In the last decade, two-dimensional materials have attracted attention in electronics and physics due to their atomically thin structures and quantum-confined electronic states. Since the intrinsic electronic properties are exceptionally large with a sub-nm thick body, the upcoming semiconducting and quantum devices from the material will have a game-changing ability. However, the intrinsic nature of its high-performance is difficult to realize because of the atomically thin body. The conventional methods to tune the electronic structures are not suitable because the high energy induces fractures of the thin-crystalline lattice. In this paper, we review tuning methods in the electronic structures of two-dimensional semiconducting materials, transition-metal dichalcogenides (TMDCs), using a gentle molecular solution technique. The method can modulate the electronic properties of the semiconductors dramatically; for example, the carrier concentration increases up to a degenerate state (metallic state), and the photoluminescence intensity increases two orders of magnitude. The molecular solution method is an option for harnessing the performance of two-dimensional materials to achieve atomically thin optoelectronics.
Due to the spread of the new coronavirus (SARS-CoV-2), new sterilization technology is gathering much attention. A wide range of light irradiation sterilization technology using ultraviolet light is very effective for prevention of virus spread. In the wavelength range of ultraviolet light (~265 nm), AlGaN-based LEDs are being actively researched. In the shorter wavelength region, mercury lamps (185 and 254 nm) and gas light sources (222 nm) are also being studied. However, in the region called vacuum ultraviolet (VUV), where the wavelength is 200 nm or less, there are few candidates for light emitting elements. MgZnO is one of the candidates because this material can theoretically emit light up to 160 nm. Emission at around 240 nm from MgZnO alloy thin films was reported in 2016, then emission at VUV region (~199 nm) was observed in 2019. It is the first to realize vacuum ultraviolet light emission by a semiconductor, and it is very promising as a sterilization device in the ultra-short-wave region.
The objective of this study was to investigate the repeatability of in situ surface modification by radical beam irradiation to reduce threading dislocation density in InN film. The growth of InN template and N radical irradiation processes were repeated twice in situ in the radio-frequency plasma-excited molecular beam epitaxy chamber before the regrowth of InN film on the N radical irradiated template. Transmission electron microscopy was applied to study dislocation behaviors of the InN film grown. In this letter, we show cross-sectional-view transmission electron microscopy evidence of the threading dislocation reduction from ~2.8×1010 cm-2 in the first irradiated InN layer to ~2.0×1010 cm-2 in the second irradiated InN layer, and to ~1.3×1010 cm-2 in the top regrown InN layer. The mechanisms of threading dislocation reduction were also studied.
Metastable α-Ga2O3 thin films with a corundum structure were grown epitaxially on flexible synthetic mica. The κ-Ga2O3 thin films were grown in a temperature range of 450‒600 °C, without a buffer layer. In contrast, the α-Ga2O3 thin films were grown in a wider temperature range of 350‒600 °C, by inserting corundum-structured α-Fe2O3 buffer layers. X-ray diffraction (XRD) rocking curve measurements revealed that the α-Ga2O3 thin film grown at 425 °C had the highest degree of crystallinity. Cross-sectional transmission electron microscopy (TEM) observations and an XRD φ scan revealed that the epitaxial relationship between the thin film and the substrate via the buffer layer was (0001) α-Ga2O3 [11‒20] || (001) synthetic mica . Furthermore, when TEM observation was performed close to the surface of the α-Ga2O3 thin film at a high magnification, a lattice image derived from the out-of-plane (0001)-plane orientation was observed. However, it was also revealed that the α-Ga2O3 thin films did not have a single crystal structure but rather an in-plane domain structure. By conducting selected area electron diffraction (SAED) at the interface area, it was determined that the α-Fe2O3 buffer layer was polycrystalline. This implies that α-Ga2O3 thin films were epitaxially grown while forming the in-plane domains on the polycrystalline buffer layers. The results of this study indicate the flexible applications of α-Ga2O3 thin films, which have significant potential for use as power sources and deep-ultraviolet devices.
We discussed on crystallization and crystal length control of fullerene(C60) by mist vapor deposition method and fabrication of n-type organic transistor. It was revealed that these crystallinities were affected by the substrate temperature and the amount of mist supplied. The n-type organic transistors using fullerene crystal were able to operate in the atmospheric condition. The electron mobility was estimated to be 7.36×10-6 [cm2/Vs]. Furthermore, the transistor characteristics have been improved by applying HMDS treatment and annealing processing. In particular, the HMDS treatment caused crystals to precipitate between the source and drain electrodes.
The extremely low-cycle fatigue (ELCF) behavior and post-fatigue microstructure of a Fe–15Mn–10Cr–8Ni–4Si austenitic alloy were investigated under a strain rate of 0.5% / sec and a maximum strain amplitude of 10% in the axial direction．(1) It was clarified that the steel damper made of Fe-15Mn-10Cr-8Ni-4Si alloy can withstand about 15 swings back even if the structure is distorted by about 10% due to a large earthquake. (2) The εpa－Nf relationship of the Fe-15Mn-10Cr-8Ni-4Si alloy showed a linear relationship, and the result that Manson-Coffin holds was obtained. (3) Even in the extremely low cycle fatigue test with a strain rate of 0.5% / sec, the test specimen temperature did not exceed 40 °C under all test conditions. Therefore, ε phase was formed in the fatigue test at all test conditions. (4) Many facets and secondary cracks were observed in the fatigue propagation region of the fracture surface. From this, it was inferred that most of the main cracks propagated at the γ/ε interface and the secondary cracks merged. As a result, the fatigue crack could not propagate linearly, and the generation of the secondary crack caused a decrease in the displacement at the tip of the crack when the stress was redistributed, thus extending the fatigue life.
Thermal sprayed ceramic coating system consists of top coat (TC), bond coat (BC), and substrate. Mechanical properties of the TC differ from those of bulk ceramic due to micro cracks and voids in the coating. A stress-strain constitutive equation of TC is one of the most important mechanical properties for stress analysis of ceramic coating system. However, in the case of the evaluation methods using a freestanding coating, the specimen breaks at a quite low strain because of rapid crack propagation. On the other hand, using a coating system specimen, the substrate prevents the specimen from rapid fracture, hence constitutive equation up to high strain is obtainable. All the previous evaluation methods using a coating system specimen are based on bending deflection, however, high accuracy is required for the evaluation of porous TBC. In this study, highly accurate evaluation method for inelastic constitutive equation using bending strain of the coating in thermal sprayed coating system was proposed. This method can evaluate the stress-strain constitutive equation of coating by calculating the coating secant modulus of elasticity from bending strain at each load. First, it was confirmed by FEM analysis that the proposed method was able to provide the accurate inelastic constitutive equation. Thereafter, we experimentally evaluated the inelastic stress-strain constitutive equations of ceramic coatings. As a result, mechanism of non-linearity of ceramic coating was able to be explained by the static friction and micro crack opening / closure, which are specific to thermal sprayed ceramic coatings.
The weight reduction of a vehicle body is expected by applying FRTP (Fiber Reinforced Thermoplastics), which have excellent mechanical properties of high specific strength and high specific stiffness. When molding structual parts of FRTP,press and injection hybrid molding is used, in which after preheated continuous fiber reinforced laminates is shaped into a complex shell shaped structure, such as ribs and bosses are added by injection molding. FRTP parts manufactured by hybrid molding often fracture at the interface between continuous fiber reinforced laminates and the injected material, thus a method for improving the strength of the rib root part is required. We have been developing the MT-RTM (Melted Thermoplastic-Resin Transfer Molding) method, which applies the RTM (Resin Transfer Molding) method to thermoplastic resins. For MT-RTM molded products, high interfacial strength can be expected, as the injected materials will be the matrix of continuous fiber reinforces laminates and no welds at the interface occurred. In this study, MT-RTM molding was applied to the hybrid molding, and the tensile test of the shell part and rib part using fiber reinforced thermoplastic resin with different fiber content as the injection material were carried out, the effect of fiber content of injection material on mechanical properties was clarified. When thermoplastic resin is injected and molded, the resin was easily impregnated into the glass fiber plain woven fabric, molded products with low porosity and high tensile strength was obtained. When a short glass fiber reinforced thermoplastic is used as an injection material for MT-RTM, the resin impregnated into the fabrics and some short glass fibers would also penetrate into tthe fabrics. The appropriate fiber content of the injection material used within our study is 20 wt%, and by using PP with MAPP, the wettability and interfacial adhesive strength are improved; and FRTP with high tensile strength is molded in the shell part and rib root part.
Due to the high cost of carbon fiber reinforced plastics (CFRP), the use of CFRP is limited to the appropriate part, which is based on the multi-material concept. We have molded the multi-material hat-shaped members by press-injection hybrid molding, in which carbon fiber reinforced thermoplastics (CFRTP) are pressed as the outer structural material followed by injection molding, and their mechanical properties have been evaluated. The selection of the stacking sequence of CFRTP, the material ratio of Al alloy and CFRTP, and its configurations are currently carried out on a trial and error basis. To reduce experimental costs, it is important to select appropriate configurations by simulation. In this study, the effects of the stacking sequence of CFRTP, material ratio of Al Alloy and CFRTP, and the stacking order on the stiffness of a hat-shaped member were analyzed by finite element method. Three points bending test for two types of model using neat PA6 and carbon fiber reinforced PA6(CF/PA6) as the injection materials were analyzed. In addition, their stiffness, specific stiffness, and specific stiffness per cost of molded part were calculated. In the analysis of the bending test of the hat-shaped member, the model using CFRTP with the stacking sequence of [±45] showed the higher stiffness. This is because the large strain at the side of the surface material was in the 40° direction. The hat-shaped member using only CFRTP with stacking sequence [±45] for the surface material served as the outer shell structure showed the highest specific stiffness. Furthermore, the stiffness, specific stiffness and specific stiffness per cost of molded part with injection material of CF/PA6 were 6.6, 6.3 and 5.0 times higher than neat PA6 model in average, due to the increased stiffness of the rib structure. In hat-shaped member, the use of carbon fiber reinforced PA6 to mold rib structure is extremely effective to obtain low cost and high stiffness member.