Yang Ruo Xi, Tan Liang Z
Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
J Phys Chem Lett. 2021 Sep 30;12(38):9253-9261. doi: 10.1021/acs.jpclett.1c02515. Epub 2021 Sep 17.
The stability of halide perovskites has been a long-standing issue for their real-world application. Approaches to improve stability include nanostructuring, dimensionality reduction, and strain engineering, where surfaces play an important role in the formation of a stable structure. To understand the mechanism we compute the lattice dynamics of the surface of CsPbI using density functional theory. We demonstrate, for the first time, that CsPbI crystals exhibit surface phonons that are localized on the outermost layers of the slabs, and we perform a complete symmetry characterization including an identification of the Raman/IR active modes. These surface phonons are present in the optically active cubic phase but are absent in the optically inactive "yellow" phase. Furthermore, we show that the surface suppresses bulk instabilities by hardening soft modes of the bulk cubic phase, resulting in phase stabilization and quenching of dynamical disorder. This study is fundamental for understanding the structural behavior of halide perovskite materials with high surface area-to-volume ratios, and for guiding stabilization strategies.
卤化物钙钛矿的稳定性一直是其实际应用中的一个长期问题。提高稳定性的方法包括纳米结构化、降低维度和应变工程，其中表面在稳定结构的形成中起着重要作用。为了理解其机制，我们使用密度泛函理论计算了CsPbI表面的晶格动力学。我们首次证明，CsPbI晶体表现出局域在平板最外层的表面声子，并且我们进行了完整的对称性表征，包括对拉曼/红外活性模式的识别。这些表面声子存在于光学活性立方相中，但在光学非活性的“黄色”相中不存在。此外，我们表明表面通过硬化体立方相的软模式来抑制体不稳定性，从而导致相稳定和动态无序的淬灭。这项研究对于理解具有高表面积与体积比的卤化物钙钛矿材料的结构行为以及指导稳定策略具有重要意义。
