Carbon nanotubes: in situ studies of growth and electromechanical properties
Weis, Johan Ek
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Carbon nanotubes have been found to have extraordinary properties, such as ballistic electrical conductivity, extremely high thermal conductivity and they can be metallic or semiconducting with a wide range of band gaps. There are however several issues that have to be solved before these properties can be fully utilised. One of these issues is that the nanotube growth temperature must be lowered in order to make the synthesis compatible with the fabrication processes used in electronics. The whole environment is heated to temperatures typically higher than 500 °C in the standard growth techniques whereas only a very localised area is heated in the technique developed here. This technique thus provides a way around the temperature issue. In the method developed here, the catalyst is deposited on top of a small metal (molybdenum) wire on the substrate. The high temperature required for nanotube growth is then reached by Joule heating by sending a current through the metal wire. This process eliminates the furnace which is used in conventional chemical vapour deposition and localises the high temperature to a very small and controlled area of the sample. Consequently, this technique is compatible with the semiconductor technology used today. Another advantage of this technique is that, since no furnace is required, a small growth chamber, which fits under a microscope, can be used. This allows in situ studies of the growth by optical microscopy and by Raman spectroscopy. By changing the carbon precursor, single- or multiwalled nanotubes can be grown. This can be important when producing devices since single-walled nanotubes predominantly are semiconducting whereas multi-walled mainly are metallic. The multi-walled nanotubes grow in a rapid and concerted process. This growth was monitored through an optical microscope. It was found that the thickness of the support layer and especially the catalyst are even more crucial parameters for nanotube growth using this local heating technique than in conventional processes. The activation energy could be extracted and was found to be 1.1-1.3 eV. The carbon nanotube growth was investigated by in situ Raman spectroscopy. The growth evolution could be well described by a model using the initial growth rate and the catalyst lifetime as parameters. The process was found to be limited by the mass transport of the carbon precursor. It was found that the molybdenum wire creates an additional pathway for the carbon cycle from gas to nanotube formation. The Raman spectra were studied at elevated temperatures. A decrease in intensity and a shift towards lower wavenumbers with increasing temperature was observed for the Stokes signal. It was found that the laser used for the Raman measurements could heat the nanotubes to high temperatures without any other heat source. Vertically aligned arrays of nanotubes were grown by conventional CVD. These arrays were actuated by applying a DC voltage between them. An effective Young's modulus of the arrays was found to be similar to that of rubber, which is orders of magnitude lower than for individual nanotubes. The capacitance between the arrays was measured to be tens of fF with a tunability of over 20%.