Guo San-Dong, Mu Wen-Qi, Guo Hao-Tian, Tao Yu-Ling, Liu Bang-Gui
School of Electronic Engineering, Xi'an University of Posts and Telecommunications, Xi'an 710121, P. R. China.
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China.
Phys Chem Chem Phys. 2022 Aug 24;24(33):19965-19974. doi: 10.1039/d2cp02724k.
The combination of piezoelectricity with a nontrivial topological insulating phase in two-dimensional (2D) systems, namely piezoelectric quantum spin Hall insulators (PQSHI), is intriguing for exploring novel topological states toward the development of high-speed and dissipationless electronic devices. In this work, we predict a PQSHI Janus monolayer VCClBr constructed from VCCl, which is dynamically, mechanically and thermally stable. In the absence of spin orbital coupling (SOC), VCClBr is a narrow gap semiconductor with a gap value of 57 meV, which is different from Dirac semimetal VCCl. The gap of VCClBr is due to a built-in electric field caused by asymmetrical upper and lower atomic layers, which is further confirmed by the external-electric-field induced gap in VCCl. When including SOC, the gap of VCClBr is increased to 76 meV, which is larger than the thermal energy of room temperature (25 meV). The VCClBr is a 2D topological insulator (TI), which is confirmed by topological invariant and nontrivial one-dimensional edge states. It is proved that the nontrivial topological properties of VCClBr are robust against strain (biaxial and uniaxial cases) and external electric fields. Due to broken horizontal mirror symmetry, only an out-of-plane piezoelectric response can be observed, when a biaxial or uniaxial in-plane strain is applied. The predicted piezoelectric strain coefficients and are -0.425 pm V and -0.219 pm V, respectively, and they are higher than or compared with those of many 2D materials. Finally, Janus monolayer VCFBr and VCFCl (dynamically unstable) are also constructed, and they are still PQSHIs. Moreover, the and of VCFBr and VCFCl are higher than those of VCClBr, and the (absolute value) of VCFBr is larger than one. According to out-of-plane piezoelectric coefficients of VCXY (X ≠ Y = F, Cl and Br), CrXY (X = F, Cl and Br; Y = I) and NiXY (X ≠ Y = Cl, Br and I), it is concluded that the size of the out-of-plane piezoelectric coefficient has a positive relation with the electronegativity difference of X and Y atoms. Our studies enrich the diversity of Janus 2D materials, and open a new avenue in the search for PQSHI with a large out-of-plane piezoelectric response, which provides a potential platform in nanoelectronics.
二维（2D）系统中压电性与非平凡拓扑绝缘相的结合，即压电量子自旋霍尔绝缘体（PQSHI），对于探索新型拓扑态以推动高速无耗散电子器件的发展而言颇具吸引力。在这项工作中，我们预测了一种由VCCl构建的PQSHI Janus单层VCClBr，它在动力学、力学和热学方面都是稳定的。在没有自旋轨道耦合（SOC）的情况下，VCClBr是一种带隙值为57 meV的窄带隙半导体，这与狄拉克半金属VCCl不同。VCClBr的带隙是由上下不对称原子层引起的内建电场导致的，VCCl中外部电场诱导的带隙进一步证实了这一点。当考虑SOC时，VCClBr的带隙增加到76 meV，大于室温下的热能（25 meV）。VCClBr是一种二维拓扑绝缘体（TI），这通过拓扑不变量和非平凡的一维边缘态得到证实。结果表明，VCClBr的非平凡拓扑性质在应变（双轴和单轴情况）和外部电场作用下是稳健的。由于水平镜面对称性的破坏，当施加双轴或单轴面内应变时，只能观察到面外压电响应。预测的压电应变系数 和 分别为-0.425 pm V和-0.219 pm V，它们高于或与许多二维材料相当。最后，还构建了Janus单层VCFBr和VCFCl（动力学不稳定），它们仍然是PQSHIs。此外，VCFBr和VCFCl的 和 高于VCClBr，并且VCFBr的 （绝对值）大于1。根据VCXY（X≠Y = F、Cl和Br）、CrXY（X = F、Cl和Br；Y = I）和NiXY（X≠Y = Cl、Br和I）的面外压电系数，得出面外压电系数的大小与X和Y原子的电负性差呈正相关。我们的研究丰富了Janus二维材料的多样性，并为寻找具有大面外压电响应的PQSHI开辟了一条新途径，这为纳米电子学提供了一个潜在平台。
