SEM image of multi-walled carbon nanotube forest on silicon substrate. (Image: Mikhail E. Kozlov, NanoTech Institute, University of Texas at Dallas)
"These and other applications relay on connectivity of carbon nanotubes in the forest structure" Mikhail Kozlov, a Research Scientist at the NanoTech Institute, University of Texas at Dallas, explains to Nanowerk. "Some degree of alignment of nanotubes in forests along with substantial entanglement creates a sophisticated conductive network and is responsible for their properties. Because of irregular structure of multi-walled carbon nanotube (MWCNT) forests and diversity of contacts between nanotubes, the characterization of bulk connectivity is a difficult task."
In order to address this issue, Kozlov performed room and low-temperature I-V measurements of MWCNT forests, numerically evaluated, and experimentally recorded dependences of differential conductance on applied bias voltage.
These measurements show that room temperature electrical properties of this nanotube network reveal quite substantial nonlinerarities that became more pronounced at sample cooling.
"The nonlinearities can be associated with the zero bias anomaly that is common for tunnel junction arrays," says Kozlov. "The anomaly shows unusual temperature dependence and can be caused by tunneling of charge carriers through numerous nanotube junctions enhanced by temperature gradients induced by measuring currents. Unlike previous studies, no insulating layer was intentionally created between the nanotube layer and electrical contacts. The observed nonlinearities can therefore be related to an intrinsic feature of MWCNT forests, i.e., three-dimensional junction arrays with large amount of junctions connected in series and parallel."
This observed phenomenon is quite typical for MWCNT forests and can be used for characterization of junction networks in the 3D carbon nanotube assemblies.
As Kozlov points out, the study shows that three-domensional carbon nanotube assemblies may provide the functionality of conventional metal-insulator-metal tunnel junction arrays that are normally fabricated by a lithographic procedure. Unique properties of such arrays are widely used in primary tunnel junction thermometers and other tunnel junction devices.