Aligned Ultralong ZnO Nanobelts and Their Enhanced Field Emission

One-dimensional (1D) semiconducting nanoscale materials have attracted considerable attention because of their importance in understanding the fundamental properties of low dimensionality in materials as well as in nanodevice applications. Many methods, including vapor–liquid–solid (VLS), vapor–solid (VS), and solution-based, have been developed to synthesize 1D semiconducting nanoscale materials such as nanoscale wires, belts, rods, tubes, and needles. Usually, these methods require templates/catalysts and tedious operational procedures. Here, we demonstrate a new strategy for the growth of aligned ultralong ZnO nanobelts, yielding an average length of 3.3 mm and widths up to 6 lm, on metal substrates in a one-step process via molten-salt-assisted template-free thermal evaporation. These ultralong nanobelts show enhanced field emission. The electric field for an emission current density of 1 mA cm is 2.9 V lm, the lowest value ever reported for pure 1D ZnO nanostructures grown on flat surfaces, corresponding to a field-enhancement factor of about 1.4 × 10. This approach is simple, efficient, and inexpensive, which significantly facilitates device fabrication. By combining a general molten-salt process, which is usually used to prepare micrometer-scale ceramic powders (although it was also used for the synthesis of ZnO nanorods in a thermal evaporation process), we have designed a new approach, molten-salt-assisted thermal evaporation, and we demonstrate that this approach can produce aligned ultralong ZnO nanobelts over a large area. The key point of this new approach is the evaporation of Zn metal powder in a liquid environment of molten sodium chloride (NaCl) salt. A side-view camera photograph of the as-grown ZnO nanobelts on the Au substrate is shown in Figure 1a, indicating that the nanobelts can grow to several millimeters in length. Figure 1b shows a top-view optical microscopy photograph, demonstrating that the ZnO nanobelts are also transparent under an optical microscope. A higher-magnification optical microscopy image of the side-view is shown in Figure 1c, indicating nominal, though imperfect, alignment. Figure 2 shows field-emission scanning electron microscopy (SEM) images of the as-grown ZnO nanobelts under different magnifications. The low-magnification image shown in FigC O M M U N IC A IO N