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MATERIALS RESEARCH

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Formation of graphite encapsulated ferromagnetic particles and a mechanism for their growth A. A. Setlur, J. Y. Dai,a) J. M. Lauerhaas, P. L. Washington, and R. P. H. Changb) Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (Received 30 June 1997; accepted 1 December 1997)

Graphite encapsulated nanoparticles have numerous possible applications due to their novel properties and their ability to survive rugged environments. Evaporation of Fe, Ni, or Co with graphite in a hydrogen atmosphere results in graphite encapsulated nanoparticles found on the chamber walls. Similar experiments in helium lead to nanoparticles embedded in an amorphous carbon/fullerene matrix. Comparing the experimental results in helium and hydrogen, we propose a mechanism for the formation of encapsulated nanoparticles. The hydrogen arc produces polycyclic aromatic hydrocarbon (PAH) molecules, which can act as a precursor to the graphitic layers around the nanoparticles. Direct evidence for this mechanism is given by using pyrene (C16 H10 ), a PAH molecule, as the only carbon source to form encapsulated nanoparticles.

I. INTRODUCTION

One of the proposed applications for carbon nanotubes and nanoparticles1,2 involves filling these structures with materials that may exhibit interesting properties due to nanometer scale confinement of the encapsulant. Nanotubes and nanoparticles filled with metal carbides have been found on the redeposited rod on the cathode when vaporizing metal/graphite anodes in helium.3–5 From these experiments in helium, it was thought that large quantities of filled nanotubes and nanoparticles could be formed only when the element evaporated with carbon formed a stable carbide or catalyzed carbon fiber growth.5 However, changing the atmosphere from an inert gas to a reactive gas affects the growth process for filled nanostructures. The formation of Cu and Ge filled carbon nanotubes in a hydrogen atmosphere revealed that hydrogen can play an important role in the formation of filled carbon nanostructures.6,7 Nanotubes filled with pure Cu or Ge, elements which do not form a stable carbide or catalyze carbon fiber growth, were found in the soot deposited on the chamber walls. In these experiments, polycyclic aromatic hydrocarbon (PAH) molecules and metal nanoparticles were thought to be the precursors for filled carbon nanotubes. Subsequently, pyrene (C16 H10 ), a PAH molecule, and metal nanoparticles were used to form filled nanotubes.6,7 The encapsulation of ferromagnetic materials has become a widely studied topic for their potential applications in recording media, magnetic inks, or ferrofluids.

a)

On leave from the Laboratory of Atomic Imaging of Solids, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China. b) Author to whom correspondence should be addressed. J. Mater. Res., Vol. 13, No. 8, Aug 1998

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