Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters.

Nanoscale electron field emitters are known to produce more stable electron emissions than conventional emitters. This has been attributed to size effects; nanoscale emitters can operate with a small emission current and a low extraction voltage, which reduces the bombardment of residual gas ions on the emitter tip. However, our experiments discovered that nanoscale LaB6 emitters had extremely stable emissions, suggesting that chemical effects are present in addition to size effects. This suggests that during operations, a material other than LaB6 may be deposited on the surface of the tip to enhance the stability of emissions. Therefore, we searched for possible materials theoretically within the La-B-O ternary system and found that lanthanum oxides (LaO) and oxygen-deficient La2O3 (La2O3-x ) had good electrical conductivity and a low work function comparable to that of LaB6. These lanthanum oxides are chemically less reactive to residual gases than LaB6. Thus, if they are present on the LaB6 surface, they could stabilize electron emissions without diminishing the emission performance. These findings suggest that lanthanum oxides could be used for electron field emitters.

Reduction in Work Functions of Transition-Metal Carbides and Oxycarbides upon Oxidation.

Herein, the work functions of group 4 and group 5 transition-metal (Ti, Zr, Hf, V, Nb, and Ta) carbides and transition-metal oxycarbides (TMCOs) were investigated by first-principles calculations for their potential application as electron emitters. The work functions of both groups decreased as the substitution of carbon atoms with oxygen proceeded, and the reduction in group 4 was more than that of group 5. In particular, ZrC1-x O x and HfC1-x O x (x ≥ 0.25) exhibited work functions of less than 3 eV, which were comparable with those of LaB6- and ZrO-coated tungsten. The reduction in the work functions could be explained by the rigid-band model of the electronic density of states. The increase in valence electrons increased the Fermi energy, while it demonstrated a less significant influence on the vacuum potential, resulting in a reduction in the work functions. The phonon dispersion curves indicated that the NaCl-type group 5 TMCOs were less stable than the group 4 TMCOs. This agrees with the experimental findings that TaC1-x O x was not synthesized and NbC1-x O x was synthesized only for smaller values of x (i.e., x < 0.28). From the viewpoints of the work functions and structural stabilities, group 4 (Ti, Zr, and Hf) TMCOs exhibit better potential for application as electron emitters than group 5 (V, Nb, and Ta) TMCOs.

A stable LaB6 nanoneedle field-emission point electron source.

A material with a low work function exhibiting field-emission of electrons has long been sought as an ideal point electron source to generate a coherent electron beam with high brightness, long service life, low energy spread, and especially stable emission current. The quality and performance of the electron source are now becoming limiting factors for further improving the spatial resolution and analytical capabilities of the electron microscope. While tungsten (W) is still the only material of choice as a practically usable field emission filament since it was identified more than six decades ago, its electron optical performance remains unsatisfactory, especially the poor emission stability (>5% per hour), rapid current decay (20% in 10 hours), and relatively large energy spread (0.4 eV), even in an extremely high vacuum (10-9 Pa). Herein, we report a LaB6 nanoneedle structure having a sharpened tip apex with a radius of curvature of about 10 nm that is fabricated and finished using a focused ion beam (FIB) and show that it can produce a field emission electron beam meeting the application criteria with a high reduced brightness (1010 A m-2 sr-1 V-1), small energy spread (0.2 eV), and especially high emission stability (<1% fluctuation in 16 hours without decay). It can now be used practically as a next-generation field-emission point electron source.

Effect of defects in the formation of AlB2-type WB2 and MoB2.

Tungsten diboride, WB2, usually has a hexagonal structure with the space group P63/mmc (number 194); and molybdenum diboride, MoB2, has a trigonal structure with R3̅m (number 166). Other than these phases, both diborides are reported to have a phase with an AlB2-type structure (P6/mmm, number 191). AlB2-type MoB2 is easy to synthesize and has been extensively studied, whereas AlB2-type WB2 is very difficult to synthesize and has appeared only once in a report by Woods et al. in 1966 (Woods, H. P.; Wawner, Jr., F. E.; Fox, B. G. Science1966, 151, 75.) We have investigated these diborides by means of first-principles calculations and found that boron defects are responsible for the difference in their synthesizability. AlB2-type MoB2 became stable enough with some boron defects added, while AlB2-type WB2 became minimally stable, suggesting it may not actually exist. Following our calculations, we attempted to synthesize AlB2-type WB2 with the optimum quantity of boron defects but observed no trace of it. We conclude, from both calculations and experiments, that AlB2-type WB2 does not exist stably in the W-B phase diagram and that the compound produced in Woods et al.'s report might have contained some impurities.

Structural stability of boron clusters with octahedral and tetrahedral symmetries.

The structural stability of cagelike boron clusters with octahedral and tetrahedral symmetries has been investigated by means of first-principles calculations. Twenty-eight cluster models, ranging from B(10) to B(66), were systematically constructed using regular and semiregular polyhedra as prototypes. The binding energies per atom were, on the whole, slightly lower than those of icosahedral clusters B(80) and B(100), which are supposed to be the most stable in the icosahedral group. The larger clusters did not always have higher binding energies. Isothermal molecular dynamics simulations were performed to determine the deformation temperatures at which clusters began to break or change their structures. We found eight clusters that had nonzero deformation temperatures, indicating that they are in metastable states. The octahedral cluster B(18) had the highest deformation temperature among these, similar to that of icosahedral B(80) and B(100). The analysis of the electronic structure of B(18) showed that it attained this high stability owing to Jahn-Teller distortion.

First-principles study of the electronic structure and cluster formation in expanded liquid boron.

The electronic structure of liquid boron and cluster formation in expanded liquid boron have been investigated with first-principles molecular dynamics simulations. The calculated electronic density of states (DOS) exhibits a metallic feature, while liquid boron is known experimentally to be semiconductive. Since the DOS is not very sensitive to density, the electronic states near the Fermi level will consist mainly of dangling bonds, which explains the difference between the calculated and experimental results. Many types of clusters are formed in expanded liquid boron. This formation occurs in a very different way from that at low temperatures because expanded liquid boron has a high temperature and pressure that are close to the liquid-gas critical point. As the density is reduced, the coordination number in boron clusters decreases to about 2, indicating that the cluster geometry tends to be one- rather than two-dimensional, which is the most stable form at low temperatures. In fact, the analysis of small clusters proved that one-dimensional forms are dominant over two- and three-dimensional forms. This is because one-dimensional geometries have a more flexible structure and a high entropy value that consequently reduces the free energy at high temperatures.

Theoretical study of the stability of lithium atoms in alpha-rhombohedral boron.

The stability of lithium atoms in alpha-rhombohedral boron was investigated by first-principles calculations of total energies and molecular dynamics (MD) simulations. In the case of a low concentration (1.03 at. %), Li at the center of the icosahedral B12 site (the I-site) had a negative binding energy, which suggests Li at the I-site is unstable. However, MD simulations at temperatures below 750 K indicated that Li is still confined in the B12 cage under these conditions, which means Li at the I-site is metastable. Over 800 K, Li began to move away from the B12 site and settled at the tetrahedral site (the T-site) or at the octahedral site (the O-site). Li at the T-site also had a negative binding energy, but MD simulations indicated it was metastable up to 1400 K and did not move to other sites. Li at the O-site was energetically the most favorable, having a positive binding energy. In the case of a high concentration (7.69 at. %), the I-site changed to an unstable saddle point. At this concentration, the T-site was metastable and the O-site became the most stable. In MD simulations at 1400 K, Li atoms at the O-site never jumped to other sites regardless of concentration. Considering these facts, the diffusion coefficient of Li in alpha-rhombohedral boron would have to be very small below 1400 K.