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.
ABSTRACT
A variety of structures of meta-aminobenzoate molecules adsorbed on the Cu(110) surface have been characterized by scanning tunneling microscopy (STM) at a wide range of surface coverages, from a single molecule to saturated phases. At the start of molecular domain formation, individual molecules thermally diffuse to form chain structures via intermolecular hydrogen bonding. At higher surface coverages, there coexist three well-ordered phases, namely [Formula: see text] and chiral [Formula: see text] phases. The molecular orientation on the surface also varies with surface coverage. Flat-lying molecules are mainly observed at low surface coverage, while upright molecules start to appear as the surface becomes more highly covered. Our experimental findings and structural analysis are well supported by high-resolution STM images measured at 4.7 K and by molecular packing models with precise lattice parameters.
