Secondary Electron Intensity Contrast Imaging and Friction Properties of Micromechanically Cleaved Graphene Layers on Insulating Substrates S. GUPTA, 1,3,4 E. HEINTZMAN, 1 and J. JASINSKI 2 1.—Department of Physics and Astronomy, Western Kentucky University, Bowling Green, KY 42101, USA. 2.—Department of Chemical Engineering and Conn Center for Renewable Energy Research, University of Louisville, Kentucky, KY 40292, USA. 3.—e-mail: sgup@rocketmail. com. 4.—e-mail: sanju.gupta@wku.edu We report on the surface properties (friction and work function) of microme- chanically cleaved graphene layers placed on thermally gown thick insulating (295 nm of SiO 2 ) 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 (l) and electronic work function (U), 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 (0.03), in contrast to multilayer (0.027) and thicker graphite (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 insu- lating 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 con- ventional optical method in its capability to characterize graphene over sub-1- lm 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. Key words: Graphene, micromechanical cleavage, atomic force microscopy, scanning electron microscopy, work function, friction coefficient, Raman mapping, HRTEM, SAED INTRODUCTION Graphene is a prototype two-dimensional carbon system described as a one-atom-thick layer of the layered mineral graphite. 1 Among the family of advanced carbons, it is one of the youngest allo- tropes, alongside diamond, graphite, fullerenes, and carbon nanotubes. 2 Since its introduction in 2004, exhibiting an electrical field effect in an atomically thick layer, its various unique and superlative properties have created higher expectations for a (Received January 25, 2014; accepted June 8, 2014; published online July 2, 2014) Journal of ELECTRONIC MATERIALS, Vol. 43, No. 9, 2014 DOI: 10.1007/s11664-014-3277-0 Ó 2014 TMS 3458