May 21, 2009
Nanomanipulators target individual CNTs
Most electrical measurements made on an individual carbon nanotube (CNT), or any type of nanowire, usually require an insulating substrate onto which the CNT is placed. Low-resistance contacts are lithographically made to the nanotube and conventional electrical measurements can then be performed. But what happens if the presence of the substrate itself can mask aspects of the electron's interaction with its environment and affect the resulting current – voltage characteristics? One obvious answer is to remove the substrate itself and use a free-standing electrode geometry. This is exactly what researchers at the Advanced Technology Institute at the University of Surrey, UK, have done. By locating high-precision nanomanipulators in a scanning electron microscope they have probed the electron transport properties of individual multiwalled CNTs. What has been seen is a power law behaviour of the current with voltage with a high exponent as high as 5 and a transition in the current-voltage characteristic to one that also exhibits an Ohmic behaviour at higher currents and voltages.
A power law behaviour in transport measurements of CNTs is often taken as the signature of the presence of a Luttinger liquid (LL). When electrical measurements are made on a substrate the electron–electron interactions are effectively cut off over a screening length that is roughly twice the distance from the nanotube to the ground plane in the substrate. By making measurements off substrate the electron–electron interactions are effectively cut off by the intrinsic length of the LL itself.
As such, off-substrate measurements are able to probe the electron–electron interactions that are masked by the presence of a ground plane. However, the much larger exponents seen in this study are not consistent with a LL description and an alternative explanation based in Environmental Quantum Fluctuation (EQF) theory has to be used. In the EQF description the exponent is associated with the tunnelling contact not the bulk and the high values of the exponent reported here are associated with impedance reflections at the contact.
Making measurements off substrate is therefore able to distinguish between the TLL and EQF theories. Using nanomanipulation is an ideal way to probe the electrical properties of without-substrate effects.
The researchers presented their work in Nanotechnology.
About the author
David Carey is senior lecturer in electronic engineering specializing in carbon-based nanomaterials and their electronic properties, including carbon nanotubes, nanotube-based composites, graphene and CVD growth of carbon materials. From 2002 until 2007 he held an EPSRC Advanced Research Fellowship and was appointed lecturer in electronic engineering in 2004. He is course director for the MSc in Nanotechnology and Nanoelectronic Devices.