GROWTH MECHANISMS OF SINGLE-WALL CARBON NANOTUBES IN A CHEMICAL VAPOR DEPOSITION ( CVD ) PROCESS ON Fe / Mo-Al CATALYST

The formation mechanisms involved in the growth of single-walled carbon nanotubes (SWNTs) by chemical vapor deposition (CVD) was studied. Transmission electron microscopy (TEM) was used to analyze the encapsulated metal catalyst particles found within the tubes, and the dimensions and location of these particles was determined. SWNTs were found to have encapsulated particles in the end of tubes, with large length to diameter ratios. As a result of these observations, we concluded that SWNTs are formed via an open-ended, base-growth mechanism (VLS mechanism). Additionally, we have demonstrated the formation of two kinds of bundles of SWNTs (Parallel bundles and as-rope bundles). SWNTs grown with thermal CVD on Fe/Mo-Al catalyst did not contain similar elongated particles or particles along the middle of the tubes, indicating that these new growth mechanisms are only applicable in the case of tubes grown via vapor phase CVD growth methods.

But one major challenge is to control the growth of SWNTs, in particular concerning their diameter and helicity.To achieve a controllable growth of the CNTs with high quality, understanding of their growth mechanism is of importance, which still remains an open question [1].Naturally, the growth mechanism of nanotubes is not well understood.It may be different depending on which method is used.
It is known that the arc-discharge and laser-ablation method lead to growth of MWNT without using metal catalyst whereas for carbon nanotubes to be synthesized with the CVD method, the catalyst particles are necessary.In contrast, for the growth of SWNTs, catalysts play an important role for all three methods mentioned above [1,[6][7][8].
Carbon nanotubes produced using the CVD method exhibit high purity, high yield, and

EXPERIMENTAL
The nanotubes were grown by a thermal CVD of methane at atmospheric pressure.

Carbon nanotubes growth
Catalyst materials were deposited onto the Using TEM grids as substrates for the growth of carbon nanotubes is a very simple approach.
The TEM grids are thin metal foils with punched holes.The grids have a diameter of 3.05 mm and a thickness of 12 to 15 m.The melting point of the grids' metals is higher than 1000°C which means that the grids should withstand the growth process.

Catalyst on the copper grid
The suspension catalyst was covered on thecopper grid by the micropipette and characterized with TEM.

TEM images of carbon nanotube on the molybdenum grid
The results of the experiments with TEM grid can be summarized as follow: The    Từ khóa: Ống nano carbon đơn thành, cơ chế phát triển từ gốc, kính hiển vi điện tử truyền qua.
etc.Dai et al. and Kukovitsky et   al. [10]  have put forward vapor-liquid-solid (VLS) mechanism.In this mechanism, liquid catalytic particles at high temperature accepted carbon atoms from the vapor, causing the liquid to become supersaturated, the supersaturated carbon atoms then deposited to form CNTs. The liquid catalytic particles acted as the medium for transport from the vapor to the crystal and the CNTs grew by the deposition of supersaturated carbon atoms.In VLS model, molecular decomposition and carbon solution are deposited at one side of the catalytic particle.Carbon diffuses from the side where it has been decomposed to another side where it is precipitated from solution.The metal-support interactions are found to play a determinant role for the growth mechanism.In the present work SWNTs have been synthesized by the catalytic decomposition of methane, over Fe-Mo-Al catalyst in a tube furnace, which allows continuous control of the CNT synthesis in real time.The properties of CNTs have been studied using SEM, RAMAN and TEM.Based on our TEM results, a growth mechanism is described.

Si/SiO 2
substrate were calcined in Ar environment at 400°C for 15 minutes, cooled to room temperature, and put it inside a 3 inches diameters quartz tube mounted in an electric tube furnace.The quartz was heated from room temperature to 900°C under Ar flow at a flow rate of 1000 sccm.The reaction began when adding H 2 :CH 4 (250:1000sccm)for the desired reaction time (14 mins).The flow was then switched to Ar and the furnace was cooled to room temperature.TEM and Raman spectroscopy.The growth mechanism of SWNTs in the methane CVD process was determined systematic by TEM imaging of nanotube ends.

Figure 1 .
Figure 1.TEM images of catalyst on the copper grid TEM images shown catalyst particles with different sizes.Diameter of catalytic grains in fig.1a was smaller than that of alumina particles (13nm).So, these particles are the grains of active catalyst.

Figure 2
Figure 2 is micrograph of the nanotubes growth sample prepared in our process.However, those nanotubes are multi-walled or single-walled tubes were firstly verified Raman spectroscopy.This technique is useful in distinguishing between the MWNTs and SWNTs because the spectra of them contain special vibration modes.

Figure 3 .
Figure 3. Raman spectroscopy of CNTs products We were characterized SWNTs properties by a Raman microscope system (YVON) at an excitation wavelength of 514.5 nm.
TEM pictures are taken at the Center for Nanoscale characterization, MINATEC/LETI.Long CNTs can be seen in the figure 4 that were produced by CVD process at 900°C.The length of this particular CNT is about few ten of micrometers.

Figure 10 .Figure 11 .Figure 12 .Figure
Figure 10.A schematic growth mechanism of individual SWNTs from catalyst nanoparticles For bundles of single-walled carbon nanotubes, the growth mechanism is the same as with individual SWNTs.

2.1. Materials
nanoparticles are mixed in 30 ml of methanol and sonicated for  1/2 hr.Then, the liquid catalyst is deposited onto the substrate by micropipette.