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44249-AC5
Fundamental Aspects of Adsorption on Individual Carbon Nanotubes

David Cobden, University of Washington (Seattle)

At room temperature, the electrical properties of single-walled carbon nanotubes are surprisingly sensitive to all kinds of vapors, including the noble gases. Opportunities for making sensors and gas or ionic storage are apparent. However, the mechanisms behind this sensitivity are still unclear. The molecules should not bind directly to the surface of nanotubes at this temperature. At cryogenic temperatures well below the physisorption energy, on the other hand, one expects the molecules to form a cylindrical monolayer on the surface of an individual nanotube, while 1D chains of molecules form in the grooves between parallel bundled nanotubes where the binding is stronger. Such cylindrical monolayers and chains represent unusual confined systems in which fundamental questions about the behavior of matter in reduced dimensionality can be studied. On graphite many molecules, including the noble gases and oxygen, can move quite freely over its smooth inert surface. They arrange themselves into phases which are two-dimensional (2D) analogs of 3D solids and liquids. These phases and the sharp transitions between them were widely studied two to three decades ago.

The main aim of this project is to study the formation of, and the phase transitions within, monolayers on the surface of individual nanotubes and small nanotube bundles. The density of adsorbed molecules is determined from the frequency shift of the natural oscillations of the nanotubes due to mass loading when they are suspended freely between two electrical contacts. The variation of frequency with gas pressure and temperature is measured to cover the phase diagram of the monolayer. At the same time we hope to elucidate the mechanisms of coupling of gas molecules to the electrons in the nanotubes by simultaneously studying the electrical conductance of the nanotube. Our studies have begun with noble gases, whose behavior is simplest. We will later turn to oxygen, which has more complex phases, with the addition of magnetism, stearic effects, and ability to dope the nanotube.

We perform electrical transport measurements on nanotubes grown by chemical vapor deposition suspended between platinum contact pads, using a technique based on that invented by Dai's group at Stanford. The nanotubes are not exposed to any processing between growth and measurement, to keep their surface nearly pristine. We have constructed two measurement systems: one with a probestation setup for initial characterization of the transport and vibrational resonances in a controlled gas environment near room temperature, and the other in a vacuum can on a cryocooler for low temperature work. After a good deal of fabrication work, we have successfully detected the vibrational resonances in a number of individual nanotubes using a version of the electrical mixing technique introduced by McEuen's group at Cornell. The resonant frequencies change with gate voltage, typically from about 50 MHz at Vg = 0 V to 300 MHz at Vg = 10 V, in accordance with the behavior observed at Cornell, because the tension depends on the attractive electrostatic force to the underlying gate. Importantly, we have observed some resonances with very high quality factor Q (>1000 in vacuum). This is higher than any value reported for nanotubes in the literature, and will allow us to detect changes in the monolayer density of less than 0.1%. Stability of the resonances in time is a limitation: at a fixed gate voltage some of the resonances are stable in time over a period of hours to within a comparable precision; others are not. We have exposed one suspended nanotube to argon gas at room temperature and found that the resonance disappears at a pressure of about 1 mbar, consistent with damping by the gas viscosity. We are now ready to move the devices into the low temperature setup and begin to form monolayers and detect them through the frequency shift.

This was the first grant obtained by the PI as junior faculty and was very helpful in setting up his lab and attracting an excellent graduate student, Zenghui Wang, who is fully supported by the grant and whose thesis will be based entirely on it. The project relies on the participation of emeritus Professor Oscar Vilches who brings unique expertise in absorption at low temperatures.

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