Layer‐Defining Strategy to Grow Two‐Dimensional Molecular Crystals on a Liquid Surface down to the Monolayer Limit

Two-dimensional molecular crystals (2DMCs) open a new door for the controllable growth of 2D materials by molecular design with a energy gap and solution processability. However, the growth of 2DMCs with defined molecular layers remains full of challenges. Herein, we report a novel method to produce various 2DMCs with a defined number of molecular layers. When the surface tension and viscosity are tuned to control the spreading of the solution on the liquid surface, large-area quasi-freestanding 2DMCs from bulk size down to the monolayer limit are obtained, which makes it possible to probe the intrinsic layer-dependent optoelectronic properties of organic semiconductors down to the physical limit, and paves the way for the application of 2DMCs in new optoelectronic devices and technologies.