Formation of Various Nanocarbon and Composite Structures by Laser Ablation

Abstract

Nanocarbon and composite materials have attracted considerable research attention due to their unique properties and potential applications. Laser ablation of graphite or graphite containing a small amount of metals can produce various nanocarbon structures such as single-walled (SW) (Guo et al., 1995) and multi-walled carbon nanotubes (MWCNTs) (Hirahara et al., 2000), SW carbon nanohorn (CNH) aggregates (Iijima et al., 1999) and polyhedral graphite (PG) particles (Kokai et al., 2003). In this chapter, we describe a simple method to form various nanocarbon and composite structures using laser ablation in highpressure Ar gas up to 0.9 MPa, with particular focus on the composite nanostructures. Graphite targets containing metals or compounds (Si, Fe, Co, Ni, Cu, Ag, B4C, Y2O3, La2O3, and Gd2O3) were used as source materials. To effectively form nanocarbon and composite structures by laser irradiation onto the targets at room temperature, we used a continuous wave Nd:YAG laser to eject C and other species with relatively low kinetic energies into the high-pressure Ar gas. As a result, hot C and metal species are confined in a space surrounded by Ar gas. For example, the initial temperature of the vaporized C species was ~5000 °C estimated for laser irradiation from emission spectroscopy (Kokai et al., 2001). In addition, emission imaging and shadowgraphic studies implied that vaporized species had low expansion velocities of 102–103 cm/s (Kokai et al., 1999) due to the high-pressure Ar gas restricting their expansion. Unlike a laser ablation method combined with an electric furnace for SW and MWCNT growth, where vaporized C and metal species kept at 800–1300°C are essential for an efficient CNT growth (Gorbunov et al., 1999), the heat sources available for the growth of various nanocarbon and composite structures in this study are laser-ablated C and metal species themselves confined by high-pressure Ar gas. The control of the resident densities and the maintenance of high-temperature of laser-ablated C and metal species, based on adjusting the metal content in graphite and the Ar gas pressure, results in various composite nanostructures with high yields in the deposits. The nanostructures were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and x-ray diffraction (XRD) patterns, and Raman spectroscopy. Depending on the type and content of metals and compounds, characteristic nanocarbon and composite structures such as hybridized SWCNH particles and Cuor SiC-filled one-dimensional (1D) structures were formed. We discuss the metaldependent growth of the various nanocarbon and composite structures with an emphasis on graphitization processes such as thermal graphitization, catalytic graphitization and vapor-