1. Introduction

Nanoparticles of carbides are used in various applications. For example, silicon carbide is used as light-receiving devices and light-emitting devices. Also transition metal carbides, such as molybdenum carbide, are expected as an alternative of noble metal catalyst [1]. Various methods to fabricate carbide nanoparticles are reported, such as chemical vapor deposition (CVD) and reduction method. To find a new simple way to fabricate carbide nanoparticles, we focused on the high temperature and pressure state induced by femtosecond laser ablation in liquid environment [2]. In this work [3], we report the fabrication of molybdenum carbide nanoparticles by laser ablation of molybdenum in hexane.

2. Experiment and method

In the experimental setup, femtosecond laser pulses (pulse width, ~700 fs; center wavelength, 1045 nm; repetition rate, 100 kHz) from a Yb-fiber laser system was irradiated to a molybdenum plate mounted inside a quartz cell filled with hexane. The laser beam was focused on the molybdenum surface with a fluence of 2.23 J/cm². In the high-temperature-high-pressure (HTHP) laser ablation center, active species of ions, atoms, and clusters ablated from Mo and hexane will strongly react with each other to form metal carbide Nanoparticles, and the irradiated substrate surface will also be carbonized simultaneously. The surface morphology of the laser-irradiated Mo substrate was characterized by SEM. XRD, Raman spectrum, and XPS were performed to identify the compounds. The collected nanoparticles were progressively analyzed by a STEM and EDX.

3. Results and discussion

After irradiation, we observed nanoparticles deposited on the molybdenum plate by SEM. The nanoparticles have an average diameter of 61 nm, most of the particles are smaller than 100 nm, and some huge particles (>100 nm) can also be observed. On the other hand, the formation of laser-induced periodic surface structure (LIPSS) was confirmed on the laser-irradiated area. The XRD pattern indicated that the nanoparticles were MoC in a fcc structure. From The results of XPS (Fig. 1), additional Mo-C bond of C 1s was observed except for the measurement of the unprocessed area, indicating the carbonization for both the nanoparticles deposition and LIPSS surface. In addition, the C-C bond of sp³ originating to diamond locating at 285.4 eV was detected for the laser irradiated area revealing the graphite-diamond transition happened under the HTHP conditions. The results of Raman spectroscopy have also supported the formation of MoC and amorphous carbon structures. The nanoparticles were also analyzed by STEM and EDX. The result identified that the particles have a core-shell structure, with MoC nanoparticles embedded in amorphous carbon shell (Fig. 2). The electron diffraction pattern obtained with the STEM image indicated these particles have high crystallinity.

4. Conclusion

By using a one-step method of laser ablation on Mo substrate in hexane, we simultaneously achieved carbonization of Mo nanoparticles and Mo substrate surface. This simple synthesis method for MoC may provide a new choice for the preparation of MoC-based devices and nanomaterials, which can accelerate the revolution of replacing noble-metal-based catalysts with more economical Mo-based catalysts.

[1] C. Wan, et. al., Angew. Chem., 126, 6525-6528 (2004).

[2] A. Hu, et. al., Diamond Relat. Mater., 18, 991-1001 (2009).

[3] Y. Tanaka, et. al., ACS Omega, 8 (8), 7932-7939 (2023)

Atmospheric-pressure plasma jet (APPJ) using helium gas can be used for remote plasma processing in open air conditions. A large number of application examples using APPJ have been reported. The APPJ is known to be formed as a result of propagation of plasma bullets. Plasma bullets are generally launched from the nozzle of a narrow DBD tube with helium gas. This scheme has not been changed for approximately two decades.

Recently, we have discovered that plasma bullets can be launched vertically from a planar surface of a dielectric plate if we employ the pulse power with very high dV/dt [1]. We call them surface-launched plasma bullets (SLPBs).

We can propose various new possibilities by eliminating the restriction of “only launching from the nozzle”. For example, the SLPBs can hydrophilize the entire inner surfaces of 3D-printed PLA bone-regeneration scaffolds (continuous porous dielectrics) while conventional APPJs cannot [2]. Furthermore, it may be possible to design the shape of the bullets in contrast to the fact that the shape of conventional plasma bullets is limited to elongated balls or comets. In this study, we investigated the possibility of generating sheet-shaped plasma bullets.

The shape of the reactors examined in this study are shown in Fig. 1. The grounded electrode (GND) is on the top and powered electrode (HV) is on the bottom. Although it is impossible in principle to change the potential distribution of a metal electrode, we examined effect of spatial profile of the bottom-electrode voltage as a study that can be done only by simulation. The discharge gas was a mixture of 0.1% N2 and 99.9% He. The bottom-electrode potential (i.e. applied voltage) was ( 15 kV ){ 1 - exp( - t/T ) }, where T was 10 ns. The spatial profile was the Gaussian profile of exp( - r² / ( 2R² )), where R was infinity or 20 mm. Numerical simulation was performed using an axisymmetric fluid model coupled with Poisson equation and Boltzmann solver (BOLSIG+). Gas-phase reactions in this model were electron impact ionization of He, excitation of He to metastable state, and penning ionization of N₂ by metastable He. Photo ionization was not involved in the model.

Figure 1(a) shows the results for R = infinity, i.e. V(r) = constant. In this case, the bullet was not launched. Figure 1(b) shows the results for R = 20 mm. In this case, the bullet was launched. The shape of the bullet was dome shape. The results obtained in this study suggest that sheet-like plasma bullet, which corresponds to large volume atmospheric pressure plasma, may be possible if we can properly design the electrode-potential spatial profile.

Acknowledgments: This work has been supported by the MEXT/JSPS KAKENHI (19H01888, 19K03811, 20K20913, 23H01166), and the Joint Usage / Research Program of Center for Low-Temperature Plasma Science, Nagoya University.

[1] Shirafuji, T. and Oh, J.-S. 11th Int. Conf. Reactive Plasmas / 2022 Gaseous Electronics Conf., ER5.00006 (2022).

[2] Shirafuji, T. et al. 42nd Int. Symp. Dry Process, H-1 (2021).

INTRODUCTION

Label-free imaging of biological specimens enables to deeply understand biological activity. Conventional optical microscopy is a powerful tool to observe specimens under ambient atmosphere. However, its spatial resolution is restricted to ~200 nm by the diffraction limit of light. Electron beam excitation assisted (EXA) optical microscopy has been developed to overcome the restricted resolution on the label-free observation of biological specimens under ambient atmosphere [1–3]. EXA microscopy consists of an electron microscopy, an optical microscopy, and a luminescent film between the microscopy. The luminescent film plays important roles: an optical probe and a separator between vacuum and air. For the former role, an intense and optically homogeneous cathodoluminescence (CL) in area is required to the luminescent film for the optical probe of EXA microscopy. The present work employed Gd³⁺-doped YAlO₃ perovskite (YAP) thin film on LaAlO₃ (LAO) buffer layer as the luminescent thin film.

METHODS

YAP and LAO thin films were fabricated by a radio frequency magnetron sputtering method. Following deposition, the samples were annealed in a quartz tube furnace at 1273 K for an hour with heating rate of 5 K min^-1 and cooling rate of 10 K min^-1. YAP/LAO systems were fabricated onto silicon nitride (SiN) membrane on Si support. Mechanical strength of SiN membrane is sufficient to sustain an atmospheric pressure and vacuum. CL spectra were acquired by field emission scanning electron microscopy (SEM) in combination with a parabolic mirror, a monochromator, and a photomultiplier tube (PMT). On the EXA system, CL emission was collected by PMT through the objective and CL images were reconstructed by detecting the CL intensity at each spots along with electron beam scanning.

RESULTS

YAP thin film exhibits strong and sharp CL peaks at around 318 and 630 nm. These peaks originate from radiative transitions of Gd³⁺ ions [3, 4]. Firstly, YAP/LAO systems were annealed under different atmosphere such as vacuum, Ar, O₂ and N2. CL intensities of O₂- and Ar-annealed samples were stronger than those of other samples. SEM images revealed that O₂-annealed YAP/LAO system had flat surfaces compared with samples annealed under other atmospheres. Therefore, annealing atmosphere of YAP thin film was determined to O₂. CL intensity of YAP/LAO system is twice that of YAP without LAO buffer layer. In addition, pre-annealing of LAO buffer layer in vacuum prior to the YAP deposition enhanced CL intensity by 4 times compared with the sample without LAO buffer layer. These results mean that existence of LAO buffer layer and pre-annealing of the LAO layer enhance the CL intensity of YAP thin film. X-ray diffraction pattern revealed that the YAP thin film without LAO layer was amorphous in structure. On the other hand, a peak indexed as (121) of YAP appeared for YAP thin film with LAO buffer layer. Therefore, LAO buffer layer would promote the crystallization of YAP host leading to the improvement of quantum efficiency in Gd³⁺ ions.

Next, YAP/LAO system was applied to EXA microscopy. The luminescent film was deposited on the vacuum side to prevent contamination of the luminescent film from biological specimens, and vice versa. Figure show EM and CL images on the same area of the prepared luminescent film. Because luminescent area corresponds to spatial distribution of the Gd³⁺ color center, surface morphologies (e.g., surface roughness) of YAP thin film hardly affect CL mapping. Luminescence within a few micrometers is confirmed, and thus the prepared YAP/LAO thin film can be utilized as optical probe on the EXA microscopy.

REFERENCES

[1] W. Inami et al., Opt. Express 18, 12897 (2010).

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[Introduction]

Perovskite ReNiO₃ (Re=rare-earth) is known to display a metal-insulator transition with temperature change except for Re=La, and this is accompanied by a structural phase transition [1]. Furthermore, recently, it was reported that the electrical resistance of SmNiO₃(SNO) thin films increases by hydrogenation with Pt catalysts [2]. When hydrogenated ReNiO₃ is denoted as ReNiO₃H_x, this increase in resistance was thought to be due to the Mott transition, in which the eg orbital of Ni ion becomes half-filled due to the electron donation to Ni from hydrogen when x=1.

There is a previous study [3] that investigated the relationship between the hydrogen concentration and electrical resistance, but the hydrogen concentration was measured after the hydrogenated sample was exposed to air, which may have resulted in hydrogen desorption. We investigated the relationship between the resistance and hydrogen concentration by performing hydrogenation without exposing the sample to the atmosphere, and the results showed that the resistance increased for x < 0.5 and then decreased for x~1 [4]. However, a problem is that the studies used Pt catalysts, and the hydrogen concentration in the sample was not uniform.

[Problem & Purpose]

The purpose of this study is to elucidate the mechanism of hydrogen-induced changes in electrical conduction properties from the viewpoints of rare earths and hydrogen concentration. To overcome this problem, experiments without Pt catalysts were conducted to investigate the relationship between hydrogen concentration and electrical resistance.

[Experiment]

SmNiO₃ thin films epitaxially grown on an LaAlO₃ substrate by pulsed laser deposition were used in this study. Hydrogenation, electrical resistivity, and hydrogen concentration were measured in situ without exposure to air to minimize hydrogen desorption. For uniformly hydrogenate the sample, atomic hydrogen exposure was used for hydrogenation, and depth profile of hydrogen concentration was measured by nuclear reaction analysis (NRA).

[Results & Discussion]

Atomic hydrogen exposure increased the electrical resistance, and NRA experiments indicated that hydrogen concentrations were increasing. The experimental results show that the electrical resistance increases rapidly at a hydrogen concentration of x~0.5 (Figure), and the temperature dependence of the electrical resistance also changes to an Arrhenius-type one. From the analysis of the Arrhenius plot, its pre-exponential factor, and activation energy were mapped to hydrogen concentration. We will discuss the electronic mechanism by comparing the previous results with those of the previously performed results on NdNiO₃ thin films.

[References]

[1] J. B. Torrance et.al., Phys. Rev. B 45 8209(R) (1992)

[2] J. Shi et al. Nat Commun 5, 4860 (2014).

[3] J. Chen et al. Nat Commun 10, 694 (2019).

[4] I. Matsuzawa et al., Phys. Rev. Mater. in press (2023)

Femtosecond (fs) pulsed lasers exhibit efficient ablation and decrease the formation of particulates, which provides significant advantages over nanosecond (ns) pulsed lasers. Due to this, there has recently been an upsurge in interest in the use of femtosecond lasers in pulsed laser deposition (PLD). However, numerical analysis for this deposition technique is relatively scarce. Existing models to predict the stoichiometry of the thin films produced by ns-PLD include a collision-based model and peak-fitting method. However, these models were not yet tested for low-energy fs-PLD. Comparing the experimental data with the aforementioned models will help us understand the stoichiometric transfer mechanism in fs-PLD. As a result, this will improve the controllability of processing materials with complex compositions.In this work, we examined the stoichiometric models as applied with low energy fs- PLD. Specifically, this study investigates the spatial variation in the elemental ratio of deposited bismuth strontium calcium copper oxide (BSCCO), a high-temperature superconducting material. BSCCO was deposited in various substrate locations during a single deposition. The deposited thin films were characterized using Energy Dispersive X-ray analysis (EDX) to measure the amount of deposited elements. The results obtained from the experiments were then compared with three models: the collision-based model, the revised collision-based model, and the peak-fitting model. The amount of elemental atoms as measured by EDX compared to the predicted values of the models is shown in Figure 1. The left figure shows the deposited elements on various substrates placed at different positions along the x-axis. The figure on the right shows the variation along the y-axis. The x-axis showed a more significant decrease in elemental atoms than the y-axis. This is due to the use of an elliptical laser beam. As a result, the plasma plume generated during ablation is not symmetric on the two axes and spreads out more widely along the y-direction. Additionally, the deposited film exhibits fluctuations in Ca atom values. The low atomic mass of Ca results in more violent dispersion than other elements.As the substrate shifted away from the center, all numerical models displayed a declining trend. According to the results for various substrate positions, the peak fitting analysis is the closest to the experimental data among the three models. Even with a modification, the collision-based model overestimates the amount of deposited material. On the other hand, the high laser irradiation used in the model may have contributed to the overcompensation of the peak fitting method. An additional adjustment can be included in the fitting analysis for improved approximation, which provides for taking into account the elliptical form of the laser beam.

Sealing inspections and leak testing have been carried out to ensure the reliability of the products in various industrial fields such as automotive industry, refrigeration and air conditioning, food, medicine, electrical devices, plant engineering, hydrogen handling, aerospace industry, and nuclear technology. The methods include waterproof tests based on IP code, environmental testing at high temperature and humidity condition, babble testing, leak testing with pressure difference method (air leak test), helium leak test, and so on. Not only reliability of inspections/testing but also the reduction of measurement time is often required to keep the productivity of production line or to develop new products efficiently. In this session, the latest trends of these methods and data analyses are presented by nine presenters. I would like to introduce three topics before them.

First is the difference between leakage and permeation. The leakage is unexpected gas/liquid flows through leaks or pinholes on a wall which are generated by welding defects, gluing defects, sticking contaminations at sealing parts, insufficient tightening, and so on. The difference of pressure or gas concentration across the wall become a driving force of the leakage. The permeation is, on the other hand, passaging of gas through a solid barrier in which the diffusion of the gas and various surface phenomena are involved. Preventing both leakage and permeation is important to keep gas and liquid tightness well and both are discussed in this session.

Second is a recent trend that scientific explanations and validations for the reliability of sealing products are strongly required. The medical field, for example, considers that complete packaging is to prevent the ingress of microorganisms and to prevent the quality deterioration of the product due to the ingress/transfer of gas/other substances by conforming to the maximum allowable leakage limit of individual preparation packaging, and the product should be ensured to meet physicochemical and microbiological specifications by data [1,2]. Analyses of the results of sealing inspections and leak testing based on fluid dynamics, gas molecular dynamics, diffusion and permeation of gas, surface tension of liquid and so on are carried out not only in the medical field, but also in various industrial fields.

Third is the recent activity of measuring instruments field. Japan Measuring Instruments Federation (JMIF) is starting to discuss establishing a standard relating to testing method of the measuring instruments which includes sealing parts. The validation method of sealing inspection with a reference sample with a specified small hole will be standardized. Here, calculating gas flow rate through the small hole by pressure difference and by gas diffusion is necessary and the method will be stated in this standard. Such calculations and experiments by using a reference sample will be useful to determine the maximum allowable leakage limit.

Collaborating among manufacturers of sealing products, leak testing companies, and researchers in vacuum and surface science will encourage better understanding of the phenomena such as leakage and permeation and find better ways to increase the reliability of sealing products. I hope that this session will be a good opportunity to share and discuss the latest techniques and problems related to sealing inspections and leak testing.

[1] The United States Pharmacopeia (USP), <1207> Package Integrity Evaluation, Sterile Products (2016).

[2] The Ministry of health, labour and welfare, The Japanese Pharmacopoeia, seventeenth edition (2016).

Many electronic devices with internal cavity are sealed in a vacuum state or filled with inert gas to protect the internal elements and circuits so that they can operate stably over a long period of time. Hermetic tests using air, helium gas, or liquid are used to inspect the sealing performance and ensure their quality. In general, there are two main types of tests: gross leak tests to detect large defects and fine leak tests to detect minute defects. In addition, due to the recent miniaturization of electronic devices and changing circumstances, hermeticity testing methods have become more complex.

Immersion testing using organic solutions containing fluorine, called fluorine-based inert liquids, is the mainstay of gross leak testing for electronic devices, and perfluorocarbons (PFC) or perfluoropolyether (PFPE) are often used as media. These liquids are characterized by low surface tension and viscosity, and can detect minute defects better than using water.

However, most fluorine-based inert liquids are highly volatile and have a very high global warming potential, so environmental concerns have often been an issue. In addition, the availability of fluorine-based inert liquids has deteriorated due in part to the tightening of PFAS regulations. At the same time, from the perspective of SDGs, many companies are beginning to seek alternative gross leak test methods to replace the immersion test using fluorinated inert liquids.

One alternative method for the gross leak tests is the pressure decay test method¹⁾. This is commonly referred to as the air leak test and involves applying compressed air to a test specimen and monitoring the pressure change. Since the tracer gas is compressed air, this test is environmentally friendly. In the immersion test, it is visually inspected to see if bubbles are generated from any defect, and it is difficult to identify the defect size and quantitatively control the product quality through the amount of leak.

In fine leak testing, the helium leak test method, which uses helium gas as the tracer gas, is commonly used, and the bombing²⁾ test is the most common method for electronic devices. The completed devices are placed in a sealed chamber and pressurized with helium gas for a certain period of time. The pressure is then relieved and each specimen transferred to another chamber which is connected to the evacuating system and a mass-spectrometer-type leak detector. When the chamber is evacuated, any tracer gas which was previously forced into the specimen will thus be drawn out and indicated by the leak detector as a measured leak rate. This test method requires that the measurement be completed with helium gas remaining inside the test specimen with defects. The smaller the test specimen, the smaller the internal cavity and the amount of helium gas that can enter, and thus the shorter the time required for the measurement.

Although it is essential to conduct both gross leak and fine leak tests to guarantee the hermeticity of electronic devices, the results of each test cannot be generally compared because of the different measurement conditions and gas types used. Here, by converting the results of each test into equivalent standard leak rate³⁾, it is possible to compare the results of different test methods under the same conditions and express them on the same abscissa scale, for example, as shown in Figure 1. By quantifying and visualizing each test result and their measurable areas, it can be seen that the upper measurement limit of the fine leak test is above the lower measurement limit of the gross leak test, and that there must be an overlapping area between each test.

References

1) JIS Z2332:2012 Leak testing method using pressure change

2) JIS Z2331:2006 Method for helium leak testing

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Many automotive components are required to be airtight against gases and liquids, and leak testing is carried out during the manufacturing process to ensure quality. The bell jar method of helium leak testing (JIS Z 2331) is widely used for leak testing of various automotive components as it allows quantitative leak tests of 10^-4 Pam³s^-1 or less. This paper describes the performance of the vacuum chamber leak testing system and the innovations in leak testing for each specimen under test.

The Bell Jar method for detecting leaks involves placing the test object inside a vacuum chamber and pumping down the air from the chamber. After conducting a test to ensure leak-tightness, introduce helium gas into the object's interior and use a helium leak detector to detect the gas (see Fig. 1). If a leak rate higher than the specified limit is detected, a failure decision is made. We can manufacture leak testers up to the 1 × 10^-9 Pam³s^-1 range, with high process capability of master leaks(±5%,Cp≧1.33), fail-safe, no error and machine times in the tens of seconds (<30 seconds). Note: Cp (process capability). An index that quantitatively evaluates the capabilities of a process in the field of quality control. Cp=(Upper standard limit- Lower standard limit)/6σ

Note that, gas pressurisation or depressurisation of the specimen during leak testing should be the same as the actual operation of the specimen. The Bell Jar method is used to leak test pressurised and hydrostatic components by introducing helium gas above atmospheric pressure into the component to be tested. The vacuum hood method shall be used for samples with an internal pressure below atmospheric pressure and the bombing method for sealed products.

When conducting helium leakage testing, it is crucial to obtain the characteristics of each tested specimen and develop a response accordingly. Some of these devising in leak testing are described below. If the test item is made of a resin material or other material with high gas permeability, it will be difficult to determine whether the gas is diffusing through the resin material or leaking from holes or cracks when the test gas is sealed inside the test item and leaks are detected (see Fig. 2(a)). Such transmissions and leaks can be distinguished by introducing helium gas into the tested object after the leak detector has started detecting the gas. This is based on the fact that leakage has a faster response time than transmission.

When the specimen is a regulator or check valve, leakage takes place during the short time between introducing the gas into the specimen and the valve plug closing (see Fig. 2 (b)). In such a test, helium gas is introduced into the sample, the leak gas is evacuated by a vacuum pump attached to the vacuum chamber during the temporary leak, and the leak is detected by a leak detector after the leak has stopped.

Recently, high-pressure injection of gasoline and diesel oil in engines and high-pressure hydrogen in hydrogen fuel cells have been advancing. As a result, quality assurance of ultra-high pressure components has become necessary when operating under ultra-high pressure conditions. We are therefore developing a method to detect leaks using liquids, which are safer than gases when under high pressure. Furthermore, to incorporate leak testing without using helium gas - which is expensive and in unstable supply - we have devised an alternate leak testing approach that utilizes argon or nitrogen as a tracer gas instead.

Introduction

As vehicles have become more autonomous driving, the need for electronic devices, especially sensor devices, would be extremely important. Most sensor devices as MEMS are with a vacuum sealed chip. If gases get into the device (external leakage) or if outgassing remains inside the device, it causes degradation and reliability is significantly reduced. On the other hand, advanced semiconductor devices have achieved significant improvements through bonding technologies such as "wafer to wafer" and mounting technologies such as "chip let". However, outgassing between bonding or during assembly is a major concern as it affects the quality of the manufacturing process. To maintain the quality of such devices, it is necessary to measure residual or entrapped gases, in the seal or bonding area and between each layer. This presentation describes the technology for leak testing and mass gas analysis to ensure the reliability of sensor devices for automotive applications and bonding devices for semiconductors.

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

This is dummy. I am suppose to rewrite it later.

Helium leak testing is used in a variety of industries, including vacuum equipment and components, automotive, air conditioning and refrigeration, electronic devices and components, food, medical, energy, and general industry. Helium leak detectors have been developed for these various applications.

In recent years, the automotive industry has seen an increase in the number of products requiring helium leak testing due to the major shift to next generation vehicles such as electric and fuel cell vehicles. In particular, helium leak testing is increasingly being used for rechargeable batteries leak testing.

In this presentation, I will describe the latest trends in helium leak testing and helium leak test systems used in production lines such as rechargeable batteries, and report on the performance and features required of helium leak detectors.

Lithium-ion battery, hydrogen fuel cell, and new refrigerant technologies plays significant role within the framework of Sustainable Development Goals (SDGs), particularly concerning the objectives of "Affordable and Clean Energy" and "Climate Action".

While lithium-ion batteries are widely used to supply electric power to lots of things, it also importantly used to store the electrical energy generated from newly renewable energy source like solar or wind. Yet, amid their manifold advantages, lithium-ion batteries also present challenges that requires careful consideration. One notable concern pertains to electrolyte leaks, an issue that can compromise both battery performance and human safety. The electrolyte can inadvertently escape, leading to reduced efficiency and potential health hazards. The challenges of the leak testing of lithium-ion batteries are either indirect or insufficient. Conventional methods utilizing helium as a tracer gas can sometimes fall short under specific conditions. Additionally, pouch-type batteries are particularly problematic to test due to their susceptibility to expansion and rupture when placed under vacuum. Innovative leak testing solutions are being developed to address the challenges of electrolyte leaks, while flexible chamber allow pouch type battery to be tested under vacuum without expanding.

Hydrogen fuel cell is one of the promising renewable energy sources. However, Hydrogen is a flammable gas, especially if it come to contact with oxygen in certain level of concentration. To ensure the safety of the fuel cell, it is necessary to ensure the certain level of the leak for each part of the fuel cell. The bipolar plate of the fuel cell is usually tested with helium leak detector with vacuum chamber method. Bipolar plate exhibits a multifaceted structure, comprising dedicated channels for hydrogen, air or oxygen, and coolant. In addition to assessing each channel for leaks to the external environment, it is also necessary to assess for potential leaks into adjacent channels. Bipolar plates and electrode membrane assembled as a fuel cell stack and further leak tested with sniffer helium leak detector. Extending beyond the core fuel cell components, the hydrogen tank and the intricate network of fluid loops also warrant thorough scrutiny. In this regard, both hydrogen and helium sniffer leak detectors play pivotal roles, enabling the detection of any potential gas leakage from the system.

Hydrofluorocarbon (HFC) refrigerants were developed as a replacement for chlorofluorocarbon (CFC) to combat ozone layer depletion problem. While it has been a major environmental success story of ozone layer restoration, subsequent findings revealed HFCs as potent greenhouse gases, with some have global warming potential (GWP) of thousands time higher than carbon dioxide. Governments across the globe are phasing out or reducing of HFCs. Alternatives are hydrofluoroolefins (HFOs), carbon dioxide, and hydrocarbons. To prevent leaky product entering the market, leak testing for each component and assembly is paramount. At the individual part level like heat exchanger, expansion valve and compressor, usually with helium leak detector. Product can undergo further test after welding, followed by subjecting the finalized product to final leak test with the intended refrigerant gas where infrared-based sensors or quadrupole mass spectrometer-based sensors leak detector can be employed. Among the emerging alternative refrigerants, carbon dioxide has garnered substantial attention. Nevertheless, this selection presents a challenge stemming from carbon dioxide's natural background concentration of approximately 400 ppm, which is compounded by emissions from proximate equipment and human operators.

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Leak testing with tracer gas

Tracer gas is defined as “in the case of the vacuum method, gas that is sprayed onto the outer surface of the test piece to be leak tested and enters the test piece through the leaking point. In the case of pressure testing, it is introduced into the test piece to be gas that is detected after it is released from a leaking point to the outside.” In order to detect the gas, the difference in physical properties of the gas is used. Therefore, in this lecture, classification by detection method and representative leak test methods will be explained, and the latest trends in leak detectors will be introduced.

Efforts of the leak testing (LT) department at the Japanese Society Non-Destructive Inspection

This section describes the JIS / NDIS standards related to the Leak testing (LT = Leak testing within the Association) category. Next, an overview of the activities of committees held at the Association and the leak testing certification system will be introduced.

In a society aiming to be carbon neutral, hydrogen is expected to be a clean energy source, but it also has problems, such as its susceptibility to explosion and hydrogen embrittlement of metals. The questions “where is hydrogen trapped in materials and where does it cause embrittlement?” and “Where is the location of the hydrogen leak from the gas line?" can be answered by measuring the location of hydrogen. However, as is well known, that it is difficult to specify the location of hydrogen. Operando hydrogen microscope (OHM) is a hydrogen visualization equipment using electron stimulated desorption (ESD) method [1,2]. In the equipment, permeated hydrogen atoms through the sample membrane are ionized by incident electron on the sample surface, desorbed into UHV environment and detected. By using the electron source of a scanning electron microscope (SEM) as the excitation source for the ESD, we visualize the surface hydrogen distribution from the hydrogen desorption position.

By OHM, we measured the diffusion coefficients of austenite-dominant regions and martensite-dominant regions in stainless steel, cold-worked dual-phase SUS304, based on changes in the amounts of hydrogen permeating time from the backside of the sample [3]. We used electron backscattering diffraction (EBSD) to determine the martensite/austenite ratio and compare the structure distribution and hydrogen distribution measured in OHM. Then, we were able to create a model of hydrogen diffusion through a dual-phase region [4]. We also observed the effect of surface film to reduce hydrogen desorption. We have created a chromium oxide film on the surface of SUS316 steel and confirmed that the release of hydrogen inside the material was suppressed to less than half. We also found that there was hydrogen release from hole-like-defects where the chromium oxide was incomplete [5]. This study found that the outgassing from the chromium oxide surfaces was not uniform, but from specific locations. In this way, OHM measurements can also be used to identify the location of gas leaks, not averaged outgassing.

The standard conductance element (SCE) is a filter of open type standard leak, and it uses sintered stainless steel as a leak medium [6]. We attempted to visualize where the gas flows in SCE, regarding the pores of the sintered body as the gas leakage points of the SCE. The figure shows SEM picture of the surface of the SCE (SCE-ICF34-04-S1, Puaron Japan co.,ltd) (left) and hydrogen distribution at the same position with the SEM (right), at the conditions of hydrogen supplying pressure of 100 Pa, SCE temperature of 373 K and ESD signals integrated 3024 maps in 168 hours. High concentration of hydrogen was measured on the pore edge area of the micro particle surface of sintered body that constitutes the SCE. We assume that the hydrogen as the adsorption source is only from the hydrogen gas flowed through the filter pores and it adsorbs with the same adsorption ratio on the SCE surface and inner surface of the pores. The hydrogen pressure inside the pore and near the exhaust port of the pore is higher than the hydrogen pressure in the measurement chamber. It is considered that the hydrogen adsorbed on the way through the pore and desorbed by ESD around the SCE pores. This is a reason why the signal is increasing at around the SCE pore. It will be possible to confirm with OHM how hydrogen flowed through the leak pores.

[1] N. Miyauchi, et al., Scripta Materialia 144 (2018) 69

[2] A. Nakamura, et al., J.P. Patent 06796275, 2020

[3] N. Miyauchi, et al., Applied Surface Science 527 (2020) 146710

[4] A. N. Itakura, et al., Scientific Reports 11 (2021) 8553

[5] N. Miyauchi, et al., Applied Surface Science 492 (2019) 280

[6] H. Yoshida, et al., Vacuum, 86 (2012) 838

Today, people are concerned about energy sources. Fossil fuels are limited and have an impact on the environment. Discussions in society and at scientific meetings often focus on how to produce energy in a sustainable way. On the other hand, reducing energy losses is equally important to regulate energy demand in our society.

One of the main sources of energy loss is friction. Liquid lubricants are used to reduce friction when two solid objects slide over each other. Most lubricants are low-vapor-pressure hydrocarbons modified with a small amount of polar compounds such as carboxylic acid. The polar modifiers are adsorbed onto the surface of the sliding solids to form monomolecular [1] or multimolecular [2] layers. The adsorbed layers and the hydrocarbon fluid covering the layers form an easily sheared film to minimize adhesion and wear.

In the present study, we observed the mechanical response at lubricant-solid interfaces using an atomic force microscope (Shimadzu, SPM-8100FM). As model lubricants, liquid hexadecane (C₁₆H₃₄) was modified with palmitic acid (C₁₅H₃₁COOH) of 0-0.5 weight%. (110)-oriented rutile TiO₂ wafers were prepared by annealing in air at 800°C for 10 h. Impurities segregated on the surface of annealed wafers were removed by etching with sulfuric acid (95.0+ %). Additional annealing provided atomically flat (110) terraces separated with single height steps on the etched wafers (Fig. 1).

In model lubricants containing 0.5 wt% palmitic acid, round domains were sometimes observed on the TiO₂ terraces. Systematic observations have being made to identify palmitic acid concentrations that are favorable or unfavorable for domain formation.

In addition to topographic imaging, force curves were obtained in the model lubricants. The force pushing the AFM tip away from the TiO₂ surface was quantified in the range of 0-100 nN as a function of tip-surface distance. The force response observed in pure hexadecane showed smooth curves that could be fitted with the Hertz model based on a double layer interface, lubricant over TiO₂. In the lubricants modified with palmitic acid (> 0.1 wt%), the observed force curves were bent with a break. The bent curves are attributed to the mechanical response of the triple layers; lubricant over adsorbed palmitic acid on top of TiO₂.

This study was supported by JSPS KAKENHI grant numbers 21K18935 and 23H05448. This abstract was English edited with DeepL Write beta.

[1] W. B. Hardy, I. Doubleday, Proc. Royal Soc. London. A 100, 550 (1922).

[2] C. M. Allen, E. Drauglis, E. Wear 14, 363 (1969).

The analysis of the diradical state of functional open-shell molecules is important for understanding their physical properties and chemical reactivity [1]. The diradical character is an important factor in the functional elucidation and design of open-shell molecules. In recent years, attempts have been made to immobilise functional open-shell molecules on surfaces to form devices [1]. However, the influence of surface interactions on the diradical state remains unclear. In this study, approximate spin-projected density functional theory calculations with dispersion correction and plane-wave basis (AP-DFT-D3/plane-wave calculations) were performed using physisorption models (p-benzyne/MgO(001) and p-benzyne/SrO(001)) to investigate variations in the diradical character caused by physisorption. A decrease in the diradical character caused by molecular distortion, variation in the diradical character caused by intermolecular interaction, and decrease in the diradical character caused by interaction with the surface were identified, resulting in a decrease in the diradical character in all models. This is different from the s-electron systems reported in previous studies [2,3], where the diradical character increased by adsorption. This is because the singly occupied molecular orbitals of p-benzyne have a horizontal distribution with respect to the surface and p-benzyne is a through-bonded diradical. The energy change due to molecular cohesion in the adsorption model was small, whereas the change in the diradical character due to cohesion was significant. This implies that it is difficult to immobilise diradicals on surfaces in a completely isolated state. In other words, it is a manifestation of the ability to tailor molecular functions derived from the diradical state by changing the adsorption structure and self-assembly by the surfaces. Furthermore, the interaction with the surfaces induces electron delocalisation to π-conjugated orbitals and intramolecular charge polarisation. This indicates that the contribution of the electron configurations, which are too small in the gas phase, is amplified by the surface interactions. Hence, the stability between the phases of materials can be tuned through inert ionic surface interactions. The difficulty of studying excited states by DFT methods requires more accurate methods, such as multi-referenced approaches using cluster models, to investigate in detail the tuning of open-shell electronic states by adsorbing on a surface. The present study is regarded as an elementary demonstration. We explicitly showed that in a real molecule, the effects of surface interactions on the diradical state are not negligible and that the theoretical design of molecular devices with functional open-shell molecules immobilised on the surface requires a detailed investigation of the surface adsorption effects in terms of their diradical states.

[1] T. Stuyver, B. Chen, T. Zeng, P. Geerlings, F. De Proft and R. Hoffinann, Chem. Rev., 2019, 119, 11291–11351.

[2] K. Fujimaru, K. Tada, H. Ozaki, M. Okumura and S. Tanaka, Suf. Interfaces, 2022, 33, 102206.

[3] K. Tada, H. Ozaki, K. Fujimaru, Y. Kitagawa, T. Kawakami and M. Okumura, Phys. Chem. Chem. Phys., 2021, 23, 25024–25028.

Introduction

Schottky electron sources are widely used as electron sources for high-resolution electron microscopes. Since electron emission is achieved by a combination of heat and electric field, tungsten, which has a high melting point, is used as the electron source material. On the other hand, because tungsten has a high work function of approximately 4.5 eV, its surface is coated with Zr-O to lower the work function. The Zr-O is supplied to the electron source by thermal diffusion from a ZrO₂ reservoir which is located several hundred micrometers away from the emission area. Although the coverage of Zr-O is closely related to the lowering of the work function, i.e., the performance of the electron source, the detailed diffusion mechanism is not understood. In this study, the diffusion state of Zr was investigated using AES.

Experimental

We investigated the diffusion of Zr and O at the electron emitting surface, W(100). A single crystal W(100) cut to 2×0.2×0.1 mm and ZrO₂ sintered in the middle was used as the measurement sample. The sample was heated in vacuum for more than 12 hours, and after quenching, the amount of Zr was measured using AES. The diffusion distance of Zr was investigated by measuring at different distances from the ZrO₂ sintered material. The heating temperature was varied around 1800 K, which is used for actual electron source, and in the range of 1600-2000 K to investigate the diffusion distance. The diffusion distance was also examined in terms of the dependence of the diffusion distance on the atmospheric vacuum by adjusting the sample by changing the vacuum level to 10^-8, 10^-7, and 10^-6 Pa.

Results and discussions

Figure 1(A) shows the measured AES spectrum. In the AES spectrum, signals from W of the base material and diffused Zr were observed. As the distance from the ZrO₂ reservoir increased, the Zr signal became weaker, and the W signal became stronger. Figure 1(B) shows a relationship between the distance from the ZrO₂ reservoir and the peak intensity ratio of Zr and W. The coverage is constant below 600 µm, but once the coverage becomes low, it is found to decrease rapidly. This behavior cannot be described by a simple equation consisting of a balance between diffusion and evaporation. One possible mechanism is that evaporation is suppressed in areas of high coverage and is stimulated when coverage decreases. Detailed mechanism will be reported in the presentation. The results of the heating temperature dependence showed that below 1800 K, there was no change in the coverage and diffusion was about 600 µm. However, as the temperature was increased to 1900K and 2000K, the coverage decreased immediately after the constant region disappeared. This behavior can be expressed by the diffusion-evaporation equation, and evaporation was dominant at these temperatures. The vacuum dependence of the Zr coverage was investigated, and it was found that the Zr diffusion was inhibited under poor vacuum conditions, as the coverage was no longer a constant region.

Summary

・We investigated Zr diffusion on W(100) surface by using AES.

・The diffusion of Zr cannot described simple equation based on diffusion-evaporation balance.

・The temperature and vacuum dependence of Zr coverage was also investigated. Diffusion of zirconia is inhibited at higher temperatures and poorer vacuum.

Reference

S. Matsunaga et al., J. Vac. Sci. Technol. B 39, 062806 (2021)

Astatine-211 has been touted as a promising radionuclide for cancer treatment, and experiments have been done to show is effectiveness in treating cancer [1,2]. Many experiments bind ²¹¹At to a carrier, most notably gold nanoparticles, to allow the targeting of cancerous areas via cancer-specific targeting molecules. In our previous works, we have studied the adsorption of astatine on gold and found that astatine absorbs stronger to gold than iodine [3]. This is interesting since generally, adsorption of elements of the same group lower down in the periodic table yields smaller adsorption energy [4].

An interesting note is that since astatine is almost at the bottom of the halogen group, it has electrons in the 4f orbital. Unlike the lighter halogens, the presence of 4f orbital induces lanthanoid shrinkage, causing the atomic radius of elements with electtrons in the orbital to become smaller. The effect is noted to go beyond the lanthanoid series, as the subsequent elements also have smaller atomic radii. This lanthanoid shrinkage is also responsible for the difference in adsorption trend of astatine on gold compared to iodine on gold. Iodine, which has no electrons in the 4f orbital, does not get the shrinking effect. More details will be presented in the meeting.

References

[1] L Dziawer, P. Kozminski, S. Mecznska-Wielgosz, M. Pruszynski, M. Lyczko, B. Was, G. Celichowski, J. Grobelny, J. Jastrzebski, A. Bilewicz, RSC Adv. 7, 41024 (2017).

[2] H. Kato, X. Huang, Y. Kadonaga, D. Katayama, K. Ooe, A. Shimoyama, K. Kabayama, A. Toyoshima, A. Shinohara, J. Hatazawa, K. Fukase, J. Nanobiotechnol 19, 223 (2021).

[3] J. Tanudji, S. M. Aspera, H. Kasai, e-J. Surf. Sci. Nanotech. 21 (2023).

[4] I. A. Pasti, S. V. Mentos, Electrochemical Acta 55, 1995 (2010).

The surface electrical conductivity is affected by the local defect structures, such as atomic defects and steps. Since the coherence length of electrons becomes longer, quantum and non-local phenomena such as electron localization and confinement effects, in which the properties of electrons as waves become prominent, are expected at low temperature. In order to study these phenomena in nanometer scale, scanning tunneling potentiometry (STP) has been developed, which allows us to directly access the surface transport properties in real-space imaging. This microscope, which is based on scanning tunneling microscopy (STM), enables us to make an image of topograph and electrochemical potential of a sample surface simultaneously with atomic scale special resolution and uV level high potential sensitivity under the current flowing parallel to the sample surface. An STP is thus quite powerful for elucidating the mechanism of the surface conductivity and local distribution of resistance in the atomic scale. So far, however, low-temperature STP have not been performed so much on clean surfaces that hold metallic surface states, which can be fabricated by the deposition of metal elements on semiconducting substrates. In spite of expected stable performance and reduced leak current through the substrate, which makes the data analysis simple, LT-STM was hard to be operated because of technical difficulties.

In order to investigate the surface state conductivity and to capture their quantum and non-local phenomena, we have developed low-temperature (LT) ultrahigh-vacuum (UHV) STP on a step-by-step basis. Firstly, by implementing an STP circuit we developed [1,2] to an existing room-temperature(RT) UHV-STM system, and the STP observations on the Si(111)-7x7 surface at RT have been achieved [2]. At the next step, we improved the circuit and installed it to an existing LT-UHV-STM whose lowest temperature is ~4.2K and succeeded in STP measurements on the striped incommensurate (SIC) phase of Pb/Si(111) surface at T=18.4K (Fig.1). In this presentation, we'll report on the progress of the development of LT-UHV-STP.

[1] M. Hamada and Y. Hasegawa: Jpn. J. Appl. Phys. 51 (2012) 125202.

[2] M. Hamada and Y. Hasegawa: Phys. Rev. B 99, 125402 (2019).

It has been known that Pb monolayers on Si(111) shows various phases depending on nominal coverages. When the nominal coverage is higher than 1.2ML, it shows superconducting transition [1]. Moreover, it has been known that the behavior of superconducting vortices at steps depends on the coverage. For √3 × √43 reconstructed phases, whose nominal coverage is 1.23ML, vortices are repelled by steps [2], suggesting that superconducting properties are strongly separated by steps. On the other hand, for striped incommensurate (SIC) phases, whose coverage is 1.33ML, vortices are just trapped by steps keeping their original circular shape [3], suggesting weaker separation by steps. These facts indicate that the difference of small amount of coverage makes significant influence on the superconducting states, and thus it is quite important to understand local conductivities of steps and terraces on these surfaces.

Scanning tunneling potentiometry (STP) is a powerful tool to measure the surface conductivity. STP allows us to measure electrochemical potential in nanometer scale during flowing in-plane current. Recently we have developed low temperature STP (LT-STP) systems. We used low conductive Si(111) as a substrate in order to negate the conductivity through the substrate. As electrodes to flow the in-plane current, we deposited Ta pads on the Si(111) substrate before loading into the UHV chamber.

Here in this research, we measured surface conductivity on several coverages of Pb monolayers on Si(111). From the results, we found that the step conductivity is modified by small difference of the coverages. We will show the details of the measurements in our presentation.

References:

[1] T. Zhang, et al., Nature physics 6, 104 (2010).

[2] F. Oguro, M. Haze et al., Physical Review B 103, 085416 (2021).

[3] Y. Sato, M. Haze et al., Physical Review Letters 130, 106002 (2023).

We observed the light emission from the indium ultra-thin films induced by scanning tunneling microscopy (STM) and revealed its process, combining the scanning tunneling spectroscopy (STS).

STM is used for researching surface structures, electronic properties, and reactions, while providing information on electro luminescence as a metal-insulator-metal junction[1]. STM-induced luminescence(STML) studies on bulk metal surfaces were reported and its process is understood by the radiative decay of the tip-surface junction plasmon excited by inelastic tunneling. It is of great importance to investigate the effect of altering the sample metal to a thin film because quantum size effects are expected to affect the electronic properties and thus the nature of the light emission. In this study, we created In thin films on Si(1119 substrate and investigated the luminescence properties on them.

We prepared √7×√3 In/Si(111) superstructure which is known to indium double layer structure[2]. We also prepared three-eight atomic layer films layer by layer, depositing to the double layer structure at low temperature. Each film extended over tens of nanometers and exhibited √3×√3 periodicity in STM image, indicating that atomically flat In(111) film is formed.

STS spectra acquired at each film showed some peaks. Analyzing with phase quantization rules[3], it is revealed that the peaks are originated from quantum well states, which come from the confinement of indium sp electrons in direction to the film thickness.

STML from the films was observed and its peaks presented bias voltage and film thickness dependence peculiar to thin films. STML spectra also exhibited the tip shape dependence of intensity distributions. Combining with the STS results, the light emission is attributed to the radiative decay of the junction plasmon which is excited by inelastic tunneling to the QWSs.

[1] R. Berndt, et al., Phys. Rev. Lett., 67, 3796 (1991)

[2] T. Shirasawa, et al., Phys. Rev. B., 99, 100502(R) (2019)

[3] M. Milun, et al., Rep. Prog. Phys., 65, 99 (2002)

Spin injection into materials by transferring spin angular momentum of circularly polarized light (CPL) attracts much attention because it can be detected without an external electric/magnetic field at room temperature. In materials having strong spin-orbit interaction (SOI) with spin-splitting bands, asymmetric electron excitation occurs due to the spin-selective excitations by CPL. Such excited carriers form helicity-dependent photocurrent (HDP). HDP has been demonstrated in topological insulators and Rashba surface/interface systems so far [1-5]. Pure spin current can also be induced by CPL under some conditions, which can be detected electrically by the inverse spin Hall effect.

In this report, we observe HDP induced by irradiating infrared CPL laser (λ = 1550 nm) in an atomic bilayer material, Si(111)-√3×√3-(Tl, Sn) surface superstructure, which has spin-splitting surface bands due to giant Rashba effect [6]. We fabricated the sample by molecular beam epitaxy (MBE) method in an ultra-high vacuum (UHV) chamber and then measured it in situ electrically with irradiating the laser (Fig. 1(a)). HDP, detected with clamp electrodes at both ends of the sample (y-direction), increased with the laser spot going to both edges of the sample in the x-direction (Fig. 1(a)), and the sign of HDP reversed between right and left edges. HDP became larger when CPL was irradiated more obliquely, that is, the in-plane component of the injected spin increased (Fig. 1(b), (c)). These behaviors are unusual because in previous reports [1,7] enhancement and reversal of HDP at sample edges occur with out-of-plane spin component and HDP should decrease when CPL is obliquely illuminated. Here, we have successfully explained such behaviors as a superposition phenomena by precession of electrons’ spin due to the strong SOI and spin Hall effect at both edges of the sample.

References

[1] D. Fan et al., Phys. Rev. Res. 2, 023055 (2020). [2] K. N. Okada et al., Phys. Rev. B 93, 081403(R) (2016). [3] W. Wu et al., Opt. Express 30, 15085-15095 (2022). [4] S. D. Ganichev and W. Prettl, J. Phys. Cond. Mat. 15, R935 (2003). [5] I. Taniuchi et al., arXiv:2308.02485 (2023). [6] D. V. Gruznev et al., Phys. Rev. B 91, 035421 (2015). [7] J. Yu et al., Nano Lett. 17, 12, 7878–7885 (2017).

The surface states of topological insulators are characterized by a distinctive linear band dispersion. Under the influence of a warping effect, some of these states can be deformed into either a hexagonal [1] or hexagram [2] shape as their energy deviates from the Dirac point. Consequently, the physical properties of topological insulators are commonly analyzed in the context of these distorted surface states.

In the present study, we investigated the electronic structure of Bi-doped PbSb₂Te₄ [Pb(Bi_0.2Sb_0.8)₂Te₄] through the observations of quasiparticle interference (QPI) in scanning tunneling microscopy/spectroscopy [Figs. 1(a) and 1(b)] [4]. Analyzing the QPI patterns uncovered that the QPI dispersions exhibit an almost linear shape, with the Dirac point positioned within the bulk band gap, as illustrated in Fig. 1(c). Through QPI simulations employing the T-matrix formalism [5] and leveraging a surface state deduced from density functional theory calculations, we ascertained that the Dirac cone of the topological surface state, particularly in energy regions proximate to the bulk states, not only undergoes deformation due to the warping effect but also experiences a significant reduction in its density of states at the vertices of the hexagonal Dirac cone [Fig. 1(d)]. This diminishment is attributed to pronounced hybridization with bulk states. The detailed results will be discussed in the presentation.

References

[1] Y. L. Chen et al., Science 329, 659 (2010).

[2] Y. L. Chen et al., Science 325, 178 (2009).

[3] L. Fu, Phys. Rev. Lett. 103, 266801 (2009).

[4] Y. Hattori, K. Sagisaka, S. Yoshizawa, Y. Tokumoto, K. Edagawa, accepted in Phys. Rev. B.

[5] Y. Kosaka et al., Phys. Rev B 95, 115307 (2017).

Bi₂Se₃ has a layered structure with a unit of -Se-Bi-Se-Bi-Se- five layers (one quintuple layer (QL)) and the QLs are weakly bound together by van der Waals forces. Bi₂Se₃ is one of the typical three-dimensional topological insulators, which have a bulk band gap with protected surface states on their surface due to the spin-orbit interaction and time-reversal symmetry. There exist spin-momentum-locked surface states, which constrain the spin orientation perpendicular to the electron momentum. When a spin current flows through the surface state, the spin current is converted into an electric current, which is known as the inverse Edelstein effect and is expected to have applications in spintronics. We have developed a spin-polarized atomic hydrogen beam [1] that can select a single electron spin state. This beam is expected to enable directly inject spin currents on topological insulator surfaces without a ferromagnetic interface as in spin pumping.

First, we conducted the analyses of structural changes and hydrogen desorption characteristics of atomic hydrogen exposed Bi₂Se₃ thin films. Bi₂Se₃ thin films with a thickness of about 100 nm were epitaxially grown on Si(111) substrates by MBE [2] and exposed to atomic hydrogen at 300 K and 90 K. LEED results showed that the intensity of the spots decreased after atomic hydrogen exposure, which indicated that the crystallinity decreased. By annealing at around 570K, the crystallinity was restored. LEED I-V and the spot-to-spot distance showed no interlayer and intralayer relaxation in the Bi₂Se₃ thin film. The TDS spectrum of Bi₂Se₃ films after the atomic hydrogen exposure showed desorption peaks of hydrogen selenide and a small amount of bismuth (hydride) in addition to hydrogen. The results indicated that exposure to atomic hydrogen caused the etching of the Bi₂Se₃ thin film. In the case of atomic hydrogen exposure at 295 K, one desorption peak was observed, whereas at 90 K, an additional peak appeared on the low temperature side. Based on the LEED and TDS results and the previous theoretical study [2], it is suggested that atomic hydrogen exposure to Bi₂Se₃ at 295 K results in hydrogen adsorption only on the surface, while at 90 K, hydrogen intercalation between the QLs occurred in addition to hydrogen adsorption. Finally, we report on the results of spin injection into Bi₂Se₃ thin films using a spin-polarized hydrogen atom beam.

References

[1] Y. Nagaya, et. al., J. Chem. Phys. 155, 194201 (2021).

[2] Lee, K.W, Lee, C.E. Sci Rep 10, 14504 (2020).

Two-dimensional (2D) materials formed epitaxially on single-crystal surfaces can have stress domains originating from the lattice mismatch between the lattices of 2D material and substrate surface. Epitaxial silicene sheet formed spontaneously on epitaxial ZrB₂(0001) thin film grown on Si(111) substrate has a characteristic domain structure [1], which also seems to exist in those formed by Si deposition on ZrB₂(0001) single crystal surface [2]. Scanning tunneling microscope (STM) observations of ZrB₂(0001) thin film surface under ultrahigh vacuum, after natural oxide removal, reveal stripe domains with width of approximately 3 nm [1]. The stripe domain boundaries running along arm-chair directions in silicene honeycomb lattice are part of the continuous, buckled honeycomb lattice which contain partial dislocations [3]. The transformation process of this stripe domains into a “single-domain” through adsorption of a small amount of silicon atoms [4] was monitored by in-situ real time STM observations at room temperature to investigate how dislocations react and eventually annihilate in a strained, buckled honeycomb lattice. The observation revealed the stepwise reactions of partial dislocations leading to the nucleation of a single-domain island, followed by the island extension through propagation of edge dislocations at its frontiers. The identification of dislocation annihilation process in epitaxial silicene sheet provides insights into how crystallographic defects can be healed in 2D materials.

[1] A. Fleurence et al., Phys. Rev. Lett., 108, 245501 (2012).

[2] T. Aizawa, S. Suehara, and S. Otani, J. Phys.: Condens. Matter, 27, 305002 (2015).

[3] A. Fleurence and Y. Yamada-Takamura, 2D Mater., 8, 045011 (2021).

[4] A. Fleurence et al., App. Phys. Lett., 108, 151902 (2016).

This talk reports the structure determination of two-dimensional (2D) materials fabricated on substrate surfaces using a positron beam.

Total-reflection high-energy positron diffraction (TRHEPD) is a surface structure analysis tool by utilizing the positive charge of the positron (Fig. 1) [1-4]. The positron, the antiparticle of the electron, has the same mass, elementary charge, and spin as the electron, but the positive charge, opposite to the electron. The crystal potential of every materials acts as a potential barrier to the positron beam. Especially at low grazing incidence, the total reflection occurs. The critical angle for total reflection can be estimated via Snell's law. For example, when a positron beam with an energy of 10 keV is incident on a Si surface, the critical angle corresponds to 2.0°. Under total reflection conditions, the penetration depth of the positron beam is less than approximately 0.5 Å, which is comparable to the thickness of one atomic layer. Thus, the spot intensities in the diffraction pattern under total reflection conditions contain information about the atomic configurations of the topmost layer only. Beyond the critical angle, the penetration depth of the positron beam gradually increases with increasing glancing angle. Hence, information about the underlying layers are involved in the spot intensities at higher glancing angles. Through rocking curve (spot intensity versus glancing angle) analysis including these glancing angle regions, TRHEPD enables to determine the atomic coordinates of 2D materials and the interface layer with the substrate.

In this talk, we will show the recent results of the structure determination of quasicrystal graphene [5] and intercalated graphene, together with the origin of the surface sensitivity of TRHEPD method.

References

[1] A. Ichimiya, Solid State Phenom. 28&29, 143 (1992).

[2] A. Kawasuso and S. Okada, Phys. Rev. Lett. 81, 2695 (1998).

[3] Y. Fukaya, A. Kawasuso, A. Ichimiya, and T. Hyodo, J. Phys. D: Appl. Phys. 52, 013002 (2019).

[4] Monatomic Two-Dimensional Layers: Modern Experimental Approaches for Structure, Properties, and Industrial Use, edited by I. Matsuda (Elsevier, Amsterdam, 2019).

[5] Y. Fukaya, Y. Zhao, H.-W. Kim, J. R. Ahn, H. Fukidome, and I. Matsuda, Phys. Rev. B 104, L180202 (2021).

Silicene and germanene are novel two-dimensional honeycomb sheets composed of Si and Ge, respectively [1]. Theoretical studies have shown that the electronic states of free-standing silicene and germanene, as well as their ribbons, possess interesting electronic properties similar to graphene, such as Dirac cones [2]. On the other hand, these materials prefer to form sp₃-like hybridized orbitals rather than sp₂ orbitals, and thus they have a low buckling structure that differs from graphene. Owing to this low buckling structure, the energy gap can be tuned by an external electric field perpendicular to the surface [3,4]. Furthermore, these materials have larger spin–orbit interactions than graphene. Therefore, it is expected that the quantum spin Hall effect can be observed in these materials at experimentally feasible temperatures [5]. As stated above, these two-dimensional honeycomb sheets have very interesting electronic properties. Therefore, these materials are currently attracting a great deal of attention as future nanoelectronic devices of atomic layer thicknesses.

Silicene and germanene are generally fabricated on metal substrates [6-8]. However, it has been reported that the electronic states of silicene on metal substrates can be dramatically altered by strong interactions with surface atoms [9,10]. Accordingly, it is difficult to measure the intrinsic electronic properties. A hydrogenation of CaSi₂ [11] or CaGe₂ [12] crystal is one of the most promising methods to fabricate a free-standing silicene or germanene. The hydrogenation yields crystals composed of hydrogenated silicene (silicane) or germanene (germanane), and the free-standing silicane/germanane can be easily exfoliated from the crystal because the interlayer interaction is a van der Waals interaction. In this presentation, we report on the hydrogen desorption characteristics of monolayer and multilayer silicane and germanane crystals. The calculations show a way to create free-standing silicene/germanene from monolayer silicane/ germanane [13,14].

Hydrogenated silicene (silicane) and germanene (germanane) have relatively large energy gaps and exhibit the usual semiconducting electronic states. On the other hand, silicene and germanene are equivalent to a single bilayer on (111) plane of the bulk structure without hydrogen termination, and it is well known that they exhibit topological electronic states. Then, what kind of electronic states do the ultra-thin crystals stacked in (111) direction have? In this presentation, I will also present the results on the electronic structures of the ultra-thin film crystals using the first-principles calculations [15].

References

[1] K. Takeda and K. Shiraishi, Phys. Rev. B 50, 14916 (1994). [2] S. Cahangirov et al., Phys. Rev. Lett. 102, 236804 (2009). [3] M. Ezawa, New J. Phys. 14, 033003 (2012). [4] A. Hattori et al., J. Phys.: Condens. Matter 31, 105302 (2019). [5] C.-C. Liu, W. Feng, and Y. Yao, Phys. Rev. Lett. 107, 076802 (2011). [6] P. Vogt et al., Phys. Rev. Lett. 108, 155501 (2012). [7] A. Fleurence et al., Phys. Rev. Lett. 108, 245501 (2012). [8] J. Yuhara et al., 2D Mater. 8, 045039 (2021). [9] Z.-X. Guo et al., J. Phys. Soc. Jpn. 82, 063714 (2013). [10] M. X. Chen and M. Weinert, Nano Lett. 14, 5189 (2014). [11] H. Nakano and T. Ikuno, Appl. Phys. Rev. 3, 040803 (2016). [12] E. Bianco et al., ACS Nano 7, 4414 (2013). [13] M. Araidai, M. Itoh, M. Kurosawa, A. Ohta, and K. Shiraishi, J. Appl. Phys. 128, 125301 (2020). [14] M. Itoh, M. Araidai, A. Ohta, O. Nakatsuka, and M. Kurosawa, Jpn. J. Appl. Phys. 61, SC1048 (2022). [15] D. Ishihara, M. Araidai, A. Yamakage, and K. Shiraishi, in preparation.

Germanene is a two-dimensional (2D) sheet of Ge atoms arranged in a corrugated honeycomb lattice. The spin-orbit coupling opens a sizable bandgap between the Dirac cones, which could make germanene behave as a 2D topological insulator (a quantum spin Hall insulator) at experimentally accessible temperatures. In addition, its high carrier mobility as well as compatibility with the Si technology make germanene a promising candidate for electronics applications. However, lack of a layered form of Ge prevents germanene from being produced by exfoliation. So far, germanene has been mainly synthesized by Ge deposition on various substrates including Ag(111), and it can also be grown by Ge segregation on Ag(111) thin films from the Ge(111) substrate [1]. However, there seems to be controversy about the interpretation of the Ge-induced surface structures formed on Ag(111) [2,3]. High solvability of Ge in Ag causes Ag-Ge surface alloy, which complicates the structural determination.

Raman spectroscopy is a versatile tool to investigate 2D materials, and is expected to provide important clues to resolve the above controversy. However, germanene’s Raman spectra cannot be measured in air due to fast oxidation. In this paper, we first investigate formation processes of different surface structures on Ag(111) thin films during Ge segregation using low-energy electron microscopy (LEEM). Based on this knowledge, we fabricate various Ge-induced surface structures and identify their vibrational properties using ultrahigh vacuum Raman spectroscopy [4].

Figures 1(a)-1(c) show LEEM images during annealing the Ar-sputtered Ag(111)/Ge(111) sample at around 200 °C. In Fig. 1(d), the same region was imaged by the dark-field mode using the first-order diffraction spot of Ag(111). The bright and dark regions are Ag grains with different orientations. In Figs. 1(a)-1(c), the dark contrast around the grain boundaries corresponds to the “√3×√3” phase. Its spread from the grain boundaries indicates preferential diffusion of Ge atoms through them. The continuous annealing led the surface structure to develop into the √3×6√3 striped phase. Further annealing at higher temperatures caused the striped phase to change into the so-called disordered hexagonal or quasi-freestanding phases. When the sample was annealed at even higher temperatures like 600 °C, three-dimensional Ge islands were formed on the surface. Annealing at the highest temperature just before the Ge island formation caused the (7√7×7√7)R19.1° structure after cooling to room temperature.

Raman spectra of the striped phase are featureless in the range of 100-300 cm^-1, indicating that the striped phase is not germanene, but Ag-Ge surface alloy. Disordered hexagonal or quasi-freestanding phases showed Raman peaks assignable to germanene. The germanene Raman peaks intensified and shifted to higher wavenumbers as the annealing temperature increased, and the spectral shape continuously changed into that of the (7√7×7√7)R19.1° structure along with the emergence of new peaks. The peak shift could be due to the reduction of the tensile strain in germanene, and the zone folding of the phonon dispersion could cause the multiple peaks.

References

[1] J. Yuhara, H. Shimazu, K. Ito, A. Ohta, M. Araidai, M. Kurosawa, M. Nakatake, and G. Le Lay, ACS Nano 12, 11632 (2018).

[2] C.-H. Lin, A. Huang, W. W. Pai, W.-C. Chen, T.-Y. Chen, T.-R. Chang, R. Yukawa, C.-M. Cheng, C.-Y. Mou, I. Matsuda, T.-C. Chiang, H.-T. Jeng, and S.-J. Tang, Phys. Rev. Mater. 2, 024003 (2018).

[3] K. Zhang, R. Bernard, Y. Borensztein, H. Cruguel, and G. Prevot, Phys. Rev. B 102, 125418 (2020).

[4] S. Mizuno, A. Ohta, T. Suzuki, H. Kageshima, J. Yuhara, and H. Hibino, Appl. Phys. Express 14, 125501 (2021).

Graphene has a unique electronic band structure, called the Dirac cone, and is being studied as a next-generation semiconductor material owing to its high carrier mobility. [1] However, there is a major problem in using graphene as a device material because it has no bandgap. In the past decade, other monolayer sheets of group IV materials such as silicon (Si), germanium (Ge) and Tin (Sn) were also experimentally formed. Monolayer honeycomb sheet of Ge, called germanene, were predicted to have very curious physical properties superior to graphene. In this study, We first report on the atomic and electronic band structure of germanene formed on Al(111) substrate measured using low-energy ion scattering spectroscopy and angle-resolved photo emission spectroscopy, respectively. Although germanene [3] have been reported and attracted large interests of researchers, they can exist only on certain substrates and in a vacuum condition. On the other hands, Si and Ge sheets terminated by hydrogen or functional groups, called silicane and germanane, are stable even in air while the physical properties are quite different from those in bulk Si and Ge. As the second topic, we report on the electrical characteristics of hydrogenated germanane (GeH) and methylated germanane (GeCH₃) which are semiconductor of about 1.6 eV and 1.8 eV direct bandgaps, respectively, even in a multilayered structure. We also report the photocurrent characteristics of GeCH₃ thin film by illumination of monochromatic light of various wavelengths.

Solid surfaces have been significant research targets to elucidate our fundamental understanding of physics and chemistry that have led to developments of our technology, such as electronics and catalysis. The unique environment has also allowed us to discover novel interface materials that do not exist in nature. For example, three-dimensional (3D) diamond silicon becomes two-dimensional (2D) silicene [1] after deposition onto a metal substrate. A bilayer ice, known to form only under high-pressure, can be generated in vacuum on crystal surfaces [2]. Such exotic materials have drastically renewed our aspects in surface science and boosted the research fields.

In recent years, discovery of various new low-dimensional boron-based materials, including borophene; a monoatomic layer of boron [3], has fascinated the scientific community. The chemical diversity of boron allows for the creation of various stable structures even in low dimensions. Such examples can be seen in superstructures such as B/Ag(111) surface, forming several structures of borophene with unique physical properties[4]. Boron is also known to make various compounds, and exploration of 2D forms of metal borides [5] have also emerged to enrich the exploration of this material group.

In the present research, we focused on the surface ordered phase on Cu(111) by boron deposition in ultrahigh vacuum [6,7]. Through a combination of the surface analyses by positron diffraction and photoelectron spectroscopy, we have determined the formation of 2D copper boride at the surface [8]. It has been known that the boron and copper phases are completely separated in 3D, due to their small difference in electronegativities and large difference in atomic size. The unexpected 2D material is composed of alternating boron and copper atomic chains with a zig-zag structure. The first-principles calculations successfully reproduced the experimental results and further unveiled that the 1D boron is energetically stabilized by electron doping from the surrounding copper atoms, leading to a π-type bounding state. This unique electrophilic character leads us to build the Bumulene model, based on the carbon chains in Cumulene [9]. Since the 2D copper boride forms on a surface, the substrate-induced evolution of the electronic states was examined in detail by the calculation and observed by photoemission band mapping [10]. We believe our comprehensive work would provide a new aspect of surface science on the most well-known Cu(111) crystal and opened a new boron science in ≦ 2D.

In this presentation, I would like to describe details of our research on 2D copper boride on Cu(111). In addition, I would also like to introduce our discovery of a new surface phase of B/Cu(110) [11], as a precursor to further exploring the details of boron configuration on the Cu surface.

References:

[1] Y. Fukaya et al., Phys. Rev. B 88, 205413 (2013).

[2] R. Ma et al., Nature 577, 60 (2020).

[3] I. Matsuda, K. Wu, Eds. 2D Boron: Boraphene, Borophene, Boronene; Springer International Publishing: Cham (2021).

[4] B. Feng et al., Phys. Rev. B 118, 096401 (2017).

[5] F. J. Tuli et al., Surf. Sci. 713, 121906 (2021).

[6] C. Yue et al., Fundamental Research 1, 482 (2021).

[7] R. Wu et al., Nat. Nanotechnol. 14, 44 (2019).

[8] Y. Tsujikawa et al., Phys. Rev. B 106, 205406 (2022).

[9] C. H. Hendon et al., Chem. Sci. 4, 4278 (2013).

[10] Y. Tsujikawa et al., e-J. Surf. Sci. Nanotechnol. https://doi.org/10.1380/ejssnt.2023-058

[11] Y. Tsujikawa et al., Surf. Sci. 732, 122282 (2023).