Tokyo Institute of Technology Hitosugi Lab

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研究内容

Long-range goal

Our principal objective is to develop new functional materials and interfaces that will outperform existing materials, including superconductors, magnetic materials, photo functional materials, ion conductors, and dielectric materials. Our group’s approach combines atom-by-atom engineering with ensemble-measurement techniques. To explore new functional materials, we exquisitely tailor material structures with atomic precision by evaluating both their atomic-scale electronic structures and their macroscopic physical properties. Based on these microscopic and macroscopic properties, we aim to establish future guiding principles for material design and to initiate new fields of research in materials science.

 

Strategies

Our strategies to achieve those goals consist of:

  1. Novel-material synthesis with atomic precision. We focus on designing novel material systems in thin films, heterostructures, low-dimensional structures, surfaces, and interfaces.
  2. Characterization of the synthesized materials. We use our custom-built analytical instruments to both probe the designed materials’ properties and to elucidate the effects of atomic-scale structure and chemistry on these properties.
  3. Synergy between analytical results and materials design. We use our experimental results to improve and to design subsequent material systems, until the desired properties are obtained.

 

Research fields

While applications of designed materials in electronic devices, spintronics, and energy fields have been a major goal in condensed-matter physics, researchers have only begun to utilize oxide nanostructures to design novel functionalities beyond those in surfaces and interfaces. We introduce the concept of “atom-by-atom engineering” of oxide heterostructures, thin films, surfaces, and interfaces to this emerging field. We also extend this approach to other materials including nitrides, hydrides and carbon materials such as graphene. With this approach, we intend to develop future “green materials”.

 

A. Characterization of electronic states of functional thin films, interfaces, surfaces & exploration of new functional materials

  • We have designed and built a low-temperature scanning tunneling microscope with pulsed laser deposition capabilities in ultrahigh vacuum. This unique system enables us to study the local electronic structures of pristine oxide thin films. Using this system, we aim to elucidate the role of the atomic-scale electronic structures of oxide thin films and heterostructures on their macroscopic physical properties. Our current interests also include graphenes, magnetic molecules, and amorphous clusters. Furthermore, we use angle resolved photoemission and transport-measurement techniques as complementary approaches to understanding electronic and magnetic properties of these materials.

A. 機能性薄膜/界面/表面の電子状態評価と新物質開拓 [新規デバイス開発]

B. Ion-conducting materials

  • Lithium ion batteries are important materials for electric cars and for realizing solar energy society. To this end, high, fast-charging battery capacity remains an essential feature that must be greatly improved. We aim to improve the capacity of Li ion batteries up to ten times that of current Li ion batteries. Since the role of most battery-material physical properties on battery performance are unknown, we synthesize ion-conducting materials by controlling atomic-scale structures (e.g., by controlling the anisotropy of crystals or grain boundaries in heterostructures) to reveal properties relevant to ion conductivity. Our ultimate goal is to elucidate the mechanism of battery operation in the atomic-scale perspective and to open a new frontier for studying electrochemical atom engineering.

 

Research highlights

"Atomic-scale visualization of initial growth of homoepitaxial SrTiO3 thin film on an atomically ordered substrate"
Ryota Shimizu, Katsuya Iwaya, Takeo Ohsawa, Susumu Shiraki, Tetsuya Hasegawa, Tomihiro Hashizume, Taro Hitosugi
ACS Nano, ACS Nano 5, 7967 (2011).
The interfaces of transition metal oxide heterostructures have exhibited various physical properties that are different from their constituent materials. To further explore multifunctional properties that utilize the interplay between spin, charge orbital and lattice degrees of freedom, it is crucial to grow oxide thin films in an atom-by-atom manner by monitoring the initial stage of epitaxial behavior at the atomic level. In this study, we atomically visualized the initial homoepitaxial growth of SrTiO3 on an atomically-ordered SrTiO3(001) substrate surface using STM. The identical atomic structure was clearly identified on the deposited SrTiO3 film surfaces as well as on the substrate, indicating the transfer of the topmost Ti-rich structure to the film surface. Such atomically ordered SrTiO3 substrates are essential to the fabrication of atom-by-atom controlled oxide epitaxial films and heterostructures.
“Fabrication of highly conductive Ti1-xNbxO2 polycrystalline films on glass substrates via crystallization of amorphous phase grown by pulsed laser deposition”
T. Hitosugi, A. Ueda, S. Nakao, N. Yamada, Y. Furubayashi, Y. Hirose, T. Shimada, and T. Hasegawa
Appl. Phys. Lett. 90, 212106 (2007).
rom a practical point of view, transparent conducting oxide (TCOs) films need to be coated on inexpensive substrates, particularly glass, because of increasing technological demands for flat panel displays and solar cells. In this Letter, Nb-doped anatase TiO2 films with high electrical conductivity and transparency were successfully fabricated on nonalkali glass using PLD and subsequent annealing in a H2 atmosphere. This result indicates that Nb-doped anatase TiO2 films have the potential to be practical TCOs that could replace indium tin oxide.
“A transparent metal: Nb-doped anatase TiO2
Y. Furubayashi, T. Hitosugi, Y. Yamamoto, K. Inaba, G. Kinoda, Y. Hirose, T. Shimada, and T. Hasegawa
Appl. Phys. Lett. 86, 252101 (2005).
In the last decades, the industrial use of transparent conducting oxides (TCOs) has undergone a major expansion, based on the demand for devices, including flat panel displays, light-emitting devices, and solar cells. Sn-doped In2O3 (ITO) has been the most widely used because of its low resistivity and high transmittance, but limitations of the existing materials become critical in view of the increasing potential new uses for TCOs. This Letter reports on the discovery of a promising TCO, Nb-doped anatase TiO2, which possesses excellent electrical conductivity and transparency, comparable to other TCOs.
”Direct observation of one-dimensional Ga-atom migration on Si(100)-2×1-H surface: a local probe of adsorption energy variation”
T. Hitosugi, Y. Suwa, S. Matsuura, S. Heike, T. Onogi, S. Watanabe, T. Hasegawa, K. Kitazawa and T. Hashizume
Phys. Rev. Lett. 83, 4116-4119 (1999).

Atomic-scale surface migration of a Ga atom on a hydrogen-terminated Si(100)-(2×1)-H surface is

studied using low-temperature scanning tunneling microscopy and first-principles calculations. The Ga atom migrates in a linear potential well confined by adjacent dimer rows and local dihydride defects, and is observed as a continuous linear protrusion (Ga-bar structure) at a narrow range of temperatures near 100 K. The observed height modulation of the Ga-bar structure reflects the local variation in potential energy at individual adsorption sites (adsorption energy variation), which may be caused by the local stress made by surface defects or subsurface impurities.
”Jahn-Teller distortion in dangling-bond linear-chains fabricated on a hydrogen-terminated Si(100)-2×1 surface”
T. Hitosugi, S. Heike, T. Onogi, T. Hashizume, S. Watanabe, Z. -Q. Li, K. Ohno, Y. Kawazoe, T. Hasegawa, and K. Kitazawa
Phys. Rev. Lett. 82, 4034-4037 (1999).
We reported on the first observation of the Jahn-Teller distortion in pseudo-one-dimensional dangling bond (DB) structures on the Si(100)-2×1-H surface. The atomic geometry and the charge density of DB structures are characterized by low-temperature ultrahigh vacuum STM, and the origin of the charge redistribution is discussed based on the first-principles theoretical calculations. The odd-even problem, the edge effect, and the finite length of the DB structures are indispensable to understand the relaxation in the structures.
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