Ternary and multinary compounds used in polycrystalline thin-film solar cells are I-III-VI₂ semiconductors typified by CuInSe₂ (CISe), I₂-II-IV-VI₄ compounds such as Cu₂ZnSnS₄ (CZTS)，and I₂-IV-VI₃ compounds such as Cu₂SnS₃ (CTS). Their crystal structures are chalcopyrite type (space group I42d (122)) for CISe, kesterite type (space group I4 (79)) for CZTS, and monoclinic system (space group Cc (9)) for CTS. In this review, we describe the characteristics of their crystal structures and electronic structures such as band-gap energy and energy levels of valence band maxima (VBM) and conduction band minima (CBM). The differences of electronic structures between ZnSe and CuGaSe₂, CuInSe₂ and Cu₂ZnSnSe₄, and Cu₂ZnSnS₄ and Cu₂SnS₃ will be discussed on the basis of the results of theoretical calculations and molecular orbital energy-level diagrams．

Design of multiply sensing materials whose intrinsic nature is mutually tunable by chemical and physical external stimuli is a pivotal challenge in the field of materials science. A class of electron-conjugated metal-organic frameworks (MOFs) composed of electron-donors (D) and/or -acceptors (A) as “D/A-MOFs” is one of the favorable candidates for such materials. Since the charge transfer of D0A0 ↔ Dδ+Aδ- is flexible as a function of the combination of the ionization potential of D and the electron affinity of A and the Madelung packing stabilization of the ionic D+A- set, D/A-MOFs are intriguing targets for guest-sensitive functional materials with magnetic, conducting, and their synergistic properties, as well as with their porous nature for gas/solvent sorption or ionic transportation.

In order to induce various electronic phase transitions in strongly correlated electron systems, lithium-ion battery-like electrochemical cells based on highly crystalline epitaxial thin films of transition-metal oxides have been developed. The electronic states of band insulators, Mott insulators, heavy fermion metals, and superconductors are studied in detail from their transport, optical, and structural properties. This method has become a powerful tool for precise electrochemical doping of single samples and in-situ measurement of modulated electronic states. In this review, we present the results obtained in our previous studies, clarify the effectiveness and scope of application of this method, and look forward to its possible application to electrochemical devices.

Intermetallic compounds can be novel catalysts because of their unique electronic structures and ordered surface structures. In Heusler alloys (X₂YZ), novel catalysts would be discovered from many possible sets of X, Y, and Z. Component elements can be substituted by others in many cases, which enables a fine control of catalysis. I have investigated catalytic properties of Heusler alloys that were unknown as catalysts. A discovery of good catalysts for selective hydrogenation of alkynes and a systematic control of catalysis were achieved using powder catalysts synthesized metallurgically. A durability and roles of X, Y, and Z were also investigated. Recently, nanoparticle catalysts were successfully synthesized.

By chemical vapor deposition of boron nitride nanotubes and MoS₂ nanotubes on the surface of single-walled carbon nanotubes, we have synthesized heteronanotubes, which are coaxial heterostructures of different types of nanotubes. This progress opens up the possibility of material design with a high degree of freedom for quasi-one-dimensional nanotubes. Such one-dimensional van der Waals heterostructures as well as two-dimensional counterparts have attracted large attention. They are expected to lead to the development of new physical properties and device applications specific to nanotube structures. This paper introduces the synthesis, structural characterization, physical properties, and applications of heteronanotubes.

Scanning probe microscopy has been used to characterize structures and electronic properties of surfaces as well as to construct nanostructures via atom-by atom manipulation. Recent advances in tip-functionalized scanning probe microscopy allows us to measure inner structures of molecules. The bond-resolved imaging technique is of importance to investigate precursors and products during on-surface reaction. In this article, the tip-induced structural isomerization and addition reaction will be discussed.

Evaluation methods of “channel mobility” in the inversion layer of the metal-oxide-semiconductor field-effect transistors (MOSFETs) are discussed, which is one of the physical properties that is significantly related to flow of charges. With the help of fundamental and well-established knowledge in the Si devices, the mechanism of electron scattering in that layer of SiC MOSFETs is investigated by analyzing the evaluated channel mobilities.