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Energy-dispersive x-ray spectroscopy and high-angle annular dark-field in a Cs-corrected scanning transmission electron microscope are employed to characterize the atomic-scale strain distribution in the active region of the InAs-based interband cascade laser. For the first time, energy-dispersive x-ray spectroscopy is utilized for the quantitative calculation of the zero-strain region, by which the geometric phase analysis of high-angle annular dark-field imaging has been carried out. The strain distribution of the active region with high accuracy has been obtained. The analysis of the out-of-plane strain shows that the active region in the InAs-based interband cascade laser is strain-compensated, while a certain degree of elemental intermixing still exists in the active region. This detailed strain distribution can provide valuable insights into the optimization of the growth conditions for the active region such as growth temperature, V/III flux ratio, and growth process to minimize the elemental intermixing and obtain a better performance interface while maintaining the strain-compensated state. [ABSTRACT FROM AUTHOR]

Over the last two decades, Electron Energy Loss Spectroscopy (EELS) imaging with a scanning transmission electron microscope has emerged as a technique of choice for visualizing complex chemical, electronic, plasmonic, and phononic phenomena in complex materials and structures. The availability of the EELS data necessitates the development of methods to analyze multidimensional data sets with complex spatial and energy structures. Traditionally, the analysis of these data sets has been based on analysis of individual spectra, one at a time, whereas the spatial structure and correlations between individual spatial pixels containing the relevant information of the physics of underpinning processes have generally been ignored and analyzed only via the visualization as 2D maps. Here, we develop a machine learning-based approach and workflows for the analysis of spatial structures in 3D EELS data sets using a combination of dimensionality reduction and multichannel rotationally invariant variational autoencoders. This approach is illustrated for the analysis of both the plasmonic phenomena in a system of nanowires and in the core excitations in functional oxides using low loss and core-loss EELS, respectively. The code developed in this manuscript is open sourced and freely available and provided as a Jupyter notebook for the interested reader. [ABSTRACT FROM AUTHOR]

Low-energy ion beam irradiation, combined with the introduction of impurities, presents a promising approach for nanopatterning silicon (Si) surfaces. In this research, we investigate the surface evolution of Si (100) surfaces irradiated by 75 eV helium (He) plasma, in the presence of tantalum (Ta), traditionally regarded as an impurity incapable of initiating pattern formation, as evidenced in prior studies. The Ta impurities are pre-deposited onto the Si surfaces using the magnetron sputtering method, which offers a more controlled and quantifiable approach compared with the conventional co-deposition route. After irradiation at 800 K, dense cone arrays are produced on the Si surface. The growth of the cones is explored for fluence spanning (1–10) × 1025 m−2. The cross-sectional scanning electron microscope images indicate that the cone lengths and base width are well characterized by t1/2 dependence. The kinetics of the cone growth follow Fick's law, characterized by an effective diffusive mechanism with the coefficients of diffusion: D = 6.49 ± 0.83 × 10−16 m−2 s−1. Transmission electron microscope observations reveal that the cone has distinct inner and outer parts with different microstructures and a clear interface. The growth mechanism of these cones is elucidated as a composite process involving preferential sputtering, the destabilizing influence of He bubbles, and the migration of adatoms. [ABSTRACT FROM AUTHOR]

Polar discontinuities occur in oxide heterostructures due to varying net charges in the sub-unit cell layers. These polar discontinuities lead to structural reconstruction and often create diverse functionalities. This work constructs polar discontinuous in La0.67Sr0.33MnO3/SrCuO2 (LSMO/SCO) heterostructures on a (001)-orientated SrTiO3 (STO) substrate under different configurations. By changing the growth order of LSMO and SCO, we found two different compensating mechanisms for polar discontinuity. When LSMO is grown on SCO, interfacial polarity discontinuities result in the generation of a large number of oxygen vacancies within the LSMO film. Thus, the LSMO magnetism deteriorates. For the SCO/LSMO/SCO trilayer, the SCO capping layer can recover the LSMO magnetism. The scanning transmission electron microscope results show an atomic reconstruction at the SCO-on-LSMO interface and several oxygen vacancies at the SrO sublayer. The interface reconfiguration releases the polar energy, thereby inhibiting the generation of oxygen vacancies and improving the ferromagnetism of the LSMO film. Our work studies the impact of polar discontinuity at the interface, providing insights into the effects of interface polar discontinuities on functional materials. [ABSTRACT FROM AUTHOR]

Strain engineering is an alternative to chemical modification in manipulating functionalities of materials. Here, we report the lattice strain could relax the abrupt phase transition of (Ni1−xFex)1−δS and lead to an excellent negative thermal expansion (NTE) effect. The high-resolution scanning transmission electron microscope and extended x-ray absorption fine structure unambiguously demonstrate that parallel Ni3S4-type nanosheets could be introduced into the matrix lattice by increasing the δ value. By forming the Ni(Fe)-S-Ni(Fe) bonds, the (1–11) plane of nanosheets epitaxially grows on the (002) plane of the hexagonal matrix. Those bonds are strong enough to bear the large lattice mismatch along the interface as the phase transition occurs. As the concentration of nanosheets increases, the weak and inhomogeneous strain in matrix lattice becomes strong and uniform, expanding the NTE window in different ways. This result opens an unconventional avenue for designing NTE materials and extends the research scope of lattice strain as well. [ABSTRACT FROM AUTHOR]

Lead-free antiferroelectric AgNbO3 ceramics have garnered extensive attention due to their rapid charge/discharge capabilities and environmentally friendly nature, holding immense potential for energy storage applications. However, the practical utilization of AgNbO3 has been hindered by its low energy storage density. This study employed an A/B-site co-doping strategy, which yielded positive effects on the energy performance of AgNbO3 ceramics. By modifying the A/B-sites with equivalent amounts of Bi3+ and Y3+ ions, enhanced maximum polarization, improved breakdown field, and slimmer hysteresis loops were simultaneously achieved, as the combined effects of refined grain size, wider optical bandgap, and randomly distributed antiferroelectric nanodomains, which were verified through the scanning electron microscope, transmission electron microscope, and ultraviolet–visible spectrum. As a consequence, a high energy storage density of 5.40 J/cm3 accompanied by an energy storage efficiency of 56.5% was achieved in the Ag0.97Bi0.01Nb0.994Y0.01O3 ceramic under a relatively low electric field of 190 kV/cm. This study underscores the effectiveness of A/B-site aliovalent co-doping as a viable strategy for developing high-performance ceramic capacitors for energy storage applications. [ABSTRACT FROM AUTHOR]

Materials research studies are dealing with a large number of images, which can now be facilitated via image-based machine learning techniques. In this article, we review recent progress of machine learning-driven image recognition and analysis for the materials and chemical domains. First, the image-based machine learning that facilitates the property prediction of chemicals or materials is discussed. Second, the analysis of nanoscale images including those from a scanning electron microscope and a transmission electron microscope is discussed, which is followed by the discussion about the identification of molecular structures via image recognition. Subsequently, the image-based machine learning works to identify and classify various practical materials such as metal, ceramics, and polymers are provided, and the image recognition for a range of real-scenario device applications such as solar cells is provided in detail. Finally, suggestions and future outlook for image-based machine learning for classification and prediction tasks in the materials and chemical science are presented. This article highlights the importance of the integration of the image-based machine learning method into materials and chemical science and calls for a large-scale deployment of image-based machine learning methods for prediction and classification of images in materials and chemical science. [ABSTRACT FROM AUTHOR]

Near-stoichiometric chromium diboride films were subject to in situ annealing inside a scanning transmission electron microscope to access the thermal behavior of the film and embedded structural planar defects. Independent of films' stoichiometry, the planar defects were unaffected by the applied heat treatments. On the contrary, the interfaces between the boron-rich tissue phase and the CrB2 phase were reshaped in the overstoichometric CrB2 film. At high temperatures, diffusion of contact metal species (platinum) from the focused ion beam sample preparation was triggered, with subsequent migration onto the sample. This resulted in the formation of metal-rich regions as directly observed and characterized at the atomic level. We determined that platinum did not react with the diboride structure but is accommodated by various defects present in the film. [ABSTRACT FROM AUTHOR]

The present study focused on the environmentally friendly preparation, characterization, and certain biological activities of gold nanoparticles (Au-NPs) made with the Gundelia tournefortii (kenger) leaf extract. The results from devices such as transmission electron microscope, energy-dispersive x-ray spectrophotometer, and Fourier scanning electron microscope reveal the structures of nanoparticles. After an hour, the greatest surface plasmon resonance was discovered at 532.15 nm. The mean cubic crystallite size was calculated using the powder x-ray diffraction model and was found to be 23.53 nm. It was noted that the produced Au-NPs had spherical forms and hexagonal dimensions and ranged in size from 5 to 40 nm. The minimum inhibitory concentration and a colorimetric method for determining cell metabolic activity test, respectively, were used to investigate the suppressive effects of Au-NPs on the growth of pathogenic bacteria and healthy and cancer cell lines. [ABSTRACT FROM AUTHOR]

This paper describes the influence of vacuum annealing on the mechanical characteristics of silicon (Si) nanowires (NWs) fabricated using focused ion beam (FIB) technologies. Two types of Si NWs having a cross-sectional one-side length or diameter ranging from 19 to 447 nm are prepared using the direct milling and Ga ion doping functions of FIB. The Si NWs prepared are annealed at 400–700 °C in high vacuum for 10 min, followed by quasi-static uniaxial tensile testing using a microelectromechanical system based tensile test system in a scanning electron microscope. All the Si NWs fracture in a brittle manner. Young's modulus of submicrometer-sized Si NWs shows both annealing and specimen size influences in the range from 120 to 170 GPa, whereas that of nano-sized Si NWs shows only annealing influence in the range from 60 to 110 GPa. Tensile strength scatters greatly, ranging from 1.0 to 7.2 GPa, which increases with increasing the NW size. A transmission electron microscope and an atomic force microscope suggest that, by annealing, recrystallization happens in the damaged layer introduced by FIB milling and the NW surface morphology changes due to its recrystallization and gallium (Ga) ion evaporation. Fracture origin is discussed through the comparison between surface roughness and crack length estimated by the Griffith theory of brittle fracture. [ABSTRACT FROM AUTHOR]

Perovskite La2Ti2O7 nanocrystalline powder was obtained through the shock synthesis method. In the study, La2O3 and TiO2 powders were mixed through ball milling and, subsequently, shocked by a flyer at a velocity of 3.2 km/s. After shock treatment, the sample was recovered and characterized via various techniques, such as x-ray diffraction, Raman, scanning electron microscope, transmission electron microscope, and ultraviolet-visible diffuse reflection spectrum analysis, to find the presence of La2Ti2O7 nanocrystalline powder with an average grain size of approximately 30 nm in the recovered samples with different yields. Moreover, the results further confirm that the high shock temperature and long ball milling treatment induce higher activation of precursors to improve the La2Ti2O7 content in the recovered samples. By adjusting the relative density of precursor and the ball milling period, pure La2Ti2O7 nanocrystalline powder was obtained. The formation mechanism of La2Ti2O7 was carefully illustrated. This study presents a new method to synthesize La2Ti2O7 nanocrystalline powder by detonation-driven flyer impact. [ABSTRACT FROM AUTHOR]

Nanobubble formation by radiolysis of water has been observed in a transmission electron microscope using very high electron dose ratios. Here, surface micro- and nanobubbles were formed in situ in a conventional scanning electron microscope at relatively low magnifications of ∼1000. Bubble formation was observed at a low dose ratio of 1.3 × 1011 electrons/cm2 s, which is 5–7 orders of magnitude smaller than in previous studies using transmission electron microscopes. Our observations suggest that the pressure in the liquid cell is one of the important factors for the formation of bubbles. The contact angles obtained by direct observation of 69 bubbles were 50°–90°, with an average of 69°. These angles are much larger than those of the bubbles previously observed using an atomic force microscope. Despite the large contact angles (and large Laplace pressures), the micro- and nanobubble lifetimes were at least three days. Scanning electron microscopy shows potential as an important analytical tool for micro- and nanobubbles. [ABSTRACT FROM AUTHOR]

A silicon p-n junction has been mapped using electron beam induced current in both a scanning transmission electron microscope (STEM) and a conventional scanning electron microscope (SEM). In STEM, the transmission of a higher energy electron beam through the thin specimen leads to better spatial resolution and a more uniform interaction volume than can be achieved in SEM. Better spatial resolution is also achieved in the thin TEM specimens as the diffusion lengths of the minority carriers are much lower than measured in bulk material due to the proximity of specimen surfaces. We further demonstrate that a positive fixed surface charge favors surface recombination of electrons in n-type silicon and induces a surface depletion region in p-type silicon. These results have been compared to off-axis electron holography measurements of the electrostatic potentials and simulations of the internal fields. [ABSTRACT FROM AUTHOR]

Malignant central airway obstruction (MCAO) resulting from tumor metastasis and compression severely impairs respiration, posing life-threatening risks. To address this, we employed a synergistic modification strategy, combining cisplatin (CIS) and silver nanoparticles (AgNPs). Polycaprolactone (PCL) served as a drug carrier, enabling the preparation of a functional CIS@AgNPs@PCL fiber membrane-covered airway stent via electrospinning. This approach aimed to enhance the patency rate of MCAO. Characterization via ATR-FTIR, scanning electron microscope-energy-dispersive spectroscopy, and transmission electron microscope confirmed successful immobilization of CIS and AgNPs onto the stent surface. CIS@AgNPs@PCL substantially suppressed non-small cell lung cancer cells (A549), causing DNA damage, ultrastructural disruption, and over 50% apoptosis in 48 h. It also displayed potent antibacterial activity against Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans biofilms. A mouse subcutaneous tumor recurrence model assessed anti-cancer efficacy. CIS@AgNPs@PCL fiber-covered stents significantly inhibited lung cancer tissue and enhanced anti-cancer effects by up-regulating caspase-3 and Bax, while down-regulating Bcl-2. This study's functional airway stent provides a proof-of-concept for an integrated anti-cancer and antibacterial strategy. It promptly restores the lumen, inhibits biofilm formation, prevents tumor progression, and improves postoperative MCAO patency. [ABSTRACT FROM AUTHOR]

Compared to the bulky ceramic counterparts, antiferroelectric (AFE) thin films exhibit higher energy-storage performance. It has been demonstrated that the performance of the AFE thin films is strongly affected by the thickness. However, possible changes in the phase structures and microstructures accompanied by varying thicknesses have been less known, which makes it hard to fully understand the physical insight of the thickness effect. Herein, we fabricate a series of PbZrO3 (PZ) thin films by chemical solution deposition whose thickness is approximately proportional to the number of deposition layers, i.e., one to four layers with each layer of about 60 nm in thickness. The detailed structural characterization has been performed by using x-ray diffraction, a scanning electron microscope, and a transmission electron microscope. The films are composed of nanorods oriented normal to the films in which each nanorod maintains single crystalline though segmented by the horizontal interfaces between each of the two neighboring layers, suggesting nearly perfect epitaxy during multilayer growth. The multidomain state is found in three-/four-layer PZ thin films while the one-/two-layer PZ films always present a monodomain state. Meanwhile, incommensurate phases with different modulation periods have been observed in the four-layer PZ films. By comparing the as-observed structural features with the measured electrical properties of different films, it seems that microstructural evolution with the film thickness should not be negligible in evaluating the structure–property relation of PZ-based thin films. [ABSTRACT FROM AUTHOR]

Stimulated electron energy loss and gain spectroscopy (sEELS and sEEGS) are used to image the nearfield of the bonding and antibonding localized surface plasmon resonance modes in nanorod dimers. A scanning transmission electron microscope equipped with an optical delivery system is used to simultaneously irradiate plasmonic nanorod dimers while electron energy loss and gain spectra of the active plasmons are collected. The length of the nanorod dimer is varied such that the bonding and antibonding modes are resonant with the laser energy. The optically bright bonding mode is clearly observed in the resonant sEEG spectrum images and, consistent with spontaneous EELS, no direct evidence of the hot spot is observed in sEEG. s-polarized irradiation does not stimulate the energy gain of the optically dark antibonding mode. However, when phase retardation is introduced by tilting the longitudinal axis, the otherwise dark antibonding mode becomes sEEG active. [ABSTRACT FROM AUTHOR]

A BiFeO3 (BFO) film is epitaxially grown on an (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) substrate to investigate the lattice mismatch effect on the domain structure and lattice strain status within the BFO film. Atomic resolution scanning transmission electron microscope image, selected area electron diffraction (SAED) patterns, and X-ray reciprocal space mapping (XRSM) data clearly reveal that the lattice strain originating from the lattice mismatch between BFO and LSAT is relaxed by causing misfit dislocations in the BFO film. The SAED and XRSM data indicate that the crystal structure of BFO film is rhombohedral with the space group R3c. In particular, XRSM data acquired along two different in-plane orientations reveal that the BFO layer consists of two different domains that were 90° off each other with respect to the surface normal orientation. An atomistic model based on the crystal orientation relation found by SAED and XRSM shows that (1) the ferroelectric polarization axes of both domains are 35.6° with respect to the BFO film surface and (2) the two domains are consistent with the so-called 71° (and/or 109°) ferroelectric domains reported previously. The lattice mismatch of ∼2.8% calculated based on the epitaxial relation is proposed to be too large to be stored as elastic strain energy within the BFO layer. [ABSTRACT FROM AUTHOR]

High-angle annular dark-field (HAADF) imaging and electron energy loss spectroscopy (EELS) in a Cs-corrected scanning transmission electron microscope were utilized to analyze the interfacial atomic structure of InAs/GaSb superlattices (SLs) grown by metalorganic chemical vapor deposition (MOCVD) on InAs substrates. Despite high growth temperature, narrow interface (IF) widths of less than 2.5 monolayers (MLs) and 3.8 MLs were extracted from HAADF and EELS, respectively, indicating that the IF quality of MOCVD-grown InAs/GaSb SLs is comparable to those grown by molecular beam epitaxy. GaAs-type IFs are considered to account for the narrow IF width. In addition, GaSb-on-InAs IFs were found to be sharper and more strained than InAs-on-GaSb IFs, which is correlated with the special gas supply and switching sequence during MOCVD growth. The strain profile deduced from the HAADF image suggests that little Sb is incorporated into InAs sublayers and 7% In is incorporated into GaSb sublayers. [ABSTRACT FROM AUTHOR]

Ca9Zn4.5Sb9 Zintl compound is one of the promising thermoelectric materials with excellent performance, which can be further improved by Eu alloying. In this work, a series of Ca9−yEuyZn4.5+δSb9 compounds is investigated by using the advanced Cs-corrected high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) imaging technique at an atomic level. The HAADF-STEM images show that alloying Eu will lead to preferential substitution of Eu atoms at Ca(1) and Ca(5) sites. The further structural analysis and first-principles density functional theory calculation elucidate the origin that the Eu atoms are prone to replace Ca(1) and Ca(5) sites in Ca9−yEuyZn4.5+δSb9. The preferential substitutions may provide a new insight into revealing the structural transformation commonly found in Zintl phase thermoelectric materials. [ABSTRACT FROM AUTHOR]

Ca9Zn4.5Sb9 Zintl compound is one of the promising thermoelectric materials with excellent performance, which can be further improved by Eu alloying. In this work, a series of Ca9−yEuyZn4.5+δSb9 compounds is investigated by using the advanced Cs-corrected high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) imaging technique at an atomic level. The HAADF-STEM images show that alloying Eu will lead to preferential substitution of Eu atoms at Ca(1) and Ca(5) sites. The further structural analysis and first-principles density functional theory calculation elucidate the origin that the Eu atoms are prone to replace Ca(1) and Ca(5) sites in Ca9−yEuyZn4.5+δSb9. The preferential substitutions may provide a new insight into revealing the structural transformation commonly found in Zintl phase thermoelectric materials. [ABSTRACT FROM AUTHOR]

When impinging on optical structures or passing in their vicinity, free electrons can spontaneously emit electromagnetic radiation, a phenomenon generally known as cathodoluminescence. Free-electron radiation comes in many guises: Cherenkov, transition, and Smith–Purcell radiation, but also electron scintillation, commonly referred to as incoherent cathodoluminescence. While those effects have been at the heart of many fundamental discoveries and technological developments in high-energy physics in the past century, their recent demonstration in photonic and nanophotonic systems has attracted a great deal of attention. Those developments arose from predictions that exploit nanophotonics for novel radiation regimes, now becoming accessible thanks to advances in nanofabrication. In general, the proper design of nanophotonic structures can enable shaping, control, and enhancement of free-electron radiation, for any of the above-mentioned effects. Free-electron radiation in nanophotonics opens the way to promising applications, such as widely tunable integrated light sources from x-ray to THz frequencies, miniaturized particle accelerators, and highly sensitive high-energy particle detectors. Here, we review the emerging field of free-electron radiation in nanophotonics. We first present a general, unified framework to describe free-electron light–matter interaction in arbitrary nanophotonic systems. We then show how this framework sheds light on the physical underpinnings of many methods in the field used to control and enhance free-electron radiation. Namely, the framework points to the central role played by the photonic eigenmodes in controlling the output properties of free-electron radiation (e.g., frequency, directionality, and polarization). We then review experimental techniques to characterize free-electron radiation in scanning and transmission electron microscopes, which have emerged as the central platforms for experimental realization of the phenomena described in this review. We further discuss various experimental methods to control and extract spectral, angular, and polarization-resolved information on free-electron radiation. We conclude this review by outlining novel directions for this field, including ultrafast and quantum effects in free-electron radiation, tunable short-wavelength emitters in the ultraviolet and soft x-ray regimes, and free-electron radiation from topological states in photonic crystals. [ABSTRACT FROM AUTHOR]

Images and spectra obtained from aberration corrected scanning transmission electron microscopes (STEM) are now used routinely to quantify the morphology, structure, composition, chemistry, bonding, and optical/electronic properties of nanostructures, interfaces, and defects in many materials/biological systems. However, obtaining quantitative and reproducible atomic resolution observations from some experiments is actually harder with these ground-breaking instrumental capabilities, as the increase in beam current from using the correctors brings with it the potential for electron beam modification of the specimen during image acquisition. This beam effect is even more acute for in situ STEM observations, where the desired outcome being investigated is a result of a series of complicated transients, all of which can be modified in unknown ways by the electron beam. The aim in developing and applying new methods in STEM is, therefore, to focus on more efficient use of the dose that is supplied to the sample and to extract the most information from each image (or set of images). For STEM (and for that matter, all electron/ion/photon scanning systems), one way to achieve this is by sub-sampling the image and using Inpainting algorithms to reconstruct it. By separating final image quality from overall dose in this way and manipulating the dose distribution to be best for the stability of the sample, images can be acquired both faster and with less beam effects. In this paper, the methodology behind sub-sampling and Inpainting is described, and the potential for Inpainting to be applied to novel real time dynamic experiments will be discussed. [ABSTRACT FROM AUTHOR]

The influence of postgrowth thermal annealing on GaSbBi Schottky barrier diodes has been investigated. The effects of the annealing temperature and time on the material quality and electrical characteristics of the diodes have been studied. The I-V characteristics indicated a better ideality factor and less leakage current at the reverse bias, as the annealing temperature increased up to 500 °C for a duration of 30 min. X-ray diffraction and scanning transmission electron microscope measurements were performed to verify that the bismuth composition was unaffected during the annealing process. Energy dispersive x-ray analysis indicated that Sb clustering occurs at high annealing temperatures, resulting in a concomitant degradation in the electrical performance. The optimum electrical characteristics of the diode were obtained with an annealing temperature of 500 °C for 30 min, resulting in an ideality factor of 1.3 being achieved. [ABSTRACT FROM AUTHOR]

High-quality flexible membranes have promoted a myriad of applications in soft electronics or spintronic devices. Nevertheless, magnetic membranes that can withstand strong folding and rolling distortions have rarely been reported. Here, we found that the few to tens of nanometer thick LaMnO3 membranes with single-crystalline qualities exhibit superflexibility, demonstrated by self-folding and rolling into few-micron and sub-micron features. The combined scanning transmission electron microscope and selected area electron diffraction experiments simultaneously confirm the 180° folded single crystalline structure and the associated bending curvature and strain as large as 2 μm−1 and 4%. Furthermore, the scanning electron microscope revealed that as the membrane thickness decreases from 40 to 20 nm and 8 nm, the 180° folding is replaced by self-rolling into few-micron size tubes. Magnetization measurements revealed a large saturation (remnant) magnetization enhancement of 21% (34%) achieved in a macroscopically forced bending state under a similar bending strain of 4.9%. This work demonstrates the superflexibility of manganite oxide membranes which promise superior potential in flexible magnetic device applications. [ABSTRACT FROM AUTHOR]

Good specimen quality is a key factor in achieving successful scanning transmission electron microscope analysis. Thin and damage-free specimens are prerequisites for obtaining atomic-resolution imaging. Topological insulator single crystals and thin films in the chalcogenide family such as Sb2Te3 are sensitive to electron and ion beams. It is, therefore, challenging to prepare a lamella suitable for high-resolution imaging from these topological insulator materials using standard focused ion-beam instruments. We have developed a modified method to fabricate thin focused ion-beam (FIB) lamellae with minimal ion-beam damage and artifacts. The technique described in the current study enables the reliable preparation of high-quality transmission electron microscope (TEM) specimens necessary for studying ultra-thin surface regions. We have successfully demonstrated that the careful selection of FIB milling parameters at each stage minimizes the damage layer without the need for post-treatment. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the degradation mode and mechanism of AlGaN/GaN based high electron mobility transistors (HEMTs) during high temperature operation (HTO) stress. It demonstrates that there was abrupt degradation mode of drain current during HTO stress. The abrupt degradation is ascribed to the formation of crack under the gate which was the result of the brittle fracture of epilayer based on failure analysis. The origin of the mechanical damage under the gate is further investigated and discussed based on top-down scanning electron microscope, cross section transmission electron microscope and energy dispersive x-ray spectroscopy analysis, and stress simulation. Based on the coupled analysis of the failure physical feature and stress simulation considering the coefficient of thermal expansion (CTE) mismatch in different materials in gate metals/semiconductor system, the mechanical damage under the gate is related to mechanical stress induced by CTE mismatch in Au/Ti/Mo/GaN system and stress concentration caused by the localized structural damage at the drain side of the gate edge. These results indicate that mechanical stress induced by CTE mismatch of materials inside the device plays great important role on the reliability of AlGaN/GaN HEMTs during HTO stress. [ABSTRACT FROM AUTHOR]

Nanoscale thermometry, an approach based on non-invasive, yet precise measurements of temperature with nanometer spatial resolution, has emerged as a very active field of research over the last few years. In transmission electron microscopy, nanoscale thermometry is particularly important during in situ experiments or to assess the effects of beam induced heating. In this article, we present a nanoscale thermometry approach based on electron energy-loss spectroscopy in a transmission electron microscope to measure locally the temperature of silicon nanoparticles using the energy shift of the plasmon resonance peak with respect to the zero-loss peak as a function of temperature. We demonstrate that using non-negative matrix factorization and curve fitting of stacked spectra, the temperature accuracy can be improved significantly over previously reported manual fitting approaches. We will discuss the necessary acquisition parameters to achieve a precision of 6 meV to determine the plasmon peak position. [ABSTRACT FROM AUTHOR]

In this paper, a paper-based ultracapacitors were fabricated by the rod-rolling method with the ink of carbon nanomaterials, which were synthesized by arc discharge under various magnetic conditions. Composites of carbon nanostructures, including high-purity single-walled carbon nanotubes (SWCNTs) and graphene flakes were synthesized simultaneously in a magnetically enhanced arc. These two nanostructures have promising electrical properties and synergistic effects in the application of ultracapacitors. Scanning electron microscope, transmission electron microscope, and Raman spectroscopy were employed to characterize the properties of carbon nanostructures and their thin films. The sheet resistance of the SWCNT and composite thin films was also evaluated by four-point probe from room temperature to the cryogenic temperature as low as 90 K. In addition, measurements of cyclic voltammetery and galvanostatic charging/discharging showed the ultracapacitor based on composites possessed a superior specific capacitance of up to 100 F/g, which is around three times higher than the ultracapacitor entirely fabricated with SWCNT. [ABSTRACT FROM AUTHOR]

AlN/GaN/AlN quantum disks (Q-disks) embedded in self-assembled hexagonally shaped GaN nanowires have been grown on the Si (111) substrate by plasma-assisted molecular beam epitaxy. To correlate the structural and optical properties of individual Q-disks inside the nanowire, highly spatially resolved cathodoluminescence (CL) spectroscopy in a scanning transmission electron microscope has been performed at 18 K. CL spectrum linescans along a single nanowire clearly identify the emission from the GaN base around 354 nm as well as two recombination channels at 347 nm and 330 nm directly correlated with the AlN/GaN/AlN Q-disk. A detailed characterization of these individual quantum objects is presented. [ABSTRACT FROM AUTHOR]

Rectangular-shaped oxygen O2+ ion beam thinning for scanning transmission electron microscope (STEM) sample preparation was investigated using a projection ion beam optical system equipped with a duoplasmatron gas ion source. The ion current can be increased by increasing the area of the rectangular-shaped oxygen ion beam and thereby overcome the low brightness of the ion source. Rectangular-shaped O2+ ion beams with different amounts of projection beam edge blur were formed in the X and Y directions by using a limiting mask between the ion source and projection mask. Gallium-contamination-free STEM sample preparation (rough, medium, and fine milling) was demonstrated using a projection ion beam optical system and three types of rectangular-shaped oxygen ion beams. [ABSTRACT FROM AUTHOR]

The rapid growth of the field of time-resolved and ultrafast electron microscopy has been accompanied by the active development of new instrumentation. Recently, time-resolved microscopes equipped with a field emission gun have been introduced, demonstrating great potential for experiments that benefit from the high brightness and coherence of the electron source. Here, we describe a straightforward design of a time-resolved transmission electron microscope with a Schottky field emission gun and characterize its performance. At the same time, our design gives us the flexibility to alternatively operate the instrument as if it was equipped with a flat metal photocathode. We can, thus, effectively choose to sacrifice brightness in order to obtain pulses with vastly larger numbers of electrons than from the emitter if for a given application the number of electrons is a crucial figure of merit. We believe that our straightforward and flexible design will be of great practical relevance to researchers wishing to enter the field. [ABSTRACT FROM AUTHOR]

In this study, a femtosecond laser etching method is proposed to pattern an aluminum nitride (AlN) film, grown by metal-organic chemical vapor deposition on sapphire. Via control of typical pulse energies, the designed pattern was precisely written in the AlN film. The morphology of the patterned structures was characterized using a three-dimensional laser scanning confocal microscope and a scanning electron microscope; crystalline quality and film strain were analyzed using a Raman spectrometer and a transmission electron microscope. The results indicate that consistent morphologies were achieved with only slight changes to the crystalline quality. The tensile stress of the AlN film was released, and the film surface was slightly compressed after laser patterning. Thus, femtosecond etching has the potential to be an in situ patterning method during film growth. [ABSTRACT FROM AUTHOR]

Supercapacitors or electrochemical capacitors are receiving greater interest because of their high-power density, long life, and low maintenance. We have synthesized CuS nanoparticles and graphene oxide (CuS–GO) nanocomposites for supercapacitor applications because of their low cost and excellent electrochemical properties. The phase purity of each material was determined using powder XRD studies. The bandgap was determined by UV-visible spectrophotometric studies. Scanning electron microscope and transmission electron microscope images revealed the nano-scale morphology of the synthesized particles. All the electrochemical measurements were conducted in a standard three-electrode configuration, using a platinum wire as the counter electrode and Hg/HgO as the reference electrode. CuS and its composites with graphene oxide on nickel foam were used as working electrodes. All the electrochemical measurements were performed in 3M KOH solution. The CuS–GO nanocomposite electrode showed a specific capacitance of 250 F/g, 225 F/g, 182 F/g, 166 F/g, 161 F/g, and 158 F/g at a current density of 0.5 A/g, 1 A/g, 5 A/g, 10 A/g, 15 A/g, and 20 A/g, respectively. CuS–GO electrodes showed a specific capacitance retention of 70% after 5000 charge–discharge cycles at a current density of 5 A/g. [ABSTRACT FROM AUTHOR]

A perpendicularly magnetized film is essentially important for spintronics applications, such as spin-transfer-torque type magnetoresistive random access memories, STT-MRAMs, and large perpendicular anisotropy energy, K u , with small saturation magnetization, M s , is required for reducing the critical current for the STT induced magnetization switching phenomenon. In this study, an equiatomic intermetallic compound, MnGaGe, was investigated in epitaxially grown film samples. Perpendicularly magnetized films with an M s value of about 260 emu/cm3 at 300 K were fabricated onto MgO (100) single crystal substrates. The maximum value of K u was 8.1 × 106 erg/cm3 at room temperature. Microstructure analysis using a scanning transmission electron microscope atomically revealed the Cu2Sb type crystal structure in the film sample. First principles calculations were also carried out to discuss the electronic structure. Using the analysis of the projected density of states for 3d-orbitals, the hybridization of the 3d-orbitals was suggested as a possible factor for promoting magnetocrystalline anisotropy. The MnGaGe film with the large K u and relatively small M s would be potentially used for spintronics applications. [ABSTRACT FROM AUTHOR]

Monodispersed magnetite nanoparticles were obtained by investigating the different synthesis parameters of the solvothermal method. The morphology and chemical structure of the Fe3O4 nanoparticles were characterized by Fourier transform infrared spectrometer (FT-IR), X-ray diffraction (XRD), Scanning electron microscope (SEM), Transmission electron microscope (TEM), Dynamic Light Scattering (DLS) and Vibrating sample magnetometer (VSM). Results indicated that without the addition of water, it forms the minor phase of iron instead of magnetite. Furthermore, the size of nanoparticles is tunable in the range of 117.7 to 217.6 nm by changing the amounts of water, PSSMA and NaOH. The amount of PSSMA restricts the growth of the nanoparticles and narrow the size distribution and a hydrophilic surface was obtained. Synthesized magnetite nanoparticles were successfully conjugated with carbon dots. The resulting nanoparticles exhibited good fluorescence characteristics. Cytotoxicity tests confirmed that nanoparticles are non-toxic. Both magnetite and C-dots/Fe3O4 nanoparticles are good potential candidates for biomedical applications specifically for bioimaging and biosensing in the future. [ABSTRACT FROM AUTHOR]

Mapping individual radiative recombination channels at the nanoscale in direct correlation with the underlying crystal structure and composition of III–V semiconductor nanostructures requires unprecedented highly spatially resolved spectroscopy methods. Here, we report on a direct one-by-one correlation between the complex radial structure and the distinct carrier recombination channels of single GaAs-AlGaAs core-multishell nanowire heterostructures using low temperature cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope. Based on an optimized focused ion beam fabrication of the optically active specimen, we directly visualize the radial luminescence evolution and identify four distinct emission lines, i.e., the near band edge and defect luminescence of the GaAs core (819 nm, 837 nm), the emission of the single embedded GaAs quantum well (QW, 785 nm), and the AlGaAs shell luminescence correlated with alloy fluctuations (650–674 nm). The detailed radial luminescence profiles are anticorrelated between QW luminescence and core emission, illustrating the radial carrier transport of the core-shell system. We inspected in detail the low-temperature capture of excess carriers in the quantum well and barriers. [ABSTRACT FROM AUTHOR]

This study investigates the effect of high-temperature (350–500 °C) annealing on CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) directly formed on a flexible polyimide substrate, which has superior thermal tolerance. As the annealing temperature increases, the tunnel magnetoresistance (TMR) ratio enhances and reaches up to ∼200% at an annealing temperature of 450 °C. The annealing temperature dependence is similar to that of MTJs fabricated in the same way on a thermally oxidized silicon substrate. Images taken by a scanning transmission electron microscope confirm the improvement of the crystallization of the CoFeB and MgO layers, which can be an important factor in enhancing the TMR ratio. Furthermore, the endurance of the flexible MTJ against repeated stretching of its substrate is investigated. The TMR ratio shows no change during and after a 1000-cycle application of a tensile strain larger than 1%. The high TMR ratio and strain endurance demonstrated in this study suggest that the flexible MTJ structure is a promising candidate for a future strain-sensing device. [ABSTRACT FROM AUTHOR]

Increasing the process temperature in CO/NH3 plasma etching has been investigated to suppress an etch stop in high-density magnetic tunnel junction stack (MTJ stack) patterning with a Ta mask. In a previous study, the occurrence of an etch stop was observed when using a 100 nm space pattern; specifically, the etching depth was unchanged with the 100 nm space pattern when the etching time was increased, although when wider spaces were used, the etching depth increased proportionally. In this study, differences in the etch stop depth with 100 nm space patterns were examined by changing the electrode temperature from 120 to 300 °C. The etch stop depth became deeper as the electrode temperature was increased and it was found that a high-temperature CO/NH3 process was an effective way to prevent the etch stop effect. However, at 300 °C, the MTJ stack's thickness was observed to expand, which may induce a deterioration of the junction's magnetic properties. A scanning transmission electron microscope image and an energy dispersive x-ray spectroscopy image of the MTJ stack revealed that nitridation of the MTJ stack was the reason of its expansion in thickness. Moreover, pure N2 plasma irradiation of the MTJ stack clearly indicated that this thickness expansion occurred for an electrode temperature of over 265 °C. Finally, the results demonstrate that, for a CO/NH3 process with an electrode temperature of 250 °C, it is possible to etch a 52-nm-thick MTJ stack without either etch stop or film thickness expansion occurring. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides such as molybdenum disulfide (MoS2) may see service in the heart of next-generation nanoelectronic devices, where highly localized power dissipation can produce nontrivial temperature gradients over nanometer-scale distances. Here, we demonstrate that MoS2 is a promising target for plasmon energy expansion thermometry (PEET), a high-spatial resolution temperature mapping technique employed in a scanning transmission electron microscope (STEM) equipped with electron energy loss spectroscopy (EELS). We first use a calibrated, commercial MEMS-style TEM sample heater chip to measure the temperature dependence of the MoS2 bulk plasmon. We corroborate the chip's temperature calibration with Raman thermometry and determine the bulk thermal expansion coefficient (TEC) of MoS2 in the temperature range of 300–1100 K. Applying this TEC value to PEET measurements on a suspended MoS2 flake, we map 70–90 K/μm temperature gradients with a submicrometer spatial resolution. [ABSTRACT FROM AUTHOR]

We investigated the nanostructure of AlGaN/GaN strained-layer superlattice (SLS) cladding in the GaN-based violet laser diode (LD) and the AlInGaN-based ultraviolet (UV) light emitting diode (LED) with a scanning transmission electron microscope (STEM). In the p-SLS cladding, comprising 34 pairs of p-Al0.1Ga0.9N/p-GaN:Mg layers in the GaN-based LD, the Al0.1Ga0.9N and GaN layers were distinguished as dark and bright bands ∼6 nm wide in the high-angle annular dark-field (HAADF) STEM images. Threading dislocations (TDs) were observed. Among TDs that came from the underlying layer, some run outside through the SLS, and the others disappeared within the SLS, which discloses a role of the SLS in suppressing defect propagation. A HAADF-STEM image of the TD with a dark line along the center of a bright contour was found. The dark line, which was darker than the surrounding matrix, is striking. One of the probable explanations for the dark line that may be considered is local segregation of light atoms (Mg or Al) in Cottrell atmosphere around the dislocation core. In the HAADF-STEM image of the UV LED wafer, the AlInGaN and AlInGaN:Si layers in the MQW were definitely resolved, appearing as dark and bright bands. HAADF-STEM also distinguished between the AlGaN and GaN layers in the p-SLS cladding in the UV LED wafer. [ABSTRACT FROM AUTHOR]

98 Mn 0.02) O 3, PSmT fibers were prepared by sol-gel method. The micrographs obtained using scanning electron microscope show that PSmT ceramic fibers are round and dense. The diameter of the fibers was in the range of 30-35µ m. The crystalline grains size is∼ 2.5µ m. The micrographs obtained using transmission electron microscope also unveiled the layer-by-layer 90° domains in the grains. X-ray diffraction patterns of the fibers show that PSmT ceramics have a pure perovskite structure. The c/a ratio of the unit cell was 1.04. The PSmT fiber/epoxy 1-3 composites were fabricated by filling the ceramic fiber bundle with epoxy. [ABSTRACT FROM AUTHOR]

We report electronic transport mapping in a single dielectric layer of a polycrystalline BaTiO3 multilayer ceramic capacitor (MLCC) by electron beam induced current (EBIC) measurements using a scanning transmission electron microscope. Ga+ focused ion beam-lift out techniques with organometallic Pt-deposition are used to extract and electrically connect to these devices while maintaining high (>gigaohm) resistance between electrodes. Different modes of EBIC are observed depending on device resistivity. We demonstrate the use of EBIC resulting from secondary electron emission as a method for performing resistance contrast imaging (RCI), with resistive grain boundaries appearing as steps in EBIC contrast. These RCI maps are also used to calculate the potential and electric field of the device under an arbitrary bias. A mix of high- and low-resistance ohmic as well as rectifying grain boundaries is observed. These results help to better establish the distribution of resistivities critical to the prevention of performance-limiting current leakage in MLCCs. [ABSTRACT FROM AUTHOR]

Several distinct mixing processes and resulting microstructures have recently been reported in multicomponent glasses prepared from multiple metal-organic frameworks. Here, two illustrative examples of multicomponent zeolitic imidazolate framework (ZIF) glasses, the (aTZIF-4-Co)0.5(agZIF-62)0.5 blend and the ag[(ZIF-67)0.2(ZIF-62)0.8] flux melted glass, are studied. These materials are characterized by quantitative X-ray energy dispersive spectroscopy in the scanning transmission electron microscope. By advancing a partial ionization cross section methodology using standards of arbitrary morphology, quantitative nanoscale elemental analysis throughout the glass volume is achieved. In turn, phase diagrams describing the mixing states are presented, offering mechanistic insight into the formation of the observed microstructures. Significant miscibility was observed in ag[(ZIF-67)0.2(ZIF-62)0.8]. These findings establish phase-segregation and interdiffusion as two processes in multicomponent glass formation, which explains the different outcomes observed in blending and flux melting. [ABSTRACT FROM AUTHOR]

Subsampled image acquisition followed by image inpainting in a scanning transmission electron microscope is a novel approach to control dose and increase the image frame rate during experiments, thereby allowing independent control of the spatial and temporal dose envelope during image acquisition. Here, subsampled imaging is shown to permit precise in situ observations of the fundamental kinetic processes behind nucleation and growth of silver (Ag) nanoparticles from an aqueous solution. At high sampling-levels, nanoparticles can be observed with morphologies that are consistent with strong interface interactions, i.e., rafts and pillars, whereas at low sampling-levels, the particles exhibit regular spherical morphologies. The relative numbers of rafts/pillars and regular nanoparticles, their sizes, and their incubation times can be attributed to local changes in the molar concentration of the Ag ions in the aqueous solution; higher sampling-levels significantly increase the reactants in the vicinity of the window, leading to rapid supersaturation and the precipitation on the window surface. These precisely controlled kinetics highlight subsampled imaging as a method by which the driving force for nucleation and growth (i.e., the electron beam) can be disentangled from the spatial/temporal resolution of the observation in all in situ experiments, providing a pathway to identify and quantify the importance of individual kinetic factors behind nucleation and growth in a wide variety of complex materials systems and architectures. [ABSTRACT FROM AUTHOR]

We report on surface plasmons in the visible range in hexagonal close-packed (HCP) gold nanoparticles, synthesized by metastable solidification of Au-Ge eutectic liquid catalyst droplets at the tips of germanium nanowires at the cessation of vapor-liquid-solid nanowire growth. Using 300 kV electrons in a transmission electron microscope in scanning transmission mode with deep subnanometer spatial resolution, we characterize plasmons and interband transitions. Compared to conventional face-centered cubic (FCC) gold nanoparticles, plasmons and interband transitions are observed at different energies in HCP gold nanoparticles. We confirm lattice parameters unique to HCP gold by convergent beam electron diffraction and x-ray diffraction. Energy dispersive spectroscopy measured in a scanning transmission electron microscope verifies minimal impurity concentration in the HCP gold nanoparticles. These experimental results confirm that HCP gold is structurally distinct from FCC plasmonic materials, and also exhibits different electronic and optical properties. [ABSTRACT FROM AUTHOR]

With rapidly increasing numbers of studies of new and exotic material uses for perovskites and quasicrystals, these demand newer instrumentation and simulation developments to resolve the revealed complexities. One such set of observational mechanics at the nanoscale is presented here for somewhat simpler material systems. The expectation is that these approaches will assist those materials scientists and physicists needing to verify atomistic potentials appropriate to the nanomechanical understanding of increasingly complex solids. The five following segments from nine University, National and Industrial Laboratories both review and forecast where some of the important approaches will allow a confirming of how in situ mechanics and nanometric visualization might unravel complex phenomena. These address two-dimensional structures, temporal models for the nanoscale, atomistic and multiscale friction fundamentals, nanoparticle surfaces and interfaces and nanomechanical fracture measurements, all coupled to in situ observational techniques. Rapid future advances in the applicability of such materials science solutions appear guaranteed. [ABSTRACT FROM AUTHOR]

A facile manganese ion Mn(II)-assisted assembly has been designed to fabricate microbowls by using graphene oxide nanosheets as basic building blocks, which were exfoliated ultrasonically from the oxidized soot powders in deionized water. From the morphology evolution observations of transmission electron microscope and scanning electron microscope, a coordinating-tilingcollapsing manner is proposed to interpret the assembly mechanism based on attractive Van der Waals forces, π-π stacking, and capillary action. It is interesting to note that the as-prepared microbowls present a room temperature superparamagnetic behavior. [ABSTRACT FROM AUTHOR]

A silicon-nanocolumn structure was fabricated by using neutral beams and a ferritin-iron-core mask. The etching process was observed by scanning electron microscope and x-ray photoelectron spectroscopy (XPS), which revealed that the iron core can be used as an etching mask for more than 30 min of etching. Extrapolation of the XPS results shows that it is possible to fabricate nanocolumns about 180 nm high. The diameter at the top of the nanocolumns was about 7 nm after etching for 5 min or longer, while it was about 12 nm after etching for 2.5 min. It is thus concluded that the current etching condition is applicable for fabricating nanocolumns about 20–180 nm high and 170-nm-high nanocolumn was fabricated. Densely distributed nanocolumns with a narrow gap of 5.5 nm between them could be etched successfully. To demonstrate that nanocolumns can be used for devices and circuits, a regularly arranged nanocolumn array was fabricated successfully. Etching damage was investigated by transmission electron microscope (TEM) and electron-spin resonance (ESR). The TEM observation revealed that the interior and subsurface of the nanocolumns kept a single-crystal structure completely, and no crystalline defects were observed. A small amount of Pb centers were found by ESR measurement and attributed to silicon dangling bonds at the nanocolumn sidewalls. The dangling-bond density was not increased by neutral-beam etching and stayed low (i.e., 2.7×1011 cm-2) after etching. Damage-free etching was thus achieved by neutral beam. This result may be attributed to the absence of UV photons in the neutral beam. Such a sub-10-nm process with an arranged pattern and low etching damage has great potential in regard to the development of future nanometer-scale devices. [ABSTRACT FROM AUTHOR]

The authors have investigated the dry etch mechanisms of complex poly-Si/TiN/HfO2 gate stacks and the issues that are correlated with the introduction of a thin metal layer in the gate stack. Based on atomic force microscopy (AFM) and scanning electron microscope measurements, they will first show that a mixture of HBr and Cl2 at low rf bias power is required to successfully pattern the TiN layer without damaging the HfO2 gate oxide. Second, it is demonstrated that the introduction of a metal layer in the gate stack prevents charging effects during the last etching steps of the silicon part of the gate. Transmission electron microscope measurements and x-ray photoelectron spectroscopy analyses of the gate sidewalls show that the thickness of the silicon sidewall passivation layer decreases during the O2 free metal etching step potentially inducing silicon gate profile distortion such as notch. However, the notch can be eliminated by etching the Si/TiN gate in a single step process instead of stopping at the TiN surface. Finally, AFM measurements show that during the TiN etching step, a low rf bias power is required to prevent damage (punching through) of the HfO2 layer. However, even under these conditions, a significant silicon recess (oxidation of the c-Si beneath the HfO2 layer) is observed even if TiN is etched in an O2-free chemistry. [ABSTRACT FROM AUTHOR]

Gold nanotubes with SiOx nanowires as sacrificial templates have been synthesized. SiOx nanowires were functionalized by 3-aminopropyl trimethoxysilane that generates a charged surface. The attachment of negatively charged Au nanoparticles was followed. The coverage of Au nanoparticles was initially less than 30%. Further coverage was achieved by the reduction of gold hydroxide to grow the continuous nanoshell on Au nanoparticles, which serve as nucleation sites. The final coverage of Au nanoshells on SiOx nanowires depends strongly on the relative amount of SiOx nanowires in gold hydroxide solution. Both transmission electron microscope and scanning electron microscope images revealed the formation of Au nanotubes with the removal of SiOx nanowires by etching in a dilute HF solution. [ABSTRACT FROM AUTHOR]