Experiment Research On The Mechanical Properties Of Graphene

Graphene is an kind of material with great potential application for electric devices because of its special structure and superior properties in many aspects. In recent years, with the progress of researches on graphene's mechanical properties, its elastic modulus can be now directly measured by different methods, instead of deducing from the values of carbon nanotubes in the past. Besides there exists much difference among all kinds of testing methods, the elastic modulus of graphene still remain unknown. It is necessary to investigate it mechanical properties to provide basis for theoretic calculation in its applications.In this thesis, the advantages and disadvantages of different kinds of testing methods for thin films is analyzed. Given the conditions of our lab, the most proper method is applied to investigate mechanical properties of graphene.Atomic force microscope is also applied to study the elastic modulus of graphene films made by mechanical exfoliation. Our results show that the behavior of the graphene sheet under different maximum loads is found to exhibit three different stages, In the first stage, an unstable state is demonstrates at small maximum loads. This state is assumed to be caused by ubiquitous small wrinkles over the entire suspended graphene sheet, which require a certain load to stretch them flat. Second, a stable state is found, which demonstrates elastic-like deformation in the force-displacement behavior. In the third stage, a plastic deformation state is observed at large maximum loads. the dependence of the elastic modulus on graphene layer number is also investigated, discussing the possibility of interlayer sliding. It is found that the elastic modulus of monolayer graphene, bilayer graphene and four layer graphene are all identical and equal to the value for bulk graphite in our experiments. Our results demonstrate that there is no interlayer sliding and strong interaction between layers. Furthermore, thermal-dependent elastic modulus of monolayer graphene is also investigated in this article. Results show that force-displacement curves under increasing temperatures display a softening effect and the elastic modulus of the monolayer graphene decreases with the increase in temperature due to thermal vibrations of atoms.What's more, the friction properties of monolayer graphene and bilayer grapehene on SiO2 substrate and those in suspended conditions are investigated. The results show that the friction force of monolayer graphene supported by SiO2 substrate is lower than bilayer graphene supported by SiO2 substrate and two kinds of suspended graphene films, while there shows no significant difference between monolayer graphene and bilayer graphene in suspended conditions.