Measurements of the Electrical Conductivity of Monolayer Graphene Flakes Using Conductive Atomic Force Microscopy.

Hyunsoo Park,Go Yamamoto,Soomook Lim,Changgu Lee,Ji Won Suk

doi:10.3390/nano11102575

Hyunsoo Park, Go Yamamoto + Show 3 more

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https://doi.org/10.3390/nano11102575

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Abstract

The intrinsic electrical conductivity of graphene is one of the key factors affecting the electrical conductance of its assemblies, such as papers, films, powders, and composites. Here, the local electrical conductivity of the individual graphene flakes was investigated using conductive atomic force microscopy (C-AFM). An isolated graphene flake connected to a pre-fabricated electrode was scanned using an electrically biased tip, which generated a current map over the flake area. The current change as a function of the distance between the tip and the electrode was analyzed analytically to estimate the contact resistance as well as the local conductivity of the flake. This method was applied to characterize graphene materials obtained using two representative large-scale synthesis methods. Monolayer graphene flakes synthesized by chemical vapor deposition on copper exhibited an electrical conductivity of 1.46 ± 0.82 × 106 S/m. Reduced graphene oxide (rGO) flakes obtained by thermal annealing of graphene oxide at 300 and 600 °C exhibited electrical conductivities of 2.3 ± 1.0 and 14.6 ± 5.5 S/m, respectively, showing the effect of thermal reduction on the electrical conductivity of rGO flakes. This study demonstrates an alternative method to characterizing the intrinsic electrical conductivity of graphene-based materials, which affords a clear understanding of the local properties of individual graphene flakes.

Highlights

Since the intriguing electronic properties of graphene have been experimentally observed in mechanically cleaved graphene [1,2,3], it has gained significant attention in various applications owing to its remarkable electrical, mechanical, thermal, and optical properties [4,5,6,7]
High-quality sub-monolayer graphene was synthesized on copper foil using lowpressure chemical vapor deposition (LPCVD) [8]
The growth time was adjusted to synthesize flower-shaped sub-monolayer graphene; the injection of methane was stopped before the individual graphene flakes merged to form a continuous layer

Summary

Introduction

Since the intriguing electronic properties of graphene have been experimentally observed in mechanically cleaved graphene [1,2,3], it has gained significant attention in various applications owing to its remarkable electrical, mechanical, thermal, and optical properties [4,5,6,7]. Two dominant methods for obtaining monolayer graphene in large quantities or large areas have been developed to overcome the limitations of mechanical cleavage from graphite. Chemical vapor deposition (CVD) on metals produces high-quality monolayer graphene with a nearly unlimited length [8]. The exfoliation of oxidized graphite into a monolayer and the subsequent reduction produce large quantities of graphene flakes [9]. Electrical conductivity is one of the most important properties of synthesized graphene. The intrinsic electrical conductivity of graphene is significantly affected by its atomic and chemical structures. The presence of grain boundaries in polycrystalline CVD-grown graphene deteriorates its electrical conductivity owing to the scattering at the grain boundary interfaces [10]. Graphene oxide (GO) obtained by exfoliation of

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