Synthesis, characterization and field emission properties of rare -earth hexaboride nanowires.

摘要

Rare-earth hexaborides are a family of compounds which have low work function, high melting point, and high mechanical strength. These properties are highly suitable for electron field emission applications. To explore this possibility, we developed a chemical vapor deposition method and produced three different types of 1D nano-structures, which include single crystalline nanowires of LaB6, CeB6, and GdB6; polycrystalline nanowires of YB12, LaB12, MgO, and Mg3N 2; core-shell nanocables of MgB2-in-MgO and LaB6-in-CNT. TEM, SEM, EELS and EDX techniques were applied to characterize structural and chemical information about the synthesized nano-structures. Vapor-solid growth, catalyst-assisted vapor-liquid-solid growth and CNT-assisted template growth are proposed to be accounted for the formation mechanisms of these 1D nano-structures and the theoretical predictions match the experimental observations quantitatively. To fabricate a single nanowire field emitter, direct contact, electron beam lithography and focused ion beam welding techniques were used to attach a single LaB6 nanowire to the tip of a tungsten wire. Cold field emission properties were measured from such made single nanowire emitters. Work function values of 2.6 eV and 1.5 eV were obtained from a LaB 6 nanowire emitter and GdB6 nanowire emitter respectively. An Emission Current density as high as 5x105 A/cm 2 was obtained from a single LaB6 nanowire emitter, under an extraction voltage of 800 V. Emission current stability was also studied for the nanowire emitter and the results indicate surface-contamination induced emission current fluctuations. A home-designed TEM in-situ field emission measurement holder was fabricated for a JEOL 2010F HRTEM. Field electron emission was performed on a single LaB6 nanowire field electron emitter simultaneously with high resolution TEM imaging. Image contrast changes were observed at under-focus imaging conditions when a series of negative biases were applied to the nanowire emitter. Such contrast changes can be used to qualitatively image the charge density distribution on the nanowire emitter tip during field electron emission.