Fabrication and Engineering of Janus Transition Metal Dichalcogenides as Novel 1D and 2D Materials

Atomic layer materials composed of two-dimensional sheets with atomic thickness have been a significant research focus due to their unique shapes and excellent properties. We have conducted extensive research on the synthesis of carbon nanotubes [1,2] and graphene nanoribbons [3-8] with advanced plasma CVD. More recently, our research has expanded to include new atomic layer materials [9-17]. Transition metal dichalcogenides (TMDs), which are two-dimensional sheet structures where transition metals are sandwiched between chalcogen atoms, have recently garnered significant attention as two-dimensional materials with various functionalities such as semiconductors, metals, superconductors, and topological insulators. While conventional TMDs consist of chalcogen atoms of the same type on both the top and bottom layers, a new material called Janus TMDs has been discovered, in which the chalcogen atoms on the top layer are replaced with different chalcogen atoms. This unique material, possessing polarization in the out-of-plane direction despite being monolayered, has been the subject of extensive theoretical research. However, experimental studies have not progressed significantly due to the difficulty of synthesizing high-quality samples.<br/>In response to this challenge, we have recently developed an in-situ observation apparatus that allows direct observation of the optical spectra of TMDs during the Janusization reaction. Our in-situ monitoring system allows us to measure Raman and photoluminescence spectra during the formation of Janus TMDs using a mild H₂ plasma reaction. By utilizing this in-situ monitoring system for Janus TMD formation, we have successfully created novel 1D Janus TMDs. When Janus formation was carried out using TMD nanotubes as the initial material, it was confirmed that the sulfur on the outermost surface was replaced with selenium while maintaining the tube structure, marking the first successful creation of Janus TMD nanotubes [15]. Additionally, it was discovered that after forming conventional 2D Janus TMDs, the introduction of a liquid followed by drying led to a transformation into a scroll structure due to inherent strain effects. Atomic structure analysis revealed that the Janus structure was maintained in each layer of the scroll, also marking the successful creation of a novel 1D material, the Janus TMD nanoscroll [16]. Additionally, we have demonstrated that a unique moiré potential structure can be observed in the heterostructure composed of 2D Janus TMD and conventional TMD layers [17]. These 1D and 2D Janus TMDs are expected to exhibit various exotic physical properties in the future.<br/> <br/>[1] T. Kato and R. Hatakeyama, ACS Nano 4 (2010) 7395.<br/>[2] B. Xu, T. Kaneko, Y. Shibuta, T. Kato, Sci. Rep. 7 (2017) 11149.<br/>[3] T. Kato and R. Hatakeyama, ACS Nano 6 (2012) 8508.<br/>[4] T. Kato and R. Hatakeyama, Nature Nanotechnol. 7 (2012) 651.<br/>[5] H. Suzuki, T. Kaneko, Y. Shibuta, M. Ohno, Y. Maekawa, and T. Kato, Nature Commun. 7 (2016) 11797.<br/>[6] H. Suzuki, N. Ogura, T. Kaneko, T. Kato, Sci. Rep. 8 (2018) 11819.<br/>[7] Q.-Y. Li, <i>et al.</i>, ACS Nano 13 (2019) 9182.<br/>[8] T. Kato, <i>et al.</i>, Commun. Mater. 3 (2022) 103.<br/>[9] T. Kato and T. Kaneko, ACS Nano 8 (2014) 12777.<br/>[10] T. Kato and T. Kaneko, ACS Nano 10 (2016) 9687.<br/>[11] T. Akama, W. Okita, R. Nagai, C. Li, T. Kaneko, T. Kato, Sci. Rep. 7 (2017) 11967.<br/>[12] C. Li, T. Kameyama, T. Takahashi, T. Kaneko, T. Kato, Sci. Rep. 9 (2019) 12958.<br/>[13] X. Qiang, <i>et al.</i>, Sci. Rep. 11, 22285 (2021).<br/>[14] X. He, Y. Iwamoto, T. Kaneko T. Kato, Sci. Rep. 12 (2022) 11315.<br/>[15] Y. Nakanishi, <i>et al.</i>, Adv. Mater. 35, 2306631 (2023).<br/>[16] M. Kaneda, <i>et al.</i>, ACS Nano 18, 2772 (2024).<br/>[17] W. Zhang, <i>et al.</i>, Small Structures 5, 2300514 (2024).