Direct imaging of current paths in multiwalled carbon nanofiber polymer nanocomposites using conducting-tip atomic force microscopy

Using conducting-tip atomic force microscopy (C-AFM), we study the spatial distribution of current paths and local electrical properties in carbon nanofiber/polymer nanocomposites. Previous studies of similar systems were hindered by a polymer-rich skin layer that exists at the nanocomposite surfaces. We present an experimental technique using oxygen plasma etching to controllably remove this polymer skin layer. After this treatment, we can directly probe the microscopic transport characteristics of the nanocomposite using C-AFM. The C-AFM results show that the electrical transport is solely carried by the carbon nanofiber (CNF) networks in the nanocomposites. In addition, high-resolution C-AFM maps show nonuniform distribution of current along the length of some CNFs, suggesting the presence of a heterogeneously distributed adsorbed polymer layer around nanofibers. Finally, two probe conductivity measurements in which one electrode (the C-AFM tip) is contacting a single constituent conducting particle were performed to study local conductivity. Results indicate that Ohmic pathways exist in the conducting network of the nanocomposite to the lowest measured nanofiber concentrations. However, non-Ohmic behavior indicating tunneling transport may also be present, especially near the percolation threshold.