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Letters to the Editor
A plasma enhanced chemical vapor deposition process to achieve branched carbon nanotubes Y. Abdia, S. Mohajerzadeha,*, J. Koohshorkhia, M.D. Robertsonb, C.M. Andreic a
Nano-Electronics Center of Excellence, Department of Electrical and Computer Engineering, University of Tehran, North Kargar Avenue, P.O. Box 14395/515, Tehran, Iran b Department of Physics, Acadia University, Wolfville, Nova Scotia, Canada B4P 2R6 c Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7
A R T I C L E I N F O
A B S T R A C T
Article history:
Vertically aligned carbon nanotubes (CNTs) have been grown on silicon substrates using
Received 4 October 2007
nickel as the catalyst layer, acetylene as the carbon source, and hydrogen as the carrier
Accepted 30 June 2008
gas. The quality of the CNTs has been examined using transmission and scanning electron
Available online 15 July 2008
microscopy and a tip-growth mechanism with an inner tube diameter of 5–8 nm was observed. The effect of plasma hydrogenation as a post-growth treatment was shown to lead to total or partial removal of the nickel seeds from the CNT tips. Using sequential hydrogenation and growth, it has been possible to achieve tree-like nanostructures. 2008 Elsevier Ltd. All rights reserved.
Carbon nanotubes (CNTs) have emerged as a promising technology for the fabrication of field-emission devices and displays, gas sensors, drug delivery and handling devices, hydrogen storage, ballistic transistors and nanolithography [1–3]. Since 1991, several techniques have been introduced for the growth of single and multi-wall carbon nanotubes including thermal catalyst-based chemical vapor deposition, laser vaporization deposition, arc-discharge and plasma enhanced chemical vapor deposition methods [4,5]. We have recently reported the vertical growth of CNTs on (1 0 0) silicon substrates by direct-current plasma enhanced chemical vapor deposition (dc-PECVD) where a mixture of acetylene (C2H2) and hydrogen (H2) was used as the carrier gas and the CNTs were used in the fabrication of inherently gated field-emission devices with high on–off ratios. In addition, the preparation of isolated patterned nanotubes for nanolithography has been successfully demonstrated [6]. In this letter, the asgrown nanotubes were exposed to hydrogen plasma as a post-growth treatment and the subsequent CNT growth led
to the formation of nanotubes on top of the previous nanostructures giving the appearance of tree-like features. Silicon -oriented wafers are used as the substrate for the growth of CNTs. The cleaned wafers were placed in an electron-beam evaporation system in order to deposit a thin layer of nickel (Ni) to act as the catalyst for CNT growth. For this investigation, the Ni thickness is set at 5 nm. Subsequently, the specimens were placed in a dc-PECVD chamber and a two-step pre-growth treatment of the surface was performed. The surface of the sample was initially treated with H2 at a flow rate of 35 sc cm for 10–15 min with the substrate temperature maintained at 650 C. This step was followed by a hydrogen plasma treatment with a power density of 5.5 W/ cm2 for 10–15 min in order to create nano-islands of Ni. The growth of CNTs was performed immediately after the hydrogenation step using the same reactor with a mixture of H2 and C2H2 gasses at a pressure of 2.8 · 102 Pa and the same temperature. The flow rate of these gases was kept at 35 and 5 sc cm. In addition to this normal growth condition, a post-growth
* Corresponding author: Fax: +98 21 8801 1235. E-mail address:
[email protected] (S. Mohajerzadeh). 0008-6223/$ - see front matter 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.06.059
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hydrogenation step has been practiced on certain samples at the same processing temperature, a H2 flow rate of 35 sc cm and a plasma power of 5.5–6.5 W/cm2. The formation of tree-like structures is feasible by performing a second growth of nanotubes on such processed samples. Scanning electron microscopy (SEM) was performed in a CAMSCAN2300 unit at an electron accelerating voltage of 25 kV. The transmission electron microscopy (TEM) diffraction contrast images and electron diffraction patterns were acquired with a Philips CM30 TEM unit operated at 300 kV. Fig. 1a depicts the ordered growth of honeycomb microstructures, obtained by patterning the Ni seed layer prior to the growth. Part (b) shows an array of circular rings of CNTs, achieved by using standard photolithography to pattern photo-resist onto circular disks and then using an oxygen plasma ashing treatment to hollow out the disks. Inset in Fig. 1b presents a magnified view of a circular ring of nanotubes. In order to study the physical structure of CNTs in more detail, cross-sectional samples were prepared using a cleavage technique and studied by TEM. Fig. 2a shows both bright field and dark field TEM images of typical CNTs grown perpendicular to the surface of the silicon substrate. As observed in these images, the Ni seed has been lifted upwards during the growth of the CNTs and was positioned at the
Fig. 2 – (a) TEM bright field and annular dark field images of typical nanotubes displaying the location of the Ni seed at the top of the CNT. (b) A bright field image of a CNT highlighting the presence of Ni midway up the CNT (left) and an EDX line scan using the Ni Ka peak confirming the presence of the Ni (right).
Fig. 1 – (a) Growth of ordered CNT structures. (b) An array of CNT rings on silicon substrate. Inset highlights one of the rings with a diameter of 1.5 lm.
top of each tube, confirming a tip-growth mechanism. The mechanism of the CNT growth has been previously discussed by Chhowalla et al. [7] where both tip and root growth conditions are possible, depending on the adhesion of the seed layer to the substrate. As observed from these images, the grown CNTs have a tapered structure with a diameter of about 50 nm at half height and an inner hole running the length of the CNTs ranging in diameter from 5 to 8 nm. Fig. 2b shows another bright field image of a nanotube where part of the Ni seed has been trapped within the hollow core of the CNT. Adjacent to the TEM image is a line scan of the intensity of the Ni Ka X-ray peak as a function of position along the CNT, evidencing part of the Ni trapped in the middle of the CNT during the growth. A hydrogenation treatment after the growth of CNTs was found to have significant effects on both the CNT and the Ni seed located at the tip. Part (a) in Fig. 3 shows the SEM images of an island of CNTs taken with the secondary electron (left
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Fig. 3 – (a) Secondary (left) and back-scattered electron (right) SEM images of a cluster of CNTs. (b) SEM image of a similar cluster after post-growth hydrogenation. The Ni seed has been removed from the tip of the CNTs and is now located on their side.
image) and back-scattered electron (right image) detectors. The presence of Ni at the tips of CNTs is readily observable in the more compositionally sensitive back-scattered electron image. Presented in Fig. 3b is a SEM image of a similar island of CNTs, exposed to a post-growth hydrogenation step at a plasma power density of 6.5 W/cm2 and a temperature of 650 C for 10 min. As observed in this image, the Ni seed has tipped onto the side of the nanotube leaving the top surface of the nanotube fully open. Thus, this post-growth hydrogenation treatment can be used as a mid-growth step where the growth of CNTs can be continued from the top side of the original CNT in order to achieve complex structures. Fig. 4 displays the process flow to realize secondary growth of CNTs and obtain tree-like structures. First the initial CNTs are uniformly coated with a layer of titanium-dioxide (TiO2) by means of an atmospheric-pressure chemical vapor deposition (APCVD) technique. This deposition step, achieved by a
Fig. 4 – The processing steps used to prepare tree-like structures, starting from the growth of vertical CNTs, conformal coating with TiO2, post-growth hydrogenation in hydrogen plasma and finally the subsequent growth of CNTs.
mixture of TiCl4 and O2 at a temperature of 250 C, is crucial for the subsequent processing. The sample is then returned to the dc-PECVD reactor and a post-hydrogenation step is performed to create holes in the TiO2 layer and to allow the trapped Ni to flow out to the outer side of the covered tip. Subsequently, C2H2 is introduced into the plasma reactor to grow the second layer of CNTs from the exposed Ni seeds on top of the original nanotubes. These steps can be repeated to create branches and sub-branches with gradual decreasing diameters. In Fig. 5 we have collected several SEM images corresponding to the growth of such tree-like structures at various stages
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ones. The conditions for this growth step are set at respective flow rates of 35 and 3 sc cm for H2 and C2H2 and plasma power of 5.5 W/cm2. These steps are repeated two times and the results are shown in the middle and right side images of Fig. 5a. It is worth mentioning that a lower C2H2 flow rate is needed to achieve nanotubes with thinner diameters. Fig. 5b presents an image of secondary CNTs grown on top of the original structures, after one and two post-growth processing sequences. Inset shows a magnified view of the secondary nanotubes grown on the original structures leading to much thinner features. In summary, we have successfully grown vertically aligned CNTs and tree-like nanostructures on silicon substrates using a dc-PECVD technique. The effects of the post-growth treatment of the CNTs by plasma hydrogenation were studied and it was observed that the tip side of the CNTs open up and Ni can be evacuated from the tip side. Lastly, the evolution of tree-like structures by a sequential hydrogenation and growth has been demonstrated. These tree-like structures can be used for improved field emission and gas detection devices.
Acknowledgement The financial support of this work has been provided in part by Iran National Science Foundation.
R E F E R E N C E S
Fig. 5 – (a) Selected SEM images highlighting the preparation of tree-like structures. (Left) an individual nanotube coated with TiO2. Inset shows the CNT after an exposure to the post-growth hydrogenation step. Middle and right images show the subsequent growth of CNTs. (b) SEM image of a cluster of tree-like nanostructures. Inset shows a more magnified view of individually placed branched nanotubes.
of preparation. The left image in Fig. 5a shows a CNT after coating with a layer of TiO2 with a thickness of 100 nm. Inset highlights the tip after the post-hydrogenation step was carried out where the trapped Ni has been partially released to the surface of each nanotube. Also as seen, the tip side of the original CNT has expanded due to such post-growth plasma hydrogenation. To avoid an explosive evacuation of the Ni from the tip side (similar to Fig. 3b), the hydrogenation plasma power was kept at a lower value of 5.5 W/cm2. After this post-growth hydrogenation and without breaking vacuum, the sample was exposed to a mixture of H2 and C2H2 to perform the second growth of the CNTs on top of the original
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