Preparation Of Suspended Graphene And Study Of Its Friction And Mechanical Properties

Graphene has great application potential in micro/nano electromechanical systems as a solid lubricant.In this paper,a circular hole with a diameter of 3μm was prepared by contact photolithography and wet etching on Si O₂/Si substrate,and graphene was covered over the hole to form a baseless suspended graphene structure.After measuring and comparing the friction and mechanical properties of suspended and supported graphene,the following results were obtained:（1）In the absence of substrate support,the friction of graphene was significantly reduced compared with that of base-supported graphene,while the surface adhesion was not significantly different.The contact area between AFM tip and suspended graphene is larger by DMT calculation.Because the in-plane tension of the suspended graphene is greater than that of the supported graphene,resulting in a significant reduction in folds,the surface friction of the suspended graphene is reduced.At the same time,the friction-enhancing effects that had been seen on the supporting graphene were no longer seen on the suspended graphene.In addition,with the increase of graphene thickness,the external stiffness gradually increases.This reduces the friction difference between the suspended graphene and the supported graphene.（2）The adhesion and friction of the pre-worn tip on the graphene surface were significantly higher than that of the new tip,but the influence of suspension on the friction of the pre-worn tip was weakened.According to the calculation of van der Waals force and capillary force,it is found that the increase of tip radius will lead to the increase of van der Waals force and capillary force on the surface of graphene,which will significantly increase the adhesion force,while the increase of adhesion force directly leads to the increase of friction force.By comparing the surface friction of suspended graphene with supporting graphene with different thickness and different tip sizes,the important influence of folds on the friction of graphene was studied and the mechanism of folds was deeply discussed.（3）The mechanical properties of graphene were studied by nano indentation with atomic force microscope.The relationship between normal deformation and load of suspended circular film under point load is derived.The normal force-displacement curve of the suspended graphene films was measured by the nanometer indentation module of atomic force microscope.Young’s modulus of the graphene films was calculated by fitting the theoretical model and experimental data.Further indentation experiments were carried out to obtain the maximum fracture stress by crushing graphene and the fracture strength was calculated.