Shi Xiaobo, Yin Huabing, Jiang Shujuan, Chen Weizhen, Zheng Guang-Ping, Ren Fengzhu, Wang Bing, Zhao Gaofeng, Liu Bo
Institute for Computational Materials Science, School of Physics and Electronics, International Joint Research Laboratory of New Energy Materials and Devices of Henan Province, Henan University, Kaifeng 475004, China.
Phys Chem Chem Phys. 2021 Feb 7;23(5):3637-3645. doi: 10.1039/d0cp06116f. Epub 2021 Feb 1.
Due to their broken out-of-plane inversion symmetry, Janus two-dimensional (2D) materials exhibit some exceptional and interesting physical properties and have recently attracted increasing attention. Herein, based on first-principles calculations, we propose a series of Janus 2D titanium nitride halide TiNXY (X, Y = F, Cl, or Br, and X ≠ Y) monolayers constructed from 2D ternary compounds TiNX (X = F, Cl, or Br), where the halogen atoms X or Y are located on each side of the monolayer, respectively. Our calculations confirm that the Janus monolayers are both dynamically and thermally stable. As compared with those of perfect TiNX monolayers, the band-structure changes of Janus TiNXY monolayers are very limited and the corresponding bandgaps only increase by about 0.1-0.2 eV. Meanwhile, the Janus TiNXY monolayers show remarkable out-of-plane piezoelectricity by virtue of their broken centrosymmetry. The calculated out-of-plane piezoelectric coefficient d is as high as 0.34 pm V, which is larger than those of most 2D piezoelectric materials reported previously. In addition, it is found that the formation of Janus structures could effectively improve the carrier mobility. The hole mobilities along the x-direction (y-direction) of Janus TiNFCl and TiNFBr monolayers reach as high as 5402 (5118) and 5538 (4135) cm V s at 300 K, respectively, which is almost twice as large as those of perfect TiNX monolayers. The giant out-of-plane piezoelectricity and high carrier mobility of Janus TiNXY monolayers suggest that these novel 2D materials could be promising for applications in electronic and piezoelectric devices.
由于其面外反演对称性的破缺，Janus二维（2D）材料展现出一些特殊且有趣的物理性质，最近受到了越来越多的关注。在此，基于第一性原理计算，我们提出了一系列由二维三元化合物TiNX（X = F、Cl或Br）构建的Janus二维卤化钛氮化物TiNXY（X、Y = F、Cl或Br，且X≠Y）单层，其中卤原子X或Y分别位于单层的每一侧。我们的计算证实这些Janus单层在动力学和热学上都是稳定的。与完美的TiNX单层相比，Janus TiNXY单层的能带结构变化非常有限，相应的带隙仅增加约0.1 - 0.2 eV。同时，Janus TiNXY单层由于其中心对称性的破缺而表现出显著的面外压电性。计算得到的面外压电系数d高达0.34 pm V，大于先前报道的大多数二维压电材料。此外，发现Janus结构的形成可以有效提高载流子迁移率。在300 K时，Janus TiNFCl和TiNFBr单层沿x方向（y方向）的空穴迁移率分别高达5402（5118）和5538（4135）cm² V⁻¹ s⁻¹，几乎是完美TiNX单层的两倍。Janus TiNXY单层巨大的面外压电性和高载流子迁移率表明这些新型二维材料在电子和压电器件应用中具有广阔前景。
