The Influences of H2Plasma Pretreatment on the Growth of Vertically Aligned Carbon Nanotubes by Microwave Plasma Chemical Vapor Deposition
© to the authors 2008
Received: 2 April 2008
Accepted: 12 June 2008
Published: 24 June 2008
The effects of H2flow rate during plasma pretreatment on synthesizing the multiwalled carbon nanotubes (MWCNTs) by using the microwave plasma chemical vapor deposition are investigated in this study. A H2and CH4gas mixture with a 9:1 ratio was used as a precursor for the synthesis of MWCNT on Ni-coated TaN/Si(100) substrates. The structure and composition of Ni catalyst nanoparticles were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The present findings showed that denser Ni catalyst nanoparticles and more vertically aligned MWCNTs could be effectively achieved at higher flow rates. From Raman results, we found that the intensity ratio of G and D bands (I D/I G) decreases with an increasing flow rate. In addition, TEM results suggest that H2plasma pretreatment can effectively reduce the amorphous carbon and carbonaceous particles. As a result, the pretreatment plays a crucial role in modifying the obtained MWCNTs structures.
KeywordsMultiwalled carbon nanotubes H2pretreatment Raman spectroscopy Scanning electron microscopy Transmission electron microscopy
Carbon nanotubes (CNTs)  undoubtedly occupy a unique position among advanced materials because of its novel electrical, mechanical, and chemical characteristics [2–4]. With these useful properties, CNTs are good candidates for various applications, such as field-effect transistors , sensors , field-emission displays [7, 8], and nanoscale interconnects .
CNTs can be synthesized by a variety of techniques, such as arc discharge, laser ablation, and plasma-enhanced and thermal chemical vapor depositions (CVDs) [10–13]. Although the former two techniques are suitable for large-scale production of CNTs, they cannot be used for self-assembly on material surfaces. CNTs synthesized by CVD are known to be longer than those obtained by other processes. It is possible to grow dense arrays of aligned CNTs by CVD , as well. Therefore, CVD is one of the prominent methods for synthesizing high-purity, high-yield CNTs for practical applications. Meanwhile, control of the CNT structure has a technical advantage in that the structural diversity leads to different electronic and mechanical characteristics. Several attempts have been made to control the structure of CNTs by various methods, including the pretreatment of the metal films on which CNTs are grown  and the direct control of structure by varying synthesis parameters . In particular, plasma etching can be used to transform a catalytic layer into catalytic nanoparticles, which might be applied to the density control of CNTs. In addition, however, to avoid the formation of metal silicide at a high temperature, a buffer layer was adopted in the annealing process .
In this study, the effects of H2flow rate during plasma pretreatment on the synthesis of MWCNTs on a Ni/TaN/Si substrate by using a microwave plasma chemical vapor deposition (MPCVD) system are investigated. The structure and composition of Ni catalyst nanoparticles are investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Raman spectroscopy equipped with a charge-coupled device detector is used to study the effect of flow rate on the intensity ratio of G and D bands (I D/I G), which, in turn, measures the amounts of the amorphous carbon and carbonaceous particles in the MWCNTs.
The substrates used in the experiments were 6-inch p-Si(100) wafers which were cleaned using standard RCA cleaning procedures to remove chemical impurities and particles. For the growth of MWCNTs, three steps were followed: (1) a 7-nm layer of nickel (Ni) and a 20-nm layer of tantalum nitride (TaN) were deposited on the substrate in a PVD system (800 W at a sputtering pressure of 6.4 × 10−3torr). (2) the Ni-coated substrate was submitted to a procedure called hereafter as pretreatment, which consisted of its annealing at 550 °C for 10 min in a H2plasma. The pretreatment was performed at different H2flow ratios (100, 200, and 300 sccm) in a 915-MHz microwave plasma chemical vapor deposition (MPCVD) system. This procedure converted the Ni layer in Ni nanoparticles distributed on the substrate surface. (3) Methane gas was then admitted in the plasma chamber (90 sccm H2and 10 sccm CH4) for the CNTs growth with the substrate kept at 550 °C for 10 min (The total pressure in the chamber was kept at 20 torr, while the gas flow rates were increased at step 2 and 3). Ni catalyst nanoparticles were examined by scanning electron microscopy (SEM, Hitachi S-4000) and high-resolution transmission electron microscopy (HRTEM, JEOL, JEM-2100F). Synthesis of aligned MWCNTs was investigated by means of SEM and TEM. In addition, Raman spectroscopy was performed in a Renishaw 1000 Spectrometer equipped with a charge-coupled device detector and operated at a wavelength of 514.5 nm and at a power of 100 mW.
Results and Discussion
In this study, we confirm the strong dependence of the catalyst morphology on the process parameters. In previous results, there were evidences that the morphology of the catalyst was dependent on the H2 plasma treatment time , H2 concentration , and H2 gas flow rate . In this article, we kept the substrate temperature (550 °C) and treatment time (10 min) the same as in the prior report  and choose the H2 flow rate as the single parameter. The synthesis of MWCNTs by CVD often involves three main steps: (1) decomposition of hydrocarbon gas at the surface of the catalyst nanoparticles; (2) diffusion of resultant carbon atom in the nanoparticles to form the nucleation seed; and (3) precipitation of carbon atoms at the nanoparticle interface to form MWCNTs. It is well known and often proposed that the size and chemical composition of metal nanoparticles determine the diameter and structural nature of the MWCNTs .
In summary, we combined SEM, Raman and TEM techniques to investigate the effects of H2flow rate during plasma pretreatment on the synthesis of the MWCNTs.
We synthesized MWCNTs by using MPCVD on Ni/TaN/Si substrates. From SEM observations, higher flow rates lead to denser Ni catalyst nanoparticles. In addition, the results of Raman spectra and TEM indicate that the morphologies of MWCNTs transform from amorphous carbon to a crystalline graphite structure or finite-sized graphite structure, depending on the H2flow rate during plasma pretreatment. A decrease in the number of defects and optimized morphologies therefore is believed to play a significant role in improving the field-emission characteristics observed in the future.
This work was partially supported by the National Center for Theoretical Sciences of Taiwan and the National Science Council of Taiwan and I-Shou University, under Grants No. NSC97-2218-E-214-003, NSC96-2218-E-214-002, ISU97-07-01-04 and ISU97-02-20. Technical support from the National Nano Device Laboratories contract NDL-95S-C-067 is also acknowledged.
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