Tracing the graphitization of polymers: A novel approach for direct atomic-scale visualization

Due to its excellent physical, chemical, and electrochemical characteristics, pyrolytic carbon has become a promising material for a wide range of advanced technologies. Pyrolytic carbon can be obtained through the pyrolysis of a polymeric carbon precursor at high temperatures and in inert atmosphere. By tuning the pyrolysis conditions, the hybridization of carbon atoms and thus the physicochemical properties of the derived carbon can be tailored. Advancing its development requires a deeper understanding of the graphitization process. In this context, an in situ microstructural analysis of the pyrolysis process is needed. This work presents the microfabrication of suspended polymer thin film structures on transmission electron microscopy heating chips, by two-photon polymerization 3D printing. We visualized graphitization of these films during in situ transmission electron microscopy heating studies. The favorable identified conditions are a thin film with a thickness of around 700 nm pre-pyrolysis, a pyrolysis profile reaching a maximum temperature of 1300°C and a minimum of 2 h of dwell at this temperature. An increase in the number of stacked graphene layers was observed over dwell time. Overall, the developed method has the potential to enable the visualization of graphitization of different polymer precursors and thus help predict the microstructure and properties of pyrolytic carbon depending on its fabrication conditions.