A research team led by Professor Rongjin Li and Professor Wenping Hu from Tianjin University reported the controllable growth of molecularly thin organic single crystals. The research result was published online on Angewandte Chemie under the title of “Layer-defined Strategy to Grow Two-dimensional Molecular Crystals on LiquidSurface down to Monolayer”.

Two dimensional molecular crystals (2DMCs), as the counterpart of 2D atomic crystals (2DACs, such as graphene), are periodically arranged monolayer or few-layered organic molecules held together by weak interactions in a 2D plane.2DMCs enables the probe of the physical properties of organic materials in the 2D limit, which cannot be done by polycrystalline thin films. In applications,2DMCs combine the solution processability of polycrystalline thin films and the high performance of bulk single crystals. 2DMCs might be the ideal material for many optoelectronic applications, including field-effect transistors and sensors. However, as an emerging type of materials, their controllable growth has been challenging.

The research team focused on 2DMCs and have made several key breakthroughs. The team discovered the key role of surface energy in determining crystal morphology and proposed the “phase separation” molecular design for 2DMCs. They also developed the space-confined self-assembly strategy for the preparation of large area 2DMCs.

To solve the challenge of layer number control, they proposed the growth of 2DMCs on a viscous liquid surface.By controlling the spread of the solution on the surface of glycerol, quasi-freestanding 2DMCs with controllable layers from bulk down to the monolayer limit are achieved for the first time, which enables the investigation of the layer-dependent optoelectrical response of organic semiconductors. The ultrathin channels are found to show a low leakage current in photodetectors (the leakage current of the monolayer channel is 1/1000 of that of the bulk channel), which can be used to detect extremely weak light. The preparation of 2DMCs with controllable layer numbers paves the way for the applications of 2DMCs in new optoelectronic devices and technologies.

Link to the paper:https://doi.org/10.1002/anie.201909552.

By Yao Jiarong from the School of Science

Editors: Eva Yin & Doris Harrington