Subsurface Imaging of Functionalized and Polymer-Grafted Graphene Oxide

We investigate the surface and subsurface morphology of stearylamine-modified graphene oxide sheets and polystyrene-grafted functionalized graphene oxide sheets through atomic force microscopy (AFM) operated, in multi-set point intermittent contact (MUSIC) mode. This allows a depth-resolved mapping of the nanomechanical properties of the top surface layer of the functionalized graphene oxide sheets. On the surface of stearylamine-functionalized graphene oxide sheets, we can distinguish areas of hydrophilic graphene; oxide from hydrophobic areas functionalized with stearylamine. We find that every sheet of graphene oxide is functionalized with stearylamine on both sides of the sheet. The exposure of polystyrene-grafted functionalized graphene oxide to chloroform vapor during the AFM measurement causes a selective swelling and a softening of the polystyrene envelope. The depth-resolved mapping of the tip sample interaction allows the shape of the folded and wrinkled graphene oxide sheets within the polystyrene envelope to be imaged; furthermore, it allows the thickness of the swollen polystyrene envelope to be measured. This yields the swelling degree, the grafting density, and the average chain conformation of the grafted polystyrene chains, which we find to be in the brush regime. Our work demonstrates a versatile methodology for imaging and characterizing functionalized and polymer-grafted two-dimensional materials on the nanometer scale.