The growth of single-walled carbon nanotubes on a silica substrate without using a metal catalyst

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The growth of single-walled carbon nanotubes on a silica substrate without using a metal catalyst Huaping Liu 1, Daisuke Takagi 2, Shohei Chiashi 3, Yoshikazu Homma

*

Department of Physics, Tokyo University of Science, Shinjuku, Tokyo 162-8601, Japan

A R T I C L E I N F O

A B S T R A C T

Article history:

Single-walled carbon nanotubes (SWCNTs) have been directly grown on a SiO2 substrate

Received 12 March 2009

using the chemical vapor deposition (CVD) of ethanol without a catalyst. Care was taken

Accepted 27 August 2009

to exclude the possibility that the SWCNT growth was induced by conventional metal cat-

Available online 31 August 2009

alysts such as Fe, Co and Ni resulting from the contamination. Pretreatment of the SiO2 at 950 C or a higher temperature in H2 before CVD was critical for the synthesis of SWCNTs. After CVD process, nano-scale carbon particles were produced besides SWCNTs. Based on these results, we propose that the annealing of SiO2 substrates in H2 at high temperature generates defects on their surfaces, and these defects provide nucleation sites for the formation of carbon nanoparticles and assist the formation of carbon nanocaps, thus leading to the SWCNT growth.  2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Much effort has been made to develop technical and viable methods for synthesizing carbon nanotubes (CNTs) that have exhibited a wealth of fascinating electrical, optical and mechanical properties with a broad range of applications [1]. The efficient and controlled growth of CNTs is a prerequisite to their industrial application. Because of an incomplete understanding of the growth mechanism of CNTs, the controlled growth of CNTs still remains a great challenge. Currently, there are three major methods for CNT synthesis: arc discharge [2], laser ablation [3], and chemical vapor deposition (CVD) [4]. In these techniques, metal catalysts, typically Fe, Co and Ni, are usually required. To clarify the role of catalysts and understand the growth mechanism of CNTs, many scientists are exploring metal-free method to grow CNTs [5–14]. A comparison between the metal-catalyst-free growth and metal-assisted growth of CNTs will give an understanding

of the growth mechanism in depth, and thus we can find out the approach for the controlled growth of CNTs. Additionally, the metal-free growth of CNTs can also avoid the detriment effect of metal particles resides on CNT-based electronic devices. The metal-free growth of CNTs on a Si/SiO2 wafer is very interesting, which is compatible with the current siliconbased microelectronics technology. Recently, solid semiconductor nanoparticles [5] and various kinds of oxide nanoparticles [6–8] have been reported to be active for the growth of single-walled carbon nanotubes (SWCNTs) in CVD. More interestingly, Lin et al. [9] revealed that dense CNTs can be produced on a porous carbon substrate without any catalyst by CVD method. These findings demonstrate that porous structures or nano-scale curvatures are critical for the nanotube growth instead of metal particles. If so, the metal-free growth of CNTs on SiO2 substrates can be achieved by making nanostructures such as nano-curvatures or holes on their

* Corresponding author: Fax: +81 3 5261 1023. E-mail address: [email protected] (Y. Homma). 1 Present address: National Institute of Advanced Industrial Science and Technology, Nanotechnology Research Institute, Tsukuba, Ibaraki, 305-8561, Japan. 2 Present address: NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan. 3 Present address: Department of Mechanical Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan. 0008-6223/$ - see front matter  2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.08.039

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surfaces. Most recently, it was demonstrated that SWCNTs could be grown on the deposited SiO2 films [7] or on the scratched SiO2 substrates [7,8]. The authors proposed that the SiO2 particles, formed after annealing the deposited SiO2 film in H2 or scratching the SiO2 substrate surfaces, are critical for the SWCNT growth. These findings support our hypothesis. However, we have to point out that the deposition of a SiO2 film by sputtering or scratching of the SiO2 substrate surface, in general, possibly introduces metal contamination. Such contamination introduction should be avoided as much as possible. In addition, the reason for the SWCNT nucleation on SiO2 still remains unclear. The catalytic activity of SiO2 in the CNT growth need to be further investigated. In this study, we present that dense SWCNTs have been directly grown on thermally grown SiO2 substrates by the low temperature (850 C) CVD of ethanol without a catalyst following the pretreatment of SiO2 substrates in air at high temperature (950–1000 C) and subsequently in Ar/H2 at the same temperature. Neither the deposition of additional SiO2 films nor the scratching procedure of SiO2 substrates is needed, and thus the possibility of introducing metal contamination is decreased. We have performed careful investigations, including the precise analysis of the metal contents after different CVD stages by total reflection X-ray fluorescence (TXRF), to rule out the possibility of the conventional metals inducing the nanotube growth. We found the annealing treatment of SiO2 substrates in H2 at 950 C or a higher temperature was critical for the growth of SWCNTs. However, we did not detect SiO2 particles on the annealed SiO2 substrate. After CVD process, nano-scale carbon particles were produced besides SWCNTs. Based on these results, we propose that the annealing treatment of SiO2 substrates in H2 ambience generates defects on their surfaces, which provide stack-up sites for the deposited carbons self-assembling into SWCNTs.

2.

Experimental details

The growth of SWCNTs was performed on Si (1 0 0) substrates (p type, 4.5 X cm) with a thermally grown SiO2 layer of 100 nm. Si/SiO2 substrates were ultrasonically cleaned in acetone solution and then loaded into a horizontal furnace with a quartz tube of 38 mm in inner diameter. The furnace was then heated up to 950–1000 C at 50 C/min in air. After that,

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the substrates were annealed in air at 950–1000 C for 30 min at the atmospheric pressure and subsequently in the Ar/H2 mixture gas (3% by volume) at the same temperature for 10 min at 9 · 104 Pa. Then, the temperature was set at 850 C. When the temperature was stabilized at 850 C, the growth of nanotubes was initiated by introducing 70 sccm Ar/H2 bubbling through ethanol solution. The background pressure is fixed at 5.3 · 102 Pa. After the growth of 30 min, the samples were cooled in Ar/H2 to room temperature. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) were used for the observation of the morphologies and structures of the produced CNTs. Raman scattering was used for characterizing CNTs with an excitation wavelength of 532 nm at room temperature. Chemical state changes of the SiO2 substrate surface after annealing treatment were measured by X-ray photoelectron spectroscopy (XPS). In order to determine if the substrates were contaminated by the conventional metal catalysts, we analyzed the impurities on the SiO2 substrate surfaces after annealing treatment and CVD process using total reflection X-ray fluorescence (TXRF).

3.

Results and discussion

3.1.

Characterization of CNTs grown without catalyst

As shown in Fig. 1, a large-area and dense CNT film has been synthesized by the annealing treatment of SiO2 substrates in air for 30 min at 950 C and subsequently in Ar/H2 for 10 min at the same temperature followed by the CVD process of 30 min at 850 C. It is worth emphasizing that CNTs are uniformly grown on whole substrate surfaces. From Fig. 1b, the produced nanotubes are randomly distributed and entangled with each other. Thus, it is difficult to evaluate their length. Meanwhile, we note that most of the produced CNTs appear thicker, while a few nanotubes look brighter and thinner. Considering CNT image formation caused by the electronbeam induced current on the SiO2 surface [15], the thicker images indicate that the nanotubes contact the substrate surface, whereas the thinner images suggest the corresponding nanotubes are raised off the substrate surface because of the high nanotube density. The structures of the produced CNTs have been characterized by the Raman spectra. As shown in Fig. 2a, the sharp and

Fig. 1 – SEM image of carbon nanotubes synthesized on annealed SiO2 substrates without catalyst. (a) Low-magnification image. (b) High-magnification image.

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150 200 250 300 Raman shift (cm -1 )

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SWCNTs. Using the relationship for SWCNT diameter d = 248/xRBM [16], where xRBM is the frequency of the RBM in cm 1, the SWCNTs with a large diameter distribution ranging from 1 to 1.8 nm are demonstrated to be synthesized. It is known that the G-band in Raman spectra is derived from the tangential motion of carbon atoms in a nanotube while the D-band is derived from the defects of nanotubes or other carbon materials. The area ratio of G-band to D-band is usually used for evaluating the quality of the produced nanotubes. In the present Raman spectrum, the high area ratio of G-band to D-band peak area (18.6) reflects that the produced SWCNTs are of good quality. We also performed TEM observation on the as-grown samples. Samples for electron microscopy were prepared by scratching off nanotubes onto micro-grids. A typical TEM image of the produced CNTs is shown in Fig. 2b. TEM observation reveals that almost all the produced nanotubes are SWCNTs, and they are organized in small bundles. However, the produced SWCNT bundles are covered with much amorphous carbon, which also contributes to the D-band of the Raman spectra in Fig. 2a, indicating that the actual quality of SWCNTs should be higher. To identify if any particles are present at the ends of SWCNTs, we also tried to observe the ends of nanotubes. However, as shown in Fig. 2c, the ends of nanotubes usually were anchored on the supporting film on micro-grids and covered with a thick layer of amorphous carbon. It was difficult to observe the tip structures of nanotubes and clarify the presence of catalyst particles by TEM.

3.2.

Fig. 2 – (a) Raman spectra and (b), (c) TEM images of catalystfree grown carbon nanotubes. The inset in (a) is the corresponding radial breathing mode of Raman spectra.

branched G-band and the multi-peaks of radial breathing mode (RBM) of the Raman spectra indicate the generation of

Evaluation of contamination

We have to emphasize that we repeated the experiment many times, and densely populated SWCNTs were wellreproducible. We also employed SiO2 substrates provided by different suppliers to grow nanotubes using the same procedure. The same results were achieved. Si wafers with a SiO2 buffer layer are usually used as the substrates of a catalyst in the preparation of nanotubes. In order to exclude the possibility of metal contamination inducing the nanotube growth, we performed elemental analysis of the SiO2 substrate surface, especially Fe, Co and Ni, after the annealing treatment stage and the CVD process by TXRF. Results are summarized in Table 1. It is clearly shown that Co and Ni contents on substrates are very low and even undetectable after CVD process, thus ignorable for the nanotube growth. However, because Fe is a common element in the environment, Fe contamination is more serious than Co and Ni. From Table 1, Fe content increases about 15 times after the annealing treatment and the CVD process of 30 min. The aforementioned Raman results show that the SWCNTs ranging from

Table 1 – Elemental analysis of SiO2 substrates after annealing treatment and CVD process by TXRF. Stages No treatment Annealing treatment in air and Ar/H2 Annealing treatment + CVD of 30 min

Co content(·1010atoms/cm2) <0.35 <0.35

Ni content(·1010atoms/cm2) <0.3 0.99

Fe content(·1010 atoms/cm2) 5.0 27.0

<0.35

0.57

74.7

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Excitation wavelength 532 nm

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Raman shift (cm -1) Fig. 3 – SEM images of the nanotubes grown on (a) Si (1 0 0) and (b) sapphire A-face using the same CVD conditions as for the SiO2. (c) Raman spectrum of the products on sapphire substrates.

1 to 1.8 nm in diameter have been produced (Fig. 2a). Based on the relationship between the catalyst particle size and nanotube diameter, the corresponding iron particle size should be larger than the nanotube diameter if the nanotubes are produced by the contaminated Fe. If we assume an iron particle with 2-nm diameter, which is a modest estimation considering that Raman could not detect larger diameter nanotubes, it contains about 800 iron atoms. By calculation, the contaminated Fe can form 0.6 iron particles with 2-nm diameter in 1 lm2 area before the nanotube growth. Even after CVD process of 30 min, only 10 iron particles with 2-nm diameter can be formed in 1 lm2 region, which impossibly produce

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Fig. 4 – SEM images of carbon nanotubes grown under different annealing temperatures: (a) 1000 C and (b) 900 C.

dense nanotubes, even assuming that all the as-formed Fe particles have produced CNTs. For comparison, we also employed Si (1 0 0) and sapphire (A-face) substrates to grow nanotubes following the same CVD process. As shown in Fig. 3a and b, dense nanotubes were grown on Si substrates while no nanotube growth was observed on sapphire substrates. Raman spectrum (Fig. 3c) further reveals that no nanotubes but microcrystalline graphite were deposited on sapphire substrates [17,18]. Since the high temperature annealing treatment in air can thermally oxidize the surface of Si substrates into SiO2, it is reasonable that the growth of CNTs is similar with that on SiO2 substrates. It is well known that the conventional metal catalysts supported by sapphire substrates (A-face) can easily catalytically grow CNTs [19]. No nanotube growth on sapphire substrates (A-face) further confirms that no serious contamination of the conventional metal catalysts occurred through the whole CVD process.

3.3.

Growth condition dependence

Since the metal-free growth of nanotubes on a SiO2 substrate by CVD is a new discovery, it is necessary to systematically investigate the nanotube growth behavior. We firstly studied the effects of the annealing temperature of the substrates and the CVD growth temperature on the nanotube yields. The SEM images of the produced SWCNTs, when the

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annealing temperature is 1000 C, 950 C, and 900 C, are shown in Figs. 4a,b and 1, respectively. As shown in Figs. 4a and 1, dense nanotubes can always be obtained after SiO2 substrates have been annealed at 950–1000 C, while, with a decrease in the annealing temperature to 900 C, few nanotubes could be synthesized (Fig. 4b). These results indicate that the annealing treatment at a high temperature, which is unnecessary for the nanotube growth from the conventional catalysts, is critical for the successful catalyst-free growth of nanotubes on SiO2 substrate. The effect of the growth temperature on the nanotube yield was investigated by varying the growth temperature from 800 C to 900 C with the other parameters fixed. As shown in Fig. 1, dense SWCNTs were obtained at 850 C, while few nanotubes were produced at 800 C and 900 C (not shown). Because no assisting of the catalytic decomposition of ethanol by catalyst is expected, a higher growth temperature (850 C) is essential to induce the pyrolysis of ethanol. However, when the growth temperature is too high (900 C or higher), a strong Raman spectrum (Fig. 5) of microcrystalline graphite has been obtained [17,18]. Therefore, a too high temperature (900 C or higher) would induce a higher decomposition rate of carbon source, resulting in the coverage of substrates by a thick carbon layer in a short time and thus no nanotube growth. In contrast, the conventional catalysts (Fe, Co, and Ni) can easily catalytically grow CNTs in a wide temperature range including 800 C and 900 C.

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Excitation wavelength 532 nm

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Raman shift (cm ) Fig. 5 – Raman spectrum of the products on SiO2 substrates at high growth temperature (900 C or higher).

Besides the annealing temperature and growth temperature, we also observed a strong dependence of the nanotube growth on the annealing gas environment. The experimental results of the gas environment influences are summarized in Table 2. With the annealing treatment only in air or Ar/H2 before CVD, no nanotubes were obtained (samples 2 and 3). However, during CVD process, dense nanotubes were still obtained even without introducing Ar/H2 (sample 4), suggesting that Ar/H2 ambience is not indispensable for the nanotube growth any more after the annealing treatment stage. In order to determine which composition of the Ar/H2 mixture gas during annealing treatment is critical for the growth of SWCNTs, we replaced Ar/H2 mixture gas with pure Ar (99.999%). The result revealed that no nanotubes were synthesized (sample 5), implying that H2 ambience during the annealing treatment of SiO2 substrate is essential. For the conventional catalysts, although the pretreatment in H2 or air can improve their activity and increase the nanotube yield, the growth of nanotube can be easily observed even without the thermal treatment in H2 or in air. The effect of gas environment on the nanotube yield reflects that the nanotubes were not grown from the conventional metal catalysts, and excludes the possibilities of Fe evaporation in the furnace during the high temperature process and Fe contamination derived from Ar/H2 gas flow. Since the annealing treatments both in air and in Ar/H2 are very important for the successful metal-catalyst-free growth of SWCNTs, it is necessary to further investigate the effect of the annealing time in air and Ar/H2 on the nanotube yield. We varied the annealing time in air from 1 to 60 min. We found that dense nanotubes were always obtained, and that the nanotube yield almost did not change with an increase in the annealing time. Therefore, the annealing time in air has no much effect on the nanotube growth, implying that the role of the annealing treatment in air is possibly to clean the substrate surface. Short annealing time in air is enough to remove the hydrocarbon contamination [20]. On the contrary, the annealing time of SiO2 substrates in Ar/H2 has a great influence on the nanotube density. As shown in Figs. 6 and 1, the yield of SWCNTs decreases clearly with decreasing the annealing time in Ar/H2 from 10 to 1 min. With an increase in annealing time to 30 min, although dense nanotubes have been synthesized, their morphologies become worse. Therefore, the proper annealing treatment in H2 ambience is important for the metal-free growth of SWCNTs. The above results show that the pretreatment of SiO2 substrates in air and subsequently in Ar/H2 for 10 min at 950 C or a higher temperature, immediately followed by CVD growth

Table 2 – Experimental procedures for SWCNT growth.

a

Sample

Heating gas(atm. Pressurea)

Annealing gases (atm. pressure)

Growth gas(5.3 · 102 Pa)

CNT growth

1 2 3 4 5

Air Air Ar/H2 Air Air

30 min, 40 min, 40 min, 30 min, 30 min,

Ar/H2 bubbling through ethanol Ar/H2 bubbling through ethanol Ar/H2 bubbling through ethanol Ethanol vapor Ethanol vapor

Dense CNTs No CNTs No CNTs Dense CNTs No CNTs

atm. pressure – Atmospheric pressure.

air + 10 min, Ar/H2 air Ar/H2 air + 10 min, Ar/H2 air + 10 min, Ar

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Fig. 6 – SEM images of carbon nanotubes grown after annealing treatment of SiO2 in Ar/H2 for different time: (a) 1, (b) 5 and (c) 30 min.

3.4.

Origin of CNT nucleation

Recently, nano-scale curvatures were supposed to provide a platform for the formation of carbon caps and the subsequent growth of CNTs [5,6]. Most recently, it was found that dense nano-scale SiO2 particles were formed after annealing the deposited SiO2 film in H2 at 900 C [7] or scratching SiO2 substrates [7,8], and these nano-scale SiO2 particles were demonstrated to be active for the growth of SWCNTs. In the present work, one of the possible explanations for the catalyst-free growth of SWCNTs is that the annealing treatment at high temperature locally evaporates SiO2 substrate surfaces, resulting in the formation of nano-scale structures with a high curvature such as nanoparticles or nanoprotrusions, and these nanoparticles or nanoprotrusions serve as the catalyst for the growth of SWCNTs. To examine this hypothesis, we analyzed the surface roughness change of SiO2 substrates after the annealing pretreatment in air and subsequent in Ar/H2 by AFM. However, almost no change in the roughness of SiO2 substrate surface was observed before and after the annealing treatment (Table 3). Therefore, the metal-free growth of SWCNTs on SiO2 substrates might not be induced by the roughness change of substrates. It is also possible that nano-scale protrusions, if formed during annealing treatment, cannot be detected by AFM because of the rough surfaces of SiO2 substrates. Accordingly, at present, we cannot rule out the possibility that nanoprotrusions contribute to the nanotube growth yet. Note that H2 ambience during the annealing stage is critical for the successful growth of SWCNTs. It is necessary to clarify if SiO2 has been reduced by H2 at high temperature,

which possibly causes the formation of silicon carbide particles when carbon source is introduced, leading to the nanotube growth. For this reason, we investigated the chemical states of the surface of SiO2 substrates immediately after the annealing treatment at 950 C in Ar/H2 using ex-situ XPS characterization. However, the XPS Si2p spectrum shows only a single symmetric peak (Fig. 7), corresponding to SiO2. To investigate the role of H2, we further performed the comparative XPS studies of the surface compositions of fresh substrate, the substrate just after oxidation, and after further H2 reduction. However, no variations have been found. Because of ex-situ characterization, oxidation in air before the XPS measurement is inevitable. We shall say that the amount of reduced SiO2 is less than that of re-oxidized Si in air, even if reduction takes place. Therefore, it is difficult to detect the surface composition change of SiO2 substrates after the annealing treatment. From Fig. 2, we know that the SWCNTs with the diameter ranging from 1 to 1.8 nm were synthesized. If the nanotubes

Intensity (arb. unit)

at 850 C, are optimum parameters for the metal-free growth of SWCNTs. Especially, the annealing treatment in H2 ambience at high temperature is essential, which is much different from the nanotube growth from the conventional catalysts, providing evidences that the growth of SWCNTs was not induced by the conventional metal catalysts coming from contamination.

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Binding Energy (eV) Fig. 7 – Si2p peak of XPS spectrum of SiO2 substrate after annealing treatment in air for 30 min and in Ar/H2 for 10 min at 950 C.

Table 3 – Roughness analysis of SiO2 substrates after annealing treatment. Treatment conditions of SiO2 substrates No treatment Annealing in air for 60 min and in Ar/H2 for 60 min

Peak-to-valley (nm)

Root mean square roughness (nm)

1.2 1.2

0.248 0.245

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Fig. 8 – Cross-sectional TEM image of a Si/SiO2 substrate after nanotube growth. were grown from some catalyst particles, according to the relationship between catalyst particle size and nanotube diameter, the corresponding catalyst particle size should also be at least 2 nm, which could be observed by TEM. Therefore, after the nanotube growth of 30 min, the cross-sections of the SiO2 substrates have been characterized by TEM. Before TEM observation, an amorphous protection layer was deposited on the as-synthesized nanotube film. Unfortunately, we did not observe any particle structures on the substrate surface (Fig. 8). Considering that the nucleation of SWCNTs could provide important information about where the nanotubes have been grown, we investigated the morphology evolution of SWCNTs by changing the growth time. The SEM images of the asgrown SWCNTs after introducing carbon source for 4, 10, 15,

and 20 min are shown in Fig. 9. As shown in Fig. 9a, only a few short SWCNTs were synthesized after the introduction of carbon source for 4 min, and also some circular structures have been produced. With an increase in the growth time to 10 min, both the nanotube density and lengths were increased (Fig. 9b). A clear increase in the density of circular structures could also be observed. After the growth of 15 min, the nanotube density further increased (Fig. 9c). On increasing the growth time to 20 min, we obtained dense SWCNTs (Fig. 9d), whose morphology is similar to that of the nanotubes grown for 30 min (Fig. 1). Therefore, after the growth of 20 min, the density of the synthesized SWCNTs possibly saturates but we cannot determine whether the nanotubes continue to grow in length with time because of their high density and curved shapes. As shown in Fig. 9, the production of few nanotubes after the CVD process of 4 min reflects that the nucleation stage of most of the produced SWCNTs are longer than 4 min, which are much longer than that of the nanotubes grown from the conventional catalysts under the same conditions. An increase in the density of SWCNTs with CVD time (Figs. 9b–d) indicates that new SWCNTs continuously nucleate with increasing the growth time. In other words, the nucleation events are random and hence we have a coexistence of SWCNTs nucleated at different times and grown for correspondingly different time periods. Because of its resolution limit, SEM cannot provide more detailed information about the nanotube nucleation. Therefore, AFM was employed for further characterizing the produced nanotubes after introducing carbon source for 4 and 10 min. The corresponding AFM images are shown in Fig. 10. By the comparison between SEM (Figs. 9a and b) and AFM images (Fig. 10) of the produced nanotubes, it is

Fig. 9 – SEM images of SWCNTs produced after different growth time: (a) 4 (b) 10 (c) 15 and (d) 20 min.

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easily known that the circular structures shown in SEM images are curved nanotubes. Meanwhile, from the AFM images, we can also observe particle structures, whose density increases with CVD time (Figs. 10a and c). The profiles of the particle structures (Figs. 10b and d) indicate that their heights range from several nanometers to several tens of nanometers. However, the above TXRF result gives evidence that the contaminated metal impossibly form so many particles. Furthermore, TEM and SEM characterizations show that no particles structures are observed. In addition, after heating in air at 850 C, almost all the particles disappeared. These results suggest that the particle structures with around several nanometers in height are carbon particles while those with several tens of nanometers in height should correspond to overlapped and entangled nanotubes. From Fig. 10, it is worth noting that most of the produced nanotubes connect with a particle at one of their ends, implying that carbon particles are likely responsible for the growth of nanotubes. Several theoretical papers have discussed different models for the catalyst-free growth mechanism of SWCNTs [21,22], suggesting that the H-free graphitic patch is a very promising precursor of various sp2 network systems such as fullerenes, nanotubes and other carbon related materials, with the different final structures depending on the experimental conditions such as temperature or pressure, initial stacking manners and interlayer distances between graphitic sheets. In our present work, since the annealing treatment of SiO2

substrates in H2 at a high temperature is critical for the successful growth of SWCNTs, we speculate that the annealing of SiO2 substrates in H2 locally creates defects such as oxygen vacancies on their surfaces. The defects expose Si dangling bonds to the carbon source, leading to the nucleation of nano-scale carbon particles around the defect sites in the CVD process as shown in Fig. 10. It should be noted that very recently diamond nanoparticles were found to act as the seed of CNT growth when the particle surface kept the sp2 state by reducing hydrogen termination [23]. Although the formation mechanism of SWCNT on the p-bond relaxed surface of the diamond particle has not been clear enough, it is speculated that a small domain of graphene is formed on the p-bond relaxed surface of the diamond, and the graphene patch including five-membered rings lifts off the diamond surface and becomes a CNT cap [23]. The same process may work on the carbon nanoparticles, which can be amorphous or defective graphitic carbon. Although we have no direct experimental evidence to demonstrate the proposed mechanism, it does not contradict the XPS, TEM, and AFM results. In addition, the proposed mechanism can explain why SWCNTs could not be grown on the sapphire substrate (Fig. 3). The sapphire surface exhibits a regularly ordered step terrace structure and is flat at an atomic level [24,25]. Most recently, Tsukamoto and Ogino [26] reported that the graphene can be more closely attached to the sapphire surface than the SiO2 surface, indicating that the sapphire surface possibly has higher adhesion force with graphite than the SiO2 surface. Therefore,

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25

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Fig. 10 – AFM images of the produced nanotubes after introducing carbon source for (a) 4 and (c) 10 min. (b), (d) are the corresponding profiles of the particles in (a) and (c), respectively. Scale bars in (a) and (c) are 200 nm.

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the graphitic carbon layer was almost uniformly deposited on the sapphire substrate that the domain sizes of the graphitic carbon were too large to form CNT caps.

[10]

4.

[11]

Conclusions

The metal-catalyst-free growth of dense SWCNTs on SiO2 substrates by ethanol CVD has been demonstrated. Metal-free grown SWCNTs on SiO2 substrates show peculiar growth behaviors different from that grown from the conventional metal catalyst. Annealing treatment of SiO2 substrates in H2 at high temperature is found to be critical for the successful growth of SWCNTs. We systematically analyzed various possible reasons for the SWCNT growth on the annealed SiO2 substrates, and proposed that the annealing treatment of SiO2 substrates in H2 at high temperature created defects on SiO2 substrate surfaces, which assisted the formation of carbon particles and subsequent carbon nanocaps, leading to the SWCNT growth. The successful metal-free synthesis of SWCNTs by CVD opens a new way to synthesize metal-free SWCNTs and gives new insights into the SWCNT growth mechanism.

[12]

[13]

[14]

[15]

[16]

[17] R E F E R E N C E S

[18] [1] Saito R, Dresselhaus G, Dressselhaus MS. Physical properties of carbon nanotube. Singapore: World Scientific; 1998. [2] Ebbesen TW, Ajayan PM. Large scale synthesis of carbon nanotubes. Nature 1992;358:220–2. [3] Thess A, Lee R, Nikolaev P, Dai H, Petit P, Robert J, et al. Crystalline ropes of metallic carbon nanotubes. Science 1996;273:483–7. [4] Li WZ, Xie SS, Qian LX, Chang BH, Zou BS, Zhou WY, et al. Large-scale synthesis of aligned carbon nanotubes. Science 1996;274:1701–3. [5] Takagi D, Hibino H, Suzuki S, Kobayashi Y, Homma Y. Carbon nanotube growth from semiconductor nanoparticles. Nano Lett 2007;7:2272–5. [6] Liu HP, Takagi 1 D, Ohno H, Chiashi S, Chokan T, Homma Y. Growth of single-walled carbon nanotubes from ceramic particles by alcohol chemical vapor deposition. Appl Phys Express 2008;1:014001. [7] Liu B, Ren W, Gao L, Li S, Pei S, Liu C, et al. Metal-catalyst-free growth of single-walled carbon nanotubes. J Am Chem Soc 2009;31:2082–3. [8] Huang S, Cai Q, Chen J, Qian Y, Zhang L. Metal-catalyst-free growth of single-walled carbon nanotubes on substrates. J Am Chem Soc 2009;131:2094–5. [9] Lin JH, Chen CS, Ma HL, Chang CW, Hsu CY, Chen HW. Selfassembling of multi-walled carbon nanotubes on a porous

[19]

[20]

[21] [22] [23] [24]

[25]

[26]

carbon surface by catalyst-free chemical vapor deposition. Carbon 2008;46:1619–23. Larciprete R, Lizzit S, Botti S, Cepek C, Goldoni A. Structural reorganization of carbon nanoparticles into single-wall nanotubes. Phys Rev B 2002;66:121402. Kusunoki M, Rokkaku M, Suzuki T. Epitaxial carbon nanotube film self-organized by sublimation decomposition of silicon carbide. Appl Phys Lett 1997;71:2620–2. Derycke V, Martel R, Radosavljevic´ M, Ross FM, Avouris Ph. Catalyst-free growth of ordered single-walled carbon nanotube networks. Nano Lett 2002;2:1043–6. Koshio A, Yudasaka M, Iijima S. Metal-free production of high-quality multi-wall carbon nanotubes, in which the innermost nanotubes have a diameter of 0.4 nm. Chem Phys Lett 2002;356:595–600. Jin CH, Suenaga K, Iijima S. How does a carbon nanotube grow? An in situ investigation on the cap evolution. ACS Nano 2008;2:1275–9. Homma Y, Suzuki S, Kobayashi Y, Nagase M, Takagi D. Mechanism of bright selective imaging of single-walled carbon nanotubes on insulators by scanning electron microscopy. Appl Phys Lett 2004;84:1750–2. Jorio A, Saito R, Hafner JH, Lieber CM, Hunter M, McClure T, et al. Structural (n, m) determination of isolated single-wall carbon nanotubes by resonant Raman scattering. Phys Rev Lett 2001;86:1118–21. Barbarossa V, Galluzzi F, Tomaciello R, Zanobi A. Raman spectra of microcrystalline graphite and a-C:H films excited at 1064 nm. Chem Phys Lett 1991;185:53–5. Pocsik I, Hundhausen M, Koos M, Ley L. Origin of the D peak in the Raman spectrum of microcrystalline graphite. J NonCryst Solids 1998;227–230:1083–6. Ago H, Nakamura K, Ikeda K, Uehara N, Ishigami N, Tsuji M. Aligned growth of isolated single-walled carbon nanotubes programmed by atomic arrangement of substrate surface. Chem Phys Lett 2005;408:433–8. Takagi D, Homma Y, Hibino H, Suzuki S, Kobayashi Y. Singlewalled carbon nanotube growth from highly activated metal nanoparticles. Nano Lett 2006;6:2642–5. Zhang P, Crespi VH. Nucleation of carbon nanotubes without pentagonal rings. Phys Rev Lett 1999;83:1791–4. Kawai T, Miyamoto Y, Sugino O, Koga Y. First-principles study of the (2, 2) carbon nanotube. Phys Rev B 2001;66:033404. Takagi D, Kobayashi Y, Homma Y. Carbon nanotube growth from diamond. J Am Chem Soc 2009;131:6922–3. Yoshimoto M, Maeda T, Ohnishi T, Koinuma H, Ishiyama O, Shinohara M, et al. Atomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabrication. Appl Phys Lett 1995;67:2615–7. Shiratsuchi Y, Yamamoto M, Kamada Y. Surface structure of self-organized sapphire (0 0 0 1) substrates with various inclined angles. Jpn J Appl Phys 2002;41:5719–25. Tsukamoto T, Ogino T. Morphology of grapheme on stepcontrolled sapphire surface. Appl Phys Express 2009;2:075502.