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.

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Metal alloys with atomic scale ordering (ordered intermetallics) have emerged as a new class of high performance materials for mediating electrochemical reactions. However, ordered intermetallic nanostructures often require long synthesis times and/or high temperature annealing to form because a high-activation energy barrier for interdiffusion must be overcome for the constituent metals to equilibrate into ordered structures. Here we report the direct synthesis of metastable ordered intermetallic Pd31Bi12 at room-temperature in minutes via electrochemical deposition. Pd31Bi12 is highly active for the reduction of O2 to H2O, delivering specific activities over 35× higher than those of commercial Pt and Pd nanocatalysts, placing it as the most active Pd-based catalyst, to the best of our knowledge, reported under similar testing conditions. Stability tests demonstrate minimal loss of activity after 10,000 cycles, and a retention of intermetallic crystallinity. This study demonstrates a new method of preparing ordered intermetallics with extraordinary catalytic activity at room temperature, providing a new direction in catalyst discovery and synthesis.

Strain-engineering is an effective strategy for manipulating the optical properties of low-dimensional materials. The integration of two-dimensional (2D) transition metal dichalcogenides (TMDs) with other low-dimensional materials can yield nonplanar assemblies that manifest a diverse range of interfaces and strain characteristics. Here we identify anomalous photoluminescence from nonplanar assemblies of 2D MoSe2 monolayers with silicon nanowires (SiNWs). Near-field scanning optical microscopy identifies pronounced photoluminescence (PL) at 1.38 eV that emanates from the nonplanar region between the 2D monolayer and SiNW. Notably, this anomalous emission is distinct (by nearly 200 meV) from the characteristic A exciton emission of monolayer MoSe2. Scanning transmission electron microscopy reveals an apparent straining of the MoSe2 unit cell across a 3–5 nm wide linear region coincident with the nonplanar boundary. The unusual room-temperature PL may be ascribed to a new excitonic state localized within this nanostrained region of the 2D MoSe2 monolayer. This work highlights the importance of nanoscale manipulation of strain in low-dimensional materials to elicit desired control over excitonic and other properties.

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