Syntesis of Carbon Nanostructures Near Room Temperature Using Microwave PECVD

Abstract

Carbon nanostructures (nanotubes, nanofibers and nanosponges) were synthesized onto Si (001) substrates using a microwave assisted plasma enhanced chemical vapor deposition (PECVD) from C₂H₂-Ar mixtures at low substrate temperatures (120 °C). Catalytic films (Ni and Cu) 3 nm thick were used. Different structures were formed, depending on the C₂H₂ partial pressure. Atomic force microscopy (AFM) and scanning electron spectroscopy (SEM) were employed for the morphological characterization of the catalytic films and the carbon nanostructures, respectively. Raman spectroscopy was used to identify carbon hybridization states. AFM was used to observe the morphology of the catalytic films. At low C₂H₂ partial pressures, nanotubes with nanospheres in their tips, growing from nanoholes were seen. With increasing C₂H₂ pressures, longer nanotubes were observed, reaching lengths from 300 to 500 nm. In their growth, the nanotubes laterally touch each other, forming nanotube bundles, or nanofibers. For the higher C₂H₂ partial pressures, dense sponge-like (nanosponge) structures, consisting of a large number of bundles, are formed. From the Raman spectra, a mixture of sp2 and sp3 hybridizations were identified. Furthermore, the low substrate temperature at which depositions can be carried out, makes possible growth of carbon nanostructures on materials to which high temperatures would be deleterious.

Keywords:
nanofiber; nanosponge; nanospheres; PECVD; microwave plasma; acetylene; AFM; SEM; Raman spectroscopy; carbon hybridization states

1 Introduction

Since the discovery of fullerenes¹, and the first reports on the synthesis of carbon nanotubes², investigations in carbon-based nanostructures have dramatically increased. As a consequence the use of carbon in new materials³ and composites⁴ has grown steadily. Such structures found promising electrical⁵, gas sensors^6,7, supercapacitors⁸ and nanomedicine⁹ applications.

Such carbon structures can be synthesized by the technique of plasma enhanced chemical vapor deposition (PECVD) and can be exemplified by diamond-like carbon¹⁰, carbon nanofibers¹¹, carbon nanotubes¹², amongst other forms of carbon¹³. PECVD is indeed of great interest, as depositions can be carried out at relatively low substrate temperatures, thus avoiding the sometimes inconvenient temperature effects in materials in which the carbon structure is grown. Furthermore, due to the usually high deposition rate of PECVD^14-18, the structures can be formed at short times, which is of interest in the industrial point of view.

In the PECVD technique, the carbon nanostructures strongly depend on the precursor gas (carbon-containing gas, usually a hydrocarbon, whose decomposition generates the carbon structures), diluent gas, such as Ar and H₂, power applied to the discharge and substrate temperature. Chen and colleagues¹⁴ have used CH₄-H₂ mixtures to synthesize nanofibers. Choi et al.¹⁵. treated at 600°C a Ni catalyst layer deposited onto Ti-coated glass with hydrogen plasma at different microwave powers prior to carbon nanotubes synthesis using CH₄ as a precursor gas. Wang & Moore¹⁶ used radio frequency-PECVD with methane and mixtures of argon with hydrogen to obtain carbon nanotubes and carbon nanofibers at 140 to 180°C using Ni and FeNi catalyst films onto glass substrates. Hoffmann et al.¹⁷. obtained carbon nanotubes at relatively low temperature (~120° C) using a DC-PECVD system and mixtures of acetylene and ammonia onto prepatterned Ni catalyst particles in a SiO₂/Si substrate.

Regarding carbon nanofibres, they are formed from a catalytic process involving ultrafine particles (few nm-diameter) of Ni, Cu or another catalyst. Growth occurs through an epitaxial process starting with the adsorption of carbon on the catalyst surface, originating from hydrocarbon decomposition, either in a glow discharge or by a thermal process¹⁹.

According to the literature^14-17,20,21 Ni is widely used as a catalyst for nanotube and nanofiber growth. Their structure can be controlled according to the catalyst pretreatment. With the use of radio frequency plasmas, nanofibers and nanotubes^16,18 were obtained in various conditions such as different precursors, gas pressures and substrate temperatures. Microwave plasmas have also been used to synthesize nanofibers²², nanotubes¹⁵ and nanosheets²³, but with a high substrate temperature (450 to 600 °C.).

It has been shown that PECVD is promising for the deposition of nanotubes, nanofibres and other carbon nanostructures, as it can be carried at lower substrate temperatures as compared to those in CVD and arc discharge processes. The elemental composition and shape of the nanoparticle catalyst deserves more research to reach carbon structures with fewer defects to facilitate their integration in Si-based devices. In this work we report an investigation on the growth of carbon nanostructures (nanotubes, nanofibers and nanosponges) from C₂H₂-Ar mixtures using a microwave PECVD system of in-house design. All nanostructures were formed onto Si (001) substrates coated with Ni or Cu films. Their morphology was characterized by AFM and SEM, while chemical bonding was investigated by Raman spectroscopy.

2 Experimental

Figure 1 is a schematic representation of the PECVD system. It consists of a reactor (quartz tube with substrate holder and microwave generator), vacuum pump of 10 m³/h pumping speed, and gas source. A Pirani gauge coupled directly to the quartz chamber was used for pressure measurements. For plasma generation, a microwave source ( 2.45 GHz, 500 Watt) was employed. Acetylene (C₂H₂, 99,99% pure) was used as the precursor gas, mixed with argon (99,99% purity). The substrates, Si (001), 1 × 1 cm², were cleaned at the RCA Cleaning Center of the Center for Semiconductor Components (CCS) of the Universidade Estadual de Campinas (Unicamp). Depositions were carried out at various C₂H₂ partial pressures in the C₂H₂-Ar mixtures, with the total pressure kept at ~210 mTorr for all depositions. The reactor base pressure was 60 mTorr. In all depositions, the applied power was kept constant, and the substrate temperature did not exceed 120 °C, as measured with a type-K thermocouple.

Figure 1
Schematic representation of the PECVD system.

Using a dc magnetron sputtering system, working at a base pressure of 1 × 10^–7 Torr, Ni and Cu catalytic films were deposited. Their thicknesses were measured with a quartz microbalance.

To analyze the morphology of the silicon substrate and those of the Cu and Ni films, AFM images in dynamic mode were obtained, using a Shimadzu microscope model SPM9700, of the Universidade Federal do Triângulo Mineiro - UFTM. The average roughness of the Si and Ni and Cu films were determined using the 9700 Scanning Probe Microscopy software provided by the Shimadzu Corporation.

The morphology of carbon nanostructures surfaces were investigated by scanning electron microscopy using the Dual Beam FIB/SEM (Focused Ion Beam/Scanning Electron Microscopy) Model Nanolab Nova 200 of CCS-Unicamp. All images were obtained in the secondary electron mode, using an electron beam of 5 kV and 0.4 nA. We used the software UTHSCSA Image Tool²⁴ to estimate the average diameters and height of the nanostructures and their standard deviations. A number of particles (20) was then chosen and their sizes individually measured and stored by the software.

Chemical bonding in the carbon nanostructures was investigated with a confocal Raman Spectroscope, model Ntegra Spectra from NT MDT.Co of CCS-Unicamp. The 473.8 nm line of a semiconductor laser was used for excitation.

3 Results and Discussion

Various samples (~26), deposited at different C₂H₂ partial pressures and during the same time interval (20 minutes), on either Ni or Cu 3 nm thick catalytic films were investigated in this work. As shown in Table 1, samples are labeled by a number (the C₂H₂ partial pressure in mTorr), followed by the catalyst chemical symbol. The Ar and C₂H₂ partial pressures used in the depositions are also given. In all depositions, the substrates did not exceed the temperature 120 °C

Table 1
Argon and C₂H₂ partial pressures in the synthesis of each sample. Thickness of Ni and Cu catalytic films: 3 nm. Deposition time: 20 minutes. Maximum temperature during depositions: 120 °C.

Sample	Catalyst	Argon pressure (mTorr)	Acetylene pressure (mTorr)
34Ni	Ni	110	34
34Cu	Cu	110	34
64Ni	Ni	70	64
64Cu	Cu	70	64
70Ni	Ni	70	70
70Cu	Cu	70	70
79Ni	Ni	60	79
79Cu	Cu	60	79
100Ni	Ni	50	100
100Cu	Cu	50	100

Figure 2 shows AFM pictures of the bare Si substrate, and of the 3 nm Cu and Ni films. An average surface roughness of 0.178 nm was measured for the Si surface, typical of Si wafers (Figure 2a). For the Cu and the Ni films (Figures 2b, c) the average roughnesses were 0.358 and 0.227 nm, respectively. As revealed by the pictures, the films consist of metal particles that completely cover the substrates. Figures 2b, c are representative of the surfaces of all catalytic films of this work as they were deposited onto a Si wafer which was subsequently cut into pieces of 1×1cm².

Figure 2
AFM images of the Si substrate (a) and Ni (b) and Cu (c) 3 nm catalytic films.

Different nanostructures were obtained depending on the C₂H₂ partial pressure. The lowest one (34 mTorr) is responsible for the structures shown in Figure 3. Nearly-spherical fine particles of relatively uniform diameter (42 ± 5 nm) are formed on top of the Ni film (Figure 3a). A completely different structure is exhibited by the film grown onto the Cu catalyst. As can be seen in Figure 3b, such structure shows nanoholes of a rather large hole size distribution (308 ± 56 nm average).

Figure 3
SEM images of structures grown at 34 mTorr C₂H₂ onto 3 nm Ni (a) and Cu (b) catalytic films.(Samples 34Ni and 34Cu).

Nanoholes are also observed in films grown over both catalysts at the C₂H₂ pressure of 64 mTorr as shown in Figure 4. Average hole diameters of 276 ± 77 and 217 ± 40 nm were measured in samples 64Ni and 64Cu, respectively.

Figure 4
SEM images of structures grown at 64 mTorr C₂H₂ onto 3 nm Ni (a) and Cu (b) catalytic films.(Samples 64Ni and 64Cu).

The early stages of formation of nanotubes with nanospheres in their tips are shown in Figure 5 for samples synthesized at 70 mTorr C₂H₂. The average diameters of the nanospheres are 56 ± 12 and 46 ± 6 nm for samples 70 Ni and 70Cu, respectively.

Figure 5
SEM images of structures grown at 70 mTorr C₂H₂ onto 3 nm Ni (a) and Cu (b) catalytic films.(Samples 70Ni and 70Cu). The inset is a magnified picture of the region showing two nanotubes growing from the nanohole resulting from coalescence of two holes.

The reasons for the formation of nanoholes and nanospheres, as observed in Figures 4 and 5, are not completely clear at the moment. Probably, it can be attributed to local nonuniformities of the catalyst film resulting in higher intensity of the catalytic process in some areas. In turn, this can lead to fast increase of the local temperature ("hot spots"), as the process to obtain graphitic layers during precipitation from solid solutions of carbon in metals like Ni or Cu is known to be highly exothermic²⁰. Finally, this local overheating of the sample can result in more intense and dense formation of carbon nanotubes near the center of the hot spot area eventually forming agglomerated structures such as nanospheres . Some certain amount of carbon-carbon species present on the sample surface can contribute to the fast nanotubes growth described above. However, there is no carbon species enough coming into the catalytic particle to sustain the nanotube growth and generate long tubes. Since the process starts at the base (base-growth model discussed below) the carbon found at the nanotube top does not precipitate after its growth, remaining on the tip in a nanosphere format. As nanospheres are relatively weakly linked to the surface, they may be lost during post-processing manipulations. This can explain the lack of nanospheres in some cases.

Figure 6 shows images of films grown at 79 mTorr C₂H₂. Figures 6a, b correspond to samples 79Ni and 79Cu, respectively. The high concentration of carbon or carbon-containing species in the discharge, compared to that of the previous depositions, provided a more extensive and complete formation of carbon structures. The average heights of nanofibers were 343 ± 32 nm and 381 ± 16 nm for samples 100Ni and 100Cu, respectively. While the average diameter calculated at half height for samples 79Ni and 79Cu were 47 ± 14 and 28 ± 8 nm, respectively.

Figure 6
SEM images of structures grown at 79 mTorr C₂H₂ onto 3 nm Ni (a) and Cu (b) catalytic films.(Samples 79Ni and 79Cu). The upper and lower insets are from the same samples (79Ni and 79Cu, respectively) and were taken with the microscope electron beam at perpendicular incidence. The magnification of the insets differ from those of the larger images.

We present herein some ideas on the dynamics of the sponge-like structures. In the early stages of their formation, only single nanotubes grow as exemplified in Figure 5. As deposition goes on, more single nanotubes are formed and their heights increase. At the same time, their associated holes increase in diameter and eventually coalesce, forming larger holes with two or more nanotubes, depending on the number of holes undergoing coalescence. In the inset of Figure 5a, coalescence of two nanoholes with their corresponding nanotubes can be seen. Figure 7 (to be later discussed) exemplifies coalescence of more than two holes. As the nanotubes grow, they laterally touch each other from the same hole, forming nanotube bundles. With continuation of the deposition process, hole coalescence is increased and bundle growth continues, reaching a stage in which bundles from neighbor holes touch each other forming an entangled sponge-like structure as seen in Figure 6. It is clear that this structure partially or totally blocks the arrival of precursor species to the catalytic surfaces, either extinguishing, or greatly reducing, nanotube formation.

Figure 7
SEM images of structures grown at 79 mTorr C₂H₂ onto 3nm Ni catalytic film (sample 79Ni, uppermost picture) and at 100 mTorr C₂H₂ onto 3nm Ni (b) and Cu (c) catalytic films. The inset is a magnified picture of the region showing a nanosphere.

Figure 6 shows the nanofibers, or nanotube bundles which do not uniformly cover the substrates forming a carbon sponge-like structure on surface. The bundle coverage shows cracks, as seen in the upper inset of the figure, taken from sample 79Ni at a magnification lower than that of Figure 6a. The regions uncovered by the bundles (Figures 6a, b and its inset) are actually portions of the cracks. We have never observed nanotubes (and their associated nanospheres) at early stages of growth in the substrate regions of the cracks. This strongly indicates that formation of cracks is a post-deposition process, occurring spontaneously or during sample handling. If cracks were formed during deposition, the observation of nanospheres would be highly probable.

It is interesting to note that despite the blocking effect of the nanotube bundles to the transit of gas-phase nanotube precursor species to the catalyst surface, nanotubes can be observed in their early stages of growth among bundles in high density regions. This is illustrated in the micrographs of Figure 7, for samples synthesized at 79 mTorr C₂H₂ (sample 79Ni, Figure 7a) and at 100 mTorr C₂H₂ (samples 100Ni and 100Cu, Figure 7b, c, respectively). Comparing the images, it becomes evident that the population of the nanospheres in the samples deposited at higher C₂H₂ pressure are incomparably higher. It seems that the higher density of nanotube precursor species in the plasma at 100 mTorr C₂H₂ pressure overruns the blocking effect of the bundles to reach the Ni and Cu surfaces.

As for the larger holes in Figures 7b, c, their diameters are about 300 and 400 nm, respectively, and several nanotubes grow from their bottoms. According to our previous discussion on the dynamics of formation of the sponge-like structures, these holes result from coalescence of more than two holes, as in the early stage of nanotubes formation when a single hole can be seen for each nanotube. Furthermore, the structures seen in Figures 7b, c, can be considered as bundle precursors, i. e., bundles in their early stage of formation.

From the high definition of the SEM images (Figures 3-7), the so-called base growth mechanism^16,19,25 is evident as no catalytic metal particles can be observed on the nanotube tips. At least two reasons point to this growth mode. The first one is the low substrate temperature that did not exceed 120 °C in any deposition, as previously noted. Owing to the low temperature, the catalyst nanoparticles do not melt, and nanofiber growth occurs by dissociation of the precursor species and subsequent carbon diffusion in the particles. The other reason is the strong adhesion of the metal particles to the Si substrate, typical of the sputtering process. In fact, Wang & Moore¹¹ have attributed to this high adhesion the base growth mechanism they observed in the formation of nanotubes grown on Fe and Ni catalysts sputter-deposited onto Si substrates.

Figure 8 shows the Raman spectrum for sample 79Ni. Pronounced peaks are seen readily identified as the D (disorder) and G (graphitic) bands at 1367 and 1587 cm^–1, respectively^26-28. The high intensity of the G band is considered as an evidence of the high sp2 carbon density in the nanofibers²⁷ while the overlapping of the G and D bands is interpreted as a mixture of sp2 and sp3 bonding states²⁸.

Figure 8
Raman spectrum of nanofibers deposited at 79 mTorr C₂H₂ (sample 79Ni).

The carbon nanofibers shown in Figures 6a, b and 7a formed by defective nanotubes bundles have sp2 carbons in its crystal lattice, evidenced by the band at 1587 cm^–1 (G band) observed in Figure 8. The latter has a higher intensity than that of the shoulder formed by the D band at 1367 cm^–1 indicating a combination of sp3 and sp2 hybridizations in the nanofiber structure. But in this case, the linewidths are larger than those in crystalline 2D graphite, observed in crystalline carbon nanotubes²⁷ due to the presence of residual amorphous carbon.

4 Conclusions

We have shown that carbon nanotubes can be successfully synthesized using microwave-assisted PECVD. The PECVD system herein used is simple and inexpensive. Furthermore, the procedure adopted does not require any elaborated previous substrate treatment such as thermal annealing or holographic processes. The low substrate temperature (120 °C, at most) during deposition is an advantage to avoid the inconvenient temperature effect in the substrate in which nanotubes are to be deposited.

Aside from single nanotubes, different carbon nanostructures (nanotube precursors, nanoholes, nanofibers and nanosponges) were observed depending on the C₂H₂ partial pressure on the gas mixture. The well-defined images of the carbon nanotubes and of their coalescence provide strong evidence for the base-growth mechanism proposed in other investigations.

The dynamics of carbon structures, has been discussed and we have seen that nanohole coalescence is an important step in carbon bundle formation. Furthermore, we have observed nanoholes without nanotubes. We propose that nanotubes rising from the bottom of holes existed previously existed but they can be lost during post deposition manipulation.

From the Raman spectrum, the sp2-nature of the carbon nanotubes bundles and the existence of mixed sp2-sp3 bond states were clearly evidenced.

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Publication Dates

• Publication in this collection
  Jul-Aug 2015

History

• Received
  24 June 2015
• Reviewed
  10 Aug 2015