Colloidal dispersions of substrate-free gas-phase synthesized graphene: dispersion formulation for thin film formation

Substrate-free graphene synthesized in the gas-phase is a promising material with high specific surface area, exceptional purity, and conductivity. Despite these desirable properties for various applications, the transformation from the powder to stable dispersions and functional thin films remains an obstacle for the technical integration of this material. In this work, this challenge is addressed through systematic dispersion, stabilization, and thin film formation studies. Space- and time-resolved analytical centrifugation was used as an in-situ method to quantify the sedimentation of graphene. Remarkable stability was achieved only when agglomeration of graphene particles was suppressed in either solvents such as N-(2-hydroxyethyl)-2-pyrrolidone, a non-toxic analog to N-methyl-2-pyrrolidone or using polymers and surfactants as stabilizers like carboxymethyl cellulose, polyvinylpyrrolidone, and N-didodecyldimethyl-ammonium bromide in aqueous dispersions. These optimized dispersions were used to fabricate thin films via Langmuir deposition, spray coating and ink-jet printing. Hall effect conductivity measurements in van der Pauw geometry, novel to this class of thin films, and surface analysis revealed conductive, yet rough and porous layers formed by spray coating (mobility up to ≈ 2 cm 2 /V∙s, carrier density ≈ 50∙10 14 1/cm 2 ) and compact micro-rough layers by Langmuir deposition (mobility ≈ 19 cm 2 /V∙s). This study provides insights into dispersion-process-structure-functionality relations and forms the framework for scalable dispersion formulation and thin film formation. The findings gained address major challenges for the device-integration of free-standing graphene for catalytic, adsorption and electronic applications.