Synthesis of highly oriented and dense conical carbon nanofibers by a DC bias-enhanced microwave plasma CVD method Goufang Zhong * , Takayuki Iwasaki, Hiroshi Kawarada, Iwao Ohdomari School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan Available online 9 August 2004 Abstract Conical carbon nanofibers (CCNFs) have been synthesized on Si substrates coated with a Fe thin film of a few nanometers in thickness by a DC bias-enhanced microwave plasma chemical vapor deposition (CVD) method using H 2 and CH 4 as reactant gases under a chamber pressure of about 230 Pa. Without DC bias, only catalytic nanoparticles could be observed by field emission scanning electron microscopy (FE-SEM) on substrate surface after growth. Highly oriented and dense CCNFs could be deposited when a negative DC bias in the range of 150 – 230 V was applied to the substrate holder during growth. The average density of CCNFs was measured as being of the order of 10 10 cm  2 , while the average length was of the order of 10 2 nm, and the conical angle was in the range of 10 – 15j. The relationships between the density, the root diameter and the length of CCNFs are discussed. D 2004 Elsevier B.V. All rights reserved. Keywords: Conical carbon nanofibers; DC bias; Microwave plasma CVD; Density; Diameter; Conical angle 1. Introduction Due to their outstanding chemical and physical proper- ties, carbon nanofibers (CNFs), including carbon nanotubes (CNTs), have a large number of potential applications such as in nanoelectronics, nanoscale structural materials, field emission devices, hydrogen storage, and sensors [1]. For field emission applications, a threshold voltage as low as 2 – 3 V has been achieved by using CNTs as emitters [2,3]. However, when a high current density was obtained, emis- sion failure on some emission sites was observed during our field emission measurement. This is attributable to the burning out of CNFs at those sites due to the poor contact between the CNTs and the substrate, which may result in the poor thermal stability of CNFs. Recently, patterned growth of vertically aligned CNFs was demonstrated by plasma- enhanced CVD [4–6]. We noted that these CNFs possess a more or less conical shape, i.e., a relatively large contact area with the substrate surface, which may be helpful for increasing the thermal stability of CNFs for field emission. As the patterned growth of CNFs was realized by means of E-beam lithography, the growth area was somehow limited to submicron or nanometer size. In this study, we demonstrate the synthesis of highly oriented and dense CCNFs on centimeter-sized Si substrates coated with Fe of a few nanometers in thickness by conventional magnetron sputtering by the DC bias-en- hanced microwave plasma CVD method. The density, diameter, length and conical angle of CCNFs were mea- sured, and their relationships were discussed. 2. Experiments In this study, Si(100) wafers coated with a nanolayer of Fe at room temperature by magnetron sputtering were used as substrates. A DC bias-enhanced microwave plasma CVD apparatus with a quartz tube chamber was employed for the synthesis of CCNFs. The steps for the synthesis of CCNFs are as follows. First, the quartz tube chamber is evacuated by a rotary pump after placing the substrates on the substrate holder which functions as the negative bias electrode. Then, mixture gases (100 sccm in total) of H 2 and CH 4 are introduced into the chamber to keep the chamber pressure at about 230 Pa. Finally, the microwave plasma at a power of 500 W is introduced into the quartz tube chamber, and a 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.06.051 * Corresponding author. Tel./fax: +81-3-5286-3391. E-mail address: zhong@kaw.comm.weseda.ac.jp (G. Zhong). www.elsevier.com/locate/tsf Thin Solid Films 464 – 465 (2004) 315 – 318