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Role of carbon atoms in plasma-enhanced chemical vapor deposition for carbon nanotubes synthesis
Thin Solid Films 515 (2006) 1314 – 1319 www.elsevier.com/locate/tsf
Role of carbon atoms in plasma-enhanced chemical vapor deposition for carbon nanotubes synthesis M.A. Bratescu a,⁎, Y. Suda b , Y. Sakai b , N. Saito c , O. Takai d a
b
n-Factory Co., Ltd., 5-36-4 Kawana-cho, Showa-ku, Nagoya, 466-0856, Japan Graduate School of Information Science and Technology, Hokkaido University, North 14 West 9, Sapporo 060-0814, Japan c Department of Molecular Design and Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan d EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan Received 17 October 2005; received in revised form 9 January 2006; accepted 9 March 2006 Available online 19 April 2006
Abstract The role of carbon atoms in a dc plasma-enhanced chemical vapor deposition for carbon nanotubes (CNTs) synthesis was investigated. It was observed that at 1.33 kPa pressure of CH4 gas in plasma, a high value of the ratio between the intensities of the graphite peak (G peak) and the disorder peak (D peak) in the Raman spectrum corresponds to the maximum value of the excited C number density in the vicinity of the Si substrate. It was found that a CH4 gas pressure higher than 1.33 kPa leads to an increase of the relative density of the C2, C3 molecules and the clusters, and to a decrease of the C excited atom number density in plasma. The presence of a high amount of sp2-graphite in the composition of CNTs observed in Raman spectrum was also confirmed by the measurement of the IR-active G peak at 1584 cm- 1 in the transmission spectrum. © 2006 Elsevier B.V. All rights reserved. PACS: 81.07.De.; 52.70.Kz.; 81.15.Gh. Keywords: Carbon; Chemical vapor deposition (CVD); Methane; Nanostructure; Optical spectroscopy; Plasma processing and deposition; Clusters; Hydrocarbons
1. Introduction Carbon nanotubes (CNTs) are increasingly becoming one of the most important materials for various applications in electronics, optics, and biophysics. Up to now, several synthesis methods for CNTs growth, such as arc-discharge [1], laser ablation [2], chemical vapor deposition (CVD) [3], and plasmaenhanced CVD (PECVD) [4,5] have been investigated. All of these techniques require high temperatures for the growing process, which were obtained in electric furnaces or with a hot filament. In our experiment we used dc plasma, which provides the advantage that the metal catalyst was deposited by Ar sputtering inside the same vacuum chamber where CNTs were produced on a Si substrate heated by Joule effect. The PECVD arrangement offered the possibility to study plasma composition ⁎ Corresponding author. Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. Tel.: +81 52 789 3259; fax: +81 52 789 3260. E-mail address: [email protected] (M.A. Bratescu). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.03.022
in the vicinity of the Si substrate by spectroscopic methods in the same time with CNTs deposition process. In the present work, the number density of the C excited atoms near the Si substrate was measured, during CNTs formation, by laser absorption spectroscopy (LAS) method [6–8]. The plasma composition and the relative number density of the CH radical, the C2, C3 molecules and the clusters were measured by optical emission spectroscopy (OES) method [9]. The morphology of the synthesized CNTs was probed with a scanning electron microscope (SEM). Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR) characterized the composition of CNTs. In the present paper, it was found that an increased CH4 gas pressure leads to an increase of the relative density of the C2, C3 molecules and the clusters. The C excited atom number density in plasma had a maximum value at a pressure of 1.33 kPa. It was observed that the optimum conditions for CNTs growth were at a CH4 gas pressure higher than 1.33 kPa, since the ratio between the intensities of the graphite peak (G peak) and the disorder peak (D peak) in the Raman spectrum had high values.
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To understand the processes for the CNTs formation in a dc PECVD, the measurements of the C atom number density, the C2, C3 molecules, the CH radical and the clusters relative number densities will be discussed in connection with the Raman data, the FT-IR spectrum and the morphological analyses. 2. Experimental details 2.1. CNTs preparation The experimental setup for CNTs synthesis and plasma diagnostics is schematically shown in Fig. 1. CNTs were produced in dc plasma in CH4 gas. The cathode was a cylindrical rod of Ni (99.99 %) with a diameter of 15 mm. The anode plate was made from stainless steel, with a diameter of 60 mm. The distance between electrodes was 7 mm. The Si substrate (Nilaco, 100, 10 × 10 × 0.625 mm, and resistivity ≤ 0.02 Ω m) was set in the space between the cathode and the anode, at a distance of ∼ 3 mm far from the cathode. The experiment was realized in two steps: (i) the deposition of the Ni thin film in dc Ar plasma and (ii ) the heating of the Si substrate and the CNT production in dc CH4 plasma. In the first step, the Ni thin film was deposited on the Si substrate by cathode sputtering in dc Ar plasma at 20 mA discharge current, in 1.10 kPa pressure and during 10 min. The Ni film thickness can be controlled by the sputtering time. The film thickness was roughly evaluated from the surface profile measurement of a Ni film which was deposited during 2 h. A surface measuring instrument (SURFTEST SV-600, Mitutoyo) was used to measure the surface profile. The Ni film was deposited on a Si substrate which was half covered with kapton tape, in dc Ar plasma, at 20 mA discharge current and in 1.10 kPa pressure. After the Ni deposition, the kapton tape was removed. The difference between the surface levels with and without the Ni film was measured. The Ni film thickness for the CNTs production was estimated to be ∼ 5 nm. After the Ni thin film deposition, the Si substrate was heated at 750 °C, using a Joule heater. For this purpose a dc stabilized power supply, at 3 A constant current was used. The substrate temperature was measured with an optical pyrometer. Typical resistance, of 500 Ω before heating, of the substrate including
Fig. 1. Experimental setup.
Fig. 2. Dependence of the C excited atoms number density on p for different values of i.
the contact resistance decreased at ∼ 3 Ω after heating, due to the increased temperature of the Si substrate [10]. A stable CH4 discharge was obtained for gas pressures ( p) from 0.27 kPa up to 2.67 kPa and for discharge currents (i) from 5 to 30 mA. The deposition time for CNT production was 1 h. During the deposition, the Si substrate was kept at a constant temperature. 2.2. Plasma diagnostics Plasma diagnostics consisted in the measurements of the number density of the C excited atom in the vicinity of the Si substrate by LAS [6] and the measurements of the relative emission intensities of the excited CH radical, the excited C2, C3 molecules and the clusters by OES [9]. The number density of the excited C atom on 31P10 level (NC) near the Si substrate was measured with a commercial diode laser (DL: Hitachi 8325G, 40 mW maximum output with the central wavelength of 830 nm at 25 °C). The DL wavelength (λ = c/ ν) was tuned to the resonance absorption of the C atom (31P10 →31S0) at λ0 = 833.744 nm. The laser beam was focused within a diameter of ∼0.1 mm in the vicinity of the Si substrate. The DL intensity was adjusted to be lower than the value that provides the saturation of the absorption signal using corresponding neutral density filters. A photodiode (PD) and a lock-in amplifier (NF Electronic Instruments 5610B) were used to acquire the absorption signal [6]. The signal to noise ratio (S /N) of the C(31P1o) atom absorption signal was ∼10. The value of NRC was calculated by the following formula: NC ¼ ð8kgi =gj k20 Aji LÞ absorption line ½1−IðmÞ=I0 dm, where gi (=3) and g j (=1) are the statistical weights of the lower and the upper levels, respectively. Aji (=0.351 ×108 s− 1) is the Einstein coefficient for the C atom transition at λ. L is the absorption length, which is assumed to be equal to the diameter of the cathode (∼15 mm). I0 is the laser intensity before entering the plasma and I(λ) is the laser intensity after passing through the plasma. The dependence of NC on p and i is shown in Fig. 2. The emission spectrum of dc CH4 plasma was analyzed in a range between 300 and 800 nm using a Hamamatsu PMA-11 (Photodiode Multichannel Analyzer) with a 2 nm spectral resolution. The emission spectra were corrected with the spectral sensitivity of the CCD detector by the acquisition software. The light background was subtracted from each recorded spectra.
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Fig. 3. Typical OES spectra of CH4 plasma (i = 30 mA and p = 1.33 kPa).
The most intense OES lines of the species produced by the decomposition of the CH4 molecule were: CH(A2Δ) radical at 431.4 nm (A2Δ → X 2Π), 388.9 nm (B2Σ → X 2Π) and 314.3 nm (C 2Σ → X 2Π), the C2(A3Πg) molecule at 473.7, 516.5, and 563.5 nm (Swan System: A 3 Πg → X′ 3 Πu), the C3( 1 Πu) molecule at 405.0 nm (Comet Head System: 1Πu → 1Σg+), and H atom at 656.3 nm (Hα) and 486.1 nm (Hβ). OES contains a lot of Ni emission lines from the sputtered atoms excited in plasma (Fig. 3). In the spectral region from 670 to 800 nm, a blackbody radiation was observed at different p and i. This continuum spectrum was attributed to the incandescence of hot hydrocarbon clusters or nanoparticles. The relative intensity of the clusters in plasma was described by the OES intensity around 800 nm. The relative densities of the C2, C3 molecules and clusters were expressed as the ratios between the OES intensity of the C2, C3 molecules (IC2, IC3) and the clusters (Iclusters), respectively and the OES intensity of the CH radical (ICH). IC2/ICH and IC3/ICH ratios could represent the relative densities of the corresponding molecule because the excitation energies of the CH radical on A2Δ level, the C2 molecule on A3Πg level, and the C3 molecule on 1Πu level relative to the ground level, have almost the same values, ∼3 eV. The dependences of the relative densities of the C2, C3 molecules and the clusters on p and i are shown in Fig. 4(a) and (b).
Fig. 4. Dependence of the relative densities of the C2, C3 molecules and the clusters (a) on p, at i = 30 mA and (b) on i, at p = 1.33 kPa.
from A1g breathing mode of single-wall CNTs [12]. The second-order Raman peaks of CNTs occurs between 2000 and 3800 cm− 1. The peaks observed at 2668 and 3185 cm− 1 are the second harmonics of D peak and G peak, respectively. The peak at 2927 cm− 1 is approximately the sum of the D and G peak frequencies. It is commonly used in the CNTs production and purification experiments to characterize the structural integrity of the sp2hybridized carbon atoms of the nanotubes by the ratio between the intensities of G peak and D peak [13,14]. Fig. 6 shows the dependence of IG/ID on different p and i = 15–30 mA.
2.3. CNTs analysis CNTs were analyzed by Raman (JASCO NRS-1000HS) and FT-IR (JASCO FT/IR 660 Plus) spectroscopy and scanning electron microscope (SEM Hitachi S4800). Fig. 5 shows a typical Raman spectrum of CNTs obtained in the experiment. The Raman spectra of CNTs have several peaks located between 200 and 3800 cm− 1 [11,12]. The most intense peaks occur at 1334 cm− 1 (D peak), which comes from carbonaceous particles and indicates the presence of defects in graphite structure, and 1589 cm− 1 (G peak), which represents E2g Raman scattering mode of sp2-hybridized carbon and comes from the tangential motion of hexagons. Besides the D and G peaks, there are several smaller peaks at 283, 310, 337, 371 and 520 cm− 1. The peak at 520 cm− 1 is due to the Si substrate. The other four peaks are characteristic features arising
Fig. 5. Typical Raman spectrum of CNTs produced in dc plasma, for p = 1.33 kPa and i = 15 mA.
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Fig. 6. Dependence of IG /ID on p for different values of i.
FT-IR spectroscopy is also used for the investigation of the CNT composition [15,16]. In the present experiment, the functional groups that were detected in the FT-IR transmission spectrum of the CNT are: D peak at 1300 cm− 1, G peak at 1584 cm− 1, carboxylic carbonyl group at 1727 cm− 1, and CHx group at around 3000 cm− 1 (Fig. 7). From the transmission spectrum measured by FT-IR, IR absorption coefficient of CHx band, α(ω), was calculated. The thickness of the deposition was considered the same as the length of CNTs which was estimated to be ∼ 15 μm from the morphological analysis by SEM (Fig. 8(a) and (b)). The number R of C–H bonds per unit dx, where volume was calculated as: NCH ¼ A absorption band aðxÞ x A (= 1.35 × 1021 cm− 2) is a constant of the C–Hx stretching mode absorption around 3000 cm− 1 and ω is the corresponding wavenumber [17,18]. NCH was found to be ∼ 2 × 1023 cm− 3 for a deposition at i = 30 mA and p = 1.33 kPa. The obtained CNTs on the Si substrate exhibit an alignment of the tubes on the Si surface as can be seen by SEM analysis (Fig. 8(c) and (d)). 3. Experimental results and discussion The highest value of NC (∼108 cm− 3) was obtained for p ∼ 1.33 kPa. NC slightly depended on i in the range from 5 to 30 mA. As pressure increased ( p N1.33 kPa), NC decreased and the relative densities of the C2, C3 molecules and the clusters increased (Figs. 2 and 4). The observed continuum spectrum which corresponds to the blackbody radiation is usually fit with a Planck function to determine particle temperature and to obtain a crude estimation of the particle size [19]. Similar results with the present OES results have been reported in Ref. [20], in a laser vaporization of graphite/(Ni, Co) system for single-wall CNTs synthesis. Some of the most significant chemical reactions in gas-phase plasma, which can explain the appearance of the C and H atoms, the C2 and C3 molecules, the CH radical and the hydrocarbon clusters are the electron impact reactions (which conduct to dissociation, excitation, and ionization processes), the neutral– neutral reactions and the ion–neutral reactions. On the surface, the chemical interaction of neutrals and ions with the catalyst and the substrate are the most significant reactions [5,21]. The C excited atoms are mainly produced in the gas-phase plasma by the dissociation of the CH4 molecule, the larger
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hydrocarbon radicals and the clusters, through electron impact processes [21]. A small amount of the C excited atoms which could appear in plasma from the re-sputtered C atoms deposited on the Si substrate could not be separately measured. A detail calculation of the hydrocarbon species number density produced in CH4 plasma takes into account both the electron energy distribution function and the energy dependent reactive collision cross section. The calculated number density of different radicals and hydrocarbon clusters in a CH4 plasma increases with the increase of the gas pressure. The present experimental results on the pressure dependence of the relative densities of the C2, C3 molecules and the clusters shown in Fig. 4(a) correspond to the theoretical calculated data [21,22]. The IG/ID ratio had a high value in case of the CNTs obtained in a CH4 plasma for p = 1.33 kPa and i ∼ 15–30 mA. For the same pressure and i = 20 mA, a maximum value of NC was measured. For p N 1.33 kPa, NC significantly decreases (Fig. 2), the relative densities of the C2, C3 molecules and the clusters increase and the IG/ID ratio values are comparable (Fig. 6). As already mentioned, the G peak reflects the structural integrity of the sp2-hybridized C atoms of CNT and the D peak indicates disordered sp2-hybridized C atoms [13]. In our experiment, the IG/ID ratio values cannot completely describe the structural integrity of sp2-hybridization of C atoms because a large amount of amorphous carbon was formed in the same time with CNTs production (Fig. 8). In the optimum plasma conditions for the CNTs production, at p N 1.33 kPa and i ∼ 15–30 mA, it is difficult to attribute a higher contribution in the CNTs formation process to the C atoms or to the hydrocarbon clusters and radicals. In a dc PECVD system the control of the CNTs formation is governed by the processes that occur in thermal CVD (arrival of excited species on the substrate surface, catalytic dissociation, departure of undissociated species, formation of carbon film, diffusion of C atoms around the catalyst, incorporation of C atoms in the grapheme tube) and by the additional effects due to the plasma (electric field and partial ionization of the gas, cathode sputtering and chemical etching) [23]. A catalytic growth model on CNT explains that in order for a tube to grow, the C atoms have to be transported to the growing edge of the tube [24]. It is assumed that the hydrocarbon clusters and the larger C molecules are dissociated into clusters of few atoms
Fig. 7. A baseline corrected FTIR spectrum of CNT deposition at p = 1.33 kPa and i = 30 mA.
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Fig. 8. SEM images of CNTs obtained for: (a) and (b) i = 30 mA and p = 1.33 kPa; (c) i = 15 mA and p = 1.33 kPa; and (d) i = 15 mA and p = 1.33 kPa.
that adsorb on the growing CNT more probably at the tube surface than directly at the tube edge. The small peaks in Raman spectrum which correspond to the A1g breathing mode of a single-wall CNT (Fig. 5), could probe that a small quantity of single-wall CNTs may be present in the deposition [12]. From SEM analysis in Fig. 8, we can observe that the obtained CNTs are mixed with amorphous carbon. FT-IR analyses confirm that a large amount of amorphous carbon– hydrogen was also deposited in the same time with CNT production (Fig. 7). The calculated number density NCH was a typical value for an amorphous carbon–hydrogen film [17]. The presence of a high amount of sp2-graphite in the composition of CNTs was also confirmed by the IR-active peak corresponding to G peak at 1584 cm− 1 (Fig. 7) [15]. The IR-active peak at 1727 cm− 1 corresponds to nonconjugated carboxylic carbonyl group and could be explained by the oxidation of the Si substrate before deposition [15,16]. The Ni atoms, mainly sputtered in Ar plasma before CNTs production, represent the metal catalyst and are uniformly distributed on the Si surface. The orientation of CNTs on the Si surface could be explained by a temperature gradient during the heating due to the electrical connections, but we have no clear evidence of this effect yet. 4. Conclusions In this paper we studied a dc PECVD for the CNTs synthesis. The optimum experimental conditions for the CNTs growth correspond to 1.33 kPa pressure of CH4 gas and 15–30 mA
discharge currents. The length of the synthesized CNTs in PECVD in CH4 gas was ∼ 15 μm. The C excited atoms number density decreased when the gas pressure was higher than 1.33 kPa and the relative number density of the C2, C3 molecules and the clusters to the number density of CH radical increased with gas pressure. The experimental results showed that a high value of the IG/ID ratio was obtained when the number density of the C excited atoms in the vicinity of the Si substrate was maxim. The presence of amorphous carbon–hydrogen mixed with CNTs in the deposition was confirmed from SEM and FT-IR analysis. The IR-active peak at 1584 cm− 1 which corresponds to the G peak showed that a large quantity of the sp2-graphite was present in the deposition. In Raman spectra, some characteristics peaks of the single-wall CNTs have been detected, which confirm that CNTs contain a small amount of single-wall CNTs. Acknowledgements This work was supported partially by Grant-in-Aid for Scientific Research (C) of JSPS, Japan.
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Role of carbon atoms in plasma-enhanced chemical vapor deposition for carbon nanotubes synthesis M.A. Bratescu a,⁎, Y. Suda b , Y. Sakai b , N. Saito c , O. Takai d a
b
n-Factory Co., Ltd., 5-36-4 Kawana-cho, Showa-ku, Nagoya, 466-0856, Japan Graduate School of Information Science and Technology, Hokkaido University, North 14 West 9, Sapporo 060-0814, Japan c Department of Molecular Design and Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan d EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan Received 17 October 2005; received in revised form 9 January 2006; accepted 9 March 2006 Available online 19 April 2006
Abstract The role of carbon atoms in a dc plasma-enhanced chemical vapor deposition for carbon nanotubes (CNTs) synthesis was investigated. It was observed that at 1.33 kPa pressure of CH4 gas in plasma, a high value of the ratio between the intensities of the graphite peak (G peak) and the disorder peak (D peak) in the Raman spectrum corresponds to the maximum value of the excited C number density in the vicinity of the Si substrate. It was found that a CH4 gas pressure higher than 1.33 kPa leads to an increase of the relative density of the C2, C3 molecules and the clusters, and to a decrease of the C excited atom number density in plasma. The presence of a high amount of sp2-graphite in the composition of CNTs observed in Raman spectrum was also confirmed by the measurement of the IR-active G peak at 1584 cm- 1 in the transmission spectrum. © 2006 Elsevier B.V. All rights reserved. PACS: 81.07.De.; 52.70.Kz.; 81.15.Gh. Keywords: Carbon; Chemical vapor deposition (CVD); Methane; Nanostructure; Optical spectroscopy; Plasma processing and deposition; Clusters; Hydrocarbons
1. Introduction Carbon nanotubes (CNTs) are increasingly becoming one of the most important materials for various applications in electronics, optics, and biophysics. Up to now, several synthesis methods for CNTs growth, such as arc-discharge [1], laser ablation [2], chemical vapor deposition (CVD) [3], and plasmaenhanced CVD (PECVD) [4,5] have been investigated. All of these techniques require high temperatures for the growing process, which were obtained in electric furnaces or with a hot filament. In our experiment we used dc plasma, which provides the advantage that the metal catalyst was deposited by Ar sputtering inside the same vacuum chamber where CNTs were produced on a Si substrate heated by Joule effect. The PECVD arrangement offered the possibility to study plasma composition ⁎ Corresponding author. Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. Tel.: +81 52 789 3259; fax: +81 52 789 3260. E-mail address: [email protected] (M.A. Bratescu). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.03.022
in the vicinity of the Si substrate by spectroscopic methods in the same time with CNTs deposition process. In the present work, the number density of the C excited atoms near the Si substrate was measured, during CNTs formation, by laser absorption spectroscopy (LAS) method [6–8]. The plasma composition and the relative number density of the CH radical, the C2, C3 molecules and the clusters were measured by optical emission spectroscopy (OES) method [9]. The morphology of the synthesized CNTs was probed with a scanning electron microscope (SEM). Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR) characterized the composition of CNTs. In the present paper, it was found that an increased CH4 gas pressure leads to an increase of the relative density of the C2, C3 molecules and the clusters. The C excited atom number density in plasma had a maximum value at a pressure of 1.33 kPa. It was observed that the optimum conditions for CNTs growth were at a CH4 gas pressure higher than 1.33 kPa, since the ratio between the intensities of the graphite peak (G peak) and the disorder peak (D peak) in the Raman spectrum had high values.
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To understand the processes for the CNTs formation in a dc PECVD, the measurements of the C atom number density, the C2, C3 molecules, the CH radical and the clusters relative number densities will be discussed in connection with the Raman data, the FT-IR spectrum and the morphological analyses. 2. Experimental details 2.1. CNTs preparation The experimental setup for CNTs synthesis and plasma diagnostics is schematically shown in Fig. 1. CNTs were produced in dc plasma in CH4 gas. The cathode was a cylindrical rod of Ni (99.99 %) with a diameter of 15 mm. The anode plate was made from stainless steel, with a diameter of 60 mm. The distance between electrodes was 7 mm. The Si substrate (Nilaco, 100, 10 × 10 × 0.625 mm, and resistivity ≤ 0.02 Ω m) was set in the space between the cathode and the anode, at a distance of ∼ 3 mm far from the cathode. The experiment was realized in two steps: (i) the deposition of the Ni thin film in dc Ar plasma and (ii ) the heating of the Si substrate and the CNT production in dc CH4 plasma. In the first step, the Ni thin film was deposited on the Si substrate by cathode sputtering in dc Ar plasma at 20 mA discharge current, in 1.10 kPa pressure and during 10 min. The Ni film thickness can be controlled by the sputtering time. The film thickness was roughly evaluated from the surface profile measurement of a Ni film which was deposited during 2 h. A surface measuring instrument (SURFTEST SV-600, Mitutoyo) was used to measure the surface profile. The Ni film was deposited on a Si substrate which was half covered with kapton tape, in dc Ar plasma, at 20 mA discharge current and in 1.10 kPa pressure. After the Ni deposition, the kapton tape was removed. The difference between the surface levels with and without the Ni film was measured. The Ni film thickness for the CNTs production was estimated to be ∼ 5 nm. After the Ni thin film deposition, the Si substrate was heated at 750 °C, using a Joule heater. For this purpose a dc stabilized power supply, at 3 A constant current was used. The substrate temperature was measured with an optical pyrometer. Typical resistance, of 500 Ω before heating, of the substrate including
Fig. 1. Experimental setup.
Fig. 2. Dependence of the C excited atoms number density on p for different values of i.
the contact resistance decreased at ∼ 3 Ω after heating, due to the increased temperature of the Si substrate [10]. A stable CH4 discharge was obtained for gas pressures ( p) from 0.27 kPa up to 2.67 kPa and for discharge currents (i) from 5 to 30 mA. The deposition time for CNT production was 1 h. During the deposition, the Si substrate was kept at a constant temperature. 2.2. Plasma diagnostics Plasma diagnostics consisted in the measurements of the number density of the C excited atom in the vicinity of the Si substrate by LAS [6] and the measurements of the relative emission intensities of the excited CH radical, the excited C2, C3 molecules and the clusters by OES [9]. The number density of the excited C atom on 31P10 level (NC) near the Si substrate was measured with a commercial diode laser (DL: Hitachi 8325G, 40 mW maximum output with the central wavelength of 830 nm at 25 °C). The DL wavelength (λ = c/ ν) was tuned to the resonance absorption of the C atom (31P10 →31S0) at λ0 = 833.744 nm. The laser beam was focused within a diameter of ∼0.1 mm in the vicinity of the Si substrate. The DL intensity was adjusted to be lower than the value that provides the saturation of the absorption signal using corresponding neutral density filters. A photodiode (PD) and a lock-in amplifier (NF Electronic Instruments 5610B) were used to acquire the absorption signal [6]. The signal to noise ratio (S /N) of the C(31P1o) atom absorption signal was ∼10. The value of NRC was calculated by the following formula: NC ¼ ð8kgi =gj k20 Aji LÞ absorption line ½1−IðmÞ=I0 dm, where gi (=3) and g j (=1) are the statistical weights of the lower and the upper levels, respectively. Aji (=0.351 ×108 s− 1) is the Einstein coefficient for the C atom transition at λ. L is the absorption length, which is assumed to be equal to the diameter of the cathode (∼15 mm). I0 is the laser intensity before entering the plasma and I(λ) is the laser intensity after passing through the plasma. The dependence of NC on p and i is shown in Fig. 2. The emission spectrum of dc CH4 plasma was analyzed in a range between 300 and 800 nm using a Hamamatsu PMA-11 (Photodiode Multichannel Analyzer) with a 2 nm spectral resolution. The emission spectra were corrected with the spectral sensitivity of the CCD detector by the acquisition software. The light background was subtracted from each recorded spectra.
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Fig. 3. Typical OES spectra of CH4 plasma (i = 30 mA and p = 1.33 kPa).
The most intense OES lines of the species produced by the decomposition of the CH4 molecule were: CH(A2Δ) radical at 431.4 nm (A2Δ → X 2Π), 388.9 nm (B2Σ → X 2Π) and 314.3 nm (C 2Σ → X 2Π), the C2(A3Πg) molecule at 473.7, 516.5, and 563.5 nm (Swan System: A 3 Πg → X′ 3 Πu), the C3( 1 Πu) molecule at 405.0 nm (Comet Head System: 1Πu → 1Σg+), and H atom at 656.3 nm (Hα) and 486.1 nm (Hβ). OES contains a lot of Ni emission lines from the sputtered atoms excited in plasma (Fig. 3). In the spectral region from 670 to 800 nm, a blackbody radiation was observed at different p and i. This continuum spectrum was attributed to the incandescence of hot hydrocarbon clusters or nanoparticles. The relative intensity of the clusters in plasma was described by the OES intensity around 800 nm. The relative densities of the C2, C3 molecules and clusters were expressed as the ratios between the OES intensity of the C2, C3 molecules (IC2, IC3) and the clusters (Iclusters), respectively and the OES intensity of the CH radical (ICH). IC2/ICH and IC3/ICH ratios could represent the relative densities of the corresponding molecule because the excitation energies of the CH radical on A2Δ level, the C2 molecule on A3Πg level, and the C3 molecule on 1Πu level relative to the ground level, have almost the same values, ∼3 eV. The dependences of the relative densities of the C2, C3 molecules and the clusters on p and i are shown in Fig. 4(a) and (b).
Fig. 4. Dependence of the relative densities of the C2, C3 molecules and the clusters (a) on p, at i = 30 mA and (b) on i, at p = 1.33 kPa.
from A1g breathing mode of single-wall CNTs [12]. The second-order Raman peaks of CNTs occurs between 2000 and 3800 cm− 1. The peaks observed at 2668 and 3185 cm− 1 are the second harmonics of D peak and G peak, respectively. The peak at 2927 cm− 1 is approximately the sum of the D and G peak frequencies. It is commonly used in the CNTs production and purification experiments to characterize the structural integrity of the sp2hybridized carbon atoms of the nanotubes by the ratio between the intensities of G peak and D peak [13,14]. Fig. 6 shows the dependence of IG/ID on different p and i = 15–30 mA.
2.3. CNTs analysis CNTs were analyzed by Raman (JASCO NRS-1000HS) and FT-IR (JASCO FT/IR 660 Plus) spectroscopy and scanning electron microscope (SEM Hitachi S4800). Fig. 5 shows a typical Raman spectrum of CNTs obtained in the experiment. The Raman spectra of CNTs have several peaks located between 200 and 3800 cm− 1 [11,12]. The most intense peaks occur at 1334 cm− 1 (D peak), which comes from carbonaceous particles and indicates the presence of defects in graphite structure, and 1589 cm− 1 (G peak), which represents E2g Raman scattering mode of sp2-hybridized carbon and comes from the tangential motion of hexagons. Besides the D and G peaks, there are several smaller peaks at 283, 310, 337, 371 and 520 cm− 1. The peak at 520 cm− 1 is due to the Si substrate. The other four peaks are characteristic features arising
Fig. 5. Typical Raman spectrum of CNTs produced in dc plasma, for p = 1.33 kPa and i = 15 mA.
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Fig. 6. Dependence of IG /ID on p for different values of i.
FT-IR spectroscopy is also used for the investigation of the CNT composition [15,16]. In the present experiment, the functional groups that were detected in the FT-IR transmission spectrum of the CNT are: D peak at 1300 cm− 1, G peak at 1584 cm− 1, carboxylic carbonyl group at 1727 cm− 1, and CHx group at around 3000 cm− 1 (Fig. 7). From the transmission spectrum measured by FT-IR, IR absorption coefficient of CHx band, α(ω), was calculated. The thickness of the deposition was considered the same as the length of CNTs which was estimated to be ∼ 15 μm from the morphological analysis by SEM (Fig. 8(a) and (b)). The number R of C–H bonds per unit dx, where volume was calculated as: NCH ¼ A absorption band aðxÞ x A (= 1.35 × 1021 cm− 2) is a constant of the C–Hx stretching mode absorption around 3000 cm− 1 and ω is the corresponding wavenumber [17,18]. NCH was found to be ∼ 2 × 1023 cm− 3 for a deposition at i = 30 mA and p = 1.33 kPa. The obtained CNTs on the Si substrate exhibit an alignment of the tubes on the Si surface as can be seen by SEM analysis (Fig. 8(c) and (d)). 3. Experimental results and discussion The highest value of NC (∼108 cm− 3) was obtained for p ∼ 1.33 kPa. NC slightly depended on i in the range from 5 to 30 mA. As pressure increased ( p N1.33 kPa), NC decreased and the relative densities of the C2, C3 molecules and the clusters increased (Figs. 2 and 4). The observed continuum spectrum which corresponds to the blackbody radiation is usually fit with a Planck function to determine particle temperature and to obtain a crude estimation of the particle size [19]. Similar results with the present OES results have been reported in Ref. [20], in a laser vaporization of graphite/(Ni, Co) system for single-wall CNTs synthesis. Some of the most significant chemical reactions in gas-phase plasma, which can explain the appearance of the C and H atoms, the C2 and C3 molecules, the CH radical and the hydrocarbon clusters are the electron impact reactions (which conduct to dissociation, excitation, and ionization processes), the neutral– neutral reactions and the ion–neutral reactions. On the surface, the chemical interaction of neutrals and ions with the catalyst and the substrate are the most significant reactions [5,21]. The C excited atoms are mainly produced in the gas-phase plasma by the dissociation of the CH4 molecule, the larger
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hydrocarbon radicals and the clusters, through electron impact processes [21]. A small amount of the C excited atoms which could appear in plasma from the re-sputtered C atoms deposited on the Si substrate could not be separately measured. A detail calculation of the hydrocarbon species number density produced in CH4 plasma takes into account both the electron energy distribution function and the energy dependent reactive collision cross section. The calculated number density of different radicals and hydrocarbon clusters in a CH4 plasma increases with the increase of the gas pressure. The present experimental results on the pressure dependence of the relative densities of the C2, C3 molecules and the clusters shown in Fig. 4(a) correspond to the theoretical calculated data [21,22]. The IG/ID ratio had a high value in case of the CNTs obtained in a CH4 plasma for p = 1.33 kPa and i ∼ 15–30 mA. For the same pressure and i = 20 mA, a maximum value of NC was measured. For p N 1.33 kPa, NC significantly decreases (Fig. 2), the relative densities of the C2, C3 molecules and the clusters increase and the IG/ID ratio values are comparable (Fig. 6). As already mentioned, the G peak reflects the structural integrity of the sp2-hybridized C atoms of CNT and the D peak indicates disordered sp2-hybridized C atoms [13]. In our experiment, the IG/ID ratio values cannot completely describe the structural integrity of sp2-hybridization of C atoms because a large amount of amorphous carbon was formed in the same time with CNTs production (Fig. 8). In the optimum plasma conditions for the CNTs production, at p N 1.33 kPa and i ∼ 15–30 mA, it is difficult to attribute a higher contribution in the CNTs formation process to the C atoms or to the hydrocarbon clusters and radicals. In a dc PECVD system the control of the CNTs formation is governed by the processes that occur in thermal CVD (arrival of excited species on the substrate surface, catalytic dissociation, departure of undissociated species, formation of carbon film, diffusion of C atoms around the catalyst, incorporation of C atoms in the grapheme tube) and by the additional effects due to the plasma (electric field and partial ionization of the gas, cathode sputtering and chemical etching) [23]. A catalytic growth model on CNT explains that in order for a tube to grow, the C atoms have to be transported to the growing edge of the tube [24]. It is assumed that the hydrocarbon clusters and the larger C molecules are dissociated into clusters of few atoms
Fig. 7. A baseline corrected FTIR spectrum of CNT deposition at p = 1.33 kPa and i = 30 mA.
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Fig. 8. SEM images of CNTs obtained for: (a) and (b) i = 30 mA and p = 1.33 kPa; (c) i = 15 mA and p = 1.33 kPa; and (d) i = 15 mA and p = 1.33 kPa.
that adsorb on the growing CNT more probably at the tube surface than directly at the tube edge. The small peaks in Raman spectrum which correspond to the A1g breathing mode of a single-wall CNT (Fig. 5), could probe that a small quantity of single-wall CNTs may be present in the deposition [12]. From SEM analysis in Fig. 8, we can observe that the obtained CNTs are mixed with amorphous carbon. FT-IR analyses confirm that a large amount of amorphous carbon– hydrogen was also deposited in the same time with CNT production (Fig. 7). The calculated number density NCH was a typical value for an amorphous carbon–hydrogen film [17]. The presence of a high amount of sp2-graphite in the composition of CNTs was also confirmed by the IR-active peak corresponding to G peak at 1584 cm− 1 (Fig. 7) [15]. The IR-active peak at 1727 cm− 1 corresponds to nonconjugated carboxylic carbonyl group and could be explained by the oxidation of the Si substrate before deposition [15,16]. The Ni atoms, mainly sputtered in Ar plasma before CNTs production, represent the metal catalyst and are uniformly distributed on the Si surface. The orientation of CNTs on the Si surface could be explained by a temperature gradient during the heating due to the electrical connections, but we have no clear evidence of this effect yet. 4. Conclusions In this paper we studied a dc PECVD for the CNTs synthesis. The optimum experimental conditions for the CNTs growth correspond to 1.33 kPa pressure of CH4 gas and 15–30 mA
discharge currents. The length of the synthesized CNTs in PECVD in CH4 gas was ∼ 15 μm. The C excited atoms number density decreased when the gas pressure was higher than 1.33 kPa and the relative number density of the C2, C3 molecules and the clusters to the number density of CH radical increased with gas pressure. The experimental results showed that a high value of the IG/ID ratio was obtained when the number density of the C excited atoms in the vicinity of the Si substrate was maxim. The presence of amorphous carbon–hydrogen mixed with CNTs in the deposition was confirmed from SEM and FT-IR analysis. The IR-active peak at 1584 cm− 1 which corresponds to the G peak showed that a large quantity of the sp2-graphite was present in the deposition. In Raman spectra, some characteristics peaks of the single-wall CNTs have been detected, which confirm that CNTs contain a small amount of single-wall CNTs. Acknowledgements This work was supported partially by Grant-in-Aid for Scientific Research (C) of JSPS, Japan.
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