Scanning probe microscopy (SPM) is one of the powerful techniques to investigate structures of material surfaces as well as adsorbed species at the atomic and submolecular scale. It also provides information on local electronic and mechanical properties using spectroscopic measurements of scanning tunneling microscopy (STM) and atomic force microscopy (AFM). However, one of the weak points of the high-resolution SPM is that the sample must be atomically flat and homogeneous. It is thus challenging to observe samples such as nanoparticles, porous materials, and nano-scale devices. Such a problem is known as the “materials gap.” To overcome this issue, our group is currently developing methodologies for the characterization of samples prepared in solution processes such as oxide nanoparticles, carbon black particles, and porous organic thin films. In this talk, I will discuss our recent study on porous organic thin film using atomic force microscopy (AFM) and molecular dynamics (MD) simulation.

Two-dimensional metal-organic frameworks (2D-MOFs), covalent-organic frameworks (2D-COFs), and hydrogen-bonded organic frameworks (2D-HOFs) are useful materials for various applications such as adsorbents, separation membranes, catalysts, and sensor devices. Structural characterization is one of the keys to the improvement of crystallinity and properties, but obtaining atomic-scale structural information on organic porous films, in both horizontal and vertical directions, is challenging. In this work, we prepared hydrogen-bonded organic thin films at the air/liquid interface using a Langmuir trough (Fig. 1 (a), (b)) and characterized them using ambient AFM. The evidence of A-A stacked honeycomb structures was obtained by resolving periodic pores over the films with a thickness variation of more than several nanometers (Fig. 1 (c), (d)) [1]. AFM images of samples prepared with different parameters provided a hint for understanding the film formation mechanism. Our expectation was supported by molecular dynamics simulations (Fig. 1(e)) [2], which revealed that the molecules are self-assembled without external forces in the quasi-two-dimensional system. In addition to hydrogen bonding and π-π interaction, long-range Coulomb interaction was found to play a vital role in the film growth.

This study was a good example of applications of ambient SPM measurements of solution-processed samples to extract atomic-level information.

References

[1] Yamanami et al., Langmuir 38, 1910 (2022).

[2] Matsui et al., AIP Advances 12, 105109 (2022).

Spatially arranged surfaces on the micro-rod structure, which

was three-dimensionally (3D) architected on a Si(110) substrate have been thoroughly investigated by a system with micro-beam reflection high-energy electron diffraction (μ-RHEED) and scan- ning electron microscopy (SEM). The combination of μ-RHEED and SEM realized analytical structure investigation of 3D surfaces with the spatial resolution of sub micrometer for the 3D rectangular shaped rod consisting of a (110) top surface (20 μm wide) and {111} vertical side surfaces (10 μm wide). Exhaustive mapping revealed the peculiar reconstructed surface structures: Si(110) “16 × 2” single domain and {35 47 7} facet surfaces lo- cally appeared on the interconnected edge region on the 3D structure in addition to the “16 × 2” and 7 × 7 super structures on flat top (110) and side {111} surfaces, respectively. The for- mation mechanism for “16 × 2” single-domain structure near the corner edge of the (110) surfaces and {35 47 7} facets on the corner edges between (110) and {111} surfaces were discussed from the viewpoint of the surface stability on the 3D geometrical shaped Si structure.

Nb has the highest superconducting transition temperature among elemental metals. That facilitates applications using superconducting junctions such as the superconducting quantum interference device (SQUID). Although the interfacial properties are important for such junctions [1], the surface of Nb is covered with oxide layers even in a vacuum condition, preventing preparation of well-defined junctions. A method to prepare a clean surface was recently established [2]. In this method, however, the crystal needs to be annealed at 2410℃, only 70℃ below its melting point. Such sophisticated methods may not be preferred for applicational purposes. Thus, we aim to clarify the oxide structure to utilize it as a well-defined interface.

We performed atomic force microscopy (AFM) and scanning tunneling microscopy (STM) measurements for the oxygen-induced NbO surface on Nb(110). The STM image in Fig. 1 represents the topmost atoms, which are referred to as the Nb^* chain atoms [3-5]. The observed chain structure is consistent with the previously proposed models. Furthermore, AFM is capable of detecting O atoms and possibly low-lying Nb atoms, and it clarified that the previously proposed models are not enough to fully explain the present observations. We propose a new structure model based on the density functional theory calculations.

References

[1] J. Brand et al., Phys. Rev. Lett. 118, 107001 (2017).

[2] A. B. Odobesko et al., Phys. Rev. B 99, 25335 (2016).

[3] I. Arfaoui et al., Surf. Sci. 557, 119 (2004).

[4] A. S. Razinkin and M. V. Kuznetsov, Phys. Met. Metallogr. 110, 531 (2010).

[5] S. Berman et al., Phys. Rev. B 107, 165425 (2023).

Two-dimensional (2D) covalent organic frameworks (COFs) opened a new pathway to generate 2D nanosheets on a substrate using a single-molecule sublimation method in a vacuum. This led to nanoarchitecture with single-molecule accuracy. Such a growth process in a vacuum is essential for developing application device products. Primarily, those COFs are known to be used as nanoporous to trap guest atoms inside, activating the functionality for many applications such as spintronics, biomedicine, drug delivery, catalysts, and water purification. This study examined transition-metal cobalt (Co) atom deposition on the 2D COFs on an atomically flat and clean Cu(111) substrate in a vacuum at room temperature. Surface morphology was monitored using scanning tunneling microscopy (STM), and electronic structures were measured using photoelectron spectroscopy (PES). An initial growth process of Co on 2D COFs is crucial for further transition-metal nanocluster applications. We found that Co atoms have not stayed inside the nanoporous but prefer to be trapped by the edges of the 2D COFs. Once the Co was trapped, other atoms gathered, leading to nanocluster formation with a size of ~3 nm. Although the Co/Cu(111) formed bilayer 10-20 nm size triangular nano-islands, the height was restricted to one monolayer height for the Co nanocluster on 2D-COFs, indicating Co interaction below the COFs. No strong electronic hybridization between Co atoms and 2D COFs was confirmed; therefore, 2D-COFs were not disturbed by the transition metal adsorption. This is a clear contrast to the π-conjugated molecular case, where strong π-d hybridization destroyed the molecular structures.

Physical properties found in H-related materials such as high-temperature superconductivity have been studied intensively [1]. Hydrogen ion implantation at low temperature, where the thermal H₂ desorption is suppressed, is a promising method to realize H-rich materials [2]. Experimental and theoretical structure analyses are important to understand and explore fascinating properties of hydrides. Channeling NRA is an ion beam analysis that can determine the lattice location of H in crystals. In this study, we fabricated PtH_x by the H ion implantation at low temperature and investigated its structure by Channeling NRA and DFT calculations.

The sample we used is Pt(100) single crystal. H absorption by the H ion implantation at low temperature was observed near the surface in nuclear reaction analysis (NRA). We performed the Channeling NRA around the surface normal direction using ¹⁵N²⁺ beam with an energy of 6.45 MeV. As shown in Fig. 1(a), decrease or increase in NRA yield at the <100> axis channeling is expected for the H atoms in octahedral (O) or tetrahedral (T) site, respectively. Figure 1(b) shows incident angle dependence of the NRA and RBS yields simultaneously obtained around the <100> axis. The change in the NRA yield was small, indicating that the H atoms are randomly distributed in the crystal. To theoretically investigate the H location, we calculated the site energies of O and T sites in Pt by DFT calculations. Neglecting the zero-point energy, the energies of the two sites coincide within ~50 meV. These results showed that the implanted H atoms occupy both O and T sites in PtH_x formed at low temperature. We will also discuss the H dynamics in Pt.

[1] A. P. Drozdov et al., Nature 525 (2015) 73.

[2] Y. Yamashita et al., Phys. Rev. B 104 (2021) L041111.

An incommensurate structure or a commensurate structure with very long periodicity is difficult to handle, because the unit cell cannot be defined or contains a lot of surface atoms. One way to tackle the problem is to start with a nearby commensurate structure with short periodicity and consider the deviation from it. For example, (22&times√3) ("herringbone") reconstruction on a clean Au(111) surface can be considered as slight deviation from bulk-truncated, (1&times1) structure. Due to tensile stress on the surface, the topmost Au layer is compressed in [-110] direction, and 23 Au atoms in the direction are accommodated within 22 periods on the substrate. After the reconstruction, all surface Au atoms cannot occupy stable fcc sites, but some are pushed to metastable hcp sites and even Au atoms on bridge sites are produced at the boundaries between fcc and hcp domains. The reconstruction is imaged by STM as bent atom rows with two brighter stripes corresponding to Au atoms on bridge sites. (Note that so-called herringbone reconstruction contains two steps: one is uniaxial compression along <-110> resulting in (22&times√3) and the other is connection of two domains which differ in compression directions by 120&deg. We focus on the first step.)

In this paper, we consider what happens if the initial structure is (√3&times√3)R30&deg with four surface atoms in a unit cell, and compare the result with the structure derived from (1&times1). Figure illustrates the initial (1&times1) (a) and (√3&times√3)R30&deg (b) structures. As for the (1&times1) structure (a), surface atoms are close-packed in [-110] direction, and occupy only fcc sites. We numbered the leftmost fcc sites with black letters along the [-110] direction, and also numbered surface atoms with red letters. Both numbers correspond to each other due to the (1&times1) periodicity. When the surface layer is compressed in [-110] direction, e.g., the 17th surface atom is pushed to the 16th fcc site. Note that only the shift by full one period is allowed. At the same time, middle atoms numbered 8 or 9 are moved by half a period, and become very close to hcp sites. Thus, bent atom rows travel through fcc-hcp-fcc sites.

In (√3&times√3)R30&deg case illustrated in Fig. (b), surface atoms occupy four different sites including the most stable fcc site, and the close-packed direction is changed to [11-2] due to 30&deg rotation of the √3 unit cell. In [-110] direction, three substrate fcc sites correspond to four horizontal surface atom rows. When the overlayer is compressed in [-110], a shift by half a period is sufficient to produce the surface atom row which is energetically-equivalent to the 0th row. For example, the shift of the 22nd (4N+2 where N is an integer) surface row to the 16th fcc site produces the same arrangement of surface atoms as the 0th row. Moreover, a half-period shift of the 20th (4N) row generates the equivalent but antiphase arrangement to the 0th row. These half-period shifts are in marked contrast with full-period shifts required in the (1&times1) case. The half-period shift causes a quarter period shift of the 9th and 11th atom rows, and places them near hcp sites. The mechanism is similar to that on the (1&times1) surface. However, the resultant arrangement is different. Surface atoms form stripes which run perpendicular to [-110] and parallel to [11-2]. Surface atoms are aligned along [-12-1] direction in one stripe, and aligned in [2-1-1] in the other, and form a chevron-like pattern as a whole. The reconstruction may be considered as two-dimensional twinning which has a symmetry axis of [11-2]. We will show that the above model can be applied to striped incommensurate (SIC) phases of Pb on Si(111) and Pb on Ge(111) systems.

By using multiple scattering calculations, we have reported that RHEED is quite useful for determination of adsorbed hydrogen positions on metal surfaces [1,2]. A hydrogen is the lightest atom and its scattering factor is much smaller than those of the substrate metals. In the case of LEED, relatively large scattering amplitude of H in the forward direction gives the changes in the intensity-energy (I-V) curves which can be used for analysing the hydrogen position. The use of large scattering amplitude of H in the forward direction is the same in the case of RHEED. In addition, in RHEED by rotating the sample, it is easy to choose diffraction conditions sensitive to H positions, where reflected intensity curves are modified largely by adsorbed H position.

In this presentation, by using a multiple scattering calculation, we will study whether we can determine subsurface H positions just below metal surfaces by RHEED. It is important to know the positions of subsurface hydrogens, which are located just one or two monolayers below the substrate. We take H absorbed at Pd(100) as an example, since no definite positions of the subsurface H are reported so far. We assume that the H coverage is 1.5 ML; 1 ML of H are pre-adsorbed at the hollow sites on the surface and 0.5 ML of H are absorbed below the surface. As the absorption sites, two very likely sites are considered. One is the octahedral (Oh) site and the other is the tetrahedral (Td) site. The rocking curves and the azimuthal plots from the surfaces with absorbed H at Oh sites and those at Td sites are compared. In order to see how large the differences are in RHEED intensities from surfaces with absorbed H at two different sites, i.e., Oh and Td sites, figure shows one example of the azimuthal plots from the surfaces with 0.5 H absorbed at the two subsurface sites around the [10]-azimuth. The solid line shows the 00 intensities from the surface with 1 H at hollow sites with 0.11 Å high with 5% relaxation of the topmost Pd layer. The dotted and the dashed lines show the 00 intensities from the same surface but with absorbed 0.5 H at Oh sites and that at Td sites, respectively. The latter two 00-azimuthal plots clearly show different variations, which can be used for identifying the absorbed site. In a similar manner, the rocking (intensity vs. glancing angle) curves from the surface with absorbed H at Oh sites and those at Td sites are different enough to distinguish the two sites by choosing diffraction conditions sensitive to subsurface H. We can also determine the height of absorbed H by choosing diffraction conditions sensitive to subsurface H height. Then, we can conclude that RHEED is quite useful to determine subsurface H position at Pd(100) as well as the adsorbed H position.

References

[1] T. Kawamura, K. Fukutani, Surf. Sci. 688 (2019) 7–13.

[2] T. Kawamura, Y. Fukaya, K. Fukutani, Surf. Sci. 722 (2022) 122098.

INTRODUCTION

Gallium nitride (GaN) has many unique properties, such as a wide direct band gap, high thermal conductivity, extremely stable chemical characteristics, and strong radiation resistance. For such GaN-based devises, proper doping is necessary to improve electrical conductivity.[1] In the case of n-type GaN, Si is widely used to form the shallow donor states in the bandgap of GaN. In the case of p-type GaN, on the other hand, Mg is usually used due to the formation of shallow acceptor levels in the bandgap of GaN. For the atomic structure and the chemical states of active and inactive dopant sites in GaN, various doping sites have been proposed in previous studies. However, the atomic structure and chemical state of the active and inactive dopant sites of GaN have not yet been clarified, owing to the lack of direct evidence. The local atomic structures and chemical states of the dopant atoms should be clarified to obtain GaN-based devices with high performance.

In the present study, we employed photoelectron holography (PEH) to obtain 3D local atomic structure around the Mg dopant in GaN. In addition, by employing the SPEA-MEM/SPEA-L1 method to experimentally obtained PEH for the dopant atoms in GaN, we tried to directly observe and visualize the local 3D atomic structure for the active and inactive Mg dopant sites for GaN.

EXPERIMENTAL

Mg-doped GaN (0001) substrates were prepared by a hydride vapor phase epitaxy method with an epitaxial layer thickness of 100 nm. The Mg dopant concentration was 2.1 x 10²⁰ cm^-3. Hall effect measurements revealed a hole concentration of 2.6 x 10¹⁹ cm^-3 for Mg dopant.

The PEH measurements were performed at the BL25SU beamline of SPring-8. We used the Scienta-Omicron DA30 as an electron analyzer. The incident photon energies were changed so that Ga 3p and Mg 2p core level kinetic energies became ~ 800 eV for the PEH simulations.

RESULTS AND DISCUSSIONS

We measured the Mg-KLL Auger spectra of Mg-doped GaN. In the spectra, two peaks were observed; peak α and peak β. After annealing at 800 ℃, the areal intensity of peak β increased while that of peak α decreased. According to previous studies, high-temperature annealing can activate a part of the inactive dopant sites, increasing the hole concentration. Therefore, peak β should be the active site of the Mg-dopant. By measuring all azimuth angles and polar angle of such components. Then we obtained PEHs for components α and β, which is shown in Fig. 1. As can be seen, component α does not have any clear hologram patterns or Kikuchi lines, whereas component β exhibits clear hologram patterns. These patterns are very similar to the simulated PEH for Mg atom substituting a Ga atom in GaN (Mg_Ga). Since component β is the active site in Mg-doped GaN, Mg_Ga should be the active site in Mg-doped GaN. According to our photoelectron spectroscopy, the intensity ratio of component β is about 27%. This is very close to the electrical activation rate of 26.2% obtained from the Hall effect measurements. Accordingly, we can conclude that the active dopant site in Mg-doped GaN is Ga_Mg. From analysis of the PEH for component α, we found that the component α may be attributed to Mg_Ga-V or Mg_Ga-H. [2]

References

[1]. Tang Jingmin and Yamashita Yoshiyuki, ACS Appl. Electron. Mater. 3, 4618 (2021).

[2]. Tang Jingmin et al., ACS Appl. Electron. Mater. 4, 4719 (2022).

The Slow Positron Facility (SPF) at the Institute of Materials Structure Science (IMSS) in KEK recently developed a system of low-energy positron diffraction (LEPD), which is the positron counterpart of low-energy electron diffraction (LEED), for the surface atomic structure determination [1]. It is considered that positron diffraction is an ideal method for the study of surface atomic arrangements [2].

The LEPD experimental station at SPF has been significantly developed lately to enable the execution of the so-called quantitative I-V analysis for the determination of precise surface structures. In case of I-V analysis the intensities of the diffracted spots are measured with the change in beam energy and analyzed. Recently we have conducted the experiment on the clean Cu(001) and Pb adsorbed Cu(001)-c(2x2) surface. The LEPD diffraction pattern of both the surface structures are obtained in 2eV steps for a span of 100 seconds each using the dedicated HEX-MCP delay line detector. The experimental data are still being analyzed but its recent progress and the I-V analysis will be discussed during the presentation. In addition to this, we have carried out the first trial of LEPD-ARPES multiprobe research where the clean Cu(001) sample prepared at the LEPD station has been transported by a UHV transfer vessel to BL-13B in the Photon Factory (PF) at IMSS, and its electronic band structure has been observed by angle-resolved photoemission spectroscopy (ARPES).

The successful LEPD experiment has been performed after the recent upgradation of the LEPD experimental station which now consists of two dedicated chambers for LEPD measurement and sample preparation. The sample manipulators at both the sample preparation chamber and LEPD measurement chamber have been upgraded which allows precise and highly reproducible adjustment of sample position and orientation. This helps to attain the normal beam incidence onto the sample to equalize the intensities between symmetrical diffraction spots necessary for I-V analysis. The manipulator of the LEPD chamber along with the upgraded sample holder can rapidly cool the sample to -181 °C within 30 minutes which is crucial as the intensities of LEPD spots are affected by the Debye-Waller factor as in in LEED. Moreover, the manipulator of the preparation chamber has the access to electrically heat the sample up to ~1200 °C, besides cooling. The sample holders are specially designed to accommodate efficient temperature change as well as compatible with Omicron-type sample holder which is being used in several stations in PF. The preparation chamber is also equipped with an Ar⁺ sputtering, a triple-pocket electron beam evaporator for thin film growth on the surface, three different gas introduction systems, and additional ports available on request. The prepared sample surfaces can be evaluated using an LEED-AES spectrometer. The preparation chamber is also attached to the adjacent load lock chamber for additional sample storage, and it can be connected to the UHV portable transfer vessel for the transfer of sample to other stations such as for ARPES measurement. The successful implementation of the new LEPD experimental station for LEPD I-V analysis and the ability to conduct multiprobe analysis on the same sample is an important step towards the development and understanding of new functional surface materials.

Reference

[1] K. Wada et al., e-J. Surf. Sci. Nanotech. 16, 313 (2018). [2] S. Y. Tong, surface science 457, L432 (2000).

Heterogeneous catalysis is a promising technology in biomass valorization owing to the high reactivity, facile post-separation process, and exceptional catalyst recyclability, which outperform homogeneous catalysts. Various functionalized catalytic materials have emerged as promising substitutes for homogenous liquid acid catalysts. These materials include carbon materials, metal–organic frameworks, and metal nanoparticles. Among these, carbon materials such as carbon nanotube, graphene oxide, and carbon black have been investigated as catalysts for cellulose hydrolysis to produce glucose.

Carbon materials exhibit notable catalytic activity upon functionalization with active acidic groups like hydroxyl groups (-OH), carboxyl groups (-COOH), and sulfonic groups (-SO₃H). For carbon catalysts, hydroxyl and carboxyl groups play pivotal roles as connection points, facilitating access to cellulose, while sulfonic groups serve as active sites, mediating the cleavage of the linkages between the glucose units in cellulose. Consequently, the surface modification of carbon materials with these functional groups, i.e., the carbon sulfonation process, is essential for producing efficient carbon acid catalysts. Conventional methodologies for carbon sulfonation include hydrothermal and reflux methods that require the use of hazardous chemicals such as concentrated sulfuric acid (95–98%), chlorosulfonic acid, fuming sulfuric acid, or 4-benzenediazoniumsulfonate under elevated temperatures for dozens of hours. Consequently, as an eco-friendly and efficient carbon sulfonation process, the application of gas–liquid interfacial plasma (GLIP) has been investigated.

Within the framework of the GLIP process, plasma is generated between the tips of multi-needles and the surface of dilute sulfuric acid, which contains carbon particles in a N₂/Ar gas mixture. The optimal catalyst synthesized from graphene nanoplatelets by the GLIP process achieved 41.5% conversion, with a high glucose selectivity of 84.3%, which is superior to the performance of catalysts synthesized by the hydrothermal method. The GLIP process required a sulfuric acid concentration of 1 mol/L, reaction temperature of 40°C, and reaction duration of 0.75 h, whereas the hydrothermal method required a sulfuric acid concentration, temperature, and reaction duration of 18 mol/L, 200°C, and 24 h, respectively. Thus, the GLIP process is not only a safer, but also a more proficient alternative. Remarkably, the catalyst synthesized by the GLIP process exhibited superior recyclability of 95.9% over three cycles for cellulose hydrolysis.

Of late, we investigated the mechanism of carbon sulfonation in the GLIP processes. Active species produced near the gas–liquid interface, such as •OH, SO₃, and HOSO₂•, probably play important roles in carbon sulfonation. Efforts are being taken to identify the gas-phase active species using Fourier transform infrared spectroscopy and quadrupole mass spectrometry and the liquid-phase counterparts using electron spin resonance spectroscopy.

1. Introduction

Diamond-like carbon (DLC) films have been widely used in industrial applications, especially cutting tools, sliding parts within automobile engines, and biomedical devices due to their unique properties. DLC films with a large amount of sp³ C bonds could have very high hardness and good wear characteristics. Because the ratio of sp³ C bonds to sp² C bonds strongly depends on the numbers of the incident carbon ions with an optimum kinetic energy, the discharge plasma with large number of carbon ions is required for the synthesis of dense and hard DLC film.

High-power impulse magnetron sputtering (HiPIMS) is one of ionized physical vapor deposition methods and is expected to synthesize hard DLC films. Up to now, a short-pulse HiPIMS was proposed to prepare hard DLC films [1], and then we proposed double pulse HiPIMS at a short pulse width and a relatively large target current [2]. In this study, DLC films are synthesized by a multi-pulse HiPIMS to achieve harder DLC films. The objective is to synthesize harder DLC films through the investigation on the relationship between the microstructure of DLC film and the setting parameters, such as the pulse number of HiPIMS, pulse interval, and pulse width.

2. Experiment

DLC thin films were fabricated via multi pulse HiPIMS at Ar gas pressures of 0.6 Pa and average power of 80 W. The multi pulse HiPIMS plasma was produced in a cylindrical vacuum chamber with 150 mm inner diameter and 220 mm height. As a typical example, the temporal waveform of the target current I_T(t) is shown in Fig. 1. The first HiPIMS plasma temporally evolved during a pulse-on time of the voltage source with a width of about 13 μs and the discharge current reached about -45 A corresponding to 1.7 A/cm². After the application of the first-pulse voltage for HiPIMS, multi-pulse voltages were applied to produce the multi pulse HiPIMS. The peak target current of the second and subsequent HiPIMS was about -40 A. The optimum energy of the energetic carbon ions for bombardment could be controlled by applying a negative pulse voltage of -100 V to the substrates. The film structure and the film density were analyzed using the Raman spectroscopy and X-ray reflectivity measurement.

3. Results and Discussion

Figure 2 shows the relationship between Raman parameters estimated from the measured Raman spectrum and N, where the number N is the number of short-pulse HiPIMS per one period. The measured Raman spectrum was assigned to the G (graphite) peak at 1540-1560 cm⁻¹ and D (disorder) peak at 1350-1370 cm⁻¹, respectively. According to the model proposed by Ferrari and Robertson [3], the I(D)/I(G) ratio is correlated with the size of sp² C network organized in aromatic rings, where I(D) and I(G) are the intensities of the D peak and G peak, respectively. In addition, the FWHM of the G peak provides insight into the structural disorder. As shown in Fig. 2, the FWHM of the G peak gradually increased from 192.1 cm^-1 to 203.7 cm^-1 with the increase in N. On the other hand, the I(D)/I(G) ratio was less sensitive to N for N ≧2, but the I(D)/I(G) ratio of the film synthesized at N ≧2 were smaller than that of the film synthesized at single pulse HiPIMS. These results indicate that the hardness of the films synthesized at N ≧2 is higher than that of the film synthesized by single HiPIMS due to the increase in the ratio of sp³ C bond to sp² C bond.

4. Conclusion

The structure of DLC film and the film density depended on the pulse number of the multi pulse HiPIMS.

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1. Introduction

Reactive magnetron sputtering, which is one of physical vapor deposition techniques, has been widely used to fabricate metal oxides such as titanium oxides, vanadium oxides, and tungsten oxides, due to its versatility and possibility of large-scale fabrication. In reactive magnetron sputtering with a metal target, Ar gas is generally employed as an ambient gas, whereas oxygen gas is used as the reactive gas. Both the deposition rate and the film properties significantly are sensitive to the mixing ratio of the reactive gas such as oxygen gas in reactive magnetron sputtering including reactive high-power impulse magnetron sputtering (HiPIMS). Reactive HiPIMS system with a pulsed oxygen gas flow control has been proposed to deposit metal oxide films [1,2], indicating the usefulness for the deposition in a transition region close to the metallic mode. However, there seems to be a room to research the growth of metal oxide thin films with a high degree of control and good reproducibility. In this study, we propose a HiPIMS combined with reactive multi pulse magnetron sputtering (mPMS) with a low target current and fabricate zirconium oxide film, which is characterized by wear resistance, large band gap and wide spectral transparency range.

2. Experiment

The magnetron sputtering system with a zirconium target with 76 mm diameter was set in a cylindrical vacuum chamber with 150 mm inner diameter and 280 mm height. The distance between the substrates (Si and glass) and the target was 70 mm. The flow rate of Ar gas was about 15 sccm, whereas the oxygen gas flow rate was 0.23 sccm. Then, the total gas pressure was 0.7 Pa. Figure 1 shows the typical waveform of the target current in HiPIMS combined with mPMS, where the pulse number N of mPMS is 7. The maximum of instantaneous power reached about 17 kW in HiPIMS and 1 kW in mPMS, respectively. The repetition rate was 400 cycle/s.

3. Results and Discussion

Figure 2 shows the deposition rate as a function of N. The deposition rate linearly increased from 0.9 nm/min at N=0 to 1.6 nm at N=6 with the increase in the pulse number N, and then it abruptly increased. This may be caused by the change in the rate of the oxide cover on the target surface (target poisoning). In other words, this result may indicate that the deposition process, which is classified into three regions such as metallic region, transition region and metal oxide region, can be controlled by changing the pulse number of mPMS. In the XRD patterns for the films fabricated at various N, a weak peak at about 28°, which is attributed to (11-1) preferred orientation of ZrO₂, was observed. The weak XRD peak observed for the films fabricated at N≦6 suggested that films were nano-crystallized.

4. Conclusion

Reactive HiPIMS combined with multi pulse magnetron sputtering can be expected to be suitable for fabricating metal oxide films such as zirconia and to achieve the control of film property depending on the pulse number of pulse magnetron sputtering.

Acknowledgements

This work is partially supported by JSPS KAKENHI Grant Number JP23K03817.

References

[1] N-W Pi, M. Zhang, J. Jiang, A. Belosludtsev, J. Vlček, J. Houška and E. I. Meletis, Thin Solid Films, 619 ,239 (2016).

[2] J. Houška, D. Kolenatý J. Vlček, and R. Čerstvý, Thin Solid Films, 660, 463 (2018).

Inward plasma is a very unique plasma processing technique capable of performing etching by irradiating local plasma near the capillary inlet against the flow of gas while sucking the etching gas [1]. Fig. 1(a) shows the basic schematic diagram of the inward plasma. By arbitrarily adjusting the pressure and the gap in the capillary tube (the distance between the sample and the end face of the capillary tube) to supply gas from the sample surface and high frequency power to RF electrodes surrounding the capillary, localized plasmas equivalent to the inner diameter of the capillary tube are generated against the gas flow in the aforementioned gap area. As the superiority of the inward plasma, by performing etching while sucking gas from the sample surface side, and that the processed surface is clean, the temperature rise of the sample during etching as compared with the general down-flow type plasma [2], it is possible to greatly change the flux ratio of the charged particles and radicals and the like. From these features, the inward plasma has been used for wiring exposure of semiconductor devices as preprocessing in failure analysis until now. Recently, the application to the semiconductor front-end processing device manufacturing is also expected. When it is used in the manufacturing process of semiconductor devices, contamination and damage caused by the manufacturing process become a problem, so it is necessary to grasp the damage caused to the device by each process. However, the plasma density, ion energy, and so on, which are factors of stress [3], have not been quantitatively evaluated. The reason is that the Langmuir probe method [4] using a single probe is known as a measurement method of plasma density and ion energy, but in the inward plasma, the plasma emission region from the tip of the capillary to the sample stage is only in the order of several hundred um. Therefore, in addition to the difficulty of placing the probe collector itself, it also affects the plasma itself. Therefore, we prepared an electrostatic type ion energy analyzer (IEA) shown in Fig. 1 (b), and decided to measure the ion energy. By placing a negatively biased (V_G) grid-electrode on the sample stage, we can obtain the ion current I_c by catching the positive ions back into the collector. In this study, RF frequency 13.56 MHz, RF power 20 W, the chamber pressure 7 Pa and the inner diameter of the capillary were set to 4 mm, V_G = -20 V and Ar gases were localized plasmas in the ceramic made capillary tubes. In this plasma configuration, the potential of the plasma can be controlled by applying a bias-voltage V_a to the electrodes inserted into the capillary tube. Fig. 1(c) shows the voltage/current properties obtained when Fig. 1(d) is V_a = 0 V in the case of V_a = 120 V. As for the high-energy component of ions, since V_c appears from 0 V or higher, the change in | I_c | due to the high-energy component was compared by drawing a tangent to V_c = 50 ~ 150 V section where I_c is the smallest and taking the leak current into account. We could observe that the largest value of the ion energy is larger in Fig. 1(c) where the plasma-potential is larger. Further, from Fig. 1 (d), the ion saturation current was found to be approximately 10 uA, when estimating the plasma density through Bohm flux, the plasma density becomes about 4.2×10¹⁴ m^-3, the smallest maximal ion energy depending on V_a became about 20 eV. By using IEA, quantitative results were obtained for the first time on the stressors of the inward plasma. In this lecture, we report on IEA of the inward plasma.

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Introduction

The performance of magnetic materials can be improved by understanding the local magnetic domain structure. Magnetic domain contrasts can be observed with a scanning electron microscope (SEM) by detecting secondary electrons (SEs) deflected by the Lorentz force derived from leakage magnetic fields of magnetic specimens. This type of magnetic contrast is generally known as type-I magnetic contrast ^[1]. To obtain SEM images with a clear magnetic contrast using the type-I method, SEs need to be detected under directional-selective detection conditions ^[2]. Areas where SEs are deflected toward a detector are observed as bright, while areas where the SEs are deflected away from the detector are observed as dark.

Experimental Procedures

To establish a convenient observation method for obtaining magnetic domain images using a conventional SEM, we observed a surface-polished Nd-Fe-B alloy specimen using FE-SEM SU5000 (Hitachi High-Tech Corp.). In this study, we sought an optimal condition for observing SEM images with clear magnetic contrast. In the conventional method, a lower detector (mounted beneath an objective lens) is used as shown in figure 1 (a). On the other hand, in this study, we developed a novel observation method using a top detector (mounted above the objective lens) in deceleration mode while tilting the specimen as shown in figure 1 (b). To verify the angular range of detected SEs with our method, we analyzed the initial emission angles of the detected SEs by their trajectory calculations. We calculated the SE trajectories with a 3D electro-optical simulator from the specimen to the top detector considering the electrostatic and magnetic fields distributed in the calculation area.

Results and Discussion

By using the tilting-deceleration method developed in this study, SEM images with a clearer magnetic contrast were obtained, and fine magnetic domains were clearly visualized. On the basis of a calculation analysis of SE trajectories, it was clarified that in the tilting-deceleration condition, SEs were directional-selectively detected, and the tilting-deceleration method much more effectively obtained SEM images with clear magnetic contrast than the conventional method. Furthermore, we found that the anisotropy caused by directional-selective detection can be reduced by combining magnetic domain contrast images of two orthogonal orientations ^[3]. Figure 2 shows comparison results of an SEM image with the tilting-deceleration method (Fig. 2 (a)), a Kerr microscope image (Fig. 2 (b)), and a spin-SEM ^[4] image (Fig. 2 (c)). These images were acquired in the same field of view of the same specimen. The observation technique developed in this study has great advantages because various analysis methods conventionally used in SEMs, such as elemental analysis using energy dispersive X-ray spectroscopy (EDX) and crystal orientation analysis using an electron backscattered diffraction (EBSD), can be easily applied in the same region of interest.

References

[1] L. Reimer, Image Formation in Low-Voltage Scanning Electron Microscopy, Society of Photo Optical (1993).

[2] D. E. Newbury, Magnetic Contrast in the SEM, Advanced Scanning Electron Microscopy and X-Ray Microanalysis, Springer (1986).

[3] H. Morishita, et al., Improvement of type-I method for observing magnetic contrast using scanning electron microscope under tilting-deceleration condition, J. Magn. Magn. Mater., Vol. 546, 168733 (2022).

[4] T. Kohashi, et al., Magnetism in grain-boundary phase of a NdFeB sintered magnet studied by spin-polarized scanning electron microscopy, Appl. Phys. Lett. 104, 232408 (2014).

X-ray absorption near-edge structure (XANES) probes the unoccupied electronic states in a local, i.e. element and orbital-selective way. XANES analysis provides detailed information in the geometrical and electronic structure of active sites, complex matter and nanomaterials, especially when combined with transmission X-ray microscopy. Here we review our recent work on XANES on boron nitride ]1] and titanate nanostructures [2].

The structure of hexagonal boron nitride (hBN) and that of individual boron nitride nanotubes (BNNT) is analyzed using linearly polarized XANES and first-principles calculations. We find that hBN has AA' stacking in agreement with the literature. The spectra of the multi-wall BNNT show a pronounced polarization dependence which is qualitatively well reproduced in the calculations. We show that BNNT have a mixed stacking sequence which explains some of the differences seen between the hBN and BNNT line shapes.

Next we focus on nanostructured titanium dioxide in the TiO2–B phase, which is a promising anode material for lithium ion batteries. The structural and electronic changes between nanoribbons in TiO2–B and the thermodynamic stable anatase phase are studied using nanoscale XANES and first-principles calculations. The oxygen K-edge spectra of the two phases display marked differences which are very well reproduced in the calculations. Strong linear dichroism is observed in single nanoribbons, reflecting preferential O-2p to Ti-3d bond orientation in the low symmetry crystal structures. A simple bond counting model is developed which semiquantitatively accounts for the major dichroic effect. It is shown that the crystal orientation of the nanoparticles can be inferred from the dichroic spectra.

[1] Polarization dependent X-ray absorption near-edge spectra of boron nitride nanotubes, Peter Krüger, Yuya Maekawa, Adam Hitchcock and Carla Bittencourt, Radiat. Phys. Chem. 175, 108129 (2020).

[2] Chemical Bond Modification upon Phase Transformation of TiO2 Nanoribbons Revealed by Nanoscale X-ray Linear Dichroism, P. Krüger, M. Sluban, P. Umek, P. Guttmann and C. Bittencourt, J. Phys. Chem. C 121, 17038-17042 (2017)

I will present some examples of characterization of spintronic devices, such as observation of the magnetic vortex core by magnetic force microscopy (MFM) [1], observation of the current-induced domain wall motion by MFM [2,3], observation of the current-induced magnetic vortex core switching by MFM [4], observation of the current-induced dynamics of magnetic vortex core by time-resolved X-ray microscope [5], observation of the skyrmion Hall effect by Kerr microscopy [6], compositonal-gradient-induced Dzyaloshinskii–Moriya interaction investigated by scanning trasmission electron microscope [7], visualization of the three-dimensional shape of skyrmion strings by magnetic X-ray tomography [8]. The work is support by MEXT, JSPS, NEDO, MEXT X-NICS Grant Number JPJ011438, the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University, and the Collaborative Research Program of the Institute for Chemical Research, Kyoto University. [1] T. Shinjo et al., Science 289, 930 (2000). [2] A. Yamaguchi et al., Phys. Rev. Lett. 92, 077205 (2004). [3] H. Tanigawa et al., Appl. Phys. Express 1, 011301 (2008). [4] S. Kasai et al., Phys. Rev. Lett. 97, 107204 (2006). [5] K. Yamada et al., Nature Materials 6, 269 (2007). [6] Y. Hirata et al., Nature Nanotechnology 14, 232 (2019) [7] D.-H. Kim et al., Nature Materials 18, 685 (2019) [8] S. Seki et al., Nature Materials 21, 181 (2022).

As a new family of quasicrystals (QCs), ultra-thin oxide quasicrystal films on Pt(111) were discovered by Förster in 2013 [1]. Identifying the geometrical structure is an inevitable challenge not only for fundamental studies of this material’s unique chemical and physical properties, but also for industrial applications. In general, QC structures can be expressed as cluster tiling. The clusters are arranged quasi-periodically for a QC, while clusters are arranged periodically for a crystalline approximant. Structural analysis of the clusters in a crystalline approximant is the first step in determining the QC geometrical structure, as it is easier to perform density functional theory simulations and diffraction experiments because of the periodicity of the approximant. Recently, QCs, crystalline approximant, and QC related structure have been named as “hypermaterial”.

In this work, we report the growth of well-ordered, large-area, ultra-thin Ba-Ti-O films on Pt(111), changing from the oxide crystalline approximant (OCA) phase into the oxide quasicrystal (OQC) phase and a (2×2) periodic structure on annealing. We examined the elemental atomic density of Ba, Ti, and O by scanning tunneling microscopy (STM), low-energy electron diffraction, Auger electron spectroscopy, and X-ray photoelectron spectroscopy (XPS). For quantitative analysis, Rutherford backscattering spectroscopy was employed to obtain calibration XPS intensity curves as a function of elemental atomic density. STM images reveal a wide-scale ultra-thin OQC film without BaO islands, and magnified STM images display typical OQC clusters. The ultra-thin OQC film has been prepared on annealing an oxide crystalline approximant with decreasing Ti concentration. We also found that the ultra-thin Ba-Ti-O film forms a (2×2) superstructure on annealing the ultra-thin OQC film. For quantitative analysis, the XPS peak intensity was measured as a function of elemental atomic density. The elemental atomic densities for the ultra-thin OQC film were determined to be 8 × 10¹⁴, 4 × 10¹⁴, and 3 × 10¹⁴ atoms/cm² for Ba, Ti, and O, respectively [2]. The structural models of OQC and OCA ultrathin Ba-Ti-O films have also been proposed [3].

Recently, the growth of ultra-thin Ce-Ti-O and Yb-Ti-O films on Pt(111) has also been studied in order to explore the rare earth oxide hypermaterials [4]. In both cases, OQC-related structures have been prepared. These results will also be shown in the talk.

REFERENCES

[1] S. Förster, K. Meinel, R. Hammer, M. Trautmann, W. Widdra, Nature 2013, 502, 215.

[2] J. Yuhara, K. Horiba, R. Sugiura, X. Li, T. Yamada, Phys. Rev. Mater. 2020, 4, 103402.

[3] X. Li, K. Horiba, R. Sugiura, T. Yamada, J. Yuhara, Appl. Surf. Sci. 2021, 561, 150099.

[4] X. Li et al., Phys. Chem. Chem. Phys. (2023) (in press).

There is an essential need for studying a micro-sized region in sample, especially when the sample properties have spatial variation and/or the sample is finely structured in a micron scale. X rays focused down to a micron scale provide a unique means to study the electronic structure of such samples through absorption and photoemission spectroscopies. To study the electronic structure deeply, sample is often rotated around a certain axis. When the sample rotation is combined with linearly polarized x rays, absorption spectroscopy gives information about orbital and magnetic anisotropy. On the other hand, combining with photoemission spectroscopy, one can perform angle-resolved photoemission spectroscopy (ARPES) and visualize the band structure of a material. The difficulty to perform such measurements is that the x-ray spot on the sample needs to be fixed while rotating the sample. Since it is challenging to fix the sample position in a micron scale during rotation, alternative methods are desired.

To perform x-ray absorption experiments at a fixed sample position but with changing relative angles between crystalline axis and x-ray polarization, we developed a methodology to rotate the angle of linear polarization at SPring-8 BL07LSU. Utilizing a unique segmented cross undulator at BL07LSU, we generated linearly polarized x rays from the interference between left and circular polarized light. By regulating the phase difference between circularly polarized light with opposite helicity, we realized generation of linearly polarized soft x rays at an arbitrary azimuthal angle. This development has allowed us to perform polarization-angle dependent x-ray absorptions study to detect orbital and magnetic anisotropy of h-BN [1] and NiO [2], respectively, at a fixed sample position. We also implemented this polarization angle-rotation scheme in resonant photoemission spectroscopy measurements, and revealed the orbital anisotropy of an Fe-based monolayer magnetic film [3]. Furthermore, we constructed a scanning x ray spectroscopy system equipped with a Wolter mirror that can focus x rays below 1 mm [4]. All combined, electronic anisotropy in a submicron spot could be investigated by x-ray absorption-based spectroscopies. As for ARPES, recent developments on electron analyzer has made it possible to deflect photoelectrons and thereby scan photoemission angles electronically without sample rotation. Therefore, one can acquire band structure and Fermi surface mapping in two-dimensional momentum space for a fixed sample spot. Using this ARPES technique and 10-micron sized beam at MAX IV BLOCH beamline, we visualized spatially varying carrier concentrations and antiferromagnetic states in a copper-oxide high-temperature superconductor [5]. This methodology is useful to evaluate several different physical quantities at various sample positions and correlate them to examine their causal relationships.

[1] Y. Kudo. MH et al., Nucl. Instrum. Methods. Phys. Res. A 1018, 165804 (2021).

[2] Y. Kudo, MH et al., e-J. Surf. Sci. Nanotechnol. 20, 124 (2022).

[3] MH et al., J. Phys.: Condens. Matter 35, 425001 (2023).

[4] H. Ando, MH et al., e-J. Surf. Sci. Nanotechnol. 21, 200 (2023).

[5] M. Miyamoto, MH et al., in preparation.

Non-linear optical effects have provided us with an abundance of phenomena that are not achievable in the range of conventional linear optics. Second harmonic generation (SHG) is a second-order non-linear optical effects that doubles the frequency of light (from hν to 2hν) and occurs with broken inversion symmetry, such as surfaces/interfaces. Taking advantage of the characteristics, in the range of visible light, it has been used not only as a technique of the up-conversion method but also as a tool for probing surfaces/interfaces selectively.

With the advent of X-ray Free Electron Laser (XFEL) such as SACLA, SHG experiments have been extended into the X-ray region [1]. By utilizing the inner-core shell excitation resonance, element selectivity was demonstrated by soft X-ray SHG spectroscopy measurements [2]. Although the soft X-ray SHG method has been expected to be valuable for materials science, it is difficult to observe the weak soft X-ray SHG signals because of the contamination by external effects, such as second-order light generated at the XFEL beamline. In this research, we developed a new detection system for soft X-ray SHG spectroscopy employing an ellipsometry method [3] that opens a new research field of non-linear X-ray spectroscopy [4,5], for example in Li-ion batteries [6], solar cells [7], and spintronics [8].

This system was constructed at soft X-ray beamline BL1 at SACLA (See Figures). The incident light was guided through various filters that reduce the intensity to prevent the sample damage (Al filter) and cut off the second-order light from the beamline (Si filter). The beam was focused before irradiation on the sample and reflected at an angle of 45^o from the surface plane. The 2hν component of the beam from a sample was selectively reflected by a Ru/B₄C multilayer mirror and subsequently detected at a multi-channel plate. A unit of the mirror and the detector rotates with a rotation angle θ around the optical path to selectively detect X-rays with specific polarization [9]. To demonstrate the system, a GaAs crystal was selected, and the incident photon energy was set to hν = 75 eV to meet the resonance condition with the As M-edge. By choosing the X-ray polarization, it was elucidated that pure SHG signals can be observed without specific filters that remove the second-order light from the beamline. Furthermore, the detection system becomes a highly sensitive SHG measurement apparatus when it is combined with a Wolter mirror that focuses soft X-ray beam into sub-μm size [10]. In this presentation, we explain the newly developed system in detail and introduce state-of-the-art soft X-ray non-linear spectroscopy research.

[1] A. Zong et al., Nat. Rev. Mater. 8, 224 (2023). [2] Sh. Yamamoto et al., Phys. Rev. Lett. 120, 223902 (2018). [3] T. Sumi et al., e-J. Surf. Sci. Nanotech. 20, 31 (2021). [4] E. Berger et al., Nano Lett. 21, 6095 (2021). [5] C. B. Uzundal et al., Phys. Rev. Lett. 127, 237402 (2021). [6] C. Woodahl et al., Nat. Mater. 22, 848 (2023). [7] M. Horio et al., Appl. Phys. Lett. 123, 031602 (2023). [8] T. Sumi et al., Appl. Phys. Lett. 122, 171601 (2023). [9] M. Araki et al., e-J. Surf. Sci. Nanotech. 18, 231 (2020). [10] S. Egawa et al., Opt. Express 23 33889 (2019).

With the recent miniaturization of semiconductor devices, understanding the physical and electrical properties of semiconductor devices, such as the dopant concentration, dopant distribution and defect level distribution, at the nanoscale has become important. Among the physical properties of semiconductors, information on semiconductor interface states is particularly important. For example, in semiconductor devices such as field-effect transistors, the presence of semiconductor interface states is known to significantly affect device operation characteristics. Therefore, direct observation of semiconductor surfaces with nanoscale spatial resolution will become even more important for understanding and controlling the effects of these properties on devices and for evaluating semiconductor device operation.

Recently, we proposed high—low frequency Kelvin probe force microscopy (high—low frequency KPFM) as a technique to solve the above problem [1]. High—low frequency KPFM is a method for measuring the magnitude and direction of band bending due to interface states by applying low-frequency and high-frequency AC bias voltages between the tip and the sample with respect to the cutoff frequency of carrier transport between the bulk and interface states and measuring the difference in CPD by KPFM. In high-low frequency KPFM, frequency modulation (FM) KPFM (FM-KPFM) combined with FM-AFM is used to detect the tip-sample interaction force. FM-KPFM has several advantages, namely, high sensitivity to the electrostatic force gradient, high detection sensitivity using a cantilever with a weak spring constant at the first resonance, ease of implementation in adding FM-AFM, and no need to enhance the bandwidth of the cantilever deflection sensor. FM-KPFM is used to apply an AC bias voltage at frequencies lower than the cutoff frequency of carrier transport, and heterodyne FM-KPFM, based on the heterodyne effect (frequency conversion effect) between mechanical oscillation of the cantilever and electrostatic force oscillation, is used to apply an AC bias voltage at frequencies higher than the cutoff frequency of carrier transport. To date, high—low frequency KPFM has successfully visualized the surface band bending of pn-patterned silicon substrates [1]. However, in high—low frequency KPFM, the CPD is compensated by a DC bias voltage, so a certain DC voltage, determined by the CPD, is applied to the semiconductor sample. Therefore, the surface potential of the semiconductor is fixed at a certain energy, and only the surface state near the Fermi level of the surface is reflected in CPD measurements, making measurement of the energy distribution of the interface states within the band gap difficult. Thus, a method for measuring the energy distribution of the interface states must be developed.

In this study, we propose high-low KPFS using high- and low-frequency AC bias voltages to measure the interface state density inside semiconductors [2]. We derive an analytical expression for the electrostatic force between the tip and the sample that takes into account the charge transfer between the bulk and interface states in the semiconductor. We show that the electrostatic force between the tip and the semiconductor sample strongly depends on the capacitance of the charge depletion region on the surface and that the analysis of the electrostatic force at low- and high-frequency AC bias voltages can provide information on the interface state density in the semiconductor band gap (Fig.1). We also demonstrate using a pn-patterned silicon substrate that the interface state density can be measured.

References

[1] R. Izumi, Y. J. Li, Y. Naitoh, Y. Sugawara, Microscopy, 2022, 71, 98.

[2] R. Izumi, M. Miyazaki, Y. J. Li, Y. Sugawara, Beilstein Journal of Nanotechnology, 2023, 14, 175.

Recently developed Spectrum Imaging for Auger electron spectroscopy (AES) would be a next-generation analysis method providing cube data image with a wide spectrum at all pixels [1]. By the post data processing, the cube data can provide any total spectrum within any regions as you selected, and it can also reconstruct another meaning map by focusing on a slight difference of direct/differential spectra, for example, energy-filtered secondary electron images, elemental maps, chemical state maps and REELS maps.

In this presentation, we will explain the overview of Spectrum Imaging and its possibilities, and introduce some applications. As a further step, we acquired high energy resolution spectra at a pass energy of 10 eV with a semiconductor sample and evaluated the accuracy of this method. As a result, a distribution of p-type/n-type areas was obtained at a pn junction area by the Si KLL peak shift of 0.6 eV due to the fermi level difference clearly.

References

[1] Noboru Taguchi, Tatsuya Uchida, Konomi Ikita, AkihiroTanaka, Nobuyuki Ikeo, Kazushiro Yokouchi and Kenichi Tsutsumi, Ultra microscopy, 233,113450 (2022).

Photoelectron momentum microscope (PMM) is an instrument built on a new concept based on imaging-type photoelectron spectroscopy and microscopy techniques to visualize the electronic state in k reciprocal lattice space of a selected microscopic region [1]. We constructed a PMM station at the soft X-ray undulator beamline BL6U [2-4] of UVSOR synchrotron facility [5]. PMM offers a new approach for μm-scale momentum-resolved photoelectron spectroscopy (MRPES) [6-12]. A key feature of the PMM is that it can very effectively reduce radiation-induced damage by directly projecting a single photoelectron constant energy contour in reciprocal space with a radius of a few Å^-1 or real space with a radius of a few hundred μm onto a two-dimensional detector. This approach was applied to three-dimensional valence band structure E(k) and E(r) measurements (stereography) as functions of photon energy (hν), its polarization (e), detection position (r), and temperature (T). In this presentation, we describe some examples of possible measurement techniques using UVSOR PMM.

We are taking a new step forward from the conventional framework of studying electronic properties of various materials by means of μm-scale valence band mapping and momentum-selective photoelectron microscopy [4,9,12]. With a hν range up to 800 eV covered by the BL6U, core-level excitation of a variety of important elements including C, N, O and transition metals is possible. Specific atomic sites and electronic states can be selectively characterized by the Resonant momentum-resolved photoelectron spectroscopy [10,11]. Furthermore, a branch was added to BL7U, an undulator-based vacuum ultraviolet (VUV 6-40 eV) beamline (Fig.1). In addition to grazing-incidence soft X-ray excitation, normal-incidence VUV with variable polarization (horizontal/vertical/circular) excitation is also available at the same focal position of the PMM.

1) C. Tusche, et al., e-J. Surf. Sci. Nanotech. 18, 48 (2020).

2) FM, et al., Jpn. J. Appl. Phys. 59, 067001 (2020).

3) S. Makita, et al., e-J. Surf. Sci. Nanotech. 19, 42 (2021).

4) FM, et al., J. Phys. Soc. Jpn. 91, 094703 (2022).

5) H. Ota, et al., J. Phys.: Conf. Ser. 2380, 012003 (2022).

6) FM and S. Suga, Phys. Rev. B 105, 235126 (2022).

7) T. Kato, et al., Phys. Rev. Lett. 129, 206402 (2022).

8) O. Endo, et al., J. Phys. Chem. C 126, 15971 (2022).

9) E. Hashimoto, et al., Jpn. J. Appl. Phys. 61, SD1015 (2022).

10) FM, et al., J. Phys. Soc. Jpn. 90, 124710 (2021).

11) Y. Hasegawa, et al., e-J. Surf. Sci. Nanotech. 20, 174 (2022).

12) FM, et al., Rev. Sci. Instrum. 94, 083701 (2023).

"Atomic-resolution holography [1]” is a powerful technique that realized an analysis of the local 3D atomic arrangement around not only constituent atoms in a crystal but also isolated atoms such as dopants. In photoelectron holography, the angular distribution of photoelectrons emitted from the target atom is used as a hologram, which can be used to directly derive the three-dimensional atomic arrangement around the emitter atom. Hologram requires at least ±60° cone angular distribution to reconstruct acurate 3D atomic arrangement with a kinetic energy of about 600 eV. DIANA [2] has been used to measure holograms effectively because it can display the angular distribution of ±60° at once. The energy resolution of DIANA is 1 %, which is insufficient to resolve chemical shift. Hence we are developing a new high-energy-resolution display analyzer CoDELMA [3, 4], which is shown in Fig. 1. CoDELMA is the only two-dimensional electron spectrometer that can display the angular distribution of particular high-energy electrons with a high-energy-resolution of ΔE/E = 1/500 over a wide two-dimensional angular range of ±50° at once.

The use of a synchrotron facility is another obstacle to the use of holography for routine analysis. Hence we are developing atom-holography microscope (Fig. 1) by a combination of a scanning electron microscope (SEM) and CoDELMA. This new microscope enables us to easily analyze the 3D atomic arrangement around specific atoms at each nano region observed by SEM in the laboratory. Recent results are shown in the presentation.

References:

[1] H. Daimon, Jpn. J. Appl. Phys. 59, 010504 (2020).

[2] H. Daimon, Rev. Sci. Instrum, 59 (4) 545-549 (1988).

[3] H. Matsuda, L. Tóth, and H. Daimon, Rev. Sci. Instrum. 89, 123105 (2018).

[4] H. Matsuda, H. Daimon, et al., J. Electr. Spectr. Rel. Phenom. 264, 147313 (2023).

Introduction

In the field of materials science, it has been attempted to create stronger and lighter materials by controlling the amount and bonding state of incorporated hydrogen in metallic materials. One of the methods to investigate diffusion properties of the hydrogen atoms in the metallic materials and their desorption behavior at the material surface is the thermal desorption spectroscopy (TDS), in which a time varying desorption rate is measured while heating the material linearly, so that the desorption spectrum is measured. However, it is difficult to determine several parameters related to the diffusion and the desorption from a single measured spectrum because the internal state of the material changed during the TDS measurement. In this study, we are developing an inverse analysis method to determine the several parameters from a single TDS measurement. For the method of the inverse analysis, a real number genetic algorithm (RGA) with adaptive domain method (ADM) [1] has been employed.

Analysis model

At time t in the TDS, the concentration distribution of dissolved hydrogen C(z,t) at depth z in the target material follows the diffusion equation with the diffusion coefficient D =D₀ exp(-E_d/kT) in the metal sample, where D₀ is the frequency factor of diffusion, E_d is the activation energy of diffusion, k is the Boltzmann constant, and T is the absolute temperature which depends on time during TDS measurement. The hydrogen atoms recombining at the surface and desorbed as molecules are represented by dC/dt=-2k_surfC²(0,t), where k_surf is the recombination coefficient. If the initial concentration distribution is given by C (z,0)=A z exp(-(z-z_c)²/(2w² )), the desorption rate v(t)=k_surfC²(0,t) can be expressed by the 6 parameters of A, z_c, w, D₀ , E_d, and k_surf. To determine these parameters, RGA with ADM was used, in which many parameter sets are examined, and the domains of the parameters are narrowing according to the differences between the measured spectrum and the spectrum of the forward analyses from the parameter sets. For the fast forward analysis, the boundary integral equation is adopted by using the fundamental solution of the diffusion equation. To evaluate the performance of the inverse analysis system, we used a simulated spectrum which was prepared from given parameters instead of an actual measured spectrum.

Results and Discussion

Inverse analyses by RGA cooperated with ADM for different initial random seeds were performed on the simulation data. In most of results, the desorption rate spectrum could be expressed with small differences from the simulated data by the inverse analyses with difference seeds as shown in Fig.1; however, the estimated parameters were not unique. The activation energy E_d in the diffusion coefficients converged within a range close to the given true value, but other parameters were different for each random seed.

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With the recent development of wave number-resolved photoelectron emission spectroscopy, precise measurements of electronic states in the full wavenumber space (kx,ky,kz) of various functional materials, which were previously unobservable, are rapidly progressing experimentally [1]. Since the late 2010s, Time of flight (ToF) wavenumber-resolved photoelectron spectroscopy measurements have been actively pursued in Germany, and advanced analytical theories, including orbital tomography, continue to be published in Austria. In Japan, a spin-resolving wave number-resolved photoelectron emission spectroscopy has been developed at UVSOR (the Institute for Molecular Science), and will soon be operational. While conventional ARPES measurements can only provide photoelectron states in a limited and narrow direction in a single measurement, wave number resolved photoelectron spectroscopy can simultaneously measure all photoelectron states emitted in all directions of the electron energy analyzer with high efficiency. The revolutionary measurement evolution achieved by using a photoelectron emission electron microscope (PEEM) as the first stage of the analyzer has made it possible to obtain highly reproducible and reliable experimental results that are approximately 10,000 times more efficient than conventional photoelectron spectroscopy measurements and free from surface damage caused by prolonged light exposure.

On the other hand, because many approximations are used in the photoelectron spectral analysis theory, there are reported cases of discrepancies between the latest high-precision experimental data and calculation results. The photoelectron spectral analysis theory requires consideration of the electronic state of the solid surface (initial state), the interaction between electrons and light, and the photoelectron state (final state). In particular, since the photoelectron state has been largely approximated, there is a strong need for calculations that incorporate the "true photoelectron state" including decay effects as well as non-free electron and self-energy effects of the photoelectron state [2,3].

Our group has performed calculations that incorporate the many-body interactions that photoelectrons undergo from surrounding electrons during propagation into the photoelectron state using multiple scattering method [4,5]. These interactions have produced peak shifts and intensity reductions in the photoelectron spectra. In 2019, we developed a theory to replace the previous work on photoelectron emission from the inner shell of atoms to analyze wave number-resolved photoelectron spectra of organic molecular thin films. We developed a theory for photoelectron spectral analysis that formulated the photoelectron emission process from molecular orbitals and created an original program. Furthermore, we calculated the structure of organic molecules adsorbed on the surface based on density functional theory. After incorporating this into the original program as the starting state, the photoelectron spectral analysis described above was performed [6]. The origin of the peak, which could not be reproduced by the previous theory (plane wave approximation), was found to be due to the scattering of photoelectrons. They also succeeded in identifying the positions of adsorbed molecules.

Based on these results, our group analyzed ToF wavenumber-resolved photoelectron spectra measured by a group at the University of Würzburg, Germany, in 2020 [7]. It clarified that changes in the surface charge distribution at the fs level cause molecular deformation and rotation. I have been involved in measuring physical properties using light since my graduate school days [8,9]. These will be combined in an oral presentation.

[1] S. Suga and A. Sekiyama, Springer Series in Optical sciences 176, (2013).

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Sensing atomic-scale magnets through mechanical interaction offers the possibility of controlling new magnetic devices with low electrical current and high-density data storage [1, 2]. However, it is extremely challenging to study such weak interaction [3]. Here, we probe the z-component of the spin in finite Fe chains on a Ir surface by the spin-polarized STM and the non-contact atomic force microscopy (NC-AFM) at 4.5 K under external magnetic fields. We show NC-AFM images and tip-sample distance dependence of magnetic signals measured with spin-polarized tips. Our finding reveals the mechanical spin interactions between a tip and a sample at the atomic scale.

References:

[1] P. Rabl et al., Nat. Phys. 6, 602-608. (2010).

[2] M. Menzel et al., Phys. Rev. Lett, 108, 197204 (2012).

[3] U. Kaiser et al., Nature 446, 522-525 (2007).

The quantum properties of ultrathin metal films and surface superstructures have generated significant interest due to the sensitivity of confined two-dimensional (2D) electronic states to subtle differences in film thickness. In/Si(111) shows metal-insulator transition [1, 2] and two-dimensional superconductivity [3] at different thickness. In/Si(111)-(√7 × √3)-rect structure is an indium double-layer structure with atomically sharp In/Si interface [4]. The atomic arrangement of this structure closely resembles that of bulk In with a body-centered tetragonal (bct) structure. This similarity raises the possibility of further growth of (001) oriented In films upon additional deposition. However, previous studies have reported the preferential growth of three-dimensional (3D) islands at and above room temperature [5]. As a result, the layer-by-layer growth on the double-layer structure has been considered challenging. A few preceding studies reported that a triple-layer structure with (6 × 6) periodicity fully cover the (√7 × √3)-rect structure by deposition at temperatures below 200 K [5, 6], while the detailed atomic and electronic structure have not yet been clarified. In the present study, we investigated the triple-layer structure using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and angle-resolved photoelectron spectroscopy (ARPES).

Occupied state STM images showed that a hexagonal array of spots covered the (√7 × √3)-rect substrate at 80 K. On the other hand, empty state STM images did not reveal a (6 × 6) but a (11 × 11) periodicity of the triple-layer structure. We also confirmed the (11 × 11) superlattice by LEED. The large superlattice is explained by commensurate lattice matching between the Si and In hexagonal lattices, namely, 11a_Si = 13a_In (a_Si = 3.84 Å, a_In = 3.25 Å). The model shows a moiré modulation with a pseudo-(5.5 × 5.5)-periodicity.

ARPES measurements demonstrated that the Fermi surface consist of two circles (k_F ~1.32, 1.46 Å^-1) and warped hexagons. The warped hexagons are related to the circular Fermi surfaces by folding back at the first SBZ boundaries of Si(111) and hexagonal In lattices. The pair of two Fermi circles with different radii is considered as a characteristic feature of triple-layer structure [7].

References

[1] H. W. Yeom et al., Phys. Rev. Lett. 82, 4898 (1999).

[2] S. Terakawa et al., Phys. Rev. B 100, 115428 (2019).

[3] T. Uchihashi et al., Phys. Rev. Lett. 107, 207001 (2011).

[4] J. W. Park and M. H. Kang, Phys. Rev. Lett. 109, 166102 (2012).

[5] A. Pavlovska et al., J. Vac. Sci. Thechnol. B 20, 2478 (2002).

[6] T. Suzuki and K. Yagyu, Surf. Sci. 726, 122174 (2022).

[7] S. Terakawa et al., Phys. Rev. B 105, 125402 (2022).

In recent years, molecular two-dimensional structures on metal surfaces have been actively investigated in the field of organic molecular electronics and spintronics. Organic molecules on metal surfaces forms self-assemble structures that differ from the bulk due to the presence of molecular-substrate interactions in addition to intermolecular interactions. In other words, molecular two-dimensional structures with arbitrary symmetry can be constructed by regulating the symmetry and intermolecular interactions of the organic molecules used and the molecule-substrate interactions. This is expected to lead to the development of novel properties and functions. In particular, the honeycomb lattices are two-dimensional structures with strongly isotropic lattices proposed by graph theory, and their physical properties have been evaluated in various fields.

In this study, we discuss about the structural and electronic properties of self-assembled organic molecular monolayers on Ag(111), focusing on the C3 symmetric triptycene derivative: Trip-Phz (see Fig.1 insert) by means of scanning tunneling microscopy (STM) measurements, tight binding (TB) calculations and density functional theory (DFT) calculations.

Trip-Phz have been found to form a honeycomb lattice in crystals precipitated from TTF solution [1]. The three-fold symmetry of the Trip-Phz molecule and the symmetry of the honeycomb lattice shows that they form a Kagome lattice when the bonding points between the molecules are taken as vertices (red dash line in Fig.1). The band structure of the Kagome lattice has not only in the Dirac band, but also a flat band [2]. This band structure is expected to show strongly correlation phenomena such as magnetism, superconductivity, and fractional quantum Hall effect.

In this presentation, we comprehensively discuss the film structure and electronic state of the self-assembled molecular structure by combining experimental results from STM measurements and theoretical results from TB calculations and DFT calculations.

In the STM measurement, we deposited a very small amount of Trip-Phz on Ag(111) and observed the molecular self-assembled structure and its electronic state in the sub-monolayer region. In this region, three self-assembled structures were observed: a one-dimensional chain structure, a two-dimensional stripe structure, and a two-dimensional structure with honeycomb lattice. In particular, the honeycomb lattice showed 7x7R21.8° /R38.2° superstructures on the Ag(111) surface. These two domains exhibited a chiral structure with each other.

As noted above, this honeycomb lattice shows that they form a Kagome lattice due to the three-fold symmetry of Trip-Phz. Moreover, Dirac band and flat band are confirmed in the TB calculation for this Kagome lattice. Therefore, we performed DFT calculations to confirm whether this electronic states are maintained even on the Ag(111) substrate. As a result, it was confirmed that the Trip-Phz honeycomb lattice maintains Dirac-like band dispersion and flat band even on the Ag(111) substrate for both HOMO and LUMO states.

It is presumed that this is achieved by the π-conjugated plane of the Trip-Phz molecule adsorbing perpendicularly to the Ag(111) surface, thereby suppressing the molecule-substrate interaction.

Details will be reported in the presentation.

References

[1] R. Ushiroguchi, Y. Shuku, R. Suizu, and K. Awaga, Cryst. Growth Des., 20, 7593 (2020)

[2] T. Niziguchi, M. Maruyama, S, Okada, and Y Hatsugai, Phys. Rev. Mat., 3, 114201 (2019)

Bulk iron telluride (FeTe) shows a bicollinear antiferromagnetic (AFM) order under 65 K [1]. The bicollinear AFM order can be tuned to a magnetic helix order by adding extra Fe [1, 2]. Recent angle-resolved photoemission and scanning tunneling spectroscopy research indicate that FeTe also shows Kondo-lattice behavior [3]. In our previous research, we found that SrTiO₃-√ 13⨯√ 13 substrate leads to a lowered C₂ symmetry on monolayer iron selenide (ML FeSe); furthermore, the electron doping from SrTiO₃-√ 13⨯√ 13 substrate surface locally modulates the superconducting gap size of the ML FeSe [4]. Considering these facts, several questions are raised: 1. Can the SrTiO₃-√ 13⨯√ 13 substrates also lower the rotational symmetry on the isostructural ML FeTe? 2. Can the Kondo-lattice behavior be confirmed on the ML FeTe film? (The AFM ordering has been confirmed [5].) 3. Can the SrTiO₃ substrate locally modulate the ground state of ML FeTe?

To clarify the above points, we performed low-temperature scanning tunneling microscopy/spectroscopy (STM/STS, T: 5 K ~ 77 K) experiments to measure the surface and electronic structure of ML FeTe/SrTiO₃-√ 13⨯√ 13.

The following are the main results of our present study:

1. In the atomically resolved STM images, as shown in Fig. 1(a), we observed that bright atoms form  2⨯ 3 cells arranged in √ 13⨯√ 13 periodicities. The bright  2⨯ 3 cells lead to a C₂ rotational symmetry on the monolayer FeTe film, which originates from the SrTiO₃ substrate.

2. The STS spectra obtained on the monolayer FeTe show asymmetric dip-like features near the Fermi level, as shown in Fig. 1(b), which fit well with the Fano functions, illustrating a Kondo scenario that localized Fe spins are screened by the conduction electrons.

3. The spatial dependence of the Fano-shaped STS spectra was observed to follow the √ 13⨯√ 13 periodicities and two types of spectra were confirmed (Fig. 1(b)), indicating that the SrTiO₃ substrate can modulate the interaction between the localized spin and conduction electrons mentioned above.

The details concerning the similarities and differences between the two cases of ML FeSe/SrTiO₃ and ML FeTe/SrTiO₃will be discussed in the presentation.

References:

[1] Shiliang Li et al., Phys. Rev. B 79, 054503 (2009).

[2] Mostafa Enayat et al., Science 345, 653-656 (2014).

[3] Younsik Kim et al., Nat. Commun. 14, 4145 (2023).

[4] Wen Si et al., Phys. Rev. B 105, 104502 (2022).

[5] S. Manna et al., Nat. Commun. 8, 14074 (2017).

TiO₂ has promising applications in solar conversion and hydrogen storage^1,2. Black titania formation due to hydrogen doping is reported to reduce the band gap from 3.1 eV to 1.54 eV, which significantly improves solar absorption efficiency. TiO₂ is also capable of storing hydrogen up to 1.4 w.t. % which is facilitated by hydrogen diffusion at 7 MPa and 450^oC. In order to investigate the hydrogen effect at higher concentration, hydrogen ion irradiation at ultrahigh vacuum environment is employed. This technique eliminates the necessity of using high pressure, high temperature, and Pd overlayer which facilitates hydrogen dissociation. Therefore, we investigate the electrical conductivity and electronic properties changes of TiO₂(100) by hydrogen ion irradiation.

Hydrogen ion irradiation (2 keV) on Nb-doped rutile TiO₂(100) at 295 K causes its measured electrical resistance to decrease, which is due to electron donations from the irradiated hydrogen to TiO₂. The donated electron reduces the Ti⁴⁺ to Ti³⁺ which is observed by measuring the chemical state change of Ti 2p core level in x-ray photoelectron spectroscopy (XPS). In ultraviolet photoelectron spectroscopy (UPS), the donated electron in the Ti 3d orbital forms an in-gap state (IGS) at 1 eV below the Fermi level. Moreover, the UP spectra also shows that the valence band bends downward which suggests electron donations to the TiO₂. These results indicate that Ti³⁺ state behaves as a small polaronic center which contributes to the conductivity.

In contrast to the results at 295 K, the hydrogen ion irradiation at 125 K causes the electrical resistance to increase. The XP spectra shows that the Ti³⁺ chemical state increases after the irradiation. However, it is observed that the IGS forms at a deeper energy level (1.3 eV below the Fermi level) compared to the former IGS by irradiation at 295 K. This suggests that there are at least two Ti³⁺-related IGS whose transport properties are different. Upon annealing to 295 K and cooling again to 125 K, the IGS composition changes permanently. Therefore, the deeper IGS is likely to be a metastable state.

References:

1. Wang et. al., Advanced Functional Materials 2013, 23, 5444-5450

2. Sun et al., The Journal of Physical Chemistry C 2011, 115, 25590-25594

Recently, the appearance of spin-triplet Cooper pairs in single atomic-layer (SAL) superconductors due to the combination with Rashba-type spin-splitting has become a topic of great interest since its first discovery in the SAL (Tl,Pb) structure on semiconductor substrates. In the search for other structures exhibiting these phenomena, several (Pb,Bi) SAL structures have been found to be promising candidates due to their strong spin-orbit coupling. In theoretical studies, the (Pb,Bi)/Ge-√3×√3 has been predicted to exhibit chiral topological superconductivity in addition to the emergence of a van Hove singularity [1-3]. Experimentally, the (Pb,Bi)/Ge-2×2, -2√3×2√3 mixed structure was successfully fabricated and its surface structure confirmed by scanning tunneling microscopy [4]. As for the electronic properties of these structures, no experimental results have been reported.

For this study, we measured the electronic structure of various (Pb,Bi) SAL structures using angle-resolved photoemission spectroscopy (ARPES). We successfully fabricated the (Pb,Bi)/Ge-1×1, -√3×√3 and -2×2 structures, for which the corresponding low-energy electron diffraction (LEED) patterns are shown in Figures 1 (a)-(c). Figure 1 (d) shows the core-level photoemission spectra of the Pb and Bi 5d peaks. These spectra were taken at the beamline BL-13 of the Saga Light Source with a beam energy of hν = 70eV. To determine the Pb to Bi composition ratio of each structure, we compared the intensities of these photoelectron spectra. We found that the (Pb,Bi)/Ge-2×2 (-√3×√3) structures has a ratio of 0.5:1 (1.85:1), while previous reports state a 1:1 (3:1) ratio [1-4]. In addition, we found that the (Pb,Bi)/Ge-1×1 structure has a similar Pb to Bi ratio with 2.04:1 as the -√3×√3 structure. Moreover, we observed that all of our structures have a metallic band dispersion that did not match the reported ones from previous studies.

In this presentation, we will show the electronic states of these structures and compare them to calculated band structures and discuss the relationship between the electronic structures and composition ratios.

[1] W. Qin et al., Nat. Phys. 15, 796 (2019).

[2] L. Li et al., Phys. Rev. B 102, 035150 (2020).

[3] S. Sun et al., Phys. Rev. B 103, 235149 (2021).

[4] A. Mihalyuk et al., Front. Mater. 9, 882008 (2022).

Molecules show strong infrared absorption when adsorbed on metal nanoparticles or on the surface of discontinuous metal thin films. The phenomenon was reported first by Hartstein et al. in 1980 [1] and is known as surface-enhanced infrared absorption (SEIRA). SEIRA has been used in various fields, such as the detection of trace chemical species and the tracking of chemical reactions on electrode surfaces [2]. Our previous study suggested that SEIRA occurs not only on metal nanostructured surfaces but also on non-metal nanostructured surfaces [3]. Therefore, we conducted infrared absorption spectroscopy measurements on line and space nanostructured surfaces of silicon, a semiconductor material.

The line and space nanostructures were fabricated in the Takeda–Sentanchi Building's super clean room at the University of Tokyo. Each structure occupies 1600 × 1600 μm² on a silicon substrate. The line width of the line and space structure was 100 nm, with four different line periods of 350 nm, 600 m, 850 nm, and 1100 nm. Polyacrylic acid (PAA; Wako Pure Chemical Industries Ltd.) was spin-coated onto the substrate with the line and space nanostructures. A 200 μL droplet of 1 to 10 g L^-1 PAA ethanol solution was dropped on the substrate. The infrared absorption spectroscopy was performed by p-polarized multi-angle incidence resolved spectroscopy (pMAIRS).

The enhancement factors of infrared absorption for the line and space nanostructures were found to be three to four. Comparing the enhancement factor of the peaks around 902 cm^-1 and 1710 cm^-1, the peak around 902 cm^-1 was more enhanced. Longer line periods give a greater enhancement factor. However, when the line period is more than about one-tenth of the excitation wavelength, the enhancement effect is smaller.

References

[1] A. Hartstein, J.R. Kirtley, J.C. Tsang, Phys. Rev. Lett. 45, 201 (1980).

[2] M. Osawa, Appl. Phys. 81, 163 (2001).

[3] T. Shimada, H. Nagashima, Y. Kumagai, Y. Ishigo, M. Tsushima, A. Ikari, Y. Suzuki, J. Phys. Chem. C 120, 534 (2016).

Recently, lead halide perovskites debuted in the field of condensed matter physics and materials chemistry, and they are a new class of semiconductor materials for a wide range of applications such as photovoltaics, photodetectors, light-emitting diodes, and lasers. High-quality thin films and nanocrystals can be easily synthesized by simple low-temperature solution processes. Grains in polycrystalline thin films and nanocrystals possess extremely low-density defect states, and they exhibit high photoluminescence quantum yields even at room temperature. Defect-tolerant halide perovskites show complex and fascinating optical and transport properties reflecting their unique ionic crystal structures. We determined the multi-band electronic structure, exciton binding energy, and reduced exciton mass of halide perovskites by using nonlinear optical spectroscopy [1-3] and magneto-optical spectroscopy [4]. The multi-level electronic structure originating from a large spin–orbit interaction was also revealed by optical Stark spectroscopy of nanocrystals [5]. Strong exciton-phonon coupling causes unique optical and thermal properties [6-8]. Furthermore, nanocrystal quantum dots and atomically thin two-dimensional layers of lead halide perovskites show the superior luminescence properties. Bright nanocrystals display highly efficient single-photon emission [9], and trions and biexcitons cause photoluminescence blinking [10]. Stable spin-polarized excitons in two-dimensional layers show ultrafast expansion at room temperature [11]. In this talk, we discuss the fundamental optical properties of lead halide perovskites and the photocarrier dynamics in perovskite solar cell devices. We also discuss the impact of the surface and interface states on the optical and transport properties of halide perovskite materials and solar cell devices.

Part of this work was supported by JSPS KAKENHI (Grant No. JP19H05465) and JST-CREST (Grant No. JPMJCR21B4).

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Heavy fermion (HF) systems in rare-earth (RE) intermetallic compounds originating from hybridization between localized f-electrons and conduction electrons, namely c-f hybridization, are central topics in the field of the strongly-correlated electron systems [1]. At low temperatures, depending on the strength of the c-f hybridization, the physical properties change from itinerant f electrons because of the Kondo effect or magnetic order originating from the magnetic moment of localized f- electrons due to Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. The competition between itinerant and localized characters of the f-electrons make a quantum critical point (QCP), resulting in the emergence of fertile quantum phenomena such as non-Fermi liquid behavior, and non-BCS HF superconductivity [2].

On the other hand, the dimensionality in the system characterizes the fundamental physical property. In low-dimensional systems, the enhancement of the electron-electron correlation and/or breaking of the inversion symmetry leads to novel quantum states. The combination of the HF state and low dimensionality modifies the ground state of the system because the order parameter of these systems is much more sensitive to dimensionality [3]. The ground state of two-dimensional (2D) HF can be easily controlled to the vicinity of a quantum critical point, which is the host to realize unconventional physical properties such as HF superconductivity, by simple external fields such as gate-tuning, and surface doping in addition to traditional external perturbations; temperature, pressure, and magnetic field [4-6].

The fabrication of artificial low-dimensional strongly-correlated electron systems and the quantization of a three-dimensional HF state by quantum confinement suitable methods to investigate the novel electronic phase. In the Ce-based artificial superlattice, the suppression of antiferromagnetic (AFM) ordering as well as the increase of the effective electron mass with decreasing of the thickness of the Ce-layer [7] has been reported. To understand the fundamental properties of 2D HF systems, it is necessary to clarify the electronic band structure and the formation mechanism of the HF. However, the details have remained unclear due to the lack of promising materials and the extremely low transition temperatures of less than a few K to HF even in known materials [7-8].

In this study, we report the HF electronic structure of a novel Yb-based monoatomic layer Kondo lattice; synchrotron-based angle-resolved photoelectron spectroscopy (ARPES) on monoatomic layered YbCu₂ on Cu(111). From ARPES, The 2D conducting band and the Yb 4f state, located very close to the Fermi level, are observed. These bands are hybridized at low-temperature, forming the 2D HF state, with an evaluated coherent temperature of about 30 K. The effective mass of the 2D state is enhanced by a factor of 100 by the development of the HF state. Our study provides a new candidate as an ideal 2D HF material for understanding the Kondo effect at low dimensions [9].

References

[1] P. Coleman et al., Journal of Physics: Condensed Matter 13, R723 (2001).

[2] C. Pfleiderer, Rev. Mod. Phys. 81. 1551-1624 (2009).

[3] S. Sachdev, Science 288, 475 (2000).

[4] W. Zhao, W. et al., Nature 616, 61–65 (2023).

[5] P. J. W. Moll et al., Nature Communications 6, 6663 (2015).

[6] B. G. Jang et al., npj 2D Materials and Applications 6, 80 (2022).

[7] H. Shishido et al., Science 327, 980 (2010).

[8] M. Neumann et al., Science 317, 1356–1359 (2007).

[9] T. Nakamura et al., arXiv: 2306.06984 (2023).

Graphene-based two-dimensional superconductors are promising for diverse applications owing to their inherent transparency and flexibility. Among these materials, C₆CaC₆, a bilayer graphene intercalated with calcium on a SiC substrate, exhibits the highest T_c, although the reported value has scattered from 4 to 8.8 K [1–4]. Early studies primarily accepted the free-standing C₆CaC₆ model based on the observation of its electronic structure [5]. Recent studies also imply the intercalation at the interface of C₆CaC₆ and the SiC substrate [2, 3]. Furthermore, since the formation of multiple two-dimensional layers at the SiC/graphene interface without intercalation in between the graphene layers has also been reported for some elements (confinement epitaxy) [6], a systematic investigation of the interface structure upon Ca deposition to the graphene/SiC systems is desired. In this investigation, we have successfully acquired compelling evidence of manifestation of confinement epitaxy in the superconducting C₆CaC₆/SiC system. We meticulously tracked the progression of the C₆CaC₆ growth on SiC using a sophisticated ultrahigh vacuum multi-probe system combining electron diffraction, photoemission spectroscopy, and transport measurements. We found that surplus deposition of calcium alters the electronic state of C₆CaC₆ from the band structures depicted in Fig. 1(a) to that elucidated in Fig. 1(b); a X-shaped metallic (XM) band emerges alongside theα*, β*, and interlayer (IL) band intrinsic to C₆CaC₆. Furthermore, the hybridization of the XM and β* bands leads to the emergence of flat bands. Through comprehensive electronic state analysis, we deduce that the XM band originates from the metallic calcium sublayer positioned between C₆CaC₆ and the calcium terminated SiC, as illustrated schematically in Figs. 1(c) and (d). A comparative analysis of the transport characteristics of C₆CaC₆ structures, both in the presence and absence of the Ca sublayer is presented in Fig. 1(e), which unveils the impact of the Ca sublayer in augmenting T_c. Two distinct categories of many-body phenomena are envisaged as contributing T_c: the intensified pairing potential ascribed to the flat band and the reduction of the scattering potential on the substrate surface owing to the existence of the interfacial metallic layer. This infers that the uncharacterized interface structure in prior investigations was responsible for the T_c scattering. [1] S. Ichinokura et al., ACS Nano 10, 2761 (2016). [2] Y. Endo et al. Carbon 157, 857 (2020). [3] H. Toyama et al., ACS Nano 16, 3582 (2022). [4] X. Wang et al., Nano Letters 22, 7651 (2022). [5] K. Kanetani et al., Proceedings of the National Academy of Sciences 109, 19610 (2012). [6] N. Briggs et al., Nature Materials 19, 637 (2020).

The interaction between an organic and an inorganic material is important to understand the mechanism of forming interfacial electronic states. Although the impact of the weak interaction on the electronic state has hardly been investigated, non-negligible features have been observed in occupied states of an organic molecule and inorganic substrate. For example, angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) of the pentacene (PEN) monolayer (ML) on graphite shows that the spectral fine features of the highest occupied molecular orbital (HOMO) state are modified from those of an isolated gaseous molecule [1-3]. A simulated photoemission spectrum including interfacial interactions is in good agreement with the experiment [4]. These results indicate that deep insight into the fine features of ARUPS is necessary to fully understand the electronic structure of weakly interacting molecular systems beyond the simple approximation. The unoccupied states are expected to be significantly affected in the spectral features due to a larger spread and overlap of the electron cloud against these tiny changes in the occupied valence states. The unoccupied state is accessible in some specific cases by so-called angle-resolved secondary-photoelectron spectroscopy [5] although photoelectron spectroscopy is known to detect the band structure of the occupied state in general.

In the LEED measurement, it is observed that the PEN (ML) structure is arranged from the liquid-like disorder (above the transition temperature, 130 K) to the incommensurate ML crystal (below transition temperature). Figs. 1 (a–c) show three characteristic ARUPS of PEN/HOPG taken at the crystalline phase. The dispersive convex (hole-like) band appears in the kinetic energy range of 1.4 to 2.4 eV as a constant final state (CFS) overlapping with the discrete non-dispersive HOMO (00) where the vibronic coupling fine features (01) and (02) are resolved. The convex band is absent in the valence band of pure graphite in both the experiment and theory [6] in the observed energy and momentum region. The CFS dispersive band consists of positive and negative intensities depending on the excitation energy, indicating Fano resonance [7] involving a discrete molecular state that couples a continuum state in different excitation paths (Figs. 1 (d, e)). In both ML phases, a characteristic asymmetric Fano profile with respect to the excitation energy is observed. In a photoelectron emission, several examples are has been reported for core excitation with the Auger decay process [8] and valence excitation of a rare gas superstructure on the surface [9], however no results has yet been reported for the complicated molecular orbital-related systems so far. The continuum state at the PEN/graphite interface could originate from a newly formed conduction band at the weakly bound interface, indicating a strong impact of the weak electronic coupling on the wave function connection via a larger spread of the unoccupied states demonstrating measuring a fingerprint of the weak interaction at the vdW interface. We discuss a concept to describe the impact of weak interaction on the electronic states by Fano profile analysis.

References

[1] H. Yamane, et al., Phys. Rev. B 72, 153412 (2005).

[2] S. Kera, et al., Progress in Surface Science 84, 135 (2009).

[3] P. Puschnig, et al., Science 326, 702 (2009).

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[7] U. Fano, Phys. Rev. 124, 1866 (1961).

[8] F. Matsui, et al., J. Phys. Soc. Jpn. 90, 124710 (2021).

[9] F. Patthey, et al., Phys. Rev. Lett. 82, 2971 (1999).

Methanol plays critical roles in chemical industries but the precise behavior of the synthesis under operating conditions remains unelucidated. The methanol synthesis is initiated by the CO₂ hydrogenation on Cu-based catalyst, a gas-surface reaction which proceeds by Eley-Rideal (E-R) type mechanism. Through molecular-beam experiment, it has been shown that the dissociative and associative gas-surface reactions can be precisely controlled by adjusting the translational, rotational, and vibrational energies of the molecular reactant. The result shows that the hydrogenation can be promoted by increasing the vibrational and translational energies of hot CO₂, in which the vibrational energy is thought to play more significant role [1]. However, the molecular beam experiment only allows the control of the average of CO₂ vibrational energy. Considering there are multiple vibrational modes of CO₂ molecule, the question is raised on which mode that actually contributes to the promotion of the reaction.

To elucidate the importance of vibrational mode in the CO₂ hydrogenation, a set of molecular dynamics (MD) simulations driven by machine-learning potential has been performed. Unlike the molecular beam experiment, MD allows precise control of initial vibrational mode of CO₂ (i.e., bending and stretching modes) combined with translational energy and incident angles. Ultimately, the dependence between reaction probability and specific mode of CO₂ can be obtained.

The active and on-the-fly learning scheme is implemented to generate the database efficiently. This scheme utilizes the quantification of uncertainty provided by the Gaussian Process framework that learn the energy and forces of various atomic environments in order to generate the accurate machine-learning potential. The results of MD shows that the successful hydrogenation of CO₂ (Fig. 1) as well as the desorption of CO₂ due to the Pauli repulsion (Fig. 2) can be simulated with satisfying accuracy under experimental surface temperature. Fig.3 and Fig.4 shows the high similarity of the kinetic energy profile obtained by machine-learning MD with the ab-initio MD. The details on the behavior of reaction as the result of varying the normal mode excitation will be presented in the conference presentation, especially the combination of initial states that might reduce the required translational energy for the hydrogenation.

[1] Quan, J. et al. Nat. Chem. 11, 722–729 (2019)

Diffusion of Hydrogen and its dynamics in metals have been in focus owing to their rich quantum behavior [1]. Despite several studies, a physical understanding of the H diffusion in metals is still lacking. Platinum being known for its catalytic properties has been less understood for its interaction with H. Moreover, the studies on the PtH_x system are limited to specific surfaces in the thermal regime (>100K) [2,3]. Here, we present the H diffusion in PtH_x of different H concentrations (x) over a wide range of temperatures (5 – 200K).

Hydrogen diffusion in a thin polycrystalline Pt (25 nm) film has been investigated through resistance relaxation experiments at temperatures in the range (5 – 200 K). The film is hydrogenated by a low-energy ion-irradiation technique at an energy of 500 eV at 25 K, resulting in metastable PtH_x. Arrhenius analysis of the H-hopping rate extracted from the exponential fitting of the relaxation data (Fig.1) shows a leveling off to a weak temperature-dependent regime below 140 K, commensurate with a mechanism dominated by quantum-rich processes. In the classical regime (>140 K), the H hopping rate shows a H concentration dependence (x) whereas this dependence becomes negligible in the quantum regime. The activation energy E_A extracted from the Arrhenius slope shows a decrease from 115 meV to 70 meV when x in PtH_x sample rises from 0.23 to 0.45 H/Pt, indicating modification of energy level of H-atoms at the metastable sites in the PtH_x by a repulsive H-H interaction at higher x.

References:

[1] K. Christmann, Surf. Sci. Rep. 9 (1988) 1.

[2] A. P. Jardine et al., Phys. Rev. Lett. 105, (2010) 136101.

[3] G. Kallen et al., Phys. Rev. B, 65, (2001) 033406.

The interaction of O₂ with various metal surfaces induces changes in its chemical stability and reactivity [1]. And the ability to control such processes bears on the chemical economy. Alloying of pristine metals provides one of the simplest and oldest way to do so (cf., e.g., [2-5]). Unraveling the stereochemistry of the processes involved would be imperative for understanding the mechanisms behind these interactions [6]. The dynamics of reactant molecules (esp., the orientation and the movement of molecules in 3D space) play an important role in reactions. The small rotational energy excitations involved (ca. less than a few meV) render the reactants susceptible to dynamical steering [1,7-9]. This makes direct comparison with theory rather challenging [1,10]. To directly probe and observe the (polar and azimuthal) orientation dependence of O₂ adsorption on Cu(110) and Cu₃Au(110), we prepared space quantized O₂ molecular beams by sorting the quantum states of the O₂ (cf., e.g., [5] and references therein) via Richtungsquantelung (space quantization), as first introduced by the 1922 Stern-Gerlach experiment [11,12]. We found that chemisorption proceeded rather favorably with the O-O bond axis oriented parallel (vs. perpendicular) to the surface, and also for O-O bond axis oriented along [001] (vs. along [-110])[5]. Alloying with Au introduced a higher activation barrier to chemisorption. This hinders the surface from further oxidation, and azimuthal anisotropy becomes almost negligible. The presence of Au also prevented cartwheel-like rotating O₂ from further reactions. More details will be presented at the conference.

[1] Montemore, M.M. et al., Chem. Rev. 118, 2816−2862 (2018).

[2] Tsuda, Y. et al., Phys. Chem. Chem. Phys. 16, 3815− 3822 (2014).

[3] Okada, M. et al., Sci. Rep. 6, 31101 (2016).

[4] Tsuda, Y. et al., Sci. Rep. 11, 3906 (2021).

[5] Tsuda, Y. et al., JACS Au 2, 1839-1847 (2022).

[6] Vattuone, L. et al., Angew. Chem. 121, 4939−4942 (2009); Prog. Surf. Sci. 85, 92−160 (2010).

[7] Diño, W.A. et al., Prog. Surf. Sci. 63, 63−134 (2000).

[8] Kasai, H. et al., Prog. Surf. Sci. 72, 53− 86 (2003).

[9] Diño, W.A., J. Phys.: Condens. Matter 14, 4379−4384 (2002).

[10] Ertl, G. Surf. Sci. 6, 208−232 (1967); Surf. Sci. 299−300, 742−754 (1994).

[11] Gerlach, W., Stern, O., Z. Phys. 9, 349–352 (1922).

[12] Hershbach, D. In: Molecular Beams in Physics and Chemistry, B. Friedrich, H. Schmidt-Böcking (eds.) (Springer, Cham, 2021) 1–22.

Friction is a ubiquitous phenomenon caused by the relative motion of two bodies and involves the dissipation of energy. Although this phenomenon is well known and has been studied for many years, it is not easy to understand because of the multiple contributions at different scales. To understand the phenomenon, it is ideal to study systems defined at the atomic scale, a typical example of which is the manipulation of a molecule on surfaces using atomic force microscopy (AFM) or scanning tunneling microscopy (STM). Such molecular manipulation has the advantage of producing fascinating nanostructures on surfaces, and its elementary processes have been studied. One of the most impressive manipulation studies is the measurement of the force required to move a molecule with AFM, i.e. the observation of static friction [1]. On the other hand, the dynamics and energy dissipation during molecular manipulation, i.e. kinetic friction, has not been extensively studied due to the hurdle of measurement and the difficulty of interpretation based on ab initio calculations.

Here we have studied the manipulation dynamics of a single CO molecule on a Cu(110) surface with a metallic tip by combining AFM, inelastic electron tunneling spectroscopy, and density functional theory calculations [2,3]. We found the following: (1) When the tip is far away from the surface, a CO molecule is adsorbed on a top site, but when the tip approaches the molecule just above the top site, the adsorption site switches to the bridge site. (2) When the oscillating tip is brought close to the molecule, the switching between the top and bridge sites occurs in correlation with the oscillation of the tip, and an energy dissipation signal is observed. (3) When the tip is placed close to the neighboring top site, molecular manipulation occurs from the top site to the neighboring top site. (4) In this case, even when the oscillating tip is brought close to the molecule, the switching occurs only once, so the energy dissipation is not measured in the time-averaged measurements. (5) Such a bridge site plays an important role as an intermediate state in molecular manipulations where the tip is scanned laterally. (6) By considering this intermediate state, the dynamics of molecular manipulations, including the difference between static and dynamic friction, can be understood. (7) Such an intermediate state has also been observed for CO manipulation on a Cu(111) surface, suggesting the generality of our results.

References

[1] M. Ternes, C. P. Lutz, C. F. Hirjibehedin, F. J. Giessibl, A. J. Heinrich. Science 319, 1066-1069 (2008).

[2] N. Okabayashi, T. Frederiksen, A. Liebig, F. J. Giessibl, accepted in Phys. Rev. Lett.

[3] N. Okabayashi, T. Frederiksen, A. Liebig, F. J. Giessibl, accepted in Phys. Rev. B

Angle-resolved photoemission spectroscopy (ARPES) is one of the main experimental techniques for probing the electronic structure of crystal surfaces and oriented molecules. In recent years ARPES has become very popular because of great instrumental advances, e.g. with photoelectron momentum microscope. While normal ARPES lacks elemental and local information, such information may partially be gained by performing ARPES at a core-level resonance. Although this is well known for long, generally no qualitative analysis of the data is done, because of a lack of computational methods for resonant ARPES (RARPES) in solids. The theoretical challenge is that a realistic description of RARPES must include both longe-range band structure [1] and short-range electron correlation effects, where the latter give rise to pronounced multiplet structures in the resonant spectra. Here, I present our recent advances in the theory of RARPES and resonant Auger electron spectroscopy and diffraction from valence bands. We have developed a theoretical method for RARPES which combines a ligand field multiplet calculation to describe the resonant photoemission process at the core-hole site [2]. The emitted photoelectron wave is propagated using multiple scattering theory which accounts for the photoelectron diffraction effects [3]. We have applied the theory to resonant photoemission with circular polarized light from a Ni surface either magnetized or non-magnetized. For the magnetized surface [2], we have reproduced the complex angular dependence of the circular dichroism (CD) signal, which is dominated by a dipole-type angular distribution due to XMCD effect in the core-excitation step. Moreover the fast angular variations of the CD are also qualititively reproduced and are explained to be due to photoelectron scattering, as in the Daimon effect. For the non-magnetized Ni surface, we could explain the strong Daimon effect which was recently reported for the Ni-2p3d3d and Ni-2d3p3d resonant spectra and the CD sign reversal observed at certain binding energies [4]. These phenomena are well reproduced by our theory and explained it in terms the angular momentum transfer from the photon to the excited electron or the ionized surface [5]. Finally we discuss the CD in normal Auger emission as well as in resonant spectator Auger emission and show that the main effect can be explained in a two-step model where the CD is determined by the core-hole alignement with the adsorbed photon field. The figures shows the calculated Cu 2p3d3d normal Auger spectra for different 2p-core-hole initial states. The black lines are the emission intensities and the red lines are the intensity multiplied by the magnetic quantum number of the emitted electron.

[1] F. Da Pieve and P. Krüger, Phys. Rev. Lett. 110, 127401 (2013). [2] R. Sagehashi, G. Park and P. Krüger, Phys. Rev. B 107, 075407 (2023). [3] P. Krüger et al. Phys. Rev. Lett. 108, 126803 (2012). [4] F. Matsui et al., Phys. Rev. B 97, 035424 (2018). [5] P. Krüger, in preparation.

It is interesting to see how surface modification or reconstruction can be used to manipulate the physical properties and electronic properties of unoccupied states on solid surfaces. An important representative of unoccupied surface states is the image potential state. This state exhibits unique characteristics, such as long lifetime and strong confinement near the surface, making it valuable for understanding the forces experienced by the electron in the vicinity of the metal.

In condensed matter physics, the periodicity of atoms results in the opening of bandgaps, a common occurrence. However, the image potential state has not shown bandgap opening due to the long-range periodic potential within the material. Nevertheless, surface reconstruction, such as the formation of a (5x1) superstructure with Iridium (Ir), can potentially affect the image potential state by introducing additional confinement and modifying electron wavefunctions.

In this study, we have employed two-photon photoemission (2PPE) spectroscopy to investigate the states above the Fermi level. Our observations revealed that the Ir(001)-(5x1) reconstructed surface exhibits a band gap, which arises from the surface reconstruction, whereas such a band gap is not seen on Ir(001)-(1x1) surface. This difference can be attributed to the distinct surface reconstructions of the two structures. This study of the Ir system with surface reconstructions, such as the (5x1) superstructure, offers a platform to investigate electronic properties and phenomena associated with confined electron dynamics.

In the pursuit of innovative materials for magnetic data storage and spintronic devices, the ability to tailor magnetic domain shapes and sizes becomes crucial. Promising candidates are materials with intrinsic anisotropies or competing interactions, as they offer the potential to host diverse domain phases that can be precisely selected through external tuning parameters such as temperature and magnetic field. In this study, we employ vector magnetic fields to directly visualize the magnetic anisotropy in unconventional magnetic materials, Fe4GeTe2, Cr2Ge2Te6, and Co3Sn2S2. Our findings demonstrate the practical control of both global and local domain shapes and sizes through external field manipulation, along with the domain transition from stripes to bubble domains. These results pave the way for future applications of tailored magnetic domains, presenting exciting prospects for the field of spintronics device applications.

Low-temperature solid oxide fuel cells (LT-SOFCs), which are SOFCs operating below <500C, face two major challenges in performance due to low operating temperatures: 1) reduced oxygen ion conductivity and 2) sluggish oxygen reduction reaction (ORR). The former is partially solved by introducing a thin-film electrolyte, but the latter is still a major issue. Therefore, the use of metal catalysts is essential to enhance ORR kinetics at the cathode, despite the issue of thermal degradation of such metal catalysts. Various methods for stabilizing metal catalysts at elevated temperatures have recently been reported, including exsolution, infiltration, and vapor-phase deposition. By adopting such a method, the thermal degradations of nanostructured metal catalysts can be effectively suppressed; in some cases, a simultaneous enhancement in the surface activity at the electrode surface has been reported. Among them, atomic layer deposition (ALD), a modified chemical vapor deposition method, can be an effective technique for the surface modification of nanostructured metal catalysts. ALD uses a sequential dose of the precursor and oxidant, and each chemical reacts via a self-limiting reaction on the substrate surface. High-quality films with uniform and conformal surface coverage even on nanostructured surfaces, can be obtained while precisely tailoring the thickness and composition. In this study, we applied an ALD YDC thin-film overcoating onto the Pt cathode of LT-SOFCs. The doping concentration was precisely controlled by varying the ALD cycle ratio between CeO₂ and Y₂O₃ cycles. The effects of the doping concentration of ALD YDC on the ORR activity and thermal stability were analyzed. Electrochemical characterization showed that the ORR activity of the Pt cathode with 19 mol ALD YDC overcoating improved by a factor of five compared to that of the bare Pt cathode. The thermal stability of the low-to-medium-doped ALD YDC-coated Pt cathode was higher than that of the bare Pt cathode or the cathode with excess doping. Therefore, the ORR kinetics were improved by applying the ALD YDC films onto the Pt cathode, which may be due to the enhancement of individual steps at the Pt surface and Pt cathode/electrolyte interface. In addition, the thermal stability of the Pt cathode was enhanced by maintaining fine nanoporous structures even after operation at 450 C. As a result, the ORR kinetics and thermal stability of the cathode were simultaneously improved by ALD YDC overcoating at an optimal concentration. The materials and process solutions for manufacturing active and stable catalysts with metal–oxide interfaces shown in this study could contribute to the development of catalyst/electrode components in energy conversion systems, especially those operated at elevated temperatures.

As the pressing need for sustainable energy solutions is increasing, solid oxide cells (SOCs), including solid oxide fuel cells (SOFCs) and solid oxide electrolytic cells (SOECs), are actively attracted to move toward clean and renewable energy as the future green energy devices. The key of the SOC is an air electrode, a fundamental component that facilitates the oxygen reduction reaction (ORR). Perovskite oxides, in particular, Sr-based perovskite oxides such as SFM (Sr₂Fe_1.5Mo_0.5O_6-δ), BSCF (Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ), and LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ) stand out as high-performance air electrode materials, boosting remarkable electrochemical properties that position them as ideal candidates for enhancing the ORR performance of SOCs.A notable challenge inherent to Sr-based perovskite oxides employed as SOC electrodes is the poor stability. The Sr segregated on the surface under the high operation temperature and the segregated SrO further reacted with H₂O and CO₂ in the air forming insulating impurity phases (Sr(OH)₂, SrCO₃) on the surface, which passivate the ORR activity.The coating on the perovskite is an efficient way to prohibit the degradation behavior of the perovskite oxide air electrode. Thermochemically stable materials coating layers act as physical protection layers to prevent the segregated SrO on Sr-based perovskite oxides from reacting with CO₂ and H₂O in the air and forming impurities. However, the coating layer will be occupied at the reaction site will lead to poor ORR activities. Hence, the fabrication of finely covered, ORR active, and thermochemically stable thin film on the Sr-based perovskite oxides surface is the key to this coating strategy.In this study, we applied defects-rich ZrO_2-x film on the SFM perovskite surface by plasma-enhanced atomic layer deposition (PE-ALD). The 15 nm ZrO_2-x film covered the porous SFM air electrode uniformly. The defects in the ZrO_2-xfilm were directly observed by TEM, enabling the ZrO_2-x an oxygen ionic conductor, rather than completely insulating. The distribution of relaxation times analysis (DRT) reveals the oxygen ions immigration process was enhanced after coating the ZrO_2-x thin film on the SFM. The polarization resistance of the 15 nm ZrO_2-x coated SFM air electrode shows the same as the bare SFM electrode at 0 hour but only 34 % of the bare SFM after operation under 800 ℃ for 50 hours. The results showed that coating ZrO_2-x film on the SFM surface effectively promoted the stability of the SFM perovskite oxide air electrode. This innovative approach holds immense potential for elevating the performance and long-term durability of high-temperature energy devices, especially air electrodes within solid oxide cells (SOC) or metal-air batteries.

[Introduction]

There has been growing interest in “soft materials”, which have potential to solve global energy and environmental problems facing our daily life. They include dye-compounds, organic semiconductors and inorganic-organic hybrid perovskites for solar cell devices, polymer gels and ionic liquids as electrolyte for secondary batteries, and metal borohydrides as hydrogen storage media. Since most of these soft materials are implemented in a thin film form into the devices, a new vacuum deposition process lowering the “activation barrier” against the first attempt to fabricate thin films of soft-materials, particularly those of newly-synthesized ones, is highly required in the R&D levels. This background is behind a recent attention to “infrared (IR) laser deposition”, a vacuum deposition process that has been newly developed in our research group for soft-materials deposition [1].

[Infrared laser deposition]

The first plot type of our IR laser deposition system appeared around in the early 2000s, whose deposition is based on the thermal process. At that time, there was the other vacuum deposition system with IR laser by a different group, which was called “IR laser deposition” as well, but more precisely, “picosecond resonant IR laser deposition”, used for polymer deposition, and rather based on the ablation-like process [2]. Figure shows a schematic of our IR laser deposition system with ellipsometer to monitor the deposition amount. The set-up is almost the same as that of pulsed laser deposition with UV laser (UV-PLD), which is popularly used for deposition of ceramics films, such as oxides and nitrides. Compared with resistive heating-type evaporators such as Knudsen cells, the IR laser deposition in the similar way to the UV-PLD allows its deposition to be efficiently controlled by digital (on/off) operation of the laser and source materials to be easily changed by “target” exchange without breaking the vacuum.

[Applications]

IR laser deposition has been found useful for deposition of various kinds of soft-materials, spanning low molecular weight organic semiconductor and EL compounds [1, 3], inorganic-organic hybrid perovskites [4] and metal borohydrides [5, 6], as well as ionic liquids [7, 8]. The IR laser deposition is also applicable to the deposition of ionic solids such as alkali-halides [9] and halide perovskites [10]. Furthermore, the compact nature of the system has led to the development of various in situ IR laser deposition systems combined with synchrotron XRD [11] and IR absorption spectroscopy [12, 13].

[REFERENCES]

[1] S. Yaginuma et al., J. Phys.: Conf. Ser. 59, 520 (2007).

[2] R. F. Haglund, Jr., et al., JLMN-Journal of Laser Micro/Nanoengineering, 2, 234 (2007).

[3] S. Yaginuma et al., APEX 1, 015005 (2008).

[4] K. Kawashima et al., Sci. Technol. Adv. Mater, 18, 307 (2017).

[5] H. Oguchi et al., ACS Appl. Electron. Mater. 1, 9, 1792 (2019).

[6] R. Nakayama et al., Cryst. Growth Des. 22, 6616 (2022).

[7] S. Maruyama et al., ACS Nano. 4, 5946 (2010).

[8] Y. Ishikawa et al., Chem. Phys. Lett. 754, 137691 (2020).

[9] S. Kato et al., Cryst. Growth Des., 10, 3608 (2010).

[10] T. Dazai et al., ACS Appl. Electron. Mater. published online (2023).

[11] T. Miyadera et al., APL Mater. 8, 041104 (2020).

[12] K. Shimada et al., , ACS Appl. Mater. Interfaces in press.

[13] K. Seta et al., Cryst. Growth Des. 23, 3731 (2023).

In the evolving landscape of material science, the intersection of microscopic imaging and machine learning is proving to be a game-changer. This presentation seeks to illuminate how these modern technologies can substantiate and even surpass insights previously reliant on human expertise. The first segment explores our ground-breaking methodology for estimating exposure temperatures in power plant piping. We have achieved evidence-based temperature estimates with a remarkable accuracy of ±10 degrees by utilizing machine learning algorithms to analyze precipitates on metal surfaces. Moreover, we identify the critical image features that are most contributory, providing quantifiable evidence to support expert assessments. The second part focuses on discerning the causes of metal fractures. Using machine learning, we delve into identifying and justifying image features that can effectively infer the reason for material failure. These methods empower researchers with evidence-based arguments, previously dependent on experiential interpretation, thereby revolutionizing material analysis and diagnosis. Real-world case studies will be presented to demonstrate these technological advances' practical applications and transformative potential.

Data analysis is crucial for the interpretation of complex data by sophisticated surface analysis techniques such as time-of-flight secondary ion mass spectrometry (ToF-SIMS). Multivariate analysis is still powerful for such purposes, although recent machine learning techniques may provide more flexible applications. I’d like to talk about machine learning contributions to the ToF-SIMS data analysis and its future development after I briefly introduce how multivariate analysis support the complex data interpretation. Data analysis learning methods are generally divided into three categories, unsupervised learning, supervised learning, and reinforcement learning. For the analysis of surface analysis data, unsupervised learning is mainly useful for extracting features including those related to unknown materials or unknown factors, while supervised learning is helpful for determination, identification and investigation of the relationship between the results by multiple methods. In terms of reinforcement learning, I’d like to discuss for what purposes reinforcement learning is more helpful than other learning methods. In addition, machine learning applications to other surface analysis techniques such as operando hydrogen microscopy based on electron stimulated desorption (ESD), scanning electron microscopy (SEM) and the analysis of the raw data, including all Kikuchi patterns, of electron backscatter diffraction (EBSD) will also be introduced.