The multi-walled carbon nanotubes (MWNTs) were grown on glass substrate at low temperature using plasma chemical vapor deposition (CVD) for the application of electronic device such as a field emission display (FED). However, low temperature deposition results in an involvement of amorphous carbon as well as a number of defects in MWNTs.
A synthesis of MWNTs by microwave plasma-enhanced chemical vapor deposition using $CH_{4}/H_{2}/NH_{3}$ gases on Ni/Cr-coated glass at low temperature, was investigated by optical emission spectroscopy (OES) and quadrupole mass spectroscopy (MW). It was found that the MWNTs were grown within a very restrictive range of the gas composition. It was also observed that amorphous carbon mostly cover the surface of Ni catalysts and the MWNTs grow only at which no amorphous carbons exist on Ni catalyst particles. Optical emission lines were observed primarily form atomic hydrogen $H_α$, molecular hydrogen, and CN radicals. The quadrupole mass spectrum also showed that the formation of $C_{2}H_{2}$ and HCN. An addition of a small amount of $NH_{3}$ resulted in a decrease of $C_{2}H_{2}$, which could be used to estimate amounts of carbon sources present in the plasma for the growth of MWNTs, and increases of CN and $H_α$ radicals acting as etching species of amorphous carbon. These results show that the etching species as well as the growth species are necessary and the ratio between two species have to be in appropriate condition for the synthesis of carbon nanotubes at low temperature. Therefore, it is worth quantitatively while characterizing growth and etching parameters for the synthesis of MWNTs at low temperature even though it is difficult to suggest the quantitative value and active species of both processes exactly. The optimum $C_{2}H_{2}/$H_α$ ratio in the gas mixture for the growth of MWNTs at low temperature was found to be 1:5 in this study.
Electron cyclotron resonance chemical vapor deposition (ECR-CVD) was used for the synthesis of carbon nanotubes at low temperature, since the plasma zone in the ECR-CVD is far from the substrate, giving rise to the benefit of maintaining the low substrate temperature during deposition. As a result, different typed of carbonaceous materials were synthesized by ECR-CVD on Ni-coated glass substrate with radio-frequency (RF) self-biasing using a $CH_{4} gas or a gas mixture of $CH_{4}$ and Ar. Negative self-biases were applied to the substrate by radio-frequency (RF) plasma for ion bombardment of the growing surface. It was observed that ion bombardment by RF biasing to the substrate had a great effect upon the growth of carbon nanotubes and their morphologies. Vertically aligned filamentous carbon was synthesized at the RF bias voltages equal to or higher than -100V, while a translucent carbonaceouc material was produced in a fence-like shape at the RF bias of -50V, called as a carbon nanowall. High resolution transmission electron microscopy (HRTEM) revealed that the degree of ordering of graphene layers in the synthesized MWNTs increased with RF bias. Amorphous carbon was mostly found within the carbon nanowall grown at the RF bias of -50V. On the other hand, defective carbon nanorods and well-graphitized MWNTs were produced at the RF bias of -100 and -200 V, respectively. In addition, HRTEM indicated that the distance between two graphene layers of carbon nanorod is much larger than that of well-graphitized MWNTs having a typical value of graphite. Raman spectroscopy analysis revealed that the G and D peaks shifted upward in position at higher RF biases, implying the average C-C bond length in the graphene layers of the MWNTs was shortened with an increase of RF bias voltage. The high crystallinity and shorter C-C bond length of the MWNTs at higher RF biases seemed to be caused by bombardment of energetic ion to substrate. Structural characteristics of defective carbon nanorod and well-graphitized MWNTs were investigated by electron energy loss spectroscopy (EELS). In EELS, the energy of+plasmon peak obtained from carbon nanorod shifts to a lower value of 23.8 eV, compared to 25.5 eV from well-graphitized MWNT. In addition, low-energy loss plasmon due to electrons at 6 eV was not observed in carbon nanorod, but clearly defined for well-graphitized MWNT. X-ray photoelectron spectroscopy (XPS) showed that the delocalization of electrons becomes more pronounced as the structure of carbonaceous film evolves into more crystalline phase from carbon nanowall to well-graphitized MWNT with increasing RF bias voltages.
The internal energy transferred from the ionic kinetic energy was estimated using plasma parameters measured by double Langmuir probe. It was found that the average value of internal energy comes to be compatible with C-H bonding enthalpy at the RF bias of -50V, corresponding to the beginning of graphitization within synthesized carbonaceous material. From these results, it can be suggested that ionic bombardment energy at high RF bias voltages provide sufficient internal energy for dehydrogenation molecules, and thus well-graphitized MWNTs can be synthesized even at temperature as low as 400℃.
In the field emission (FE) measurements, the lower threshold electric fields and the higher emission current density were obtained form the MWNTs grown at lower self-bias voltages. The electric fields to obtain 1A/ ㎠ were 4.6 and 11.1 V/m for the carbon nanorod and nanotubes grown at the RF bias of -100 and -200V, respectively. The emission areas, calculated from the Fowler-Nordheim (F-N) plots, decreased noticeably with higher RF bias voltages, while the field enhancement factor were unchanged. Therefore, it is likely that the larger emission areas and the resultant lower threshold electric field at lower RF bias voltages originate from the numerous defects on the nanorods or nanotubes.
