Two-dimensional halide perovskite single crystals: principles and promises

In the last few years, two-dimensional (2D) metal halide perovskites (MHPs) are gaining tremendous attention and are replacing their three-dimensional (3D) congeners rapidly. These next-generation halide perovskites can circumvent the limitations of the 3D MHPs such as stability and structural diversity. The incorporation of bulky organic cation can not only prevent the moisture penetration into the crystal lattice and thus providing greater stability but also expands the field of hybrid semiconducting materials by offering structural diversity. These unique features render even higher tuneability and improved photophysical properties and as a result intensive investigations have been made for 2D MHP-based polycrystalline thin films. However, single crystals based on two-dimensional halide perovskites are still emerging and have great potential for future device applications. Along with the hybrid halide perovskites, the all inorganic halide perovskites are also blossoming. In this review, we have discussed exclusively the development of hybrid as well as all inorganic two-dimensional halide perovskites. First, we have discussed the crystal structure of 2D perovskites. In the next section, different growth procedures reported for preparation of single crystals are discussed. We then highlight the effect of doping on single crystals and their optoelectronic properties. Finally, we discuss the current challenges and future perspectives to further develop 2D single crystals for their efficient use in various devices.