Emerging opportunities in the two-dimensional chalcogenide systems and architecture

Inspired by the triumphs - and motivated by the need to overcome the limitations - of graphene, the science and engineering community is rapidly exploring the landscape of other potential two-dimensional materials, particularly in their single - or few layer form. Dominating this landscape are the layered chalcogenides; diverse in chemistry, structure and properties, there are well over 100 primary members of this materials family. Driven by quantum confinement, single layers (or few, in some cases) of these materials exhibit electronic, optical, and transport properties that diverge dramatically from their bulk counterparts. The field has evolved considerably since the time when single or few layer flakes were “synthesized” by the scotch-tape mechanical cleavage method. New and more sophisticated methods for controlled synthesis (or thinning), deposition and chemical exfoliation have been developed that can “dial” the number of layers with large areal coverage on diverse substrates. Further, the 2D chalcogenide layers are being used as “substrates” onto which other dimensionally confined structures are being integrated in the spirit of nanoscale composites. Some composite structures exhibit synergy of multiple functionalities of the individual components, while in other cases they represent quantum coupling or unusual behavior that is contrary to nominal synergy or the proportional contribution of individual components. Last but not the least, there remain many structural and chemical combinations that are yet to be explored with deeply intriguing properties or phenomena that are waiting to be revealed. Thus, it is timely to review the status of the field; particularly in the context of synthesis, geometric architecturing and characterization of 2D layered systems. Herein we review the evolving architecture of two-dimensional chalcogenide materials. We outline classes of specific materials and the evolution of their properties as they transition from nominally three to two-dimensionality, and especially in their single (or few) layer form. A variety of vapor-phase synthetic methods for the direct growth of large area single layers and the typical techniques for their characterization are presented. Lastly, we examine the potential of these materials as the fundamental building blocks of two-dimensional heterostructures and multi-dimensional nanocomposites. However, we also emphasize the need for fundamental experimental and theoretical undertaking to probe the classical problems like basic characterization and the dynamics of nucleation and growth in these 2D systems for realizing complex architecturing and resultant technologically useful phenomena and properties.