Spherical Aberration Correction Research Articles

Development of electrostatic spherical aberration corrector using annular and circular electrodes

The spherical aberration corrector for high‐resolution electron microscopes has been newly designed and constructed. This aberration correction device is formed by an electrostatic lens consisted of only two electrodes having annular and circular holes and a constant‐voltage supply. We call it ‘annular and circular electrodes (ACE) corrector’. The ACE corrector has big advantages in easy alignment and very low‐cost over the conventional Cs‐correctors. In this study, we have constructed the corrector for the scanning transmission electron microsope (STEM) probe‐forming optical system. The developed device is very small (less than several tens millimeter) so that this can be assembled at the tip of the conventional aperture holder and installed in a 200kV‐STEM column. In experiments using specimens of nanocrystals, we have successfully controlled the spatial resolution of the dark‐field STEM images by changing the voltage applied to the corrector and also have imaged lattice fringes. These results clearly demonstrate that our developed device can correct effectively the inherent aberration of the objective lens of the STEM. Copyright © 2016 John Wiley & Sons, Ltd.

Single Cobalt Atoms with Precise N-Coordination as Superior Oxygen Reduction Reaction Catalysts.

A new strategy for achieving stable Co single atoms (SAs) on nitrogen-doped porous carbon with high metal loading over 4 wt % is reported. The strategy is based on a pyrolysis process of predesigned bimetallic Zn/Co metal-organic frameworks, during which Co can be reduced by carbonization of the organic linker and Zn is selectively evaporated away at high temperatures above 800 °C. The spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurements both confirm the atomic dispersion of Co atoms stabilized by as-generated N-doped porous carbon. Surprisingly, the obtained Co-Nx single sites exhibit superior ORR performance with a half-wave potential (0.881 V) that is more positive than commercial Pt/C (0.811 V) and most reported non-precious metal catalysts. Durability tests revealed that the Co single atoms exhibit outstanding chemical stability during electrocatalysis and thermal stability that resists sintering at 900 °C. Our findings open up a new routine for general and practical synthesis of a variety of materials bearing single atoms, which could facilitate new discoveries at the atomic scale in condensed materials.

Single Cobalt Atoms with Precise N‐Coordination as Superior Oxygen Reduction Reaction Catalysts

AbstractA new strategy for achieving stable Co single atoms (SAs) on nitrogen‐doped porous carbon with high metal loading over 4 wt % is reported. The strategy is based on a pyrolysis process of predesigned bimetallic Zn/Co metal–organic frameworks, during which Co can be reduced by carbonization of the organic linker and Zn is selectively evaporated away at high temperatures above 800 °C. The spherical aberration correction electron microscopy and extended X‐ray absorption fine structure measurements both confirm the atomic dispersion of Co atoms stabilized by as‐generated N‐doped porous carbon. Surprisingly, the obtained Co‐Nx single sites exhibit superior ORR performance with a half‐wave potential (0.881 V) that is more positive than commercial Pt/C (0.811 V) and most reported non‐precious metal catalysts. Durability tests revealed that the Co single atoms exhibit outstanding chemical stability during electrocatalysis and thermal stability that resists sintering at 900 °C. Our findings open up a new routine for general and practical synthesis of a variety of materials bearing single atoms, which could facilitate new discoveries at the atomic scale in condensed materials.

Development of high-resolution cathodoluminescence system for STEM and application to plasmonic nanostructures.

A high-resolution cathodoluminescence (CL) system for scanning transmission electron microscope (STEM) has been developed by employing a field emission gun and a spherical aberration corrector, which realizes a probe size of 1 nm even at an accelerating voltage of 80 kV and beam current of the order of 1 nA. Angle resolved measurement of light emission from a sample in the STEM is possible by combining a parabolic mirror and position-controlled pinhole. CL spectra are successively acquired by a highly sensitive charge-coupled device while scanning the incident electron beam or pinhole, which enables various detection modes, i.e. (i) angle resolved spectral pattern, (ii) beam scan spectral image and (iii) photon map. In order to calibrate the acquired spectrum, the correction function is created from the comparison between the observed and theoretical spectra of the transition radiation. Furthermore, the modification of polarization by the parabolic mirror is discussed. Some examples of the applications of the STEM-CL system to plasmonics are presented to demonstrate the unique measurement features of the CL system, i.e. (i) multipole modes in silver nanoparticles, (ii) surface plasmon polariton modes in a 2D plasmonic crystal and (iii) localized surface plasmon modes in a gold bow tie nano-antenna.

High spatial resolution imaging of helium isotope by TOF‐SNMS

Laser ionization mass nanoscope is a time‐of‐flight sputtered neutral mass spectrometer associated with laser post‐ionization by tunneling effect. A spherical and chromatic aberration corrector is installed in the primary ion column. The lateral spatial resolving power of He imaging of solid surface has been evaluated by scanning image using a probe diameter of 90 nm from crater edge slope of a He ion‐implanted Si substrate. Helium distribution from the scanning image is quantitatively equivalent with depth profiling analysis from surface of the same substrate, indicating that spatial resolving power of 20 nm for depth resolution has been achieved on the He scanning image through use of oblique incident effect of the primary beam. Copyright © 2016 John Wiley & Sons, Ltd.

Spherical aberration from trajectories in real and hard-edge solenoid fields

For analytical, real and hard-edge solenoidal axial magnetic fields, the low-energy electron trajectories are obtained using the third-order paraxial ray equation. Using the particle trajectories, it is shown that the spherical aberration in the hard-edge model is high and it increases monotonously with hard edginess, although the focal length converges, in agreement with a recent field and spherical aberration model. The model paved the way for a hard-edge approximation that gives correct focal length and spherical aberration, which is verified here by the trajectory method. In essence, we show that exact hard-edge fields give infinite spherical aberrations.

Effect of focusing condition on molten area characteristics in micro-welding of borosilicate glass by picosecond pulsed laser

The characteristics of the molten area are attributed not only by laser energy condition but also the focusing condition. In this study, a picosecond pulsed laser of 1064 nm in wavelength and 12.5 ps in pulse duration was used as a laser source for joining glass material. Influence of focusing condition on micro-welding of glasses was experimentally investigated by using an objective lens with and without spherical aberration correction, and its molten area was characterized. The usage of objective lens with spherical aberration correction led to a larger molten area inside the bulk material of glass even under the same pulse energy, which related to the efficient micro-welding of glass materials. In addition, an optical system with the spherical aberration correction led to a stable absorption of laser energy inside the bulk glass material, stabilizing the shape of molten area, which resulted in the reliable weld joint. On the other hand, breaking strength of the specimens with spherical aberration correction was higher than that without spherical aberration correction. Therefore, it is concluded that the focusing condition with spherical aberration correction led to the larger and stable molten area, which resulted in higher joining strength in micro-welding of glass materials.

A facile method for the selective decoration of graphene defects based on a galvanic displacement reaction

Inherent defects, such as grain boundaries (GBs), wrinkles and structural cracks, present on chemical vapor deposition (CVD)-grown graphene are inevitable because of the mechanism used for its synthesis. Because graphene defects are detrimental to electrical transport properties and degrade the performance of graphene-based devices, a defect-healing process is required. We report a simple and effective approach for enhancing the electrical properties of graphene by selective graphene-defect decoration with Pd nanoparticles (Pd NPs) using a wet-chemistry-based galvanic displacement reaction. According to the selective nucleation and growth behaviors of Pd NPs on graphene, several types of defects, such as GBs, wrinkles, graphene regions on Cu fatigue cracks and external edges of multiple graphene layers, were precisely confirmed via spherical aberration correction scanning transmission electron microscopy, field-emission scanning electron microscopy and atomic force microscopy imaging. The resultant Pd-NP-decorated graphene films showed improved sheet resistance. A transparent heater was fabricated using Pd-decorated graphene films and exhibited better heating performance than a heater fabricated using pristine graphene. This simple and novel approach promises the selective decoration of defects in CVD-grown graphene and further exploits the visualization of diverse defects on a graphene surface, which can be a versatile method for improving the properties of graphene. Decorating defects with palladium nanoparticles enhances the electrical properties of chemical vapour deposited graphene. Unlike other methods of synthesizing graphene, chemical vapour deposition can produce large areas of the two-dimensional material, but it also tends to create defects. Han-Bo-Ram Lee from Incheon National University and Taeyoon Lee from Yonsei University in South Korea, together with colleagues across the country, have shown that palladium nanoparticles automatically attach themselves to imperfections in chemical vapour deposited graphene, thereby improving its electrical properties. They chemical vapour deposited graphene on copper and dipped it in a palladium chloride solution. The greater chemical reactivity of defect sites in graphene causes palladium atoms to selectively attach to imperfections such as wrinkles, cracks and grain boundaries, creating nanoparticles at these sites. The researchers anticipate that the method could be used in the manufacture of smart mirrors and displays. Selective decoration of palladium nanoparticles on diverse graphene-defect sites. Graphene grown by chemical vapor deposition method has structural imperfections, such as grain boundaries, wrinkles and topological cracks, which cause reduced graphene-based device performance. Herein, we selectively decorated graphene defects with metal nanoparticles (Pd and Ag) by using a wet-chemistry-based galvanic displacement reaction within a few minutes without generating damage on graphene. The defect-decorated graphene showed a noticeable improvement in electrical characteristics, and defect-decorated graphene-based transparent heater showed better heating performance than a heater fabricated using pristine graphene.

Cooled Dyson long-wave infrared push-broom imaging spectrometer by re-imaging

A cooled long-wave infrared push-broom imaging spectrometer with an F-number of 2 was designed based on the Dyson configuration. A three-mirror off-axis aspherical optical system that provided excellent slit-shaped images was selected as the fore telescope objective. The re-imaging method was applied to obtain a cold stop efficiency of 100%, and the corrector lens in traditional Dyson imaging spectrometers was replaced with re-imaging lenses to correct spherical aberrations. The designed imaging spectrometer provided a spectral resolution of 25nm at a range of 8–12μm and possessed a relatively small volume.

Dimensionally stable and light-colored polyimide hybrid reinforced with layered silicate

Light-colored polyimide hybrids with a quaternary alkylammonium modified montmorillonite, Cloisite® 20A (C20A) were successfully synthesized via a solution intercalation method. The structural and morphological features of the PI/C20A hybrids were characterized by Fourier transform infrared (FTIR) spectroscopy, wide-angle X-ray diffraction (WAXD), field emission scanning electron microscopy (FE-SEM), and spherical aberration correction scanning transmission electron microscope (STEM). The dimensional stabilities were strongly dependent upon the chemical and morphological structures which were influenced by the Cloisite® 20A loading content. A critical Cloisite® 20A concentration was obtained for the evolution of both the structural and physical properties of the PI/C20A hybrids. The dynamic stress behaviors were investigated in situ by using the wafer bending method during thermal imidization of the softbaked PI/C20A hybrid precursor and thermally cured hybrid films using a thin film stress analyzer. The PI/C20A hybrid films exhibited stability at 510 °C, high glass transition and relatively low coefficient of thermal expansion, and an improved solvent resistance capacity. Open image in new window

Recent advances in Lorentz microscopy

Lorentz transmission electron microscopy (LTEM) has evolved from a qualitative magnetic domain observation technique to a quantitative technique for the determination of the magnetization state of a sample. In this review article, we describe recent developments in techniques and imaging modes, including the use of spherical aberration correction to improve the spatial resolution of LTEM into the single nanometer range, and novel in situ observation modes. We review recent advances in the modeling of the wave optical magnetic phase shift as well as in the area of phase reconstruction by means of the Transport of Intensity Equation (TIE) approach, and discuss vector field electron tomography, which has emerged as a powerful tool for the 3D reconstruction of magnetization configurations. We conclude this review with a brief overview of recent LTEM applications.

Spherical aberration correction with threefold symmetric line currents

It has been shown that N-fold symmetric line current (henceforth denoted as N-SYLC) produces 2N-pole magnetic fields. In this paper, a threefold symmetric line current (N3-SYLC in short) is proposed for correcting 3rd order spherical aberration of round lenses. N3-SYLC can be realized without using magnetic materials, which makes it free of the problems of hysteresis, inhomogeneity and saturation. We investigate theoretically the basic properties of an N3-SYLC configuration which can in principle be realized by simple wires. By optimizing the parameters of a system with beam energy of 5.5keV, the required excitation current for correcting 3rd order spherical aberration coefficient of 400mm is less than 1AT, and the residual higher order aberrations can be kept sufficiently small to obtain beam size of less than 1nm for initial slopes up to 5mrad.

A1Development of an Aberration corrected 1.2-MV Holography Electron Microscope

A 1.2-MV cold field-emission holography electron microscope equipped with a spherical-aberration corrector has been developed. The microscope has the following superior properties: stabilized accelerating voltage (stability: 0.3 ppm peak to peak), minimized electrical and mechanical fluctuation, and a field emission gun with high stability and brightness [ 1 ]. Information transfer of 43 pm was accomplished by using W{633} chromatic lattice fringes. When this developed FE-TEM was applied to observations of GaN [411] thin samples, the projected Ga atom positions were visualized with 44 pm separation—the smallest separation ever observed. This resolution is an important base performance for effective electron holography observations. The microscope enables performing electromagnetic field observations at high-resolution that were not possible with 300-kV TEMs in various types of research. This research was supported by a grant from the Japan Society for the Promotion of Science (JSPS) through the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program),” initiated by the Council for Science and Technology Policy (CSTP).

A2Installation of Bio-High Voltage Electron Microscope at Korea Basic Science Institute

A 1.2-MV cold field-emission holography electron microscope equipped with a spherical-aberration corrector has been developed. The microscope has the following superior properties: stabilized accelerating voltage (stability: 0.3 ppm peak to peak), minimized electrical and mechanical fluctuation, and a field emission gun with high stability and brightness [1]. Information transfer of 43 pm was accomplished by using W{633} chromatic lattice fringes. When this developed FE-TEM was applied to observations of GaN [411] thin samples, the projected Ga atom positions were visualized with 44 pm separation—the smallest separation ever observed. This resolution is an important base performance for effective electron holography observations. The microscope enables performing electromagnetic field observations at high-resolution that were not possible with 300-kV TEMs in various types of research. This research was supported by a grant from the Japan Society for the Promotion of Science (JSPS) through the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program),” initiated by the Council for Science and Technology Policy (CSTP). [1] K. Kasuya et al, J. Vac. Sci. Technol. B 32, 031802 (2014). Figure: 1.2-MV Holography Electron Microscope

Epitaxial ferromagnetic Fe3Si on GaAs(111)A with atomically smooth surface and interface

Single crystal ferromagnetic Fe3Si(111) films were grown epitaxially on GaAs(111)A by molecular beam epitaxy. These hetero-structures possess extremely low surface roughness of 1.3 Å and interfacial roughness of 1.9 Å, measured by in-situ scanning tunneling microscope and X-ray reflectivity analyses, respectively, showing superior film quality, comparing to those attained on GaAs(001) in previous publications. The atomically smooth interface was revealed by the atomic-resolution Z (atomic number)-contrast scanning transmission electron microscopy (STEM) images using the correction of spherical aberration (Cs)-corrected electron probe. Excellent crystallinity and perfect lattice match were both confirmed by high resolution x-ray diffraction. Measurements of magnetic property for the Fe3Si/GaAs(111) yielded a saturation moment of 990 emu/cm3 with a small coercive field ≤1 Oe at room temperature.

Simulation of the Through-Focus Modulation Transfer Functions According to the Change of Spherical Aberration in Pseudophakic Eyes

To evaluate the effects of spherical aberration (SA) correction on optical quality in pseudophakic eyes, we simulated the optical quality of the human eye by computation of the modulation transfer function (MTF). We reviewed the medical records of patients who underwent cataract surgery in Asan Medical Center, retrospectively. A Zywave aberrometer was used to measure optical aberrations at 1-12 postoperative months in patients with AR40e intraocular lens implants. The MTF was calculated for a 5 mm pupil from measured wavefront aberrations. The area under the MTF curve (aMTF) was analyzed and the maximal aMTF was calculated while changing the SA ( $-0.2{\sim}+0.2{\mu}m$ ) and the defocus (-2.0 ~ +2.0 D). Sixty-four eyes in 51 patients were examined. The maximal aMTF was $6.61{\pm}2.16$ at a defocus of $-0.25{\pm}0.66D$ with innate SA, and $7.64{\pm}2.63$ at a defocus of $0.08{\pm}0.53D$ when the SA was 0 (full correction of SA). With full SA correction, the aMTF increased in 47 eyes (73.4%; Group 1) and decreased in 17 eyes (26.6%; Group 2). There were statistically significant differences in Z(3, -1) (vertical coma; P = 0.01) and Z(4, 4) (tetrafoil; P = 0.04) between the groups. The maximal aMTF was obtained at an SA of $+0.01{\mu}m$ in Group 1 and an SA of $+0.13{\mu}m$ in Group 2. Optical quality can be improved by full correction of SA in most pseudophakic eyes. However, residual SA might provide benefits in eyes with significant radially asymmetric aberrations.

Effects of Focusing Condition on Micro-Welding Characteristics of Borosilicate Glass by Picosecond Pulsed Laser

Glass materials are widely used in products such as optical components and semiconductor devices. In these products, precision welding techniques of glass are required to manufacture small and complicated shape. The laser welding method can perform the joining without an intermediate layer and an adhesive agent. In addition, an ultra-short pulse laser can reduce the heat affected zone with the high space accuracy. However, heating and cooling cycles are repeated even in the case of ultra-short pulsed laser. The temperature distribution and change of molten area are influenced not only by laser energy condition but also focusing condition. Therefore in this study, effects of focusing condition of laser beam on micro-welding characteristics of glass were experimentally investigated by using a picosecond pulsed laser. A usage of object lens with the spherical aberration correction led to a large molten area even at the same pulse energy, which related to the efficient welding of glass materials. An optical system with the spherical aberration correction led to stabilizing the shape of molten area, which resulted in the reliable weld joint.

Correction of depth-induced spherical aberration for deep observation using two-photon excitation fluorescence microscopy with spatial light modulator.

We demonstrate fluorescence imaging with high fluorescence intensity and depth resolution in which depth-induced spherical aberration (SA) caused by refractive-index mismatch between the medium and biological sample is corrected. To reduce the impact of SA, we incorporate a spatial light modulator into a two-photon excitation fluorescence microscope. Consequently, when fluorescent beads in epoxy resin were observed with this method of SA correction, the fluorescence signal of the observed images was ∼27 times higher and extension in the direction of the optical axes was ∼6.5 times shorter at a depth of ∼890 μm. Thus, the proposed method increases the depth observable at high resolution. Further, our results show that the method improved the fluorescence intensity of images of the fluorescent beads and the structure of a biological sample.

Gas-assisted laser cutting of medium-section metals using spherically aberrated beams

We analyzed the effects of the focal point aberrational offset in optical transport systems for high-power lasers. Transverse and near-axial laser intensity distribution transformations in the presence of both positive and negative spherical aberrations were numerically calculated and experimentally demonstrated for different strengths. We show that spherical aberration yields considerable asymmetry of the focused beam’s caustic. Several optical transport systems with identical optical parameters (excluding the noncorrected axial beam spherical aberration) were designed. We examined the effects of the laser intensity profiles produced by these systems on the quality of oxygen-assisted laser cutting of medium-section mild steel. We show that high-quality cuts can be obtained for different shapes of laser intensity distribution. However, the greater the refocusing magnitude introduced by the spherical aberration correction, the more precisely the focal point position must be maintained during the laser cutting process.

可调微流控光学变焦透镜

A new kind of optofluidic varifocal lens driven by hydraulic pressure drop was put forward in this paper. The whole microfluidic device consisting of micro-channels and micro-cavities was made of polydimethylsiloxane( PDMS) material and produced by soft lithography. COMSOL simulation showed the deformations of micro-cavity under different hydraulic pressure drops,forming convex lens and concave lens. The focal lengths and light path diagrams of all sorts of lens were obtained by using the ZEMAX optical simulation software. In the end,an effective method to correct spherical aberration of convex lens was proposed.