1-Dimensional carbon nanomaterials, such as carbon nanotubes (CNTs) and carbon nanofibers (CNFs), have attracted great attention in materials science since the discovery of CNTs [1]. They have been synthesized from a gas phase at growth temperatures usually higher than 500oC. However, for a variety of applications, especially for flexible plastic-based devices, synthesis at lower temperatures, ideally at room temperature, needs to be achieved.
In the previous papers, we demonstrated that Ar+ ion irradiation to bulk carbon and carbon-coated substrates induced the formation of conical protrusions and single CNFs pointing in the ion-beam direction grew on the cone top without any catalyst even at room temperature [2]. The ion-induced CNFs thus synthesized were typically 20-50 nm in diameter and micrometer range in length. They were characterized by the amorphous nature and the hollow-less structure. As is well-know, for the CNT growth in chemical vapour deposition, catalyst metals are necessary. Thus, a simultaneous metal supply during the ion-induced CNF growth must be very interesting; does the metal supply trigger the change in crystalline structure of growing CNFs? In the present work, we tackled this important subject.
Graphite plates were Ar+ ion bombarded with and without a simultaneous supply of metal atoms at room temperature. The graphite surfaces sputtered without any metal supply were covered with densely distributed CNF-tipped conical projections similar to the previous results. By contrast, the graphite surfaces sputtered with a simultaneous metal supply were characterized by densely distributed conical (without CNF on top), needle-like, fibrous and CNF-tipped conical structures, depending on the metal species and the supply rate. Transmission electron microscope observation revealed a graphitized structure of metal-doped CNFs and sometimes an encapsulation of short metal nanowires in the CNFs. Since doping of any kinds of materials into CNFs is possible in principle, it was believed that this ion-irradiation method would open up a new approach to fabricate 1-D nano-functional materials at room temperature.
[1] S. Iijima, Nature 354 (1991) 56.
[2] M. Tanemura, et al., Appl. Phys. Lett. 84 (2004) 3831, 86 (2005) 113107, 87 (2005) 193102. |