Effects of the electrical conductivity and orientation of silicon substrate on the synthesis of multi-walled carbon nanotubes by thermal chemical vapor deposition
© Choi et al.; licensee Springer. 2013
Received: 21 December 2012
Accepted: 16 February 2013
Published: 27 February 2013
We studied the effects of the electrical conductivity and orientation of silicon substrate on both catalytic Fe thin film and the structure and morphology of multi-walled carbon nanotube (MWNT) grown by low-pressure chemical vapor deposition. Both p-type Si(100) and Si(111) substrates with three different doping concentrations (high, low, undoped) were used to evaluate the formation of catalytic nanoparticles and the growth of MWNTs. The morphology of catalytic nanoparticles such as size and density was characterized by field-emission scanning electron microscopy, Cs-corrected energy-filtered transmission electron microscopy, and X-ray photoelectron spectroscopy. Structural characteristics of MWNTs grown on different combinations of silicon substrate orientation and electrical conductivities (σ) were also systematically analyzed. Based on the experimental results, growth modes of MWNTs could be controlled by choosing an appropriate combination of σ and orientation of Si substrates.
KeywordsMulti-walled carbon nanotube Catalytic nanoparticle Substrate effect
A large number of experimental parameters for multi-walled carbon nanotubes (MWNTs) grown by chemical vapor deposition (CVD) have been investigated including the type of thickness of catalytic metal films [1, 2], the substrate temperature [3, 4], the ammonia gas flow rates [5, 6], and supporting substrate, etc. [7, 8]. Among those parameters, the control of the catalyst particles is one of the most important factors that determine the structure and morphology of MWNT properties such as lengths, diameters, and density [9–11]. However, a basic growth mechanism explaining the way metallic atoms interact with carbon to nucleate, grow, and heal carbon nanotubes (CNTs) still needs to be understood. Previously, we investigated the effect of the electrical conductivity of the Si(100) substrate on the control of the growth of MWNTs and found that as the electrical conductivity of the silicon substrate increased, the average diameter of the MWNTs also increased while the density of MWNTs decreased . Accordingly, the electrical conductivity (σ) of the substrate can be treated as a parameter for controlling the growth of MWNTs, which is another important parameter related to crystallographic orientation of the exposed substrate surface. Different orientations of silicon substrate play a role in CNT growth resulting from different surface energies. In this study, we report the effects of σ and orientation of the silicon substrate on the growth of MWNTs by thermal CVD. We also describe the role of proposed parameters that govern their growth kinetics and the knowledge about these.
Results of the Hall measurement by van der Pauw method 1 cm × 1 cm size
2.7 × 1012
6.7 × 10-4
1.8 × 1015
9.8 × 10-2
6.0 × 1019
4.3 × 102
1.0 × 1012
1.7 × 10-4
1.0 × 1015
6.1 × 10-2
3.4 × 1019
8.9 × 102
Argon (Ar) gas was flowed into the chamber at a flow rate of 1,000 sccm in this experiment . At the same time, while ammonia (NH3) gas with a flow rate of 140 sccm was flowed into the reactor, the substrates were heated up to the growth temperature of 900°C for 30 min and then maintained at 900°C for 5 min. Acetylene (C2H2) gas was supplied to synthesize MWNTs with a flow rate of 20 sccm for 10 min at 900°C [15, 16]. After the growth of MWNTs, the chamber was cooled down to room temperature and purged with Ar ambient. This work has focused on the size contribution and formation of catalyst particles by supporting substrate orientation and conductivity. However, the samples must be taken to the instrument for ex situ analysis. Therefore, we have endeavored that the exposure of samples to air and moisture was minimized. Once the samples were taken out from the chamber and cooled off to room temperature, each sample was divided into small pieces for the characterization by field-emission scanning electron microscopy (FE-SEM; Hitachi S-4300SE, Hitachi, Ltd., Chiyoda-ku, Japan), Cs-corrected energy-filtered transmission electron microscopy (JEM-2200FS, JEOL Ltd., Akishima-shi, Japan), and X-ray photoelectron spectroscopy (XPS; AXIS Nova, Kratos Analytical Ltd., Manchester, UK). The XPS analysis was carried out using an Al K (1,486.6 eV) X-ray (hν = 1,486.6 eV) photoelectron spectrometer. The base pressure of the XPS system was 5.2 × 10-9 Torr.
Results and discussion
The contrary tendency of Fe particle size according to substrate orientation could be explained that agglomeration and segregation of Fe particles were affected by atomic density, surface energy, and thermal conductivity of different Si surface orientations at the same thermal condition. The binding energy between Fe film and Si(100) substrate is smaller than that between Fe film and Si(111) substrate. In addition, the surface energy of Si(100), 2.13 J/cm2, is almost twice higher than that of Si(111), 1.23 J/cm2. Accordingly, it is expected that the catalytic particles could more easily migrate on Si(100) surface by thermal energy. Under these conditions, there exists a high probability of Fe particle agglomeration. Indeed, it was observed that the average diameter of Fe particles on Si(100) substrate was larger than that on Si(111) substrate. When the metal thin film is annealed, particles are formed by film coarsening, and then, they could agglomerate or break down through surface migration, driven by a thermally activated process resulting in a minimization of the surface energy of the metal film-substrate system.
Generally, the diameter and length of carbon nanotubes were affected by catalytic metal particle sizes in the early stage of growth. Since the average Fe particle size on Si(100) substrate is larger than that on Si(111) substrate, MWNTs grown on Si(100) have larger diameter and shorter length than those grown on Si(111) substrate. As the electrical conductivity of Si(100) substrate increased, Fe particle size is increased, so carbon nanotubes with a short length and large diameter were grown. However, on the other hand, in the case of Si(111) substrate, as the electrical conductivity increased, smaller Fe particles were formed. Accordingly, MWNTs with small-diameter and long carbon nanotubes were synthesized.
In this study, we report the effects of the orientation and electrical conductivity of silicon substrates on the synthesis of MWNTs by thermal CVD. It was found that the size and distribution of Fe particles on silicon substrate could be controlled by varying both orientation and σ. Accordingly, it is possible that the growth of MWNTs by thermal CVD could be also controlled by using the orientation and σ. In the case of Si(100) orientation, it was found that as the electrical conductivity of Si(100) substrates increased, the vertical growth of MWNTs was restrained while the radial growth was enhanced. On the other hand, in the case of Si(111) orientation, the situation is reversed. In this case, it was found that as the electrical conductivity of Si(111) substrates increased, the vertical growth of MWNTs was enhanced while the radial growth was restrained. More detailed investigation on this matter is in progress.
As a result, a strong correlation exists between the growth modes of the MWNTs and the combination of σ and orientation of the silicon substrate. Our results suggest that the combination of σ and orientation of the silicon substrate can be considered as an important parameter for controlling the growth modes of CNTs fabricated by thermal CVD, without the need to alter other growth parameters.
This research was supported by the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (grant no. 20120482). The authors wish to thank Ms. Hyesoo Jeong for plotting the particle distribution.
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