Research On Nano-Tribology Of Graphene Based On Atomic Force Microscope

In this paper,graphene on silica substrate was prepared by the method of mechanical exfoliation.The structure and morphology of graphene samples were characterized by Raman spectrometer and atomic force microscope.Atomic force microscope was used to study the adhesive properties of the graphene samples under different conditions and the influences of defects like fold,wrinkle,edge and step-edge found in the graphene samples on the frictional properties.The results of the research on graphene adhesion showed that the adhesion under nitrogen atmosphere was smaller than that under air atmosphere.This was becauce that the formation of water meniscus between probe tip and graphene surface could be hindered under nitrogen atmosphere.What's more,the adhesion of graphene edge was larger than that of the inner region.The adhesion measured with the same probe on copper-based graphene was larger than that of the silicabased graphene.No significant relationship was found between graphene adhesion and the number of layers.And the larger the curvature radius of the probe tip,the greater the adhesion.The adhesion of folded graphene along with its frictional properties were investigated under nitrogen atmosphere.The results showed that the adhesion was independent of the layer number of folded graphene.The frictional properties of each area of the folded graphene were far beyond the silica substrate.The friction coefficients of the single layer,the fold on single layer,the double layers and the fold on double layers regions were successively decreased,which were0.049,0.031,0.023 and 0.021 respectively.The friction forces were successively decreased as well.The frictional property of the graphene fold was weaker than the unfolded graphene of same number of layers due to the weaker bonding force between the layers,and at the same time,no obvious relationship between friction and adhesion was found in each area of folded graphene.Studies on measuring the adhesion with a sharp tip or a spherical tip showed that the measurement history of adhesion had little influence on the subsequent measurement of adhesion.Studies on freshly folded graphene in the air showed that the friction force of the folded region was significantly higher than that of the unfolded region.In the atmospheric environment,the characteristics of the friction properties of wrinkle,edge,and step-edge were studied by line scanning.The results showed that the friction of the wrinkle on graphene was larger than that of the wrinkle-free area,and it increased with the increasement of the load.When the probe tip slid perpendicularly to the wrinkle,the friction force at the wrinkle was the largest,showing obvious friction anisotropy;When the probe tip slid from a thin layer of graphene to a thick layer of graphene,the lateral signal enhanced significantly at the step-edge,and became stronger when the load increased.However,the step-edge of graphene did not show obvious friction anisotropy;For the edge of graphene,during the process where tip slid from the substrate to graphene,due to the elastic deformation of graphene,a frictional transition zone was formed at the edge.