In the past few years, there is increasing interest about high entropy alloys (HEA) as promising materials for various applications. High entropy alloys, a new class of alloys, are composed of five or more component atoms with almost similar compositions. These alloys form single phase solid solution due to the increased configurational entropy of such a multicomponent system [1]. With this background, HEAs offer the opportunity to discover materials with appropriate functionality for a specific application. It is possible to combine cheap elements and tune their properties. However, it also entails that investigating these alloys would be a challenging task due to the numerous elements in the periodic table and the possible combinations of such elements to form HEA.

In this study, we aimed to evaluate the reactivity of HEA which is composed of transition metals that are abundant in Southeast and East Asia regions for its potential application as a catalyst. Specifically, we investigated the adsorption of H atom on CoCuFeMnNi alloy by performing density functional theory (DFT) and machine learning (ML) methods. Previous experimental works have synthesized CoCuFeMnNi and its corrosion behavior and mechanical properties have been studied [2-3]. We first evaluated the stability of the alloy by applying the Hume-Rothery rules and calculating thermodynamic parameters relevant to HEAs. We have verified that CoCuFeMnNi will tend to form a solid solution and will be stable as face-centered cubic crystal. We calculated, via DFT, the adsorption energies of H atom on the hollow site of a random subset of the CoCuFeMnNi surfaces. From the results, we observed that the presence of Cu in the vicinity (nearest neighbor) of H reduces the adsorption strength while Mn and Fe as nearest neighbors enhance the H adsorption. We employed ML algorithm to predict the remaining adsorption energies. In the implementation, we defined microstructure data based on the local environment of the adsorbed H on the surface as features [4]. We have verified the validity of the algorithm by establishing its accuracy and obtaining good agreement between the DFT calculated and ML predicted values. The analyses of the results will be presented at the conference.

References

[1] D.B. Miracle, O.N. Senkov, Acta Materialia 122, 448 (2017).

[2] S. Ozturk, A. Furkan, S. Onal, S.E. Sunbul, O. Sahin, K. Icin, Journal of Alloys and Compounds, 903, 163867 (2022).

[3] R. Sonkusare, P.D. Janani, N.P. Gurao, S. Sarkar, S. Sen, K.G. Pradeep, K. Biswas, Materials Chemistry and Physics, 210, 269 (2018).

[4] D. Roy, S.C. Mandal, B. Pathak, ACS Applied Materials & Interfaces 13, 56151 (2021).

[Introduction] Silver (Ag) is an established industrial catalyst to produce ethylene oxide (EO) through direct oxidation of ethylene with gaseous oxygen. Ethylene epoxidation occurs through insertion of an oxygen atom into a π-bond of ethylene on Ag surfaces. In previous studies, it was revealed that atomic oxygen species at Ag surfaces extract hydrogen from ethylene resulting in full oxidation to carbon dioxide (CO₂) and water (H₂O). On the other hand, covalent oxygen species such as a sulfate (SO₄) was proposed to provide an oxygen atom to the p-bond of ethylene to produce EO [1]. Recently, a carbonate (CO₃) species was also suggested as a candidate of such oxygen-donor species [2]. In this study, we report new insights into behavior of the covalent oxygen species contributing to ethylene epoxidation on Ag(110) obtained from in situ measurements of near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS).

[Methods] We measured the surface species on a Ag(110) model catalyst under ethylene epoxidation conditions (total pressure, P = 0.2 Torr or 1 Torr, sample temperature, T = 370 ~ 520 K) using NAP-XPS and PM-IRRAS. The catalytic activity was also monitored using a quadrupole mass spectrometer (QMS) at the same time. NAP-XPS measurements were conducted at BL-13B of the Photon Factory (PF) at High Energy Accelerator Research Organization (KEK) [3]. In order to obtain information from the outermost surfaces, we tuned the incident X-ray energy as the kinetic energy of photoelectron to be about 100 eV. Second, PM-IRRAS measurements were carried out with a FTIR spectrometer (Nicolet: iS50) and a photo elastic modulator (Hinds: PEM-100). All PM-IRRA spectra were measured with 500 scans at a resolution of 4 cm^-1. To distinguish EO (m/z = 44) from CO₂ (m/z = 44) by QMS, we used deuterated ethylene (C₂D₄) instead of normal ethylene (C₂H₄), where deuterated EO (C₂D₄O) is detected as m/z = 48.

[Results and Discussion] NAP-XP spectra of O 1s, C 1s and S 2p regions for Ag (110) surfaces in the presence of reactant gases (0.16 Torr O₂ and 0.04 Torr C₂D₄) at 470 K indicate formation of CO₃ and SO₄ (Figure (a) and (b)) accompanied by oxidation of Ag atoms. In this case, the CO₃ species is formed from a reaction between O₂ and C₂D₄ on the Ag surface, on the other hand, the sulfur (S) of SO₄ is assumed to segregate from the Ag bulk followed by oxidation. To obtain further information, we measured PM-IRRA spectra simultaneously with catalytic activity using QMS where production of EO and CO₂ (total oxidation product) were clearly detected above 450 K. Under this reaction condition, PM-IRRA spectra exhibit peaks associated with the CO₃ and SO₄, which is consistent with the XPS results. In situ measurements to investigate correlation between behavior of the IRRAS peaks and the EO production were conducted as a function of temperature, which showed clear correlation. Detailed analyses results on the in situ observations are presented in the talk.

[References] [1] (a) T. E. Jones et al. ACS Catal. 2015, 5, 5846. (b) T. E. Jones et al. ACS Catal. 2018, 8, 3844. [2] K. Isegawa et al. J. Phys. Chem. C 2021, 125, 17, 9032. [3] R. Toyoshima et al. J. Phys.: Condens. Matter. 2015, 27, 083003.

Introduction. Hydrogen spillover is a dynamic phenomenon which is initiated by the dissociation of hydrogen molecules followed by the migration onto the reducible metal oxide supports.[1] Spilled hydrogen atom shows specific behavior compared with gaseous hydrogen, thus it dramatically promotes the performance of various functional materials, such as heterogeneous catalysts, hydrogen storage materials and hydrogen fuel cells. Because of the present movement toward a hydrogen-based society, hydrogen spillover has received increasing interest both in academia and industry.[2] However, understanding of its dynamic behavior, such as at what temperature it can take place, and which pathway it follows, is still lacking because the observation method is not well established.[3] Our group has reported Ru and Ni, an immiscible metal combination, specifically form RuNi solid solution alloy by an assist of hydrogen spillover on the surface of TiO₂.[4] In this work, we evaluated hydrogen spillover pathways in typical reducible metal oxide such as TiO₂, CeO₂ and WO₃ by combining Ru-Ni alloy nanoparticle formation, in-situ techniques, kinetic analysis, and density functional theory calculation.

Experimental/methodology. RuCl₃·3H₂O and NiCl₂·6H₂O were supported on TiO₂ by conventional impregnation method, after which the sample were reduced under H₂ dosage at a heating rate of 5 °C/min up to 300 °C to obtain RuNi/TiO₂. Catalysts with CeO₂ and WO₃ as their supports were also prepared by the same method. In order to evaluate the alloy effect of Ru and Ni in the catalytic reaction, the prepared catalyst was applied to the hydrogen production reaction from ammonia borane (AB ; NH₃BH₃).

Results and discussion. RuNi/TiO₂ and RuNi/CeO₂ showed enhanced activity over those of corresponding monometallic Ru catalysts in the hydrogen production reaction from AB indicating the formation of RuNi solid solution alloy nanoparticles. On the other hand, RuNi/WO₃ didn’t show any activity improvement compared with that of Ru/WO₃ indicating the segregation of Ru and Ni. In-situ XAFS measurement showed that Ru³⁺ and Ni²⁺ were simultaneously reduced over TiO₂ and CeO₂ although the redox potential of Ni²⁺ is much lower than that of Ru³⁺. In contrast, Ru³⁺ and Ni²⁺ were sequentially reduced according to their distinct redox potentials. These results suggest that hydrogen spillover occurred on the surface of TiO₂ and CeO₂, while hydrogen spillover is absent on the surface of WO₃. Systematic characterization combining in-situ DRIFT and MS measurements under the H/D exchange reaction were performed in order to study the dynamic behavior of hydrogen spillover over each support. In-situ DRIFT measurements under the H₂/D₂ switched atmosphere revealed that hydrogen spillover takes place at 50 °C, 150 °C, and 250 °C on the surface of TiO₂, CeO₂, and WO₃, respectively. Moreover, MS measurements demonstrated that hydrogen spillover on TiO₂ and CeO₂ preferentially proceeded on their surface rather than within their bulk phases. Conversely, hydrogen spillover favorably occurred within the bulk prior to the surface over WO₃. (Figure 1).[5]

References

[1] R. Prins, Chem. Rev., 112, 2714 (2012).

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Single-atom alloy (SAA), which consists of a small amount of active metal atomically dispersed in a more inert host metal, has garnered interest as a catalyst for oxygen reduction reaction (ORR) [1]. In the past years, researchers aimed to characterize SAAs with good reactivity and stability. The stability can be analyzed using the segregation energy, which measures the preference of the dopant atom to segregate to the topmost layer of the surface. While segregation happens on pristine surfaces, it is important to note that adsorbates can also induce dopant segregation. Due to the complexity of the SAA system, identifying the significant factors influencing dopant segregation remains a challenge [2].

Hence, we investigated dopant segregation and identified the significant factors influencing it by performing density functional theory (DFT)-based calculations and machine learning (ML) methods. We generated SAA surfaces of Ag, Au, Co, Cu, Ir, Ni, Pd, Pt, and Rh and used O and OH as adsorbates, key ORR reactants. We calculated the adsorption energies and the segregation energies with and without the presence of the adsorbates. We considered a set of 44 features encompassing the elemental, energetics, and electronic properties of the SAAs. We performed a two-stage feature selection method for both O- and OH-SAA systems which reduced the features to the top five most influential – formation energies, metallic radius difference, d-band centers of the dopant at the surface, and subsurface layer, the difference in surface energy between the host and dopant atom, and difference in the total number of d-electrons between the host and dopant atom. Using these identified features, we implemented various ML models – linear regression (LR), support vector machine regression (SVR), Gaussian process regression (GPR), and extra trees regression (ETR) – to predict adsorbate-induced dopant segregation energies. We found that the SVR model, both for O-SAA and OH-SAA, exhibited the best performance among the models. For O-SAA, SVR performance metrics are R²=0.92, RMSE=0.11, and MAE=0.09 for the train set; and R²=0.94, RMSE=0.10, and MAE=0.07 for the test set while the performance metrics for OH-SAA are R²=0.81, RMSE=0.016, and MAE=0.13 for the train set; and R²=0.91, RMSE=0.13, and MAE=0.10 for the test set. Also, we identified Rh-Au(111) as a potential ORR catalyst based on the criteria – good reactivity for ORR catalysis and good stability with and without adsorbates.

References:

[1] Hannagan, R. T., Giannakakis, G., Flytzani-Stephanopoulos, M., & Sykes, E. C. H. (2020). Single-Atom Alloy Catalysis. Chemical Reviews. doi:10.1021/acs.chemrev.0c00078

[2] Butler, K. T., Davies, D. W., Cartwright, H., Isayev, O., & Walsh, A. (2018). Machine learning for molecular and materials science. Nature, 559(7715), 547–555. doi:10.1038/s41586-018-0337-2

Introduction

Hard protective coatings have been widely used in diverse applications to improve the performance and durability of workpiece. Among all hard coating materials, multicomponent transition metal nitrides have drawn much of the attention due to their superior mechanical strength[1], chemical inertness[2], oxidation resistance[3] and thermal stability[4], which are all desirable material properties for anti-wear applications. However, these properties strongly depend on the chemical composition of the coatings. Owing to the chemical complexity of multicomponent nitride, the search for extraordinary coating materials is a rather time-consuming task. Thus, Accelerating the progress of investigating the compositional space of multicomponent nitride is crucial.

Results

In this study, an experimental combinatorial approach is adopted to efficiently explore the compositional space of multicomponent nitrides as illustrated in Fig.1. Two materials systems, HfNbTiVZrN and AlCrSiTiN, are fabricated through reactively co-sputtering technique. From the elemental quantification results, both HfNbTiVZrN and AlCrSiTiN exhibit wide composition spread for the metallic and metalloid elements.

For HfNbTiVZrN, the crystal structure is identified to be single phase rock-salt structure regardless of the chemical composition with the crystallite size from 4.4 to 30.9 nm. The hardness of HfNbTiVZrN exhibits a strong dependence on the composition and crystallite size with values ranges from 22.4 GPa to 39.3 GPa, showing significantly better mechanical properties than the constituent binary and ternary nitrides. Among all the compositions, Hf_3.5Nb_2.6Ti_22.9V_4.9Zr_17.7N exhibits the optimum hardness of 39.3 GPa. Scanning transmission electron microscope analysis indicates that the metallic elements are randomly distributed in the cation sub-lattice. The solid solution of atoms with different sizes could effectively hinder the movement of the dislocations, strengthening the mechanical properties.

For AlCrSiTiN, the incorporation of metalloid element (i.e. silicon) would induce the spinodal decomposition, leading to the formation of nanocomposite structure (nano-crystalline metal nitride/amorphous silicon nitride). These nanocomposite coatings would have the benefits of grain size refinement, precipitation hardening, grain boundary reinforcement, and nanostructure toughening. The hardness and fracture toughness of AlCrSiTiN are thus found to be strongly correlated to the silicon contents of the coatings, showing the optimum values at silicon contents around 8 at. %. In order to evaluate the performance of AlCrSiTiN at high temperature, oxidation test and high temperature tribological test are performed at 700℃. The results suggest that coatings with high toughness would have favorable high temperature wear resistance. On the other hand, the type of oxidation products would also influence the tribological behaviors at high temperature. It appears that avoiding adhesive wear behaviors (i.e. strong bonding between the formed oxide and testing counterpart) is beneficial for high temperature wear resistance.

References

[1] S.-Y. Hsu, Y.-T. Lai, S.-Y. Chang, S.-Y. Tsai, J.-G. Duh, Surface and Coatings Technology, 442 (2022) 128564.

[2] Y. Zhao, S. Chen, Y. Chen, S. Wu, W. Xie, W. Yan, S. Wang, B. Liao, S. Zhang, Vacuum, 195 (2022) 110685.

[3] W. Shen, M. Tsai, K. Tsai, C. Juan, C. Tsai, J. Yeh, Y. Chang, Journal of the Electrochemical Society, 160 (2013) C531.

[4] P.-K. Huang, J.-W. Yeh, Scripta Materialia, 62 (2010) 105-108.

Amyloid fibirl formation is a common phenomena of proteins obserbed on biomembranes. This phenomena is induced by a consecutive process of nucleation and elongation. The nucleation of proteins has been reported to occur on (bio)membranes. Of lipids in biosystem, glycolipids is an important species to predominate the accumulation and conformation change of proteins. Recently, beta-octyl-glucoside (b-CG), a derivative of cholesterol (Ch) that Ch is linked to glucose by a 1-4 glucoside bonding, played a role for a stress reponse of membrane surface against the heat and other environmental cahnges [1]. Such derivatives of lipids was embedded into lipid membranes to enhance the binding of amyloid beta peptides (Aβ) to membranes. Here, Aβis a causative protein of Alzheimer's disease. This was because glucoside cluster effect might promote the binding of peptides to membranes via a formation of hydgene bonding network.

Amyloid fibirl formation of Aβ was first examined in the presence and absence of b-CG-embedded DMPC liposomes. Unique self-assemblies of Aβ was observed, e.g. spherulitic fibrillar aggregates (spherulite) [2]. To clarify the mechanism of such self-assemblies, the kinetic analysis of fibril formation of Aβ. A concentration dependench of b-CG on nucleaton rate was observed.Alternatively, Aβ could interact with b-CG-embedded membranes in a manner of concentration dependency of b-CG.As a control, Ch-embedded membranes indicated the fibril formation propensity of Aβ different from that of b-CG-embedded membranes.Therefore, glucose group linked to Ch might act as the binding part with Aβ.This finding was supported by the peptide-adsorption experiment based on a quarzcrystal microbalance method and surface pressure measurement.

Alternatively, fluorescence probe-labelled Aβ was used to visualize how the lateral diffusion of Aβ was influenced by b-CG. First, b-CG was clearly seprarated from DMPC lipids, which was called b-CG rich domain. Next, we monitored the lateral diffusivity of labelled-Aβ with TIRFM and observed a localization of Aβ on the b-CG-rich domain. In addition, Aβ could be laterally diffused on DMPC and from DMPC to b-CG rich domain, whereas the lateral diffusion of amyloid beta peptiteds from b-CG rich domain to DMPC was not observed. These observations suggested that association process of Aβs occurred on b-CG rich domain rather than DMPC region. Lateral diffusivity of Aβ was determined by the orientation of Aβ into the surface of lipid membranes. As a cmparison, the orientation of Aβ into polymer membranes (polylactic acid, polymethyl methacrylate, and polyvinylpyrroridone) depended on the entanglement of Aβ with polymer membranes [3].

Further time-development of the associated Aβ formed the nuclei and fibrils. Finally, the self-assemblies of amyloid beta peptides on the b0CG embedded membranes induced the spherulitic fibrillar aggregate.

[1] H. Akiyama et al., J. Biol. Chem., 295, 5257-5277 (2020)

[2] T. Shimanouchi et al., Biophys. Biochim. Acta, 1870, 140816 (2022)

[3] T. Shimanoujchi et al., Appl. Sci., 11, 4480 (2021)

Introduction

All cells in our body are covered by cell membranes. Membrane proteins in cell membranes are essential for biological activities because they are responsible for signal transduction through the membranes. The lipid bilayer, which is the basic structure of the cell membrane, can be artificially formed and is widely used as a model cell membrane system. When isolated or synthesized membrane proteins are embedded in the lipid bilayer, this membrane will be a useful functional analysis system for various membrane proteins. We have reported stable artificial lipid bilayer and proposed them as a membrane platform for evaluating the functions of the membrane proteins, especially ion channels. Recently, we have constructed a membrane system in which a lateral voltage can be applied in addition to a conventional transmembrane voltage. The lateral voltage effectively enhanced the transmembrane current in ion-channel-incorporated lipid bilayer systems [1]. However, the working mechanism of the lateral voltage to increase the channel activities has not yet been clarified. In this study, we constructed a simulation model of an artificial membrane platform using continuum modeling and simulated the effect of the lateral voltage on the membrane properties of lipid bilayer.

Methods

An artificial bilayer lipid membrane is formed across a microaperture in a 12 µm-thick Teflon film. In order to apply the lateral voltage inside the lipid bilayer, two titanium (Ti) electrodes are deposited around the aperture, and a protective SiO₂ layer is formed on the surface of the electrodes. The diameter of the microaperture is 80-150 µm. The thickness of Ti electrodes and SiO₂ insulating layers is 200 nm and 300 nm, respectively. We modeled the artificial lipid bilayer platform using the finite element analysis software COMSOL Multiphysics 5.4. To describe the processes in this system, we used two physics modules of electrostatic and transport of diluted species which are described by the Poisson equation and Nernst-Planck equation, respectively.

Results and Discussion

First, we constructed a model in which a transmembrane voltage is applied to a bilayer lipid membrane of 5 nm thick and 200 nm wide. Both sides of the membrane were filled with KCl buffer (102.5 nm long and 200 nm wide). The relative permittivity of the bilayer lipid membrane and KCl buffer was 2.2 and 75, respectively. A potential of +100 mV was applied to the KCl buffer at one side of the membrane, and the KCl buffer at the other side was grounded at 0 mV. The ionic concentrations of potassium and chloride ions in the buffer were set to 150 mM. The Poisson-Boltzmann equation was incorporated to reproduce the electric double layer at the electrode-KCl buffer interface. Changing the ionic concentrations of the buffer from 150 mM to 10 mM resulted in an expansion of the thickness of the electric double layer from about 3 nm to about 20 nm, indicating that the electric double layer was introduced into the lipid bilayer model. In the presentation, we will also discuss the results when a lateral voltage is applied to the inside of the lipid bilayer.

References

[1] T. Ma, et al., Faraday Discuss., 233, 244-256 (2022).

The cell membrane is the outermost layer of cells and functions as reaction fields for the transportation of materials, information, and energy into and out of cells. The lipid bilayers are the fundamental structure of the cell membranes. It is a self-assembled structure formed by amphiphilic molecules such as phospholipids. Intracellular and extracellular cations absorb on the lipid bilayers, causing changes in physical properties and structure, such as fluidity and phase separation in the bilayer membrane. Recently, molecular simulations of lipid bilayer systems containing water have been conducted. However, the treatment of interactions between lipids and ions in such systems is less well-established, necessitating relevant experimental data. We aim to reveal the ion coordination to the lipid molecules in lipid bilayers and its effect on the molecular orientation using X-ray absorption spectroscopy (XAS) in water^[1].

The XAS experiments were performed on the soft X-ray in-vacuum undulator beamline at UVSOR-III Synchrotron, BL3U. Vesicle suspensions of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-di-O-hexadecyl-sn-glycero-3-phosphocholine (DietherPC) were prepared in a buffer solution (100 mM NaCl, 25 mM HEPES, pH 7.4/ NaOH). A monolayer of supported lipid bilayer (SLB) was prepared on both sides of Si₃N4 membranes in the cell for XAS measurement by the vesicle fusion method^[2]. O K-edge XAS were measured in the range of 527 - 535 eV. The liquid layer between the Si₃N₄ membranes was compressed to < 100 nm by the pressure outside the cell^[1].

The O K-edge XAS spectra of DOPC-SLB and DietherPC-SLB yielded absorptions in the range of 531 - 533 eV. We attributed it to three components, two derived from the P=O group and one derived from the C=O group in the DOPC molecule by comparing the XAS spectra of DOPC- and DietherPC-SLBs. We measured DOPC-SLB in the buffer solutions with the Na⁺ concentration ([Na⁺]) of 2.1 - 510.4 mM. The X-ray incident angle (T) was set at T = 35 ° to exclude the effect of molecular orientation on the XAS spectra intensity. Characteristic high-energy peak shifts and intensity changes of the P=O^- derived components at the [Na⁺] regions of < 50 mM, 50 - 100 mM, and > 100 mM. We have calculated the inner-shell spectra using the atomic configuration of DOPC bilayers in water including ions that was determined by dissipative particle dynamics simulations with effective parameters based on the fragment molecular orbital method. The inner-shell spectra of oxygen atoms in the phosphate group showed high-energy shift depending on the distance from Na⁺ that was consistent with the experimental results. These results indicate that the ion coordination on lipid molecules is evaluated by the XAS spectra in water.

References

[1] M. Nagasaka et al., J. Electron Spectrosc. Relat. Phenom. 224, 93 (2018).

[2] R. Tero et al., Materials 5, 2658 (2012).

The lipid bilayer is a self-assembled structure of amphiphilic lipid molecules, such as phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol (Chol), in aqueous solution, and is the fundamental structure of biomembranes represented by cell membranes. Lateral organization of lipids and proteins in cell membranes are key factors for the control and efficiency of the transportation substances and signals into and out of cells. To understand the processes of these cell membrane reactions, artificial lipid bilayer systems have been applied. The supported lipid bilayer (SLB) is one of the artificial bilayer systems existing at solid-liquid interfaces. Physical and chemical properties of the solid substrate surface affect the structure and characteristics of SLB [1]. Therefore, chemical modification and microfabrication on the substrate surfaces are applied to patterning of SLB itself and also inner domains in SLB. We have reported formation and physical properties of SLB on graphene oxide (GO) that was deposited on a thermally oxidized Si wafer (SiO2/Si) [2]. GO is a chemical derivative of graphene modified with hydrophilic functional groups such as hydroxy, carboxy, and carbonyl groups. In this talk, localization of specific lipid domains in multicomponent lipid bilayers on GO and its mechanism are described [3].

GO was prepared by the chemical exfoliation method and casted on a SiO2/Si substrate. Binary SLBs of dioleoyl-PC (DOPC) and dipalmitoyl-PC (DPPC), or ternary SLBs of egg-derived PC, egg-derived SM and Chol were prepared on GO/SiO2/Si by the vesicle fusion method and observed with an atomic force microscope (AFM) and a fluorescence microscope.

The transition temperature between the liquid crystal (Lα) and gel phases of DOPC and DPPC are -17 °C and 41 °C, respectively, and thus the phase separation occurs in DOPC+DPPC-SLB at 25 °C. The AFM topography of DOPC+DPPC-SLB on GO/SiO2/Si (Figure 1) shows that two regions with different height existed on GO, while its height was uniform on the bare SiO2 surface. Observation of fluorescence microscopy fluorescence recovery after photobleaching showed that the DOPC+DPPC-SLB on the bare SiO2 region was in the fluid Lα phase, and thus indicates the gel-phase domains were condensed on GO. We attribute the domain localization to the preferential nucleation on GO at the initial step of the domain growth, based on the dependence of the localization efficiency on the cooling rate. This mechanism was supported by the results in PC+SM+Chol-SLB: domains of the less fluid liquid ordered phase were localized on GO, leaving more fluid liquid disordered bilayers on the bare SiO2 surface.

[1] R. Tero, Materials. 5, 2658 (2012).

[2] R. Tero, Vac. Surf. Sci. (in Japanese) 66, 343 (2023).

[3] R. Tero, Y. Hagiwara and S. Saito, Int. J. Mol. Sci. 24, 7999 (2023).

Figure 1. AFM topography of DOPC+DPPC-SLB on GO/SiO2/Si. Scale bar: 500 nm.

Carbon-based nanomaterials, such as graphene and carbon nanotubes (CNTs), are promising for the energy-related applications including lithium-ion batteries (LIBs) and supercapacitors. Those graphitized nanocarbon are usually synthesized at elevated temperatures. However, from a view point of the green (environmentally friendly) process, even the energy-related nanomaterials should be synthesized at lower temperatures, ideally at room-temperature (RT).

Ar⁺ ion irradiation onto the solid surfaces entails the formation of various shapes of surface nanostructures, such as well-known conical nanostructures at RT. Such a formation of conical structures is dramatically enhanced by the simultaneous supply of C during the Ar⁺ ion irradiation [1]. The composition of the ion-irradiated surfaces is readily controllable by a simultaneous supply of the “third” material during the ion irradiation [2, 3]. By the simultaneous C and Ni supply during the Ar⁺ ion irradiation to Au substrate, for instance, the conically textured Au surface was covered with C layers with a dispersion of Ni nanoparticles (NPs), and the C layers showed the spontaneous local graphitization [4]. Very interestingly, the Ni NP including C thus prepared at RT showed the excellent supercapacitor property [4]. Surprisingly, the NPs were still metallic in the C matrix even after the long exposure to air.

Encouraged by this fact, we challenged the synthesis of Li including C nanocomposites by the ion irradiation method. In this case also, the metallic Li was clearly visible by high resolution transmission electron microscope, with a lattice distance of 0.24 nm corresponding to the (110) plane of Li after the exposure to air without any glovebox [5], and the solid electrolyte interphase (SEI) was instantaneously formed at the very initial stage of the charge-discharge cycles [1]. So, the ion-induced C-Li can be attractive as the anode material for the LIB battery for easy and safe handling.

By pre-coating C layers also, Ar⁺ ion irradiation onto Au substrate induced the enhanced formation of conical protrusions. Au conical arrays thus prepared were used as a substrate for the surface enhanced Raman spectroscopy, and the ultrafast and ultrasensitive detection of COVID-19 virus was achieved [6]. Thus, the C-based nanocomposites fabricated by ion-irradiation method at RT are promising for the energy- and bio-related applications.

[1] W. M. Lin, et al., Appl. Surf. Sci. 613 (2023) 156011. [2] S. Sharma, et al., Carbon, 132 (2018) 165. [3] M. Z Yusop, et al., ACS Nano 6 (2012) 9567. [4] T. Akiyama, et al., RSC Adv. 12 (2022) 21318. [5] S. Sharma, et al., Nanomaterials, 10 (2020) 1483. [6] Y. Yang, et al., Nano-Micro Lett. 13 (2020) 109.

This work was partially supported by JSPS Grant-in-Aid for Scientific Research (B) Grant No. 20H02618.

The Fermi surface is one of the key issues that govern the electronic behavior of the solid-state material. Angle-resolved photoelectron spectroscopy (ARPES) can determine the two-dimensional Fermi surface as well as the band structure in a plane parallel to the sample surface. To determine the surface normal dispersion, it is necessary to scan the photon energy. Although it is well established how to determine the dispersion along the k_z line at k_x=k_y=0 including the Γ point, a method to obtain the whole three-dimensional (3D) Fermi surface is not well developed and worthy of investigation. Among the transition metal dichalcogenides, (1T-)TiSe₂ exhibits a unique behavior at its CDW (Charge Density Wave) transition which gives rise to the 2a × 2b × 2c superlattice formation due to the interlayer periodic distortion. It is therefore interesting to study the change in the 3D Fermi surface of TiSe₂ due to the CDW transition. Experiments were carried out at BL6U and BL5U of UVSOR, where the 3D (k_x, k_y and BE) ARPES spectra near the Fermi level were recorded with scanning the photon energy. The total size of the storage required for the complete set of spectra reaches several tens of G bytes, and a dedicated program has been developed. Calculated Fermi surfaces of TiSe₂ (without CDW) are shown in Fig.1 (a). Fig.1(b) shows a 3D surface of TiSe₂ obtained from the photoelectron intensity at the Fermi level as a function of the 3D electron momentum (k_z is determined assuming an internal potential of 13eV) [a movie is available at [https://www.youtube.com/watch?v=M8TvNQn8Pg8]. A tilted oval box centered on an L point, reproducing well the DFT calculation, is clearly recognized. Figure 1(c) shows a 2D section of the volume data from Figure 1(b) along the Γ-A-L-M plane. A tilted oval shape around the vacant L point is seen as is predicted by the DFT calculation. In the oral presentation we will discuss a temperature dependence of the 3D Fermi surface of TiSe₂.

Various kinds of two-dimensional transition metal dichalcogenides (TMDCs) have been investigated for the application of electronical and optical thin film devices. We are interested in the heterostructural thin films combining different TMDCs single-layers and performed some experiments for the purpose of developing our knowledge about its physical properties. In this report, we will discuss the structure and properties of heterostructural WS₂/MoS₂ thin films grown in-plane by chemical vapor deposition (CVD).

The CVD growth was performed at atmospheric pressure using ~40 sccm 3% H₂/Ar as carrier gas. MoO₃ and (NH₄)₂WS₄ as precursors were placed directly under a quartz glass substrate mounted face-down. Sulfur precursor was placed at the edge of the furnace and heated independently with heating tape. During the deposition, the furnace was heated to around 875℃ while the sulfur precursor was kept at around 300℃. The deposition time was approximately 15 minutes.

After CVD growth, we observed of the substrate by optical microscope and found out an "infinite-symbol-type" thin film (Fig. 1a). Noting the difference in color between the center and the outer of this thin film, we performed several surface analyses to investigate this reason. First, we used Raman spectroscopy to evaluate analyses related to elemental composition and crystal structure. Next, we used photoluminescence (PL) spectroscopy to evaluate electronic and luminescence properties. For the spectra obtained in these experiments, if differences in the position and shape of the peaks are observed, we can suggest that the composition differs between the center and the outer of this film.

For Raman measurements, the laser power was 6 mW, the irradiation time was 1 sec, and the number of integrations was 20. On the other hand, for PL measurements, the laser power was set to 0.3 mW, 10 sec, and 5 times, respectively. All measurements were performed in air at room temperature, and the peak position was calibrated with the Si 520.5 cm^-1 peak of the standard sample.

To investigate the composition of the entire thin film, we performed Raman mapping (Fig. 1b) using the peak of E¹_2g mode of MoS₂ and 2LA(M) mode of WS₂. This mapping image shows that the main component in the center is MoS₂, while the outer is WS₂. Furthermore, a slight distortion of the crystal can be generated. First, at the black-filled boundary between MoS₂ and WS₂, the E¹_2g mode of MoS₂ and the 2LA(M) mode of WS₂ are shifted due to distortion caused by lattice mismatch between MoS₂ and WS₂. Second, in the outer WS₂ thin film, intensity brightening and darkening with 3-fold symmetry were also observed. Therefore, it is possible that the structure of the WS₂ thin film is slightly different in light and dark, and we expect to observe differences in electrical and spectral properties in the future.

Referring to the mapping image, we measured Raman spectra at points A and B (Fig. 1c), and the peaks appeared in Raman spectra at points A and B corresponded to MoS₂ (E¹_2g, A_2g) and WS₂ (2LA(M), E¹_2g, A_2g) vibrational modes, respectively. Therefore, this thin film is considered to have a heterostructure consisting of a single thin film of MoS₂ in the center and WS₂ on the outer.

Clear differences were also obtained in PL measurements (Fig. 1d). The spectral shapes are close to those reported previously, respectively.

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With the increasingly serious environmental and resource issues, traditional fossil fuels are gradually unable to meet our needs. Therefore, finding alternative green energy has become an important development direction. Formic acid (FA) is a multifunctional molecule that can be converted into various valuable substances. Two catalytic conversion reactions of formic acid are known: (1) dehydration reaction: HCOOH → CO + H₂O and (2) dehydrogenation reaction: HCOOH → H₂ + CO₂. Dehydration reaction can bring us CO, which can be used as a fuel and also plays an important role in the chemical industry and synthesis. Dehydrogenation reactions can generate H₂, which has received widespread attention and research in recent years as the most representative clean energy source.

Catalysts play an important role in various conversion reactions, and different catalytic reactions depend on different types of catalyst. The two-dimensional nano sheet materials obtained by the stripping of layered materials, owing to their large surface area and special electronic state, are expected to be used in various fields such as catalysts and electronic devices. In our laboratory, the hydrogen boride sheet (HB) was experimentally prepared by using strong acidic cation exchange resin to exfoliate and exchange the magnesium ions of layered material MgB₂ with protons.¹ The structure and special electronic state of HB make it promising as a new type of non-metallic two-dimensional layered catalyst.

In previous studies in our laboratory, it has been proved that hydrogen boride (HB) sheets play a role as a solid acid catalyst^{2, 3} and the sheets are chemically stable against water.⁴ Therefore, HB is expected to have a catalytic effect on the dehydration reaction of FA. In this study, the effect of HB on the catalytic conversion of FA will be investigated.

Firstly, we investigated the thermal decomposition characteristics of FA and found that FA undergoes a weak self-decomposition reaction starting from 260 °C. Then prepared HB sheets were loaded into a fixed bed reactor. As pre-treatment, the sample was heated at 300 °C in argon gas environment for 1 hour and cooled to room temperature, testing within the range of 120 - 300 °C with the introduction of FA.

We found that the conversion of FA began at 120 °C after the addition of HB, and CO was selectively generated with high conversion within the high-temperature range. The W/F (weight of catalyst / ethanol flow rate) was changed by adjusting the FA flow rate. It was found that under different W/F conditions, the product selectivity did not change significantly, But the conversion rate has changed within the low temperature range. The small amount of generation of CH₄ during the 1 μl/min experiment may be due to the large amount of hydrogen released by HB during the initial heating period which may convert CO to CH₄. After a period of time, the HB sheet reached a stable state and no CH₄ was generated (0.5 μl/min and 2 μl/min experiment). This means that FA is likely to be converted to CO with a conversion of over 90% and a selectivity of 100% under the catalysis of HB. And continuous stability tests have proven the good catalytic stability of HB. Our experimental results show that HB has good catalytic performance for the dehydration reaction of formic acid. In the presentation, detail reaction features will be shown.

References:

1) H. Nishino, et al., J. Am. Chem. Soc. 139 (2017) 13761.

2) A. Fujino, et al., ACS Omega. 4 (2019) 14100-14104.

3) A. Fujino, et al., Phys. Chem. Chem. Phys. 23 (2021) 7724-7734.

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A single-walled carbon nanotube (SWCNT) has the shape of rolled-up graphene and right- or left-handed structures depending on the way it is rolled-up. Circular dichroism (CD) spectra have been observed using SWCNT with a structure that is rolled-up in a particular way [1]. On the other hand, the evanescent field of an optical nanofiber (ONF) is known to be elliptically-polarized (EPL) with polarization direction locked to the propagation direction of light within the fiber [2]. In previous work, optical isolators were realized using EPL and cold atoms with in states which exhibited large CD [3]. We propose that a similar non-reciprocal optical effect could allow the creation of optical isolators at room temperature using SWCNTs of a given chirality coupled to the evanescent field of a nanofiber [4]. In this study, we will explore the principle of such a device and proposals for improving its performance.

Figure 1 shows a conceptual diagram of the device. SWCNT was deposited on the ONF as depicted in Fig. 1. By rotating the linear-polarized light (LPL) passing into the ONF, the mode’s evanescent field intensity distribution rotates around the ONF axis. The transmittance was minimized when the EPL coincided with the position where the SWCNTs were attached in the left hand system. On the other hand, the transmittance did not change in the corresponding right hand system. In the presentation, we will present the results of research on the performance evaluation of this non-reciprocal optical system and discuss the interaction between CD of SWCNT and EPL.

Introduction : Two-dimensional (2D) materials show unique properties such as unique electronic states and a large surface area. Therefore, they have potential applications in the development of superior electronic devices and catalysts. Among them, rhombohedral boron sulfide (r-BS) is a nonmetallic layered material composed of boron and sulfur that has been experimentally shown to exhibit excellent performance as an OER catalyst when mixed with graphene [1]. In addition to this, r-BS is theoretically predicted to exhibit excellent thermal conductivity [2] and high hydrogen storage performance via alkali modification [3]. It has also been shown experimentally that r-BS can be easily exfoliated from the bulk into nanosheets and that its band gap depends on the number of layers [4]. These properties make r-BS promising for applications. On the other hand, it is not yet clear experimentally which type of carrier properties r-BS exhibits as a semiconductor. The purpose of this study is to synthesize r-BS and evaluate its carrier properties by measuring Seebeck coefficient and electrochemical measurements using photocurrent.

Method : Powdered amorphous boron and sulfur were mixed at an atomic ratio of 1 : 1 and formed into pellets. The sample pellets were then packed into an h-BN cell, and high-pressure synthesis was conducted using a belt-type high-pressure synthesis apparatus at 5.5 GPa and 1873 K for 40 minutes. After synthesis, the cell was quenched and sample was taken from the cell. The sample was used to evaluate the structure by X-ray diffraction measurements, and its carrier properties were evaluated by Seebeck coefficient measurements and linear sweep voltammetry (LSV) using the sample as an electrode.

Result : X-ray diffraction results show that the peaks shown by the obtained sample indicate that the synthesized sample is r-BS single-phase. The measured Seebeck coefficient was in the range of 500~520 μV/K, suggesting that the r-BS carrier is a hole. In addition, a p-type rectification profile was observed in the LSV in the dark condition (Figure). The current was larger for the positive bias condition because the holes contributed to the current, while the photo-electrochemical current was more pronounced for the negative bias condition. These results indicate that r-BS is a material with p-type semiconductor properties.

Conclusion : In this study, we synthesized r-BS and clarified its physical properties by measurement. Specifically, the Seebeck coefficient and LSV results indicate that r-BS is a p-type semiconductor. Details of other measured properties will be discussed on the presentation day.

References:

[1] L. Li, et al., Chemical Engineering Journal 471 (2023) 144489.

[2] P. Mishra, et al., Sustain. Energy Fuels 4 (2020) 2363-2369.

[3] P. Mishra, et al., J. Appl. Phys. 123 (2018) 135903.

[4] H. Kusaka, et al., J. Mater. Chem. A 9 (2021) 24631-24640.

Introduction

Generation of nanosized structures is technologically important, and the controlled formation of structures in solids on a nanometer scale is critical to modern technology. Such structures may have properties different from those of the bulk materials. Nanohole-based nanomaterials with a size less than the wavelength of an excitation laser beam, for example, are promised for applications such as chemical and biological sensing, membrane biorecognition, unique optical responses under laser excitation, etc.

High-energy particle irradiation is one of method to generate interesting nanomaterials. Aggregation of surface vacancies produced homogeneously by ion sputtering may produce pits that can become rather deep. Electron irradiation, which can induce back sputtering on the surface of thin foil specimens, is also one of the techniques used for nanometer scale etching, lithography and hole formation, and intense convergent electron beams have been utilized, so far. Parallel electron beams of 500–1000 nm diameter have been shown to produce pits on the exit surface of Au(111) foils by sputtering over the electron energy range of 0.4–1.1 MeV.

Self-organization is a method to produce characteristic structures and several self-organization phenomena of defect clusters under high-energy particle irradiations such as voids, bubbles, and stacking fault tetrahedra have been reported so far. Spontaneous well-ordered periodicity can be developed on a broad surface by ion beam sputtering and a numerical model has been proposed as the formation process.

Here, we present our studies on the evolution of nanosized structures resulting from the sputtering of atoms from the exit surface of thin metal specimens during homogeneous electron irradiation, focusing on a novel self-organization phenomenon, which can occur on the electron irradiated surface [1–6] and give data on the temperature dependence of the formation of nanoholes for gold.

Results and Discussion

We found a new type of self-organized nanostructure formation on the exit surface of thin gold foils irradiated with high doses of 360–1250 keV electrons at temperatures of about 100 K. Fig. 1 shows a self-organized nanostructure generated on Au(001) foil surface at 95 K. The structure consists of aligned nanogrooves, which develop parallel to the surface, and nanoholes and hillocks, which grow parallel to the electron beam. The nanogrooves show strong irradiation-direction dependencies on their growth. They grow along [100] and [010] directions for [001] irradiation, along [100] for the [011] irradiation, whereas no clear grooves are formed for [111] irradiation. The widths of nanogrooves and holes are between about 1 and 2 nm, which are the smallest ones generated on metal surfaces so far. The final structures of the thin foils under electron irradiation are nanoparticles or nanowires. This method has been utilized to produce long gold nanowires for investigations of the interesting physics such as the electron transport properties and the multi-shell structure. Temperature dependence of the nanostructure for gold indicates that the effect of surface diffusion becomes significant above 240 K.

Furthermore, the self-organized structures for silver, copper, nickel, and iron are investigated. The formation of nanoholes and nanogrooves basically originates in the sputtering at the electron exit surface. The difference in the anisotropic growth of the nanogrooves and nanoholes among the kinds of metals should be attributed to the irradiation-induced anisotropic flow of point defects and some related factors, which are attributed to the nature of metals.

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A bucky paper (BP), which consists of a large amount of carbon nanotubes (CNTs), is a potential candidate for electrical devices due to its high electrical conductivity and flexibility. BP devices are often used in the atmosphere. Although, the effect of water vapor on the resistivity of BPs has been observed, there is little agreement on its physical origin [1, 2]. The effect of water vapor on the resistivity of CNTs has also been discussed for individual SWCNTs. First-principles calculations have shown that charge transfer occurs between an individual CNT and adsorbed water molecules [3]. In this study, we measure the electrical resistivity of different types of BPs in a wide range of humidity and clarify the physical origin.

We prepared two types of BPs with different crystallinity. One was prepared by super-growth (SG) methods and dispersed in ethanol (SG-E), while the other was prepared by eDIPS methods and dispersed in denatured alcohol (eDIPS-D). SG-E consisted of low-crystallinity CNTs with low G/D ratio. e-DIPS consisted of high-crystallinity CNTs with high G/D ratio. First, we measured the resistivity by the four-terminal measurement method under controlled humidity at RH = 20, 40, 60, 80 % and at constant temperature T ~ 24℃ as shown in Fig. 1 (a,b). Second, we focused on charge transfer as the cause of the decrease in electrical resistivity with increasing humidity. For this reason, we performed Raman measurement to observe the presence of G-band shift as shown in Fig. 1 (c,d). The resistivity of SG-E decreased with increasing the humidity as shown in Fig. 1 (a), while that of eDIPS-D was almost unchanged as shown in Fig. 1 (b). The G-band shift was not observed in SG-E, while a shift of about 2 cm^-1 shift from 1532 cm^-1 was observed in eDIPS-D. This shift suggests charge transfer from water molecules to CNT by humidification. Consequently, the G-band shift was observed only in the samples where the resistivity was almost unchanged with increasing humidity. In the presentation, we will discuss about these unexpected results of the G-band shift. Furthermore, we will investigate the carriers and their mobility of BP by Hall effect measurements and discuss the effect of charge transfer on the humidity dependence of resistivity.

[1] A. Zahab et al., PHYSICAL REVIEW, 62, 15, 10000 (2000).

[2] Y. Kakogi et al., Meeting abstract of IVC (2022).

[3] D. Iwasaki et al., Appl. Phys. Express 10, 045101 (2017).

Introduction:

Zeolitic imidazolate frameworks (ZIFs) have gained prominence due to properties like surface area, tunable pores, and stability. Among ZIFs, ZIF-8, composed of zinc ions and 2-methylimidazolate (Hmim) ligands, stands out. Synthesis choices impact ZIF-8 properties. The use of methanol, while effective, presents several concerns. It is flammable, toxic, and sourced from non-renewable resources, hence raising issues of safety, environmental impact, and sustainability. These challenges necessitate the exploration of safer and more environmentally friendly alternatives to methanol. Water seems viable, but its use is not straightforward, demanding a thorough investigation. This study addresses water-based ZIF-8 synthesis challenges and compares its properties with methanol. By analyzing physicochemical attributes and precursor ratios, we seek optimal synthesis routes, enhancing ZIF-8 performance.

Results:

Yield increases with molar ratio growth for aqueous and methanolic solutions. Water-based ZIF-8 samples exhibit higher yields, partly due to higher molecular weight compounds. XRD patterns confirm ZIF-8 and a new structure for aqueous samples, identified as ZIF-L. Methanol's acidity favors ZIF-8 formation, giving methanolic samples higher ZIF-8 yields. Crystallite sizes decrease with higher ratios. Methanol's suitability for ZIF-8 synthesis is confirmed.

Conclusions:

This study aims beyond solvent comparison, exploring fundamental traits for optimized water-based synthesis. ZIF-L and methanolic ZIF-8 production under specific conditions are demonstrated. Proposed mechanisms align with the results. This work advances eco-friendly MOF synthesis, contributing to green chemistry discourse.

References:

1. Tezerjani et al. Different solvent effects on zeolitic imidazolate framework-8 synthesis, RSC Adv. 11, 19914 (2021).

2. Jian et al. Water-based synthesis of ZIF-8 with high morphology, RSC Adv. 5, 48433 (2015).

3. Ahmad et al. Enhanced performance of amine-functionalized ZIF-8-decorated GO for ultrafiltration, Separation and Purification Technology 239, 116554 (2020).

Metrology is the basis of science and technology. In order for vacuum gauges to measure the correct value, they must be traceably calibrated to vacuum standards based on physical phenomena. Vacuum standards are managed and developed in National metrology institutes in each country. In Japan, National Metrology Institute of Japan (NMIJ) is one of the organizations of National Institute of Advanced Industrial Science and Technology (AIST). NMIJ has several pressure standards, such as mercury manometer, pressure balances, static expansion system, and orifice-flow method. As a future primary standard, optical pressure standard is under development. I am working the following three studies.

First one [1] is improving the static expansion system for calibrating customers gauges such as capacitance diaphragm gauges (CDGs) and spinning rotor gauges (SRGs). The static expansion system consists of the small-volume chamber, middle-volume chamber, and large-volume chamber. It generates vacuum pressures, such as 1 mPa and 1 Pa. Though principle is simply written as Boyle’s law, it is according to the equation of state of gas. So, the chambers are surrounded by the aluminum box and form box. In the presentation, the procedure of calibrations and the calibration results for CDGs and SRGs are shown. In addition, the comparison results with the vacuum standards in other national metrology institutes (NMIs) are shown. The vacuum standards are equivalent to those of other NMIs in the world within their uncertainties.

Second one [2] is the dimensional measurements of piston-cylinder for pressure balances. Pressure balances generate pressure according to the definition of pressure. This pressure standards cover the pressure range 10 kPa to 1 GPa. When ones need high pressure, the piston-cylinder with small diameter and heavy weight. Here, a point is the accuracy of the effective areas of the piston-cylinder. It is determined by the mercury manometer. On the other hand, it is known the effective area can be calculated from the radius in each position of piston-cylinder. For the dimensional measurements, I prepared a modified roundness measuring machine and the micro-CMM. In the presentation, the radius data maps of piston-cylinder are shown as the preliminary results.

Third one [3] is developing the optical pressure standard. Pressure is evaluated from gas refractive index, thermodynamic temperature, and gas polarizability, according to the equation of state of gas and the Lorentz–Lorenz equation. Recently, the Helium polarizability was calculated with quantum technique. So, some researchers call this method “quantum pascal”. It can be a primary standard without mercury. And, its uncertainty can be smaller than those of the static expansion system and pressure balances in the pressure range from 1 Pa to 100 kPa. One of the core techniques is gas refractive index measurements. As the measurement method, I use Fabry–Perot cavities. The gas refractive index is evaluated from the ratio of the cavity optical length under vacuum and the length under gas-filled pressure. In the presentation, the preliminary results of comparison between the developed optical pressure standard and the static expansion system are shown.

[1] Yoshinori Takei, Hajime Yoshida, Eiichi Komatsu, and Kenta Arai, "Uncertainty evaluation of the static expansion system and its long-term stability at NMIJ", Vacuum 187, 110034, (2021).

[2] Yoshinori Takei, Yohan Kondo, Hiroaki Kajikawa, and Tsukasa Watanabe, "Dimensional measurement of piston/cylinder for improving pressure standards -roundness and straightness measurements and an attempt of diameter measurement-", Proceedings of JSPE, D84, (2022).

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Introduction

The age of autonomous driving would be just approaching. The role of automotive sensor devices would be very important. Sensor devices have chips that are vacuum sealed. If gases get into, or outgassing remains on the chip, it will be damaged, and the quality will deteriorate. And then, the safety of the vehicle is compromised. Leak testing of sensor devices is necessary to ensure vehicle safety. We have developed a high-precision mass gas analyzer which can measure E-15Pa-m3/sec (He) to ensure vehicle safety.

Achieving Ultra-micro leakage measurement using 0.2% BeCu technology

In the ultra-high vacuum (UHV) and Extremely high vacuum (XHV) region of E-7 Pa or lower, when conducting residual gas analyses with a standard quadrupole residual gas analyzer, outgassing from the sensor tip ion source cannot be ignored, and it is difficult to perform high-precision gas analyses under such conditions. The most important element for solving this problem is a flange-integrated ion source in which radiant heat from the thermionic cathode filament cannot reach the other electrode. A thermionic cathode filament/grid is surrounded by a low-thermal-radiation, high-thermal-conductivity 0.2% beryllium-copper alloy (BeCu) as shown to the right in Fig 1. This is constructed such that heat generated from the filament does not flow to the quadrupole analyzer section and the secondary electron multiplier (EM). As a result, WATMASS outgassing decreases to approximately 1/10000 that in a conventional residual gas analyzer (stainless steel material), and high precision gas analyses in the XHV region can be performed. Utilizing this performance, we have developed a WATMASS GA System that can analyze sealing devices such as MEMS and other devices with non-destructive and destructive analysis with E-16Pa m3/s (He). Details would be announced on the day.

References:

1) Fumio Watanabe, Investigation, and reduction of Spurious peaks caused by electron – stimulated desorption and outgassing by means of a grid heating method in a hot-cathode quadrupole residual gas analyzer. J. Vac. Sci. Technol. A20, 1222 (2002)

2) Fumio Watanabe: J. Vac. Sci. Technol. A22 (2004) 181 & 739. 3) Fumio Watanabe: J. Vac. Soc. Jpn, Vlo. 56, No. 6, 2013

4) Hajime Yoshida, Kenta Arai: AIST Ultra - Fine leak calibration unit

The Japan Surface Vacuum Society industrial award was presented to Shimadzu in 2022. This talk briefly introduces the background to the establishment of the standard and its main contents.

A turbomolecular pump is a vacuum pump capable of producing ultrahigh vacuum by rotating the rotor at high speed. Rotational energy of the rotor is very large, and there is a risk of pump falling and debris scattering by the failure of the rotor.

In response to these risks, from 2003 to 2005, the Japan Vacuum Industry Association held working sessions with committee members from turbomolecular pump manufacturers, sellers, AIST, etc., and published a document on the results of activities in March 2005. This guideline introduces a wide range of methods for measuring rapid shutdown torque at the time of rotor breakdown performed by manufacturers in Japan at that time, and summarizes what should be included in the test report.

Based on this, the Japan proposed a project to create an ISO standard, and a three-year activity began in 2006. I was appointed as the project leader by the ISO Technical Committee TC112 (vacuum technology). In September 2007, we rented a conference room at AIST, where engineers from Japan, Germany, and France manufacturers, as well as Japan corporations of overseas manufacturers, gathered to discuss Japan proposals. After four rounds of international vote revisions, with the addition of the test method submitted later by Germany, the international standard was published in 2010 as ISO 27892.

ISO 27892 describes two destructive test methods for forcing rotors to break. The first method is to add notches in the rotor or shaft in advance to cause centrifugal destruction. The second method is to hit the rotating rotor blades with a foreign object and destroy the turbine blades. There are two main types of pump fixing methods when measuring rapid shutdown torque: one method is to measure the torsional torque acting between the pump and the test frame by mounting it on the test frame (See figure below.), and the other is to measure the torque acting between the floor and the pump by fixing the pump to the floor. The more suitable test method should be selected by the manufacturer based on the intended use of the product. ISO 27892 describes a method of measuring rapid shutdown torque using a strain gauge or force sensor and other means.

ISO 27892 lists the items to be included in the test report. For example, destructive torque measured by the method or notes on the design for safe mounting to parts (bolts, clamp, and others) on the pump where rapid shutdown torque is transmitted directly.

Safety is not directly guaranteed in this international standard. If each manufacturer tests according to the standard, products that fall off or scatter debris when the rotor is destroyed will be disappear from markets. It is hoped that this will exert a checking effect.

1) Tomoaki Urano, Turbomachinery 23, 635 (1995)

2) Working on Safety Assurance of Turbomolecular Pumps, "Guidelines for Safety Assurance of Turbomolecular Pumps" (Japan Vacuum Industry Association, 2005)

3) ISO 27892: 2010 Vacuum technology — Turbomolecular pumps — Measurement of rapid shutdown torque (2010)

A vacuum firing, which is the high-temperature heating of vacuum chambers or vacuum materials in a high vacuum furnace, has been known as the process of reducing the amount of dissolved hydrogen in the bulk to obtain low outgassing. For example, the CERN applied vacuum firing to stainless steel at 950°C for a few hours [1]. The Japan Proton Accelerator Research Complex (J-PARC) applied vacuum firing at 650-750°C for 10 hours to titanium [2]. These conditions are comprehensively determined by taking into account the reduction of hydrogen concentration in the bulk, diffusion distance relative to a material thickness, mechanical strength, sensitization in the case of stainless steel, etc. The reduction of the hydrogen concentration in some materials by vacuum firing has been reported in previous research [3, 4]. A few research compares the outgassing rate of vacuum-fired stainless steel with non-vacuum-fired one in the same experimental condition [5, 6]. From those research, vacuum-firing is an effective treatment to obtain ultra-high vacuum or extreme-high vacuum in a vacuum system. Detailed research about the outgassing characteristics of the vacuum-fired vacuum chamber is worthwhile. Furthermore, the mechanism of the outgassing reduction by the vacuum firing should be performed from a surface microscopic point of view.

Our previous research focused on the titanium chamber [7]. The build-up result after baking showed that the vacuum-fired titanium chamber has lower outgassing for typical gas species, such as H₂, H₂O, CO, and CO₂ than the non-vacuum-fired one. In addition, X-ray photoelectron spectroscopy (XPS) revealed that the reformed surface titanium oxide layer after vacuum firing is thinner than that before vacuum firing. Thus, we concluded that the low outgassing of the vacuum-fired titanium after baking is the result of the titanium getter function. That is, the vacuum-fired titanium can be activated by baking more easily than the non-vacuum-fired one because the oxygen in a thinner oxide film is easy to diffuse into the bulk and the pure titanium, which has the getter function, appears on the surface.

In this research, the effect of the vacuum firing (850℃ for 10 h) on the stainless steel SUS304 is investigated from the vacuum and surface point of view. The build-up test of the stainless steel vacuum chamber clearly showed the outgassing suppression by the vacuum firing. Especially, the hydrogen outgassing, which was the main component after baking, was much reduced. Thermal desorption spectroscopy showed that the vacuum firing reduced the desorption of H₂, H₂O, CO, and CO₂ with high desorption energy even after air exposure. Especially the effect on the H₂ was very large. X-ray photoelectron spectroscopy (XPS) showed the increase of ferric oxide and the decrease of chrome oxide on the near surface of the vacuum-fired stainless steel. On the other hand, the XPS also showed that the chrome oxide was systematically increased by heating from 200℃ to 400℃.

In summary, the outgassing reduction mechanism by vacuum firing is considered as follows.

1. Reduction of the outgassing rate by vacuum firing is the result of the hydrogen reduction in the bulk due to the diffusion to the vacuum phase during the vacuum firing and re-formation of the surface Fe-oxide and Cr-oxide as a diffusion barrier from the gas phase to the bulk. The thermal desorption of the other molecules by vacuum firing has also contributed to the outgassing reduction.

2. Reduction of the outgassing rate by higher temperature baking is the result of the decrease of adsorbed/absorbed gas molecule density due to the thermal desorption and the increase of the surface Cr-oxide as a diffusion barrier against hydrogen from the bulk.

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The vacuum system of the SuperKEKB accelerator has been operating smoothly since 2016. However, occasional abnormal pressure increases caused by air leakage from flanges and discharges inside of beam pipes have been observed several times, resulting in significant vacuum issues. The anomalous pressure behavior was predominantly noted during high-current beam storage, subjecting vacuum components to substantial thermal load, or when a high-current beam has been abruptly aborted, inducing significant thermal stress. Detecting early signs of these abnormal pressure behaviors would enable proactive countermeasures. Given this context, a pressure anomaly detection program utilizing machine learning has been proposed and is currently under development.

The program primarily comprises three sequential steps. Firstly, regression curves for the pressure behavior of each vacuum gauge are derived using normal data (reference data), collected several days before the data to be evaluataed (check data), based on an appropriate model. Second, a two-layer feedforward neural network (FNN) is constructed to classify the check data into two categories: “normal” and “abnormal”. Input parameters include the root mean square error (RMSE) calculated from the derived regression curve and others relevant factors. The FNN weight parameters, determing the classification decision boundary, are learned from historical data during instances of actual vacuum issues. Finally, the program is integrated into the accelerator control system, displaying the names of abnormal vacuum gauges on a graphical user interface (GUI) panel to trigger alarms.

A pivotal element of program development lies in crafting suitable models for pressure behavior within the reference data to generate accurate regression curves. Rational and realistic models based on physical phenomena yield precise anomaly detection. To start, the operational state is divided into two segments: the "Storage part", where the beam is injected (or re-injected) into the ring and stored, and the "Tail part", immediately following beam abort. For the Storage part, pressure (P) behavior is described as a function of beam current (I). P can be expressed as P = P_b + ΔP_s (I) + ΔP_t (I), where P_b represents the base pressure, ΔP_s indicates pressure increase due to synchrotron radiation, and ΔP_t signifies pressure rise from component heating. ΔP_s results from photon-stimulated desorption (PSD) and is proportional to I. ΔP_t originates from thermal gas desorption and is experimentally found to be roughly proportional to the square of the component temperature rise (ΔT) in our case [1]. As ΔT is proportional to the electromagnetic field’s power induced by the beam, it is proportional to I squared divided by the number of bunches (N_b). On the other hand, for the Tail part, pressure is a function of time (t) after beam abort. Pressure (P) can be represented as P = P_b + ΔP_v (t) + ΔP_w (t), where ΔP_v and ΔP_w denote pressure decrease due to the evacuation of gas molecules existed in space and adsorbed on beam pipe surface, respectively. ΔP_v is proportional to −exp(−at), where a is a constant tied to space volume and pumping speed.

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1. Introduction

A Ti-Zr-V type non-evaporable getter (NEG) coating, which was developed at CERN, is a breakthrough vacuum technology because it makes the vacuum chamber wall a getter surface by a low activation temperature around 180-300 ℃ [1, 2]. The oxide surface layer of the NEG coating reduces to metallic Ti, V, and Zr by the activation and thus becomes a getter. In more detail, the displacement reaction of Zr on oxidized Ti or oxidized V is the reason for the low activation temperature of the Ti-Zr-V coating [3]. On the other hand, the deterioration of the getter coating, which means the decrease of the sticking factor, is generally said to be caused by the increase of the oxygen concentration in the film. The Zr oxide of thin NEG films such as 30 nm or 87 nm was difficult to reduce to the metallic Zr compared to that in thick films such as 203 nm or 1100 nm after the same activation condition [4]. Those activation and deterioration mechanism analyses were performed by photo electron spectroscopy with an X-ray tube or synchrotron radiation as a photon source.

2. Measurements

We performed a sequence of measurements with XPS to understand more detail about the activation and deterioration mechanism. The sample of a titanium plate with Ti-Zr-V coating of 1 μm thickness was prepared. The NEG was coated on the titanium plate sample by the DC magnetron sputtering with NEG alloy [5]. The sample was set in the surface science station in the BL23SU of SPring-8. At first, the XPS measurements for the sample surface were performed during the sample temperature was raised to 250℃. After that, the XPS was subsequently performed during the injection of oxygen gas into the chamber while keeping the sample temperature at 250℃, which corresponds to the accelerated deterioration test. This would be the first time that the surface oxidation process was measured in situ by XPS. After that, the depth profile of the sample was measured with another XPS apparatus with an X-ray tube by argon etching.

3. Results and discussions

During the temperature rise measurement, the Zr oxide was shifted to the more oxide spectra (larger binding energy) at the same time that the Ti and V spectra shifted from the oxide to the metal. After that, when the Ti and V become more metallic with the higher temperatures, Zr also moves to the metallic side. This suggests that the surface Zr gets the oxygen from Ti oxide and V oxide at the first stage of the activation and the oxygen of the Zr oxide would diffuse to the bulk in the continuous temperature rise. During the oxygen gas injection at 250℃, the oxidation proceeded in the order of Zr, Ti, and V. These results agree with the order of the stabilities of metal oxides from the Gibbs free energy [3]. The depth profile of the sample saturated by oxygen shows that oxygen mainly forms in Zr oxide in the NEG coating. The second-biggest oxide component in the coating was Ti oxide. The V shows the metallic spectra in the coating interior even after the NEG coating was saturated by oxygen.

4. Conclusion

XPS measurement of the activation and oxidation process was performed in situ at BL23SU of SPring-8. During the oxygen gas injection at 250℃, the surface oxidation proceeded in the order of Zr, Ti, and V. The depth profile of the NEG coating sample, which was saturated with oxygen, revealed that the concentrated oxygen in the coating exists in the form of mainly Zr oxide and Ti oxide in the second place.

References

[1] C. Benvenuti, et al., VACUUM 50, 57 (2001).

[2] O. B. Malyshev, et al., Journal of Vacuum Science & Technology A 27, 321 (2009).

[3] E. Belli, et al., Physical Review Accelerators and Beams 21, 111002 (2018).

[4] C-C. Li, et al., Thin Solid Films 515, 1121 (2006).

[5] J. Kamiya, et al., e-Journal of Surface Science and Nanotechnology 20, 107 (2022).

Refractory metal nitride, RMN, coatings possessed excellent characteristics and were employed in a wide range of applications, including aerospace, electronics, thermal management, protective layers, ...etc. The fabrication methods for the RMN films were mostly magnetron sputtering techniques with controls on source gases, power modulation, facility geometry, thermal process,... and so forth. This study focused on the TaN and MoN based RMN coatings produced by magnetron sputtering with various input power modulation, including radio frequency, and high intensity plasma impulsed magnetron sputtering, i.e. RFMS and HiPIMS, respectively. The microstructure evolution from amorphous, nanocrystalline, equiaxial-grain, columnar features, in terms of the form of input power and the level of power density. In general, an amorphous/nanocrystalline microstructure feature could be deduced under a low RF power, while a higher level of RF power enhanced the crystallization of RMN layers. In contrast, the use of the HiPIMS power resulted a columnar multiphase RMN film due to higher pulse energy for the coexistance of nitride phases. Recent findings on microstructure evolution and characteristics of the RMN coatings will be intensively discussed.

INTRODUCTION

To suppress the increased atmospheric CO₂ levels, emission reduction as well as catalytic convert CO₂ into valuable fuels and chemicals are urgently required. Photocatalytic CO₂ reduction, using ZrO₂-based photocatalysts, is a promising solution for sustainable, carbon-neutral cycles.^[1] A photocatalyst combining ZrO₂ with Ni exhibited one the highest photocatalytic activity from CO₂ to CH₄ in literature.^[2] The reaction pathways from CO₂ to CO and CH₄ were reported using ZrO₂ photocatalyst.^[3] The research employed density functional theory (DFT) to explore CO₂ reduction mechanisms on ZrO₂ catalysts comprising oxygen vacancies (V_O) on several monoclinic ZrO₂ surfaces. It is known that monoclinic (0 0 1) ZrO₂ is photoactive, whereas tetragonal (1 0 1) ZrO₂ is thermally active. This study compares V_O' effects on monoclinic ZrO₂ (0 0 1) and tetragonal ZrO₂ (1 0 1) surfaces during the CO₂ reduction. In this presentation, we focus on monoclinic ZrO₂ (0 0 1) and tetragonal ZrO₂ (1 0 1) surfaces in view of specific μ₂ oxygen vacancies serving as CO₂ adsorption sites as well as enabling CO₂ to CO conversion. Furthermore, following steps towards CH₄ formation over supported Co nanoparticles (Co–ZrO₂) are discussed.

METHODS

Spin-polarized periodic DFT calculations were performed using VASP. DFT-D3 accounted for van der Waals interactions via the projector-augmented wave method. It comprehends the reaction energy at surface, structural optimization, and adsorption energy calculations on each adsorption site. Nudged Elastic Band (NEB) calculations were conducted to analyze the interactions among different adsorption sites in the reaction coordinate. Monoclinic-phase ZrO₂ (0 0 1), tetragonal ZrO₂ (1 0 1) surfaces with (2 × 2 × 2) unit cells were used, modeling surface V_O^•• by removing a top-layer O atom.

RESULTS AND DISCUSSION

The obtained reaction pathways are depicted in Scheme 1. CO₂ decomposition starts with the most stable M-shaped (μ-Zr)₂-CO₂. Subsequently, the effect of proton addition under UV–visible light irradiation was evaluated. The adsorbed CO₂ was converted into hydroxy carbonyl species (OCOH) through a reaction with photogenerated holes on ZrO₂ and splitting of H₂ (Scheme 1(d)). Following subsequent steps to species (k) and (l), CO was released. This process revealed that the ZrO₂ (0 0 1) surface can reduce CO₂. Next, the regeneration of V_O^•• site releasing water was considered. From species (m), protonation of the hydroxy species led to water release, regenerating original V_O^•• site (Scheme 1(b)). Please note that relatively large energy for water release was partially compensated by the adsorption energy of CO₂ on V_O^•• site similar to the photocatalytic cycle using monoclinic ZrO₂ (1 1 1).^[3]

Corresponding to experiments reported CO₂ to CH₄ and CO to ethene and propene on Co-ZrO₂, the reaction mechanism is also discussed at the presentation.

REFERENCES

[1] H. Zhang, Y. Izumi et al., J. Am. Chem. Soc., 141, 6292 (2019).

[2] H. Zhang, Y. Izumi et al., Angew. Chem. Int. Ed., 60, 9045 (2021).

[3] K. Hara, Y. Izumi et al., J. Ph

Introduction

The utilization of biomass-derived polyols includes the conversion to alkene molecules through catalytic removal of oxygen atoms known as deoxy-dehydration (DODH) reaction. For this reaction, ReO_x-Au/CeO₂ catalyst, consisting of rhenium (Re) oxide and gold (Au) nano-particles (NPs) supported on cerium dioxide (CeO₂), was found highly active [1]. It is proposed that the ReO_x is partially reduced via hydrogen spillover from the Au NPs under the presence of molecular hydrogen and that the partially reduced ReO_x acts as an active site for the DODH reaction [2]. It has not been unveiled, however, how the Au NPs promotes the hydrogen-induced reduction and how the Au-NP-activated hydrogen reduces the ReO_x and the CeO₂ surfaces. In this study, the chemical states of Re and Ce of the ReO_x-Au/CeO₂ model catalyst under exposure to molecular hydrogen were studied using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS).

Experiment

The ReO_x-Au/CeO₂ model catalysts were prepared by vacuum deposition of Re and Au on CeO₂ thin films with a thickness of 10 nm formed on Si substrates. After co-deposition of Re and Au, the CeO₂ thin films were thermally treated at 300 ºC in air. Characterization of the sample surfaces with XPS and AFM revealed that the ReO_x is deposited with coverages of sub-monolayer and the Au atoms form NPs with a diameter of approximately 10-12 nm and an average spacing of 45 nm. The hydrogen-induced reduction process was traced with NAP-XPS at BL-13B of the Photon Factory under 0.1 Torr H₂ and at three different temperatures (100 ºC, 140 ºC and 250 ºC). According to a recent report [3], hydrogen spillover takes place at 100 ºC and 140 ºC on CeO₂, while it does not occur at 250 ºC and instead of that water desorption takes place.

Results and Discussion

In the beginning, the ReO_x is pre-treated by heating in gaseous oxygen to be oxidized to a Re oxidation state of 7+. Then the oxidized surfaces were exposed to molecular hydrogen with a pressure of 0.1 Torr. Fig.1(a) shows time evolution of Re 4f_7/2 XPS taken at 140 ºC. The oxidation state of Re was reduced from 7+ to 6+ and 4+, while components of the lower oxidation states such as 2+ and 0 were not observed. Comparison of evolution of the reduced component of Re between with and without the Au NPs revealed that the presence of the Au NPs obviously enhances the reduction rate of Re, which supports hydrogen activation and following hydrogen spillover by the presence of the Au NPs. It was also confirmed that the higher temperature induces the higher reduction rate as shown in Fig. 1(b). The higher reduction rate observed at 250 ºC suggest that direct reduction of ReO_x by molecular hydrogen might contribute dominantly at such a high temperature. While at low temperatures below 150 ºC, where the hydrogen spillover takes place [3], the hydrogen-spillover-induced reduction should be taken into accounts, which will be discussed in the presentation.

References

[1] Ota, N. et al. ACS Catal. 2016, 6, 3213-3326. [2] Nakagawa, Y. et al. ACS Catal. 2018, 8, 584-595. [3] Beck, A. et al. ACS Nano 2023, 17, 1091-1099.

The conformation and alignment of molecules in organic materials play a critical role, influencing the macroscopic properties of these materials. As two-dimensional (2D) materials offer a simplified model of their three-dimensional (3D) counterparts, researchers have extensively investigated the atomic-scale arrangement and alignment of molecules in 2D assemblies using scanning tunneling microscopy (STM).^[1] This research is focused on the systematic exploration of the structural configuration and alignment of a donor-acceptor-type molecule, specifically 4-(3,3-dimethyl-2,3-dihydro-1H-indol-1-yl)benzonitrile (IBN), within both 2D and 3D assemblies.^[2] The study focused on examination of 2D assembly of IBN on the Au(111) via STM and 3D assembly of IBN in a single crystal using X-ray crystallography (Fig. 1).

Our investigation has elucidated that IBN maintains a planar conformation in both 2D and 3D assemblies, and the dipole moment of IBN in the 2D and 3D assemblies are essentially indistinguishable. Interestingly, in both the 2D and 3D assemblies, IBN molecules align themselves in a manner that effectively cancel the net dipole moment, despite the distinct variations in their self-assembled motifs. In the 2D assemblies, the orientation and self-assembled structure of IBN are subject to influence from the surface density of IBN. Furthermore, these factors are intricately interconnected with the crystal orientation and superstructure of the Au(111) substrate, owing to the strong interaction between IBN and the Au(111) surface. Moreover, insights from scanning tunneling spectroscopy have disclosed that the coordination structure is not included in the self-assembled arrangement of IBN on Au(111) .

References

[1] W. H. Soe, Y. Shirai, C. Durand, Y. Yonamine, K. Minami, X. Bouju, M. Kolmer, K. Ariga, C. Joachim, W. Nakanishi, ACS Nano, 11, 10357 (2017).

[2] W. Nakanishi, Y. Matsushita, M. Takeuchi, K. Sagisaka, Phys. Chem. Chem. Phys., 25, 13702 (2023).

Introduction

Reducing the use of expensive and rare precious metals is essential for the cost reduction and widespread use of various catalyst systems. One of the simple ways to reduce the amount of catalyst metal is by highly dispersing ultrafine catalysts to achieve high specific surfaces. It is well known that catalytic activities of highly dispersing ultrafine catalysts strongly depend on their sizes.[1] Therefore, controlling and maintaining the size of highly dispersing ultrafine catalysts are essential for practical applications. It has been reported that the dynamic behaviors of agglomeration and redispersion of sub-nanometric Pt catalysts depend on atmospheres.[2] Reductive atmospheres enhance catalyst agglomeration, while catalyst redispersion occurs in oxidative atmospheres. In this study, we investigated molecular adsorbate dependence of the stability of sub-nanometric Pt catalysts. We calculated the Pt-Pt binding energy in Pt dimers with various molecular adsorbates using first-principles calculations based on density functional theory.

Methods

We performed the first-principles calculations, implemented in the plane-wave and projector-augmented wave method code, the Vienna Ab-initio Simulation Package (VASP). We considered the van der Waals interaction with the non-local correlation functional rev-vdW-DF2. We adopted graphene as catalyst support materials. We considered H₂, O₂, H₂O, OH, CO, CO₂, N₂, and NO as molecular adsorbates.

Results

In the case of Pt dimers on pristine graphene, the calculated Pt-Pt binding energy is 0.60 eV/atom, which is significantly smaller than the atomization energy of bulk Pt of 5.51 eV/atom.[3] We revealed that H₂, OH, and NO adsorption on Pt dimers does not affect the Pt-Pt binding energy. The Pt-Pt binding energy changes with these molecular adsorbates are less than 0.05 eV. On the other hand, O₂, H₂O, CO, and N₂ adsorption decreases the Pt-Pt binding energy. The corresponding Pt-Pt binding energy with O₂, H₂O, CO, and N₂ are 0.41, 0.36, 0.41, and 0.37 eV/atom, respectively. In the case of O₂ and H₂O adsorption, the most stable dissociation structures are those in which the adsorbed molecule dissociates as the Pt dimer dissociates. For CO₂ adsorption, the Pt-Pt binding energy increases to 1.15 eV/atom. This is because spontaneous CO₂ bending can be observed only on the Pt dimers, resulting in stable adsorption. It has been reported that such CO₂-bending adsorption structures are also stable on other transition metal dimers.[4]

References

[1] Y. Watanabe, X. Wu, H. Hirata, and N. Isomura, Catal. Sci. Technol. 1, 1490 (2011). [2] L. Liu, D. N. Zakharov, R. Arenal, P. Concepcion, E. A. Stach, and A. Corma, Nat. Commun. 9, 574 (2018). [3] L. Schimka, R. Gaudoin, J. Klimeš, M. Marsman, and G. Kresse, Phys. Rev B 87, 214102 (2013). [4] Y. Pan, C. Liu, T. S. Wiltowski, Q. Ge, Catal. Today, 147, 68 (2009).

Introduction The relationship between chemical reactions and ion formation at the junction between the electrode and the molecular film is considered an important issue for improving the performance of rechargeable batteries and realizing many types of micro-molecular devices. In this study, we focus on hydrogen atom (H) addition reactions in imidazole-terminated alkanthiolate self-assembled monolayers (Im-SAMs) on Au substrates. While hydrogenated imidazoles stabilize as imidazolium cations in acidic solutions, it is not clear whether charge transfer from the Au substrate, across the alkyl insulating layer, to the imidazole groups is possible in non-aqueous systems. In our previous experiments, the H-addition reaction proceeded when the Im-SAM was irradiated with hydrogen atoms [1]. To characterize the products, we calculated the total energies and molecular orbitals of the chemical species likely to be produced in this reaction. The results were useful to examine the possibility of charge transfer.

Results and Discussions For the molecular orbital calculations, B3LYP/6-31++G(d,p) level of calculation was used as the density functional and basis set. To simplify the calculations, we picked up molecular model based on 1-methylimidazole. Two corrections were made when comparing the total energies of each molecular model. First, the energies of the unadded hydrogen atoms alone were added together. When electrons were transferred to the substrate, a negative value was added to the total energy as stabilizing energy for the work function φ of the substrate . In terms of total energy, the first radical species produced (Fig. 1b) is lower than the imidazole form (Fig. 1a), suggesting that the H addition reaction proceeds sufficiently. In contrast, the cationic species (Fig. 1c) were estimated to have a higher energy of the individual molecule. However, when the stabilization energy for the work function φ due to the transfer of excess electrons to the substrate was taken into account, the energy was found to be lower than that of the radical species. In other words, the formation of cationic species with electron transfer was confirmed to be possible. Moreover, the reaction involving the addition of another H atom (Figs. 1d and 1e) was suggested, while the reaction had an activation energy. For this reaction, it is expected to involve the transfer of electrons from the substrate and further progress to a stable neutral state.

REFERENCES

[1] R. Muneyasu et al. (in preparation for publication).

Metal borides have been investigated over the past decades since the material group shows varieties of physical phenomena, such as superconductivity and quantum criticality. Recently, two-dimensional boron networks in the crystals have paid attention among researchers to examine the topology physics and to utilize them as mother materials for synthesis of the functional atomic sheets. An example is YCrB₄, which is one of the XYB₄-type ternary borides with X= rare-earth metal and Y= transition metal. The boron layer consists of pentagons (5-membered rings) and heptagons (7-membered rings). The tiling arrangement belongs to the non-symmorphic symmetry group and our theoretical work has successfully made a topological classification of the boron sheet depending on chemical environments [1,2]. Recently, we synthesized a wide-area atomic sheet of HB by the ion-exchange reaction and the novel material has captured interests for real-world applications [3-5]. On the other hand, the as melt samples through arc-melting (without subsequent post-annealing), are essentially polycrystalline, inherently exhibit the YCrB4 phase as a major phase and minor ones with amorphous phases arising from oxides and with other metal borides. To make the better understanding of bulk and surface properties of YCrB₄, a large size growth of the single crystal has been called for.

In this study, we have grown YCrB₄ crystals using the arc-melting and post-annealing method at various conditions and we have succeeded in synthesizing 0.2~0.3 mm-sized single-crystals, as confirmed by SEM, EDX, and XRD. The two-dimensional (2D) flakes, prepared by mechanical peeling of the crystal, have allowed us to prepare the 2D platform and to make the detailed surface analysis. After the surface cleaning by Ar-sputtering and post-annealing, we have discovered formation of the boron-terminated surface and found the metallicity by micro-focused photoelectron spectroscopy at synchrotron radiation beamline BL-28A at KEK-PF. In the presentation, bulk/surface electronic states of the YCrB₄ crystal are described in detailed with the experimental results and theoretical calculations. The surface electronic structure is discussed by comparisons with borophene layers, discovered on crystal surfaces [6-8].

Reference:

[1] I. Tateishi, X. Zhang et al., Molecules 27, 1808 (2022).

[2] Y. Ando, X. Zhang et al., Phys. Rev. B 106, 195106 (2022).

[3] X. Zhang et al., Molecules 28, 2985 (2023).

[4] X. Zhang et al., J. Phys. Chem. C, 126, 12802 (2022).

[5] M. Niibe, X. Zhang et al., Phys. Rev. Mater. 5, 084007 (2021).

[6] Y. Tsujikawa, X. Zhang et al., Surf. Sci. 732, 122282 (2023).

[7] Y. Tsujikawa, M. Horio, X. Zhang et al., Phys. Rev. B 106, 205406 (2022). [8] I. Matsuda, K. Wu ed. 2D Boron: Boraphene, Borophene, Boronene (Springer, 2021).

Hard X-ray photoelectron spectroscopy (HAXPES) has been attracting considerable attention since it can probe the chemical and electronic states of the bulk and buried interface lying at depths of several tens nm due to its large probing depth ^[1]. In the last decade, HAXPES has been applied to various research fields ^[2,3]. The utilization of HAXPES measurements has expended in the several beamlines at SPring-8. This spring, SPring-8 BL46XU has been upgraded by renewing its optics and instrumentation, resulting in a significant performance improvement. This beamline is available to users from July 2023 as the second dedicated HAXPES beamline "HAXPES II” at SPring-8 following BL09XU. The beamline layout is shown in Fig.1. The features of the upgrade of BL46XU are as follows.

· A high-throughput HAXPES system specialized for automated measurements was installed in the upstream experimental hutch(EH) 1, and an atmospheric pressure HAXPES system that enables measurements under a gas atmosphere was installed in the downstream EH 2.

· Two types of double channel-cut monochromators (Si220 and Si311) were installed into the optics hutch, allowing the user to select the optimum excitation X-ray conditions (energy, resolution, and flux) according to experimental purpose.

· Wolter focusing mirrors were installed at the front of the HAXPES apparatus in both EHs 1 and 2, enabling the use of high-flux and highly stable X-rays.

This upgrade will enable users to use the appropriate instruments in different ways according to the measurement target and analysis purpose, which lead to further achievements and improved convenience for users in the future. In this presentation, we introduce the beamline specifications obtained through the commissioning and the latest measurement results using the upgraded HAXPES instruments.

References

[1] K. Kobayashi, Nucl. Instrum. Methods Phys. Res. A. 601, 32 (2009).

[2] K. Kobayashi, M. Yabashi, Y. Takata, T. Tokushima, S. Shin, K. Tamasaku, D. Miwa, T. Ishikawa, H. Nohira, T. Hattori, Y. Sugita, O. Nakatsuka, A. Sakai, and S. Zaima, Appl. Phys. Lett. 83, 1005 (2003).

[3] N. Yabuuchi, K. Shimomura, Y. Shimbe, T. Ozeki, J.Y. Son, H. Oji, Y. Katayama, T. Miura, and S. Komaba, Adv. Energy. Mater. 1, 759 (2011).

Electron backscatter diffraction (EBSD) is a surface analysis method that provides detailed information on the crystal orientation and strain of a solid sample and is applied to the evaluation of the crystal structure of various solid samples. Kikuchi diffraction patterns are generated from each measurement point on the surface of a solid sample depending on the crystal structure and the angle. Main Kikuchi bands in the Kikuchi patterns are generally used for indexing the crystal structures. A pattern matching method based on the simulation of Kikuchi patterns from the type of crystal lattice of the assumed sample is now available from 2022. This method has increased the possibility of indexing complex regions that had been difficult to index in the past. However, it is still difficult to evaluate unknown structures because only crystal structures predicted in advance can be simulated. Therefore, we proposed an analytical method to clarify the crystal structure by multivariate analysis, considering all Kikuchi patterns obtained from all measurement points of the sample as variables [1]. In this study, we aim to extract more detailed crystal structures by applying autoencoder which is an unsupervised learning method based on artificial neural networks. In this study, austenitic stainless steel (SUS304) with dislocations produced by cold working [1] was analyzed using EBSD (Symmetry, Oxford Instruments) with a scanning electron microscope (SU3500, Hitachi High Tech Corp.). The thickness and he grain size of the sample were 100 µm and 50–150 µm, respectively. The accelerating voltage was 20 kV, the step size was 1 µm, and the raster size was 74 × 82 µm². All Kikuchi pattern maps at all measurement points were analyzed at once. The pixels of each Kikuchi pattern map were the variables and the pixels of the measured area were the samples. The original Kikuchi pattern maps were reduced by integrating 8×8 pixels to make the number of the variables smaller than the sample number. The data set was analyzed using sparse autoencoder of Neural Network toolbox of Matlab and our original program coded in Python 3 [2,3]. As a result, the measurement area of the sample was divided into several regions in the features (the middle layer) and a Kikuchi pattern map corresponding to each divided region were extracted in the decoder weights. To obtain clear images for sample regions and Kikuchi pattern maps, large epoch number was required. References 1) S. Aoyagi, D. Hayashi, Y. Murase, N. Miyauchi, A. N. Itakura, e-Journal of Surface Science and Nanotechnology, 21(3) 128-131 (2023). https://doi.org/10.1380/ejssnt.2023-023. 2) S. Aoyagi, K. Matsuda, Rapid Communications in Mass Spectrometry, 37(4), e9445 (2023). https://doi.org/10.1002/rcm.9445 3) S. Aoyagi, D. Hayashi, A. Nagataki, T. Horiba, M. Saito, e-Journal of Surface Science and Nanotechnology, 21(1) 9-16 (2022).

Hydrogen causes hydrogen embrittlement of metals, causing cracks and fractures. In order to clarify the hydrogen embrittlement of metals, it is necessary to visualize and analyze the crystal structures, crystallographic orientation, and hydrogen distribution. In this study, we aimed to obtain information related to the crystal structure and crystallographic orientation that is correlated with the distribution of hydrogen by image fusion [1] of the images obtained with scanning electron microscope (SEM), and electron backscattered diffraction (EBSD), and time-course hydrogen distributions in metals using an operando hydrogen microscope [2] based on electron stimulated desorption (ESD). The hydrogen gas was supplied to backside of a pure vanadium plate, penetrates the sample inside, and arrives at the surface. The hydrogen distribution at the vanadium surface was measured with the operando hydrogen microscope for 40 hrs. Hydrogen maps were measured every 150 s, a SEM image, and EBSD images including inverse pole figure (IPF) Kernel Average Misorientation (KAM), band contrast, and Euler angle maps were integrated using image registration (Matlab). The fused image data set was analyzed using multivariate analysis and the relationship between crystal structures and hydrogen permeation was indicated. References [1] T. Akiyama, N. Miyauchi, A.N. Itakura, T. Yamagishi, and S. Aoyagi, Journal of Vacuum Science & Technology B, 38, 034007 (2020) [2] N. Miyauchi, K. Hirata, Y. Murase, H.A. Sakaue, T. Yakabe, A.N. Itakura, T. Gotoh, S. Takagi, Scripta Materialia 144, 69 (2018).

To promote materials development based on data-driven science, it is essential to develop technologies for extracting features from high-dimensional measurement data such as images and spectra. There is a need for a system that automatically converts high-dimensional measurement data into features and stores the table data under AI-ready status. A cloud system, Research Data Express (RDE) [1], is being developed to register experimental and computational data quickly. The RDE has workflows to automatically extract features from high-dimensional measurement data.

As one of the feature extraction tools, we have developed an automatic spectral decomposition tool using reference data associated with physical states such as electron bound state and crystalline structure, etc. The developed tool can be used as part of the workflow functionality of the RDE. This spectral decomposition has high interpretability because it uses reference spectral data. The figure shows four concepts in the design of this spectral decomposition tool. Four concepts are presented as follows:

・[Reference] A spectral decomposition tool using a basis (reference data) linked to a physical state.

・[Integration] Multiple spectra can be integrated and shared parameters can be controlled.

・[Selection] Reference spectra from candidates based on data can be selected automatically.

・[Customization] Analysis models (peak, BG, noise models) can be customized. This spectral decomposition tool enables the automatic analysis of spectral data using reference data. In this presentation, examples of its use will be presented on the subjects of X-ray diffraction data and X-ray photoelectron spectroscopy data.

[1] Research Data Express (RDE), https://dice.nims.go.jp/services/RDE/

Neutron-irradiated graphite has attracted attention as a useful material for creating new carbon phases. Recently, neutron-irradiated highly oriented pyrolytic graphite (n-HOPG) has been shown to transform into transparent amorphous diamond fragments under multiple processes of shock compression and rapid quenching [1]. Also, it has been reported that the n-HOPG can be transformed into nano-polycrystalline diamond [2] and compressed graphite [3], by applying high-pressure and high-temperature treatments. Both the structures are stable at room temperature and ambient pressure. However, the structure and the formation mechanism, especially the compressed graphite, has not yet been clarified.

The purpose of this study is to clarify the effect of irradiation defects on structural changes in n-HOPG under high pressure and high temperature by utilizing in-situ XRD and TEM-EELS to elucidate the formation mechanism of stable compressed graphite.

From the in-situ XRD experiments performed at SPring-8, it was found that graphite G(002) peak is represented by two components for the irradiated samples, whereas only one component for the un-irradiated samples. Under in-situ experiment, a recovery of irradiation defects was also suggested at around 600℃. On the other hand, the compressed graphite was also synthesized by high-pressure and high-temperature treatments of n-HOPG and un-irradiated HOPG at 15 GPa and 1500℃.

The structural and electronic properties of the specimen were subsequently characterized by TEM-EELS. Fig. 1 suggests the formation of compressed graphite and the presence of a slight sp²-σ* component, unlike graphite.

Reference

[1] K. Niwase et al., Phys. Rev. Lett. 102, 116803 (2009).

[2] M. Terasawa et al., Diam. Relat. Mater. 82, 132 (2018).

[3] K. Niwase et al., J. Appl. Phys. 123, 161577 (2018).

Vanadium dioxide (VO₂) thin films exhibit a metal–insulator transition (MIT) with sensitivity to the lattice strain [1, 2]. Substrates with step and terrace structures could be an attractive platform for growing high-quality thin films using the atomically-ordered surface structures. Thus, a prominent lattice strain effect could be derived using VO₂ thin films on these substrates. In this study, we grew VO₂ thin films on TiO₂(110) substrates with step and terrace structures to investigate their crystallinity and MIT dynamics in real space, by using X-Ray Diffraction (XRD) and scattering-type Scanning Near-field Optical Microscopy (s-SNOM). VO₂ thin films were grown by pulsed laser deposition under the substrate temperature, the laser frequency, and the partial oxygen pressure of 723 K, 2 Hz, and 0.95 Pa, respectively. The crystallinity was characterized using X-ray diffractometer (Malvern Panalytical Empyrean, Cu Kα irradiation generated at 45 kV / 40 mA) while heating and cooling the sample between 60 ℃ and 120 ℃. Throughout the procedure, the sample was gradually heated from ambient temperature to each designated temperature point (60, 65, 70, 75, 80, 85, 90 ℃) while repeatedly obtaining XRD patterns in a 2θ range at 20.00°-100.00° with an angle scanning speed of 1°/min. To ensure the complete metal-insulator transition of VO₂, the sample was further heated to 120°C and held at this temperature to attain an XRD pattern. Subsequently, the cooling process was initiated, mirroring the above-described measurements. The sample was cooled from 90°C down to 60°C, employing the same measurement parameters detailed earlier.

Regarding the s-SNOM measurements, an optical parametric oscillator (Levante, APE) underwent pumping by a 1030-nm Yb:KGW oscillator (FLINT, Light Conversion). This process resulted in the generation of near-infrared signal and idler pulses. By employing difference frequency generation (Harmonixx DFG, APE) between these signal and idler pulses, mid-infrared pulses were produced. These pulses were tunable across a range of 4 – 15 μm in the central wavelength λ₀. For the subsequent experiments, mid-infrared pulses with λ₀ = 6.1 μm (~1634 cm-1), a spectral bandwidth of 200 cm-1 FWHM, and a repetition rate of 80 MHz were utilized.

Subsequently, the vertically polarized mid-infrared pulses, attenuated to below 5 mW, were directed into an infrared scattering scanning near-field optical microscopy system (IR s-SNOM; neaSNOM microscope, neaspec GmbH). To facilitate this, the mid-infrared pulses were focused onto the apex of a metallic tip (Arrow-NCPt, NanoAndMore Japan) within a tapping-mode AFM configuration. Scattered pulses emanating from the tip apex were then captured by an HgCdTe detector (IRA-20-00103, Infrared Associate). In order to ensure the detection of signals originating from the highly localized near-field, the detector signal underwent further lock-in demodulation using harmonics of the tapping frequency ω_t.

In terms of AFM operation, a tapping frequency of approximately ω_t ~ 225 kHz and a tapping amplitude of A ~ 84 nm were employed. The lock-in demodulation was detected at the third harmonics of the tapping frequency (3ω_t), contributing to the overall sensitivity of signal detection.

Figure 1 shows the temperature dependence of X-Ray Diffraction spectra of VO₂/TiO₂(110) for the heating process from 60 ℃ to 120 ℃. A noticeable shift in the VO₂(220) diffraction peak was observed, which can be attributed to the structural transition from the insulator phase (at lower temperatures) to the metallic phase (at higher temperatures). Specifically, the pronounced (220) diffraction peak exhibited an angle of 2θ = 57.56° at 60°C, while this angle shifted to 2θ = 57.32° at 120°C.

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Zirconia (ZrO₂) is a widely used ceramic material owing to its high strength and toughness, but overcoming the difficulty in mechanical processing is desired. In this context, previous studies revealed that the ductility of yttria-stabilized zirconia is greatly enhanced by the application of an electric field [1]. However, the microscopic mechanism of this phenomenon has not been well understood yet. As a first step to clarify the mechanism of the ductility enhancement, in this study, we have constructed a machine learning potential to predict the behavior of ions in ZrO₂ under electric fields.

We use a machine learning potential because we can expect to achieve high prediction accuracy and relatively low computational cost simultaneously. As for the type of machine learning potential, we have adopted the high-dimensional neural network potential (HDNNP) [2] in this study. Our group proposed a modified HDNNP scheme to examine ionic motions under applied electric fields by adding a neural network (NN) to predict the Born effective charges [3]. We have adopted this modified scheme in this study. First, ZrO₂ structural data including those with oxygen vacancies were generated using classical molecular dynamics (MD) calculations [4] for a variety of crystal structures (orthorhombic, rhombic, monoclinic, and cubic). Next, density functional theory (DFT) calculations were performed on the structural data obtained by the MD calculations to obtain training data on the energy, forces acting on respective ions, and the Born effective charges. The NN to predict the Born effective charges and HDNNP were constructed using the training data from DFT calculations (8000 structures).

We have confirmed that the constructed HDNNP has good prediction accuracy for both energy and forces: the root mean square errors for energy and force are 5.55 × 10^-3 eV/atom, 1.14 × 10^-1 eV/Å for training data, respectively. Note that we have considered the charge states of O vacancies of 0, +1, and +2 though they take the charge state of +2 in bulk ZrO₂. It is also worth noting that we have obtained anomalously large Born effective charge values for some structures with the 0 and +1 charge states compared to the value for the defect-free bulk. We have found that the anomalous values are caused by the metallic nature due to the Fermi level above the bottom of conduction band (see Figure 1) and insufficient computational conditions for such a case. In fact, we have obtained reasonable values of Born effective charges by setting severer computational conditions such as the number of k points, and successfully constructed the NN to predict Born effective charge. In the presentation, we will discuss behavior of the Born effective charge in detail and also present the results of MD calculations under the application of an electric field.

This study was supported by JST CREST Programs "Novel electronic devices based on nanospaces near interfaces" and "Strong field nanodynamics at grain boundaries and interfaces in ceramics" and JSPS KAKENHI Grant Numbers 19H02544, 20K15013,21H05552, 22H04607, and 23H04100.

References [1] H. Motomura, et. al., J. Eur. Ceram. Soc., 42, 5045 (2022). [2] J. Behler and M. Parrinello, Phys. Rev. Lett., 98, 146401 (2007). [3] K. Shimizu, et al., arXiv.2305.19546 (2023). [4] Y. Wang, et. al., Phys. Rev. B, 85, 224110 (2012).

Titanium dioxide (TiO₂) have crystalline structures to indicate photocatalytic effects, that are Anatase and Rutile. Anatase is more photocatalytic than Rutile. Loading Au nanoparticles (Au NPs) onto Anatase TiO₂, photocatalysis is enhanced [1, 2]. This phenomenon is contributed to by SPR (Surface Plasmon Resonance) [3]. SPR phenomenon is the resonance excitation of localized free charge oscillations (surface plasmons) on a metal surface by incident light interacting with the metal surface. As a consequence, SPR phenomenon leads to an increase in light absorption and enhancement of electric field intensity at the surface [2, 4]. In previous studies, Au deposited on silicon substrate, and on TiO₂ thin film of sapphire substrate were performed simultaneous Scanning Near-field Optical Microscopy (SNOM) / Atomic Force Microscopy (AFM) measurements. From the measurements conducted, it was observed that the electric field intensity of Au was smaller than that of the substrate. The reduction in Au electric field intensity was attributed to a decrease in the near-field signal of Au [4, 5]. However, the reasons for the decrease in Au near-field signal remain uncertain, although it has been determined to exhibit dependency on nanoparticle size. In this study, we focus to elucidate the relationship between the size and height of Au NPs obtained from AFM and the near field light intensity obtained from SNOM. To further clarify, we intend to understand the mechanism by which Au NPs contribute to the enhancement of catalytic activity in Anatase TiO₂.

In this experiment, Anatase TiO₂ was epitaxially grown on Nb doped SrTiO₃ (100) (0.05 wt%) substrate by Pulsed Laser Deposition (PLD). Following the deposition, measured by Reflection High-Energy Electron Diffraction (RHEED), Anatase TiO₂ (001) - (1×4) surface reconstruction is observed. RHEED image of Anatase TiO₂ (001) - (1×4) reconstruction surface shows Figure1 (a). Subsequently, Au NPs were deposited in vacuum and anneal at 400 degrees in a furnace, Au -supported Anatase TiO₂ was made. The produced Anatase TiO₂ was characterized by using Scanning Near-Field Optical Microscopy (SNOM) / Atomic Force Microscopy (AFM). SNOM involves bringing a probe into contact with the sample surface, detecting the scattering light of near-field light generated at the probe's tip, and performing Fourier series expansion on the scattered near-field light to map the intensity of higher-order harmonics. On the other hand, AFM oscillates the probe at a constant frequency and makes contact with the sample surface to measure the topography of the sample surface. The SNOM/AFM measurement setup used in this experiment shows Figure1 (b). A incident laser with a wavenumber (k) of 1634 cm^-1 detects through a beam splitter, focusing the laser through an off-axis parabolic mirror. The scattered near-field light from the interaction of the probe and the sample surface is directed to a detector, allowing for the observation of the near-field intensity (third harmonic frequency spectrum). The obtained distributions of near-field intensity and topography from SNOM/AFM were plotted to illustrate the relationship between Au NPs size, height, and near-field light intensity. Figure1 (c, d) illustrates the SNOM/AFM simultaneous measurement images of Anatase TiO₂ with a 2 Å deposition of Au NPs. Figure1 (c) is the image displays the third harmonic near-field intensity (SNOM Image), while Figure1 (d) is the topography image obtained from AFM (AFM Image). From Figure1 (c, d), tendency is observed that the intensity of near-field intensity decreases as Au NPs size increases.

References

[1]: Jingtao Zhang, et al., Material Letters, 162, 235 (2016)

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Scanning probe microscopes are used to image sample surfaces at the nano to atomic level. Until now, precise measurements were conducted under ultra-high vacuum and extreme low-temperature environments. Recently, capturing phenomena occurring at room temperature, such as surface diffusion, surface dynamics, biological observations, and drug reactions, has become important. However, at room temperature, thermal drift causes the imaging area to shift, making long-term observation of changes in the same region at the nanometer scale very challenging. In this study, we report the development of an automatic correction technique for thermal drift using feature point matching.

In our method, multiple feature points are extracted from continuously acquired images, and pairs are formed by matching these points. Subsequently, only effective pairs are detected by filtering, and the drift amount and drift velocity are calculated from the positional displacement between the pairs. Experiments were conducted on an Si(111)-(7x7) surface with Sn deposition, using a scanning tunneling microscope (STM) operating under room temperature ultra-high vacuum conditions.

Figures 1 and 2 show the results of thermal drift correction over approximately three days. Figure 1 displays continuously acquired images over the measurement period, confirming that the scanned area is fixed and that the drift has been corrected at the atomic level. Figure 2 illustrates the changes in the X and Y positions of the piezoelectric scanners used to correct the thermal drift amount in the experiment of Figure 1. The nonlinear thermal drift amount occurring beyond the scanning area (22.5 nm²) is confirmed to be corrected. This method has been found to enable the observation of the diffusion of surface-adsorbed atoms.

Faceted macrosteps formed at equilibrium are considered to be caused by step-step attraction [1]. However, the explicit value of the step-step attractive energy has not yet been obtained. This is because macrostep formation at equilibrium is one of the difficult problems arising from the surface roughness associated with the crystal morphology.

The Mermin-Wagner theorem states that the long-range order is destroyed by thermal fluctuations in low-dimensional systems with short-range forces, such as the crystal surface or interface. The thermal roughening transition is known as the topological order phase transition with the Berezinskii-Kosterlitz-Thouless (BKT) universality class [2]. In addition, the equilibrium crystal shape (ECS), which is the shape of a crystallite with the lowest surface free energy, causes the faceting transition just at the thermal roughening transition temperature T_R [3].

In our previous work for a lattice model with microscopic step-step attraction, which is caused by quantum mechanics, the macrostep is formed at equilibrium as a result of the first-order shape transition, i.e., two-surface coexistence [4]. To obtain the phase diagram (faceting diagram), we need to calculate the surface free energy in detail with high confidence in the surface entropy. Although the lattice model of the surfaces is on the square lattice, we calculated the surface free energy [5,6] with high accuracy using the product wave function renormalization group method [7], which is an extension of the tensor network method.

Using the phase diagram [5,6], we show how to estimate the step-step attractive energy [6]. As an example, the method is applied to the Si(113)+(114) surface, where the phase diagram was obtained experimentally [1]. The step-step attractive energy is then estimated to be approximately -123 meV [6].

References

[1] Song, S.; Mochrie, S. G. J. Tricriticality in the orientational phase diagram of stepped Si (113) surfaces. Phys. Rev. Lett. 1994; 73: 995—998.

[2] van Beijeren, H. Exactly Solvable Model for the Roughening Transition of a Crystal Surface. Phys. Rev. Lett 1977, 38, 993—996.

[3] Rottman, C.; Wortis, M. Statistical mechanics of equilibrium crystal shapes: Interfacial phase diagrams and phase transitions. Phys. Rep., 1984, 103, 59—79.

[4] Akutsu, N. Thermal step bunching on the restricted solid-on-solid model with point contact inter-step attractions. Appl. Surf. Sci., 2009, 256, 1205—1209. ibid Non-universal equilibrium crystal shape results from sticky steps. J. Phys. Condens. Matte, 2011, 23, 485004, 1—17.

[5] Akutsu, N. Faceting diagram for sticky steps. AIP Adv., 6, 035301 (2016).

[6] Akutsu, N.; Akutsu, Y. Slope—Temperature Faceting Diagram for Macrosteps at Equilibrium. Sci. Rep., 2022, 12, 17037, 1—11.

[7] Nishino, T.; Okunishi, K. Product wave function renormalization group. J. Phys. Soc. Jpn., 1995, 64, 4084—4087. Hieida, Y.; Okunishi, K.; Akutsu, Y.

Magnetization process of a one-dimensional quantum antiferromagnet: The product-wave-function renormalization group approach. Phys. Lett. A,1997, 233, 464—470.

Accurate measurement of the local work function (LWF) is crucial for enhancing the performance of electronic/optoelectronic devices. Currently, LWF can be measured by scanning tunneling microscopy (STM) or atomic force microscopy (AFM). In STM, the local barrier height (LBH), which refers to the average of LWFs between the tip and the sample, is measured from the tunneling current as a function of tip-sample separation I_t(z). Conversely, the AFM-based Kelvin probe technique measures the local contact potential difference (LCPD), which represents difference of LWF between the tip and the sample surface. However, in the both cases, direct determination of the LWF requires pre-calibration of the tip’s LWF, while it is known that the condition of the tip apex and its associated LWF often change during the measurement.

In this study, we developed alternative method to directly measuring the LWF, based on frequency modulation AFM with a conductive tip. Ultrafast voltage pulses, synchronized with the cantilever oscillation, are applied to the sample (Figure 1). By sweeping the trigger delay of the pulse application, and measuring the tunneling current, we can obtain I_t(z), allowing for the determination of LBH. Importantly, this procedure can also measures the LCPD, by recording the energy dissipation of the oscillating cantilever [1]. This method thus allows simultaneous measurements of LBH and LCPD, enabling the direct evaluation of LWF without the conventional pre-calibration. Further details of our method and the results will be presented in the session.

References:

[1] E. Inami and Y. Sugimoto, Phys. Rev. Lett. 114, 246102 (2015)

To improve characteristics of field emitters, such as high brightness and small source size, methods to fabricate nano-tips have been investigated. One of these methods is field-assisted oxygen etching [1] which utilizes the oxidation of tungsten and the field evaporation by applying the field of the order of several tens V/nm to the tip under oxygen environment. The final shape of the tip depends on methods which are the decreasing bias method to prevent the evaporation of the apex for extreme sharp tip [2] and the fixed bias method to form a nano-protrusion on the large base tip [3]. Additionally, Rahman et al. reported that the tips fabricated by the decreasing bias method enable to extract the electron beam by applying only 4 V [2], which is speculated that the results relate to composition of the tip apex. In this study, we analyze the composition of tips fabricated by fixed and decreasing bias methods by using a pulsed voltage atom probe.

W<011> tips were prepared by electrochemical etching of polycrystalline tungsten wire (0.15 mmφ). The prepared tips were introduced into a chamber consisting of a field-ion microscope (FIM) and a pulsed-voltage atom probe in which tip was cooled with liquid nitrogen. We observed He-FIM images of the W tips, then the surface of the W tip was cleaned by the field evaporation. After that, two methods of the field-assisted oxygen etching were conducted. In both methods, oxygen at a pressure of the order of 10^-4 Pa was admitted to the chamber, and a positive bias corresponding to the helium best image field (BIF) was applied to the tip. In the decreasing bias method, the applied voltage was gradually reduced to keep the field strength of the apex at the He-BIF and to prevent the field evaporation of the atoms at the apex, because the tip becomes sharper than the initial shape due to the field evaporation of oxidized shank of the tip. When the apex was terminated with only several atoms the oxygen etching was stopped by turning off the O₂ gas supply. In the fixed bias method, the constant positive bias corresponding to the He-BIF was applied to the tip during the etching. When a nano-protrusion was formed on the tip apex, the etching was stopped by shutting the valve of oxygen gas. The atom probe analysis of prepared tips was performed without exposure to the air. The flight distance of the atom probe is about 450 mm, and mass resolution, m/Δm, for W³⁺ is estimated to be 80.

Figures 1(a) and (b) shows mass-to-charge ratio spectrums obtained for nano-tips fabricated by decreasing and fixed bias method, respectively. For the tip fabricated by the decreasing bias method as shown in Fig. 1(a), only pure tungsten was detected until the radius of curvature achieved 14 nm confirmed by FIM observation after atom probe analysis. On the other hand, for the tip fabricated by the fixed bias method, a ladder chart as shown in Fig.1 (c) indicates that pure tungsten and tungsten oxide were detected alternately, although FIM image of the nano-protrusion shows clean surface. The detection of tungsten oxide ions during atom probe analysis of the nano-protrusion indicates that the tip base is covered with tungsten oxide. The difference in detected ions species between two methods is attributed to tip form after field-assisted oxygen etching. The oxidation reaction occurs at the shank of tips. The decrease in bias etched region shifts from the apex to shank where the field strength is low, therefore, oxide layer at the apex is removed. For the fixed bias method, the oxidation and the field evaporation of oxide move toward a boundary between the nano-protrusion and the base. The field strength at the base tip may be relatively low, where oxygen molecules adsorbed and oxide layer did not field-evaporate.

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Introduction

Vibrational sum-frequency generation (V-SFG) spectroscopy is a powerful technique for investigating molecules at interfaces and surfaces [1]. Multiplex V-SFG measurements have been performed by using a broadband infrared (IR) pulse and a narrowband visible pulse to generate broadband SF lights from the molecules [2]. The IR pulse is usually prepared by difference frequency generation (DFG) with nonlinear crystals such as AgGaS2 and GaSe [3]. However, the IR pulse's bandwidth limits the V-SFG spectral range at most 400 cm^-1. In addition, the phonon absorption of these crystals below 500 cm^-1 prohibits low-frequency IR light generation, making it challenging to perform SFG measurements at the low-frequency region. Therefore, most current SFG studies focus on intramolecular vibrations at the high-frequency region (> 2000 cm^-1). It would be helpful to expand the detection bandwidth of V-SFG spectra and achieve low-frequency V-SFG measurements, including intermolecular vibrations and frustrated translations/rotations. In this study, we have developed ultra-broadband V-SFG spectroscopy with a broadband IR pulse generated by two-color laser-induced air plasma. We demonstrated ultra-broadband V-SFG spectroscopy of polymethylmethacrylate (PMMA) film on a GaAs substrate between 550-3000 cm^-1.

Method

Our newly developed V-SFG measurement system uses a broadband IR pulse generated by two-color laser-induced air plasma [4]. The Ti:sapphire laser's output was split into two, and about 90% of the laser output passed through a β-BaB₂O₄ crystal to generate a second harmonic (SH) light. The SH light (400 nm) and fundamental light (800 nm) with a parallel polarization direction were focused onto the air in an N₂-purged box to induce plasma. Another part of the laser output was used to prepare a narrowband visible pulse by a band-pass filter with a half-width half maximum of 1.2 nm. The two pulses were focused onto the sample surface to induce the SFG polarization at the surface. The emitted SFG lights, after being filtered by a short-pass filter, were detected by a polychromator.

To demonstrate ultra-broadband V-SFG spectroscopy, our developed system measured bare GaAs(100) and PMMA film on GaAs(100). The PMMA film 10-µm thick was made by drop casting of 2wt% PMMA in toluene solution on a GaAs(100) substrate. During the V-SFG measurements, the time delay between the two incident pulses was varied, with the visible pulse following the IR pulse.

Results and Discussion

The SFG spectrum of the bare GaAs substrate showed a broadband spectral shape, which reflects the spectrum of the IR pulse generated by air plasma [5]. This result indicates that our SFG system's detection range is roughly 550-4000 cm^-1. The V-SFG spectrum of the PMMA film deposited on GaAs(100) at the 0 ps delay time showed a broad spectrum, which is like that of the bare GaAs substrate; this broadband spectrum of the PMMA/GaAs mainly originates from the non-resonant SFG signal from the GaAs substrate. In contrast, the SFG spectrum of the PMMA on the GaAs substrate at > 0.8 ps delay time showed sharp peaks. By comparing these observed peaks with those of Raman and IR absorption spectra of a PMMA film, we conclude that we have successfully detected ultra-broadband V-SFG spectra of the PMMA film between 550-3000 cm^-1.

References

[1] Y.-R. Shen, Second Harmonic and Sum-Frequency Spectroscopy, WORLD SCIENTIFIC, 2022.

[2] T.A. Ishibashi and H. Onishi, Appl. Phys. Lett. 81, 1338 (2002).

[3] K. Madeikis, et al., Opt. Express 29, 25344 (2021).

[4] J. Dai, N. Karpowicz, and X.C. Zhang, Phys. Rev. Lett. 103, 023001 (2009).

[5] E. Matsubara, M. Nagai, and M. Ashida, J. Opt. Soc. Am. B 30, 1627 (2013).

Recently, electrochemical carbon dioxide (CO₂) reduction reaction using photocatalysts to convert CO₂ into useful chemicals has attracted much attention. Au/p-GaN photocatalysts, in which gold nanoparticles (AuNPs) are supported on p-type gallium nitride (p-GaN), show high activity for CO₂ reduction due to the plasmon excitation of AuNPs followed by selective hole transfer to the p-GaN substrate remaining high-density electrons at the AuNP surfaces^[1]. The plasmon-assisted electrochemical CO₂ reduction using Au/p-GaN catalysts has been reported to selectively produce carbon monoxide (CO)^[1]. However, the chemical structure of proton donor required for CO₂ reduction with the Au/p-GaN catalysts is not clear. It was proposed that multiple protons are required for CO₂ reduction to proceed^[2]. Therefore, the understanding of proton donor is important for controlling CO₂ reduction. In this study, we attempted to observe the proton donor in a near surface region of the AuNPs by in-situ surface-enhanced Raman spectroscopy (in-situ SERS) under electrochemical conditions.

The Au/p-GaN catalyst was prepared by vacuum deposition of Au on p-GaN substrates according to the previously reported method by Joseph et al^[1]. The Au/p-GaN catalyst exhibited absorption peak at about 570 nm due to a plasmon resonance of AuNPs. In-situ SERS measurements were performed in a CO₂-saturated 0.5 M NaHCO₃ solution (pH=7.4), with CO₂ bubbling during the measurement to prevent pH change in the bulk. The Au/p-GaN catalyst was used as a working electrode, Pt wire as a counter electrode, and Ag/AgCl electrode as a reference electrode. All SERS spectra were collected at a constant potential. All the potentials are referred to the Ag/AgCl standard electrode potential unless noted otherwise.

Fig. 1 shows potential dependence of Raman bands at 1358 cm^-1 and 1380 cm^-1 assigned to bicarbonate (HCO₃^-) and carbonate (CO₃^2-), respectively, in the near surface region of AuNPs. The peak area of HCO₃^- decreases as the potential becomes more negative, while the peak area of CO₃^2- exhibits almost no change. This result indicates that the HCO₃^- is transformed to the CO₃^2- if the peak area of Raman band is assumed to be proportional to the molecular density and the newly transformed CO₃^2- diffuses out from the near surface region due to a repulsion from the negatively charged AuNPs’ surface. The transformation of HCO₃^- to CO₃^2- can be related to the CO₂ reduction, because the bulk pH is constant during the in-situ SERS measurement. This is interpreted as deprotonation of HCO₃^- in the near surface region of AuNPs with acting as a proton donor during the CO₂ reduction. Furthermore, the local pH near the electrode surface was calculated from the peak area ratio of HCO₃^- and CO₃^2- at each potential by the Henderson-Hasselbarch equation. The surface pH was found to increase as the potential becomes more negative. This result suggests that not only the HCO₃^- was converted to a CO₃^2- (eq 1), but also that the HCO₃^- acted as a proton donor (eq 2).

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X-ray photoelectron spectroscopy (XPS) normally requires ultra-high vacuum to suppress scattering of photoelectrons. Such a requirement creates a gap between the XPS experimental condition and the realistic working environments of functional materials such as catalysts. To meet the demand of studying functional materials at work, the technique of ambient pressure X-ray photoelectron spectroscopy (APXPS) has been developed. At SPring-8 BL07LSU, which was operated as a University of Tokyo soft X-ray outstation beamline[1] until August 2022, an APXPS system was constructed and enabled measurements under 100 mbar of gas pressure[2-4]. To further enhance the capability of this APXPS system and realize XPS measurement at 1 bar, we are modifying the design and developing a new APXPS system at the new synchrotron facility NanoTerasu, currently under construction in Tohoku university. The greatly enhanced brilliance in the soft X-ray region makes NanoTerasu an ideal place to set up a soft X-ray APXPS system.

In APXPS systems, ultra-high vacuum must be maintained inside electron analyzer and the beamline, while the sample is placed under near ambient pressure. To realize this, a SiN window and aperture nozzle are placed at the sample-beamline and sample-analyzer interfaces. To secure enough photoelectron signals at 1 bar, the key point is to suppress attenuation of incident X-rays as well as scattering of outgoing photoelectrons by gas molecules. Therefore, we modified the sample environment in which the SiN window and nozzle can be brought as close as 10 mm and 15 um, respectively, to the sample while they were 23 mm and 50 um at SPring-8 BL07LSU. To irradiate the sample in front of the nozzle with X-rays, we adopted extremely grazing incidence (< 5 degrees) condition while the grazing angle of incident X-rays was set at 22 degrees at SPring-8 BL07LSU. With these modifications and high brilliance of NanoTerasu, we expect realistic possibilities of APXPS measurements at 1 bar. In addition, we are modifying the system from the cell type, where only a small cell around the sample in the analysis chamber is filled with gases, to the back-filling type, where the whole analysis chamber is filled with gases. The modification not only simplifies the system and makes operation easier, but also increases the space around the sample. This will enhance the flexibility of measurement conditions, which for example enable electronic field application or the study of liquid-solid interfaces.

[1] S.Yamamoto et al. J.Synchrotron Radiat. 21, 352 (2014)

[2] T. Koitaya, S. Yamamoto et al., Top. Catal. 59, 526-531 (2016).

[3] S. Yamamoto et al., Phys. Chem. Chem. Phys. 20, 19532-19538 (2018).

[4] Y. Ishihara et al., Adv. Mater. Interfaces 10, 2300258 (2023).

Nanoporous materials such as zeolites and nanostructured composite materials are becoming more important in the field of catalysis. The understanding of the surface structure characteristics and the precise analysis of the local arrangement of elements in these material systems are indispensable for the deep understanding of the structure performance relationships, which depends on the availability of sophisticated analysis techniques. However, the zeolites are usually insulating and electron beam sensitive material. Therefore, it is one of the difficult samples to observe fine details of the surface.

In order to solve these problems and obtain surface information that governs the properties, the development of a high-resolution low landing voltage SEM is desirable. On the other hand, in the low landing voltage SEM, the effects of objective lens aberration and chromatic aberration become significant, and the electron probe diameter increases, resulting in a decrease in spatial resolution. For this reason, we have been working to improve the spatial resolution of the SEM under low landing voltage conditions. We have developed an objective lens called the super hybrid lens (SHL): a compound lens consisting of both magnetic and electrostatic lenses. The SHL is capable of producing a small probe size even at low incident voltages (for example 0.7 nm at 1 keV) and moreover, beam deceleration mode allows imaging down to 10 eV. However, it was difficult to reduce electron charge-up effect even under low voltage conditions at high magnified imaging. Therefore, we have developed an electron energy selectable detector which is installed in the electrostatic lens of SHL, the so-called upper hybrid detector (UHD). The UHD enables the reduction of the electron charge-up effect during high resolution observation, owing to detecting higher energy electrons that are expected to reduce the electron charge-up. Figure 1 shows high magnification images of Zeolite ZSM-5. The image of UHD clearly shows fine steps on crystal surfaces, while a conventional detector so-called upper electron detector (UED) image shows an uncleared surface owing to detecting secondary electrons below 70 eV even at landing energy of 500 eV.

In this report, we will introduce a recent low landing voltage SEM for observing the fine structures of materials such as zeolites.

The optical microscope is widely used for the observation of the structures in liquid samples. However, due to the limited resolution of optical microscopes, it is difficult to observe particles in a liquid when their size is smaller than 1 um. Especially, for samples containing nano-scale structures dispersed in a liquid, the resolution of an optical microscope is insufficient, making it necessary to employ a high-resolution scanning electron microscope (SEM) for observation. Nevertheless, SEM observation is conducted under vacuum conditions, requiring the liquid-dispersed samples to be dried. During the drying process, deformation and aggregation of the samples can occur, posing a challenge to preserve their original shape. Recently, we have tried to use capsule-type holder (Aquarius Starter Kit, FlowVIEW Tek) for observing structures in liquid. Because liquid samples can be sealed within the holder under atmospheric pressure conditions, it is possible to achieve in-situ observation of liquid samples through a silicon nitride membrane window with a thickness of 20 nm (according to the sample, 30 nm and 50 nm are also available). In contrast, this capsule-type holder is suitable for high-resolution observations as well. The cross-sectional schematic diagram of the holder is shown in Fig 1a [1-2]. In this study, we attempted to observe and analyze particles dispersed in a liquid using a FE-SEM (JSM-IT800<i>, JEOL Ltd.) employing this liquid-sample-dedicated holder. Moreover, in order to capture the dynamic changes of particles, a high responsibility detector (scintillator backscattered electron detector (SBED)) was used. As an example, we observed a liquid makeup primer emulsion. The SEM image (Fig 1b) shows that the dispersion state of particles contained within the makeup primer emulsion can be clearly confirmed. In addition, we also investigated the relationship between incident voltage and image formation, as well as the feasibility of Energy Dispersive X-ray Spectroscopy (EDS) analysis (JED-2300, JEOL Ltd.) during observations of liquid samples. Through the membrane window, elements present in the sample, from light elements (such as carbon, oxygen) to heavy elements, can all be detected by EDS analysis.

References

[1] E. J. Lang, N. M. Heckman, T. Clark, B. Derby, A. Barrios, A. Monterrosa, C. M. Barr, D. L. Buller, D. D. Stauffer, N. Li, B. L. Boyce, S. A. Briggs, K. Hattar, Nucl. Instrum. Methods Phys. Res., Sect. B 537, 29, (2023).

[2] N. Asano, J. Lu, S. Asahina, S. Takami, Nanomaterials, 11, 908, (2021).

The investigation of electronic properties like band structures on surfaces within three-dimensional (3D) shaped structures has attracted the attention of researchers in semiconductor field. This interest arises from the potential to utilized surface carrier transport for advancing 3D transistors in order to break the limitation of downsizing 2D planar-type device. On these geometrically 3D shaped structures, it becomes quite important the creation of atomically flat and well-ordered facet surfaces in design and the construction of functional films on these facet surfaces, for device application. In addition, on artificially designed 3D structures, it could be expected the singular states arising from 3D geometry such as edges between facet surfaces, for fundamental interesting. Despite the significance of these 3D shaped structures, there is a notable scarcity of reports pertaining to the creation of 3D sample and evaluation of electronic bands.

Recently, our group has succeeded in creating 3D-Si structures by photo-lithography technique and surface reconstruction consisting of atomically ordered Si{111}7×7 facet surfaces with sharp facet lines [1-4]. By using laboratory (He source) based angle-resolved photoelectron spectroscopy (ARPES), we have initially reported a successful creation of Si{111}7×7 clean facet surfaces showing surface and bulk electronic bands [4]. In this work, we demonstrate the construction of an epitaxial grown film on Si{111} facet surfaces and film band structures, which promise electronic devices using 3D shaped high-crystalline films in future. In addition, we display valence band and core level spectra arising from the singular region as the facet edges.

In this experiment, we have constructed 3D-Si facet-lined structures on Si(001) substrates with (111) and (-1-11) facet surfaces (Fig. 1(a)) showing 7×7 reconstruction in low-energy electron diffraction (LEED) (Fig. 1(b)), and created well-ordered √3×√3-Ag as a prototype of a functional thin-film. In UVSOR BL5U, we performed the surface preparation and ARPES measurements. Indeed, after the flashing and degassing in ultra-high vacuum, we obtained clear spots Si{111}7×7 and √3×√3-Ag reconstruction and successfully obtained the surface and bulk band structure, in both facet surfaces. In order to survey specific states arising from the singular 3D regions such as edges, we measured Si 2p core levels depending on the optical geometry to discuss chemical components at 6 K (Fig. 1(c)). We will report the analyzed details with the band dispersion results.

References

[1] A. N. Hattori, K. Hattori, et al., Appl. Phys. Express 9, 085501 (2016).

[2] A. N. Hattori, K. Hattori, et al., Surf. Sci. 644, 86 (2016).

[3] S. Takemoto, K. Hattori, et al., Jpn. J. Appl. Phys. 57, 090303 (2018).

[4] K. Hattori, A. N. Hattori, et al., e-J. Surf. Sci. Nanotechnol. 30, 214 (2022).

1. Introduction

Controllable wettability was realized on laser-treated copper (Cu) substrate surfaces with postprocessing [1,2]. In these reported methods, laser ablation technology was used for fabricating micro/nano structures and followed by postprocessing with a chemical agent or heat treatment to add the hydrophobic base to the surface. The wettability and morphology of these hydrophobic surfaces have been extensively studied, but the influence of surface chemical composition has not been clarified. In this study, we realized the superhydrophobic surface by simply irradiating the Cu substrate surface with a femtosecond laser beam under low fluence, and without any postprocessing. During the laser irradiation, the laser fluence was kept low to avoid laser ablation, and even the laser-induced periodic surface structures were not observed on the low-fluence treated surface. On the other hand, the low-fluence treated surfaces were confirmed with the formation of hydrophobic cuprous oxide (Cu₂O) [2].

2. Experiment and results

The Cu substrate surface was scanned by femtosecond laser pulses following the pattern with a size of 0.5 mm x 0.5 mm with a wavelength of 1030 nm, a pulse duration of 700 fs, and a repetition rate of 100 kHz. The pulse energy is set to 1 µJ which is much lower than the ablation threshold energy obtained experimentally. The discoloration of the irradiated surface was observed by a microscopy image. According to the SEM image, nano protuberances were confirmed instead of the general microstructure due to laser ablation. According to the measurement of Raman spectroscopy (Fig. 1 (a)) and XRD (Fig. 1 (b)), we confirmed three vibrational modes attributed to Cu₂O and an XRD peak associated with the lattice spacing of Cu₂O (1 1 1), respectively. The depth of the oxide layer was also confirmed by the measurement of STEM-EDX. As shown in Fig. 1(c), a 28.4-nm oxide layer was observed on the laser-irradiated surface, whose thickness is about 3 times the natural oxide film of the untreated substrate. These results suggest that we successfully promoted the surface oxidation of the Cu substrate by the low-fluence laser irradiation. Furthermore, the product of Cu₂O (1 1 1) has the lowest surface energy than other facets [3]. Interestingly, the facet of obtained Cu₂O surface is the same as the main facet of the unprocessed substrate. Cu substrate surfaces with different Cu₂O proportions (ratio of peak area: Cu₂O/Cu) were fabricated for evaluating the relationship between hydrophobic base proportion and the surface contact angle. With the low surface energy due to Cu₂O, the low-fluence treated Cu surface showed a controllable hydrophobicity with a controllable proportion of Cu₂O. The contact angle of laser treated substrate surface almost linearly increases with the Cu₂O proportion.

3. Conclusion

By a one-step method of low-fluence laser irradiation, we successfully fabricated hydrophobic surfaces on Cu substrates owing to the formation of low-free-energy Cu₂O. The contact angle of the Cu surface exhibited enhanced hydrophobicity after this simple one-step process, showing a linear increase with the Cu₂O proportion. This simple one-step technique would promote the application of laser-induced functional surfaces.

4. References

[1] J. Long et al., ACS Appl. Mater. Interfaces 7, 9858–9865 (2015).

[2] A. He et al., Appl. Surf. Sci. 434, 120-125 (2018).

[3] Y. Maimaiti et al., Phys. Chem. Chem. Phys. 16, 3036 (2014).