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.

The fabrication of high‐performance (opto‐)electronic devices based on 2D channel materials requires the optimization of the charge injection at electrode–semiconductor interfaces. While chemical functionalization with chemisorbed self‐assembled monolayers has been extensively exploited to adjust the work function of metallic electrodes in bottom‐contact devices, such a strategy has not been demonstrated for the top‐contact configuration, despite the latter being known to offer enhanced charge‐injection characteristics. Here, a novel contact engineering method is developed to functionalize gold electrodes in top‐contact field‐effect transistors (FETs) via the transfer of chemically pre‐modified electrodes. The source and drain Au electrodes of the molybdenum disulfide (MoS2) FETs are functionalized with thiolated molecules possessing different dipole moments. While the modification of the electrodes with electron‐donating molecules yields a marked improvement of device performance, the asymmetric functionalization of the source and drain electrodes with different molecules with opposed dipole moment enables the fabrication of a high‐performance Schottky diode with a rectification ratio of ≈103. This unprecedented strategy to tune the charge injection in top‐contact MoS2 FETs is of general applicability for the fabrication of high‐performance (opto‐)electronic devices, in which asymmetric charge injection is required, enabling tailoring of the device characteristics on demand.

Unidirectional and crystalline organic semiconductor microwire arrays were obtained by solvent vapor annealing with PMMA as the assisting layer.