First-Principles Study of Penta-PtXY (X = Se, Te; Y = S, Te; X ≠ Y) Monolayer with Highly Anisotropic Electronic and Optical Properties

Two-dimensional (2D) semiconducting materials with anisotropic physical properties have induced lively interest due to their application in the field of polarizing devices. Herein, we have designed a family of penta-PtXY (X = Se, Te; Y = S, Te; X ≠ Y) monolayers and predicted the electronic and optical properties based on the first-principles calculation. The results suggest that the penta-PtXY (X = Se, Te; Y = S, Te; X ≠ Y) monolayers are indirect-gap semiconductors with a medium bandgap of 2.29-2.66 eV. The penta-PtXY (X = Se, Te; Y = S, Te; X ≠ Y) monolayers own a remarkable mechanical anisotropy with a high Young's modulus anisotropic ratio (3.0). In addition, the penta-PtXY (X = Se, Te; Y = S, Te; X ≠ Y) monolayers exhibit a high anisotropy ratio of hole/electron mobility in the x and y directions (1.16-3.54). The results calculated by the G0W0+BSE method indicate that the single-layers also bear a salient optical anisotropy ratio (1.56-2.11). The integration of the anisotropic electronic, optical, and mechanical properties entitles penta-PtXY (X = Se, Te; Y = S, Te; X ≠ Y) monolayers as potential candidates in multifunctional polarized nanodevices.

Uniaxial compressions induced complementarity and anisotropic behaviors in CuVP2S6

Uniaxial compressions in layered materials can change their electronic structures and properties. In this work, a bimetallic compound CuVP₂S₆is simulated by using Density Functional Theory (DFT) in the presence of uniaxial compressions. Our results clearly show vertical compressions could lead to anisotropic behaviors, which include the compression effect caused by interlayer compression and the anisotropy of intralayer stretching. The vertical compressions change the V-S bonds and the P-S bonds respectively in AA and AB structures. The complementarity between intralayer stretching and interlayer compression could also result in adjustable bandgaps and degeneracy breakdown of V atoms. Results from the electron localization function analysis demonstrate that the free electrons of AA and AB structures tend to delocalize, and ionic features in V-S bonds could be weakened with increasing vertical compressions. Moreover, the two internal binding energies of AA and AB structures and the charge density difference analysis show that the anisotropy in the intralayer stretch and the charge transfer between metal atoms and S atoms increases gradually.

Origin of the Efficient Nonlinear Optical Response of Two-Dimensional Layered CuFeTe2 Nanosheets

Two-dimensional ternary transition metal chalcogenides (TMCs) have aroused great research interest owing to outstanding semiconducting properties and diverse magnetic response. CuFeTe₂, as a typical TMC, exhibits ambiguous magnetic behavior and ground state of spin density waves nature. Herein, we first report efficient nonlinear absorption and superior nonlinear refraction of CuFeTe₂ nanosheets. The nonlinear absorption and refraction coefficients reach -0.22 cm/GW and -1.66 × 10^-12 cm²/W, respectively. Semiempirical theory for direct bandgap semiconductors was applied to estimate the nonlinearities of CuFeTe₂ nanosheets. The calculation results indicate that the efficient nonlinearities stem from the free carrier induced band filling effect.

Anisotropic Infrared Response and Orientation-Dependent Strain-Tuning of the Electronic Structure in Nb2SiTe4

Two-dimensional materials with tunable in-plane anisotropic infrared response promise versatile applications in polarized photodetectors and field-effect transistors. Black phosphorus is a prominent example. However, it suffers from poor ambient stability. Here, we report the strain-tunable anisotropic infrared response of a layered material Nb₂SiTe₄, whose lattice structure is similar to the 2H-phase transition metal dichalcogenides (TMDCs) with three different kinds of building units. Strikingly, some of the strain-tunable optical transitions are crystallographic axis-dependent, even showing an opposite shift when uniaxial strain is applied along two in-plane principal axes. Moreover, G₀W₀-BSE calculations show good agreement with the anisotropic extinction spectra. The optical selection rules are obtained via group theory analysis, and the strain induced unusual shift trends are well explained by the orbital coupling analysis. Our comprehensive study suggests that Nb₂SiTe₄ is a good candidate for tunable polarization-sensitive optoelectronic devices.

Scalable Van der Waals Encapsulation by Inorganic Molecular Crystals

Encapsulation is critical for devices to guarantee their stability and reliability. It becomes an even more essential requirement for devices based on 2D materials with atomic thinness and far inferior stability compared to their bulk counterparts. Here a general van der Waals (vdW) encapsulation method for 2D materials using Sb₂ O₃ layer of inorganic molecular crystal fabricated via thermal evaporation deposition is reported. It is demonstrated that such a scalable encapsulation method not only maintains the intrinsic properties of typical air-susceptible 2D materials due to their vdW interactions but also remarkably improves their environmental stability. Specifically, the encapsulated black phosphorus (BP) exhibits greatly enhanced structural stability of over 80 days and more sustaining-electrical properties of 19 days, while the bare BP undergoes degradation within hours. Moreover, the encapsulation layer can be facilely removed by sublimation in vacuum without damaging the underlying materials. This scalable encapsulation method shows a promising pathway to effectively enhance the environmental stability of 2D materials, which may further boost their practical application in novel (opto)electronic devices.

Multielement 2D layered material photodetectors

The pronounced quantum confinement effects, outstanding mechanical strength, strong light-matter interactions and reasonably high electric transport properties under atomically thin limit have conjointly established 2D layered materials (2DLMs) as compelling building blocks towards the next generation optoelectronic devices. By virtue of the diverse compositions and crystal structures which bring about abundant physical properties, multielement 2DLMs (ME2DLMs) have become a bran-new research focus of tremendous scientific enthusiasm. Herein, for the first time, this review provides a comprehensive overview on the latest evolution of ME2DLM photodetectors. The crystal structures, synthesis, and physical properties of various experimentally realized ME2DLMs as well as the development in metal-semiconductor-metal photodetectors are comprehensively summarized by dividing them into narrow-bandgap ME2DLMs (including Bi₂O₂X (X = S, Se, Te), EuMTe₃(M = Bi, Sb), Nb₂XTe₄(X = Si, Ge), Ta₂NiX₅(X = S, Se), M₂PdX₆(M = Ta, Nb; X = S, Se), PbSnS₂), moderate-bandgap ME2DLMs (including CuIn₇Se₁₁, CuTaS₃, GaGeTe, TlMX₂(M = Ga, In; X = S, Se)), wide-bandgap ME2DLMs (including BiOX (X = F, Cl, Br, I), MPX₃(M = Fe, Ni, Mn, Cd, Zn; X = S, Se), ABP₂X₆(A = Cu, Ag; B = In, Bi; X = S, Se), Ga₂In₄S₉), as well as topological ME2DLMs (MIrTe₄(M = Ta, Nb)). In the last section, the ongoing challenges standing in the way of further development are underscored and the potential strategies settling them are proposed, which is aimed at navigating the future advancement of this fascinating domain.