Abstract
Dynamic atomic force microscopy (AFM) was employed to spatially map the elastic modulus of highly oriented pyrolytic graphite (HOPG), specifically by using force modulation microscopy (FMM) and contact resonance (CR) AFM. In both of these techniques, a variation in the amplitude signal was observed when scanning over an uncovered step edge of HOPG. In comparison, no variation in the amplitude signal was observed when scanning over a covered step on the same surface. These observations qualitatively indicate that there is a variation in the elastic modulus over uncovered steps and no variation over covered ones. The quantitative results of the elastic modulus required the use of FMM, while the CR mode better highlighted areas of reduced elastic modulus (although it was difficult to convert the data into a quantifiable modulus). In the FMM measurements, single atomic steps of graphene with uncovered step edges showed a decrease in the elastic modulus of approximately 0.5%, which is compared with no change in the elastic modulus for covered steps. The analysis of the experimental data taken under varying normal loads and with several different tips showed that the elastic modulus determination was unaffected by these parameters.
Highlights
In recent years, the study of the size-dependent properties of materials, and in particular those at the nanometer scale, have received significant attention [1,2,3]
The average measured contact stiffness over the highly oriented pyrolytic graphite (HOPG) flat terrace is 0.11 ± 0.05 N/m, which is comparable to previous studies where the contact stiffness on an atomic terrace was found to be 0.12 N/m [25]
The contact stiffness depends on the surface topography or local roughness, which can in turn change the contact area of the tip with the surface, as well as the variation in local mechanical properties [28]
Summary
The study of the size-dependent properties of materials, and in particular those at the nanometer scale, have received significant attention [1,2,3]. Through interrogation of materials at this small length scale, the development of advanced materials that have significantly improved mechanical [1], electrical [4], and bio-compatibility [5] properties have been enabled. Focusing on mechanical properties of materials, the nanoscale mechanical properties, including elastic and shear moduli, can drastically differ from their bulk values. This results in opportunities to custom design or enhance bulk engineering materials with advances discovered through nanoscale interrogation [1]. Quantitative measurements of nanoscale mechanical properties can offer important insights into material functionalities at the nanoscale with high temporal.
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