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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The optical, electronic, catalytic, and mechanical properties of transition-metal dichalcogenide (TMD) bulk crystals and monolayers have been examined extensively. TMD materials can be prepared through exfoliation, deposition from elemental and molecular precursors, and chemical conversion of corresponding transition metal oxides. Nevertheless, approaches for largescale synthesis of high-quality TMD crystals are needed to ensure the continued utilization of these materials within optoelectronic and energy conversion applications. Moreover, studies examining the principal stages of TMD nucleation and growth are comparatively rare and require further attention. We report a method for the conversion of bulk MoO 3 to MoX 2 (X = S, Se) under high-vacuum conditions and use this approach to isolate and examine intermediates formed during the chalcogenation process. Micron-scale Mo sub-oxide deposits are readily converted to MoS 2 when exposed to an atmosphere of S at 40 Torr and 300 °C. Notably, we show through elemental mapping of the cross-section of a partially converted single deposit that MoS 2 is present at higher concentrations nearest the deposit−substrate interface. We also note the formation of either distinct or alloyed TMD phases depending on whether a stepwise or concurrent chalcogenation, respectively, is performed. Our results provide an effective route toward conversion of transition metal oxides to bulk TMDs and provide insights into the mechanism of this scalable conversion process.

Topological materials, such as Cd3As2, are of great importance for next-generation computing systems where energy efficiency is paramount. For example, in spin–orbit torque magnetic access memory (SOT-MRAM), topological materials generate the spin–orbit torque necessary for magnetization switching of an adjacent magnetic layer. Current shunting at the ferromagnet–topological material interface can be detrimental for spin generation and overall SOT efficiency. Here, the current shunting effect is analyzed in permalloy- and gold-shunted Cd3As2 disk structures using angle-dependent magnetoresistance measurements. We observe Shubnikov–de Haas oscillations and weak antilocalization. Hakami–Larkin–Nagaoka and Tkachov–Hankiewicz models are used to evaluate the weak antilocalization. We confirm the effects of current shunting and discuss its detriment to the efficiency of spin-based devices.

Recent studies of transition-metal dichalcogenide (TMD) nanoribbons have stimulated the development of synthetic strategies for the controlled growth of these dimensionally restricted crystals. We demonstrate the width-controlled synthesis of MoSe 2 nanoribbons grown on a designer surface comprising Si(001) treated with phosphine. Adjustment of the H 2 partial pressure in the carrier gas stream enables the nanoribbon widths to be tuned between 175 nm and almost 500 nm. Experiments and simulations suggest that H 2 exposure increases the surface coverage of hydrogen on the Si−P dimers that normally serve as favorable regions for nanoribbon nucleation and growth. Moreover, the MoSe 2 nanoribbons exhibit an anomalous photoluminescence blue shift whose magnitude of 60 meV is similar to that reported in optical emission spectra of MoS 2 nanoribbons. These studies demonstrate that the recently developed strategy of substrate-directed growth of nanoribbons can be extended to the selenide family of TMDs. Moreover, they expand the synthetic foundation for preparing complex TMD heterostructures, which are required for optical-and quantum-based sensors, transducers, and processors.