Surface Mapping of Functionalized Two-Dimensional Nanosheets: Graphene Oxide and MXene Materials

In this study, we characterized the morphology, composition, and surface properties of individual flakes of graphene oxide and Ti₃C₂T MXene chemically modified with ethylenediamine, dopamine, and (3-aminopropyl) triethoxysilane (APTES). Individual monolayers of modified Ti₃C₂T MXene and graphene oxide nanosheets were deposited using the Langmuir-Blodgett technique. We compared the chemical surface modification of these two-dimensional (2D) flakes by employing advanced atomic force microscopy (AFM) modes, including quantitative nanomechanical (QNM) mode, Kelvin-Probe force microscopy (KPFM), and Nano-IR imaging. This approach reveals the distribution of mechanical, electrical, and chemical properties on individual flakes at the nanoscale. QNM analysis confirms that the flakes exhibited full surface coverage after the chemical modification process. In modified MXene flakes, we observed a decrease in apparent elastic modulus and an increase in adhesion of up to four times after their functionalization. Nano-IR imaging demonstrates that chemical modification uniformity is highest for graphene oxide species, while the complex surface distribution was observed for dopamine-modified MXene flakes, with a difference between the inner flat surface and their edges. KPFM indicates greater uniformity of surface electrical potential in differently modified graphene oxide, while a significant increase in surface potential of MXene flakes is seen when modified with dopamine. We suggest that a combination of the added dielectric layer and different grafting densities across the flakes is responsible for the increased or changes in apparent surface potential. Overall, a combination of AFM probing modes is needed for understanding how these functionalized nanosheets can be integrated into diverse polymer matrices.