Novel functional materials and devices in uncultivated fields and ideas

Amorphous Oxide Semiconductor (AOS)
　In 2004 and before, it had been believed “good semiconductor” can be realized only in crystalline materials such as Si, GaN, and ZnO. Notwithstanding that, we demonstrated the high-performance thin-film-transistor (TFT) can be realized by amorphous oxide semiconductor “IGZO”, In-Ga-Zn-O. Although “IGZO” has an amorphous structure, transparent flexible high-performance TFT can be produced. Our IGZO TFT is already in mass production in iPad, Surface Pro4, and 77-inch OLED TVs. In addition, we recently succeeded to demonstrate room-temperature fabrication of inorganic light-emitting semiconductor films, which will be used for optical devices and displays, replacing OLED in the future.
[Ref.] K. Nomura et al., Nature (2004), Science (2003)., K. Ide et al., Appl. Phys. Lett. (2022).

Transparent conductor using covalent bonds
　Germanium oxide (GeO_x) is known as a good electrical insulator with a wide bandgap over 6 eV. We demonstrated to convert SrGeO₃ to a good transparent conductor. Quantum calculation explains its electronic structure and why it reduce sthe bandgap down to 2.7 eV by employing the cubic SrGeO₃ structure. Like this, we are making continuous challenges to create new functional materials based on our original material design concept.
[Ref.] H. Mizoguchi et al., Nature Commun. (2011).

Ultrawide gap amorphous oxide semiconductor
　As explained above, it had been believed the AOS does not have good electronic properties, but we then overturned this misunderstanding by IGZO TFTs. Next our target is ultrawide gap AOSs, which has been believed not to been realized until now. We recently realized GaOx AOS with a large bandgap of 4.12 eV by controlling the defects and understanding the doping mechanism in AOSs.
[Ref.] J. Kim et al., NPG Asia Mater. (2017).

Energy harvesting materials using ambient “small heat“
　Although we don’t recognize, we have unlimited energy of “small heat” in our life, but we cannot use them for electronic devices at present. If we realize new energy harvesting materials and devices converting the “small heat” to useful electricity, so called IoT, inter-networking society to connect everything, will be realized as the communication devices will operate only by the ubiquitous energy without battery. In order to realize such high-efficiency and power-saving devices, we have been challenging to create high-performance thermoelectric material by our material design concept and special bulk/thin-film growth technique and also proposed new ideas and mechanism for the enhancement of energy conversion efficiency.
[Ref.] C. Yamamoto et al., Adv. Funct. Mater. (2020).Titech News, T. Katase et al., Advanced Science (2021).Titech News, M. Kimura et al., Nano Letters (2021).Titech News, X. He et al., Advanced Science (2021).Titech News

Thermal management material that automatically control heat insulation and dissipation
 In recent years, nearly two thirds of the total energy produced from fossil fuels is discarded as waste heat. Therefore, there has been increasing importance in the reduce and reuse of waste heat energy by thermal management technologies. Heat conduction in a solid depends on the temperature gradient and thermal conductivity of a material. Low thermal conductivity material is used for thermal insulation and High thermal conductivity material is used for thermal dissipation. On ther other hand, if there is a material that can change thermal conductivity largely by external stimuli, it enables the active control of heat flow, i.e. switching of heat insulation and dissipation. However, such materials are rare and challenging to develop. We recently developed a new material that can switch thermal conductivity through a reversible 2 dimentional to 3 dimentional structural phase transition in (Sn1-xPbx)Se.
[関連論文] Y. Nishimura et al., Adv. Electron. Mater. (2022).Titech News

Development of new functional materials & devices by nanoscale-controlled thin-film growth and high electric-field approach
   Unique properties, can not be observed in bulk, appear by preparing the artificial hetero-structure or by controlling the carrier densities and electric potential in materials using external electric field. We aim to develop new functional thin-films and opto-electronic/electro-magnetic devices using nanoscale-controlled thin-film growth and high electric-field approach. For example, we succeeded to realize Josephson junction and superconducting quantum interference devices of iron pnictide superconductors by forming the artificial grain boundary junctions in thin films. We also proposed a solid phase epitaxy method to regularly align different cations in thin film material and realized the single crystalline film of ferromagnetic oxide semiconductor, leading to the all oxide ferromagnetic junction devices.
[Ref.] T. Katase et al., Nature Commun. (2011)., PNAS (2014)., Adv. Electron. Mater. (2015, 2015, 2016)., Sci. Rep. (2016)., Science Adv. (2021).Titech News , X. He et al., ACS Appl. Mater. Interfaces (2022).

Computer-assisted materials science & materials design

　We use first-principles calculations, molecular dynamics simulations, device simulations, etc for designing our new materials and also for revealing the underlying mechanism of their functions. It is usually difficult to find new materials, but we challenge to develop novel functional materials by understanding “why the material show good properties”, predicting “how we can improve the material properties”, and confirming the idea experimentally. One of our approaches is shown in the electron density map of 12CaO・7Al2O3 (C12A7) with 0.4nm-size cage. Usually, electrons in the conventional semiconductor conduct through the positions of atoms, but for C12A7, the electrons pass through the cages, which can be visualized by the quantum calculation. By collaboration of experiment and theory, we always try to understand the underlying mechanisms and to find a way to develop novel functional materials.