Growth morphology evolution of ZNO nanowires synthesized by chemical vapor deposition

This chapter reviews the recent progress in the synthesis of ZnO nanowires (NWs) using vapor route, particularly chemical vapor deposition (CVD) since it is the most popular synthesis technique for ZnO NWs. The general requirements for the growth of vertically aligned ZnO NWs via heteroepitaxial approach and homoepitaxial approach will be discussed. The effect of CVD process parameters on the growth of catalyst-free ZnO NWs are investigated. These include distance between Zn precursor and substrates, oxygen flow rate, synthesis duration and amount of Zn powder. Since highly (00.2) oriented ZnO seeded layers had been used to facilitate the aligned growth of NWs, the effect of CVD process parameters, especially on the alignment of NWs could be clearly identified. Furthermore, the results show that the synthesis parameters are correlated with the supersaturation of Zn and/or O vapors and influence the morphology evolution of ZnO NWs. Thus, this systematic study provides a more in-depth analysis on the growth of catalyst-free ZnO NWs which is governed by Vapor-Solid (VS) mechanism.For comparison purpose, the growth of Au-catalyzed ZnO NWs synthesized using CVD is also discussed. The presence of Au alloy droplets (nanoparticles) has been proven to increase the growth rate of ZnO NWs. It is found that the growth of Aucatalyzed ZnO NWs is more appropriate to be explained by a combination of Vapor-Liquid-Solid (VLS) and Vapor-Solid (VS) mechanisms rather than the commonly reported VLS mechanism only. The VLS-VS mechanism is applicable for both tip-growth model and base-growth model of Au-catalyzed ZnO NWs. The competition between VLS and VS mechanisms in their effectiveness for capturing adsorbed Zn and O atoms will determine the final morphology of the ZnO NWs. The similarities and dissimilarities of the growth mechanism for both catalyst-free ZnO NWs and Au-catalyzed ZnO NWs will be discussed. Finally, different doping approaches, including their strengths and limitations in producing doped ZnO NWs will also be presented.