Introduction: Two-Dimensional Layered Transition Metal Dichalcogenides

Published as part of Chemical Reviews special issue “Two-Dimensional Layered Transition Metal Dichalcogenides”. Two-dimensional (2D) materials have attracted tremendous attention in recent years, with transition metal dichalcogenides (TMDs) representing a particularly intriguing class. (1−3) TMDs consist of a transition metal atom (such as Mo, W, or Ti) sandwiched between two chalcogen atoms (S, Se, or Te), forming an MX₂ stoichiometry. Characterized by their unique layered structures, the weak van der Waals forces between the covalently bonded atomic crystalline layers allow them to be exfoliated into single- or few-layer sheets, displaying properties that are markedly different from those of their bulk counterparts. For example, the reduced dimensionality leads to a direct bandgap in many TMDs, unlike the indirect bandgap in their bulk form, making them suitable for optoelectronic applications such as photodetectors, light-emitting diodes, and solar cells. (3−9) The unique properties and potential applications of TMDs are driving significant advancements in various fields, from electronics to energy storage and beyond. (10−16) This virtual thematic issue is dedicated to exploring the latest developments and future directions in the research and application of 2D-TMDs. The scalable preparation of the atomically thin 2D-TMDs in large quantity or large area is foundational for capturing their potential in diverse technologies. Considerable efforts have been devoted to the preparation of various forms of 2D-TMDs, including mechanical exfoliation, chemical vapor deposition (CVD), and liquid-phase exfoliation. (17−24) Mechanical exfoliation, though versatile for producing diverse flakes, is limited in scalability and reproducibility. CVD offers better control over thickness and size, making it suitable for large-area production of high quality monolayers or thin films. Liquid-phase exfoliation is advantageous for producing solution-processable TMD inks, essential for printable electronics or energy applications that require bulk quantity of monolayer or few-layer TMDs. Additionally, TMDs often exist in different phases, such as 1T, 1T′, 2H, and 3R, each with distinct chemical or electronic properties. For instance, the 2H phase MoS₂ is semiconducting, while the 1T and 1T′ phases are metallic and semimetallic, respectively. Thus, phase engineering of nanomaterials (PEN) plays a critical role in tailoring the properties of TMDs. Control over these phases can be achieved through techniques like doping, strain engineering, and chemical treatments, enabling the customization of TMD properties for specific applications. (25) Furthermore, the nonbonding van der Waals interactions between the covalently bonded TMD atomic layers allow for the flexible intercalation of foreign atoms or molecules, forming self-assembled interlayers between the crystalline atomic layers without disrupting the in-plane covalent bonds. This capability opens up another direction for tailoring and tuning the physical properties of TMDs. (11,26−29) With versatile variability in chemical compositions, layer numbers and structural symmetries, the TMD materials exhibit highly tunable electronic, optical, and mechanical properties, making them highly versatile for diverse applications from electronics to energy storage and beyond. The direct bandgap and high carrier mobility of TMDs at the limit of subnanometer thickness make them ideal for next-generation electronic and optoelectronic devices. They are being intensively explored for use in transistors, flexible displays, and photodetectors. TMD-based transistors, for example could promise reduced power consumption and increased switching speeds compared to traditional silicon-based devices. (3,30,31) The atomically thin geometry and highly surface sensitive electronic properties make 2D-TMDs an attractive material platform for chemical and biological sensors. Their ability to detect low concentrations of gases or biomolecules with high selectivity and sensitivity opens up new possibilities for environmental monitoring and medical diagnostics. (32−35) The large surface area and tunable electronic properties of 2D-TMDs make them highly tunable catalysts for diverse reactions including green hydrogen production. Additionally, TMDs have shown potential in energy storage devices such as lithium-ion batteries and supercapacitors. Their high surface area and layered structure can facilitate efficient ion transport and storage. TMD-based anodes in lithium-ion batteries, for instance, can provide higher capacity and longer cycle life compared to the conventional materials. (36−38) While it is difficult to cover all the relevant topics of this rapidly expanding field, this virtual thematic issue brings together leaders in the field of diverse backgrounds to discuss the latest developments, trends, and future directions in 2D-TMDs. From the outset, Kaihui Liu et al. addressed the critical need for scalable production of large-area TMD thin films, providing a comprehensive overview of the epitaxial growth of TMDs, including wafer-scale production and epitaxial growth of single-crystals. (21) Xidong Duan et al. systematically summarized the latest techniques for fabricating TMD heterostructures, discussing the rationale, mechanisms and advantages of each strategy, highlighted the representative applications of 2D-TMD heterostructures in various technological areas, and discussed the challenges and future perspectives in the synthesis and device fabrication of TMD heterostructures. (39) Zhaoyang Lin and Xiangfeng Duan et al. reviewed the development of solution-processable 2D-TMD inks, discussing the chemical synthesis of these inks and the techniques for their deposition and highlighting their potential for scalable and cost-effective production of thin films for diverse applications in electronics and optoelectronics. (20) The review concludes with an analysis of the key challenges and future research directions for advancing the technology of 2D-TMD inks. Hua Zhang et al. explored the critical role of crystal phases in determining the properties of TMD materials, providing a comprehensive overview of the synthetic PEN strategies for TMDs, highlighting the importance of controlling both conventional and metastable phases for applications in various fields, including electronics and catalysis, and offer perspectives on future challenges and opportunities in the domain. (25) Yuan Liu et al. examined the challenges of forming high-quality metal contacts with 2D-TMDs due to their ultrathin structures and highlighted van der Waals (vdW) contacts as a low-energy alternative to conventional metallization methods. They discussed recent advancements in vdW contacted devices, their unique transport properties, and their promise for realizing unprecedented device performance, providing a comprehensive analysis of the current research landscape and future prospects in this rapidly evolving field. (40) Yongmin He and Zheng Liu presented an overview of microcell-based studies of TMD electrocatalysts, summarizing advances in understanding TMD catalysts at the single fake (device) level, discussing challenges and future directions in this innovative research area, and highlighting the advantages of spatial confinement for catalytic site exposure. (41) Finally, Pulickel Ajayan et al. reviewed the application of 2D-TMDs in energy conversion and storage, (42) highlighting significant advancements in phase, size, composition, and defect engineering of TMDs, aimed at optimizing their performances for applications like electrocatalytic water splitting and alkali ion batteries. They also provided critical insights into ongoing research and future directions in designing TMDs for energy solutions. Despite significant progress to date, the reliable and large-scale synthesis of high-quality, defect-free TMDs remains a significant hurdle. Achieving precise and reproducible control over the phase and composition of TMDs is another challenge that needs to be addressed. (43) Moreover, integrating TMDs into existing technologies and systems requires further research to understand their long-term stability and performance. Future research in 2D-TMDs is likely to focus on improving synthesis techniques, exploring new phases and heterostructures, and developing novel applications. The ongoing advancements in characterization tools and computational methods will also play a crucial role in understanding and optimizing TMD properties. Overall, 2D-TMDs represent a vibrant and rapidly evolving field of research. Their unique properties and versatile applications have the potential to drive significant advancements across various technological domains, paving the way for innovative solutions to contemporary scientific and engineering challenges. This virtual thematic issue underscores the transformative potential of 2D-TMDs and aims to inspire further research and innovation in this dynamic field. Xiangfeng Duan received his B.S. degree from the University of Science and Technology of China in 1997 and his Ph.D. degree from Harvard University in 2002. From 2002 to 2008, he was a Founding Scientist at Nanosys Inc., a nanotechnology startup partly based on his doctoral research. Dr. Duan joined UCLA in 2008 with a Howard Reiss Career Development Chair. He was promoted to Associate Professor in 2012 and advanced to Full Professor in 2013. His research focuses on nanoscale materials and devices, with applications in next-generation electronics, energy solutions, and health technologies. Hua Zhang is the Herman Hu Chair Professor of Nanomaterials at the City University of Hong Kong. He completed his Ph.D. at Peking University (1998). As a postdoctoral fellow, he joined Katholieke Universiteit Leuven (1999) and moved to Northwestern University (2001). After working at NanoInk Inc. (USA) and the Institute of Bioengineering and Nanotechnology (Singapore), he joined Nanyang Technological University in 2006 and moved to the City University of Hong Kong in 2019. His current research interests focus on the phase engineering of nanomaterials (PEN), especially the preparation of novel metallic and 2D nanomaterials with unconventional phases, and epitaxial growth of heterostructures for various applications. This article references 43 other publications. This article has not yet been cited by other publications.