The use of organic thin films devices in various applications including photovoltaic, displays and sensors have tremendously increased owing to their relatively low cost and fabrication ease. Even though various factors affect the organic thin films performance, the morphology and crystalline ordering of its layers are particularly important since they play a significant role in determining the charger carriers’ mobility. This has attracted the attention of many researchers with the main focus being investigating their growth properties on different substrates including metals and metal oxides.
Graphene exhibit excellent chemical and physical properties thus it has been widely used as electrodes or templates in different organic devices to enhance device performance. Molecular orientation and packing on the graphene substrate influence the charge injection and charge carrier’s mobility across the electrode and organic material interface. In most cases, organic molecules with planer backbone lie on graphene thus resulting in graphene substrates being perpendicular to molecular π-π stacking. However, in rare cases like the organic field-effect transistors, parallel alignment is ideal as it enhances the charge injection and transportation. To this end, the controlling mode of crystalline ordering and alignment is vital for graphene-based organic thin films devices.
Despite substantial efforts in controlling the ordering and alignment of organic thin films with graphene surface perpendicular to the π-π stacking direction, less emphasis have been given to the parallel alignment. Generally, organic field-effect devices use aromatic molecules like metal-phthalocyanines as active layers. However, the need to enhance the performance of organic field-effect devices has led to extensive research about single crystalline structure metal-phthalocyanines thin films with graphene surface parallel to molecular π-π stacking.
Recently, Professor Wei-Dong Dou and Bi-Yun Shi at Shaoxing University developed a new fabrication method of single crystalline structure metal-phthalocyanines thin films with graphene surface parallel to the molecular π-π stacking. They purposed to control the orientation and packing ordering of the metal-phthalocyanines thin films by first depositing the copper nano grains onto the graphene surface before depositing metal-phthalocyanines thin films. In addition, they utilized atomic force microscopy and X-ray diffraction to investigate the film growth properties of metal-phthalocyanines on a graphene substrate. Their work is published in the journal, Organic Electronics.
The authors observed that initially, metal-phthalocyanines molecules adopted lying down geometries on a graphene substrate, due to strong molecule-substrate interaction, thus resulting in the formation of molecular π-π stacking that was perpendicular to the surface of graphene. However, it was switched to a direction parallel to the graphene surface when it was covered with a thin layer of copper nano grains before deposition of metal-phthalocyanines thin films.
The Wei-Dong Dou-Bi-Yun Shi study has successfully developed a new method for realizing parallel molecular π-π stacking of metal-phthalocyanines thin films on the graphene substrate. Also, it is more efficient for obtaining single crystalline metal-phthalocyanines thin films. Therefore, it will advance fabrication of high-performance graphene-based organic filed-effects devices that uses metal-phthalocyanines thin films as active layers. Their method can also be extended to other metal-phthalocyanines molecules with the same backbone that can be distinguished from each other.
Shi, B., & Dou, W. (2018). Tuning the molecular packing structure of metal-phthalocyanines on graphene by inserting interfacial copper nano grains. Organic Electronics, 56, 240-246.Go To Organic Electronics