1. Introduction

Fe₂O₃ and γ-FeOOH, major constituents of iron rust, are n-type semiconductors with a bandgap of 2.2~ 2.6 eV. Our group has reported Cu-doped p-type iron oxide thin films and homojunction solar cells with n-type iron oxide by electrochemical deposition (ECD) at constant-potential and two-step pulse potential. In previous works, we fabricated iron oxide homojunction thin film solar cells with a Mg(OH)₂ intermediate layer by drop-dry deposition (DDD) to improve device performance. However, there were no details on the properties of n-Fe-O/ or p-Cu-Fe-O/Mg(OH)₂ heterojunction. Therefore, in this work, we fabricate non-doped n-type and Cu-doped p-type iron oxide thin films by ECD at constant-potential and two-step pulse potential respectively then we stack Mg(OH)₂ by DDD, and investigate current-voltage (I-V) characteristics of these heterojunction.

2. Experimental methods

For iron oxide deposition, a three-electrode cell was used with an indium tin oxide (ITO), a platinum sheet and an Ag/AgCl electrode used as working electrode or substrate, the counter electrode and the reference electrode, respectively. The deposition solution for n-type iron oxide contained 50 mM iron sulfate heptahydrate (FeSO₄·7H₂O) and 100 mM sodium sulfate (Na₂SO₄) dissolved in approximately 50 mL water. In the deposition, constant potential of -0.86 V was applied for 10 min. The deposition solution for p-type iron oxide was prepared by adding 2 mM copper sulfate (CuSO₄) to the n-type iron oxide solution. In the two-step pulse deposition, the potential was -0.86 V for 10 s and 0 V for 10 s in a cycle repeated 30 times. Then 0.05 mL of the deposition solution contained 25 mM magnesium nitrate (Mg(NO₃)₂) and 50 mM sodium hydroxide (NaOH) dissolved in approximately 50 mL water was dropped on each iron oxide thin films. Then the substrate was heated at 60 °C using a heater plate until evaporation completely, rinsed with water and blown by nitrogen gas. The steps of the solution dropping and drying were repeated two times. The fabricated heterojunction thin films were annealed in air at 400 °C for 60 min. I-V measurements were performed on as-deposited and annealed heterojunctions.

3. Results and discussion

The results of the I-V measurements of n-Fe-O/Mg(OH)₂ are shown in Fig. 1 and 2. From these figures, rectification with threshold voltages of 1.7 mV and 0.4 mV observed for as-deposition and annealed heterojunction, respectively. The results of the I-V measurements of p-Cu-Fe-O/Mg(OH)₂ are shown in Fig. 3. In these figures, heterojunction with p-type iron oxide thin films showed ohmic I-V characteristics. In summary, n-Fe-O/Mg(OH)₂ heterojunction diode were successfully fabricated.

References

1) S. Kobayashi and Ichimura, Semicond. Sci. Tecnol. ,33,(2018),105006.

2) R. Takayanagi and Ichimura, Jpn. J. Appl. Phys. ,59, 2020),111002.

Introduction

During magnetron sputtering, localized grooves form on the target surface. It is known as the "erosion track" and determines the target usage efficiency. Many simulations have been performed on this topic, but experimental studies were scarce. We have previously measured the time evolution of the erosion profile during DC sputtering and found that the normalized profile can vary with time. The profile can also change with the discharge power. Especially in high-power pulsed magnetron sputtering (HPPMS), the high-density plasma can escape the magnetic confinement, and the profile may widen. This may affect not only the target usage efficiency but also the process balance in reactive HPPMS through the change in the effective area of the target. In this study, sputter discharges were performed using a high-power pulsed power supply, and the surface profile of the target was measured to compare the results with DC cases.

Experiment

Cu targets was sputtered using an HPPMS system, and every few hours, the target was removed from the sputtering equipment to measure the surface erosion profile. Surface profiles were measured with a height gauge (depth resolution 0.01 mm). The argon gas pressure and flow rate used for the sputtering discharge were 1.00 Pa and 20.0 sccm, respectively. The frequency of the pulse discharge was 200 Hz, the duty ratio was 5.0%, and the time-averaged power was 100 W.

Result and Discussion

Figure 1 compares data from HPPMS and DC magnetron sputtering (DCMS) with a sputtering time of 4 hours. The time-averaged power was equal to 100 W. The profiles are normalized by the height of the deepest point. The actual depth at the deepest point was 0.267 mm for HPPMS and 0.999 mm for DC. The eroded mass was 3.3 g for HPPMS and 4.05 g for DC, indicating that the effective spattering yield in HPPMS was smaller than that in DC. The full width at a half maximum of the track was 11.13 mm for HPPMS and 8.33 mm for DCMS. This suggests that the plasma generated by the instantaneous high power in HPPMS spatially spread and formed a wider erosion track.

Reference

1)Nakano, Saito and Oya, Surf. Coat. Technol., 326 (2017) 436-442

Introduction

Vanadium dioxide (VO₂) exhibits a reversible insulator-metal transition (IMT) at around 68 ℃ [1]. At this time, a crystalline structural change from a low-temperature monoclinic structure to a high-temperature tetragonal structure occurs. At low temperatures, it transmits infrared light. The source of heat from sunlight. At high temperatures, it reflects the infrared light. It has been reported that a substrate temperature of 400 ℃ or higher is required to deposit VO₂ thin films by the reactive sputtering method [2]. However, the deposition of VO₂ thin film on low-melting substrates requires a lower deposition temperature. In this study, we aim to lower the deposition temperature of VO₂ thin film in the reactive sputtering method by dynamically controlling the oxygen flow rate.

Experiment

VO2 thin films were deposited on c-Al₂O₃ substrates using reactive direct current magnetron sputtering (r-DCMS). Vanadium metal (φ50 mm, 99.9%) was used as the target, and 50 W of DC power was applied. The Ar gas flow rate and the deposition pressure were set to 5 sccm and 1.0 Pa. The discharge voltage was monitored by increasing or decreasing the O₂ gas flow rate. First, the discharge voltage reached 390 V and was held for 20 seconds. Next, the O₂ flow rate was decreased, and the discharge voltage was changed to 350 V and held for another 20 seconds. This operation was repeated for 10 minutes. The substrate temperatures were varied from 300 to 400 ℃ with a 50 ℃ step. The crystallinity of the deposited films was evaluated by X-ray diffraction (XRD). The resistance dependence against the temperature (R-T characteristic) was examined by the two-probe method.

Result and discussion

Figure. 1(a) shows the XRD patterns for the deposited samples. The diffraction peak from VO₂ (011) plane at 2θ = 27.85° was observed in the 400 ℃ and 350 ℃ samples, indicating the crystal growth of VO₂ thin films on c-Al₂O₃ substrate under 400 ℃. Fig. 1 (b) shows the R-T characteristics of the prepared samples. In the sample of 300 ℃, the IMT was not observed. In the 350 ℃ sample, 1.5 orders of magnitude change in resistance was achieved. These results correspond with the XRD results. Single-phase VO₂ thin films were successfully prepared at a low temperature of 350 ℃. The results will contribute to the deposit of VO₂ films on low-meltong temperature substrates.

References

1) f. J. Morin, Appl. Phys. Lett., 3, 34(1959)

2) C. Zhang et al. Materials. 15. 7849(2022)

Nanosheets of graphene and layered compound materials are attracting attention because of their various functions, such as high-electronic conductivity, dielectricity, and catalytic properties. They are formed by mechanical cleavage of layered materials and growth of nanoparticles using hydrothermal methods. Gallium oxide hydroxide (GaOOH) is an ultrawide bandgap semiconductor of 5.6 eV, and could be transformed by dehydration into gallium oxides of 4.2-4.9 eV. Therefore, the development of a fabrication process for GaOOH nanosheets is important for catalytic and device applications. We found that nanosheet structures grow self-assembled on the surface of oriented GaOOH gel films. Here, we present the temperature dependence of the gelation process of the GaOOH sol film and the nanosheet growth process on its surface.

A sol solution was prepared by the stirring for 1 h at 60ºC using tris-acetylacetonato gallium, monoethanolamine, and 2-propanol. After adjusting the process temperature in the range of 18 to 28ºC, thin sol films were formed on quartz substrates by spin coating. Two-dimensional gelation reaction of the sol films occurred in the in-plane direction from the edge of the substrate. The gel surfaces had grain-like structures at 18ºC, but became band-like structures at temperatures above 22ºC. The threshold temperature for nanosheet growth on the gel surface was found to be 22ºC, and the growth was anisotropic depending on the band-like structure. This suggests that the band-like gel surface is highly oriented. Since the thickness of the nanosheets was discrete at 10 nm intervals, the single layer was estimated to be 10 nm. Nanosheets prepared at 22ºC had a height of 1-4 monolayers and a width of 1-5 μm. The nanosheet density increased with increasing temperature, which may be due to the enhanced supply of GaOOH molecules from the gel film. However, the nanosheets were etched and disappeared at the temperature above 30ºC. Furthermore, thick plate-like crystals were grown on the surfaces above 38ºC. Therefore, it was found that the growth temperature of nanosheets was 22 to 28ºC. These results indicate that the oriented GaOOH gel film surface is useful for fabricating nanosheet structures.

1. Introduction

Zinc oxide (ZnO) has been actively researched for various applications in ultraviolet (UV) and visible optical regions. Moreover, ZnO-based devices were also reported as emitters of terahertz (THz) waves beyond the UV–visible region. In both the UV–visible and the THz regions, laser processing is an efficient method for realizing thin-film-based devices or nano/microstructure-based functional surfaces. Many researchers have widely reported ZnO-based thin films fabricated by pulsed laser deposition; nanosecond UV lasers were employed in most of these reports. By contrast, ZnO-based functional surface structures fabricated by direct laser irradiation upon a ZnO substrate occur seldom. Recently, femtosecond laser-induced nano-ripples on ZnO surfaces have been reported by Hang et al. and Liu et al., respectively. Meanwhile, such literature did not mention single-pulse irradiation physics and the initial laser processing phenomenon on the ZnO surface. In our work, the surface of the ZnO substrate was irradiated by a single femtosecond laser pulse with various energy.

2. Experiments and Results Femtosecond laser pulses from a Yb:KGW laser were irradiated and focused upon the surface of a ZnO substrate (orientation: 0001; hexagonal structure) by an F-Theta lens at normal incidence. The single-pulse samples were obtained by linearly scanning the focused laser beam with different pulse energy (Ep). The multi-pulse samples were obtained by continuously irradiating the focused laser beam upon the same spot, and the pulse numbers were controlled by adjusting the irradiating time. The irradiated areas were observed by a scanning electron microscope (SEM) and a confocal laser scanning microscope (CLSM) to attain the morphological details (radius, cleavage depth). The cathodoluminescence (CL) was performed to measure the luminescence properties and progressively analyze the defect of the laser-irradiated area. Figure (a) shows the SEM and CL images of the single-pulse irradiated spots with different Ep. Surface cleavages from the irradiated center were also observed once Ep was higher than 10 μJ. Figure (b) plots the cleavage depth (d) as a function of Ep. Although laser-induced cleavage was observed on the single-pulse irradiated samples, this cleavage phenomenon did not remain in the case of multiple pulses irradiation. Figure (c) presents the SEM and CL images of the multi-pulse irradiated spots on the ZnO substrate. Laser-induced cleavage persisted until the fifth pulse, and after that, the general thermal melting became dominating. Defects level may be modified by the irradiating of laser pulses. As shown in Figure (d), the UV intensity of the CL spectrum dramatically decreased by the first pulse, indicating the collapse defect level on the laser-irradiated surface. The UV peak of the CL spectrum due to the bandgap of ZnO increases with the area of melting; on the contrary, the green peak due to oxygen vacancy (Vo) decreases with the area of melting. A melting mountain with a smooth surface was observed in the enlarged SEM image of the 11-pulses irradiated spot. The enlarged CL image of this spot presents that the melting surface consisted of many nano phosphors. In the high-temperature, high-pressure center of the irradiated spot, the ablated pieces were heated and melted together, and the Vo decreased under the heating in oxygen-rich conditions. Based on the above findings, moth-eye structures on ZnO substrates were fabricated by femtosecond laser processing, but the expected improvement in transmittance was not realized for THz waves. We annealed such ZnO moth-eye structures to decrease defects and obtained remarkable improvement in transmittance. Shallow level defect responding to THz waves may be formed during the laser processing.

Gallium Nitride (GaN) is a wide-bandgap semiconductor used in electronic and optoelectronic devices. In these applications, heat management is important to achieve high performance and long lifetime, and thus deep understanding of thermal conduction behavior is required. Because the computational cost of ab initio calculations is often too high to obtain such understanding, especially in examining the effects of defects, machine learning potentials have been attracting attention in recent years, which are expected to achieve low computational cost and accuracy comparable to ab initio calculations simultaneously. In this study, we have constructed a machine learning potential for investigating the effects of defects on thermal conduction behavior in GaN, and examined its prediction accuracy.

As for the type of machine learning potential, we adopted the high-dimensional neural network potential (HDNNP) [1]. In fact, our group developed a modified scheme of HDNNP [2] that can take account of multiple charge states of defects. Note that the stable charge state of a defect can vary depending on the Fermi level of the system. The modified scheme was applied to bulk GaN crystals with N vacancies, and the results showed insufficient prediction accuracy for defect formation energies [2]. In this study, we have generated structural data for the training dataset in addition to those generated previously in our group [2] to improve the prediction accuracy: The former has been obtained from ab initio molecular dynamics (AIMD) calculations based on density functional theory (DFT) performed using the VASP package on structures containing +1 to +3 valence N vacancy defects at low temperatures, while the latter from molecular dynamics calculations using classical potentials on the Ga₁₆N₁₆, Ga₃₂N₃₂ and Ga₆₄N₆₄ models, and structures where several N atoms were removed from these models using the LAMMPS package. Phonon band structures and thermal conductivities were calculated for GaN structures containing N vacancy defects using the open-source program Phonopy/Phono3py, and compared with the DFT calculations.

Figure 1 shows the phonon bands of Ga₆₄N₆₃ with the charge state of 3+ calculated by HDNNP and DFT. The red curves in the figure denote phonon bands of the perfect crystal for comparison. The phonon bands calculated with the HDNNP agree those calculated by DFT well, including the band split near the Γ point and the appearance of flat bands at frequencies near 500 cm^-1. The root mean square error of force prediction for slightly displaced structures to calculate phonon bands was 192.9meV/Å for the previous HDNNP [2] and 109.5meV/Å for the HDNNP constructed in this study. Thus, we can say that the force and phonon prediction accuracies have been improved much. On the other hand, preliminary calculations show that the errors of thermal conductivities compared with the DFT results were larger for the present HDNNP than the previous one. In the presentation, we discuss the prediction accuracy comparison between the previous and present HDNNPs in more detail. This work was supported by the JST CREST Program “Novel electronic devices based on nanospaces near interfaces” and JSPS KAKENHI Grants Nos. 19H02544, 19K15397, 20K15013, 20H05285, 21H05552, 22H04607 and 23H04100.

References

[1] J. Behler et al., Phys. Rev. Lett. 98, 146401 (2007)

[2] K. Shimizu et al., Phys. Rev. B. 106, 054108 (2022)

Fine metal patterning is an important process that directly affects the size of electric devices in the field of organic electronics. The conventional method for metal patterning is vacuum- deposition with a shadow mask, however, this method has such problems as limited pattern shapes and a restricted resolution. We have proposed selective metal deposition using photochromic diarylethenes (DAEs), which enables metal-pattern formation based on maskless vacuum deposition [1,2]. Metal atoms don’t deposit on the colorless DAE surface, but deposit on the colored DAE surface. However, selective metal deposition using DAE can be applicated for limited metal species (Mg, Zn, Mn, and Pb); it cannot be applied to Ag and Cu [3], which have been widely used in electronics field. Perfluoropolyether (PFPE) has a low surface energy and surface softness for efficient Ag atom desorption [4]. In this study, we report the selective deposition for Ag using a PFPE-based material (KY-1901, a mold release / antifouling agent supplied from Shin-Etsu Chemical Co. Ltd.), which has a low surface energy and is vacuum-depositable.

We tried to fabricate Ag patterns with PFPE patterns on the Alq₃ film. Figure 1 shows the process of this selective deposition method. We fabricated the Alq₃ layer on a glass substrate. The PFPE pattern was prepared by vacuum-deposition using a shadowmask. Finally, Ag was evaporated onto the PFPE patterned surface without a shadow-mask. Figures 2a and 2b show the shadowmask and obtained Ag patterns, respectively. The Ag pattern was not clear, and the expansion of no-Ag deposit area was observed. PFPE is in oily state and therefore patterned PFPE could diffuse and be incorporated the into the Alq₃ layer. We therefore tested vapor deposition of viscous polymerized PFPE to prevent diffusion/incorporation of PFPE on/into the Alq₃ layer. The PFPE material has the silane coupling site, which reacts with moisture to de-alcoholate by annealing and viscosity of PFPE increases (Fig. 3). We prepared the polymerized PFPE source by pre-annealing (120℃, 120h) and evaporated it in the process shown in Fig. 1. As a result, we succeeded a fine Ag pattern on Alq₃ layer corresponding the shadow mask (Fig. 2c).

The difference in the softness between PFPE films was estimated by AFM-force curve (Fig. 4). It was observed that original PFPE/Alq₃ surface was softer (46 nN/nm) than that of the pre-annealed PFPE/Alq₃ surface (52 nN/nm). This result means that pre-annealed PFPE didn’t incorporate into Alq₃ layer, indicating that well-defined layer was prepared.

In conclusion, we proposed a method to fabricate a fine Ag pattern on organic surfaces using maskless metal-evaporation. This method would be applied to prepare a fine electrode for organic devices.

Multi-junction solar cells with multiple layers of semiconductor materials with different bandgap energies can achieve conversion efficiencies of over 40%. InGaAsN is lattice-matched to GaAs and is expected to be a material for sub-cells with a bandgap energy of 1 eV. However, high-quality crystal growth of InGaAsN with long minority carrier diffusion lengths is difficult, due to a large miscibility gap and the generation of N-related crystal defects. We had studied the high-quality crystal growth of InGaAsN by Chemical Beam Epitaxy (CBE) method. CBE is a growth method using organic compound precursors, and because of the low growth pressure, the decomposition reaction of the precursors takes place on the substrate surface. Therefore, since the crystal quality and the impurity incorporation depend sensitively on the growing surface structure of the substrate, such as the step structure, the N-related defect densities can be reduced by optimizing surface structure in CBE growth. However, hydrogen derived from organic compound precursors forms NH complex defects and plays the role of acceptor and donor. It is necessary to understand the formation conditions and defect characteristics of NH complex defects to reduce their densities.

In our previous study, we have reported that the high-density residual acceptor in p-type InGaAsN is attributed to NH defects, based on the relationship between the carrier density measured by Hall effect measurements and the NH peak intensity in the FT-IR spectra. While, the residual donor had not yet been analyzed in detail. In this study, both acceptor and donor density were calculated from the temperature dependence of carrier density and mobility by Hall effect measurement, and defect characteristics were analyzed by DLTS method.

The figure shows the temperature dependence of the mobility of InGaAsN grown on GaAs(001) vicinal substrates measured by Hall effect measurements. Two samples with different off-cut directions of [110] (A-step) and [100] (AB-step), respectively, are compared. The low-temperature mobility is limited by ionized impurity scattering of both acceptors and donors. The acceptor and donor densities were calculated from the temperature dependence of mobility and carrier densities. The calculated acceptor and donor densities were 1.87×10¹⁸(cm^-3) and 1.80×10¹⁸(cm^-3) for the AB step sample and 4.72×10¹⁸(cm^-3) and 4.50×10¹⁸(cm^-3) for the A step sample. In both samples, it was found that nearly equal amounts of high-density acceptors and donors were incorporated. The results of defect characterization by the DLTS method will also be reported.

The magnetic states of traditional spin electronic devices are usually controlled by a magnetic field or a small current. In such devices, the performance, signal-to-noise ratio, and dimension are strongly influenced by their thermal stability. To overcome the thermal disaster, magnetic devices through voltage control have been developed. One of the methods to manipulate the magnetic state of a ferromagnetic layer is through the strain effect (/magnetostriction) of the layer. In such a case, the strain of the ferroelectric layer is provided by a nearby piezoelectric (/ferroelectric) material. In this study, Ni/ZnO bilayers were grown on Si(100) substrates using radio frequency magnetron sputtering with fixed thickness and deposition temperature equal to 40nm and 50^oC, respectively, of the Ni layer. By varying the thickness and deposition temperature of the ZnO underlayer, the magnetic behaviors of the Ni layer and the leakage behavior of the ZnO layer were investigated.

The XRD revealed that the ZnO layer has a Wurtzite structure with (0001) orientation. The texture of the ZnO(0001) got better as increasing the deposition temperature, Tg. The surface roughness of the ZnO layer increased as elevating the Tg and thickness as observed by the AFM. The Ni layer displayed a fcc(111) orientation as a good texturing formation in the ZnO underlayer. A perpendicular magnetic anisotropy, PMA, was obtained for the Ni films grown on the highly texturing ZnO(0001) underlayers with higher Tg and thicknesses, as depicted in Figs (a)-(d) for the perpendicular magneto-optical Kerr effect measurements, PMOKE. Because the magnetic easy axis of Ni bulk is along [111] direction, we concluded that the PMA mainly contributed from crystalline anisotropy, although the roughness may also assist the PMA.

Interestingly, the PMOKE loops of the Ni layer displayed an exchange bias field, Hex, of around 50 Oe, as shown in Figs. (f) and (i), which indicated a formation of the NiO layer between Ni and ZnO. By applying a bias voltage across the Ni/ZnO bilayers, the coercivity, Hc, of the Ni layer decreased as the leakage current of the ZnO gets higher, as shown in Figs. (e) and (g). The thermal effect responses for the decrease of the Hc. However, the Hex was not affected by the thermal effect. For another Ni/ZnO junction with a smaller leakage current and Hc value, the Hex was affected by the positive bias voltage which resulted in a decrease of Hex. The lattice strain that comes from the ZnO underlayer may contribute to the deformation of the NiO layer and suppress the exchange coupling between the NiO and Ni layers.

In this study, the crystal anisotropy dominates the magnetic anisotropy of the Ni on the ZnO(0001) underlayer. The thermal and strain effects are two key factors for the variation of the Hc and Hex behaviors.

REFERENCES

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Most of the characteristics of f-electron compounds stem from the interplay between Ruderman-Kittel-Kasuya-Yosida (RKKY) and the Kondo effect, resulting in abundant intriguing properties such as helical magnetism, metamagnetism, and spin-triplet superconductivity. However, because their manifestation is limited at extremely low temperature range, these properties have remained within the realm of fundamental research, with limited opportunities for practical applications. Our study aims to apply spintronics techniques to f-electron compounds, artificially expressing novel functionalities. In this study, we deposited a few nanometers of the ferromagnetic Fe on the surface of the CeRu₂Si₂ to evaluate the induced magnetism at the interface. CeRu₂Si₂ is a heavy fermion compound with a Kondo temperature of 20-25 K, displaying metamagnetic transitions of f-electrons at extremely low temperatures and high magnetic fields. Using the element-selective magnetic measurement technique of soft X-ray magnetic circular dichroism (XMCD) spectroscopy, we explored the increase in metamagnetic transition temperature at the CeRu₂Si₂ surface, the emergence of novel interfacial properties, and the modulation of the magnetic state of the ultra-thin Fe layer reciprocally influenced by f-electron effects (particularly, the manifestation of new magnetic anisotropy due to strong spin-orbit interactions). Figure 1 summarizes X-ray absorption spectra (XAS) and X-ray magnetic circular dichroism (XMCD) spectra, together with XMCD hysteresis curves, obtained for CeRu₂Si₂(001)/Fe(2 nm) at different absorption edges. In the Fe L_2,3-edge XAS shown in Fig. 1 (b), sizable XMCD contrasts whose sizes are almost independent of temperature are observed. Additionally, the XMCD hysteresis at the Fe L₃ peak energy (Fig. 1 (e)) shows similar magnetic process for all the temperatures. Although a relatively large magnetic field of approximately 2.5 T is required to saturate the magnetization, this is due to the perpendicular field geometry and shape anisotropy of the thin film. The Ce M-edge XMCD spectra (Fig. 1 (a)), on the other hand, varies systematically with temperature, not necessarily following the magnetic behavior of Fe. Focusing on the hysteresis loops at the Ce M₄ peak (Fig. 1 (d)), an almost linear magnetization behavior reminiscent of paramagnetism dominates at low temperatures around 6 K, with a slight bending indicative of weak ferromagnetism. However, this bending is also observed in CeRu₂Si₂ without deposited Fe, suggesting a possibility of an intrinsic magnetic feature of CeRu₂Si₂ around the metamagnetic transition temperature. Meanwhile, at Si K-edge, small but noticeable XMCD spectra are observed at low temperatures (Fig. 1 (c)). However, the corresponding hysteresis loops show complete paramagnetic behavior dependent on temperature (Fig. 1 (f)). Possible origins of these results will be discussed in the presentation.

When a surface is irradiated with soft X-rays, phenomena such as 1) core ionization, 2) Auger processes, and 3) ion desorption occur in a short time less than 0.1 ps (Fig. 1). To elucidate these dynamics, it is necessary to simultaneously measure the photoelectrons, Auger electrons, and ions emitted from the surface (coincidence measurement). Using electron – ion coincidence spectroscopy we clarified the H⁺ desorption mechanism induced by 4a₁ ← O 1s resonant excitation of condensed H₂O [1]. On the other hand, Auger-electron – photoelectron coincidence spectroscopy (APECS) was found to be powerful for surface analysis. Using Si-L₂₃VV-Si-2p APECS, topmost-surface Si-2p photoelectron components of Si(100)-2×1 was identified [2], and local valence electronic structures of Si(111)-7×7 was investigated [3].

Around 2016 we moved to nonevaporable getter (NEG) research based on surface science. NEG is a functional material that evacuate residual gases at room temperature by forming an active surface when heated in ultra-high vacuum (UHV). Typical NEGs are Ti, Zr, V and their alloys. When NEG is deposited on the inner wall of a vacuum vessel, the vacuum vessel will evacuate the residual gases just by baking, and UHV can be maintained without electric power for several decades. Therefore, the development of NEG will contribute to CO₂ emission reduction and Sustainable Development Goals (SDGs). Commercial NEG pumps using ZrVFe alloy can be activated by heating at 400–500 °C, and pump active residual gases such as H₂, H₂O, CO₂, CO, N₂, and so on at room temperature (RT) [4]. Recently we have developed a new NEG, Pd overcoated on Ti thin film with a purity higher than 99.95% (oxygen-free Pd/Ti hereafter), which pumps H₂ and CO at RT after baking at 133 °C for 12 hours [5]. Then we developed a zero-length conflat fin-type nonevaporable getter (NEG) pump using oxygen-free Pd/Ti deposition [6]. The NEG pump was activated by baking and its pumping speeds were measured as a function of baking time and baking temperature. The NEG pump can be fully activated by baking at 150 °C for 12 h and exhibits initial pumping speeds of 2340 L s^–1 for H₂, and 1440 L s^–1 for CO. The initial pumping speeds of the oxygen-free Pd/Ti thin film after baking at 150 °C were estimated to be 3.2 L s^–1 cm^–2 for H₂ and 7.6 L s^–1 cm^–2 for CO. The morphologies of oxygen-free Pd/Ti thin films on the partition plates and the bottom were examined by scanning electron microscopy, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The Ti thin film was completely coated with Pd on the bottom, whereas the partition plates were covered by Pd/Ti nanostructures. The present NEG pump is ideal for maintaining ultrahigh vacuum below 10^–8 Pa, because its pumping speeds for H₂ and CO are quite large, and because it can be fully activated by baking at 150 °C for 12 h. The oxygen-free Pd/Ti NEG pumps have been commercialized in 2018.

References

[1] K. Mase et al., J. Chem. Phys. 108, 6550 (1998).

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For development of new efficient catalysts for energy and environmental uses, designs of “active sites", "reaction fields" and “energy injection” are essential factors. In the nanospace of zeolite, mesoporous silica and metal-organic framework (MOF), it is possible to control the structure of catalytic active sites in forms of fine particles, clusters, molecules, and atomic moieties, and also possible to control the reaction fields with unique properties such as hydrophobicity and electrostatic fields. Ultrafine semiconductor photocatalysts, single-site photocatalysts, plasmonic catalysts, and MOF photocatalysts, can be designed for H₂ production-storage-transportation, CO₂ fixation, H₂O₂ synthesis, and various environmental reactions.

Ultrafine semiconductor: Hybrid of TiO₂ photocatalyst and porous adsorbent can decompose organic pollutants diluted in air and water efficiently by adsorbing them. In particular, the cavities of the hydrophobic porous material are effective in adsorbing dilute organic compounds and promoting the photocatalytic performance.

Single-site photocatalyst: The tetrahedrally coordinated metal oxide (titanium, chromium, vanadium, and molybdenum oxide) moieties can be implanted and isolated in the silica matrixes of microporous zeolite and mesoporous silica materials and named “single-site photocatalysts”. Under light irradiation these single-site photocatalysts form the charge-transfer excited state, i.e., the excited electron-hole pair state which localizes quite near to each other, plays a significant role in various photocatalytic reactions. Photofunctional metal complex capsulated within nano-cavities can also perform as single-site photocatalyst with high activity and selectivity.

Plasmonic catalyst: Design of nanostructured plasmonic catalysts, such as nanoparticles and nanosheet morphologies, that strongly absorb visible light over a wide range of the solar spectrum due to localized surface plasmon resonance (LSPR) have been designed. A method for the synthesis of Ag nanoparticles with color dependent on the particle size and morphology, was developed with combined microwave heating and mesoporous silica materials. Plasmonic materials based on earth abundant elements found that reduced molybdenum oxide (H_xMoO_3-y) nanosheet with oxygen defects and doped hydrogen displayed intense absorption in a wide range from the visible to the near-infrared region.

MOF photocatalyst: Our group demonstrated that application of MOF materials for photocatalytic H₂O₂ production via oxygen reduction for the first time. MOF constructed from metal clusters and organic linkers offer opportunities to develop novel materials due to the advantages, such as porosity, versatility, and structural diversity. With the advantages of the unprecedented flexibility of MOF, we designed the MOF materials to improve the photocatalytic activity for H₂O₂ production by the linker functionalization, the missing-linker defects in the MOF frameworks, and the utilization of a noble two-phase reaction system using hydrophobic MOF.

[1] T. Zhang, H. Yamashita, Y. Zhao, et al., JACS Au, 2023, 3, 516.

[2] Y. Kondo, H. Yamashita, et al., Chem, 2022, 8, 2924.

[3] Y. Wen, H. Yamashita, Y. Zhao, et al., Angew. Chem. Int. Ed., 2022, 61, e202205972.

[4] Y. Chen, H. Yamashita, Z. Bian, et al., Angew. Chem. Int. Ed., 2022, 61, e202213640.

[5] H. Yin, H. Yamashita, et al., Angew. Chem. Int. Ed., 2022, 61, e2021114242.

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After the advent of graphene and other two-dimensional (2D) materials that could be directly peeled from natural 3D crystals, a paradigmatic shift was the creation of Silicene, the first artificial 2D crystal, synthesized in 2012. Since then, a number of offsprings, coined Xenes, i.e., monoelemental analogues, have emerged from the lightest Beryllene to the heaviest Bismuthene. Among those, the new 2D forms of Ge, Sn, and Pb coined Germanene, Stanene and Plumbene, have attracted huge interest, especially because they possess sizeable spin-orbit couplings, allowing for the quantum spin Hall effect, possibly at room temperature and even above. 2D topological insulators may lead to breakthrough applications from Field-Effect Transistors to spintronics. Group IV Xenes are also considered for improved lithium/sodium batteries, solar cells, and for hydrogen storage, while major applications are also envisaged in nanobiology, nanomedicine and for cancer cell theranostics. 1D nanoribbons, may host Majorana zero modes at their ends, which could be crucial for quantum computation. In my talk I will recall how Silicene was designed and describe the striking emergent properties of several Xenes, especially those of Germanene, Stanene and Plumbene, as well as their outstanding multifaceted outcomes.

We have developed microscopes with higher spatial resolution using a convergent electron beam and a functional thin film[1]. The electron beam has high convergence and can form a probe on the scale of a few nanometers. Consequently, using the electron beam allows us to achieve a spatial resolution beyond the diffraction limit of light. Furthermore, the sample can be observed by converting the quantity of the sample with the functional thin film into a quantity that can be imaged with an electron beam. By converting an electron beam into light through a fluorescent thin film, we can obtain optical information about the samples. By using a photoconductive film that utilizes the internal photoelectric effect, it is possible to convert light intensity into electricity and measure it with an electron beam. Additionally, by converting charge distribution into the thickness of the depleted layer, we can measure ion concentrations[2]. In this way, by using a highly convergent electron beam and functional thin films, we can develop super-resolution microscopes that can observe various samples and surpass the diffraction limit of light.

As an example, we will explain a super-resolution optical microscope achieved by converting an electron beam into a nano-optical spot on an ultrathin fluorescent thin film [1]. Figure shows the schematic of the developed super-resolution optical microscope. It consists of a lower scanning electron microscope and an upper optical microscope. Between the SEM and the optical microscope, a nitride silicon thin film with an ultrathin fluorescent thin film is placed. This fluorescent thin film acts as a functional thin film. By changing this thin film to another thin film, the information obtained from the microscope can be changed. In the case of a fluorescent thin film, when the thin film is irradiated with a convergent electron beam, a nano-optical spot is formed on its surface. To minimize electron scattering, both the fluorescent thin film and the nitride silicon thin film are extremely thin. Since the electron beam has a very high convergence and excites light only in a region of several nanometers, the excited optial nano-optical spot size is several tens of nanometers. Only a small portion of the observation sample, directly placed on the atmospheric side of the fluorescent thin film, is illuminated by the formed optial nano-optical spot. The light transmitted through the sample and the light scattered by the sample are detected using the upper optical microscope. The electron beam is scanned in two dimensions to acquire a 2D image. Through the conversion of a convergent electron beam into a nano-optical spot, the spatial resolution beyond the diffraction limit of light can be achieved. Using the developed fluorescent thin film-based super-resolution optical microscope, MARCO-expressing Chinese Hamster Ovary (CHO) cells were observed[3]. The evaluation of spatial resolution from the intensity profile of fiber structures resulted in 121nm. The results show that super-resolution microscopy using an electron beam and an ultrathin fluorescent film has a spatial resolution below the diffraction limit of light.

We describe a microscope with spatial resolution beyond the diffraction limit of light that uses converging electron beams and functional thin films. As an example, we introduced a super-resolution optical microscope that utilizes the optial nano-optical spot excited by the convergent electron beam. The electron beam is transformed into light through the fluorescent thin film. The observation of CHO cells demonstrated that fine cell structures can be observed with spatial resolution that exceeds the diffraction limit of light. In this way, by utilizing convergent electron beams and functional thin films, the development of microscopes capable of high spatial resolution and measurement of various physical quantities is expected.

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The combination of a single-cycle THz pulse and a scanning tunneling microscope (STM) enables to investigate ultrafast dynamics with the nanometer and picosecond resolutions [1-5]. In THz-STM, the tunnel barrier of STM junction is modulated by the oscillation of electric field of THz pulse. Due to the bandwidth of electrical circuit, conventional THz-STM only measures “time-averaged” or “net” field-driven tunneling current, thus it is difficult to determine how electrons are manipulated by positive and negative components within a single-cycle electric field oscillation. By combining a photon detection system with a THz-STM, it enables to investigate excitation and energy dissipation processes triggered by THz-field-driven tunneling [5]. In this work, we revealed field-driven ultrafast charge transfer between the STM tip and a molecule by measuring luminescence signal from the molecule (Fig. 1a).

Single-cycle THz pulses were generated by using optical rectification of laser pulses from a Yb fiber laser (1035 nm, 50 MHz, 40 W) in a LiNbO₃ prism. The carrier envelope phase (CEP) of THz pulse can be tuned by using a CEP shifter [4]. The CEP-controlled THz pulses were guided to an STM (5 K, 1 × 10^-11 Torr) and focused to the junction. The CEP of near field THz electric field was evaluated by measuring the tunneling current on a clean Ag(111) surface (Fig. 1a) [4]. The photons generated at the STM tip is collected using a lens inside the STM and directed out of the ultrahigh vacuum chamber, where it was refocused onto a grating spectrometer with a charge coupled device (CCD) photon detector.

Figure 1b shows THz-STM luminescence (THz-STL) spectra of a Pd-phthalocyanine (PdPc) on NaCl ultrathin film grown on Ag(111). The luminescence peak originated from the fluorescence of a PdPc is observed at 1.84 eV only when THz pulses were irradiated (red spectra in Fig. 1b). Figure 1c shows CEP dependence of molecular luminescence intensity, where CEP = π is defined as ΔCEP = 0. In the CEP dependence, the luminescence intensity is maximum at around ΔCEP = π/6 and zero at around ΔCEP = ± π. This result indicates that the THz waveform is responsible for the exciton formation. In this presentation, we would like to discuss the mechanism of exciton formation in a molecule by THz-field-driven ultrafast charge transfer between the STM tip and the molecule.

[1] T. L. Cocker et al., Nature 539, 263 (2016).

[2] K. Yoshioka et al., Nat. Photon. 10, 762 (2016).

[3] S. Yoshida et al., ACS photon. 8 315 (2021).

[4] K. Yoshioka et al., Nano Lett. 18, 5198 (2019)

[5] K. Kimura et al., ACS photon. 8, 982 (2021).

AFM has been a key tool for investigating nano-scale structures and mechanical properties of biological systems in liquid environment. However, conventional AFM is only accessible to surfaces and interfaces, and it is difficult to obtain information about the interior of such systems. To solve this problem, we have recently developed a needle-like long AFM tip and combined it with 3D-AFM. With these techniques, we have successfully performed 3D imaging that penetrates the living cell surface and visualizes its internal structure at the nanoscale. However, the information obtained by these techniques is limited to force distribution reflecting the three-dimensional structure, and it is difficult to simultaneously investigate physical information other than mechanical properties. One possible solution to this problem is using a quantum sensor. For example, the frequency shift of optically detected magnetic resonance (ODMR) of fluorescent nanodiamonds (NDs) is sensitive to ambient temperature and has been applied to temperature measurements in biological systems. Such a fluorescent ND attached to the AFM tip is expected to enable 3D-AFM imaging for 3D temperature mapping in cells (Fig. 1(a)).

In order to realize such a measurement, in this study, we established the method for measuring the ODMR of a fluorescent ND on an AFM tip. Here we developed two fundamental techniques, (i) fixing the fluorescent ND on the AFM tip apex and (ii) combining the ODMR measurement system with 3D-AFM. As for (i), we used a focused ion beam scanning electron microscope (FIB-SEM) equipped with a gas injection system. In its chamber, an AFM tip was placed close to one of the fluorescent NDs scattered on the substrate, and carbon gas was injected near the tip to immobilize the ND (Fig. 1(b)). Then, as for (ii), we implemented the custom-built 3D-AFM system, single photon counting module, and sample holder on the commercially available confocal microscope. In addition, to generate the radio-frequency magnetic field required for ODMR measurements, we made an antenna by patterning gold on a glass substrate and mounted it on a sample holder. With these systems, we have succeeded in detecting the ODMR of fluorescent NDs scattered on the glass substrate (Fig. 1(c)). These developed techniques should contribute the realizing intracellular 3D temperature mapping with 3D-AFM.

TiO₂ is one of the famous photocatalysts applied for various fields such as water splitting catalysts, coating materials, and solar cells. For obtaining the design guidelines of high reactive photocatalysts, it is important to understand the surface structure and the elementary process of photocatalytic reaction as the reaction field.

Rutile TiO₂(110) surface have been used as a model surface for the study of a photocatalyst. The rutile TiO₂(110)-(1×1) surface transforms to the (1×2) surface by iterations of Ar⁺ sputtering and high-temperature annealing over 900 °C. The (1×2) surface exhibits an asymmetric periodic row structure [1] accompanied by various local structures as shown in Fig. 1: the single-link structure, the double-link structure, and the line defect. Among these local structures, the line defect has been reported with no reactivity of water adsorption [2]. The origin of no reactivity of water adsorption is still controversial. In this study, we investigated the line defect on rutile TiO₂(110)-(1×2) surface using scanning tunneling microscopy (STM) and non-contact atomic force microscopy (NC-AFM).

Nb-doped (0.05 wt%) rutile TiO₂(110) substrates (Shinkosha) were used for the sample. The (1×2) surface was prepared by iterations of Ar⁺ sputtering (Ar partial pressure: 2.5 × 10^-4 Pa, Energy: 1.5 keV) and annealing at 1000 °C under the ultra-high vacuum (< 5.0 × 10^-8 Pa).

In the case of NC-AFM imaging of metal oxide surfaces, the contrast of the NC-AFM image depends on the tip apex polarity [3]. We obtained NC-AFM images of the line defect with opposite contrast to each other. This result suggests that the line defect has charge polarity. To confirm this assumption, the contact potential difference (CPD) on line defects and (1×2) periodic rows were evaluated using NC-AFM. The comparison of the CPD between line defects and (1×2) periodic rows reveals that the line defects were charged relatively negative from (1×2) periodic rows. Furthermore, the atomic-resolution NC-AFM imaging unveiled the structure of the line defect.

Reference

[1] D. Katsube, et al., Beilstein J. Nanotechnol. 9, 686-692 (2018).

[2] P. Maksymovych, et al., Chem. Phys. Lett. 382, 270-276 (2003).

[3] G. H. Enevoldsen, et al., Phys. Rev. B 76, 205415 (2007).

1. Introduction

In recent years, three-dimensional (3D) micro-fabrication technics have been developed as demand for microdevices increased. Additive manufacturing [1] techniques can fabricate 3D structures by simple process. As an inexpensive and simple fabrication process, we have developed laser-assisted electrophoretic deposition (LAEPD) [2-5]. In this method, the colloidal nanoparticles trapped by laser trapping are deposited on a substrate by electrophoresis. However, because the fabricated structures are composed of aggregates of particles, their mechanical strength is significantly lower than that of the bulk.

In this study, we developed an in-process sintering method for LAEPD using a laser for sintering by localized surface plasmon resonances. The mechanical strength of the fabricated structures was measured from the deflection of the pillar applied a calibrated load using a microcantilever. Furthermore, observation of the cross-section and analysis of the crystallization were also performed.

2. Experimental Methods

In this method, a laser for deposition (λ = 488 nm) and a laser for sintering (λ = 785 nm) are used. Those two lasers were focused on the same position on a substrate by an objective lens. Figure shows the during fabrication process. The deposition laser and the sintering laser were irradiated alternately in the cell filled with Au colloidal solution. Because the nanoparticles around the deposited structure aggregate due to heat generated by the sintering process, the colloidal solution was continuously flowed during the fabrication process to remove the aggregated nanoparticles.

3. Results and Discussion

The mechanical strength of the fabricated pillars was measured using an AFM microcantilever operated in a vacuum chamber of the scanning electron microscopy (SEM). A calibrated load was applied to the pillars by manipulating the cantilever. Young's moduli of the pillars were estimated from the pillar size measured using the SEM and the measured spring constants of the pillars. The dependence of Young's modulus on irradiation time of the sintering laser was investigate. Young's modulus of the pillars increased with increasing the sintering time. Due to the protective agent (mercaptosuccinic acid) contained in the colloidal solution, Young's modulus of the pillars fabricated without sintering shows significantly lower than the bulk value. In contrast, the highest Young's modulus of the pillar fabricated with sintering was 27.6 GPa, which was 10-times higher than that of the pillars fabricated without sintering.

To observe the internal structure of the fabricated structures, the micropillars were cut by the focused ion beam and observed using the field emission-scanning electron microscopy. The pillars fabricated with sintering have pores of about 100 nm in size, which were not observed in the pillars fabricated without sintering. The crystal orientation of the cross-section of the fabricated pillars were also analyzed using electron backscattering diffraction. It was found that the pillars fabricated with sintering had more crystal grains than those fabricated without sintering. It is thought that the local sintering process released the protective agent, resulting in metallic bonding of the nanoparticles and shrinkage and migration of the material in the deposited structure. Therefore, this method is effective in improving the mechanical strength of structures fabricated in LAEPD.

References

[1] A. Reiser, et al., Adv. Funct. Mater. 30(28) (2020) 1910491.

[2] F. Iwata, et al., Nanotechnology 20(23) (2009) 235303.

[3] K. Nakazawa, et al., Nanomanufacturing and Metrology 4(4) (2021) 271-277.

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The surfaces of inorganic nanomaterials play a key role in numerous applications, such as catalysis and optoelectronics. Dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS, Fig.1a) is a promising technique to probe the atomic-level surface structure of inorganic nanomaterials. Nevertheless, this technique is still challenging to apply for the observation of quadrupolar nuclei, such as ¹¹B, ¹⁷O, ²⁷Al, ³³S and ⁶⁷Zn, which represent over 74% of NMR-active isotopes. Recently, we have introduced an efficient DNP-SENS technique (see Fig. 1b) to detect the NMR signal of quadrupolar nuclei near surfaces, even for weakly magnetic isotopes, such as ^47,49Ti, ⁹⁵Mo and ⁶⁷Zn ^[1].

We leveraged it to gain new insights into the structure of nanomaterial surface. We recorded the ¹⁷O NMR spectrum of gamma-Al₂O₃ (Fig. 1c). The DNP-SENS experiment allowed us to observe separately the signal of non-protonated and protonated O sites, OAl_x and HOAl_y, respectively. Similar approach was applied to record the ¹⁷O NMR spectra of the surface of MoO₃/TiO₂ particles with applications in heterogeneous catalysis (Fig. 1d). These spectra permitted us to distinguish the ¹⁷O signals of MoO₃ particles and polyoxomolybdates but also to identify for the first time Brønsted acid sites, HOMo₂ and HOMo₃, at the surface of the polyoxomolybdates, which are involved in catalytic reaction. We also detected using DNP-SENS the NMR spectra of insensitive quadrupolar nuclei, ⁹⁵Mo and ^47,49Ti, near the surface of MoO₃/TiO₂.

The proposed DNP-SENS experiment was also employed to investigate the structure of the surface of quantum dots used in optoelectronics, such as Al-doped ZnO nanoparticles and ZnS nanoplatelets. These experiments allowed the first NMR detection of insensitive quadrupolar nuclei, ³³S and ⁶⁷Zn, near the surface of inorganic materials. The ¹⁷O, ²⁷Al, and ⁶⁷Zn DNP-NMR spectra of Al-doped ZnO proved that the surface region of these nanoparticles contains, besides ZnO phase, as secondary phases, such as α-Al₂O₃ and partially inverse ZnAl₂O₄ spinel. For ZnS nanoplatelets, the combination of DNP-SENS, ultra-high field NMR and DFT calculations demonstrated that a large fraction of S and Zn atoms are located near the surface covered by dodecylamine and the presence of S vacancies in these nanoplatelets.

More recently, we also combined DNP-SENS with multiple-quantum magic-angle spinning (MQMAS) technique to record high-resolution NMR spectra of quadrupolar nuclei near the surface of inorganic materials. This novel method was applied to distinguish the different sites occupied quadrupolar nuclei, such as ²⁷Al, ¹⁷O and ¹¹B, near the surface of inorganic nanomaterials, such as γ-Al₂O₃, ZnO nanocrystals, (ZnO-ZrO₂)/SiO₂ and BN/SiO₂ heterogeneous catalysts.

[1] H. Nagashima, J. Trébosc, Y. Kon, K. Sato, O. Lafon, J.-P. Amoureux, J. Am. Chem. Soc., 142, 10659–10672 (2020).

Terahertz (THz) time-domain spectroscopy has been attracting much attention in many research areas such as imaging, molecular spectroscopy, and solid-state physics because the energy covers various elementary excitations in solids and molecules. Various methods for detecting the phase-locked THz electric field have been developed, as represented by electro-optic (EO) sampling and photoconductive antennas. Because most of these detection methods use insulating solid crystals, phonon absorptions and the phase matching condition in the crystals largely disturb the time-domain waveform of the THz pulse, particularly between 5 and 15 THz. To realize a gapless detection for broadband THz pulses, Air-Biased Coherent Detection (ABCD) has been developed [1]. It is based on the interference between the THz electric field-induced second harmonic (TFISH) light from air molecules and an electric-field-induced second harmonic lights by electrodes, requiring a high bias voltage beyond 1 kV. Recently, to reduce the voltage value, Solid-State-Biased Coherent Detection (SSBCD) using insulators such as silica or diamond instead of air has been developed [2]. However, it still requires sub-kV bias and microfabrication processes. Therefore, a much simpler geometry for gapless broadband THz pulse detection is highly demanded.

Here, we investigate second-harmonic generation (SHG) light from a Pt surface in air under terahertz (THz) pulse irradiation. An output of the Ti:sapphire regenerative amplifier is divided into two beams for THz generation and for a near-infrared pulse as a fundamental light of SHG lights. Both p-polarized pulses are collinearly focused on the Pt surface in air, and THz pulse-modulated SHG intensity ΔI_2ω is measured by a photomultiplier tube. Comparing the time profile of ΔI_2ω with that evaluated by a conventional EO sampling method, we find that ΔI_2ω shows a clear time profile of the THz field. With numerical simulations of ΔI_2ω based on a wave equation, we explain the results of ΔI_2ω as interference between the TFISH light from air molecules in an optical path and a SHG light from a Pt surface. Because the THz field inside the metal is sufficiently weak, the TFISH generation in the metal is negligible. As a result, the effect of phonons is absent for the ΔI_2ω measurements, enabling a gapless detection of broadband THz pulses in the region of 0.2 – 20 THz. Thus, we have developed a new gapless detection method for broadband THz pulses by using a metal surface in air instead of the electrodes with the high bias voltage [3] and named this method Air-Metal Coherent Detection (AMCD). The present AMCD method does not suffer from phonons or phase matching in insulating solid-state optics and does not require any power supply, bias voltage, or fabrication process, but offers a simple and gapless sampling method for broadband THz pulses.

[1] N. Karpowicz et al., Applied Physics Letters 92, 011131 (2008).

[2] A. Tomasino et al., Optica 4, 1358 (2017).

[3] S. Tanaka et al., Applied Physics Letters 122, 251101 (2023). selected as a featured article.

Ferromagnetic materials find applications ranging from electric vehicle motors to magnetic memory devices, playing a key role towards a low-carbon society. Fe₇₅Co₂₅ locates on the top of Slater-Pauling curve, showing strong magnetic moment. Recent progress in Materials Informatics has been accerating the materials development, improving magnetic properties beyond conventional materials. Prior studies predicted (Fe₇₅Co₂₅)_97.5-Ir_2.5 as large magnetic material virtually by first-principles calculations and machine learning[1]. Conbinatorial sputtering system fabricated it, and shows large magnetic moment surpassing the Slater-Pauling curve experimentally. Enhancement of magnetic resistance is also reported[2]. However, the origin of the magnetic moment, namely the mechanism of electronic spin state, remains elusive. The magnetic moment of multi-element magnetic alloys depends on complex interactions between the component elements, so the mechanism of the magnetic moment is not completely understood.

To address this, we used Magnetic Circular Dichroism (MCD) to analyze the origin of FeCoIr magnetic moment. MCD enables element selective analysis of magnetic moment and resolving them into orbital and spin components quantitatively. SPring-8 is one of the world's leading facilities for MCD measurements using both soft and hard X-rays. We measured soft X-ray MCD at the L edges of Fe and Co was conducted at BL25SU, while hard X-ray MCD at the L absorption edge of Ir were conducted at BL39XU. We applied the Magnetic Sum Rule to obtain MCD spectra for the evaluating the orbital and spin magnetic moments of each elements[3]. The samples were prepared by depositing compositional gradient films of (Fe₇₅Co₂₅)_100-xIr_x (0≦x≦11) onto MgO(100) substrates using a combinatorial sputtering apparatus. The film thickness was 30 nm, and a 2 nm Ru layer was deposited on the surface as a capping layer. MCD spectra were obtained combinatorially by continuously scanning the SR beam irradiation position.

Fe's spin magnetic moment and orbital magnetic moment as a dependence of Ir concentration is shown in Figure 1. As Ir concentration increased, each magnetic moments rises. Identical increasing trends were confirmed for Co and Ir. These experimental results were self consistent with the first-principles calculations. We could indicate that Ir addition increases the spin moment and orbital moment of all elements. This study practically illuminated the changes in spin and orbital magnetic moments, bridging the gap between experimental observation and theoretical prediction.

[1] Y. Iwasaki et.al., Commun Mater 2, 31 (2021).

[2] R. Toyama et al., Phys. Rev. Materials 7, 084401 (2023)

[3] C.T.Chen et al., Phys.Rev.Lett. 75, 152 (1995).

Vanadium dioxide (VO₂) is a material that undergoes a metal-insulator transition at around 70 °C. VO₂ has attracted attention in industrial applications because the transition around room temperature is accompanied by significant changes in electrical resistivity and transmittance in the near-infrared light region. The metal-insulator transition in VO₂ exhibits thermal hysteresis, and the desired thermal hysteresis width varies depending on the application. Thus, it is important to control the thermal hysteresis width in VO₂. In this study, we found that the thermal hysteresis width is reduced by irradiating VO₂ film with 1 keV Ar⁺ ions. Soft X-ray absorption spectroscopy was also performed to investigate the valence of V before and after Ar⁺ ion irradiation.

An 80 nm thick VO₂ film was formed on a Si substrate by thermal oxidation of V thin film. The VO₂ film was irradiated with 1 keV Ar⁺ ions under 3×10^-6 Pa, and the change in thermal hysteresis width was investigated for the VO₂ film with and without irradiation. For VO₂ film irradiated in 10 minutes (dose of 3.9 × 10¹⁵ ions/cm²), the thermal hysteresis width decreased by 15 °C compared to that of VO₂ film without irradiation. From the numerical simulation using software SRIM (Stopping and Range of Ions in Matter), the surface of VO₂ film after Ar⁺ ion irradiation was considered to be oxygen deficient. To investigate the depth distribution of oxygen deficiency in VO₂ film, the valence of V in VO₂ film with and without Ar⁺ ion irradiation was investigated by three different soft X-ray absorption spectroscopies (Auger electron yield, total electron yield, and total fluorescence yield). It was found that the peak energy of the V L-edge shifts to the lower energy in the VO₂ film irradiated with Ar⁺ ions via Auger electron yield and total electron yield. However, such peak shift was not observed for the spectra obtained via total fluorescence yield. This result suggests that the reduction of V ions, which was probably caused by the formation of oxygen deficiency on the topmost VO₂ surface, was induced by Ar⁺ ion irradiation. Therefore, the thermal hysteresis width of the VO₂ film was found to be decreased by introducing the oxygen deficiencies at the topmost surface.

Germanene is a two-dimensional (2D) sheet of germanium (Ge) with a honeycomb lattice. Recent theoretical studies have predicted several interesting electronic properties of germanene, such as 2D topological insulators. However, unlike graphene, germanene is easily oxidized in air, making it difficult to realize electrical devices based on germanene. To overcome the drawback of the chemical stability of germanene, it is necessary to understand how germanene is oxidized. Therefore, we started to study germane and discovered an interesting phenomenon: oxidized germanene can be restored to good quality germanene simply by heating it in ultra-high vacuum (UHV). Figure 1(a) schematically describes the effect of oxidation and reheating in UHV of germanene. Figure 1(b) shows Ge3d X-ray photoelectron spectroscopy (XPS) spectra of germanene (black), oxidized germanene (blue), and reheated germanene. Oxidation was performed by introducing O₂ into the UHV chamber at RT. The XPS spectra clearly show that the oxidized germanene was restored simply by heating in UHV. Figure 1(c) shows the low energy electron diffraction (LEED) patterns of as-grown and oxidized germanene after heating at RT, 250 °C, and 500 °C in UHV. The LEED patterns indicate that the oxidized germanene is fully recovered after heating at 500 °C. The detailed mechanism of the recovery will be discussed in the presentation.

In this talk, I will introduce the activity of JVSS in aimed at exploring the future collaborations with other societies in Asia and America.

Two-dimensional (2D) materials have been studied extensively as monolayers, vertical or lateral heterostructures. To achieve functionalization, monolayers are often patterned using soft lithography and selectively decorated with molecules. Here we demonstrate the growth of a family of 2D materials that are intrinsically patterned. By a single step of direct selenization of a Pt(111) substrate, we first epitaxially grow a high-quality single-crystal, monolayer platinum diselenide (PtSe2), a new member of the layered transition-metal dichalcogenides (TMDs) family [1]. We then show that, by controlling the annealing temperature and deposition amount of Se on a Pt substrate, we obtain either a homogeneous monolayer PtSe2 in the 1T phase or a ‘patterned monolayer’ comprising a periodic triangular structure of alternating 1H and 1T phases. The size of the triangles can be tuned by varying the density of deposited Se atoms. The choice between a homogeneous 1T phase or patterned 1H/1T phases is reversible. We also report the fabrication of a patterned monolayer of CuSe having periodic arrays of triangular nanopores, with distinct domains. In both patterned materials, adsorption of different species at preferred pattern components has been achieved, illustrating the potential for selective/dual functionalization [2]. We also report the fabrication of the intrinsically patterned Ag5Se2, exhibiting the possibility of due functionalization. The atomic arrangement is determined as a quasi-periodic pattern of stoichiometric triangular domains with a side length of ~ 15 nm and apical offsets. The boundaries between triangular domains are sub-stoichiometric. Deposition of different molecules on the patterned Ag5Se2 exhibits selective adsorption behavior. Pentacene molecules preferentially adsorb on the boundaries, while tetracyanoquinodimethane (TCNQ) molecules adsorb both on the boundaries and the triangular domains. By co-depositing pentacene and TCNQ molecules, we successfully construct molecular corrals with pentacene on the boundaries encircling TCNQ molecules on the triangular domains [3]. This work ushers in a frontier for the fabrication of intrinsically patterned 2D materials.

References:

[1]. Y. Wang, et al., Nano Letters, 15, 4013 (2015)

[2] X. Lin, et al., Nature Materials, 16, 717 (2017)

[3] J. Lu, et al., Nano Research, 15, 6730 (2022)

In this talk, I will briefly introduce the Korean Vacuum Society (KVS), which has grown in step with Korea's science and industries, and introduce my recent research results on two-dimensional materials. Understanding and controlling the oxidation of two-dimensional materials remains a key interest in the final stage for practical application of them. The oxidation process of InSe, a representative 2D material with a Si-like bandgap, high electron mobility and gas stability, has been widely studied, but is not yet fully understood. Here, we report in-situ spectroscopy results showing the causes why different way of oxidation appear depending on the experimental conditions. Se vacancies and interlayer interactions allow oxygen adsorption on the inert InSe, leading to intra-structure deformation and weakening of interlayer interactions. These two phenomena appear with a time difference in the surface layer which is a charge transfer channel in devices, resulting in major carrier shift from p-type to n-type. We found that low-density Se vacancy defects lead to Se oxidation, whereas high-density defects triggers oxidation via In with lower electronegativity.

Halide Perovskites have attracted much attention in recent years owing to their interesting optics and electronic properties that are applicable to various applications, including solar cells, light-emitting devices, lasers, scintillation devices, electro-photocatalytic devices, and sensors. Halide perovskite solar cells (PSC) currently achieve a high power conversion efficiency (PCE), which is the highest among the third-generation solar cells. However, PSC reproducibility and long-term stability remain major problems limiting their commercialization. These problems are partially linked to the bonding nature of perovskite materials, which are polycrystalline materials formed via ionic bonding. Because of this, most preparation processes for their crystalline layers involve wet chemical processes that may leave many structural defects on the grain surface or boundaries. Various surface defects can arise during perovskite layer fabrication, multilayer/junction structure formation, storage, and device operation. Perovskite crystallization occurs rapidly (in just a few seconds), in contrast to silicon crystallization, which forms through covalent bonds. Various surface defects can arise during perovskite layer fabrication, multilayer/junction structure formation, storage, and device operation. The halide perovskites considered in this study poses an ABX₃ structure, where A represents a metal or organic monovalent cation, B stands for a lead (Pb) or tin (Sn) dication, and X represents a halide. Both experimental and theoretical studies of APbI and ApbBr, for instance, indicate that the valence and conduction bands are predominantly formed from the I-5p or Br-4p orbitals and the Pb-5p orbitals, respectively. Despite the A cation does not contribute to the valence and conduction bands, as revealed by ultraviolet photoelectron spectroscopy (UPS), it plays a role in hybridization bond formation. The absence of these ions from the surface may lead to the formation of gap states or defect states. However, the presence of surface/defect states within the perovskite layer can significantly influence charge transport and transfer within devices, especially solar cells. To date, the direct measurement of surface defects, particularly at the junction interface, cannot be easily performed. Various methods have been developed to determine and quantify defects in perovskite materials and solar cells. Here, we have investigated the effects of surface defects and defect states on PSC characteristics using transient photovoltage spectroscopy (TPV) and impedance photovoltage spectroscopy (IMVS). Through these collaborative measurements, we determined that the TPV curves may exhibit simple multiexponential decay or suppressed exponential decay. The fast-decay component can be assigned to the trapping and detrapping of charge carriers by the defect states. The slow decay can be associated with displaced ion migration at the surface or interface, which can be related to the appearance of an impedance semicircle in the low-frequency region in the Nyquist plot of the IMVS measurement results. These results indicate that the surface defect states in perovskite materials play a much more crucial role than their silicon counterparts. Effectively controlling these defect states is of utmost importance and can be achieved through various methods such as enhancing the defect passivation, mobile ion immobilization, engineering the junction interface through the utilization of other two-dimensional materials, and employing physical treatments such as plasma treatment.

References

1. Y. S. Handayani et al 2019 Mater. Res. Express 6 084009; P. Pitriana et al. 2019 Res. in Phys. 15 102592

2. R. Hidayat et al. 2020 Sci. Rep. 10 19197; A.A. Nurunnizar et al. 2021 Mater. Sci. Semicon. Process. 135 106095

This talk aims to introduce the Samahang Pisika ng Pilipinas (SPP) or the Physics Society of the Philippines to the Japan Society of Vacuum and Surface Science (JSVSS) and the rest of the invited societies such as Vacuum Society of the Philippines (VSP), Chinese Vacuum Society (CVS), Korean Vacuum Society (KVS), Taiwan Vacuum Society (TVS), Taiwan Association for Coating and Thin Film Technology (TACT), Physical Society of Indonesia (PSI), and American Vacuum Society (AVS), in the Annual Meeting of Japan Society of Vacuum and Surface Science.

Introduction of Taiwan Association for Coating and Thin Film Technology (TACT)

Since its establishment on September 3, 1999, Taiwan Association for Coating and Thin Film Technology (TACT) has grown into the most active society in the field of thin-film and coating technology in Taiwan. As one of the most influential and impactful societies in Taiwan's academia and industry, TACT not only fosters the advancement of coating technology but also provides vital technical support services to the industry. The society also actively engages in international conferences and events related to coating and thin film technology, effectively raising awareness about TACT on a global scale. Commencing from TACT 2009, a biannual international thin film conference has been convened, attracting more than 700 attendees from around the world. This conference serves as a platform for the exchange of knowledge and ideas among researchers and engineers in both academia and industry. Moreover, selected papers presented during the conferences have been featured in esteemed journals, such as “Surface & Coating Technology” and “Thin Solid Films.” Furthermore, each of TACT’s international conferences has garnered endorsements from prestigious societies like the American Vacuum Society (AVS), Thin Film Society (TFS), Korean Vacuum Society (KVS), and particularly, Japan Society of Vacuum and Surface Science (JVSS) for TACT 2023 (https://tact2023.conf.tw). We look forward to establishing collaborations with other relevant societies and extending a warm invitation to all members to participate in our upcoming conferences.

Enhanced Transmittance and Conductivity of TCO Coatings Deposited by High-Power Impulse Magnetron Sputtering A transparent conductive material, mostly transparent conductive oxide (TCO), is essential and indispensable for flexible electronics. Different from the conventional, single-layered TCO, multilayered oxide/metal/oxide (OMO) structures have been investigated as a promising alternative. The metal layer reduces the electrical resistance of the oxides, and the transparency is enhanced by suppressing the reflection from the middle metal layer and substrate. Meanwhile, due to the limited temperature tolerance of the flexible polymeric substances, a high-power impulse magnetron sputtering technique has been used to deposit the OMO structures at a substrate temperature lower than 70 oC. Various OMO multilayered structures, including ZnO/metal/ZnO, ITO/metal/ITO, are deposited on polyethylene naphthalate (PEN) and polyethylene terephthalate (PET), and soda lime glass. The effects of layer thickness and metal type on the transmission and resistivity of the obtained OMO structures are investigated. The applications of our OMO coatings in transparent micro-LED displays and wearable devices made of a wireless vertical-type LED package are also demonstrated.

References

ACS Appl. Electron. Mater. 5, 2, (2023) 905–912

ACS Appl. Electron. Mater. 3, 2, (2021) 979–987

Thin Solid Films 748 (2022) 139158

Coatings 11 (2021) 144.

This talks covers research background and current status of high-entropy alloy coatings with emphasis on (MoSiTiVZr)Nx high-entropy nitride coatings, explains the relationship between the N content and the crystal structure and properties of the film.

The use of new materials opens new ages for human beings. Thus, the exploration of new materials is crucial for developing next-generation electronics. Pioneered by graphene, new 2D materials exhibit abundant unusual physical phenomena undiscovered in bulk forms due to their unique electronic structure. Muscovite mica, a 2D layered oxide, is the most significant available natural and synthetic 2D layered single crystal. In 2016, Chu’s group was the pioneer in proposing to use muscovite mica as substrates to build up flexible electronics using inorganic materials[1], forming a research playground named “MICAtronics”. Due to its 2D feature, an atomically smooth surface can be obtained for van der Waals heteroepitaxy, and superior mechanical flexibility can be utilized for bending conditions. Various heteroepitaxies, including metal/muscovite, oxide/muscovite, and conventional semiconductor/muscovite, were shown with many device demonstrations, delivering new material solutions for flexible devices and presenting a promising future for muscovite-based flexible electronics. Owing to its natural thermal and chemical stabilities, the heterostructures made on mica show excellent environmental stability, standing out as an alternative solution to soft technology[2]. However, in these studies, muscovite is a flexible substrate, and no critical functionality of muscovite is revealed. Taking the nature of the 2D layered structure of muscovite, a van der Waals gap of 0.3 nm exists between layers, which can be viewed as a 2D cavity. The idea of this study is to use this 2D cavity to create a new material form by inserting chemical species, primarily focusing on transition metal ions. Then, a heat treatment is implemented to transfer transition metal ions into crystalline form. The atmosphere will be controlled during this process, expecting to deliver materials in metal, oxide, nitride, and carbide forms. These newly formed systems with muscovite can be viewed as an atomic composite due to the atomic layer stacking of the heterostructure. Properly designed, this atomic composite can show properties different from bare muscovite, delivering a new substrate selection for flexible electronics.

References: 1. Y. H. Chu, npj Quant. Mater. 2, 67 (2017). 2. Y. Bitla and Y. H. Chu, FlatChem 3, 26 (2017).

Introduction

Silicon dioxide (SiO₂) stands as a cornerstone in the semiconductor industry due to its indispensable roles in insulation, passivation, and gate dielectric applications. And for the fine-tuning of semiconductor device performance and nanoscale fabrication, plasma-enhanced atomic layer etching is a promising approach as it enables precise and controlled removal of a material of interest. [1] In this work, the focus is developing alternative co-reactants for the ALE of SiO₂, including those based on radical reactions. The feasibility of using sequential fluorination and low energy ion bombardment to etch SiO₂ was investigated via in-situ using Fourier transform infrared spectroscopy (FTIR) and ex-situ spectroscopic ellipsometry to monitor the surface reactions and etched thickness of the thin film, respectively.

Results

FTIR studies revealed the fluorination of SiO₂ by exposing the film to fluorine radicals, which proceeds through the removal of surface hydroxyl groups. ALE was accomplished through ion-enhanced bombardment of the fluorine-terminated SiO₂ using argon plasma, enabling a monolayer etching (etch per cycle = 0.14 nm) in a two-step process. Using trenched patterns with an aspect ratio (AR) of 3, the etch profiles showed consistent monolayer isotropic etching. Moreso, it is shown that bias-application during the ion bombardment step can enhance the etching of horizontal surfaces on trenched patterns, implying the controllability of the ion angular distribution with bias.

References

[1] K. J. Kanarik, S. Tan, and R. A. Gottscho, J. Phys. Chem. Lett., 9, 4814 (2018).

The Photon Factory, a synchrotron radiation (SR) facility in Tsukuba, has five beamlines (BLs) dedicated to photoelectron spectroscopy (PES) research, i.e., BL-2A, 3B, 13B, 27A, and 28A. Among these, two undulator BLs, 13B and 28A, have been redesigned and redeveloped since 2018 for the PES measurements using micro-focused SR beams. Micro-focusing of the SR at BL-13B was achieved by replacing a toroidal post-mirror (tangential and sagittal radii of 98.2 and 0.0698 m, respectively) with the one having the radii of 43.0 and 0.0399 m, which gives size of 78 μm (H) × 15 μm (V) for the 100-eV photon beam [1]. At BL-28A, a Kirkpatrick-Baez (K-B) mirror is used to have a microbeam with a size of 10 μm (H) × 12 μm (V) at the photon energy of 200 eV [2].

BL-13B is suitable for micro X-ray PES (μ-XPS) research because of its wide photon-energy range available between 48 and 2000 eV. The PES system at BL-28A equips an electron analyzer with a deflector electron lens mode (Scienta Omicron DA30) and, thus, is more suitable for micro angle-resolved PES (μ-ARPES) measurements. One of good examples for μ-ARPES research is the study of microcrystalline boron sulfide (BS) by Sugawara et al. [3]. They searched for large crystals from the BS powder by the two-dimensional (2D) scan while monitoring the B 1s core-level peak, and measured a band dispersion with a deflector mode without changing the tilt angle of the sample goniometer.

Fig. 1 shows an example of the μ-XPS measurements at BL-13B. 2D intensity maps of Pd and Rd 3d_5/2 core-level peaks and a C 1s peak of a PdRh alloy surface covered with acetic acid molecules were measured at the 460-eV photon energy. From the correlation analysis among these three peak intensities, acetic acid prefers to be bonded to Rh rather than Pd, because there is a linear correlation between the C 1s and Rh 3d peak intensities.

The PES system at BL-13B is equipped with a two-axis sample goniometer with wide angular ranges of the tilt rotation (−90° — +90° relative to the surface normal direction) and the azimuth rotation (−180° — +180°) [4]. A wide tilt rotation allows to acquire photoelectrons emitted to the grazing-angle direction and, therefore, to verify surface properties without being obscured by bulk components. This was demonstrated by measuring the SnO thin film whose surface was oxidized by exposing it to the atmosphere. More interestingly, an incidence angle of the light (relative to the surface plane) becomes small when the tilt rotation approaches to +90° (or −90°). If total reflection conditions for both the incidence light and the emitted photoelectrons are simultaneously fulfilled, true surface analyses are possible. This will bring new opportunities for surface science research.

References

[1] K. Ozawa et al., J. Synchrotron Radiat. 29, 400 (2022).

[2] M. Kitamura et al., Rev. Sci. Instrum. 93, 033906 (2022).

[3] K. Sugawara et al., Nano Lett. 23, 1673 (2023).

[4] Y. Aiura et al., Measurement 217, 112866 (2023).

In near-ambient pressure photoelectron spectroscopy, photoelectron intensity is greatly reduced due to scattering of photoelectrons in gas. Therefore, in general, the distance d between the sample and the aperture cone is set as small as possible within a range in which the pressure drop on the sample surface does not occur. Recently, however, we have reported that increasing d has a significant effect on the environmental charge compensation in the measurement of insulators [1-4]. There have been few reports on the detailed relationship between photoelectron intensity and d in near-ambient pressure photoemission spectroscopy. In this study, we investigated the d dependence of photoelectron intensity in near-ambient pressure photoemission spectroscopy by varying d over a wide range [5].

Experiments were carried out using a near-ambient pressure hard X-ray photoelectron spectroscopy apparatus installed at Hyogo Prefecture ID beamline BL24XU in SPring-8. The excitation X-ray energy was 8 keV. In our usual arrangement, d = 0.3 mm. Here, however, we varied d greatly in the range of 0.3 to 5 mm and investigated the gas (Ar or N₂) pressure P dependence of the photoelectron intensity from the Au plate at each d. Ta. See reference [1] for details on how to change d.

ln (I / Io) decreased linearly with P at each d, as expected from the Beer-Lambert law, I / Io = exp (- σ P d / k_B T). where I and Io are photoelectron intensities in gas and vacuum, and σ is the electron scattering cross section. From these measurements, it was found that increasing d in charging-free measurements of an insulator is also advantageous from the viewpoint of photoelectron intensity. From the slope of the ln (I / Io) vs P plot, the value of σ d was obtained for each d. Figure 1 shows the relationship between σ d and d obtained. Clearly the straight line obtained here does not pass through the origin. To reproduce this result, it is necessary to change d to d + do (do is a constant of about 1 mm) in the Beer-Lambert law. It is considered that the correction is necessary due to the effect of residual gas in the electron lens.

References:

[1] S. Suzuki et al., J. Electron. Spectrosc. Relat. Phenom. 257, 147192 (2022).

[2] K. Fujitani et al., Heliyon 9, e15794 (2023).

[3] K. Fujitani et al., Appl. Surf. Sci. 637, 157891 (2023).

[4] K. Takahara et al., Adv. X-ray Chem. Anal., Jpn 54, 75 (2023).

[5] S. Suzuki et al., J. Vac. Sci. Technol. B 41, 044204 (2023).

X-ray absorption spectroscopy (XAS) in the soft X-ray region is one of the methods, which is extensively used to analyze the chemical properties and electronic states near surface. However, to observe the reaction change over time without halting the reaction is difficult with conventional measurement of soft X-ray XAS, because it requires a considerable amount of time for obtaining one spectrum, due to measure the absorption intensity stepwise at each energy. To overcome the disadvantage of time-consuming measurements, we developed a method to illuminate the sample with dispersed soft X rays and separately collecting the fluorescent X-ray emitted at each position on the sample [1], and succeeded to acquire one spectrum at once for real-time observation. The method can also be applied for the observation of chemical reactions under the near ambient pressure air and external fields (magnetic and electric fields), because the detection of soft X-ray fluorescence has recently been realized. In addition, this method can also analyze depth profile, simultaneously with real-time observation [2].

In this study, aiming for the observation under higher gas pressure, we observed the surface oxidation of Co thin film, which proceeds to depth direction in real time under a gas pressure of ca. 10 Pa. And followingly, we prepared the configuration of apparatus, which has Si₃N₄ windows to separate the gas induced area from the high vacuum area, and, ambient air was induced to the apparatus up to 10000 Pa to observe the surface oxidation of Cu. It is known that the oxidation state of Cu has CuO and Cu₂O, therefore the time change of these component together with distribution of the components in depth direction was analyzed.

For another application example of the method, we applied it to observe the chemical states at the solid–liquid interface directly in real time and operando during the electrochemical oxygen evolution reaction (OER) for water splitting. Hydrogen generation by electrochemical water splitting has received a lot of attention recently, since hydrogen is considered as a sustainable source of renewable energy. Generally, cathodic materials for the hydrogen evolution reaction and anodic materials for the OER are combined in one device for water splitting. The OER is known to be a bottleneck in electrochemical energy conversion, and various studies have attempted to improve the performance. Therefore, it is necessary to observe the electrode surface in real-time and in operando to obtain precise information for the reaction mechanism. We achieve the real-time/operando measurement with changing the electrode potential with a time resolution of ca. 3 ~ 10 s. In this study, we focused on Co as OER catalysts, whose oxidation states and intermediates at the solid-liquid interface have not been probed directly and unclarified. A dedicated electrochemical cell [4] was attached to the XAS apparatus, and O K-edge XAS data on the surface of a Co thin film working electrode in an alkaline solution (NaOH, 0.1 M) was observed during sweeping the potential. As results, when the electrode potential was shifted to a positive value, the peaks of O K-edge for the Co electrode at around 531 eV, which is for molecule-like oxygen, and around 529 eV for possibly precursor were detected, and the peak intensity changed in correlation with the electrode potential.

References:

[1] K. Amemiya, K. Sakata, M. Suzuki-Sakamaki, Rev. Sci. Instrum. 91 093104, (2020).

[2] K. Sakata, M. Suzuki-Sakamaki, K. Amemiya, Nano Lett. 21, 7152 (2021).

[3] K. Sakata, K. Amemiya, J. Phys. Chem. Lett. 13, 9573-9580 (2022).

[4] K. Sakata and K. Amemiya, Chem. Lett. 50, 1710-1713 (2021).

X-ray ptychography is a coherent X-ray diffractive imaging technique with the object of interest scanned in small steps by an overlapping probe. The collected diffraction pattern set is reconstructed into real-space images of the sample by iterative phase retrieval calculation with a 10 nm order spatial resolution. The spectroscopic ptychography method is a combination of X-ray ptychography imaging and XAFS spectroscopy, where X-ray ptychography measurements are applied in X-ray energy around the target absorption edge. Thus, the reconstructed image stacks of the sample objects provide highly spatially-resolved XAFS spectra of the non-uniform materials in nanoscale, which is considered to be the most promising tools for visualizing mesoscopic structures and chemical states (e.g., element composition, valence, local structures, etc.). Here, we report the development of X-ray ptychography in tender X-ray regions (2.5 keV) for the first time and the application of sulfur chemical state visualization of cathode active materials of lithium-sulfur battery. Tender X-rays are useful for analyzing the chemical states of light elements because the K-edge of various light elements, such as sulfur and phosphorus, are in their energy range. This makes it possible, for example, to elucidate the reaction and degradation mechanisms of lithium-sulfur battery cathode materials, in which sulfur plays an important role in charge-discharge reactions. However, due to the limitations of optical elements, there are very few reports on high-spatial-resolution visualization studies of sulfur materials using tender X-rays. Furthermore, there have been no actual examples of X-ray ptychography using tender X-rays due to the lack of synchrotron radiation beamlines that can provide highly coherent tender X-rays. Our group has established the first X-ray ptychography measurement system using tender X-rays. The measurement system was developed at the beamline BL27SU for spectroscopy at SPring-8. In this measurement system, X-rays monochromatized by a Si(111) crystal monochromator are spatially extracted by a pinhole of about 10 μm in diameter to ensure coherence of the incident X-rays. Diffraction intensity patterns were obtained using the newly developed SOPHIAS-L two-dimensional detector for tender X-rays. The measurement accuracy was improved by introducing elemental technologies such as precision machining of pinholes and temperature stabilization of the entire optical system [1]. Next, we applied this technique to the measurement of cathode materials for lithium-sulfur batteries to visualize the heterogeneous chemical state of sulfur in the materials [2]. Sulfur-modified poly-(butyl-methacrylate) (SPBMA, main components: sulfur, carbon) particles (diameter: ~5 μm) used as cathode active material in lithium-sulfur batteries were measured. Measurements were taken at 30 points around the sulfur K-edge of 2460-2500 eV. As a result, we succeeded in reconstructing an absorption image that shows the same shape as the scanning electron microscope image, and the space-resolved X-ray absorption spectrum obtained from the absorption image shows the same shape as the reference spectrum obtained from the sample powder, confirming the high accuracy of this measurement. In order to analyze the elemental distribution and chemical state within the particles in detail, the amount of phase shift and sulfur-sulfur (S-S) and sulfur-carbon (S-C) bonding in the space-resolved X-ray absorption spectrum were reflected, respectively. Peak intensities were normalized by sulfur content and mapped in two dimensions. These distributions consistently suggest a trend toward more carbon near the center of the particle and more sulfur near the surface, consistent with the elemental distribution obtained by electron microscopy of a cross-section of another particle.

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Recently there has been an increasing demand for remote experiments among synchrotron radiation users. During the COVID-19 pandemic, when the movement of people was severely restricted, the demand was further strengthened. In response to the demand, we have made efforts for remote experiments that can be implemented in synchrotron radiation facilities.

In the Hiroshima Synchrotron Radiation Center (HiSOR), Hiroshima University, there are SR beamlines for angle (and spin) resolved photoemission spectroscopy (ARPES) and soft x-ray absorption spectroscopy (SXAS) on solid and thin-film materials. During the pandemic period, some experiments were carried out by the local beamline staff on behalf of outside users. The so-called “proxy experiments” or “agent experiments” were heavy tasks for the local staff.

There have been attempts to conduct experiments via the internet. Using remote desktop access software, beamline users can operate measurement systems. In the case of ARPES and SXAS measurements which require the ultra-high vacuum (UHV) environment, however, the data-taking part can be conducted only after their samples are transferred and fixed on the manipulator which was done by on-site staff. It is difficult to transfer and exchange the samples in a UHV chamber remotely.

Furthermore, as the ARPES and SXAS measurements are sample surface-sensitive methods, the measured sample needs to be exchanged and recleaned from time to time based on the sample surface condition. It means remote users need on-site support at almost all their beamtimes for the sample exchanges. Therefore, it is very important to realize a fully automatic sample transfer system for remote experiments. We are now developing the motorized sample transfer system in the UHV chamber.

So far test equipment that simulates a sample exchange section was assembled. We aim to build a user-friendly remote control system without major modifications to the existing UHV measurement apparatus. The test equipment consists of two CF-114-base chambers separated by a pneumatic gate valve, two motorized transfer rods, and a motorized XY stage. All motors and gate valves can be controlled by LabVIEW (National Instruments) software, and the objects inside the chamber can be observed using two USB cameras. We use a personal computer to control the motors for the sample exchange system, indicating that remote operation is now possible.

It was necessary to build an interlock system to prevent accidents during sample exchange and transfer, which happen even in on-site operations. While we set some limit switches around the motors, we could not find proper contact sensors that could be used in the UHV chamber. Thus, an interlock for collision prevention was made using an image sensor (IX-150, KEYENCE). The image sensor can be retrofitted to a viewport out of the vacuum chamber, so it is convenient for building the simple interlock system without changing the layout in the UHV chamber.

The sensor can measure the accurate depth and width of the object through the viewport. The feature is used for sample alignment and enables to carry out a semi-automatic sample exchange on the sample bank without other encoders. The semi-automatic system using the image sensor allows safe sample exchange reducing accidents caused by human error.

This work is conducted as part of Photon Factory, UVSOR, and HiSOR collaborative project for the construction of the new R&D beamline.

The waveform of electromagnetic radiation from an ultra-relativistic electron reflects the motion of the electron. This implies that, by controlling the electron motion in a magnetic field, one can control the properties of radiation waveform in the nanometer or Angstrom scale using a synchrotron light source. In recent years, we have been exploring the possibility of the spatio-temporal control of undulator radiation and have succeeded in performing coherent control and ultrafast spectroscopy of gas-phase atoms and molecules in the extreme ultraviolet and attosecond regime.

The experiments were carried at the light source development beamline BL1U in the UVSOR-III synchrotron. The temporal structure of radiation can be exploited in a time-domain measurement using a tandem undulator setup comprising two identical undulators placed in series (Fig. 1a). The individual relativistic electron passing through two undulators emits a pair of 10-cycle light wave packets [1,2], and the light wave packets naturally possess longitudinal coherence between them. The time delay between the light wave packets in pair can be controlled at the attosecond level. The use of the longitudinal coherence enables quantum control of atoms [3, 4], which were thought to be enabled only by laser sources. Furthermore, the femtosecond Auger decay of the inner-shell excited state is tracked [5] (Fig. 1b). This approach has recently been extended to controlling the interference between photoelectron wave packets [6]. This new capability of synchrotron light sources will open up the possibilities of probing and controlling ultrafast phenomena in a wide range of research fields, taking the advantage of the short wavelength radiation such as the chemical selectivity in excitation processes. Moreover, to explore new applications of spatial structure of undulator radiation, we have produced vortex beam which has a helical wave front and carries orbital angular momentum [7]. The interaction between the XUV vortex beam and atoms was studied by measuring photoelectron angular distributions of helium atoms [8].

References:

1) T. Kaneyasu et al., Sci. Rep. 12, 9682 (2022).

2) T. Fuji et al., Optica 10, 302 (2023).

3) Y. Hikosaka et al., Nature Communications 10, 4988 (2019); 12, 3782 (2021).

4) T. Kaneyasu et al., Phys. Rev. Lett. 123, 233401 (2019).

5) T. Kaneyasu et al., Phys. Rev. Lett. 126, 113202 (2021).

6) T. Kaneyasu et al., Sci. Rep. 13, 6142 (2023).

7) M. Katoh et al., Sci. Rep. 7, 6130 (2017).

8) T. Kaneyasu et al., Phys. Rev. A 95, 023413 (2017).

Introduction The solar energy is a sustainable and clean energy candidate to achieve a carbon-free society. Among all the possible approaches for solar energy conversion, the photocatalytic water splitting is a promising way to harvest solar energy and produce hydrogen. Recently, Domen and co-workers developed a photocatalyst, Rh/Cr₂O₃/CoOOH/SrTiO₃(STO):Al, which has an external quantum efficiency of 96 per cent at wavelengths between 350 and 360 nm.¹ Although it is assumed that CoOOH acts as a OER site and Rh/Cr₂O₃ acts as a HER site, actual observation of photo-induced carrier transfer to the co-catalysts has not been achieved yet. In this work, we developed a new in situ soft X-ray absorption spectroscopy (SXAS) system with the conversion electron yield (CEY) mode which provides surface sensitive chemical information to study the photoexcited carriers transfer from bulk STO to the co-catalysts especially CoOOH, the OER site.

Experiment The STO:Al photocatalyst system with co-catalysts loaded was prepared with two steps ¹; (1) preparation of STO:Al nanoparticles. Using the flux method, STO and SrCl₂ mixed with molar ratio of 1:10, and heated in an alumina crucible at 1373 K for 10 hours. (2) photodepostition for the HER and OER co-catalysts loading on the STO surface. One-hundred mg of STO:Al was dispersed in 50 ml water and performed 3 steps photodeposition in a series for Rh, Cr₂O₃, and CoOOH.¹ In situ SXAS measurements were performed at BL-13A in the Photon Factory (KEK-PF, Tsukuba). The SXAS measurement cell based on the CEY mode² was modified in the present study such that in situ XAS measurements can be conducted under irradiation of UV-Vis lights onto sample surfaces with different gas environments.

Results and Discussion The standard SXA spectra for Co L-edge were measured using both the total electron yield (TEY) method and the CEY method under dark conditions. From Co L-edge CEY-SXAS measurements, we confirmed that the Co species in the photocatalyst is associated with CoOOH. However, we found from TEY-SXAS measurements under a high vacuum condition that the Co species undergoes auto-reduction with showing a strong pre-edge peak intensity compared with the results obtained from the CEY mode. Therefore, the CEY mode measurement can give more reliable and X-ray-induced-reduction-free SXAS data. Using the CEY mode with UV-Vis irradiation, the Co L-edge XAS gives slight reduction. When the UV-Vis light was delivered on the sample surface, a main peak at 780 eV, slightly shifts to the lower energy side. As the UV-Vis irradiation time increases, the pre-edge peaks are slightly enhanced with reduction in intensity of the main peak, showing that a small amount of Co³⁺ was reduced to Co²⁺. This result indicates that the Co is not receiving photoexcited holes but electrons when the photocatalyst is irradiated by the UV-Vis light without water vapor. After saturated water vapor was introduced into the cell, the pre-edge peaks A and B originating from the Co²⁺ state drastically diminished with a slightly higher energy shift of the main peak C, showing oxidation from Co²⁺ to Co³⁺ and further towards Co⁴⁺ in the OER site as shown in Fig. 1. These spectral changes indicate that the Co is receiving the photoexcited holes from the bulk STO, and the photoexcited holes are used to oxidize the Co²⁺ to the Co³⁺ in the OER sites.

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Mitigation of industrial carbon dioxide (CO₂) emissions has been highly desired because it is essential to prevent various environmental issues, such as global warming. Recycling CO₂ into valuable chemical products is one of the promising solutions. In particular, conversion into methanol is attractive due to its high versatility in the chemical industry [1, 2].

The methanol synthesis is typically operated at 50-100 bar and 200-300 ℃ using Cu/ZnO/Al₂O₃ catalysts. Since it is an exothermic reaction, lowering the operating temperature increases the equilibrium conversion ratio. However, the current Cu-based catalysts have low activity for the dissociation of molecular hydrogen (H₂) which is one of the crucial elemental processes. Therefore, an increase in the H₂ dissociation activity should lead to lowering the reaction temperature. Recently, highly diluted bimetallic catalysts, so-called single atom alloy catalysts (SAACs) have attracted much attention because of their unique and superior activity for the H₂ dissociation and selective hydrogenation reactions [3]. For example, on the Pd/Cu SAAC, H₂ molecules are dissociated at the Pd single atom sites even at 80 K, and the H atoms spill over onto Cu sites and react with other molecules. Thus, we expected that the Pd/Cu SAAC could be a good catalyst for CO₂ hydrogenation into methanol. Here, we have investigated the surface chemistry of CO₂ and H₂ on the Pd/Cu SAAC using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS).

AP-XPS measurements were performed using the apparatus at BL07LSU of SPring-8. The Pd/Cu SAAC was prepared by vapor-deposition of Pd onto the clean Cu(111) surface at 380 K in a UHV chamber. The coverage of Pd was estimated to be ~0.02 ML based on the Pd 3d_5/2/Cu 3p peak intensity ratio. The sample was installed into an AP-cell, and the gaseous CO₂ and H₂ were introduced up to ~3.5 mbar of total pressure. The core-level spectra of C 1s, O 1s, and Pd 3d_5/2 were measured at 300, 340, and 380 K. The photon energy of 680 eV was used.

First, gaseous CO₂ was introduced at room temperature. When the CO₂ pressure reached ~1 mbar, the peaks originating from the adsorbed carbonate (CO₃) species were observed; 288.4 and 289.0 eV for C 1s, and 531.1 eV for O 1s [4]. The impurity C 1s peak was also observed at ~284.5 eV. In the O 1s region, the atomic oxygen (O_ad) species were also detected at 529.4 and 529.9 eV [4, 5]. The formation of the CO₃ and O_ad species indicates the disproportion and dissociation of CO₂.

Next, H₂ was added to supply gas, and total pressure increased up to ~3.5 mbar. The observed C 1s spectra can be deconvolution into CO₃ (288.4 eV) and formate species (HCOO, 287.7-288.0 eV [6]). The former decreased, and the latter increased with the increase in H₂ pressure. In the O 1s region, the peaks for CO₃ and HCOO were indistinguishable. The peak deconvolution also indicates the appearance of new components at lower binding energies; 285.5 eV for C 1s and 530.5 eV for O 1s. We tentatively assigned these peaks to methoxy (CH₃O) species [7]. Upon heating to 380 K, the C 1s peak for HCOO and the O 1s peak for CH₃O further developed.

The present results indicate that on the prepared Pd/Cu(111) surface CO₂ was hydrogenated into HCOO via CO₃ species, which might be further converted to CH₃O species below 380 K. So far, the development of HCOO and CH₃O species has not been reported on the bare Cu surfaces below 400 K.

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Friction is one of the most familiar physical phenomena and people have investigated the behavior of friction, that is “law of friction”, and tried to control friction from ancient age. Dry sliding friction was investigated by da Vinci, who discovered that the friction force is proportional to the loading force and does not depend on the apparent contact area. These behaviors are now called Amontons’ law [1] . The mechanism of the dry sliding friction and Amontons’ law is believed to be explained by the adhesion theory developed by Bowden and Tabor in mid 20-th century. In the theory the adhesion force at real contact points yields friction force and the real contact area and then the friction force are considered to be proportional to the loading force and do not depend on the apparent contact area. During last decade, however, it was claimed that the usual adhesion mechanism can not yield friction force proportional to the real contact area and the loading force due to randomness or incommensurate nature of the interface. Here we investigate the frictional phenomena of macroscopic objects starting from atomic model. For the atomic single rel contact point the loading force dependence of the friction force is the subject of the condition of the interface of the contact. In mesoscopic systems many real contact points emerge and the number and the area distribution of the contact depend on the nature of the surface randomness. In the case that the loading condition is uniform, Amontons’ law holds irrespective of the loading force dependence of the friction force of a single contact or details of the surface randomness. In most macroscopic systems, however, nonuniformity of the stress cause precursor slips. In that case the static friction coefficient decreases with the increasing loading force [2-4] . The law of friction depends on the scale.

[1] Dowson, D., “History of Tribology”, Longman (1979).

[2] Otsuki, M., and Matsukawa, H., “Systematic Breakdown of Amontons’ Law of Friction for an Elastic Object Locally Obeying Amontons’ Law”, Scientific Reports 3 1586 (2013).

[3] Katano, Y., Nakano, K., Otsuki, M., and Matsukawa, H., “Novel Friction Law for the Static Friction Force Based on Local Precursor Slipping”, Scientific Reports 4 6324 (2014).

[4] Iwashita, W., Matsukawa, H. & Otsuki, M. Static friction coefficient depends on the external pressure and block shape due to precursor slip. Scientific Reports 13, 2511 (2023).

Shear thinning is a non-Newtonian behavior of liquid lubricants whose viscosity decreases with the increase in shear rate. Shear thinning is widely observed for a variety of liquid systems both in nanoscopic and macroscopic experiments; the mechanisms involved in the behavior are very complex which attracts considerable attention. In this talk, shear thinning of molecularly confined films of a variety of lubricant liquids investigated using the surface forces apparatus (SFA) will be described [1-3]. The results were contrasted with the shear thinning behavior studied using different approaches for different systems; nanoscopic measurements by means other than the SFA, computer simulation of confined liquids [4,5], and simulation of macroscopic elastohydrodynamic (EHD) lubrication [6,7] will be examined.

In the SFA experiment, a liquid lubricant was injected between molecularly-smooth mica surfaces and confined by normal compression. Under applied pressure (typically in the range of ~ 10 MPa), the liquid film reached a “hard-wall” thickness (generally a few molecular-layer thick for typical lubricant liquids). Lateral sliding motions at constant sliding velocity V (typically 10 μm/s or below) were applied and friction force F was measured. In the SFA experiment, sliding film thickness D and real contact area A were directly measured by multiple beam interferometry simultaneously with friction measurement. Then, viscosity of the confined liquid η_eff (referred to as effective viscosity) was obtained using Couette flow equation, η_eff = FD/AV. Measurements were made for a variety of lubricant systems from simple liquids to polymer melts; the results showed shear thinning for all the systems in their hard-wall state. We found that the relationship between shear thinning viscosity and shear rate appeared to follow a universal function that was almost linear in log-log scale with the slope of about -0.9 [2,3]. The existence of the universal function is explained from the view that the hard-wall films are in the glass-like state where characteristic relaxation times should correlate with the time scale of submolecular rearrangements, that are not very different for various liquid lubricants.

In macroscopic EHD lubrication, extremely high pressure (the order of GPa) at contact asperities prolongs the time scale of molecular motions in the intervening liquid lubricant and the system exhibits shear thinning at very high shear rate (typically near 10⁶ s^-1 or higher) [6,7]. This shear thinning is closely related to a well-known empirical concept “limiting shear stress” [8] and has been studied extensively by experiments and simulations. Although the physical mechanisms are still a matter of debate, plausible scenarios involve a glass-like transition of liquid lubricants under extremely high pressure. This molecular view of shear thinning in EHD lubrication could have something in common with that proposed for confined liquids measured using the SFA despite very different sliding conditions.

REFERENCES

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Recent advancement of fabrication technology can provide very smooth surfaces, whose surface roughness is the nm-order. This enables mechanical devices with nm-sliding gap, which brings innovation in machine performance. In the lubrication of nm-sliding gaps, conventional fluid lubrication cannot be applied, and boundary lubrication in which the sliding surfaces are separated by an additive adsorption film becomes useful. However, the adsorption mechanism and relationship between adsorption and friction properties are not well understood due to difficulty of measurement. The microscopic properties of the adsorbed molecules are not easy to measure because the thickness of the adsorbed films is on the order of 1 nm. In addition, the adsorption process takes place in oils. In this talk, I present a new method for measuring the adsorption properties and friction properties of polymer additives using our developed vertical-objective-type ellipsometric microscopy (VEM).

Since VEM is based on ellipsometry, which is widely used for thickness measurement of thin films and provides 0.1-nm thickness resolution, high film-thickness (or gap) resolution is possible. Ellipsometry requires oblique illumination for generating ellipsometric contrast. In conventional ellipsometric microscopy, corresponding to the oblique illumination, oblique observation is used. This causes drawback of narrow field of view. For example, the field of view is on around 1 µm when an objection lens with a high NA of around 0.9. To overcome this drawback, VEM is proposed. In VEM, focusing the illumination light onto the off-axis point on the back focal plane of the objective lens can provide oblique illumination with the objective lens normal to the sample surface. This enables VEM to provide lateral resolution of the order of 0.1 µm and thickness resolution of the order of 0.1 nm.

The thickness change of adsorbing films from the oil to solid surfaces and friction properties of polyalkylmethacrylate (PAMA) polymer additives were measured by VEM, and the relationship between the adsorption and friction properties is discussed in terms of the polarity of the polymer and molecule size.

In the field of nanotribology, anisotropy of friction [1-3] has been understood by the incommensurability of the crystal lattice at the interface [4,5]. Incommensurability is a physical origin of superlubricity, ultralow friction state, which is key to the energy-saving problem. On the other hand, in the field of materials science, it has been recently clarified that the incommensurability of layered materials has played important roles in electronics, magnetics, optics, and spintronics and it has been discussed by the science of Moiré pattern. Therefore, in this work, the mechanism of superlubricity of the twisted graphene interface, the monolayer graphene sheet absorbed onto the graphite substrate surface, was studied based on Moiré science.

First, the two-dimensional map of superpositions of graphene lattices, interaction energy, and force map were calculated as a function of the twisted angle and sliding distance, which showed the long-range periodic Moiré pattern. As illustrated in Figure 1, the pseudo-AB and AA stacking regions formed honeycomb and triangular lattices, respectively. The unit-cell size of the Moiré pattern rapidly became smaller as the twisted angle increased, which could explain the anisotropic behavior of the maximum lateral force plotted as a function of the twisted angle.

Next, the anisotropy spectra of the loading and lateral forces showed a strong sheet size dependence. The larger the sheet became, the sharper the anisotropy peak became, which resulted in a wider superlubric region.

Furthermore, if the loading and lateral forces and the misfit angle were normalized, the universal scaling law of the anisotropy spectrum independent of the sheet size was obtained. Thus, it is clarified that the correlation between the true contact area (Moiré pattern) and the vertical and lateral forces is independent of the sheet size, suggesting the possibility of controlling adhesion and superlubricity based on universal scaling laws.

References

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[5] K. Shinjo and M. Hirano, Surf. Sci. 283, 473 (1993).

We have investigated the friction properties on nanoscale and macroscale using alkyl-substituted phthalocyanine derivatives. The frictional properties on nanoscale depended on the two-dimensional molecular structure of physisorbed molecules. On the other hand, the frictional properties on macroscale were independent on the physisorbed molecules. We considered the change in effective viscosity with contact pressure to compare the nanoscopic with macroscopic behavior. Recently, we have attempted to investigate the interfacial molecular behavior of a lubrication film composed of nonpolar base oil and polar additives. Nonpolar base oil molecules were involved in the formation of a lubricating film, which mainly composed of polar additives at the interface. Comparing the molecular behavior with the frictional property, this physisorbed nonpolar base oil molecules might contributed to the friction reduction.

The oxygen reduction reaction (ORR) is a key reaction in polymer electrolyte fuel cells (PEFCs) and metal–air batteries. Although platinum group metal (PGM)-based ORR electrocatalysts have been widely used even in state-of-the-art PEFCs, PGMs has been rare and expensive. For further wide-spread applications of PEFCs, non-PGM electrocatalysts with high ORR activity and product selectivity to H₂O must be developed. Natural catalysts of metalloenzymes including laccases and cytochrome c oxidases are known to catalyze the ORR and therefore encourage us to develop metalloenzyme-inspired electrocatalysts based on non-PGMs for the ORR because these metalloenzymes use copper and/or iron ions as active sites [1-3].

We have developed non-PGM electrocatalysts of Cu- Fe- and N-doped carbon nanotubes, (Cu,Fe)-N-CNT, for the ORR, inspired by the heterometallic active site of cytochrome c oxidase [4]. The co-presence of Cu and Fe active sites increase the ORR activity and selectivity to H₂O in acidic media, compared with monometallic Fe-N-CNT or Cu-N-CNT. Kinetic analysis revealed that the selective 4-electron reduction of O₂ to H₂O is dominant for (Cu,Fe)-N-CNT whereas the sequential (2+2)-electron reduction mainly proceeds for Fe-N-CNT or Cu-N-CNT, indicating that the ORR mechanism can be modulated by the co-presence of Cu and Fe active sites. The ORR mechanistic insights based on spectroscopic data obtained by in situ X-ray absorption spectroscopy and Mössbauer spectroscopy.

References

[1] M. Kato, I. Yagi, e-J. Surf. Sci. Nanotechnol., 18, 81 (2020).

[2] M. Kato, T. Murotani, I. Yagi, Chem. Lett., 45, 1213 (2016).

[3] M. Kato, M. Muto, N. Matsubara, Y. Uemura, Y. Wakisaka, T. Yoneuchi, D. Matsumura, T. Ishihara, T. Tokushima, S. Noro, S. Takakusagi, K. Asakura and I. Yagi, ACS Appl. Energy Mater., 1, 2358 (2018).

[4] M. Kato, N. Fujibayashi, D. Abe, N. Matsubara, S. Yasuda and I. Yagi, ACS Catal., 11, 2356 (2021).

Introduction

As a clean and reliable energy technology, hydrogen is regarded as an appropriate alternative to fossil fuel, while water electrolysis using intermittent electric energy represents a promising commercial technology for industrial hydrogen production [1]. Water splitting reaction consists of a hydrogen evolution reaction (HER) at the cathode and an oxygen evolution reaction (OER) at the anode. Compared with the HER, the OER has sluggish kinetics and a large reaction barrier, limiting the efficiency of electrocatalytic water splitting [2]. Thus, the development of highly active oxygen evolution electrocatalysts has become a research hotspot. Currently, noble metals and metal oxides are the most widely used catalysts for OER electrocatalysis. However, metal-based catalysts often suffer from multiple disadvantages, including high cost, low selectivity, poor durability, impurity poisoning, and fuel crossover effects [3]. Therefore, metal-free catalysts have received increasing interest as promising electrocatalysts for advanced energy conversion and storage.

Results

We focused on rhombohedral boron monosulfide (r-BS), which is a new metal-free catalyst. r-BS was synthesized by a previously reported high-pressure solid-state reaction [4], wherein a mixture of amorphous boron and sulfur with an atomic ratio of 1:1. Here, we report that r-BS mixed with graphene (r-BS+G) as a new metal-free electrocatalyst, shows promising electrocatalytic activity with much better performance than most published metal-based catalysts in 1M KOH solution [5]. As shown in Fig. 1a, the r-BS+G shows significantly enhanced OER activity with the ultralow an overpotential of 250 mV at 10 mA cm^−2, which is 50 mV better than commercial RuO₂. Furthermore, the overpotential (250 mV) of r-BS+G is much lower than those of most reported metal-free OER electrocatalysts [6]. The mixing process was just a simple sonication but probably ameliorated the conductivity to effectively used active sites of r-BS.

Stability is very important for the wide application of electrocatalysts. It may be that the catalytic activity of the r-BS+G is too high, and the catalyst's performance has declined after performing 1000 cyclic voltammetry (CV) cycles. To solve this problem, we introduced nickel foam (NF) as the self-supporting working electrode. After dropping the sample ink onto the NF, the r-BS+G-NF shows significantly enhanced OER activity with the ultralow an overpotential of 308 mV at 100 mA cm⁻², which is 53 mV better than commercial RuO₂-NF (Fig. 1b). Most importantly, the stability of the r-BS+G has been greatly improved, the r-BS+G-NF exhibited strong durability in a prolonged chronopotentiometry test at a constant current density of 100 mA cm⁻² for 100 h (Fig. 1c), which provides more possibilities for future practical applications.

References:

1) Z. Y.Yu, Y. Duan, et al., Adv. Mater. 33, e2007100 (2021).

2) P. Zhai, C.Wang, et al., Nat. Commun. 14, 1873 (2023).

3) S. Pan, H. Li, D. Liu, et al., Nat. Commu. 13, 1-10 (2022).

4) H. Kusaka, T. Kondo, et al., J. Mater. Chem. A, 9, 24631 (2021).

5) L. Li, T. Kondo, et al., Chem. Eng. J. 471, 144489 (2023).

6) Z. P. Wu, et al., Adv. Fun. Mater. 30, 1910274 (2020).

In a large, ordered two-dimensional ensemble, if synchronized unidirectional molecular motion can be selectively induced, it leads to new functionality, e.g., molecular switches that convert external stimulus into mechanical motion. In the system of copper(II) phthalocyanine (CuPc) molecules adsorbed on the transition metal dichalcogenide TiSe₂, more than half of the CuPc become positively charged as CuPc⁺ due to the hot carrier injection from TiSe₂, causing rotational motion within the large ensemble. We clarified the mechanism by integrating and measuring time-resolved(tr) photoelectron spectroscopy—tr orbital tomography, trARPES, trXPS, and trXPD [1]. However, the detail of the relaxation process of hot carriers between CuPc and TiSe₂ is not theoretically disclosed yet. Previously, we have theoretically considered the dynamic process of hot carriers between adsorbed molecules (CO₂) and substrates (ZrO₂) [2]. Based on the similar theoretical approach, this study examines hot carrier injection-induced rotation for the system with large ensembles.

In this study, the geometry optimization, static electronic structure, and ab initio molecular dynamics (AIMD) calculations were implemented through the Vienna Ab initio Simulation Package (VASP). The hot carrier relaxation dynamics is studied using non-adiabatic molecular dynamics (NAMD) simulations implemented based on the time-dependent Kohn-Sham DFT with the fewest switch surface hopping (FSSH) algorithm. In detail, the generalized gradient approximation of Revised-Perdew–Burke–Ernzerhof (RPBE) was used, in combination with Coulomb and exchange interactions of the localized d-orbitals in the transition metal elements using the Dudarev DFT+U method with an effective Hubbard parameter U_eff = 5.5 eV for Ti and U_eff = 4.0 eV for Cu. The cutoff energy was set to 500 eV. A 15 Å-vacuum layer was accommodated in the direction perpendicular to the surface to minimize the interactions between periodic images. The van der Waals interactions were included using the Grimme DFT-D3 method. To account for the charged molecular, the CuPc⁺ was modeled by the anionic pseudopotential method.

AIMD results demonstrated that charged CuPc⁺ rotated counter-clockwise and neutral CuPc⁰ rotated clockwise in view perpendicular to the TiSe₂ surface. 375 fs later, the rotation angles were +15° for CuPc⁺ and −15° for CuPc⁰, precisely reproducing the experimental results. The results of the NAMD calculations reproduce well the relaxation process after pump pulse absorption. Specifically, hot electrons were relaxed from the high level of the Ti 3d conduction band to the conduction band minimum, and hot holes were injected from the Se 4p valence band to the HOMO of CuPc at about 375 fs.

References:

[1] K. Baumgärtner, et al., arXiv:2305.07773.

[2] K. Hara, et al., J. Phys. Chem. C 2023, 127(4), 1776–1788.

It has been well established that the single-atom catalyst, particularly the transition metal (TM = Fe and Co) embedded in N-doped graphene, has the potential to reduce O₂ to H₂O with appreciable activity and stability [1]. However, substantial improvement in the oxygen reduction reaction (ORR) activity at lower overpotential are still required. The atomistic simulation that takes into account the electrochemical environment, i.e., electrode potential and electrical double-layer (EDL), are desirable to better understand the underlying reaction mechanism of the ORR [2-3]. Here, we present a density functional theory study on the mechanism of ORR on the Fe-N₄-C and Co-N₄-C in contact with an acidic solution under an applied potential, enabled by the effective screening medium method combined with the reference interaction site model (ESM-RISM). We found the solvent effect is essential in modelling the ORR, as the potential-determining step (PDS) changes from ^*O → ^*OH (O₂ →*OOH) to ^*OH → H₂O (l) (^*OH → H₂O (l)) for Fe-N₄-C (Co-N₄-C) when the implicit solvent is introduced. We also found, with the constant potential method combined with the ESM-RISM, the reaction intermediates become competitive, and the resulting PDSs are changes from *O → *OH and O₂ (g) → *OOH for Fe-N₄-C and Co-N₄-C, respectively. The calculated limiting potentials are comparable for both Fe-N₄-C and Co-N₄-C, in contrast to the results obtained using the constant (neutral) charge simulation in which the superior catalytic activity of Co-N₄-C has been predicted. The origin of the discrepancy between these results is mainly the charge state (different from the neutral one) at a fixed electrode potential, which can be determined through the constant potential method with ESM-RISM [4]. This work clarifies the roles of the electrolyte solution within the RISM framework and demonstrates the importance of the variable charge state through the constant potential calculation [5].

References

[1] T. Patniboon and H.A. Hansen, ACS Catal. 11, 13102-13118 (2021).

[2] Q. Jia et. al., ACS Nano 9, 12496-12505 (2015).

[3] J. Wang et. al., J. Am. Chem. Soc. 139, 17281-17284 (2017).

[4] S. Nishihara and M. Otani, Phys. Rev. B 96, 115429 (2017).

[5] A.F.Z. Abidin and I. Hamada, J. Phys. Chem. C 127, 13623-13631 (2023).