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

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.

Keywords: conductive atomic force microscopy; electrical conductivity; flakes; graphene; reduced graphene oxide.

Figure 1

(a) Schematic illustration of the electrical measurements of graphene flakes using C-AFM. (b) Schematic illustration of the resistance profile as a function of the distance between the tip and the electrode. (c) SEM image of the AFM tip.

Figure 2

(a) SEM image of CVD-grown monolayer graphene flakes transferred onto SiO₂. The inset shows the SEM image of monolayer graphene grown on the surface of copper foil. (b) Raman spectra of CVD-grown monolayer graphene placed on SiO₂ before and after the removal of the polymer residue. (c,d) Raman maps for the integrated intensity of (c) the G (1542–1642 cm⁻¹) and (d) D bands (1314–1414 cm⁻¹).

Figure 3

(a) AFM topography image of a CVD-grown monolayer graphene flake on SiO₂ after the removal of the polymer residue. (b) The line profile of the current and morphology of the flake along the dashed line marked in (a).

Figure 4

(a,b) AFM images for the (a) topography and (b) current distribution of the CVD-grown monolayer graphene flake on SiO₂. (c) Force–distance curve. (d) Resistance profile of the graphene flake as a function of the distance from the tip to the electrode.

Figure 5

(a–c) XPS C 1s spectra of (a) GO and (b,c) rGO flakes. Thermal annealing was performed at (b) 300 and (c) 600 °C. (d) Raman spectra of GO and rGO flakes. rGO-T300 and rGO-T600 denote rGO flakes annealed at 300 and 600 °C, respectively.

Figure 6

Deconvolution of Raman spectra of (a) GO and (b,c) rGO flakes.

Figure 7

(a,b) AFM images for the (a) topography and (b) current distribution of the rGO flake annealed at 300 °C. (c,d) AFM images for the (c) topography and (d) current distribution of the rGO flake annealed at 600 °C. (e) Force–distance curves. (f) Resistance profiles of the rGO flakes as a function of the distance from the tip to the electrode.