Metal contacts to two-dimensional (2D) semiconductors are often plagued by the strong Fermi level pinning (FLP) effect which reduces the tunability of the Schottky barrier height (SBH) and degrades the performance of 2D semiconductor devices. Here, we show that MoSi₂N₄ and WSi₂N₄ monolayers—an emerging 2D semiconductor family with exceptional physical properties—exhibit strongly suppressed FLP and wide-range tunable SBH. An exceptionally large SBH slope parameter of S ≈ 0.7 is obtained which outperforms the vast majority of other 2D semiconductors. Such intriguing behavior arises from the septuple-layered morphology of MoSi₂N₄ and WSi₂N₄ monolayers in which the semiconducting electronic states are protected by the outlying Si–N sublayer. We identify Ti, Sc, and Ni as highly efficient Ohmic contacts to MoSi₂N₄ and WSi₂N₄ with zero interface tunneling barrier. Our findings reveal the potential of MoSi₂N₄ and WSi₂N₄ as a practical platform for designing high-performance and energy-efficient 2D semiconductor electronic devices.

Photocatalytic water-splitting for hydrogen generation by sunlight provides a new route to address energy and environmental problems. In recent years, tremendous efforts have been devoted to designing highly efficient water-splitting photocatalysts (PCs). Adequate light absorption, effective photogenerated carrier separation, and sufficiently large overpotentials for water redox are crucial in achieving high solar-to-hydrogen (STH) efficiency. These parameters thus strongly influence the design of novel photocatalytic materials. Two-dimensional (2D) PCs have flourished because of their large specific surface area ratio, short carrier migration distance compared to bulk PCs, enormous design flexibility via van der Waals heterostructure (HS) engineering and many other unique capabilities that meet the criteria for high-efficiency STH conversion. In this review, we summarize the recent developments of 2D materials and HSs for water-splitting applications from a theoretical perspective. Specifically, we first discuss a number of 2D materials and HSs employed for water-splitting. We review various strategies of material design to modulate and enhance the photocatalytic performance via improving light harvesting and carrier separation, such as the introduction of defects and dopants, and the application of strain, external electric field, rotation angles and ferroelectric switching. We then discuss the methods to evaluate hydrogen evolution reaction, oxygen evolution reaction and STH efficiency. Finally, the opportunities and challenges of designing 2D materials and HSs for water-splitting are presented.

Recent experimental synthesis of ambient-stable MoSi2N4 monolayer has garnered enormous research interest. The intercalation morphology of MoSi2N4—composed of a transition metal nitride (Mo-N) inner sub-monolayer sandwiched by two silicon nitride (Si-N) outer sub-monolayers—has motivated the computational discovery of an expansive family of synthetic MA2Z4 monolayers with no bulk (3D) material counterpart (where M = transition metals or alkaline earth metals; A = Si, Ge; and N = N, P, As). MA2Z4 monolayers exhibit interesting electronic, magnetic, optical, spintronic, valleytronic, and topological properties, making them a compelling material platform for next-generation device technologies. Furthermore, heterostructure engineering enormously expands the opportunities of MA2Z4. In this review, we summarize the recent rapid progress in the computational design of MA2Z4-based heterostructures based on first-principle density functional theory (DFT) simulations—a central work horse widely used to understand the physics, chemistry, and general design rules for specific targeted functions. We systematically classify the MA2Z4-based heterostructures based on their contact types, and review their physical properties, with a focus on their performances in electronics, optoelectronics, and energy conversion applications. We review the performance and promises of MA2Z4-based heterostructures for device applications that include electrical contacts, transistors, spintronic devices, photodetectors, solar cells, and photocatalytic water splitting. We present several prospects for the computational design of MA2Z4-based heterostructures, which hold the potential to guide the next phase of exploration, moving beyond the initial “gold rush” of MA2Z4 research. This review unveils the vast device application potential of MA2Z4-based heterostructures and paves a roadmap for the future development of MA2Z4-based functional heterostructures and devices.