Direction and strain controlled anisotropic transport behaviors of 2D GeSe-phosphorene vdW heterojunctions

Tunable electronic properties and optoelectronic characteristics of MoGe2N4/SiC van der Waals heterostructure

The assembly of van der Waals (vdW) heterostructure with easily regulated electronic properties provides a new way for the expansion of two-dimensional materials and promotes the development of optoelectronics, sensors, switching devices and other fields. In this work, a systematic investigation of the electronic properties of MoGe2N4/SiC heterostructures using density functional theory has been conducted, along with the modulation of electronic properties by vertical strain and the potential application prospects in optoelectronic devices. The results show that MoGe2N4/SiC heterostructure has excellent dynamic and thermal stability and belongs to type-II band alignment semiconductors. This is extremely beneficial for the separation of photo-generating electron-hole pairs, so it has important significance for the development of photovoltaic materials. In addition, under the control of vertical strain, the semiconductor-metal transition occurs in the MoGe2N4/SiC heterostructure when the compressive strain reaches 6%. In the case of compressive strain less than 6% and tensile strain, the MoGe2N4/SiC heterostructure maintains the type-II band alignment semiconductor characteristics. Meanwhile, we find that the MoGe2N4/SiC heterostructure has optical absorption coefficients of up to 105in the visible and ultraviolet light ranges, which can improve the absorption coefficients of the MoGe2N4and SiC monolayer in some visible light regions. Finally, the optical conductivity of the MoGe2N4/SiC heterostructure exhibits significant anisotropy, with the armchair direction displaying higher conductivity within the orange light range. In conclusion, the formation of vdW heterostructure by vertically stacking MoGe2N4and SiC monolayers can effectively improve their electronic and optical properties, which provides a valuable reference for the future development of electronic devices and photovoltaic materials.

Thermal Transport in 2D Materials

In recent decades, two-dimensional materials (2D) such as graphene, black and blue phosphorenes, transition metal dichalcogenides (e.g., WS₂ and MoS₂), and h-BN have received illustrious consideration due to their promising properties. Increasingly, nanomaterial thermal properties have become a topic of research. Since nanodevices have to constantly be further miniaturized, thermal dissipation at the nanoscale has become one of the key issues in the nanotechnology field. Different techniques have been developed to measure the thermal conductivity of nanomaterials. A brief review of 2D material developments, thermal conductivity concepts, simulation methods, and recent research in heat conduction measurements is presented. Finally, recent research progress is summarized in this article.

High gas sensing performance of inorganic and organic molecule sensing devices based on the BC3N2 monolayer

Recently, a novel two-dimensional (2D) BC₃N₂ monolayer has gained a lot of attention due to its graphene-like structure, and it was first reported by using the particle swarm optimization algorithm and ab initio calculations. Combining density functional theory with the non-equilibrium Green's function method, a 2D BC₃N₂-based nanodevice has been theoretically constructed and the gas sensing performance of the BC₃N₂ monolayer for inorganic and organic molecules has been extensively investigated. The results revealed that the BC₃N₂ monolayer remains metallic with thermodynamic stability. Meanwhile, the results of sensing performance analysis show that the inorganic molecules CO, NO, and NO₂ and organic molecules C₂H₂ and HCHO have strong chemical interactions with BC₃N₂ and were chemically adsorbed onto BC₃N₂. In contrast, the interactions between NH₃, SO₂, CH₄, C₂H₄ and CH₃OH and BC₃N₂ are very weak and these molecules adopt physical adsorption. In the case of chemisorption, the electronic transport behaviors of the 2D BC₃N₂ devices are sensitive to molecules, and the gas sensitivity of BC₃N₂ is strongly anisotropic, especially for organic C₂H₂ with the gas sensing ratios from 7.30 to 10.43 (from 2.51 to 2.79) under different bias voltages along the zigzag (armchair) direction. For inorganic molecules, the gas sensing device is not particularly sensitive, and the maximum gas sensing ratio is only 1.36 for CO. Meanwhile, the large anisotropic gas sensitivity can reach up to 2.66/6.22 for electron transport along the armchair and zigzag directions for CO/C₂H₂ in the BC₃N₂-based sensing devices. Accordingly, the high gas sensitivity can be disclosed by displaying the scattering state around the Fermi level at different bias voltages during the transport process. As a result, BC₃N₂ could be used in 2D gas sensing devices, especially for sensing organic molecule C₂H₂.

Electric field controlled type-I and type-II conversion of BP/SnS van der Waals heterostructure

Type-I heterostructure, in which electrons and holes are confined in same region, is widely used in light emitting diodes and semiconductor lasers. Type-II heterostructure is widely used in photovoltaic devices because of its excellent spatial separation property of electrons and holes. Can we integrate photovoltaic, photoelectric properties with luminescent property in one device? Here we report a van der Waals heterostructure formed by black phosphorus (BP) and SnS monolayers. It is expected to realize these functions in one device. By first-principles methods, the structural stability, electronic properties and optical properties are investigated. It was found that the BP/SnS bilayer is type-II heterostructure with an indirect bandgap of 0.56 eV. Thep-like character of the band edge in BP/SnS vdW heterostructure makes it to be an excellent optoelectronic material. The type-II stability of the system can be improved by applying a negative electric field. However, when the positive electric field is bigger than 0.1 V Å^-1, the system begins to transform from type-II to type I. Therefore, by adding a gate voltage the bandgap and band alignment of this system can be controlled. The photovoltaic and photoelectric properties can be integrated in one device based on this heterostructure.

Carbon phosphide nanosheets and nanoribbons: insights on modulating their electronic properties by first principles calculations

A carbon phosphide (CP) monolayer, a 2D structure derived from the same 3-fold coordination found both in graphene and phosphorene, has been successfully synthesized in an experiment recently. In this paper, we investigated the modulation of electronic structures and transport characteristics of 2D nanosheets and quasi-1D nanoribbons of CP nanomaterials in the α-phase by using first-principles density functional theory simulation. The calculated band structures show that the band gap of 2D CP nanosheets progressively increases as the uniform biaxial strain changes from compression to stretching. However, the biaxial strain cannot change the indirect band gap behavior of the original 2D CP nanosheet. In addition, the band structures of quasi-1D nanoribbons with different styles of H-passivated zigzag edges have also been studied. The results show that the H-passivated zigzag PC ribbons with two P edges are semiconductors with indirect band gaps, and the gaps decrease with increasing width of ribbons. However, the H-passivated CP nanoribbons with one P-atom terminated edge in combination with one P-atom edge, and H-passivated CC nanoribbons with two C-atom terminated edges display metallic behaviors. The semi-conductive or metallic behaviors of zigzag CP nanoribbons can be explained by presenting the wave function of their energy band around the Fermi level. Finally, the electronic transport properties of different CP nanoribbon based nanojunctions are studied in which arise the interesting negative differential resistance or rectification effects in their current-voltage characteristic curves.

Computational insights into structural, electronic and optical characteristics of GeC/C2N van der Waals heterostructures: effects of strain engineering and electric field

Vertical heterostructures from two or more than two two-dimensional materials are recently considered as an effective tool for tuning the electronic properties of materials and for designing future high-performance nanodevices. Here, using first principles calculations, we propose a GeC/C₂N van der Waals heterostructure and investigate its electronic and optical properties. We demonstrate that the intrinsic electronic properties of both GeC and C₂N monolayers are quite preserved in GeC/C₂N HTS owing to the weak forces. At the equilibrium configuration, GeC/C₂N HTS forms the type-II band alignment with an indirect band gap of 0.42 eV, which can be considered to improve the effective separation of electrons and holes. Besides, GeC/C₂N vdW-HTS exhibits strong absorption in both visible and near ultra-violet regions with an intensity of 10⁵ cm⁻¹. The electronic properties of GeC/C₂N HTS can be tuned by applying an electric field and vertical strains. The semiconductor to metal transition can be achieved in GeC/C₂N HTS in the case when the positive electric field of +0.3 V Å⁻¹ or the tensile vertical strain of −0.9 Å is applied. These findings demonstrate that GeC/C₂N HTS can be used to design future high-performance multifunctional devices.

Vertical heterostructures from two or more than two two-dimensional materials are recently considered as an effective tool for tuning the electronic properties of materials and for designing future high-performance nanodevices.