Impedance spectroscopy of short multiwalled carbon nanotube networks deposited on a paper substrate: tracking the evolution of in-plane and thru-plane electronic properties

Multiwalled carbon nanotubes (MWNT) were deposited via dropcasting and dried using vacuum filtration. Because of the slightly larger pore size (5–10 µm) of the paper substrate used as compared to the length of the nanotubes (0.5–2 µm), a variety of MWNT networks were formed both on the surface and through the thickness of the paper. Using a combination of impedance spectroscopy, equivalent circuit modeling, and microscopy techniques, it was possible to describe in detail how the electrical properties change as a function of how the MWNTs are distributed on the porous substrate by varying the number of deposited layers (1–20) as well as the dispersion concentration (0.1–5 mg/mL). In the in-plane, four different electrical responses were observed and modeled: (1) a substrate dominated spectrum representing unconnected MWNTs, (2) one that included bundle and junction responses as well as some inductances representing sparsely distributed MWNT networks, (3) followed by a parallel RL circuit for partially connected MWNT networks, and (4) finally a series RL circuit for fully connected MWNT networks. In the thru-plane, only two different electrical responses were observed and modeled. The results for the in-plane and thru-plane properties were used to generate percolation curves that show that electrical conductivity can change as much as 10 orders of magnitude for the same exact MWNTs. These results indicate that not only do the characteristics of the nanotubes themselves play a role but also the structure of the underlying substrate and the details of how the films are deposited.