, we extended the double-scattering model to establish the imaging theory of MFs in STEM for a bilayer sample and applied this theory to successfully explain both experimental and simulated STEM images of a perovskite PbZrO3/SrTiO3 system. Our findings demonstrated that the wave vectors of electrons exiting from Layer-1 and their relative positions with the atomic columns of Layer-2 should be taken into account. The atomic column misalignment leads to a faster reduction in the intensity of the secondary scattering beam compared to the single scattering beam as the scattering angle increases. Consequently, the intensity distribution of MFs in the bright field (BF)-STEM can be still described as the product of two single atomic images. However, in high angle annular dark field (HAADF)-STEM, it is approximately described as the superposition of the two images. Our work not only fills a knowledge gap of MFs in incoherent imaging, but also emphasizes the importance of the coherent scattering restricted by the real space when analyzing the HAADF-STEM imaging.

Aliovalent Dilute Doping and Nano-Moiré Fringe Advance the Structural Stability and Thermoelectric Performance in ß-Zn4Sb3.

Thermoelectric (TE) generators have come a long way since the first commercial apparatus launched in the 1950s. Since then, the ß-Zn4Sb3 has manifested its potential as a cost-effective and environmentally friendly TE generator compared with the tellurium-bearing TE materials. Although the ß-Zn4Sb3 features an intrinsically low thermal conductivity κ, it suffers from a long-lasting structural instability issue arising from the highly mobile zinc ions. Herein, the dilute Ga dopant gives rise to the aliovalent substitution, lowers the mobile zinc ions, and optimizes the hole carrier concentration n H simultaneously. Meanwhile, the formation of nano-moiré fringes suggests the modulated distribution of point defect that results from soluble Ga in a ß-Zn4Sb3 lattice, which elicits an ultralow lattice thermal conductivity κ L = 0.2 W m-1 K-1 in a (Zn0.992Ga0.008)4Sb3 alloy. Hence, a fully dense ß-Zn4Sb3 incorporated with the dilute Ga doping reveals superior structural stability with a peak zT > 1.4 at 623 K. In this work, the aliovalent dilute doping coupled with phase diagram engineering optimizes the fluxes of moving electrons and charged ions, which stabilizes the single-phase ß-Zn4Sb3 while boosting the TE performance at the mid-temperature region. The synergistic strategies endow the ionic crystals with a thermodynamic route, which opens up a new category for high-performance and thermal robust TE alloys.

Fast determination of sample thickness through scanning moiré fringes in scanning transmission electron microscopy.

Sample thickness is an important parameter in transmission electron microscopy (TEM) imaging for interpreting image contrast and understanding the relationship between properties and microstructure. In this study, we introduce a method for sample thickness determination in scanning TEM (STEM) mode based on scanning moiré fringes (SMFs). Focal-series SMF imaging is used and sample thickness can be determined in situ at a medium magnification range, with beam damage and contamination avoided to a large extent. It provides a fast and convenient approach for determining sample thickness in TEM imaging, which is particularly useful for beam-sensitive materials.

Structural Characteristics of the Si Whiskers Grown by Ni-Metal-Induced-Lateral-Crystallization.

Si whiskers grown by Ni-Metal-Induced-Lateral-Crystallization (Ni-MILC) were grown at 413 °C, intentionally below the threshold for Solid State Crystallization, which is 420 °C. These whiskers have significant common characteristics with whiskers grown by the Vapor Liquid Solid (VLS) method. The crystalline quality of the whiskers in both methods is the same. However, in VLS, a crystalline substrate is required, in contrast to the amorphous one in Ni-MILC for the growth of single crystalline whiskers. Moreover, whiskers grown by VLS have a polygonal cross-section with their diameter determined by the diameter of the hemispherical metallic catalysts. On the other hand, in the Ni-MILC, the cross-section of the whiskers depends on the size of the NiSi2 grain from which they are emanated. This was confirmed by observing the crossing whiskers and the rotational Moiré patterns in the crossing area. The structure of disturbed short and thin nonlinear branches on the side-walls of the whiskers was studied. In the whiskers grown by the VLS method, significant contamination occurs by the metallic catalyst degrading the electrical characteristics of the whisker. Such Si whiskers are not compatible with the current CMOS process. Whiskers grown by Ni-MILC at 413 °C are also contaminated by Ni. However, the excess Ni is in the form of tetrahedral NiSi2 inclusions which are coherent with the Si matrix due to the very low misfit of 0.4% between them. These whiskers are compatible with current CMOS process and Thin Film Transistors (TFTs).

Testing of a `hard' X-ray interferometer for experimental investigations.

A `hard' X-ray LLL interferometer is tested for experimental investigations. The interferometer has both a base and a `ceiling', which are rigidly connected through columns. As a result, the interferometer does not have uncontrollable preliminary moiré. The intensity distribution is uniform in the interfering beams. It is shown that the interferometer is very sensitive to minor mechanical stresses. As a result, the interferometer must be freely placed on the goniometer head. Constant-thickness fringes are obtained using a wedge with a vertically placed apex. The volumes available for specimen placement are limited due to the existence of the ceiling. These difficulties can be overcome. The hard interferometer can be used for object and deformation investigations.

Theoretical study of the properties of X-ray diffraction moiré fringes. III. Theoretical simulation of previous experimental moiré images.

As a practical confirmation of a recently published X-ray moiré-fringe theory [Yoshimura (2015). Acta Cryst. A71, 368-381], computer simulations using this theory were conducted for previous experimental moiré images of a strained bicrystal specimen [Yoshimura (1996). Acta Cryst. A52, 312-325]. Simulated moiré images with a good or fairly good likeness are presented as a result of this simulation, in which the characteristic fringe-and-band and local strain patterns in the experimental images are reproduced well. Experimental moiré images taken when the inclination of the lattice planes was forcedly increased in one of the component crystals of the bicrystal specimen were also fairly well simulated in this computation, and their fringe patterns of inclined fringes are shown to be in accordance with the prediction by the theory. This moiré-fringe theory is thus considered to be widely applicable to the study of moiré images. Furthermore, the successful simulation of the previous experimental moiré images means that a satisfactory theoretical explanation was given for the experimental images, with respect to their characteristic global features. However, this study by the theoretical simulation shows explicitly that some significant peculiarities in the fringe profiles of the experimental images still remain unexplained by this moiré-fringe theory.

Mapping the strain distribution within embedded nanoparticles via geometrical phase analysis.

Strain variation within a nanoparticle plays a crucial role in tuning its properties. Geometrical phase analysis (GPA) is typically a powerful tool to investigate the strain in high-resolution transmission electron microscopy (HRTEM) images. It is known that the traditional GPA method measuring the displacement of lattice fringes directly in an HRTEM image is inapplicable to strain measurements on nanoparticles embedded in a matrix, where lattice fringes of nanoparticles are invisible, instead Moiré fringes are present. Furthermore, considering the small size of embedded nanoparticles, generally a few nanometers, no reference region can be chosen and utilized to calculate the relative displacement by GPA. Hence advanced methods need to be developed to break through the barriers of invisible lattice fringes and lack of a reference region. In this work, using α-Fe nanoparticles embedded in sapphire as a test object, we illustrate a GPA method dedicated to embedded nanoparticles. Both the Fourier filter method and the inverse Moiré fringes method were used to reconstruct the invisible lattice fringes of α-Fe nanoparticles. Then a computer-generated image corresponding to an unstrained α-Fe lattice was used as the reference during GPA. The GPA results indicate that there exists a compressive strain in the range of 1.5˜2% within the α-Fe nanoparticles. Our work presents an effective approach to revealing the strain distributions within embedded nanoparticles.

Theoretical study of the properties of X-ray diffraction moiré fringes. I. Corrigenda and addenda.

Seven corrections are made and several supplementary equations are added to the article by Yoshimura [Acta Cryst. (2015), A71, 368-381].

Low dose scanning transmission electron microscopy of organic crystals by scanning moiré fringes.

In the pharmaceutical industry, it is important to determine the effects of crystallisation and processes, such as milling, on the generation of crystalline defects in formulated products. Conventional transmission electron microscopy and scanning transmission electron microscopy (STEM) can be used to obtain information on length scales unobtainable by other techniques, however, organic crystals are extremely susceptible to electron beam damage. This work demonstrates a bright field (BF) STEM method that can increase the information content per unit specimen damage by the use of scanning moiré fringes (SMFs). SMF imaging essentially provides a magnification of the crystal lattice through the interference between closely aligned lattice fringes and a scanning lattice of similar spacing. The generation of SMFs is shown for three different organic crystals with varying electron beam sensitivity, theophylline, furosemide and felodipine. The electron fluence used to acquire the BF-STEM for the most sensitive material, felodipine was approximately 3.5 e-/Å2. After one additional scan of felodipine (total fluence of approximately 7.0 e-/Å2), the SMFs were no longer visible due to extensive damage caused to the crystal. Irregularity in the SMFs suggested the presence of defects in all the organic crystals. Further effort is required to improve the data analysis and interpretation of the resulting SMF images, allowing more information regarding the crystal structure and defects to be extracted.

Theoretical study of the properties of X-ray diffraction moiré fringes. II. Illustration of angularly integrated moiré images.

Using a theory of X-ray diffraction moiré fringes developed in a previous paper, labelled Part I [Yoshimura (2015). Acta Cryst. A71, 368-381], the X-ray moiré images of a silicon bicrystal having a weak curvature strain and an interspacing gap, assumed to be integrated for an incident-wave angular width, are simulation-computed over a wide range of crystal thicknesses and incident-wave angular width, likely under practical experimental conditions. Along with the simulated moiré images, the graphs of characteristic quantities on the moiré images are presented for a full understanding of them. The treated moiré images are all of rotation moiré. Mo Kα1 radiation and the 220 reflection were assumed in the simulation. The results of this simulation show that fringe patterns, which are significantly modified from simple straight fringes of rotation moiré, appear in some ranges of crystal thicknesses and incident-wave angular width, due to a combined effect of Pendellösung oscillation and an added phase difference from the interspacing gap, under the presence of a curvature strain. The moiré fringes which slope to the perpendicular direction to the diffraction vector in spite of the assumed condition of rotation moiré, and fringe patterns where low-contrast bands are produced with a sharp bend of fringes arising along the bands are examples of the modified fringe pattern. This simulation study provides a wide theoretical survey of the type of bicrystal moiré image produced under a particular condition.

Experimental and theoretical investigations of an X-ray LLL interferometer with a wedge-shaped mirror plate.

An X-ray LLL interferometer with a wedge-shaped mirror plate is experimentally and theoretically investigated. Experimentally obtained interference patterns show that the Moiré fringes are superposed on Pendellösung fringes and the period of the Pendellösung fringes is not changed after passing the analyzer plate. An eikonal theory of interference-fringe formation in an LLL interferometer with a wedge-shaped mirror plate is developed, and provides predictions that coincide with experimentally obtained results.

Theoretical study of the properties of X-ray diffraction moiré fringes. I.

A detailed and comprehensive theoretical description of X-ray diffraction moiré fringes for a bicrystal specimen is given on the basis of a calculation by plane-wave dynamical diffraction theory. Firstly, prior to discussing the main subject of the paper, a previous article [Yoshimura (1997). Acta Cryst. A53, 810-812] on the two-dimensionality of diffraction moiré patterns is restated on a thorough calculation of the moiré interference phase. Then, the properties of moiré fringes derived from the above theory are explained for the case of a plane-wave diffraction image, where the significant effect of Pendellösung intensity oscillation on the moiré pattern when the crystal is strained is described in detail with theoretically simulated moiré images. Although such plane-wave moiré images are not widely observed in a nearly pure form, knowledge of their properties is essential for the understanding of diffraction moiré fringes in general.

Structural Analysis of Highly Relaxed GaSb Grown on GaAs Substrates with Periodic Interfacial Array of 90° Misfit Dislocations.

We report structural analysis of completely relaxed GaSb epitaxial layers deposited monolithically on GaAs substrates using interfacial misfit (IMF) array growth mode. Unlike the traditional tetragonal distortion approach, strain due to the lattice mismatch is spontaneously relieved at the heterointerface in this growth. The complete and instantaneous strain relief at the GaSb/GaAs interface is achieved by the formation of a two-dimensional Lomer dislocation network comprising of pure-edge (90°) dislocations along both [110] and [1-10]. In the present analysis, structural properties of GaSb deposited using both IMF and non-IMF growths are compared. Moiré fringe patterns along with X-ray diffraction measure the long-range uniformity and strain relaxation of the IMF samples. The proof for the existence of the IMF array and low threading dislocation density is provided with the help of transmission electron micrographs for the GaSb epitaxial layer. Our results indicate that the IMF-grown GaSb is completely (98.5%) relaxed with very low density of threading dislocations (105 cm-2), while GaSb deposited using non-IMF growth is compressively strained and has a higher average density of threading dislocations (>109 cm-2).