Formation of Ni/SiO 2 and Ag/SiO 2 Nanosphere Composites
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FORMATION OF Ni/SiO2 AND Ag/SiO2 NANOSPHERE COMPOSITES S. M. Prokes and W. E. Carlos, Lenward Seals1, Stephen Lewis1, and James L. Gole1 Naval Research Laboratory, Washington, D. C. 20375 1 Georgia Institute of Technology, Atlanta, Ga. 30332-0430 ABSTRACT SiO2 nanospheres have been produced via a high temperature evaporation process and they have been Ni or Ag plated using electroless plating solutions. These samples were examined by Atomic Force Microscopy (AFM) and Magnetic Resonance (MR). The initial SiO2 nanospheres were about 30 nm in diameter, and the Ni plating layer resulted in a 25nm thick metallic Ni coverage, while the Ag coverage was estimated to be in the 150 nm range. In the case of the Ni/SiO2 nanosphere composites, the MR signals show the presence of Ni+2 and Ni+3 paramagnetic centers, seen below 40K, and ferromagnetic metallic Ni, which is seen above 40K. The dried Ni plating solution (with no SiO2) shows only the presence of paramagnetic Ni+3. These results suggest that an interfacial reaction at the surface of the SiO2 nanospheres leads to the formation of ferromagnetic Ni, which deposits onto the spheres and forms a ferromagnetic Ni/SiO2 nanosphere composite. In the case of the Ag/SiO2 nanosphere composites, no MR signal is seen from the non-magnetic Ag, but strong paramagnetic behavior has been noted for Co+2, which originates from the plating solution. INTRODUCTION It has recently been demonstrated that semiconductor-based nanostructures can be formed by a high temperature synthesis [1-7] which, as a function of experimental conditions, can be made to produce a variety of nanospheres and nanowires whose agglomeration can lead to the formation of nanotubes [5]. When starting with an initial Si/SiO2 mixture, silicon nanowires [17] sheathed in oxide have been obtained, as well as nearly monodisperse SiO2 nanosphere distributions [4,5]. In addition to semiconductor materials, the formation and incorporation of metallic nanostructures is also of interest for applications in catalysis [8] and bio-sensors [9], as well as for the study of magnetic properties. There are a variety of existing techniques which have been used to form metallic nanostructures, including ion implantation [10], electrodeposition [11], the filling of carbon nanotubes [12] or step decoration [13,14]. However, a simpler technique, which can produce large quantities of material and which results in a well characterized particle size distribution involves the electroless plating of a metal on existing nanostructures. We will show that Ni and Ag can be easily deposited in solution onto existing SiO2 nanospheres, creating a Ni/SiO2 or Ag/SiO2 composite nanostructures. In the case of Ni, these composites exhibit room temperature ferromagnetic behavior. EXPERIMENTAL DETAILS The dispersed silica (SiO2) nanospheres [4,5] were synthesized in a single step in-situ high temperature process, using a 50/50 Si/SiO2 mixture and argon carrier gas in an apparatus described previously [4,5]. Depending on the relative locations of double concentric
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