A type-II C2N/α-Te van der Waals heterojunction with improved optical properties by external perturbation

Tunability of electronic properties in the 2D MoS2/α-tellurene/WS2 heterotrilayer via biaxial strain and electric field

Alpha-tellurene (α-Te), a two-dimensional (2D) material that has been theoretically predicted and experimentally verified, has garnered significant attention due to its unique properties. In this study, we investigated the 2D trilayer MoS2/α-Te/WS2 van der Waals heterostructure with different stacking orders using first-principles calculations. Our results indicate that this heterotrilayer exhibits an intrinsic type-I band alignment and an indirect band gap similar to that of monolayer α-Te. Notably, the band edges of the heterostructure can be modulated by biaxial strain and an external electric field, enabling these edges to arise from different monolayers. This controlled manipulation facilitates the effective separation of photogenerated electron-hole pairs and prolongs the carrier lifetime. Moreover, the heterostructure can undergo a transition from an indirect to a direct band gap under biaxial compressive strain or a moderate negative electric field, and semiconductor-to-metal transition can also be achieved by intensifying the biaxial strain and external electric field. Overall, our research provides valuable theoretical insights into the potential applications of α-Te-based heterostructures, rendering them promising candidates for the next generation of nanodevices.

Optoelectronic properties and applications of two-dimensional layered semiconductor van der Waals heterostructures: perspective from theory

Van der Waals heterostructures (vdWHs) which combine two different materials together have attracted extensive research attentions due to the promising applications in optoelectronic and electronic devices, the investigations from theoretical simulations can not only predict the novel properties and the interfacial coupling, but also provide essential guidance for experimental verification and fabrications. This review summarizes the recent theoretical studies on electronic and optical properties of two-dimensional semiconducting vdWHs. The characteristics of different band alignments are discussed, together with the optoelectronic modulations from external fields and the promising applications in solar cells, tunneling field-effect transistors and photodetectors. At the end of the review, the further perspective and possible research problems of the vdWHs are also presented.

Dicarbon nitride and Janus transition metal chalcogenides van der Waals heterojunctions for photocatalytic water splitting

Two-dimensional graphene-like dicarbon nitride (C₂N) is a newly synthesized metal-free material, which has attracted significant research interest owing to the direct band gap, high carrier mobility, thermal stability, and great tunable properties. However, their application in photocatalytic water splitting has not been well explored. In this work, the properties of photocatalytic water decomposition in heterojunctions composed of C₂N and transition metal dichalcogenides (TMDs) with Janus structure MoXY (X, Y = S, Se, Te) are systematically studied by the first-principles calculations based on density functional theory. The results show that except for MoTeS/C₂N, the other five heterojunctions have type-Ⅱ band alignment, which causes electrons and holes to gather in the C₂N and MoXY layer separately. Because the coupled built-in electric field at the intra-layer and inter-layer of asymmetric TMDs with Janus structure forms van der Waals heterojunction, the external electric field is an effective means of modulating the electronic properties of the heterojunction. Under the imposition of an external electric field, the MoSeS/C₂N, MoTeSe/C₂N, and MoTeS/C₂N heterojunctions meet the band edge requirements for the photocatalytic decomposition of water. Detailed analysis demonstrates that the MoSeS/C₂N heterojunction could effectively improve the optical absorption properties of monolayer C₂N, making it a potential photocatalytic water decomposition material.

Structural, Electronic and Optical Properties of four α-Se-based Heterostructures with Hyperbolic Characteristics

The physical properties and potential applications of two-dimensional (2D) materials can be effectively modulated and enriched by constructing van der Waals heterostructures (VDWHs) with two or more 2D monolayer materials....

Boosting the photocatalytic H2 evolution activity of type-II g-GaN/Sc2CO2 van der Waals heterostructure using applied biaxial strain and external electric field

Two-dimensional (2D) van der Waals (vdW) heterostructures are a new class of materials with highly tunable bandgap transition type, bandgap energy and band alignment. Herein, we have designed a novel 2D g-GaN/Sc₂CO₂ heterostructure as a potential solar-driven photocatalyst for the water splitting process and investigate its catalytic stability, interfacial interactions, and optical and electronic properties, as well as the effects of applying an electric field and biaxial strain using first-principles calculation. The calculated lattice mismatch and binding energy showed that g-GaN and Sc₂CO₂ are in contact and may form a stable vdW heterostructure. Ab initio molecular dynamics and phonon dispersion simulations show thermal and dynamic stability. g-GaN/Sc₂CO₂ has an indirect bandgap energy with appropriate type-II band alignment relative to the water redox potentials. Meanwhile, the interfacial charge transfer from g-GaN to Sc₂CO₂ can effectively separate electron–hole pairs. Moreover, a potential drop of 3.78 eV is observed across the interface, inducing a built-in electric field pointing from g-GaN to Sc₂CO₂. The heterostructure shows improved visible-light optical absorption compared to the isolated g-GaN and Sc₂CO₂ monolayers. Our study demonstrates that tunable electronic and structural properties can be realised in the g-GaN/Sc₂CO₂ heterostructure by varying the electric field and biaxial strain. In particular, the compressive strain and negative electric field are more effective for promoting hydrogen production performance. Since it is challenging to tune the electric field and biaxial strain experimentally, our research provides strategies to boost the performance of MXene-based heterojunction photocatalysts in solar harvesting and optoelectronic devices.

Type-II g-GaN/Sc₂CO₂ van der Waals heterostructure with electronic properties has potential for nanoelectronics, optoelectronics and photovoltaic device applications.

A first-principles study on the electronic and optical properties of a type-II C2N/g-ZnO van der Waals heterostructure

The structural, electronic and optical properties of a new van der Waals heterostructure, C2N/g-ZnO, composed of C2N and g-ZnO monolayers with an intrinsic type-II band alignment and a direct bandgap of 0.89 eV at the Γ point, are extensively studied using first-principles density functional theory calculations. The results indicate that the special optoelectronic properties of the constructed heterostructure mainly originate from the interlayer coupling and electron transfer between the C2N and g-ZnO monolayers, and the photogenerated electrons and holes are located on the C2N and g-ZnO layers, respectively, which reduces the recombination probability of the electron-hole pairs. According to Bader charge analysis, there are 0.029 electrons transferred from g-ZnO to C2N to form a built-in electric field of ∼9.5 eV at the interface. Furthermore, the tunability of the electronic properties of the C2N/g-ZnO heterostructure under vertical strain and electric field is explored. Under different strains, the type-II band alignment properties of the heterostructure are retained and the vertical compressive strain has a greater influence on the bandgap modulation than the vertical stretching strain. The implemented electric field also does not change the type-II band alignment but changes the bandgap of the heterostructure from 1.30 to 0.58 eV when the electric field strength varies from -0.6 to 0.6 V Å-1. In addition, the absorption spectrum of the C2N/g-ZnO heterostructure under solar light is also studied. The absorption range of the heterostructure varies from the ultraviolet to near-infrared region with the absorption intensity in the order of 105 cm-1. All of these studies indicate that the C2N/g-ZnO heterostructure has excellent electronic and optical properties and promising applications in nanoelectronics and optoelectronics.

A C2N/ZnSe heterostructure with type-II band alignment and excellent photocatalytic water splitting performance

A type-II C₂N/ZnSe heterostructure with strong light-absorption ability, high carrier mobility and low exciton binding energy, exhibits excellent photocatalytic water splitting performance

Superior Anchoring of Sodium Polysulfides to the Polar C2N 2D Material: A Potential Electrode Enhancer in Sodium-Sulfur Batteries

Despite the high theoretical specific energy in rechargeable sodium-sulfur batteries, the shuttle effect severely hampers its capacity and reversibility, which could be overcome by introducing an anchoring material. We, herein, use first-principles calculations to study the low-cost, easily synthesized, environmentally friendly, and stable two-dimensional polar nitrogenated holey graphene (C₂N) and nonpolar polyaniline (C₃N) to investigate their performance as anchoring materials and the mechanism behind the binding to identify the best candidate to improve the performance of sodium-sulfur batteries. We gain insight into the interaction, including the lowest-energy configurations, binding energies, binding nature, charge transfer, and electronic properties. Sodium primarily contributes to binding with the nanosheets, which is in accordance with their characteristics as anchoring materials. Sodium polysulfides (NaPSs) and the S₈ cluster adsorb at the pores of C₂N, where there are six electron lone pairs, one for each N atom. The polar C₂N binds the NaPSs much strongly than the nonpolar C₃N. In contrast to C₃N, the charge population substantially modifies by adsorbing NaPSs on C₂N, with a substantial charge transfer from the sulfur atoms. The calculated work function of 6.04 eV for pristine C₂N, comparable with the previously reported values, decreases on adsorption of the NaPSs formed from battery discharging. We suggest that the inclusion of C₂N into sulfur electrodes could also improve their issue with poor conductivity.

Enhanced carrier mobility and tunable electronic properties in α-tellurene monolayer via an α-tellurene and h-BN heterostructure

Using first-principles calculations within density functional theory, we explore the electronic properties of the α-tellurene/h-BN (Te/BN) heterostructure. We find that the type-I van der Waals (vdW) Te/BN bilayer exhibits an indirect semiconductor property with a bandgap of 0.59 eV, in which both the valence band maximum and conduction band minimum originate from the tellurene monolayer. The very weak interaction between α-tellurene and h-BN monolayers is demonstrated by the small charge transfer between the interlayer. More strikingly, we find that the carrier mobilities in the Te/BN bilayer can reach up to 104 cm2 s-1 V-1, one order of magnitude larger than those in tellurene. The underlying physics is that the Te/BN bilayer dramatically increases the in-plane stiffness as well as reducing the deformation potential compared with the tellurene monolayer. Additionally, we also show that the electronic properties of the Te/BN bilayer can easily be tuned by introducing defects or dopants in the BN monolayer. For instance, the B vacancy makes the Te/BN bilayer undergo the transition from semiconductor to half-metal. Our findings will broaden the potential application of tellurene and provide theoretical guidance for the relative experimental studies on 2D heterobilayers.