LaB 6 Nanowires and Their Field Emission Properties
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0901-Rb15-06.1
LaB6 Nanowires and Their Field Emission Properties Han Zhang,1 Jie Tang,2,3 Qi Zhang,2 Gongpu Zhao,2 Guang Yang,2 Jian Zhang,2 Otto Zhou1,2 and Lu-Chang Qin1,2 1
Curriculum in Applied and Materials Sciences, 2Department of Physics and Astronomy University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, U.S.A. 3 National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan
ABSTRACT For field-induced electron emission, the two factors that enable a high emission current density at low applied voltages are (a) low work function of the emitter and (b) sharpness of the emitter tip. We have developed and applied a chemical vapor deposition method to synthesize single-crystalline LaB6 nanowires for applications as point electron emitters. The crystallographic orientation of the grown nanowires can be controlled by the catalysts used in synthesis and their typical diameter is ranged from below 20 nm to over 100 nm. The nanowires’ tip is either hemispherical or flat top with rectangular cross-section depending on the catalyst being utilized. The field emission properties have also been measured from the single nanowire emitters and the results are discussed for applications as point electron sources used in high performance electron optical instruments such as the transmission and scanning electron microscopes. INTRODUCTION Lanthanum hexaboride (LaB6) has a boron octahedron-based stable three-dimensional network embedded with La atoms. This unique arrangement allows a combination of properties including low work function, low electrical resistivity, low volatility at high temperature, high melting point and high chemical resistance. [1,2] Thermionic electron source made of LaB6 is 10 times brighter and has 10 times longer service life than the conventional tungsten-based electron filament in the electron optical instruments such as the transmission electron microscope (TEM) and the scanning electron microscope (SEM). To pursue higher performance in these microscopy applications, field emission (FE) electron sources have been introduced to achieve an even higher brightness and lower energy spread. The brightness of an FE electron gun is proportional to the current density of electrons emitted from the cathode surface, which can be described by the zero-temperature Fowler-Nordheim (F-N) equation [3] E2 10.4 6.44 × 10 7 φ 3 / 2 2 J = 1.5 × 10 −6 exp( 1 / 2 − ) A/cm , φ E φ where φ is the cathode’s work function and E is the local electric field applied on the cathode tip, which is empirically expressed by E = V / 5r [9] with V being the applied voltage and r the radius of the cathode tip. The above relation indicates that, in order to obtain a higher emission current density, one would prefer a material with a lower work function and in the form of a needle with a smaller radius. For this purpose, LaB6, being a low work function metal, is a natural
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candidate for producing point electron sources. It was demonstrated that when providing the same magnitude of brightnes
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