Experimental Characterization On Interfacial Mechanical Properties Of Graphene/Substrate Interface And Several Main Influence Factors

Practical industrial mass production of macro-sized graphene is realized with the development of the chemical vapor deposition(CVD)method for graphene preparation,and macro-sized graphene materials are expected to be increasingly applied in the domain of flexible electronic devices.The microstructure of graphene/substrate is the foundation and core of the electronic devices.The structural reliability,performance and stability of these devices are directly influenced by the interfacial mechanical behaviors of the graphene/substrate microstructure.Meanwhile,the strain regulation of the graphene and graphene heterostructure-based devices depends on the interfacial load transfer.Currently,a prominent problem in research on the properties of nanomaterials such as graphene is the large discrepancy between the extracted data used to quantitatively characterize the interfacial properties,with the difference reaching several orders of magnitude.Herein,we focus on the microstructure of CVD monolayer graphene/substrate,and systematically investigate the interfacial mechanical properties of the graphene/substrate microstructure using in situ Raman spectroscopy.Several typical influence factors on the interfacial mechanical properties of graphene/substrate are analyzed.This study is of critical scientific significance with definite engineering application value.First,the microstructure of 1-cm-long CVD monolayer graphene/PET substrate is designed and prepared to investigate the interfacial mechanical properties of graphene/substrate.The methodology of quantitative mechanical measurements of graphene using in situ Raman spectroscopy is discussed.Using a micro-tensile test and Raman spectroscopy,in situ measurements are taken to obtain the strain field of graphene subjected to a uniaxial tensile loading and unloading cycle.The interfacial stress/strain transfer and the evolution of the bonding status of the interface are analysed.Five interfacial mechanical parameters are defined and extracted to quantitatively characterize the interfacial properties of graphene/substrate.To explore the existence of the size effect of the interfacial mechanical properties,eight microstructures of PET substrate and graphene with eight different sizes ranging from microns to centimeters are designed and studied.The size effect of the interfacial mechanical properties of graphene/PET microstructure is found by a series of Raman experiments.Data reveal that the interface strength decreases by power exponent relates to the increase of graphene length.Therefore,the reason for the large discrepancy between the extracted data from previous work is well-described by the size effect found here.The experimental analysis also indicates that the fringe area of the graphene is the main factor affecting the interfacial mechanical properties and is one of the main causes for the size effect.An innovatively defined parameter called the relative critical length is used to characterize the size effect of interfacial properties of graphene/substrate.Second,a series of double cantilever beam(DCB)fracture tests are performed for seven applied separation rates to measure the adhesion energy of the graphene/PET interface.Experimental results reveal that the measured adhesion energy of the graphene/PET interface linearly increases with the increase of the applied separation rate.After the DCB tests using seven applied separation rates,the strain field of the transferred graphene on the fracture surface(adhesive)is measured and analyzed using in situ micro-Raman spectroscopy to explore the reason for the separation rate dependence of the adhesion energy.The results indicate that a high separation rate led to damage and induced residual tensile strain of the transferred graphene causing the extra energy loss,which is considered as the main cause for the separation rate dependence of the adhesion energy.Finally,a standard method of selecting the optimal separation rate in DCB tests for measuring the adhesion energy of the interface between graphene(or another nanofilm)and a substrate is proposed.Third,a theoretical model describing the Van der Waals interactions of the interface between the graphene and a substrate with corrugated surface is established.The Van der Waals interaction potential considering the 4th-order approximation is proposed using an analytical approach,indicating that the solution has the problem of no convergence when the roughness of the substrate is large.A numerical approach is then developed to keep clear of no convergence.The effective frictional coefficient and maximum interfacial shear stress(interface strength)when the graphene slides at a constant height(flat trajectory)above the substrate with a sinusoidal surface are obtained,suggesting that the two parameters are directly proportional to the roughness of the substrate.Meanwhile,the maximum interfacial shear stress and the adhesion energy are linearly related.The negative effective frictional coefficient and bifurcation phenomenon appear in certain location(the equilibrium separation)above the substrate.The situation that the graphene slides at a constant normal force(wavy trajectory)above the substrate are further studied,indicating that the shear Van der Waals force acting on the graphene is quite small so that can be ignored when the graphene slides at zero normal force along the wavy trajectory above the substrate.Therefore,it is theoretically predicted that hyperlubrication can occur in graphene/substrate microstructures.Finally,combined with the experimental results in chapter 3,this chapter explaines from the theoretical point of view that the roughness of the substrate is one of the reasons for the size effect of graphene/substrate microstructure.