Graphene Preparation And Raman Spectroscopy Of Strained Graphene

Graphene has aroused researchers’high attention due to their excellent photoelectric properties in device applications related to its unique structure.Recently,in order to further meet more demands and expand the application fields of graphene,various techniques（including doping,alloying,inducing defects,forming van der Waals heterostructure,electrostatic regulation,and strain engineering）have been exploited to modulate their properties.The purpose is to adjust the band structure to open up the band gap of graphene.Among them,strain engineering is an effective technique that changes the lattice and electronic structure and thus modulates various properties of graphene.However,doping and electrostatic regulation have poor accuracy and stability for graphene band gap control.Strain engineering,as an important strategy for tuning the lattice and electronic structure of graphene,has been widely used in the modulation of physical properties,and has strong controllability and reversibility for the regulation of graphene performance.The introduction of strain has also broadened the device applications of graphene in the field of flexible nanoelectronics and optoelectronics.Therefore,based on the latest development of strain engineering,research content of this article mainly includes the following three aspects:（1）The research on Raman spectroscopy properties of graphene produced by exfoliating graphite and chemical vapor deposition（CVD）.For the graphene produced by exfoliating graphite,Raman characterization analysis shows that G and 2D peak shift to a high-frequency（bule-shift）as the number of the layer increased.The peak position of graphene is dependent on the laser energy with the increase of energy,the G peak slightly shifts to a low-frequency（red-shift）,and the peak position of the 2D peak significantly shows a blue-shift.Further,the crystallographic orientation of graphene can be estimated according to the Raman mapping of peak D.When the angle between two adjacent edges is 30°,the two adjacent edges should have different chirality.That is,one is armchair and the other is zigzag.Contrarily,when the angle is60°,both edges should have the same chirality,either armchair or zigzag.For the graphene synthesized by CVD method,the influence of oxidation of copper substrate and the introduction of passivating agent melamine on production of graphene were explored.The experimental results show that oxidation of copper substrate and the introduction of melamine can reduce active sites and increase the area of single crystal graphene.According to the polarization Raman analysis of graphene,the experimental results show that the polarization Raman does not depend on excitation energy.Through analysis to characterize the polarization of Raman,the G peak of graphene is isotropy,which shows a round shape;the 2D peak of graphene is anisotropy,which shows a figure-eight shape.（2）The uniaxial and biaxial strain of graphene was introduced by transferring the CVD-grown graphene onto the surface of Au microparticles（Au MPs）.Through the characterization of graphene Raman spectroscopy and polarized Raman spectroscopy,we found that G peak and 2D peak red-shifts with increasing uniaxial and biaxial strain.Under uniaxial strain,the G peak splits into G^-peak and G⁺peak.For every1%applied under uniaxial strain,the peak position of G^-peak drops by 31.1 cm^-1,the peak position of G⁺peak drops by 12.8 cm^-1 and the peak position of 2D peak drops by 40.9 cm^-1.For every 1%stress applied under biaxial strain,the peak position of the G peak decreases by 6.0 cm^-1 and the peak position of the 2D peak decreases by 15.0 cm^-1.It is obvious that the tensile strain causes significant softening of graphene phonons,as reflected by red-shift of G and 2D peak.The uniaxial strain will break the sublattices symmetry of graphene,hence causing the G peak to split.The isotropy of graphene is broken,and the morphology of the polarized Raman spectrum transforms from a circle to a figure of eight under uniaxial or biaxial stress.For the unsplit G peak,the analysis of the polarization dependence of the Raman scattering process is used for the first time to determine the orientation of the graphene crystal axis.The strain is applied at an angle of 20°from the zigzag direction,whereas for the graphene under biaxial strain is applied at 14°from the zigzag direction.The Raman polarization spectra of the 2D peaks are deflected under uniaxial or biaxial stress,indicating that strain can cause the anisotropic phonon softening and Dirac cone deformation.（3）The wrinkle of graphene was introduced by transferring graphene onto the surface of polydimethylsiloxane（PDMS）.Through production of graphene and characterization of graphene Raman spectroscopy,we found that the morphology of graphene wrinkles is closely related to the layer thickness of the sample.Multilayer graphene flakes show simple ripple geometry and monolayer graphene flakes show needle-like geometry.Through analysis to characterize the wrinkles of graphene,it can be concluded that the graphene is under tensile stress at the top of the wrinkle and the graphene is under compressive stress at the bottom of the wrinkle.Through production to few-layers of graphene by mechanical exfoliation,when 40%of the pre-strain is applied,the top of the graphene wrinkle is subjected to a tensile strain of0.061%.When 60%of the pre-strain is applied,the top of the graphene wrinkle is subjected to a tensile strain of 0.052%.It indicates that the strain of the stretched flexible substrate is uncontrollable.