Recent Advances in the Asymmetric Growth of Organic Polar Crystals: A Review

Nowadays, organic polar crystals are likely to replace and surpass inorganic crystals as the new generation of functional crystal materials due to their ultrafast nonlinear optical response, low dielectric constant, and strong molecular designability. However, these organic polar crystals with excellent properties often have imperfections, such as rough surfaces, twin boundaries, and narrow-line defects in the bulk. Therefore, there is an urgency to establish methods to maximize their properties starting from the mechanism. Herein, this work first reviews asymmetric growth mechanisms of organic polar crystals, including solvent adsorption inhibition, "relay" growth, "self-poisoning", and so on. Then, the universal characterization methods of organic polar crystals in terms of crystallography and properties are described in detail. Besides, a careful examination of regulation strategies to obtain high-quality organic polar crystals shows that soft templates, polymer additives, and temperature cycling are prospective approaches. Subsequently, the potential of organic polar crystals for nonlinear optical applications has been continuously explored, and their types can be classified into organic small molecules, organic ionic compounds, and organic poled polymers. Finally, the corresponding future opportunities and challenges are put forward based on the shortcomings and application bottlenecks of the current asymmetric growth mechanism of organic polar crystals.