Professor Yinsheng Guo of the University of Nebraska - Lincoln

"Simple Visualization of Universal Ferroelastic Domain Walls"

Lead halide perovskites (LHPs) have emerged as a promising class of solution processable semiconductors, showing outstanding performance in optoelectronic applications including photovoltaics, solid state lighting, and photodetection. Domain features and domain walls in LHPs have attracted broad interest due to their potential impact on optoelectronic properties of this unique class of semiconductors. Using non-polarized light and simple imaging configurations, we directly visualize ferroelastic twin domains and their switching through multiple consecutive phase transitions. This direct optical contrast originates from finite optical reflections at the wall interface between two compositionally identical, orientationally different, anisotropic domains inside the material bulk. Our findings show these domain walls steer internal reflections and energy transport inside halide perovskites optically. First-principles calculations show universal low domain-wall energies and modest energy barriers of domain switching, confirming their prevalent appearance, stable presence, and facile movement observed in the experiments. The generality of ferroelasticity in halide perovskites stems from their soft bonding characteristics. This work shows the feasibility of using LHP twin domain walls as optical guides of internal photoexcitations, capable of non-volatile on-off switching and tunable positioning endowed by their universal ferroelasticity.