(38c) Direct Synthesis of MgxBy Nanostructures

Our group has previously demonstrated the successful synthesis of boron nanotube through CVD reaction, and proposed a mechanism involving the physical templating of boron nanotubes. To achieve the MgB2 nanostructures, most methods reported involve the post doping of Mg. The Hybrid Physical-Chemical Vapor Deposition (HPCVD) was introduced by Zheng et al. in 2002 to form MgB2 in one step. Up till now, it was mainly used to synthesize MgB2 films and crystalline whiskers on the flat substrates. In this paper, we'll present the direct fabrication of MgxBy nanostructures by applying our uniform diameter physical template of MCM-41 in the HPCVD reaction.

The heterogeneous HPCVD reactions were carried out under the diborane flow balanced in hydrogen. Different reaction temperatures and pressures were tested to optimize the fabrication process. TEM analysis shows starting from 873K reaction temperature, the MgxBy nanostructures start to fabricate. Some of the structures were darken on the outer edge, which could be nanotube structures. The yield of the nanostructures increases with the reaction temperature. The as-synthesized nanostructures have the diameters in the range of 3-4nm, and the diameter distribution doesn't vary much with the increasing reaction temperature. However, the diameter distribution broadens with the reaction pressure increase. The Raman spectrum of our as-synthesized MgxBy nanostructures shows a finger print peak at 528-1cm, which is the stretching mode of boron bonding with other elements. NEXAFS analysis on the boron k-edge indicates the formation of Mg-B bond after the HPCVD reaction and EELS analysis also confirms the existence of B and Mg in the nanostructures with slight Si contamination. Si might be incorporated in the nanostructure during the fabrication process followed the VLS theory. DC magnetization indicates the as-synthesized sample has superconductive transition temperature at 35K, close to the bulk phase MgB2 standard, and the difference might due to the Si incorporation in the nanostructures.