Two-dimensional halide perovskites: A review on their orientations

Two-dimensional halide perovskites, emerging materials with quantum well structures, demonstrate excellent physical properties, such as easily tunable band structures, hot-phonon bottleneck, photo-induced phase segregation, and spin-orbit coupling, and therefore hold great potential for various optoelectronic devices. Interestingly, this class of perovskite materials also possesses a unique anisotropic feature, and their orientations can highly affect their charge carrier behaviors and other related properties. Therefore, effectively controlling the orientation of two-dimensional halide perovskites is of vital importance. So far, various methods, on the basis of different mechanisms have been developed to modulate the orientation, particularly the out-of-plane orientation. Herein, we summarize recent advances in these strategies for enabling the out-of-plane orientation for two-dimensional perovskites. We then systematically review the possible principles for tunning the orientation, followed by an overview of the remaining problems, particularly the microscopic mechanisms behind these modulation methods and their influences, and the future perspectives.