Structure and Field‐Emission Properties of Sub‐Micrometer‐Sized Tungsten‐Whisker Arrays Fabricated by Vapor Deposition

Abstract

One-dimensional (1D) sub-micrometer/nanometer-scale materials have attracted great attention as a result of their unique properties and promising potential for a wide range of applications.[1] To date, a number of 1D materials, including carbon nanotubes (CNTs), [2] oxides (ZnO, [3] MoO x , [4] WO x [5] ), carbides (SiC, [6] W 2 C [7] ), nitrides (AlN, [8] GaN [9] ), and metals (Mo, [4a,10] W, [11] Co [12] ), have been comprehensively studied as candidates for field-emission (FE) applications. Among them, CNTs and oxide nanowires exhibited relatively low turn-on fields. However, the lack of adequate long-term and/or high-temperature FE stability and satisfactory mechanical properties has hindered their development in practical applications. The refractory metal tungsten, as the most important metallic material, has been used for FE applications (as cold and/or hot cathodes) for decades, not only because of its high electronic emissivity but also due to its exceptional thermal and chemical stabilities (such as the highest melting point, $3420 8C, and the lowest vapor pressure among all metals, its decent strength and rigidity at room and elevated temperatures, and its excellent corrosion resistance to metal and oxide vapors).[13] Presumably, 1D structures of metallic tungsten should be an ideal choice for FE applications.[11] In fact, previous studies have revealed that individual tungsten nanowires possess an ultrahigh fieldenhancement factor [11c,d] (two orders of magnitude higher than that of CNTs [2] ) and a high stability of the FE current density (at $10 6 A cm À2, a standard deviation of less than 1%, [11d] in contrast to 24% shown by individual multiwalled CNTs [2d] ). In the past few years, the techniques for the fabrication of tungsten nanowires or nanorods have been intensively developed. [11,14] Nevertheless, the controllable synthesis of 1D tungsten micrometer/nanometerscale structures for FE applications still remains a great challenge. For example, although high-temperature vapor deposition has been considered a promising process, [11e,14e] and comprehensively studied for synthesizing tungsten nanorod/nanowire arrays, how to control the configuration and crystalline structure of the products at the growth temperatures of $1500 8C is still unclear. In this Communication, we report an innovative, simple, and cost-effective vapor-deposition method for synthesizing submicrometer-sized tungsten-whisker arrays with ideal configuration and excellent FE properties. Figure 1a shows a scanning electron microscopy (SEM) image of a tungsten whisker array grown on a Si (111) substrate at 900 8C for 4 h. The synthesized whiskers have an average diameter of 180 nm (ranging from 80 to 300 nm) and a length of up to 20 mm. A high-magnification SEM image of a typical whisker (inset in Fig. 1a) shows that it is straight and has a hexagonal cross-section and pyramidal tip. Energy-dispersive spectroscopy (EDS) analyses of the whiskers reveal that they consist of pure elemental tungsten. Figure 1b is a ty...

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