ABSTRACT
Chun Li, Nishuang Liu, Longyan Yuan, and Guojia Fang Department of Electronics Science & Technology, School of Physics & Technology, Wuhan University, Wuhan, Hubei 430072, People’s Republic of China whulic@gmail.com and gjfang@whu.edu.cn
Although enormous morphologies of ZnO nanostructure have been reported, one-dimensional (1D) nanorod (nanowire [NW]) are thought to be the most common and important one for both fundamental physical property research and nanoscale device integration. From an application point of view, device performance will be reinforced if nanostructures can be shape-, size-, density-, and alignment-controllably-fabricated to a high degree of precision since it is directly related to how the nanostructures interact with each other optically, electronically, and mechanically. Furthermore, the oriented growth of nanorods will greatly facilitate device fabrication and save processing cost. Therefore, great efforts have been devoted to control the morphology and alignment of 1D ZnO nanorods (ZNRs). By virtue of their unique oriented morphology, aligned ZNRs with growth direction perpendicular or parallel to the substrate have been successfully demonstrated as UV nanolasers [5], photonic crystals [6], UV detectors [7], field emitters [8],and dye-sensitized solar cells [9]. Generally, epitaxial growth or a lattice-matched substrate is a basic requirement for the growth of aligned ZNRs. Sapphire has been commonly used as the substrate for vertical ZNR growth on which the growth is initiated, guided, and oriented by a metal catalyst particle. However, the presence of catalysts can not only be detrimental to the intrinsic physical properties of nanorods but also be harmful to their applications in nanodevices. Additionally, a sapphire substrate is insulating, relatively expensive, and not compatible with semiconductor-integrated techniques, which limited the applications of ZNRs in photonic and electronic devices. Therefore, the growth of aligned ZNRs on cheap and broad lattice-mismatched substrates is highly desirable. Recently, in order to reduce the lattice mismatch between the substrate and the ZnO crystal, an oriented ZnO thin film as a seed layer was deposited on the substrate before nanorod growth. Typically, well-aligned ZNR arrays perpendicular to the substrate can be obtained either by high-temperature (700-900°C) vapor-phase transport (VPT, also known as thermal evaporation) or by low-temperature (70-150°C) hydrothermal solution methods, thanks to the intrinsic lattice-matched crystal growth behavior of ZnO. As one of the most important applications, electron field emission (FE) based on nanomaterials attracts much attention. Quasi-1D nanomaterials, including nanotubes, NWs, and nanorods, with sharp
tips are a natural candidate for FE, which is of great commercial interest in vacuum microelectronic devices such as field emission displays (FEDs) as well as X-ray and microwave sources. Among various 1D nanostructured materials, carbon nanotubes (CNTs) have attracted extensive efforts since their discovery in 1991. CNTs are regarded as good candidates for electron emitters and generally show a low turn-on field about 0.5-2 V/µm and a high emission current density about several mA/cm2 due to their high aspect ratios, small tip radii curvatures, and low work functions. However, CNTs can be easily degraded in an oxygen ambience. As an oxide, ZnO nanostructure material shows much more thermal stability and oxidation resistance, which make it also a good candidate for cold cathode electron emission materials. In this chapter, we summarized some recent advances on ZnO-seed-layer-assisted synthesis of well-aligned ZNR arrays for FE applications. First, in order to synthesize Si-compatible, high-crystal-quality and high-aspect-ratio ZNRs with uniform diameter and length, we discussed the nanorod growth conditions and the seed layer effect for the growth of well-aligned NRs by the VPT approach. We also made a brief description of the most common hydrothermal solution method for ZNR array growth. Then, we focused on some basic aspects of FE properties of ZNRs arrays and summarized the reported techniques for achieving enhanced FE properties. Finally, we gave a short conclusion and the prospect of ZNRs for FE applications. 13.2 Synthesis of Zinc Oxide Nanorods
13.2.1 Vapor Transport MethodA considerable number of approaches, including VPT [10], metal-organic chemical vapor deposition (MOCVD) [11], electrochemical deposition [12], and hydrothermal solution techniques [13], have been demonstrated to synthesize aligned ZNRs. Among them, VPT, either based on the vapor-liquid-solid (VLS) or vapor-solid (VS) process, is considered to be the most widely used technique for controllable synthesis of aligned ZNRs. Compared with the hydrothermal solution approach, the vapor-phase-grown ZNRs are free of contaminations and have high crystal quality. The technique
based on VPT also has the advantages of a fast growth process, simple apparatus requirements, and large-scale production. In a typical VLS synthesis process, a metal catalyst thin film is used to facilitate the nucleation and oriented growth of aligned ZNRs on substrates with little lattice mismatch, such as GaN (1.8%), SiC (3.5%), and sapphire (18.4%, [01-10]ZnO || [11-20]sapphire), and even on a Si substrate, which shows a large lattice mismatch of 40.1%.The length, diameter, and density of ZNRs are usually dominated by the size of the catalyst particle and the thickness of the metal catalyst film. For catalyst-free growth of aligned ZNRs, a thin ZnO film serving as a seed layer was deposited onto a Si substrate not only to facilitate the nucleation of ZnO nuclei but also to decrease the lattice mismatch between the Si substrate and ZNRs. Usually, the growth process can be explained with the VS mechanism [14]. ZNRs can be grown on a variety of polycrystalline ZnO seed layers [15], even on a relatively poor-crystalline ZnO film thermally oxidized from a metallic Zn film with no preferred orientation growth. However, ZNRs grown on different seed layers exhibit obvious differences in growth rate, diameter, density, and alignment. The most common ZnO seed layer deposition method for the growth of well-aligned ZNR arrays was found to be pulsed-laser deposition (PLD) due to its high-quality film growth feature as well as efficient film growth speed advantage [16]. Typically, a powder mixture of ZnO and graphite with a molar ratio of 1:1 was placed in the closed end of a one-end sealed small quartz tube. A ZnO-coated Si substrate was placed onto a quartz plate that is a definite distance away from the evaporation source in the small quartz tube. Then, the small tube was pushed into a large quartz tube with the source positioned at the center of the furnace and the open end toward the gas flow. The furnace was heated at a rate of 25°C/min under a constant flow of 100 sccm Ar gas and held at 950°C for a different time duration. During the whole synthesis process the pressure in the tube was kept at about 200 Pa. Figure 13.1 (top) shows a schematic diagram of a tube furnace and the results of the substrate temperature effect on ZNR array growth by VPT [14]. It was found that the local substrate temperature will greatly influence the final morphologies of ZNRs. Figure 13.1 (bottom) shows the SEM images of typical four kinds of ZnO nanostructures: nanoridges, nanorods, nanorod-nanowall junctions, and nanotips obtained along different temperature gradient regions in the
furnace tube labeled A, B, C, and D with corresponding temperatures of 920-860°C, 850-790°C, 750-650°C, and 630-550°C, respectively. Below the temperature of 550°C (region E), there is no featured nanostructure growth except for a compact nanocrystalline film.
