Syntesis of Carbon Nanostructures Near Room Temperature Using Microwave PECVD

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 sensors6,7, 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 PECVD14-18, 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 & Moore 16 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 literature14-17,20,21 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 nanotubes16,18 were obtained in various conditions such as different precursors, gas pressures and substrate temperatures. Syntesis of Carbon Nanostructures Near Room Temperature Using Microwave PECVD


Introduction
Since the discovery of fullerenes 1 , and the first reports on the synthesis of carbon nanotubes 2 , investigations in carbon-based nanostructures have dramatically increased.As a consequence the use of carbon in new materials 3 and composites 4 has grown steadily.Such structures found promising electrical 5 , gas sensors 6,7 , supercapacitors 8 and nanomedicine 9 applications.
Such carbon structures can be synthesized by the technique of plasma enhanced chemical vapor deposition (PECVD) and can be exemplified by diamond-like carbon 10 , carbon nanofibers 11 , carbon nanotubes 12 , amongst other forms of carbon 13 .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][15][16][17][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 H 2 , power applied to the discharge and substrate temperature.Chen and colleagues 14 have used CH 4 -H 2 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 CH 4 as a precursor gas.Wang & Moore 16 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 SiO 2 /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 process 19 .
According to the literature [14][15][16][17]20,21 Ni is widely used as a catalyst for nanotube and nanofiber growth. Theirstructure 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.Carbon nanostructures (nanotubes, nanofibers and nanosponges) were synthesized onto Si (001) substrates using a microwave assisted plasma enhanced chemical vapor deposition (PECVD) from C 2 H 2 -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 C 2 H 2 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 ws used to observe the morphology of the catalytic films.At low C 2 H 2 partial pressures, nanotubes with nanospheres in their tips, growing from nanoholes were seen.With increasing C 2 H 2 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 C 2 H 2 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.

Syntesis of Carbon Nanostructures Near Room Temperature Using Microwave PECVD
Microwave plasmas have also been used to synthesize nanofibers 22 , nanotubes 15 and nanosheets 23 , 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 C 2 H 2 -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.

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 m 3 /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 (C 2 H 2 , 99,99% pure) was used as the precursor gas, mixed with argon (99,99% purity).The substrates, Si (001), 1 × 1 cm 2 , 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 C 2 H 2 partial pressures in the C 2 H 2 -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.
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 Tool 24 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.

Results and Discussion
Various samples (~26), deposited at different C 2 H 2 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 C 2 H 2 partial pressure in mTorr), followed by the catalyst chemical symbol.The Ar and C 2 H 2 partial pressures used in the depositions are also given.In all depositions, the substrates did not exceed the temperature 120 °C 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 1×1cm 2 .
Different nanostructures were obtained depending on the C 2 H 2 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).Nanoholes are also observed in films grown over both catalysts at the C 2 H 2 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.
The early stages of formation of nanotubes with nanospheres in their tips are shown in Figure 5 for samples synthesized at 70 mTorr C 2 H 2 .The average diameters of the nanospheres are 56 ± 12 and 46 ± 6 nm for samples 70 Ni and 70Cu, respectively.
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 exothermic 20 .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 C 2 H 2 .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.
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   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.Materials Research 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 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 C 2 H 2 (sample 79Ni, Figure 7a) and at 100 mTorr C 2 H 2 (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 C 2 H 2 pressure are incomparably higher.It seems that the higher density of nanotube precursor species in the plasma at 100 mTorr C 2 H 2 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  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][27][28] .The high intensity of the G band is considered as an evidence of the high sp2 carbon density in the nanofibers 27 while the overlapping of the G and D bands is interpreted as a mixture of sp2 and sp3 bonding states 28 .
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 nanotubes 27 due to the presence of residual amorphous carbon.

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 C 2 H 2 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.

Figure 1 .
Figure 1.Schematic representation of the PECVD system.

Table 1 .
Argon and C 2 H 2 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.

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

Figure 5 .
Figure 5. SEM images of structures grown at 70 mTorr C 2 H 2 onto 3 nm Ni (a) and Cu (b) catalytic films.(Samples70Ni and 70Cu).The inset is a magnified picture of the region showing two nanotubes growing from the nanohole resulting from coalescence of two holes.

Figure 6 .
Figure 6.SEM images of structures grown at 79 mTorr C 2 H 2 onto 3 nm Ni (a) and Cu (b) catalytic films.(Samples79Ni 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.

Figure 7 .
Figure 7. SEM images of structures grown at 79 mTorr C 2 H 2 onto 3nm Ni catalytic film (sample 79Ni, uppermost picture) and at 100 mTorr C 2 H 2 onto 3nm Ni (b) and Cu (c) catalytic films.The inset is a magnified picture of the region showing a nanosphere.
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.