(495i) Role of Interfacial Electronic Interactions in Graphene-Directed Assembly of Conjugated Polymers

Graphene-organic hybrid electronics is indisputably one of the most promising directions of research for development of next-generation electronics. Graphene, a Nobel prize-winning feat, possess extraordinary properties such as remarkable mechanical strength, ultrahigh conductivity and optical transparency. However, drawbacks such as lack of band-gap and low structural tunability has limited its applications. With limitless possibility of facile molecular structure and band gap tunability, conjugated polymers possess properties that complements graphene and are an attractive class of materials for graphene-organic hybrid electronics, albeit they demonstrate low electronic performance and are susceptible to degradation. In this scenario, integration of the two materials could provide a synergistic advantage, leading to electronic devices like sensors and photodetectors with superior properties. Moreover, utilization of graphene as a substrate to template self-assembly of conjugated polymers could also result in unique electronic interaction between the two materials which can be fascinating for fundamental studies and potential applications. This interaction could also modulate multiscale morphology of the polymer as well as optoelectronic properties of both the materials. While studies involving adsorption of organic molecules on graphene have been well explored previously in the case of 𝜋-conjugated small molecules, similar studies on the conjugated polymers is clearly lacking. In addition, fundamental questions on how the conjugated polymers interact with graphene and how it affects electronic structure of both materials is still not understood.

In this talk, we present graphene as a novel substrate in templating self-assembly of conjugated polymers and demonstrate the effects of the resulting interfacial electronic interaction on both conjugated polymer as well as graphene. By utilizing meniscus-guided solution coating technique, we assembled monolayer films of two donor-acceptor polymers, PII-2T and DPP-BTZ, on graphene and silicon substrates. While atomic force microscopy revealed higher fiber density and higher film coverage of PII-2T monolayer films on graphene, grazing incidence X-ray diffraction revealed that graphene results in higher crystallinity and higher alignment of polymer chains. In addition to the morphological variation due to polymer-graphene interaction, photoelectron spectroscopy measurements showed lowering of binding energies of core electrons in various atoms present in the polymer chain as well as Fermi level movement to highest occupied molecular orbital (HOMO) level, both of which suggest p-type doping of the polymer. This effect was exaggerated in the case of polymers assembled on fluorinated graphene where presence of electronegative fluorine atoms must have increased the p-type doping phenomenon of the polymer. The presence of electronic interaction between the conjugated polymer and graphene-based substrates and the resulting electronic structure modification on both sides of the interface was also corroborated by Raman spectroscopy and UV-Vis spectroscopy.