Engineered Negative Pressure in Nanoscaffolded CsPbI₃ Imparts Thermodynamic Stability and Optical Tunability to the Perovskite Phase

Strain at heterointerfaces in functional composite materials is inevitable and has in 2D geometries been intentionally engineered to improve superconductivity, ferromagnetism, and phase stability. As an example, tensile strain generated due to the mismatch in thermal expansion coefficients between thin-film CsPbI₃ and its substrate (so called substrate-clamping) has been shown to impart metastability of the otherwise unstable perovskite phase at room temperature. However, such substrate-induced stress is biaxial, resulting in an opposing strain in the third, orthogonal axis. Herein, we extend such engineered interfacial strain to all three dimensions to approximate the condition of ‘negative pressure,’ i.e., 3D tensile stress. Using a generalizable approach, we show negative pressure stabilizes the symmetric, low density, cubic perovskite phase of CsPbI₃, which is a promising absorber material for photovoltaic devices. In this work, we crystallized CsPbI₃ in the perovskite phase inside rigid oxide scaffolds at elevated temperatures and quenched them. Given the extremely low modulus of CsPbI₃, when it is synthesized within a rigid, ordered metal-oxide nanoscaffold at high temperature and then rapidly quenched, a tensile interfacial stress is generated. The three-dimensional tensile strain generated by this thermally induced stress differs from its biaxial counterpart in that it can thermodynamically favor high symmetry crystal phases i.e., the functional perovskite phase. We used X-ray diffraction and high-resolution transmission electron microscopy to identify the phase and estimate lattice expansion as a function of thermal excursions imposed on the CsPbI₃-scaffold composite. Furthermore, photoluminescence studies showed bandgap tunability as a consequence of negative pressure. Such an experimental materialization of 3D-interfacial stress or “negative pressure” to access the thermodynamically forbidden phases and tune optoelectronic properties of a perovskite material has been demonstrated for the first time in this work.