Switching polarization via inter-layer sliding or heterostructural inversion in two-dimensional SnS/SnSSe ferroelectric heterostructures with enhanced vertical polarization and ultra-low switching barriers.

Development of piezoelectric materials is limited partly due to the incompleteness of internal mechanism and the lack of vertical piezoelectricity. Herein, we theoretically identify the stable MoTO (T = S, Se, or Te) monolayers and bilayers. When two elements are given but another element can be changed, the larger the electronegativity difference ratio R ratio is, the stronger the piezoelectricity will be. Vertical piezoelectric coefficient d 33 of the MoTeO bilayer reaches 38.907 pm/V, which is 12 times larger than that of the bulk GaN. The “active asymmetric electron-transfer” strategy mainly contributes to the spontaneous remarkable piezoelectricity of MoTO. Importantly, we proposed the new method for calculating the piezoelectric coefficients of two-dimensional (2D) materials, which corresponds to the fact that 2D materials have a certain thickness. This study not only provides novel extraordinary candidates for energy conversion and touch-sensor nanodevices but also promotes a deeper understanding of piezoelectricity of 2D materials.

The integration of piezoelectric and micro-electro-mechanical system (MEMS) technologies is promoting the rapid development of the MEMS industry. However, due to the lack of excellent in-plane and out-of-plane piezoelectric coefficients, good ambient temperature stability, and high mechanical durability, the reported two-dimensional piezoelectric thin films are facing severe challenges in the development of MEMS. Therefore, based on density functional theory calculations, we simulated the VIA group (the sixth main family in the periodic table of elements) functional MXenes, namely, Sc2CXY (Sc2COS, Sc2COSe, Sc2CSO, and Sc2CSeO). The results show that this kind of material has good dynamic, thermal, and mechanical stability. Importantly, the structure exhibits good in-plane and out-of-plane piezoelectric effects due to the inversion asymmetry, bandgap adjustability, flexibility, and different electronegativity. Among them, the piezoelectric strain value of d33 is one to three orders of magnitude higher than that of d22 or d31, which makes these materials can become a strong candidate for advanced equipment such as nano-wearable electronics, robot bionic skin, and piezoelectric products for MEMS.

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We theoretically report four photocatalyst candidates, namely, Sc2COS, Sc2CSO, Sc2COSe, and Sc2CSeO. A mirror asymmetry atom structure endows Sc2COS with the largest intrinsic built-in electric field (EF) of 7.53 × 109 V/m among these monolayers, facilitating the separation of photogenerated electron–hole pairs. Sc2CSO and Sc2CSeO with smaller EF are half-reaction water splitting photocatalysts, performing the hydrogen evolution reaction. It is highly interesting that C atoms aligned to the top S or Se atoms move down by about 0.40 Å and are vertically aligned to the bottom O atoms, named as the inside vertical reconstruction. Therefore, Sc2CSO (Sc2CSeO) transforms into Sc2COS (Sc2COSe) with larger EF, suitable for photocatalytic overall water splitting. They have great optical absorption with optical absorption coefficients up to 105 cm−1. Our work reveals the physical mechanism of EF enhancement caused by inside vertical reconstruction effect facilitates overall water splitting, which provides a feasible strategy for theoretically designing intriguing photocatalysts.