Kim Doyun, Vasileiadou Eugenia S, Spanopoulos Ioannis, Kanatzidis Mercouri G, Tu Qing
Department of Materials Science & Engineering, Texas A&M University, College Station, Texas 77840, United States.
Department of Chemistry, Northwestern University, Evanston, Illinois 60201, United States.
ACS Appl Mater Interfaces. 2021 Jul 14;13(27):31642-31649. doi: 10.1021/acsami.1c06140. Epub 2021 Jun 30.
In-plane strains are commonly found in two-dimensional (2D) metal halide organic-inorganic perovskites (HOIPs). The in-plane mechanical properties of 2D HOIPs are vital for mitigating the strain-induced stability issues of 2D HOIPs, yet their structure and mechanical property relationship largely remains unknown. Here, we employed atomic force microscope indentation to systematically investigate the in-plane Young's moduli of 2D lead halide Ruddlesden-Popper HOIPs with a general formula of (R-NH)PbX, where the spacer molecules R-NH are linear alkylammonium cations (CH-NH, = 4, 6, 8, or 12) and X = I, Br, or Cl. Fixing the spacer molecule to butylammonium, we discovered that the of 2D HOIPs generally follows the trend of Pb-X bond strength, different from the tendency found in the out-of-plane moduli , showing more prominent effects of the metal halide inorganic framework on than . exhibits nonmonotonic dependence on the chain length of the linear alkyl spacer molecules, which would first decrease and plateau but then increase again. This is likely due to the competition of the bond strength and structural distortion in the inorganic layer, the relative fraction of the soft organic spacers, and the interfacial mechanical coupling associated with the interdigitation of the alkyl chains. The mechanical anisotropy of 2D HOIPs, marked by /, shows wide tunability based on structural composition, particularly for iodide-based 2D HOIPs. Our results provide valuable insights into the structure-property relationships regarding the mechanical anisotropy and in-plane mechanical behaviors of 2D HOIPs, which can guide the materials design and device optimization to achieve required mechanical performance in 2D HOIP-based applications.
面内应变常见于二维（2D）金属卤化物有机-无机钙钛矿（HOIPs）中。二维HOIPs的面内力学性能对于缓解二维HOIPs的应变诱导稳定性问题至关重要，但其结构与力学性能的关系在很大程度上仍不清楚。在此，我们采用原子力显微镜压痕法，系统地研究了通式为（R-NH）PbX的二维卤化铅Ruddlesden-Popper HOIPs的面内杨氏模量，其中间隔分子R-NH为线性烷基铵阳离子（CH-NH，n = 4、6、8或12），X = I、Br或Cl。将间隔分子固定为丁基铵时，我们发现二维HOIPs的面内杨氏模量通常遵循Pb-X键强度的趋势，这与面外模量中发现的趋势不同，表明金属卤化物无机骨架对面内杨氏模量的影响比面外模量更为显著。面内杨氏模量对线性烷基间隔分子的链长呈现非单调依赖性，即先降低并趋于平稳，然后再次升高。这可能是由于无机层中键强度与结构畸变的竞争、软有机间隔基团的相对比例以及与烷基链叉指相关的界面机械耦合所致。以面内杨氏模量与面外杨氏模量之比表示的二维HOIPs的力学各向异性，基于结构组成显示出广泛的可调性，特别是对于碘化物基二维HOIPs。我们的结果为二维HOIPs的力学各向异性和面内力学行为的结构-性能关系提供了有价值的见解，可指导材料设计和器件优化，以在基于二维HOIPs的应用中实现所需的机械性能。
