Forming extremely thin crystals of organic semiconductors under ambient conditions is now possible. The key is a tiny gap. Materials sprayed on a surface vaporize when heated, rising to a substrate a tiny distance above where they cool and form very thin crystals (Chem. Mater. 2020, DOI: 10.1021/acs.chemmater.9b05215). The method uses a simple heater and common organic semiconductor materials to make ultrathin crystals, which have better performance in devices like LEDs and field effect transistors because of their low electrical resistance. The approach could make the fabrication process easier and cheaper.
Organic semiconductor crystals, including ultrathin crystals, are generally made by depositing them from solutions or by cooling vaporized material onto a substrate. But these methods have disadvantages. The solution approach produces crystals with defects arising from incorporating solvent molecules, making them unstable. The vaporization process produces nearly flawless crystals but requires special vacuum-based equipment with controlled atmospheres, and it is still quite difficult to get crystals only a few molecular layers thick.
Yang Liu, Xutang Tao, and their colleagues at Shandong University thought they could get ultrathin crystals by adapting a method they previously developed to form pure organic crystals (Chem. Mater. 2018 DOI: 10.1021/acs.chemmater.7b04170). By creating a gap of a few hundred micrometers between the reservoir of material and the substrate on which the crystals will deposit, the researchers were able to mimic the conditions used in traditional vacuum-based processes, but without any special equipment.
The gap is so small that the molecules have nowhere else to go, explains Paddy K. L. Chan of The University of Hong Kong, who was not involved in either study. They travel almost straight up without colliding with any other molecules and are forced to crystallize on the upper substrate.
Building upon that technique, Liu and Tao’s team has now developed a method of growing extremely thin crystals that could be used for high performance and cheaper electronic devices by controlling how the organic material is distributed on the heated surface.
Instead of spreading solid powder on a surface as they had done previously, the authors dissolved two common organic semiconductors, pentacene and one known as DNTT, in toluene. They coated the lower surface with the solution using various methods including dropping and spraying, placed a cleaned silicon wafer 300 µm above, and heated the lower surface to 230 °C.
Using this technique the researchers formed much thinner crystals than with their previous approach, down to about 8 nm for DNTT and about 5 nm for pentacene—just five and three molecular layers, respectively, and were able to control where on the substrate the crystals were formed.
Field-effect transistors made of these crystals showed average charge carrier mobilities, an important measure of performance, that were comparable to those reported in other studies. The best carrier mobilities were some of the highest reported so far for ultrathin crystals of these materials. Along with the simple method of making these thin crystals, the high performance field-effect transistors could lead to cheaper and faster electronic devices.
Pentacene is “a superstar organic semiconductor which has long been considered as air-sensitive,” Liu says, so being able to apply this approach successfully to this material is a step toward it being industrially practical. In addition, Liu thinks this method can also be used for making ultrathin crystals of inorganic materials like perovskites that are used in solar cells.
The technique will be useful for making thin crystals for applications like thin film displays and sensors, which rely on high-performance field-effect transistors, says Hanying Li of Zhejiang University, who was not part of the research. Thin crystals are especially suited for flexible electronics. However, since the gap is very critical in this method, he thinks its use may be limited for applications in flexible devices, where the spacing will be difficult to control.