Formation of two-dimensional micelles on graphene:a multi-scale theoretical and experimental study

Graphene and related two-dimensional (2D) materials possess outstanding electronic and mechanical properties, chemical stability and high surface area. However, to realize graphene’s potential for a range of applications in materials science and nanotechnology there is a need to understand and control the interaction of graphene with tailored high-performance surfactants designed to facilitate the preparation, manipulation and functionalization of new graphene systems. Here we report a combined experimental and theoretical study of the surface structure and dynamics on graphene of pyrene-oligoethylene glycol (OEG) -based surfactants, which have previously been shown to disperse carbon nanotubes in water. Molecular self-assembly of the surfactants on graphitic surfaces is experimentally monitored and optimized using a graphene coated quartz crystal microbalance in ambient and vacuum environments. Real-space nanoscale resolution nanomechanical and topographical mapping of sub-monolayer surfactant coverage, using ultrasonic and atomic force microscopies both in ambient and ultra-high vacuum, reveals complex, multi-length-scale self-assembled structures. Molecular dynamics simulations show that at the nanoscale these structures, on atomically-flat graphitic surfaces, are dependent upon the surfactant OEG chain length and are predicted to display a previously unseen class of 2D self-arranged ‘starfish’ micelles (2DSMs). Whilst three-dimensional micelles are well known for their widespread uses ranging from microreactors to drug-delivery vehicles, these 2DSMs possess the highly desirable and tunable characteristics of high surface affinity coupled with unimpeded mobility, opening up strategies for processing and functionalizing 2D materials