Abstract
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 submonolayer surfactant coverage, using ultrasonic and atomic force microscopies both in ambient and ultrahigh vacuum, reveals complex, multilength-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). While 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.
Highlights
Surfactants are essential[1−4] for the efficient separation, dispersion, and functionalization of graphitic materials[5] in aqueous solution and provide enabling steps in the production of graphene devices.[6]
The recently synthesized family of surfactants[19] contain pyrene groups for stable planar anchoring to the graphene surface, connected by oligoethylene glycol (OEG) chains to hydrophilic head groups derived from Newkome dendrons.[20]
Pyrene is known to bind to graphene with a binding energy of −1.09 eV, and to experience a low energy barrier of ca. 0.01 eV to sliding and rotation parallel to the sheet.[21−24] This results in a desirable combination of (i) strong adsorption of the surfactants onto graphitic surfaces, and (ii) free lateral movement over them
Summary
Surfactants are essential[1−4] for the efficient separation, dispersion, and functionalization of graphitic materials[5] in aqueous solution and provide enabling steps in the production of graphene devices.[6]. For sensing and energy storage applications, separation of graphene sheets by surface-stable surfactant structures with a smaller thickness is highly desirable.[17,18] We report a combined experimental and theoretical study of the molecular assembly of a tailored class of surfactants on graphitic surfaces. The resulting nano to microscale structuring is shown to derive from the formation of flat 2D starfish micelles (2DSMs), whose height is much smaller than their lateral surface dimensions. These 2DSMs are not formed by transferring preformed 3D micelles onto surfaces, but instead arise from the postdeposition aggregation of surfactant molecules on the surface
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