Secondary Electron Intensity Contrast Imaging and Friction Properties of Micromechanically Cleaved Graphene Layers on Insulating Substrates

摘要

We report on the surface properties (friction and work function) of micromechanically cleaved graphene layers placed on thermally gown thick insulating (similar to 295 nm of SiO2) films on commercial Si (001) substrates. By employing atomic force microscopy (AFM) and scanning electron microscopy with varying primary-electron acceleration voltage (V (acc)) in secondary electron imaging (SEI) mode, we determined the coefficient of friction (mu) and electronic work function (I broken vertical bar), respectively, as functions of the number of graphene layers (n). The friction coefficient was deduced from line scans of friction maps obtained simultaneously while measuring AFM topography. The findings show that supported mono-, bi-, and trilayer graphene all yield similar results (similar to 0.03), in contrast to multilayer (similar to 0.027) and thicker graphite (similar to 0.015) flakes. From the SEI contrast variation, we obtained a reproducible discrete distribution of SE intensity stemming from atomically thick graphene layers on a thick insulating substrate. We were able to determine the number of graphene layers (i.e., n) from the SE intensity contrast or the SE intensity itself. Moreover, we found a distinct linear relationship between the relative SE intensity from the graphene layers and their number, provided a relatively lower V (acc) was used. The different contrast in SEI micrographs at lower V (acc) is attributed to the fact that the generation of secondary electrons emitted from the graphene was affected by the different work functions corresponding to different n values (or thickness contrast, C). This simple and facile method is superior to the conventional optical method in its capability to characterize graphene over sub-1-mu m(2) areas on various insulating substrates. These results are supplemented by optical microscopy, high-resolution transmission electron microscopy, and Raman spectroscopy and Raman mapping that yield the structural quality (or disorder) of the graphene layers, albeit semiquantitatively.