LASER-ASSISTED CHEMICAL VAPOR DEPOSITION OF CARBON COATED COBALT NANOPARTICLES
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0901-Ra09-03.1
LASER-ASSISTED CHEMICAL VAPOR DEPOSITION OF CARBON COATED COBALT NANOPARTICLES 1
O. Alm,1,* J.-O. Carlsson1, M. Boman1 Department of Materials Chemistry, The Ångström Laboratory, Uppsala University, P. O. Box 538, SE-751 21 Uppsala, Sweden
Abstract Carbon coated nanoparticles were synthesized by laser-assisted (ArF excimer laser, λ = 193 nm) chemical vapor deposition (LCVD). The particles were formed in the gas-phase by photolytic dissociation of cobaltocene in argon and the particles were deposited onto a silicon substrate. The particles were deposited at two different laser fluencies, 70 and 300 mJ/cm2. Single crystalline spherical cobalt particles with a well-defined carbon shell were observed by transmission electron microscopy (TEM) for the highest fluence, 300 mJ/cm2. The metallic nucleus phase were identified as either β-Co or Co3O4. Polycrystalline particles were deposited at 70 mJ/cm2: These particles contained α-Co, β-Co, CoO and Co3O4. The particles deposited at 300 mJ/cm2 were log-normally distributed and the total diameter had a mean geometric size of 25 nm while the nuclei had a mean diameter of 10 nm. X-ray photoelectron spectroscopy (XPS) measurements showed that the particles had a carbon content roughly ten times the amount of cobalt. Sputtering showed that both cobalt oxide and metallic cobalt was present. HRTEM micrographs of the particles revealed that only one phase was present in each nuclei, proving the nuclei were either an cobalt oxidie or metallic cobalt. Raman spectroscopy showed that that the carbon shell contained mostly amorphous carbon. Small domains of carbon of more graphitic character was embedded in the amorphous carbon shell in the 300 mJ/cm2 sample. Introduction The different and often unique physical properties (as a result from size and quantum effects) that can be obtained from nanostructured materials, compared to the bulk of the same material is a huge driving force for research. For instance the magnetic properties of nano-structured materials have a potential use in many different applications such as sensors, storage and recording systems, and medical applications (local drug delivery) [1, 2]. For these areas, nanoscale compounds of the ferromagnetic transition metals (Fe, Co, Ni) have been subject of a lot of research. Spherical crystalline nanoparticles of iron surrounded by a carbon shell have been produced by LCVD of ferrocene [3-7]. Both the stable bcc and metastable fcc phase of Fe were obtained [6]. The carbon shell, which consisted of turbostratic and amorphous carbon, protected the metallic nucleus from oxidation for several days [6]. These particles reached the boiling point during their formation [4-7] and the size distribution and phase composition [6] could be altered by tuning the laser power from the laser. Additionally a differential mobility analyzer (DMA) was used to select certain sizes of the particles, resulting in a very narrow size distribution [3]. Cobalt is another transition metal with attractive magnetic properties. Pure Co (α
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