AFM-Based Surface Potential Measurements on Carbon Nanotubes
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AFM-Based surface potential measurements on carbon nanotubes Sandra B. Schujman*, Robert Vajtai+, Yuriy Shusterman*, Sujit Biswas*, Brian Dewhirst*, Leo J. Schowalter*, Bin Q. Wei+, Pulickel M. Ajayan+ * Department of Physics, Applied Physics and Astronomy and +Department of Materials Science and Engineering Rensselaer Polytechnic Institute, Troy, NY, 12180, U.S.A.
ABSTRACT
In order to build electrical devices based on carbon nanotubes, it is crucial to understand the effects of the substrate, electrical contacts, defects and junctions on the transport properties of both multi-wall and single-wall carbon nanotubes (MWNT and SWNT). In the present study two different dispositions are analyzed. In one of them, isolated MWNT were deposited onto an insulating oxide layer over a conducting substrate and platinum electrical contacts were prepared via a focused ion beam (FIB) deposition method. An ohmic contact to the substrate allowed us to polarize it at a different potential than the nanotubes themselves. In the second one, a vertical array of MWNT was grown on a template of alumina in which one of the ends was coated with gold, to provide an electric contact. To investigate transport properties, we used an atomic force microscope (AFM) to determine the electric surface potential of MWNT. The technique employed, scanning surface potential microscopy (SSPM), applied a dc-voltage to the tip that equilibrates the local electrostatic potential on the sample so as to eliminate forces on the AFM tip caused by electric repulsion or attraction between tip and sample. There is no alteration of the sample potential caused by the tip. Surface potential measurements on the first system show that the electric transport on the MWNT has a diffusive character. The application of a (4.5V) bias between nanotube and substrate causes the resistance of the nanotube to drop by a factor of four. We can also resolve the voltage drop on the nanotube and at the contacts. The measurements in the second system (vertical arrangement) show that the nanotubes are uncapped and we can observe a contrast that varies with the application of different voltages to the back contact.
INTRODUCTION
The quest for miniaturization of electronic circuits and devices requires the development of new concepts for components and wiring of the circuitry, and new tools for characterizing those components and wires. The idea of using carbon nanotubes both as electronic devices as well as self-assembled wires is taking on more momentum, as the properties of these materials are being understood. As with any new concept, as new properties get uncovered, also new problems appear. When we think of characteristic length scales of devices on the order of nanometers, the concept of “electric contact” acquires a new dimension. New tools are required to study and characterize these new circuits. Carbon nanotubes, and structures based on them, have attracted an enormous interest because of their high thermal conductivity, the fact that by controlling their structure it is possib
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