Chakrabarti Arkita, Gawas Ramchandra, Johnson Craig L, Fafarman Aaron T
Department of Chemical and Biological Engineering, Drexel University, Philadelphia, PA 19104.
Materials Characterization Core, Drexel University, Philadelphia, PA 19104.
Proc Natl Acad Sci U S A. 2024 Nov 12;121(46):e2413942121. doi: 10.1073/pnas.2413942121. Epub 2024 Nov 7.
In this work, the phenomenon of strain induced by a mismatch in thermal expansion coefficients between a thin film and its substrate is harnessed in a new context, replacing the canonical planar support with a three-dimensional (3-D), nanoconfining scaffold in which we embed a material of interest. In this manner, we demonstrate a general approach to exert a continuously tunable, triaxial, tensile strain, defying the Poisson ratio of the embedded material and achieving the exotic condition of "negative pressure." This approach is hypothetically generalizable to materials of low modulus and high thermal expansion coefficient, and we use it here to achieve negative pressure in perovskite-phase CsPbI embedded within the cylindrical pores of anodic aluminum oxide membranes. Through controlled thermal hysteresis, the perovskite crystal structure can be continuously tuned toward higher symmetry when confined in a scaffold with pore size <40 nm, in contrast with the symmetry-reducing action of any other mechanical perturbation. We use this effect to control the octahedral rotation angle that is critical to the remarkable photovoltaic attributes of halide perovskites. Under hundreds of megapascals of apparent negative pressure, the bandgap tunability is observed to follow the same quantitative trend observed for hydrostatic positive pressure, exploring the negative pressure region and demonstrating the relative dominance of bond stretching effects over average octahedral rotation angle on electronic structure. This study reveals and quantifies the structural and electronic consequences of 3D tensile strain present by design and provides a framework for understanding adventitious strain present in all nanocomposite materials.
在这项工作中，我们在一个新的背景下利用了薄膜与其衬底之间热膨胀系数不匹配所引起的应变现象，用一种三维（3-D）纳米限制支架取代了传统的平面支撑，我们将感兴趣的材料嵌入其中。通过这种方式，我们展示了一种施加连续可调的三轴拉伸应变的通用方法，突破了嵌入材料的泊松比，并实现了“负压”这一奇特状态。该方法假设可推广到低模量和高热膨胀系数的材料，我们在此用它在阳极氧化铝膜的圆柱形孔隙中嵌入的钙钛矿相CsPbI中实现了负压。通过控制热滞回，当限制在孔径<40 nm的支架中时，钙钛矿晶体结构可以朝着更高的对称性连续调整，这与任何其他机械扰动的对称性降低作用形成对比。我们利用这种效应来控制八面体旋转角，该角度对于卤化物钙钛矿卓越的光伏特性至关重要。在数百兆帕的表观负压下，观察到带隙可调性遵循与静水正压相同的定量趋势，探索了负压区域，并证明了键拉伸效应在电子结构上相对于平均八面体旋转角的相对主导地位。这项研究揭示并量化了通过设计存在的三维拉伸应变的结构和电子后果，并提供了一个理解所有纳米复合材料中偶然应变的框架。
