Regulated and Gradual Polymorph Transition By Thermal and Plasma Treatment on Transition Metal Dichalcogenides
Authors: Lee Dongho , Seok Hyunho , Son Sihoon , Lee Seokchan , Yu Dayoung , Kim Taesung 
Published: February 2024

Transition metal dichalcogenides (TMDs) have two different phase structure which exhibit different properties. Generally, it can be classified into 1T (metallic) and 2H (semiconductor) phase structures according to the relative atomic direction of the TMDs. Currently, the utilization of two-dimensional (2D) materials is limited due to the lack of an appropriate phase control method of TMDs. There are several phase control methods reported so far (post-treatment chemical process, lattice deformation of physical crystals due to ion collision, phase control of materials through strain, etc.), but the disadvantages of requiring additional processes and low yield are limited for practical usage of those method. In this research, we conducted a study on gradual phase control of TMDs according to substrate temperature (150, 200, 250, 300, and 350°C) while other plasma parameters were kept constant (power, pressure, processing time, and gas ratio). By controlling the phase, the versatility of TMDs is dramatically improved. 1T-TMDs basal plane plays a crucial role as catalytically active site to enhance electrochemical properties more than 2H-TMDs. 1T-TMDs used as a hydrogen generation catalyst has been proposed as an alternative to noble metals (e.g., Pt, Ir, Ru). This is because 1T-TMDs have a metallic property and thus have dense catalytically active sites as edge and basal plane, which can be used as an electrochemical catalyst. However, the problem of (1T-TMDs) manufactured in the conventional way is that the unstable properties at room temperature of them. During the low-temperature plasma process, ion bombardment onto the substrate induces a grain boundary of several nano sizes to expand the active site per area of the surface crystal structure resulting in high catalytic activity. In addition, the possibility of large-scale production was presented on large-scale wafer synthesis, and the original phase of 1T-TMDs was preserved after the 1,000 HER cycle to prove robustness and durability. On the other hand, in the case of (2H-TMDs), instead of showing low catalytic activity, the existence of band gap makes them favorable for semiconductor device applications such as photodiode, transistor, and neuromorphic computing. In this study, by using the sputtered MoS2 and WS2 to control the ratio of different polymorph, memristive switching (RS) is investigated. Similarly, by ion bombardment onto substrates in a low-temperature plasma process, a few nano-sized particle boundaries were induced, creating a defect on the material surface. An experiment was conducted in which filaments are mimicking artificial neural networks and then ruptured under electrical bias. However, the reproducibility of filament formation for 2H-TMDs was relatively low, and the formed filament was relatively unstable. Therefore, it was intended to form a kind of 'road' to solve this problem. Here, the road was using a 1T-TMDs structure with high catalytic activity. The mixed phase of TMDs consist of a different phase (1T and 2H) structure, which depends on their process temperature. Diversity of process temperature results in various phase ratio into the material structure. Gradual phase change from 1T to 2H at various temperature (150, 200, 250, 300, and 350 °C) confirmed by X-ray photoelectron spectroscopy. By utilizing the mixed phase of TMDs, the metal filaments were formed along the 1T structure within 2H structure, which showing uniform RS behavior. Higher reproducibility was obtained for mixed phase structure than the 2H-TMDs and secured stability. In conclusion, by presenting a research direction on how to control the TMDs’ phases, this study can increase the utilization of the 2D material while adjusting different phases together, rather than single usage of the 2D material phases such as 1T- and 2H-TMDs.
Acknowledgement: This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education 2022R1A6A3A13063381 and 2022R1A3B1078163). And this work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2022R1A4A1031182).