Control of Cobalt Catalyst Thin Film Thickness by Varying Spin Speed
in Spin Coating towards Carbon Nanotube Growth
Nor Najihah Zulkapli
a, Mohd Edeerozey Abd Manaf,
Hairul Effendy Ab Maulod, Nor Syafira Abdul Manaf,
Raja Noor Amalina Raja Seman, Mohd Shahril Amin Bistamam,
ElyasTalib and Mohd Asyadi Azam
b1Carbon Research Technology Group, Faculty of Manufacturing Engineering, UniversitiTeknikal
Malaysia Melaka (UTeM),Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia
am051320004@student.utem.edu.my, basyadi@utem.edu.my
Keywords : Cobalt catalyst, Thin films, Spin coating, Catalytic chemical vapor deposition
Abstract. Cobalt (Co) catalyst thin film is an active metal catalyst that can be very helpful to grow carbon nanotubes (CNTs). The catalyst thin films were prepared on silicon wafers by spin coating the solution of cobalt acetate tetrahydrate and ethanol. The effects of different spin speed parameter during the spin coating process were investigated. The findings showed that the optimum thickness of the Co catalyst thin films, i.e., 12.1 nm, was obtained at the highest spin speed of 8000 rpm. The uniformity of the thin films was also found to increase with increasing spin speed. The study also demonstrated that single-walled carbon nanotubes could be grown from Co catalyst particles after the catalytic chemical vapor deposition of ethanol. The particle and thickness analysis, as performed by means of FESEM while the existence of CNTs, was performed by Raman spectroscopy.
Introduction
The synthesis of carbon nanotubes (CNTs), especially single-walled carbon nanotubes (SWCNTs), by the catalytic chemical vapor deposition (CCVD) method has been widely studied due to their extraordinary electrical, mechanical, and thermal properties, accompanied with cost effective and good quality of CNTs produced [1]. The transition metals such as Fe, Ni and Co have been extensively employed as the metal catalyst in the synthesis of SWCNTs by this method [2,3].These metals have high solubility of carbon at high temperature and high diffusion rate – features which are very helpful during the CNT growth process. The catalyst particles formed on the substrates provide active sites for the nucleation of CNTs, and the size of catalyst particles vigorously corresponds to the diameter of CNTs produced [4].In order to attain a smaller diameter of CNTs, the size of particles can be controlled by controlling the thickness of catalyst thin films on the substrates. SWCNTs can be grown from the catalyst particles that have diameters in range of 3-4 nm [5].
Other than arc-discharged and laser ablation, physical vapor deposition (PVD) is one of the attractive techniques to deposit catalyst thin films onto selected substrates. Smaller and more uniformed thickness of catalyst thin films can be easily attained by the PVD technique. Several studies have been reported regarding the formation of catalyst thin films thickness less than 1.0 nm[6,7]. However, the employment of PVD leads to the cost ineffectiveness in the electronic devices manufacturing due to the need of high power of electrical supply in order to provide a vacuum condition in the system. Thus, other deposition techniques, spin coating [8-10], dip-coating [11,12], spray coating [13] and patterning [14], which are based on the solution-based catalytic precursors have been progressively prominent to gain uniformed thin films. Out of these techniques, spin coating is extensively investigated due to the easy and simple technique to form uniformed catalyst thin films that will contribute to the direct growth of CNTs onto the substrates. It also allows obtaining smaller thickness of thin films. With those merits, it has high potential to produce CNTs for the use in the development of high performance electrochemical capacitor [15].
In this paper, the effect of spin speed on thickness of Co catalyst thin films produced by spin coating technique is investigated. The main aim for this research is to achieve smaller thickness of thin film which will be used to grow CNTs on the silicon substrates. The thickness of Co catalyst thin films over various spin speed of spin coating is studied. The ability of the produced Co catalyst particles to grow CNTs on the substrates is also discussed.
Experimental
Co catalyst thin films were prepared by diluting Co acetate tetrahydrate, Co(CH3COOCH)2.4H2O, in absolute ethanol to produce a solution with a concentration of 16.8
mmol/L. The solution was continuously stirred for 12 hours and left for 36 hours aging. The silicon wafer of 101.6mm diameter, with one sided polished and 1000 Å thermal oxide layer, was used as substrate. The substrate was cut into squares of 15x15 mm2. Next, the wafers were rinsed with
acetone and stirred with ethanol for 10 min for cleaning purpose. Then, the cleaned wafers were dried with nitrogen gas.
Co acetate tetrahydrate/absolute ethanol solution was spin-coated on the cleaned silicon wafer at spin speed of 6500, 7000, 7500, 8000 rpm for 60 s. The coated wafers were quickly pre-heated at 250°C for 5 min to vaporise the solvent, leaving Co catalyst thin films on the wafers. Then the pre-heated wafers were placed in a vacuumed furnace for heat treatment process of 450°C, 500°C, 550°C and 600°C for 10 min to obtain Co catalyst nanoparticles.
The smallest and most uniform thickness of Co catalyst thin film was then employed to study the ability of that catalyst thin film to grow carbon nanotubes by CCVD at 725°C for the growth duration of 15 min. The CCVD system was allowed to cool to room temperature under the argon flow after the reaction. A field emission scanning electron microscope (FESEM, Hitachi SU8100, HTI lab, Selangor) was used to analyze the obtained samples at 5.0kV. Raman analysis was performed by RenishawInViaRaman Microscope (Wotton-under-Edge, UK) with laser excitation of 632.8 nm wavelength HeNe laser.
Results and discussion
In order to investigate the dependence of thickness of Co catalyst thin films on spin speed of spin coating, four different spin speeds parameter were used for the coating of Co catalyst solution on the substrates. The resulted films were characterized by FESEM as shown in Fig. 1(a) to (d). A variation in the thin film thickness obtained showed that the deposited thin films were in continuous pattern. As the spin speed was increased from 6500 to 8000 rpm, the thin film thicknesses were decreased from 25.5 nm to 12.1 nm (Fig. 2(a)-(e)).These results indicate that increasing the spin speed rotation effectively decreases the thickness of thin film deposited on the silicon substrate. The thinning process is due to the developed centrifugal force that causes shear thinning of the precursor solution to form a film layer on top of the substrate. The thickness of thin film plays an important role because it determines the depth of the stacked particle on top of each other which affects the surface roughness and the uniformity of the particle distribution all over the surface of the substrate. If the thickness of the thin film is increased, there will be more stacking of particles, which causes non uniformed particle distribution on the substrate.
Fig. 1 FESEM micrographs of Co catalyst thin films at the spin speed of (a) 6500 rpm, ( b) 7000 rpm, (c) 7500 rpm, (d) 8000 rpm. The samples are cross-sectional, tilted to 45° and in
different scales.
Fig. 2 The histograms of Co catalyst thin films thickness with the spin speed of (a) 6500 rpm, (b) 7000 rpm, (c) 7500 rpm, (d) 8000 rpm and (e) graph of average thickness of Co catalyst thin
films at four different spin speeds.
The catalyst thin film with the smallest thickness which was obtained at 8000 rpm of spin speed was selected for the CCVD process to grow CNTs in order to prove that the deposited Co catalyst can be used for the CNT growth. The result of Raman spectroscopy analysis as shown in Fig. 3 shows that single-walled CNTs are successfully grown on the substrate. Using the results of Raman analysis, the structural properties of as-grown CNTs were also studied. The Raman spectrum of the sample exhibits peaks of the radial breathing mode (RBM) from 100 to 350 cm-1, which reveal the
presence of SWCNTs, a D-band at approximately 1326 cm-1, and a G-band at approximately
1587cm-1 (Fig. 3). The quality of the CNTs can be assessed by calculating the intensity ratio of the
G band to D band (IG/ID). The (IG/ID) ratio of this sample was 1.71, indicating that the as-grown
CNTs have small portion of structural defects.
0
Thickness (nm) Thickness (nm) Thickness (nm)
Fig. 3 Raman spectrum of the as-grown CNTs on the Co catalyst thin film obtained by spin coating at 8000rpm.
Conclusion
The Co catalyst thin film produced from the spin coated of 16.8 mmol/L solution of cobalt acetate tetrahydrate and ethanol at 8000 rpm for 60 s was found to have the smallest thickness of thin film with uniformed thickness. As mentioned earlier, the size of catalyst particles can be controlled by controlling the thickness of catalyst thin films. In order to obtain a smaller size of catalyst particles, the favored thickness of thin films is as small as possible. This condition leads to the growth of SWCNTs since SWCNTs can be grown from the catalyst particles that have diameters in the range of 3-4 nm. The potential of SWCNTs to grow on this catalyst thin film by ethanol CCVD was proven by the Raman spectrum of the sample that exhibited peaks of the RBM from 100 to 350cm-1, which reveal the presence of SWCNTs.
Acknowledgements
Special thanks to Carbon Research Technology (CRT), UTeM group for continuous mutual support and knowledge proof-read. The authors are also grateful for the financial support by the Ministry of Higher Education (MOE), Malaysia under the Exploratory Research Grant Scheme (ERGS) with research grant numbered E00032.
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