Van Der Waals Interfacial Mechanics In Graphene System

As the most representative 2D material,graphene has hastened many important scientific discoveries owing to its excellent physical and mechanical properties,especially in the fields of micro-nano mechanical and electronic devices(M/NEMS),nano and sub-nano scale transport,and 2D superconductivity.Due to the atomic thickness and extremely large surface area,the interfacial van der Waals(vdW)force of graphene significantly affects its elastic deformation,which determines the macro performance of materials and devices.At the nano and even sub-nano scale,the stacking effect size effect and strain effect play an important role on the physical and mechanical properties of graphene.The cross-scale research on graphene interfacial mechanics is not only of great significance to the basic disciplines such as micro-nano mechanics and physical mechanics,but also promotes the development of the new 2D M/NEMS.Focusing on the micro-nano to sub-nano scale mechanical behaviors of graphene vdW interface,this thesis studied three key scientific problems:i)the accurate measurement of interfacial mechanical parameters such as graphene interfacial adhesion energy,ii)the cleaning effect and mechanical mechanism of graphene vdW interface,iii)the atomic reconstruction of graphene and its mechanical mechanism.The specific research contents are as follows:1.Based on the micro-blister,Atomic Force Microscope(AFM)and Raman spectroscopy,the deflection and strain fields of graphene blister at micro scale were in-situ characterized.It was found that the interfacial shear resistance of graphene/graphene is weaker than graphene/SiO2.By introducing Lame solution into the mechanical model of circular blister,the model of micro-blister under sliding boundary condition was established.The interfacial adhesion energies of graphene/graphene and graphene/SiO2 were measured based on the geometric parameters of micro-blister,which were 120 ± 20 mJ/m2 and 260 ± 40 mJ/m2,respectively.We further revealed the influence of vdW interface on the elastic deformation of graphene,and tuned the friction of graphene to superlubric state by applying biaxial strain.2.The liquid nanopockets in suspended bilayer graphene interface were constructed.By charaterizing moire pattern generated by lattice mismatch of graphene,we found the cleaning effect of vdW interface.Under the mechanical stimulus by AFM tip,the nanopockets could move directionally,and adjacent nanopockets could merge with each other spontaneously with the recovery of moire pattern.These phenomena clearly confirmed the cleaning effect of graphene vdW interface.Based on the study of morphologies of nanopockets,the mechanical model of liquid nanopockets was established,and the interfacial adhesion energies of bilayer graphene with different twist angles were obtained.By introducing the edge instability effect into the nanopockets model,the long-range interaction between nanopockets was revealed,which explains the mechanism of nano-scale vdW interface cleaning.Moreover,the strain engineering on the long-range interaction between pockets was studied under in-situ loading.Based on the experimental results and mechanical model,we revealed that the long-range interaction between nanopockets is elastocapillary force,which was further verified by molecular dynamics(MD)simulation.3.By tuning the twist angle of graphene layers,the atomic reconstruction behavior at sub-nano scale was studied.The continuous-to-discrete transition of moire pattern was discovered under conductive AFM.Based on the Frenkel-Kontorova(F-K)model and the continuum mechanics,the mechanism of atomic reconstruction was revealed,that is the competition between interfacial vdW force and in-plane elastic deformation of graphene.The critical twist angle determined by the vdW force and the in-plane shear modulus was derived.Furthermore,based on the deformation of graphene moire pattern,we developed a new method to characterize strain fields at nano scale,whose accuracy could reach one ten-thousandth.We further studied the strain engineering of the atomic reconstruction behavior of graphene,and expanded the applicability of the atomic interfacial mechanical model.