Two-dimensional transition metal dichalcogenide (TMD) nanosheets.

This special issue is about two-dimensional transitionmetal dichalcogenides (2DTMDs), a family of materials consisting of over 40 compounds with the generalized formula of MX2, where M is a transition metal typically from groups 4–7, and X is a chalcogen such as S, Se or Te. Bulk TMDs have been widely studied over several decades because it is possible to formulate compounds with disparate electronic structures. In the bulk form, MX2 compounds are layered materials (or van der Waals solids) in which there is strong intralayer bonding and weak interlayer bonding. Each individual layer of the TMDs consists of three atomic layers in which the transition metal is sandwiched by two chalcogens. Furthermore, the chalcogen atoms are saturated and therefore are not highly reactive. These features allow for the attainment of individual layers of the TMDs by several exfoliation or vapor deposition methods. The isolation of monolayers of TMDs leads to the dramatic changes in their properties, primarily due to the confinement of charge carriers in two dimensions (xand y-directions) due to the absence of interactions in the z-direction. Thus, singlelayered nanosheets are two-dimensional materials that possess dramatically different Materials Science and Engineering, Rutgers University, 607 Taylor Road, Piscataway, NJ 08854, USA. E-mail: [email protected] b Center for Nanochemistry (CNC), Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, People’s Republic of China. E-mail: [email protected] c School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. E-mail: [email protected]