Metamorphic GaAs/GaAsBi Heterostructured Nanowires.

GaAs/GaAsBi coaxial multishell nanowires were grown by molecular beam epitaxy. Introducing Bi results in a characteristic nanowire surface morphology with strong roughening. Elemental mappings clearly show the formation of the GaAsBi shell with inhomogeneous Bi distributions within the layer surrounded by the outermost GaAs, having a strong structural disorder at the wire surface. The nanowire exhibits a predominantly ZB structure from the bottom to the middle part. The polytipic WZ structure creates denser twin defects in the upper part than in the bottom and middle parts of the nanowire. We observe room temperature cathodoluminescence from the GaAsBi nanowires with a broad spectral line shape between 1.1 and 1.5 eV, accompanied by multiple peaks. A distinct energy peak at 1.24 eV agrees well with the energy of the reduced GaAsBi alloy band gap by the introduction of 2% Bi. The existence of localized states energetically and spatially dispersed throughout the NW are indicated from the low temperature cathodoluminescence spectra and images, resulting in the observed luminescence spectra characterized by large line widths at low temperatures as well as by the appearance of multiple peaks at high temperatures and for high excitation powers.

Prediction of nanocomposite properties and process optimization using persistent homology and machine learning.

Physical property prediction and synthesis process optimization are key targets in material informatics. In this study, we propose a machine learning approach that utilizes ridge regression to predict the oxygen permeability at fuel cell electrode surfaces and determine the optimal process temperature. These predictions are based on a persistence diagram derived from tomographic images captured using transmission electron microscopy (TEM). Through machine learning analysis of the complex structures present in the Pt/CeO2 nanocomposites, we discovered that l2 regularization considering diverse structural elements is more appropriate than l1 regularization (sparse modeling). Notably, our model successfully captured the activation energy of oxygen permeability, a phenomenon that could not be solely explained by the geometric feature of the Betti numbers, as demonstrated in a previous study. The correspondence between the ridge regression coefficient and persistence diagram revealed the formation process of the local and three-dimensional structures of CeO2 and their contributions to pre-exponential factor and activation energies. This analysis facilitated the determination of the annealing temperature required to achieve the optimal structure and accurately predict the physical properties.

Strain visualization using large-angle convergent-beam electron diffraction.

In this study, we report a strain visualization method using large-angle convergent-beam electron diffraction (LACBED).1 We compare the proposed method with the strain maps acquired via STEM-NBD, a combination of scanning transmission electron microscopy (STEM) and nanobeam electron diffraction (NBD). Although STEM-NBD can precisely measure the lattice parameters, it requires a large amount of data and personal computer (PC) resources to obtain a two-dimensional strain map. Deficiency lines in the transmitted disk of LACBED reflect the crystalline structure information and move, curve, or disappear in the deformed area. Properly setting the optical conditions makes it possible to acquire real-space images over a broad area in conjunction with deficiency lines on the transmitted disk. The proposed method acquires images by changing the relative position between the specimen and the deficiency line and can grasp the strain information with a small number of images. In addition, the proposed method does not require high-resolution images. It can reduce the required PC memory or storage consumption in comparison with that of STEM-NBD, which requires a high-resolution diffraction pattern (DP) from each point of the region of interest. Compared with the two-dimensional maps of LACBED and NBD, NBD could detect large distortions in the area where the deficiency line curved, moved, or disappeared. The curving or moving direction of the deficiency line is qualitatively consistent with the NBD results. If quantitative strain values are not essential, strain visualization using LACBED can be considered an effective technique. We believe that the strain information of a sample can be obtained effectively using both methods.

Unsupervised machine learning combined with 4D scanning transmission electron microscopy for bimodal nanostructural analysis.

Unsupervised machine learning techniques have been combined with scanning transmission electron microscopy (STEM) to enable comprehensive crystal structure analysis with nanometer spatial resolution. In this study, we investigated large-scale data obtained by four-dimensional (4D) STEM using dimensionality reduction techniques such as non-negative matrix factorization (NMF) and hierarchical clustering with various optimization methods. We developed software scripts incorporating knowledge of electron diffraction and STEM imaging for data preprocessing, NMF, and hierarchical clustering. Hierarchical clustering was performed using cross-correlation instead of conventional Euclidean distances, resulting in rotation-corrected diffractions and shift-corrected maps of major components. An experimental analysis was conducted on a high-pressure-annealed metallic glass, Zr-Cu-Al, revealing an amorphous matrix and crystalline precipitates with an average diameter of approximately 7 nm, which were challenging to detect using conventional STEM techniques. Combining 4D-STEM and optimized unsupervised machine learning enables comprehensive bimodal (i.e., spatial and reciprocal) analyses of material nanostructures.

Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin film.

Lattice strain effects on the piezoelectric properties of crystalline ferroelectrics have been extensively studied for decades; however, the strain dependence of the piezoelectric properties at nano-level has yet to be investigated. Herein, a new overview of the super-strain of nanoporous polycrystalline ferroelectrics is reported for the first time using a nanoengineered barium calcium zirconium titanate composition (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 (BCZT). Atomic-level investigations show that the controlled pore wall thickness contributes to highly strained lattice structures that also retain the crystal size at the optimal value (<30 nm), which is the primary contributor to high piezoelectricity. The strain field derived from geometric phase analysis at the atomic level and aberration-corrected high-resolution scanning transmission electron microscopy (STEM) yields of over 30% clearly show theoretical agreement with high piezoelectric properties. The uniqueness of this work is the simplicity of the synthesis; moreover the piezoresponse d 33 becomes giant, at around 7500 pm V-1. This response is an order of magnitude greater than that of lead zirconate titanate (PZT), which is known to be the most successful ferroelectric over the past 50 years. This concept utilizing nanoporous BCZT will be highly useful for a promising high-density electrolyte-free dielectric capacitor and generator for energy harvesting in the future.

Novel image processing method inspired by wavelet transform.

I developed a novel image processing method inspired by wavelet transform that uses a concentric mother wavelet. This method can be used for noise filtering, feature extraction, image enhancement, and other applications and is effective for images with low spatial frequencies. When filtering small-sized images, setting masks for Fourier filtering is often difficult; however, the proposed method does not require mask setting, thus facilitating filtering. In practice, many concentric mother wavelets of different frequencies are prepared in advance, and the cross-correlation between the input image and prepared wavelets is calculated. Reconstruction or image processing can be easily performed by combining the obtained results for each frequency. I believe that the present method can aid image processing related to material analysis.

Localised surface plasmon resonance inducing cooperative Jahn-Teller effect for crystal phase-change in a nanocrystal.

The Jahn-Teller effect, a phase transition phenomenon involving the spontaneous breakdown of symmetry in molecules and crystals, causes important physical and chemical changes that affect various fields of science. In this study, we discovered that localised surface plasmon resonance (LSPR) induced the cooperative Jahn-Teller effect in covellite CuS nanocrystals (NCs), causing metastable displacive ion movements. Electron diffraction measurements under photo illumination, ultrafast time-resolved electron diffraction analyses, and theoretical calculations of semiconductive plasmonic CuS NCs showed that metastable displacive ion movements due to the LSPR-induced cooperative Jahn-Teller effect delayed the relaxation of LSPR in the microsecond region. Furthermore, the displacive ion movements caused photo-switching of the conductivity in CuS NC films at room temperature (22 °C), such as in transparent variable resistance infrared sensors. This study pushes the limits of plasmonics from tentative control of collective oscillation to metastable crystal structure manipulation.

Discovery of a metastable van der Waals semiconductor via polymorphic crystallization of an amorphous film.

Here we report on the growth of thin crystalline films of the metastable phase GeTe2. Direct observation by transmission electron microscopy revealed a Te-Ge-Te stacking with van der Waals gaps. Moreover, electrical and optical measurements revealed the films exhibted semiconducting properties commensurate with electronics applications. Feasibility studies in which device structures were fabricated demonstrated the potential application of GeTe2 as an electronic material.

Classification for transmission electron microscope images from different amorphous states using persistent homology.

It is difficult to discriminate the amorphous state using a transmission electron microscope (TEM). We discriminated different amorphous states on TEM images using persistent homology, which is a mathematical analysis technique that employs the homology concept and focuses on 'holes'. The structural models of the different amorphous states, that is, amorphous and liquid states, were created using classical molecular dynamic simulation. TEM images in several defocus conditions were simulated by the multi-slice method using the created amorphous and liquid states, and their persistent diagrams were calculated. Finally, logistic regression and support vector classification machine learning algorithms were applied for discrimination. Consequently, we found that the amorphous and liquid phases can be discriminated by more than 85%. Because the contrast of TEM images depends on sample thickness, focus, lens aberration, etc., radial distribution function cannot be classified; however, the persistent homology can discriminate different amorphous states in a wide focus range.

Non-negative matrix factorization for mining big data obtained using four-dimensional scanning transmission electron microscopy.

Scientific instruments for material characterization have recently been improved to yield big data. For instance, scanning transmission electron microscopy (STEM) allows us to acquire many diffraction patterns from a scanning area, which is referred to as four-dimensional (4D) STEM. Here we study a combination of 4D-STEM and a statistical technique called non-negative matrix factorization (NMF) to deduce sparse diffraction patterns from a 4D-STEM data consisting of 10,000 diffraction patterns. Titanium oxide nanosheets are analyzed using this combined technique, and we discriminate the two diffraction patterns from pristine TiO2 and reduced Ti2O3 areas, where the latter is due to topotactic reduction induced by electron irradiation. The combination of NMF and 4D-STEM is expected to become a standard characterization technique for a wide range materials.

Semiconductor nanochannels in metallic carbon nanotubes by thermomechanical chirality alteration.

Carbon nanotubes have a helical structure wherein the chirality determines whether they are metallic or semiconducting. Using in situ transmission electron microscopy, we applied heating and mechanical strain to alter the local chirality and thereby control the electronic properties of individual single-wall carbon nanotubes. A transition trend toward a larger chiral angle region was observed and explained in terms of orientation-dependent dislocation formation energy. A controlled metal-to-semiconductor transition was realized to create nanotube transistors with a semiconducting nanotube channel covalently bonded between a metallic nanotube source and drain. Additionally, quantum transport at room temperature was demonstrated for the fabricated nanotube transistors with a channel length as short as 2.8 nanometers.

Electrical conduction and field emission of a single-crystalline GdB44Si2 nanowire.

The electronic transport and field emission properties of a single-crystalline GdB44Si2 nanowire are studied. The atomic structure and elemental composition of the GdB44Si2 nanowire are characterized by transmission electron microscopy (TEM) using atomic imaging, energy-dispersive X-ray spectroscopy (EDS), and electron energy-loss spectroscopic (EELS) mapping. The electrical conductivity of the single GdB44Si2 nanowire is in the range of 46.8-60.1 S m-1. The in situ TEM field emission measurement reveals that it has a low work function of 2.4 eV. To realize a converged electron emission, a field evaporation pretreatment was used to clean the emission surface and to make a sharpened tip. The field emission probe measurement results show that the electron emission from the sharp GdB44Si2 nanowire is converged to a single field emission spot and it has a work function of 2.6 eV which is in agreement with the in situ TEM measurement. The stability of field emission current is also very good with a fluctuation of 1.4% in 20 min. With a low work function and stable emission current, the GdB44Si2 nanowire shows great promise for field emission applications.

A controllable and efficient method for the fabrication of a single HfC nanowire field-emission point electron source aided by low keV FIB milling.

A single hafnium carbide (HfC) nanowire field-induced electron emitter with a sharp tip apex is fabricated by Pt deposition and focused ion beam (FIB) milling. The structure of the electron emitter is characterized by scanning transmission electron microscopy (STEM) and atom probe tomography (APT). The HfC nanowire is single-crystalline with a thin oxide layer on its tip surface. The field emission properties are determined by using both in situ transmission electron microscopy (TEM) and a field-emission probe in a high-vacuum chamber. A high current of 173 nA was obtained at a low extraction voltage of 631 V with an emission gap of 5 mm. The emission current is stable at 60 nA for 100 min with a fluctuation of 0.7%. The deduced work function was 3.1 eV. It is suggested that the implanted Ga ions and the oxide layer induce more downward dipoles that are beneficial for lowering the work function and creating a stable surface. When the low keV FIB processing is applied, it takes within 30 minutes to finish a HfC nanowire emitter, establishing an efficient procedure for the preparation of nanowire emitters. These results provide a controllable and fast production method for the fabrication of single nanowire field-emission point electron sources.

Reversible tactile hypoesthesia associated with myofascial trigger points: a pilot study on prevalence and clinical implications.

INTRODUCTION: Tactile hypoesthesia observed in patients with myofascial pain syndrome (MPS) is sometimes reversible when pain is relieved by trigger point injections (TPIs). We aimed to investigate the prevalence of such reversible hypoesthesia during TPI therapy and topographical relations between areas of tactile hypoesthesia and myofascial trigger points (MTrP) in patients with MPS. METHODS: Forty-six consecutive patients with MTrP were enrolled in this study. We closely observed changes in areas of tactile hypoesthesia in patients who had tactile hypoesthesia at the first visit, and throughout TPI therapy. Tactile stimulation was given using cotton swabs, and the areas of tactile hypoesthesia were delineated with an aqueous marker and recorded in photographs. RESULTS: A reduction in the size of hypoesthetic area with TPI was observed in 27 (58.7%) patients. All the 27 patients experienced a reduction in pain intensity by more than 50% in a numerical rating scale score through TPI therapy. In 9 patients, the reduction in the sizes of hypoesthetic areas occurred 10 minutes after TPI. Complete disappearance of tactile hypoesthesia after TPI therapy was observed in 6 of the 27 patients. Myofascial trigger points were located in the muscles in the vicinity of ipsilateral cutaneous dermatomes to which the hypoesthetic areas belonged. CONCLUSION: Our results indicate a relatively high prevalence of reversible tactile hypoesthesia in patients with MPS. Mapping of tactile hypoesthetic areas seems clinically useful for detecting MTrP. In addition, treating MTrP with TPI may be important for distinguishing tactile hypoesthesia associated with MPS from that with neuropathic pain.

Two-dimensional Gaussian fitting for precise measurement of lattice constant deviation from a selected-area diffraction map.

Unlike X-ray diffraction or Raman techniques, which suffer from low spatial resolution, transmission electron microscopy can be used to obtain strain maps of nanoscaled materials and devices. Convergent-beam electron diffraction (CBED) and nanobeam electron diffraction (NBED) techniques detect the deviation of a lattice constant (i.e. an indicator of strain) within 0.01%; however, their use is restricted to beam-insensitive samples. Selected-area electron diffraction (SAED) does not have such limitations but has low spatial resolution and precision. The use of a spherical aberration corrector and a nanosized selected-area aperture improves the spatial resolution, but the precision is still low. In this study, a two-dimensional stage-scanning system is used to acquire arrays of diffraction patterns at different positions of the sample under fixed beam conditions. Data processing with iterative nonlinear least-squares fitting enabled the spot displacement for each point of the scan area to be measured with precision comparable to that of the CBED or NBED technique. The precise strain determination, in combination with the simplicity of the measurement process, makes the nanosized SAED technique competitive with other methods for strain mapping at nanoscale dimensions.

Chirality transitions and transport properties of individual few-walled carbon nanotubes as revealed by in situ TEM probing.

Physical properties of carbon nanotubes (CNTs) are closely related to the atomic structure, i.e. the chirality. It is highly desirable to develop a technique to modify their chirality and control the resultant transport properties. Herein, we present an in situ transmission electron microscopy (TEM) probing method to monitor the chirality transition and transport properties of individual few-walled CNTs. The changes of tube structure including the chirality are stimulated by programmed bias pulses and associated Joule heating. The chirality change of each shell is analyzed by nanobeam electron diffraction. Supported by molecular dynamics simulations, a preferred chirality transition path is identified, consistent with the Stone-Wales defect formation and dislocation sliding mechanism. The electronic transport properties are measured along with the structural changes, via fabricating transistors using the individual nanotubes as the suspended channels. Metal-to-semiconductor transitions are observed along with the chirality changes as confirmed by both the electron diffraction and electrical measurements. Apart from providing an alternative route to control the chirality of CNTs, the present work demonstrates the rare possibility of obtaining the dynamic structure-properties relationships at the atomic and molecular levels.

Real-space analysis of diffusion behavior and activation energy of individual monatomic ions in a liquid.

Investigation of the local dynamic behavior of atoms and molecules in liquids is crucial for revealing the origin of macroscopic liquid properties. Therefore, direct imaging of single atoms to understand their motions in liquids is desirable. Ionic liquids have been studied for various applications, in which they are used as electrolytes or solvents. However, atomic-scale diffusion and relaxation processes in ionic liquids have never been observed experimentally. We directly observe the motion of individual monatomic ions in an ionic liquid using scanning transmission electron microscopy (STEM) and reveal that the ions diffuse by a cage-jump mechanism. Moreover, we estimate the diffusion coefficient and activation energy for the diffusive jumps from the STEM images, which connect the atomic-scale dynamics to macroscopic liquid properties. Our method is the only available means to observe the motion, reactions, and energy barriers of atoms/molecules in liquids.

Compositional tuning in sputter-grown highly-oriented Bi-Te films and their optical and electronic structures.

Growth of Bi-Te films by helicon-wave magnetron sputtering is systematically explored using alloy targets. The film compositions obtained are found to strongly depend on both the sputtering and antenna-coil powers. The obtainable film compositions range from Bi55Te45 to Bi43Te57 when a Bi2Te3 alloy target is used, and from Bi42Te58 to Bi40Te60 (Bi2Te3) for a Te-rich Bi30Te70 target. All films show strong orientation of the van der Waals layers (00l planes) parallel to the substrate. The atomic level stacking of Bi2Te3 quintuple and Bi bi-layers has been directly observed by high resolution transmission electron microscopy. Band structure simulations reveal that Bi-rich Bi4Te3 bulk is a zero band gap semimetal with a Dirac cone at the Gamma point when spin-orbit coupling is included. Optical measurements also confirm that the material has a zero band gap. The tunability of the composition and the topological insulating properties of the layers will enable the use of these materials for future electronics applications on an industrial scale.

[Evaluation of facial nerve palsy with electrogustometry and stapedial reflex].

BACKGROUND: To clarify the prognosis of facial nerve palsy, electroneuronal tests, including electrogustometry and stapedial reflex, have been utilized. But, the relationship among these tests and patients' prognosis is not clear. METHODS: Sixty five patients with peripheral facial nerve palsy were investigated. Electrogustometry (EG), stapedial reflex (SR) and blink reflex (BR) were performed at the first visit on the consult of facial nerve palsy. The palsy scale (full score is 100 points) was evaluated 8 weeks after the onset, and we defined cure cases if score became over 90 points. The cure ratio was examined on each group of EG positive or negative, SR positive or negative and BR R1 wave positive or negative, respectively. The relationship among these three examinations was also investigated. RESULTS: There were no significant differences between the cure ratio of EG positive and negative groups. However, the cure ratio of SR positive group was significantly larger than that of SR negative group. The cure ratio of BR positive group was 100%, regardless of the result of other two tests. CONCLUSIONS: The present study suggests that SR is a more useful parameter than EG for the evaluation of the prognosis of patients with facial nerve palsy, and the pathological meaning of these 2 tests is different from that of BR.

[Effect of propofol on visual evoked potentials during neurosurgery].

BACKGROUND: Evoked potentials are used to monitor the central nervous system during neurosurgery and it is well known that they are affected by the depth of anesthesia. Many studies on the evoked potential like somatosensory evoked potential (SEP) and auditory brain stem response (ABR) are reported, but studies on visual evoked potential (VEP) are few. We investigated the influence of the propofol concentration on VEP in neurosurgical patients. METHODS: Seven patients scheduled for neurosurgery, three with cranial aneurysm and four with brain tumor, were studied. Anesthesia was maintained with intravenous propofol using target controlled infusion (TCI). We measured the change of amplitude and latency of VEP at three propofol concentrations (effect site concentrations of 1.5, 2.0 and 3.0 microg x ml(-1)), and also evaluated bispectral index (BIS) at each propofol concentration. RESULTS: Amplitude of VEP at 3.0 microg x ml(-1) propofol concentration decreased significantly compared with the amplitude at 1.5 microg x ml(-1) concentration. No significant change was observed with the latency of VEP. The value of BIS at 3.0 microg x ml(-1) propofol concentration also decreased significantly compared with 2.0 microg x ml(-1) concentration. CONCLUSIONS: Amplitude of VEP is strongly affected by the concentration of propofol. Caution should be taken in evaluating VEP in patients undergoing propofol anesthesia.