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Morphological Evolution of Neodymium Boride Nanostructure Growth by Chemical Vapor Deposition

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journal contribution
posted on 2009-06-18, 00:00 authored by Gonghua Wang, Joseph R. Brewer, Jie Ying Chan, David R. Diercks, Chin Li Cheung
Nanoscale-driven design of electron emission materials can significantly increase their overall efficiency as cathodes for field-induced electron emission by taking advantage of the field enhancement effect. The refractory nature and low work function (1.6 eV) of neodymium hexaboride (NdB6) suggest that high aspect ratio NdB6 nanostructures are potential candidates as efficient field emission cathodes. Here we report the morphological evolution of one-dimensional neodymium boride nanostructures synthesized using palladium-nanoparticle-catalyzed chemical vapor deposition as a function of reaction temperature. Scanning electron microscopy data show that judicious choices of reaction temperatures (795−940 °C) can lead to the preferential growth of curly nanowires or high aspect ratio nanowires. Transmission electron microscopy and selected area electron diffraction reveal that the crystallinity of these nanostructures changes from amorphous, to polycrystalline, to single crystalline as the reaction temperature increases. At reaction temperatures above 900 °C, single-crystalline NdB6 nanowires with preferential [100] growth direction were successfully synthesized. Energy dispersive X-ray spectroscopic data suggest that this morphological evolution was strongly influenced by the solubility profiles of Nd and B in the Pd catalyst nanoparticles at different reaction temperatures. The implication of these results on the criteria of catalyst choices for the growth of binary metallic boride nanomaterials is also discussed.