B P Querne Mateus, C Dias Alexandre, Janotti Anderson, Da Silva Juarez L F, Lima Matheus P
Department of Physics, Federal University of São Carlos, 13565-905, São Carlos, São Paulo, Brazil.
University of Brasília, Institute of Physics and International Center of Physics, Brasília 70919-970, DF, Brazil.
J Phys Chem C Nanomater Interfaces. 2024 Jul 12;128(29):12164-12177. doi: 10.1021/acs.jpcc.4c01813. eCollection 2024 Jul 25.
Two-dimensional (2D) Janus structures offer a unique range of properties as a result of their symmetry breaking, resulting from the distinct chemical composition on each side of the monolayers. Here, we report a theoretical investigation of 2D Janus ''31 monochalcogenides from group IV ( and ' = Ge and Sn; , ' = S and Se) and 2D non-Janus 3̅1 counterparts. Our theoretical framework is based on density functional theory calculations combined with maximally localized Wannier functions and tight-binding parametrization to evaluate the excitonic properties. The phonon band structures exhibit exclusively real (nonimaginary) branches for all materials. Particularly, SeGeSnS has greater energetic stability than its non-Janus counterparts, representing an outstanding energetic stability among the investigated materials. However, SGeSnS and SGeSnSe have higher formation energies than the already synthesized MoSSe, making them more challenging to grow than the other investigated structures. The electronic structure analysis demonstrates that materials with Janus structures exhibit band gaps wider than those of their non-Janus counterparts, with the absolute value of the band gap predominantly determined by the core rather than the surface composition. Moreover, exciton binding energies range from 0.20 to 0.37 eV, reducing band gap values in the range of 21% to 32%. Thus, excitonic effects influence the optoelectronic properties more than the point-inversion symmetry breaking inherent in the Janus structures; however, both features are necessary to enhance the interaction between the materials and sunlight. We also found anisotropic behavior of the absorption coefficient, which was attributed to the inherent structural asymmetry of the Janus materials.
二维（2D）Janus结构由于其对称性破缺而具有一系列独特的性质，这种对称性破缺源于单层两侧不同的化学成分。在此，我们报告了对来自IV族的二维Janus“31”单硫属化物（和' = Ge和Sn；，' = S和Se）以及二维非Janus 3̅1对应物的理论研究。我们的理论框架基于密度泛函理论计算，结合最大局域化Wannier函数和紧束缚参数化来评估激子性质。所有材料的声子能带结构都只呈现实（非虚）分支。特别地，SeGeSnS比其非Janus对应物具有更高的能量稳定性，在研究的材料中表现出出色的能量稳定性。然而，SGeSnS和SGeSnSe的形成能比已合成的MoSSe更高，这使得它们比其他研究结构更难生长。电子结构分析表明，具有Janus结构的材料的带隙比其非Janus对应物更宽，带隙的绝对值主要由核心而非表面成分决定。此外，激子结合能范围为0.20至0.37 eV，使带隙值降低21%至32%。因此，激子效应比Janus结构中固有的点反转对称性破缺对光电性质的影响更大；然而，这两个特征对于增强材料与太阳光之间的相互作用都是必要的。我们还发现了吸收系数的各向异性行为，这归因于Janus材料固有的结构不对称性。
