TSINGHUA SCIENCE AND TECHNOLOGY ISSN 1007-0214 11 /11 pp741-744 Volume 10, Number 6, December 2005

From Carbon Nanotube Crystals to Carbon Nanotube Flowers*

ZHANG Zhengjun (张政军)* *, ZHAO Ye (赵 烨), ZHOU Ya (周 雅)

Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

Abstract: We have investigated the very initial deposition stages of chemical vapor deposition (CVD) with

ferrocene (Fe(C5H5)2) and xylene (C8H10) for growing carbon nanotubes, and made clear that the mecha-

nism for the self-organization behaviors of nanotubes at different growth stages by this approach. For in-

stance, the organization of nanotubes into flower-like structures at prolonged deposition is developed from

the crystal-like structures formed at early growth stages, both of which are closely related to and determined

by the very initial deposition stages of this CVD approach. Based on this approach, ways have been estab-

lished to build up different architectures of carbon nanotubes, by controlling the initial deposition stages of

the CVD process, with which we have realized the selective growth of self-organized carbon nanotube struc-

tures. This study provides a new idea for growing carbon nanotube architectures by CVD.

Key words: carbon nanotubes; self-assembly; chemical vapor deposition (CVD)

Materials of nanometer scale dimensions, e.g., nano-tubes, nanowires, nanorods, etc., have attracted great interest due to their novel properties and potential ap-plications[1-5]. The assembly of these nanostructures on planar substrates is crucial, since it is very important to control collective behaviour for applications in nan-odevices, for instance, the growth of arrays of aligned carbon nanotubes on flat substrates for field emission applications[6-9]. Investigations on carbon nanotubes indicated that they self-organized into differently shaped structures: crystallites, spherulites, and honey-comb networks at the early growth stages, which de-veloped further at prolonged deposition into films of different morphology[10,11]. The studies gave the first ever glimpse on the early growth stages of nanotubes and provided a way to control the nanotube film morphology[10,11]. The growth mechanism of these

structures, which definitely relates to the very initial growth stage of nanotubes, however, remains still un-clear, as well as the relationship to the final morphol-ogy of the film developed from these self-organized structures. To make these clear, we need to investigate the initial stages of nanotubes growth and to follow the development of self-organized structures of carbon nanotubes at prolonged deposition. This is, however, very difficult for most chemical vapor deposition (CVD) processes.

In this letter, we report the first investigation on the very initial stages of the CVD approach using ferrocene and xylene as the catalyst and carbon source. Carbon nanotubes grown by this approach self-organized into crystal-like structures at early growth stages and developed into flower-like structures at prolonged deposition.

Carbon nanotubes are grown on thermally oxidized silicon wafers at ~ 800℃ by CVD process with ferro-cene and xylene. The detailed procedure has been de-scribed elsewhere[10-12]. Adjusting the ratio of ferro-cene over xylene resulted in the formation of crystal-lites (at ＜0.01 g/mL), spherulites (at ＞0.2 g/mL),

﹡ Received: 2005-07-11 Supported by the National Natural Science Foundation of China (No. 50201008) and the Ministry of Education of China

﹡﹡ To whom correspondence should be addressed. E-mail: zjzhang@tsinghua.edu.cn; Tel: 86-10-62772233; Fax: 86-10-62771160

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and honeycomb networks (at ratios between) of nano-tubes, which, at prolonged deposition, developed into packed nanotube films through space-filling the silica surface[10,11]. In this study, we fixed the ratio at ＜0.01 g/mL, to observe the nucleation of nanotube crystals at the very early growth stages to elucidate the growth mechanism of nanotube crystals and the relationship to the final film morphology.

Figure 1 shows scanning electron microscopy (SEM) images of carbon nanotubes deposition at a ferro-cene/xylene ratio of < 0.01 g/mL. Nanotubes grown at this ratio are self-organized into crystal-like structures at the early grow stages. These nanotubes are typically about several µm in diameter and exhibit polygonal shapes on the silica surface, see Figs. 1a and 1b. We can see that after nanotube removal, the crystallites leave perfect polygon traces on the substrate surface. At prolonged deposition, a film of densely packed, ver-tically aligned carbon nanotubes was formed[10], atop which, see Figs. 1c and 1d, flower-like structures, about several ten µm in diameter, are also formed with aligned carbon nanotubes.

The nanotube crystals growth at the early growth stages is related to the formation of dimples in a thin amorphous carbon film containing catalyst particles covering the silica surface, deposited at the very initial deposition stage, through the decomposition of xylene and ferrocene at ~ 800℃. The thin film was always observed in previous experiments, and was found atop the carbon nanotubes grown[10-13]. Figure 2a shows a typical SEM image of a dimple formed, about several µm in diameter. Note from Figs. 2a-2c that the dimple is the place in which nanotubes developed first, outside which there is almost no observable growth. The for-mation of the dimples, as shown in Fig. 2c, is due to the distribution of the catalyst particles, i.e., there were more particles inside the dimples (left part of the figure) than outside the dimples (right part of the figure). Nanotubes developed inside the dimple area, as indi-cated by arrows in Fig. 2b, were beneath the dimple, and were grown upwards aligned. Thus the growth of carbon nanotubes lifted up the dimple and cracked it from the thin film attached to the silica substrate and, as shown in Fig. 2d, resulted in the development of

Fig. 1 SEM images of (a) and (b) crystal-like structures; and (c) and (d) flower-like structures of nanotubes formed at the early growth stages and prolonged deposition, respectively, by chemical vapour deposition using ferrocene and xylene at ~ 800℃.

ZHANG Zhengjun (张政军) et al：From Carbon Nanotube Crystals …… 743

Fig. 2 (a)-(d) are SEM images showing the development of nanotube crystals from dimples formed in the thin film de-posited at the very initial stage of deposition; (e) and (f) are SEM images of traces of the dimples and nanotube crystals, showing the preferred partition of catalyst particles in these areas.

nanotubes around the crack. The carbon nanotubes un-der the film outside the crack, see Figs. 2b and 2d, were also grown upwards aligned; and therefore, lifted up the film from the silica substrate to a height reduced from the crack outwards, forming an embryo of nano-tube crystal. Further growth of nanotubes under the dimple and the film outside the crack led to the forma-tion of well-shaped nanotube crystals at the early growth stages (see Figs. 1a and 1b).

The formation of dimples in the thin film is intrigu-ing and plays a key role in the growth of well-shaped nanotube crystals. We thus removed carbon nanotubes forming the crystals and dimples by mechanical shak-ing, and observed their traces. Figures 2e and 2f show traces of a dimple and a crystal. We can see that there

are more catalyst particles in traces of dimples and nanotube crystals than in other places. Due to the pre-ferred partition of catalyst particles, carbon nanotubes first nucleated in these areas and resulted in the emer-gence of dimples, acting as seeds for the development of nanotube crystals in the early growth stages. The formation of the dimples, in the authors’ opinion, is probably due to the stress caused by the nucleation and growth of nanotubes in areas of more catalyst particles. The preferred partition of catalyst particles might be due to the surface property of the silica substrate and needs further investigation.

The growth of nanotubes flowers after prolonged deposition is also closely related to the formation of dimples in the thin film. Figure 3 shows the formation of flower-like structures. At prolonged deposition,

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nanotubes also developed from places outside the dim-ples. These were developed under the film and grown upwards aligned at comparable growth speeds. Thus the whole film was lifted up from the substrate, cover-ing the densely packed, vertically aligned carbon nano-tubes. In the dimple areas, through similar mechanisms to that of the growth of nanotube crystals, the nano-tubes developed into flower-like structures.

One may notice the similarity between nanotube flowers shown in Figs. 3a and 3b and nanotube crystals shown in Figs. 1a and 1b.

Fig. 3 SEM images showing the growth mechanism of nanotube flowers atop the film of packed nanotubes

The present study makes a clear picture for the growth of nanotube crystals at the early growth stages, and for the growth of nanotube flowers at prolonged deposition, by CVD using ferrocene and xylene. The key in growth these nanostructures is the preferred par-tition of catalyst particles on the substrate surface at the very initial stage of deposition. One might modify the substrate surface to construct desired properties[14], to grow various nanostructures of carbon nanotubes.

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