Plasma-Assisted Synthesis of Carbon Nanotubes
© The Author(s) 2010
Received: 31 May 2010
Accepted: 19 July 2010
Published: 1 August 2010
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© The Author(s) 2010
Received: 31 May 2010
Accepted: 19 July 2010
Published: 1 August 2010
The application of plasma-enhanced chemical vapour deposition (PECVD) in the production and modification of carbon nanotubes (CNTs) will be reviewed. The challenges of PECVD methods to grow CNTs include low temperature synthesis, ion bombardment effects and directional growth of CNT within the plasma sheath. New strategies have been developed for low temperature synthesis of single-walled CNTs based the understanding of plasma chemistry and modelling. The modification of CNT surface properties and synthesis of CNT hybrid materials are possible with the utilization of plasma.
CNTs are commonly synthesized by laser ablation , arc discharge  and thermal chemical vapour deposition (TCVD) [32–34] methods. The choice of the synthesis technique is highly motivated by the field of applications. In the field of microelectronic application, controllable assembly and directional in situ synthesis of CNTs are very crucial steps to incorporate CNTs directly into the integrated circuit. Chemical vapour deposition methods offer the greatest potential for large-scale and commercially viable synthesis of assembled CNTs. TCVD methods have also been traditionally used in integrated circuit manufacturing and therefore existing facilities are suitable for CNT growth.
However, the synthesis of CNTs using TCVD method requires undesirably high temperature, which damages the electronic chip. A milder synthesis condition is needed. Plasma-enhanced chemical vapour deposition (PECVD) method offers a solution to low temperature synthesis of CNTs. Likewise PECVD methods are also widely used in integrated circuit manufacturing for the growth of oxide and nitride thin films and its conversion for CNT growth will not be a major issue.
Early reports on PECVD methods required synthesis temperature as high as TCVD to grow CNTs (or CNFs). However, only PECVD methods synthesize free-standing, individual and vertically aligned (VA) CNTs. This unique feature distinguishes PECVD from TCVD and opens up the possibility of making nanodevices based on single strand CNT. Improvement in catalyst-support design, PECVD reactor setup, plasma conditions and synthesis parameters significantly help to lower the CNT synthesis temperatures to 400–500°C. Despite the advancement in PECVD studies of CNT growth, there are still many unresolved issues. For examples, the active carbon species responsible for the catalytic growth of nanotubes remain unclear. A credible low temperature (<400°C) synthesis of aligned CNT without compromising its crystallinity has not been reported. Self-termination of CNT growth due to catalyst poisoning is a common phenomenon and is it possible to avoid it and achieve uninterrupted growth in PECVD methods? The built-in electric field of a plasma sheath in a PECVD reactor has yet to be fully exploited to orient and align CNT growth. The PECVD methods also offer the opportunity to modify the properties of CNTs using plasma and create new hybrid materials. Thus, the intent of this review is to address these issues of CNT growth using PECVD methods. The review is organized as follows: “Challenges of PECVD Methods” discusses the challenges of PECVD methods with special focus on low temperature synthesis, ion bombardment effects and directional growth of CNT guided by electric and magnetic field. The modification of CNT using the plasma is presented in “Plasma Modification of CNTs”. Concluding remarks are given in “Conclusion”.
The synthesis of carbon nanotubes or nanofibres requires temperature of 700–1,000°C using thermal chemical vapour deposition (TCVD) methods. This temperature requirement far exceeds the temperature limit of microelectronic, which is typically ~400–500°C. Plasma-enhanced chemical vapour deposition (PECVD) method has been proposed as an alternative method to reduce the synthesis temperature. The plasmatic energy efficiently dissociates gas molecules at lower temperatures, and the synthesis of carbon nanotubes might occur at lower temperatures. The presence of a built-in electric field in a plasma sheath will align the growing CNTs along the field lines. Thus, PECVD methods favour low temperature synthesis of VA-CNTs.
Large-scale Monte Carlo simulations  of SWNT synthesis showed that PECVD methods were more suitable for low temperature synthesis and had two orders of magnitude higher growth rates than TCVD methods. In PECVD methods, the delivery and redistribution of carbon adatoms between the catalysts and the nanotubes’ bases were more efficiently controlled than TCVD methods. Catalyst poisoning and amorphous carbon formation were prevented and resulted in uninterrupted ultralong plasma-assisted growth of SWNTs.
Low temperature (≤500°C) growth of carbon nanotubes/nanofibres using PECVD methods
Type of plasma a
CH4:H2 = 30:50 sccm
Ni powder (4–7 mm)
C2H2/N2 1 sccm
2 nm Ni film
Ratio C2H2:NH3 = 1:4
6 nm Ni film
CH4 + H2 = 30 sccm
33 nm Ni film
C2H2:NH3 = 30:200 sccm
6 nm Ni film
C2H2:NH3 = 50:200 sccm
CH4:H2 = 0.4:20 sccm
100 nm Ni film
C2H2:NH3 = 5:20 sccm
40–100 nm Ni films
30 nm Ni film
C2H2:NH3 = 50:200 sccm
CH4:Ar = 60:15 sccm
C2H4:H2 = 10:40
1 nm Fe film
Would the growth mechanism of CNTs depend on temperature? The growth of CNTs synthesized using high temperature TCVD methods had been proposed to be a vapour-liquid-solid mechanism . The catalyst was in a liquid drop state and carbon species from the chemical vapour dissolved into it. Carbon nanotubes were precipitated from the supersaturated eutectic liquid. The activated energy for TCVD (≤700°C) was reported to be ~1.2–1.8 eV [49, 50]. Clearly, this proposed growth mechanism of CNTs was not suitable for low temperature growth (<120°C), whereby the catalysts might remain as solids at such low temperatures. Low activation energy of ~0.2–0.4 eV was reported for low-temperature plasma-assisted growth of CNTs [27, 51], which was similar to the activation energy of surface diffusion of carbon atoms on polycrystalline Ni (0.3 eV) . Hoffman et al.  suggested that the rate-limiting step for low temperature plasma-assisted growth of CNTs was the carbon diffusion on the catalyst surface.
Early attempts to use PECVD process to synthesize carbon nanotube yielded mostly VA-MWNTs and CNFs. In order to synthesize SWNTs, it required the use of special plasma configuration such as remote plasma or point arc discharge, whereby the substrates were minimally exposed to the plasma sheath. Goheir et al. [52, 53] showed that the exposure of substrates to the plasma sheath inevitably resulted in the transition of SWNTs to MWNTs, which was attributed to ion bombardment effects. In the plasma sheath, there was a high density of plasma ion flux (nion ~ 1010 cm3) which bombarded SWNTs at sufficiently high energy of ~100 eV and caused C–C bond breakage. The presence of plasma radicals such as NHx and H further chemically etched the surface of the carbon nanotubes. The growth mechanism of SWNT in a PECVD process determines the resistibility of the carbon nanotubes towards ion-etching effects. In the tip-growth mechanism, the catalyst was at the tip of the vertically growing SWNT, offered protection to walls of the nanotube from the ion-etching effects. On the other hand, in base-growth mechanism, the catalyst was adhered to the substrate, and the vertically growing SWNTs had uncapped tips, which were easily destroyed by the impinging ions. Consequently, a transition from SWNTs to MWNTs in a PECVD process was observed since the multiple layers of carbon were more resistant towards ion etching.
When the gas pressure (P) was fixed, the increment of plasma input power significantly increased the ion flux impinging SWNTs, while the ion energy was moderately increased. On the other hand, for a fixed plasma input power, the plasma sheath varied with pressure as V ∝ P 1/2. The plasma ion flux and energy can be rewritten as follow: Eion ∝ P −1/10 and nion ∝ P 5/4, which indicated that the ion-etching effects were dominated by the ion flux. The reduction of incoming ion flux was essential to the synthesis of high quality SWNTs in a plasma sheath. On the basis of Eqs. 1 and 2, there is always ion bombardment in a plasma sheath during SWNT growth but the degree of ion bombardment is minimized by tuning the sheath voltage and reactor pressure in order to achieve SWNT growth.
Luo et al.  showed that the resistibility of VA-SWNTs against ion etching was dependent on the synthesis temperature. At temperatures ≥600°C, the ion-etching effects did not damage the VA-SWNTs significantly. When the temperatures were lowered to ≤500°C, the growth rate of SWNT was reduced, and ion-etching effects became significant. The conversion of SWNTs into MWCNTs by low energy hydrogen bombardment is still possible at low temperature synthesis, particularly when the ion-etching rate is faster than the CNT growth rate.
Zhang and Qi et al.  showed that a methane plasma selectively etched metallic SWNTs while semiconducting SWNTs remained unmodified. Metallic SWNTs were irreversibly etched into hydrocarbon gas species as a result of ion bombardment of H and CH3 ion species present in methane plasma. Small-diameter SWNTs were preferentially etched over larger ones because of the higher radius curvature and strain in the C-C bonding. This finding had great implication for controlling the electronic properties of SWNTs synthesized in a PECVD process. Theoretical studies [1, 2] had predicted that ~1/3 of as-synthesized SWNTs was metallic, and the remaining ~2/3 nanotubes were semiconducting. In other words, the SWNTs synthesized in PECVD methods composed mainly of semiconducting tubes. Li and Qu et al. [59, 60] also demonstrated the preferential synthesis of semiconducting SWNTs in a PECVD, whereby the metallic SWNTs will inherently destroyed during the synthesis steps.
Several strategies had been developed to minimize the effects of reactive ion etching which were inherent in PECVD processes. Nozaki et al. [54, 55] had developed an atmospheric pressure radio frequency PECVD to synthesize VA-SWNTs. The high collision frequency of the molecules at atmospheric pressure significantly reduced the ion etching of the SWNTs. In a remote downstream PECVD process [45, 46], high density plasma was generated at a distance from the SWNT substrates such that the plasma sheath was not close to the substrates and reduced the ion-etching effects. Kato et al.  diffused the spatial distribution of plasma by forming a small hole (diameter 10 mm) in the centre of the bottom electrode while the substrate placed below it. This diffusion PECVD method reduced ion bombardment and promoted the growth of free-standing individual SWNTs.
In a PECVD process, the free-standing CNTs were aligned along the direction of the electrical field in the plasma sheath. Experimental studies showed that the CNT alignment was dependent on the catalytic nanoparticles in the tips of the tubes. Alignment of free-standing CNTs in a field was observed for tip-growth model but not base-growth model. The high polarizability of CNTs in an electric field also assisted its directional growth. However, the vertical alignment of dense CNT forests, which were synthesized in base-growth model, was also observed in a PECVD process. In this case, the alignment of CNT forests was due to the collective van dar Waals interaction among the tubes (crowding effects).
The conductivity of substrates has a strong effect on the alignment of the nanotubes within the plasma sheath. When a conductive substrate was used to synthesize CNTs in a PECVD process, the electric field lines of the plasma sheath were always perpendicular to the substrate. On the other hand, when an insulating surface was deposited on top of the conducting substrate, a phenomenon known as plasma-induced surface charging occurred in the presence of an electric field of plasma sheath. The insulating surface accumulated net negative charges quickly and repelled the electron flux. In a steady-state plasma, the potential of the insulating surface (V f ) coupled to the plasma potential (V P ) via the sheath: V f = Vp – V sh, where V P and V sh is the potential of the plasma and plasma sheath, respectively.
Similarly inclined CNTs were synthesized in the plasma sheath by orienting the electric field lines with respect to the substrate surface [66, 67]. Lin et al.  synthesized inclined CNTs by placing a tilted substrate in a plasma sheath which had electric field lines travelling vertically from the plasma to the sample stage. The corners of microstructures in the proximity of the substrates would also distort the electric field and yield inclined CNTs. The ability to control the inclination of CNT with respect to the substrate surface had important technical implications. For examples, inclined CNTs might be used as AFM probe tips and microfluidic channel as a valve or filter.
Other nano-sized graphitic materials were also attached carbon nanotubes in a PECVD process. Nano-sized graphite flakes were attached on the exterior of VA-CNTs for prolonged synthesis duration or the absence of etchant dilutant gases such as H2 and NH3 in PECVD processes [69, 70]. Malesevic et al.  also demonstrated a combined growth of carbon nanotubes and carbon nanowalls in a PECVD process. It was proposed that the excess carbon radicals over saturated the catalysts and terminated the CNT growth. The remaining carbon species were deposited in the form of graphitic sheets in the vicinity of the CNT tips.
Abdi et al.  synthesized branched carbon nanotubes, a PECVD process. A 5 nm layer of nickel catalysts were deposited on a silicon wafer. After the synthesis of VA-CNTs, a conformal layer of TiO2 was coated on the exterior of the VA-CNTs. Hydrogen plasma was used to etch the TiO2 coated VA-CNTs’ tips and exposed the embedded Ni catalysts, which were subsequently used to grow branched CNTs. Branched CNTs were expected to have improved field emission and gas detection properties.
Surface functionalization of the CNTs had been achieved using PECVD methods. Various etchants such as Ar, H2, O2 and fluoride gases had been used to modify the surface properties of CNTs [72, 73]. For examples, under optimal conditions, treatment of CNTs with Ar and H2 plasma improved its field emission properties. Li et al.  also showed that the wettability of as-synthesized CNTs could be carefully tuned from hydrophobic to hydrophilic using O2 plasma. X-ray photoelectron spectroscopy revealed that OH–C=O groups, which increased the hydrophilicity of the plasma-treated CNTs, were formed at the open tips.
This article reviewed the synthesis of carbon nanotubes using PECVD methods. The utilization of plasma helped to lower the synthesis temperature of CNTs but excessive ion bombardment hindered SWNT growth. The growth of SWNTs was achieved when ion bombardment was minimized. The rate-determining step for low temperature PECVD growth of CNTs was suggested to be surface carbon diffusion. Therefore, future work should focus on the preparation, characterization and modelling of catalysts suitable for surface diffusion mechanism. Lab-scale fabrication of horizontally aligned CNTs with in the plasma sheath was demonstrated. However, current strategy of aligning CNTs horizontally within the plasma sheath might not be practical for actual device manufacturing. The application of an external electric field to synthesize horizontally aligned CNTs might still be required for large area synthesis. When compared to wet chemical treatment, plasma treatment of VA-CNT thin films was a useful ‘dry’ method to modify its surface properties without destroying the thin film integrity.
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