The Two-Dimensional World of Materials

In everyday life, the term "two-dimensional" might refer to the world of anime or games. In the world of materials, it refers to layered structures and nanosheets, such as graphite (black lead) or graphene, which is a single atomic layer exfoliated from graphite. These materials are attracting attention because they have structures capable of "intercalation," where new molecules or ions (guests) are inserted between the host layers, and "exfoliation," where layers are peeled off to obtain thin sheets at the atomic layer level (Figure 1). I am interested in the fact that various applications of two-dimensional materials can be expected depending on what is sandwiched between which layered substances, and I would like to introduce a few of them.

The first is layered substances made of polymers that change color. Polymers called polydiacetylenes undergo a color change from blue to red when stimuli such as heat or solvents are applied. External stimuli cause molecular motion, twisting the molecular chains (conjugated chains) responsible for the polydiacetylene's coloration, which changes the light absorption wavelength and results in a color change. By intercalating guest molecules or ions between these layers, the mobility of these molecular chains and the layered structure can be altered. This allows for the control of color tone, responsiveness, sensitivity, reversibility, and other factors, thereby controlling the responsiveness to stimuli (Figure 2a). Using these, it becomes possible to visualize (image) physical stimuli such as temperature and force. For example, it is possible to image the temperature distribution generated by devices used in surgery, or the compressive stress distribution generated by brushes or complex-shaped objects (Figure 2b–e).

The second is nanosheets with a thickness close to that of an atom. Layered structures can be used to produce sheets of atomic-level thickness through exfoliation, much like peeling off sheets from a stack of sticky notes one by one. We have established a technology to create nanosheets whose surfaces are modified with molecules by intercalating organic molecules between the layers of inorganic compounds with layered structures to make them easier to peel. Furthermore, by utilizing machine learning, we have clarified the experimental conditions for controlling the ease of exfoliation and the size of the nanosheets (Figure 3a). Nanosheets with modified surfaces can be dispersed in various solvents, allowing for the creation of liquid-repellent surfaces that repel dirt and water, as well as coatings that exhibit coloration through light interference and color changes due to stimuli (Figure 3b, c).

The third is creating two-dimensional structures on our own. The chemical synthesis of perfect graphene, paved with benzene rings, is said to be one of the dreams of chemists. While we have not reached this ideal, an unexpected discovery (see Research Bulletin Kyurizukai for details) led to the creation of an "amorphous conjugated polymer network," a polymer that partially contains structures resembling slightly disordered graphene or graphene oxide (Figure 4). Such polymers can be synthesized from various combinations of molecules. By leveraging machine learning and high-throughput methods, we can create resource-risk-free energy-related materials that do not use rare metal elements, such as electrode active materials for lithium-ion secondary batteries and catalysts for producing hydrogen through water electrolysis, as well as high-strength materials.

The above research has progressed thanks to the efforts of my students and the cooperation of diverse collaborators, to whom I would like to express my gratitude. I intend to continue advancing research and education that contribute to the development of academia and society.

References and Sources

1) R. Shibata, S. Matsuda, H. Kawakubo, H. Imai, Y. Oaki, J. Mater. Chem. B 2024, 12, 10886.

2) N. Shioda, R. Kobayashi, S. Katsura, H. Imai, S. Fujii, Y. Oaki, Mater. Horiz. 2023, 10, 2237.

3) N. Ono, R. Seishima, K. Okabayashi, H. Imai, S. Fujii, Y. Oaki, Adv. Sci. 2023, 10, 2206097.

4) H. Yamanaka, H. Imai, S. Fujii, Y. Oaki, Mater. Horiz. 2026, 13, 3326.

5) Y. Haraguchi, Y. Igarashi, H. Imai, Y. Oaki, Digital Discovery 2022, 1, 26.

6) Y. Haraguchi, H. Imai, Y. Oaki, Adv. Mater. Interfaces 2022, 9, 2201111.

7) T. Sakuma, R. Sato, A. Yamaguchi, H. Imai, N. Arai, Y. Oaki, J. Am. Chem. Soc. 2025, 147, 11564.