Communications Large-Area Synthesis of Carbon Nanofibers by Low-Power Microwave Plasma-Assisted CVD** By Guofang Zhong,* Minoru Tachiki, Hitoshi Umezawa, Toyokatsu Fujisaki, Hiroshi Kawarada, and Iwao Ohdomari As it will play a very important role in the coming age of nanotechnology, the carbon nanotube (CNT) has been at- tracting the attention of researchers worldwide since its dis- covery. [1] Currently, laser ablation and DC arc plasma are commonly used for the growth of single-walled CNTs. [2±4] CVD methods, such as hot-filament and conventional mi- crowave plasma-assisted (MP) CVD, are widely employed for the synthesis of multiwalled CNTs or carbon nanofibers (CNFs) on various substrates coated with a nanocatalytic layer of transition metal such as Fe, Co, or Ni. [5±8] Although MPCVD has the advantage of being able to produce con- trolled growth of CNFs at relatively low temperatures, 500±800 C, as the substrate is too close to, or even placed in, the microwave plasma ball, i.e., it is more or less in con- tact with the microwave plasma ball, the uniform growth area and growth reproducibility of CNFs prepared by con- ventional MPCVD are far from satisfactory. Moreover, the growth temperature can hardly be decreased further. Bos- kovic et al. [9] reported the synthesis of CNFs at room tem- perature by RF plasma-enhanced (PE) CVD, but as they used micrometer-sized Ni catalysts, only a relatively large (30  40 lm 2 ) CNF growth area was reported. In this communication, a novel method of antenna edge (AE) MPCVD [10] for large-area synthesis of CNFs with high growth reproducibility at a very low microwave power input by remote deposition, is demonstrated for the first time. (The term ªremote depositionº means to deposit CNFs at a substrate distance relatively far from the micro- wave plasma ball.) Experiments show that, even at a small microwave power input of 60 W, large-area synthesis of CNFs could be achieved by remote deposition, despite the fact that the di- ameter of the microwave plasma ball was only about 10 mm. Typical scanning electron microscopy (SEM) morphologies of CNFs grown on Si substrates coated with catalytic Ni are given in Figure 1. The substrate distances and temperatures are 30 mm and 580 C for Figure 1a and 80 mm and 645C for Figure 1b. The temperature was measured by a thermocouple. As we can see, CNFs could even be synthesized at the very large substrate distance of d = 80 mm. Furthermore, CNFs could not only be depos- ited on substrates the same size as the substrate holder by remote deposition, but also have a quite uniform surface morphology over the entire substrate when d is over 40 mm. However, if the substrate is placed very close to the antenna edge so that it comes into contact with the mi- crowave plasma ball, e.g., d is less than 10 mm, no CNFs can be observed in the central part of the substrate. CNFs can only be grown on a ring-shaped area, which may be regarded as remote deposition. The aligned growth of CNFs given in Figure 1a occurred at a value of d of not more than 30 mm. The diameter and length distributions of these CNFs are quite narrow. The diameters of CNFs in Figure 1a are in the range 30±50 nm. However, when d is too large, CNFs similar to those shown in Figure 1b are deposited. The surface SEM image in Fig- ure 1b(2) shows that the diameters of the CNFs are quite disparate. Moreover, it can be seen that some CNFs, having large diameters, stand out from the dense film of CNFs. In our experiments, the largest CNF is approximately 140 nm in diameter, while the smallest is less than 10 nm in diame- ter. In Figures 1a,b, catalytic particles of Ni are clearly seen on the tips of the CNFs, which is in agreement with litera- ture reports. [11,12] This may suggest a ªtip growthº mecha- Chem. Vap. Deposition 2004, 10, No. 3 DOI: 10.1002/cvde.200304168  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 125 ± [*] Dr. G. Zhong,Dr. M. Tachiki, Dr. H. Umezawa, T. Fujisaki, Prof. H. Kawarada, Prof. I. Ohdomari School of Science and Engineering, Waseda University 3-4-1 Okubo, Shinjuku,Tokyo, 169-8555 (Japan) E-mail: zhong@kaw.comm.waseda.ac.jp [**] This work is supported in part by the Japan Society for the Promotion of Science (JSPS), and a Grant-in-Aid for Center of Excellence (COE) Research from the Ministry of Education, Culture, Sports, Science, and Technology. Cross-sectional morphologies Surface morphologies a1 a2 b1 b2 Fig. 1. Typical SEM morphologies of carbon nanofibers grown on Si sub- strates coated with 2 nm Ni by remote deposition. The substrate distances and temperatures are: a) 30 mm, 580 C, and b) 80 mm, 645 C.