Progress and Prospects in Transition-Metal Dichalcogenide Research Beyond 2D.

This review discusses recent advances and future research priorities in the transition-metal dichalcogenide (TMD) field. While the community has witnessed tremendous advances through research conducted on two-dimensional (2D) TMD crystals, it is vital to seek new research opportunities beyond developed areas. To this end, in this review we focus principally on articulating areas of need in the preparation and analysis of TMD crystals encompassing dimensionalities and morphologies beyond 2D. Ultimately, the development of new synthetic methods to control key structural features of low-dimensional TMD crystals (e.g., dimensionality, morphology, and phase) will afford access to a broader range of breakthrough properties for this intriguing material class. We begin with a brief overview of the evolution of 2D TMD research, discussing both the synthetic methods that have enabled the preparation of these materials and the manifold properties they possess. We focus the bulk of our review on discussion of recent advances associated with 1D TMD crystals, which are often referred to as TMD nanoribbons, and include a discussion of recent efforts in 0D systems. We discuss synthetic strategies that have been developed to prepare such beyond 2D crystals and highlight their unique physical and chemical properties. After reviewing the host of analytical tools available for characterization of TMD materials, we identify future analytical instrumentation needs. We conclude with a discussion of the prospects of beyond 2D TMD crystals in optoelectronics, catalysis, and quantum information science.

Substrate-directed synthesis of MoS2 nanocrystals with tunable dimensionality and optical properties.

Two-dimensional transition-metal dichalcogenide (TMD) crystals are a versatile platform for optoelectronic, catalytic and quantum device studies. However, the ability to tailor their physical properties through explicit synthetic control of their morphology and dimensionality is a major challenge. Here we demonstrate a gas-phase synthesis method that substantially transforms the structure and dimensionality of TMD crystals without lithography. Synthesis of MoS2 on Si(001) surfaces pre-treated with phosphine yields high-aspect-ratio nanoribbons of uniform width. We systematically control the width of these nanoribbons between 50 and 430 nm by varying the total phosphine dosage during the surface treatment step. Aberration-corrected electron microscopy reveals that the nanoribbons are predominantly 2H phase with zig-zag edges and an edge quality that is comparable to, or better than, that of graphene and TMD nanoribbons prepared through conventional top-down processing. Owing to their restricted dimensionality, the nominally one-dimensional MoS2 nanocrystals exhibit photoluminescence 50 meV higher in energy than that from two-dimensional MoS2 crystals. Moreover, this emission is precisely tunable through synthetic control of crystal width. Directed crystal growth on designer substrates has the potential to enable the preparation of low-dimensional materials with prescribed morphologies and tunable or emergent optoelectronic properties.