Branched polyethylenimine fragmentation enabling efficient nitrogen doping and healing of defective graphene

An emerging post-synthesis doping/healing method for defective graphene layers by rapid thermal degradation of a polymer coating is a promising approach to mitigate defects introduced at nearly every step of manufacturing workflows. However, the thermal fragmentation process of the dopant-containing polymers and their interactions with defects are still unknown, hindering optimal use and application to other dopants and two-dimensional (2D) materials beyond graphene. To this end, this work employs a tandem approach of pyrolysis-gas chromatography/mass spectroscopy (Py-GC/MS) and reactive molecular dynamics (RMD) simulations for a model system composed of nitrogen-doping branched polyethylenimine (PEI) interfacing with graphene-like structures. The analysis of pure PEI degradation is used to compare and validate the approaches, where the same trends in fragment size distribution are observed. For Py-GC/MS, PEI and reduced graphene oxide (rGO) particles are used as a representative system, identifying 13 fragments that change significantly in abundance with rGO loading. Further insight into the pathway of degradation and doping is revealed via RMD simulations, where the degradation is found to begin at or near the large branches in the PEI structure. Overall, small fragments under 100 Da, such as nitrile derivatives, are identified as likely responsible for defect doping and healing after a sequence of reactions at the gas-graphene interface. The understanding of degradation and doping pathways in this model system will certainly lead to further optimization of the process and its expansion to additional polymers and 2D materials.