Impact of surface free energy on two-dimensional crystallization

Two-dimensional (2D) molecular crystals are emerging materials with fascinating optoelectronic properties. However, growing 2D organic single crystals is challenging because little is known about the factors that determine 2D crystallization. Herein, we investigated the relationship between 2D crystallization and the surface free energy of organic semiconductors. We proposed a term called surface free energy anisotropy factor (g) to measure the tendency of 2D crystallization. Taking two [1]benzothieno [3,2-b] [1] benzothiophene derivatives as examples, we identified that the one with a larger g factor showed a higher tendency for 2D crystallization, irrespective of the external factors. This work provides a guideline for the rational design of organic semiconductors toward 2D crystallization.