Raman Spectroscopic Study Of Strain Graphene At Strong Interface With Flexible Substrates

Graphene has been regarded as an ideal material for flexible electronic device because of its excellent electrical and mechanical properties,and its application value in flexible sensors has attracted more and more interest.In the current research,the common method is to use a flexible substrate(such as PDMS)as a support layer for graphene.However,the interface between graphene and flexible substrate is very poor.Therefore,designing a reliable and strong interface,which has good stress transmission between graphene and flexible substrate,has become an urgent problem to be solved in the application of graphene.In this work,a design of reinforced composite interface structure based on graphene is proposed.The structure uses formvar film as the transition layer,which improves the interface mutation between graphene and flexible substrate.Because the interfacial interaction between graphene and the substrate can be indirectly measured by strain transfer,and the Raman spectra of graphene has high strain sensitivity,we use Raman spectroscopy to quantitatively evaluate the interface strength between graphene and flexible substrate.The single-point Raman spectroscopy of graphene can be used to determine the maximum strain of graphene,while the scanning Raman spectroscopy can determine the strain uniformity over the entire sample range.We found that under this strong interface,graphene can reach nearly 2%strain,much higher than other interfaces(0.2%-1.3%),and the strain of graphene shows good uniformity.Based on this structure,we further explored the influence of crystal orientation and grain boundary on the Raman spectra of graphene under strong interface.The single crystal graphene was stretched along the AM and ZZ directions,and the Raman characteristic peaks(G peak and 2D peak)with strain were compared.We found that the graphene was stretched along different crystal orientations under a strong interface,and the Raman spectrum changes were not significantly different In addition,we have studied the Raman spectra of bi-crystalline graphene and polycrystalline graphene.By comparing their single-point Raman spectral and scanning Raman spectroscopy of 2D peak at the grain boundary,we can conclude that the grain boundary has no effect on the Raman spectrum of the graphene under strong interface.And based on the strong interface,we investigated the cause of the G~*peak by studying the strain sensitivity of the G~*peak(the Raman characteristic peak of graphene at about 2450 cm-1).First,during the experimental process,we found that the G' peak has good strain sensitivity during uniaxial stretching:the peak position changes continuously with increasing strain,while the peak shape and peak width are unchanged.Then,based on the first-principles calculation,we found that when the strain is less than 1%,the energy of the phonons along different second-order Raman spectral transition paths(?-K-S and ?-R-M)is uniform,so no peak splitting occurs,and the theoretical peak shift rate of the iTO+iLA phonon transition is obtained.These theoretical data are in good agreement with the experimental data,indicating that the cause of the G~*peak is derived from the phonon transition of iTO+LA.