Field emission characteristics of polycrystalline LaB6 single tip and surface electric field distribution

Lanthanum hexaboride (LaB₆) as an excellent thermionic electron emitter was characterized by high brightness, thermal stability, low volatility and high mechanical strength [1]. The advantages are originated from its low work function (2.4 eV~2.6 eV), high melting point (2715 °C) and covalent bond of B-B atom in crystal structure. Recently, many researchers focused their attention on the excellent field emission properties of LaB₆ one-dimensional nanoelectronics as well as nanomachine applications . Late et al.[3] have reported the LaB₆ tips and foils grown on tungsten and rhenium substrate by Pulsed laser deposition (PLD) at temperature 700°C with average grain size ~ 125nm. However, there are some disadvantages of above-mentioned methods. The BCI₃ as the boron source is easy to absorb water and convert into boric acid, which resist the reaction. The stoichiometry of the products was hard to control by gaseous starting materials. For thin films, there have some intrinsic disadvantages, including poor adhesion to the substrate and oxide contained in the emitter. Compared to the nanowires and nanofilms, the bulk materials can provide large size, low cost, simple preparation. Therefore, there are great research prospects for polycrystalline LaB₆ Field emission properties. The polycrystalline LaB₆ bulk material was prepared by SPS method. The LaB₆ field emitting single tip has been achieved using the electrochemical etching method. Figure 1 shows the relative density of 95.5% and 99.2% corrosion morphology. It is seen from the figure 1 that the low-density samples have a large number of corrosion holes and surface defects, showing anisotropic corrosion characteristic. This is due to the corrosion process, the etching solutions immerse to the holes to lateral erosion, which directly led to the tip radius of about 10μm. The relative density of 99.2% of tip shows that cor- osion of the sample is uniform and surface is smooth, the tip radius is about 2μm. Fig. 2 shows the tip field emission current and applied voltage curves. It is seen that when the voltage is 1200 V (electric field strength of 12 V / μm) there occurs emission current and the current increased with the voltage When the voltage is increased to 1700 V (electric field strength of 17 V / μm), the emission current was 16 μA and don´t reach saturation. Fig.3 shows the results of surface electric field distribution.