Growth and Field Emission of Single‐Crystalline Hafnium Carbide Nanowires

Abstract

<100>-oriented hafnium carbide (Hf C) nanowires are synthesized on the graphite substrate by chemical vapor deposition. The dependence of temperature, Hf Cl 4 content, and flow rate of CH 4 gas on the morphology of nanowires is investigated herein. The results indicate that the Hf C nanowire often kinks to another growth direction at a high Hf Cl 4 content or a low temperature due to the destabilization of the catalyst droplet induced by either a faster growth rate or slower thermal activation. Reduced nanowire density and length are observed at the sample synthesized at a lower CH 4 flow rate. Based on the experimental results, high-crystallinity nanowires with high aspect ratios and high densities are prepared at a lower Hf Cl 4 content, higher temperature, and high CH 4 flow rate. The field-emission properties of a single Hf C nanowire are also characterized by assembling a single Hf C nanowire emitter. An extremely high field enhancement factor of 5.57 Â 10 6 m À1 is obtained herein.

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

“…The calculated values for TMCs ( x = 0) are within approximately ±0.5 eV from experimental values, and ±0.2 eV from theoretical values. − The differences from the experimental values are likely to be attributed to carbon defects in TMCs. − In the early stage of oxidation as x increases from 0 to 0.25, the work function greatly reduces for all these TMCs, which agrees well with our experimental results that the work function of HfC and ZrC reduces from 3.6 to 2.5 eV and from 3.6 to 3.02 eV, respectively. , In the case of HfC 1– x O x , x was estimated to be approximately 0.2 . In the experiments, − the ⟨100⟩ oriented nanowires were sharpened by field evaporation when surfaces other than the {100} surface may have appeared. Even if other surfaces are involved in electron emission, the tendency would be the same because the reduction in work function is brought about mostly by the increase of the Fermi energy, as shown later in Figure .…”

“…The difference in the surface energy caused LaB 6 to grow along a specific direction ⟨100⟩. , In an analogous manner to LaB 6 , we expected that group 4 and 5 TMCs could be made into nanowires because they also have a cubic structure (NaCl-type). Thus, we succeeded in fabricating HfC and ZrC nanowires growing in the ⟨100⟩ direction. − …”

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

“…Currently, W is used for the emitter tip of CFE electron guns because it can be processed to dimensions less than 100 nm, which is necessary to create a high electric field (>10 9 V m −1 ) and produce a fairly constant emission current. Many studies have been conducted to find electron source materials with superior emission properties, for example, single-atom tips, 4–6 carbon nanotubes, 7–9 and etched sharp tips of borides, 10–26 carbides, 27–39 and nitrides. 40–46…”

“…[1][2][3] Currently, W is used for the emitter tip of CFE electron guns because it can be processed to dimensions less than 100 nm, which is necessary to create a high electric eld (>10 9 V m −1 ) and produce a fairly constant emission current. Many studies have been conducted to nd electron source materials with superior emission properties, for example, single-atom tips, [4][5][6] carbon nanotubes, [7][8][9] and etched sharp tips of borides, [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] carbides, [27][28][29][30][31][32][33][34][35][36][37][38][39] and nitrides. [40][41][42][43][44][45][46] In particular, LaB 6 has been studied repeatedly since the 1970s because its low work funct...…”