A two-dimensional (2D) MoSi₂N₄ monolayer is an emerging class of air-stable 2D semiconductors possessing exceptional electrical and mechanical properties. Despite intensive recent research effort devoted to uncover the material properties of MoSi₂N₄, the physics of electrical contacts to MoSi₂N₄ remains largely unexplored thus far. In this work, we study van der Waals heterostructures composed of MoSi₂N₄ contacted by graphene and NbS₂ monolayers using first-principles density functional theory calculations. We show that the MoSi₂N₄/NbS₂ contact exhibits an ultralow Schottky barrier height (SBH), which is beneficial for nanoelectronics applications. For the MoSi₂N₄/graphene contact, the SBH can be modulated via the interlayer distance or via external electric fields, thus opening up an opportunity for reconfigurable and tunable nanoelectronic devices. Our findings provide insights into the physics of 2D electrical contacts to MoSi₂N₄ and shall offer a critical first step toward the design of high-performance electrical contacts to MoSi₂N₄-based 2D nanodevices.

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.

This Perspective reviews the fundamental physics of space–charge interactions that are important in various media: vacuum gap, air gap, liquids, and solids including quantum materials. It outlines the critical and recent developments since a previous review paper on diode physics [Zhang et al. Appl. Phys. Rev. 4, 011304 (2017)] with particular emphasis on various theoretical aspects of the space–charge limited current (SCLC) model: physics at the nano-scale, time-dependent, and transient behaviors; higher-dimensional models; and transitions between electron emission mechanisms and material properties. While many studies focus on steady-state SCLC, the increasing importance of fast-rise time electric pulses, high frequency microwave and terahertz sources, and ultrafast lasers has motivated theoretical investigations in time-dependent SCLC. We particularly focus on recent studies in discrete particle effects, temporal phenomena, time-dependent photoemission to SCLC, and AC beam loading. Due to the reduction in the physical size and complicated geometries, we report recent studies in multi-dimensional SCLC, including finite particle effects, protrusive SCLC, novel techniques for exotic geometries, and fractional models. Due to the importance of using SCLC models in determining the mobility of organic materials, this paper shows the transition of the SCLC model between classical bulk solids and recent two-dimensional (2D) Dirac materials. Next, we describe some selected applications of SCLC in nanodiodes, including nanoscale vacuum-channel transistors, microplasma transistors, thermionic energy converters, and multipactor. Finally, we conclude by highlighting future directions in theoretical modeling and applications of SCLC.

MXene, a new class of two-dimensional nanomaterials, have drawn increasing attention as emerging materials for sensing applications. However, MXene-based surface plasmon resonance sensors remain largely unexplored. In this work, we theoretically show that the sensitivity of the surface plasmon resonance sensor can be significantly enhanced by combining two-dimensional ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{T}}_x$ MXene and transition metal dichalcogenides. A high sensitivity of ${198}^{\circ }$ /RIU (refractive index unit) with a sensitivity enhancement of 41.43% was achieved in aqueous solutions (refractive index ∼1.33) with the employment of monolayer ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{T}}_x$ MXene and five layers of ${\mathrm{WS}}_2$ at a 633 nm excitation wavelength. The integration of ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{T}}_x$ MXene with a conventional surface plasmon resonance sensor provides a promising approach for bio- and chemical sensing, thus opening up new opportunities for highly sensitive surface plasmon resonance sensors using two-dimensional nanomaterials.

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.

We identify a new universality in the carrier transport of two-dimensional (2D) material-based Schottky heterostructures. We show that the reversed saturation current ($\mathcal{J}$) scales universally with temperature ($T$) as $\log (\mathcal{J}/T^{\beta })\propto -1/T$, with $\beta =3/2$ for lateral Schottky heterostructures and $\beta =1$ for vertical Schottky heterostructures, over a wide range of 2D systems including nonrelativistic electron gas, Rashba spintronic systems, single- and few-layer graphene, transition metal dichalcogenides, and thin films of topological solids. Such universalities originate from the strong coupling between the thermionic process and the in-plane carrier dynamics. Our model resolves some of the conflicting results from prior works and is in agreement with recent experiments. The universal scaling laws signal the breakdown of $\beta =2$ scaling in the classic diode equation widely used over the past sixty years. Our findings shall provide a simple analytical scaling for the extraction of the Schottky barrier height in 2D material-based heterostructures, thus paving the way for both a fundamental understanding of nanoscale interface physics and applied device engineering.