|M.Sc Thesis||Department of Materials Science and Engineering|
|Supervisor:||Prof. Kaplan Wayne D.|
Plasma enhanced chemical vapor deposition (PECVD) is the generally accepted method for producing vertically aligned CNTs. However, this method suffers from some disadvantages, mainly due to the destructive nature of the plasma that is acute at the nano-scale. This study proposed an alternative method of growing vertically aligned CNTs without plasma by inducing a static electrical field (i.e. below plasma breakdown).
Our work has shown that the application of global, static electrical fields as high as 0.16- V/μm during thermal decomposition did not result in CNT alignment. Although dense fields of curly-like CNTs with a characteristic nickel particle at the tip were obtained, no signs of field alignment were detected. This finding implies that the presence of the plasma in the PECVD process is required not only due to its electric field induction but also because its role as a preferable medium for aligned CNT growth.
Since understanding the emission mechanism of CNTs was still of interest, electron emission was measured from the non-aligned, entangled CNTs. These measurements showed typical field emission (FE) properties. Turn-on voltages as low as 4 V/μm followed by an exponential raise in current were measured. Also, the I-V figures showed a high degree of correlation with the Fowler-Nordhiem FE model.
We discovered a surprising “selective growth” behavior in which a uniform catalyst layer yielded patterned CNT growth, duplicating the lithography of the underlying substrate. This behavior was highly unexpected since the nickel catalyst, responsible for CNT growth was supported by a relatively thick titanium layer (100nm), serving as a diffusion barrier. Our analysis found this barrier to be ineffective mainly due to the strong “gettering” characteristics of titanium which modifies the composition of this layer. The full explanation of this behavior is still an open question.