The effect of the plasma on heating the growth substrate in plasma enhanced chemical vapor deposition (PECVD) of carbon nanotubes is characterized for the first time. This effect, which is commonly ignored in the nanotube/nanofiber literature, is the sole heating mechanism in this work for catalyst pretreatment and growth of straight and vertically aligned multiwalled carbon nanofibers. Significant temperatures, as high as 700 °C, are induced from a C2H2:NH3 direct current (dc) plasma with no other heat source present. To model the behavior of the plasma-heated substrate platform, we have developed a 1-D dc discharge model that incorporates a cathode platform energy balance, including ion bombardment, thermal radiation, and solid and gas conduction. The predicted gas-phase species present are correlated with the morphology of nanofibers grown by exclusive plasma heating as well as by heating from plasma in combination with a conventional resistive heater. The understanding of plasma heating and its accurate modeling are essential for reactor design for wafer scale production of vertically aligned nanofibers.

Most long-range telecommunication systems are based upon microwave links. The transmitters use microwave amplifiers which in the very near future will be required to work at up to 30-100 GHz with output power in the region of a few tens of watts. Carbon nanotubes ͑CNTs͒, which exhibit extraordinary field emission properties because of their high electrical conductivity, ideal high aspect ratio whisker-like shape for geometrical field enhancement, and remarkable thermal stability, can be used as the emitter in such applications. This article will describe the plasma enhanced chemical vapor deposition growth of vertically aligned carbon nanotubes, and how well controlled arrays of such structures can be grown. We will also describe how high current densities of ϳ1 A/cm 2 , under direct current and 1.5 GHz direct modulation, can be obtained from CNT cathodes. These CNT cold cathodes offer considerable weight and size savings over conventional hot cathodes used in microwave applications ͑e.g., SATCOM, radar͒.

This paper presents the growth of vertically aligned carbon nanotubes by plasma-enhanced chemical vapor deposition (PECVD) using Ni catalyst and C2H2/NH3 feedstock. The role of plasma in aligning the carbon nanotubes during growth is investigated both experimentally and computationally, confirming that the field in the plasma sheath causes the nanotubes to be aligned. Experiments using a plasma analyzer show that C2H2 is the dominant precursor for carbon nanotube growth. The role of NH3 in the plasma chemistry is also investigated, and experimental results show how the interaction between NH3 and the C2H2 carbon feedstock in the gas phase explains the structural variation in deposited nanotubes for differing gas ratios. The effects of varying the plasma power during deposition on nanotube growth rate is also explored. Finally, the role of endothermic ion-molecule reactions in the plasma sheath is investigated by comparing measured data with simulation results.

Neutral species and positive ions were extracted directly from a C2H2:NH3 plasma used to grow vertically aligned carbon nanotubes (CNTs) and analyzed by mass spectrometry. We observe that NH3 suppresses C2H2 decomposition and encourages CNT formation. We show that the removal of excess carbon, essential for obtaining nanotubes without amorphous carbon deposits, is achieved through gas phase reactions which form mainly HCN. We determine an optimum C2H2:NH3 gas ratio which is consistent with previous observations based upon postdeposition analysis. We find, in contrast to thin film growth by plasma-enhanced chemical vapor deposition, that the optimum condition does not correspond to the highest level of ionization. We also provide evidence that C2H2 is the dominant precursor for CNTs in our experiments.

This paper presents a number of factors which have been found to be important to the growth of carbon nanotubes and nanofibres by plasma enhanced chemical vapour deposition. The effect of the electric field in a plasma discharge on nanotube growth is investigated and shown to be important in achieving nanotube alignment. The use of a plasma discharge also enables deposition to take place at lower temperatures, facilitating the use of substrates which would otherwise be damaged. The effect of varying the ratio of carbon feedstock gas to etchant gas is investigated and the ratio is shown to be important for controlling the shape of deposited nanostructures. The effects of varying plasma power are investigated, showing that greater plasma power results in a lower growth rate. Higher levels of plasma power are also shown to cause the sidewalls of deposited carbon nanotubes to be etched. Finally, the growth rate of carbon nanotubes and nanofibres is shown to depend upon the strength of the local electric field. It is proposed that a higher field causes greater ionization within the plasma, which results in a higher growth rate. This is borne out by comparing simulation results with experimental observations.

The role of plasma in plasma enhanced chemical vapour deposition of carbon nanotubes and nanofibres is investigated with both experimental and computational diagnostic techniques. A residual gas analysis (RGA) of a 12 mbar dc discharge with a C 2 H 2 /NH 3 gas mixture is conducted near the Ni catalyst surface employed for carbon nanofibre growth. The results are corroborated with a 1D dc discharge model that solves for species densities, ion momentum, and ion, electron and neutral gas thermal energies. The effect of varying the plasma power from 0 to 200 W on the gas composition is studied. The dissociation efficiency of the plasma is demonstrated where over 50% of the feedstock is converted to a mixture of hydrogen, nitrogen and hydrogen cyanide at 200 W. Finally, the important role that endothermic ion-molecule reactions play in this conversion is, for the first time, established. Of these reactions, dissociative proton abstraction and collision-induced dissociation are of the greatest significance.

The effect of temperature on the growth of single-walled carbon nanotubes (SWCNTs) was investigated over the range of 725–900 °C. A cold-wall reactor consisting of a heated stage (on which the substrate for SWCNT growth (Al/Fe/Mo) was placed) and a showerhead (from which C2H2 was introduced vertically into the reactor) was used for the growth. The heating was found to play two roles: (1) it generated complex hydrocarbon radicals during the growth process, as well as (2) promoting catalytic nanoparticles on the substrate during the annealing process. The optimum temperature for the highest SWCNT yield was found to be ∼860 °C. For the first time, in situ mass spectroscopy was used to identify the growth precursors generated from thermal pyrolysis of C2H2 within this temperature range. The peak of the radicals found (C6H9, C5H9 and C6H13) and the highest catalyst support particle density (Fe catalyst supported on AlxOy) was correlated to the maximum yield of single walled carbon nanotubes at the optimum growth temperature of ∼860 °C. Bottom gate SWCNT-FETs (single-walled carbon nanotube based field effect transistors) were fabricated showing a high transconductance of ∼0.12 µS and on/off ratio of ∼105 which are both comparable to other state-of-the-art SWCNT-FET.