The precise determination of three-dimensional (3D) atomic coordinates is the cornerstone of materials analysis, but conventional transmission electron microscopy is limited to visualizing only two-dimensional projections. The direct observation of the true 3D atomic arrangement has, therefore, remained an elusive goal in materials science. Atomic-resolution holography is a powerful technique capable of analyzing the 3D atomic arrangement, even around isolated atoms. However, it typically requires allocated time at large-scale synchrotron radiation facilities, which has hindered its widespread adoption and integration into routine materials characterization workflows. Here, we show the realization of an atom-holography microscope that breaks this fundamental constraint, enabling atomic-resolution holography to be performed anytime and anywhere. Our novel microscope integrates a hologram-display electron spectrometer and a Scanning Electron Microscope (SEM), allowing for the visualization of the 3D atomic arrangement within a microscopic region in a SEM image. As a proof of concept, we analyzed a Kikuchi electron hologram obtained from a Bi2Se3 sample and successfully reconstructed the atomic arrangement with an accuracy of 0.1 Å. This result demonstrates that advanced analytical microscope capable of mapping the composition and 3D atomic arrangement of each pixel in a SEM image is now available anytime, anywhere, which is expected to significantly accelerate the development of highly functional materials and microdevices.

Three-dimensional (3D) local atomic structure around specific functional atom plays a crucial role in functional materials, but it has not been able to be analyzed by a standard structure analysis method of x-ray diffraction (XRD) because this kind of small amount of atom has no translational symmetry. Recently several atomic-resolution holography methods have been developed, which can directly display the 3D atomic structure around this kind of specific atoms with no translational symmetry. These are the 'photoelectron holography', 'x-ray fluorescence holography', and 'CTR holography'. Their accuracy improved dramatically by the development of new analysis codes and sensitive analyzers and detectors. A new technique of direct 3D atomic structure analysis method 'stereography of atomic arrangement' has also been developed. These techniques received renewed attention recently, and a project '3D Active-Site Science' of JSPS Grant-in-Aid for Scientific Research on Innovative Areas has been pursued for 5 years. This article reviews principles and some results of atomic-resolution holography methods.

A compact wide-acceptance angle (around +/- 50 degrees) high-energy-resolution 2D electron analyzer CoDELMA (Compact DELMA) is proposed, constructed and tested. CoDELMA is composed of a recently-proposed variable -deceleration-ratio wide-acceptance-angle electrostatic lens (VD-WAAEL), a cylindrical mirror analyzer (CMA) and a projection lens. After a brief review of VD-WAAEL, the use of a plane grid in the lens is presented as a new design option, which allows keeping the focusing angle almost constant irrespective of the deceleration ratio. A detailed consideration is given to the combination of CMA and a projection lens, which allows simultaneous analysis of wide reciprocal ( +/- 8 degrees) and real space ( +/- 5 mm) at the entrance of CMA. CoDELMA is the successor to the previously developed DELMA instrument including a concentric hemispherical analyzer. The whole system of CoDELMA becomes much more compact than that of DELMA owing to the use of VD-WAAEL and CMA, and this results in considerable reduction of the production cost. Results of test measurement of energy spectrum and angular distribution using an electron gun as an excitation source confirmed that the energy resolution and the acceptance angle are almost the same as the designed values.

The reason for the absence of superconductivity in Sr2IrO4 was estimated by photoelectron spectra and photoelectron holograms. The analysis of the La photoelectron hologram concluded that La atoms are substituted to Sr sites. Two O 1s peaks were observed and were identified as the oxygens in the IrO2 and SrO planes by photoelectron holography and density functional theory (DFT) calculations. In the Ir 4f spectrum of Sr2IrO4, an unexpected Ir3+ peak was observed as much as 50% of all of the Ir. The photoelectron hologram of Ir3+ showed a displacement of about 0.15 Å. This displacement is thought to be due to the oxygen vacancies in the IrO2 plane. These oxygen vacancies and the associated local displacement of the atoms might inhibit superconductivity in spite of sufficient electron doping.

The dependence of the forward scattering peak intensity I on Z was considered as the power law I? Z(alpha), and the value of alpha was discussed. Using the Yukawa potential and the first Born approximation alpha = 2 is obtained as in the Z(2) contrast of the HAADF-STEM. The simulation shows alpha at E-k = 600 eV and 30 keV are about 1/2 and 1, respectively. Z-dependence of the screening constant could explain alpha = 1 at E-k = 30 keV. This knowledge of Z dependence will help elemental analysis in photoelectron holography.

In recent years, the analysis of atomic arrangements of dopants using photoelectron holography has attracted much attention. A three-dimensional atomic image around the photoelectron-emitting atoms can be recon-structed from the photoelectron hologram. In this method, the theory of reconstructing an atomic arrangement from a photoelectron hologram is important. The recently developed Scattering Pattern Extraction Algorithm using L1 regularization (SPEA-L1) is capable of precisely reconstructing atomic images. We evaluated the ac-curacy of SPEA-L1 by comparing it with the Barton method using photoelectron holograms of W(110)-O. As a result, it was observed that SPEA-L1 could accurately reproduce an atomic image with a width as small as a fraction of the width of the reconstructed atomic positions compared to the Barton method. The obtained W (110)-O structure agreed well with the structure obtained by Low Energy Electron Diffraction(LEED) and cal-culations with an accuracy of 0.1 angstrom.

The electronic and atomic structures of topological insulator Bi2Se3, upon Ag atom deposition, have been investigated by combined experimental methods of scanning tunneling microscopy (STM), photoelectron spectroscopy, and first-principles calculations. We show from the results of STM that the deposited Ag atoms are stabilized beneath the surface instead of being adsorbed on the topmost surface. We further reveal from the angle-resolved photoemission spectroscopy that the Bi2Se3(0001) topological surface states stay uninterrupted after a large amount of absorption of Ag atoms. Our analysis of the photoelectron intensity of Ag core states excited by soft X-ray suggests that a large amount of deposited Ag atoms diffused into a deeper place, which is beyond the probing depth of X-ray photoelectron spectroscopy. The first-principles calculations identify the octahedral site in the van der Waals gaps between quintuple layers to be the most favorable locations of Ag atoms beneath the surface, which yields good agreement between the simulated and experimental STM images. These findings pave an efficient way to tailor the local lattice structures of topological insulators without disturbing the topologically nontrivial surface states.

To explain the deviation in the valence subband energy levels of the Si(111) inversion layer obtained by experiments and calculations, the band edge profile that has the same subband levels as the experimental results is searched through numerical calculations. The obtained band edge profile is characterized by its flat feature in the subsurface region over 1.2 nm from the surface of the Si substrate. By taking the second derivative of the obtained band edge profile, the carrier distribution in the space charge layer is obtained, and the existence of negative charges in the subsurface region is revealed. The origin of the negative charges is attributed to the inhomogeneous valence electron distribution due to the standing wave formation in the narrow space charge layer.

Atomic-resolution holography microscope is proposed, and has been composed by combining a small SEM and a new two-dimensional display-type analyzer CoDELMA. CoDELMA can display an angular distribution of emitted electrons from the sample up to ±50° with high energy-resolution at once. The electron beam size can be down to 20 nm, which ensures the spatial resolution of several tens nm. A holographic reconstruction from the angular distribution of Auger or energy-loss electrons emitted from selected atomic species reveals the three-dimensional atomic arrangement around the atom. Hence this system works as an atomic-resolution holography microscope, which can reveal the three-dimensional atomic arrangement around the selected atomic species in nm region for the first time.

Professor Charles S. Fadley (nicknamed Chuck), who was a global leader in photoelectron spectroscopy using synchrotron radiation, passed away on 1st August 2019 at the age of 77. He was a well-known founder of photoelectron diffraction, and as a front runner in photoelectron spectroscopy using synchrotron radiation he initiated and promoted several novel approaches; such as, photoelectron holography, hard X-ray photoelectron spectroscopy, soft-X-ray standing wave spectroscopy, and more. He contributed to many scientific activities and served scientific communities including this ALC conference. He was an honorable member of the JSPS 141st Committee and a laureate of JSPS 141st Committee Award. This paper summarizes his life to honor his great achievements in science and contributions to scientific communities.

We have realized a simple and compact new two-dimensional electron analyzer "a VD-WAAEL analyzer" using a variable-deceleration-ratio wide-acceptance-angle electrostatic lens (VD-WAAEL). Using an electron gun and an angle measurement tool, we confirmed that a two-dimensional angular distribution could be measured at once over a large solid angle of +/- 45 degrees. We also confirmed that energy analysis could be performed with an aperture. Here a high energy resolution of similar to 0.23% was obtained by using the aperture size of 0.8 mm. at the 40 degrees position. Moreover, a magnified image of SUS316 #100 mesh was successfully measured. The magnification ratio was 25, which was in good agreement with the calculation.

Although the local 3D atomic arrangement around specific active-site atoms in a functional materials is very important to reveal the origin of the functionality, the analysis of the atomic arrangement has not been possible so far because this local structure has no translational symmetry. Recently the analysis of this kind of local 3D atomic arrangement around specific atoms which has no translational symmetry has become possible with several atomic-resolution holographies and related techniques, which have been developed in Japan. Hence a project of "3D Active-Site Science" of JSPS Grant-in-Aid for Scientific Research on Innovative Areas has started. This review describes the characteristic of these new local 3D atomic imaging techniques and the achievements of the project especially two representative results and new notation system of active-site.

Most functional materials include “active-sites”, such as dopants, interfaces, or nanoclusters, that are structurally irregular, but essential in the materials’ functions. In the past decade, the structures and properties of these active sites have been better understood with the development of state-ofthe-art analytical methods, theoretical calculation tools, and fabrication techniques. In this special issue, we have collected the latest research articles on these actives-sites in materials from various fields such as biology and condensed matter physics. The revealed active-sites are complex nanostructures within the matrix materials, which exceeded conventional expectations. It is believed that a better understanding of this nanoscale order will lead to new material designs and development.

We have established the original methodology that enable to observe atomic orderings and arrangements of “surfaces with arbitrary directions” on 3D figured structures, by developing diffraction and microscopy techniques. An original technique, namely, the directly and quantitatively viewing of the side- and facet-surfaces in atomic scale using reflection high-energy electron diffraction (RHEED) and scanning tunneling microscope (STM), can feedback to the determination of process parameters in the etching procedure. The scientifically optimized etching recipe enabled the creation of atomically-ordered side-surfaces, which are perpendicular to planar substrate surfaces on 3D patterned Si substrates. RHEED and STM prove atomically-reconstructed Si{100}2×1, {110}16×2, and {111}7×7 side-surfaces that were realized for the first time. We have also developed the atomically-ordered 3D nanofabrication technique, where the material stacking direction is switched from the general out-of-plane to in-plane direction, and realized the formation ultra-thin epitaxial films in 3D space.

A single crystal of the double peroviskite Sr2FeMoO6 has been prepared and was studied by using two-dimensional photoelectron diffraction (2DPED) and x-ray absorption spectroscopy (XAS). We found clear evidence for disorder of Fe3+/Mo5+ by 2DPED experiments, supporting a disorder picture for Sr2FeMoO6. On the other hand, Fe 2p XAS spectra suggested the existence of Fe2+ components (i.e. the mixed valence states) at the surface, but it was suppressed in the bulk. It seems that this fact has complicated a controversy so far concerning the reduction of saturation magnetization of this material.

Series of two-dimensional Auger electron intensity angular distributions (AIAD) were measured from a polycrystalline iron surface using a focused soft X-ray beam and a display-type analyzer. The Fe(110) surface was polycrystallized by annealing up to the Martensitic transition temperature in an ultrahigh vacuum condition. After cooling the sample to room temperature, the crystal orientations of the 21 × 21 scanned points were determined by Fe LMM AIAD measurements. Domain formation of sub mm size was confirmed. The domains oriented in the [001] direction were found adjacent to the domain oriented in the [110] direction, suggesting that the Martensitic transition progressed according to the Bain relationship. The chemical and magnetic properties of domains with different surface orientations were characterized by combining diffraction measurements with X-ray absorption spectroscopy techniques. We show here how the surface oxidation reaction as well as the magnetization axis depend on the surface orientation.

New fitting analyses for peak shapes in a 2D reciprocal-space map are demonstrated to evaluate the strain, strain distribution and domain size of a crystalline ultra-thin (15 Å) film of β-FeSi2(100) grown epitaxially on an Si(001) substrate, using grazing-incidence X-ray diffraction. A 2D Laue-fit analysis taking into account instrument broadening and the double-domain effect provides residual maps as a function of the inequivalent strains ɛ b and ɛ c along the b and c axes of β-FeSi2, respectively (and domain size D), reflecting the probability of existence of homogeneous domains with fixed ɛ b , ɛ c and D, in addition to the most probable minimum residual. A 2D Laue fit with an inhomogeneous domain distribution provides a population map with ɛ b and ɛ c , reflecting strain components contributing to the film. The population map also leads to a reference residual as a guide for the strains contributing to the residual map. The advantages of the 2D Laue fits are discussed by comparison with the Scherrer, Williamson–Hall and Gaussian fitting methods for equivalent systems. The analyzed results indicate that the β-FeSi2 nanofilm was considerably small strained, which was also confirmed by transmission electron microscopy, implying a weak interface interaction between the film and the substrate.