Fumio Hosokawa

Visualization of Light Elements at Ultrahigh Resolution by STEM Annular Bright Field Microscopy

In the field of materials sciences such as studies on ceramics, semi-conducting material and metals, role of light elements is important, because it is one of mainly composing elements or determiner of character i. e. dopants. The light elements at high resolution have been observed by ultrahigh voltage electron microscopy or aberration corrected electron microscopy in Transmission Electron Microscopy (TEM), since the visualization of light requires highly resolving power. Recently, a high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) has become widely used in this field because of high-resolution capability and easily interpretable image contrast, which is roughly proportional to square of atomic number Z (Z). However, the HAADF image sometimes gives lack of light element because of excess contrast originated from Z, when the specimen contains the light and heavy elements. The TEM bright field imaging gives an image contrast roughly proportional to the Z, when the specimen is thin enough to be able to apply the ‘thin film approximation’. We have examined to apply an STEM annular bright field (ABF) imaging, which is equivalent to TEM hollow cone illumination imaging technique [1-2], to the oxide or nitride samples for simultaneous visualization of light and heavier elements. According to the article on hollow cone illumination in TEM [2], the contrast transfer in ABF expected to give better resolution than conventional BF STEM and to give non-oscillating contrast transfer, which gives easily-interpretable images unlike the BF STEM. This paper reports characteristics and the experimental result of the ABF imaging technique.

STEM imaging of 47-pm-separated atomic columns by a spherical aberration-corrected electron microscope with a 300-kV cold field emission gun

A spherical aberration-corrected electron microscope has been developed recently, which is equipped with a 300-kV cold field emission gun and an objective lens of a small chromatic aberration coefficient. A dumbbell image of 47 pm spacing, corresponding to a pair of atomic columns of germanium aligned along the [114] direction, is resolved in high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) with a 0.4-eV energy spread of the electron beam. The observed image was compared with a simulated image obtained by dynamical calculation.

Direct imaging of lithium atoms in LiV2O4 by spherical aberration-corrected electron microscopy

We visualized lithium atom columns in LiV₂O₄ crystals by combining scanning transmission electron microscopy with annular bright field (ABF) imaging using a spherical aberration-corrected electron microscope (R005) viewed from the [110] direction. The incident electron beam was coherent with a convergent angle of 30 mrad (semi-angle), and the detector collected scattered electrons over 20-30 mrad (semi-angle). The ABF image showed dark dots corresponding to lithium, vanadium and oxygen columns.

Performance of low-voltage STEM/TEM with delta corrector and cold field emission gun

To reduce radiation damage caused by the electron beam and to obtain high-contrast images of specimens, we have developed a highly stabilized transmission electron microscope equipped with a cold field emission gun and spherical aberration correctors for image- and probe-forming systems, which operates at lower acceleration voltages than conventional transmission electron microscopes. A delta-type aberration corrector is designed to simultaneously compensate for third-order spherical aberration and fifth-order 6-fold astigmatism. Both were successfully compensated in both scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) modes in the range 30-60 kV. The Fourier transforms of raw high-angle annular dark field (HAADF) images of a Si[110] sample revealed spots corresponding to lattice spacings of 111 and 96 pm at 30 and 60 kV, respectively, and those of raw TEM images of an amorphous Ge film with gold particles showed spots corresponding to spacings of 91 and 79 pm at 30 and 60 kV, respectively. Er@C(82)-doped single-walled carbon nanotubes, which are carbon-based samples, were successfully observed by HAADF-STEM imaging with an atomic-level resolution.

Measurement method of aberration from Ronchigram by autocorrelation function

Aberrations up to the fifth-order were successfully measured using an autocorrelation function of the segmental areas of a Ronchigram. The method applied to aberration measurement in a newly developed 300kV microscope that is equipped with a spherical aberration corrector for probe-forming systems. The experimental Ronchigram agreed well with the simulated Ronchigram that was calculated by using the measured aberrations. The Ronchigram had an infinite magnification area with a half-angle of 50mrad, corresponding to the convergence angle of a uniform phase.

Achieving 63 pm Resolution in Scanning Transmission Electron Microscope with Spherical Aberration Corrector

The performance of a newly developed high-resolution 300 kV microscope equipped with a spherical aberration corrector for probe-forming systems is reported. This microscope gave the highest resolution for the distance between atomic columns, as determined by a high-angle annular dark field imaging method using a GaN[211] crystalline specimen, where the distance between the neighboring columns of Ga was 63 pm.

Development of Cs and Cc correctors for transmission electron microscopy

Novel spherical aberration (Cs) and chromatic aberration (Cc) correctors, which correct aberrations using a new principle, were developed. The asymmetric Cs correctors were designed for use in the probe- and image-forming systems at 300 kV to diminish undesired parasitic aberrations. The correctors composed of non-equivalent multipoles connecting with a demagnifying transfer doublet in the system. The axial aberrations were corrected well up to the fifth order except 6-fold astigmatism (A(6)) experimentally. Next, we developed superior Cs correctors for probe- and image-forming systems of low voltage microscope that uses triple dodecapoles to correct 6-fold astigmatism (A(6)). An important feature of this system is the rotation of the 3-fold astigmatism azimuth at the second dodecapole. The optimum rotation of the three hexapole fields for the compensation of A(6) was derived from theoretical calculations. The experimental results confirmed the compensation of A(6) and the third-order Cs. Finally, a unique Cc corrector, which utilized the concave lens effect formed by a long quadrupole field, was designed. The performance of the Cc corrector was investigated using a 30-kV transmission electron microscope. The results confirmed that Cc correction was achieved.

Aberration-corrected STEM/TEM imaging at 15 kV

The performance of aberration-corrected (scanning) transmission electron microscopy (S/TEM) at an accelerating voltage of 15kV was evaluated in a low-voltage microscope equipped with a cold-field emission gun and a higher-order aberration corrector. Aberrations up to the fifth order were corrected by the aberration measurement and auto-correction system using the diffractogram tableau method in TEM and Ronchigram analysis in STEM. TEM observation of nanometer-sized particles demonstrated that aberrations up to an angle of 50mrad were compensated. A TEM image of Si[110] exhibited lattice fringes with a spacing of 0.192nm, and the power spectrum of the image showed spots corresponding to distances of 0.111nm. An annular dark-field STEM image of Si[110] showed lattice fringes of (111) and (22¯0) planes corresponding to lattice distances of 0.314nm and 0.192nm, respectively. At an accelerating voltage of 15kV, the developed low-voltage microscope achieved atomic-resolution imaging with a small chromatic aberration and a large uniform phase.

Counting lithium ions in the diffusion channel of an LiV2O4 crystal

As a new microscopic method to reveal lithium ion behavior in lithium ion batteries, we demonstrated that lithium atoms in the diffusion channel of the spinel structure (LiV2O4 crystal) were visualized and their number was countable one-by-one by using annular bright field imaging method in conjunction with a spherical aberration corrected electron microscope: the lithium column intensity varied by a step of single lithium atom in correlation with the thickness change of the LiV2O4 crystal, in accordance with theoretical image simulations.

Resolving 45-pm-separated Si–Si atomic columns with an aberration-corrected STEM

Si-Si atomic columns separated by 45 pm were successfully resolved with a 300-kV aberration-corrected scanning transmission electron microscope (STEM) equipped with a cold-field emission gun. Using a sufficiently small Gaussian effective source size and a 0.4-eV energy spread at 300 kV, the focused electron probe on the specimen was simulated to be sub-50 pm. Image simulation showed that the present probe condition was sufficient to resolve 45 pm Si-Si dumbbells. A silicon crystalline specimen was observed from the [114] direction with a high-angle annular dark field STEM and the intensity profile showed 45 pm separation. A spot corresponding to (45 pm)(-1) was confirmed in the power spectrum of the Fourier transform.