Herein, we present the basic idea of a two-step photon up-conversion solar cell, along with the theoretically calculated conversion efficiency and experimental results of a high-efficiency solar cell, proposed by us based on the above concept. The experimental results have demonstrated that the insertion of a quantum-dots layer beneath a semiconductor hetero-interface improves the efficiency of intraband transition (photon up-conversion) occurring at the hetero-interface. Finally, our ongoing efforts to realize low-cost, high-efficiency two-step up-conversion solar cells are presented.

Low-dimensional materials are expected to be new materials for next-generation devices, but their mass synthesis and implementation in high-volume or large-area devices is one of the major challenges. In this paper, the synthesis of nanosheet bundles of silicon, silicide, and related materials using layered crystals such as CaSi₂ as templates is described. Powdered bundles comprising nanosheets with a thickness of several nanometers to several tens of nanometers were obtained, and the Si and silicide nanosheet bundles were also prepared in the form of microwalls on Si substrates. This paper introduces the progress from nanosheet bundle synthesis by solid–vapor phase reaction to nanosheet bundle synthesis in aqueous solutions, metal melts, and molten salts.

In this paper, we introduce frequency modulation atomic force microscopy (FM-AFM) for visualizing atomic-scale surface structures in liquid. Recent achievements in FM-AFM development allow us to visualize intramolecular structures for organic molecules and biomolecular assemblies formed by weak intermolecular interactions at a single functional group level. In contrast, visualization of fluctuating structures and vertically extended chemical structures is difficult, even with the FM-AFM technique. To overcome the technical limitation of the FM-AFM, three-dimensional scanning AFM (3D-AFM) is developed for visualizing the spatial distribution of solvation and fluctuation structures at the solid–liquid interface. The applications of 3D-AFM techniques are also introduced.

This study aims to develop integrated gas sensors for multiple gas sensing. A structure consisting of an ionic gel and multiple electrodes was proposed to realize the integration of gas sensors. Using two of the multiple electrodes, voltage modulation by gas molecules at the ionic gal/electrode interface was detected through the drain current measurement of an externally connected transistor. The voltage modulation depends on the electrode materials and gas molecules. From several types of time-series drain currents derived from combinations of two electrodes among the multiple electrodes, the concentrations of hydrogen, ammonia, and ethanol were simultaneously estimated using a neural network model.

Molecular beam epitaxy (MBE) has long been studied and developed as a nitride-semiconductor crystal growth method along with metal–organic vapor deposition and hydride vapor deposition. This study introduces the authors’ recent achievements in nitride-semiconductor crystal growth by MBE, which is characterized by low-temperature growth and nitrogen-plasma irradiation. Direct growth of gallium nitride on a ScAlMgO₄ substrate, threading-dislocation-density reduction in indium nitride (InN) grown with in situ surface modification by nitrogen-plasma irradiation, and InN remote epitaxy using graphene are described.

Semiconducting silicide BaSi₂ is an indirect band-gap semiconductor composed of earth-abundant elements with a forbidden bandwidth of approximately 1.3 eV. It has a large optical absorption coefficient equivalent to CuInS₂, and its conductivity type and carrier density can be controlled by impurity doping using group 13 and 15 elements. In this article, an approach to thin-film solar cells on glass substrates is introduced, utilizing the knowledge gained from epitaxial films on Si substrates.