Scanning Electron Microscopy Imaging of Graphene Supported on Monocrystalline and Polycrystalline Substrates

Author:Huang Li

Supervisor:gan yang


Degree Year:2019





For supported graphene,reliable characterization of morphological and structural characteristics is critical for revealing the structure-property relationships for graphene and graphene-based devices.SEM,as a non-invasive imaging and micro-analysis technique,offers nanoscale spatial resolution,quick setup,and wafer-scale surveying capability.Indeed,SEM have been successfully used for structural characterization,growth mechanism study,contamination study,and layer number determination of supported graphene.However,the image conrast for graphene was rather poor at routine imaging parameters because of weak interaction between graphene and incident electron beam.Here,we systematically characterized graphene supported on monocrystalline,noble polycrystaline and non-noble polycrystaline substrates using SEM.The underlyng mechnism for contrsat formation as well as Vacccc and WD dependence of image contrast for graphene were revealed by tuning imaging parameters.The findings reported here will have important implications for high contrast imaging of supported graphene,differentiation graphene from substrate and determination layer-numbers of graphene using SEM.Superior image contrast of secondary electron(SE)images for few-layer graphene supported on monocrystal,including SiC,SiO2/Si and Al,were realized through simultaneously tuning two key parameters—acceleration voltage(Vacc)and working distance(WD).The overlooked role of WD in characterizing graphene was highlighted and clearly demonstrated.The underling mechanism of contrast formation for supported graphene and the influence of imaging parameters on image contrast for graphene were revealed.A unified model of Vacc and WD dependence of three types of SE collected by the standard side-attached Everhart-Thornley(E-T)SE detector was conceptually developed for mechanistically understanding the improved mass-thickness contrast for supported few-layer graphene.Under lower Vacc and/or larger WD,it is the increasing difference in the total SE collected by the detector that accounts for the improved image contrast for supported graphene.Electron channeling contrast(ECC)for the polycrystal observed in a SEM image makes it challenge to differentiate graphene from substrate for graphene supported on noble polycrystalline substrates using SEM.ECC is an orientation-dependent image contrast for polycrystalline materials.Two feasible methods that allow accurate determination of layer-numbers of graphene using SEM were proposed and clearly demonstrated.With the first method,the brightness of local grains in an SEM image was adjusted with image-processing software,making the gray-scale values of different grains equal to each other.By the second method,the sample stage was tilted to a certain angle during imaging process,giving rise to the identical orientation between neighboring grains relative to the incident electron beam.The findings reported here will have important implications for unambiguous differentiation graphene and other 2D materials from the supported polycrystalline substrate.A systematic SEM study was carried out for graphene supported on polycrystalline Cu substrates including partially coverage CVD-grown graphene on Cu substrate(CVD G/Cu),full-coverage CVD-grown graphene on Cu substrate(CVD SLG/Cu)and transferred graphene onto Cu substrate(SLG/Cu).Unexpected anomalous bright image contrast was observed in a SEM image for CVD G/Cu at small WD,making it challenge to differentiate graphene from substrate and determine layer-numbers of graphene using SEM.Imaging at larger WD affording darker appearance of graphene than Cu substrate was proposed to lessen the“twinkling”effect and thus enable unambiguous discrimination of graphene from the substrate.The intricate interplay between the electron channeling contrast(ECC),oxidation layer,and the imaging parameters was revealed by systematically tuning imaging parameters.By comparing SEM images of CVD G/Cu captured before and after oxidation,the initial oxidation site for Cu substrates was revealed to be the defect of graphene layers.