Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America RESEARCH ARTICLE Advanced Science Letters Vol. 4, 1–8, 2011 Morphology Defined ZnO Nanostructures Through Microwave Assisted Chemical Synthesis: Growth Mechanism, Defect Structure, and Emission Behaviours Ma. de Lourdes Ruiz Peralta 1 , J. García Serrano 2 , and U. Pal 1 ∗ 1 Instituto de Física, Universidad Autónoma de Puebla, Apdo. Postal J-48, Puebla, Pue. 72570, Mexico 2 Instituto de Ciencias Básicas e Ingeniería, Universidad Autónoma de Estado de Hidalgo, C.U., Carretera Pachuca-Tulancingo Km. 4.5, Hidalgo 42184, Mexico Zinc oxide nanostructures of rod, twisted-needle, petals and flower-like morphologies could be grown by microwave assisted chemical synthesis with excellent reproducibility. Each of the morphologies evolved through controlling the pH of the reaction mixture in between 5.5 and 12.0 before microwave irradiation. Morphology, structural, optical and optoelectronic properties of the nanostructures have been studied using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, micro-Raman spectroscopy and photolumines- cence spectroscopy. It has been observed that both the crystallinity and defect structures in the nanostructures depend strongly on synthesis conditions. Mechanisms of morphology evolution and photoluminescence emis- sion behaviours of the nanostructures are presented. Keywords: ZnO Nanostrictures, Microwave Irradiation, Morphology Control, Defect Structure, PL Emission. 1. INTRODUCTION Zinc oxide is a wide band gap (3.37 eV at 300K) semiconductor with large exciton binding energy (60 meV) 1 with diverse appli- cations ranging from optoelectronic devices like light-emitting diodes, 2 solar cells, 3 Schottky diodes, 4 to chemical catalysis. 5 The control over size and morphology in semiconductors of nanometer and micrometer dimensions presents a real challenge for the design of novel functional devices. Depending on the syn- thesis techniques, ZnO nanostructures with different morpholo- gies like needles, 6 rods, 7 8 flowers, 9 and belts, 10 have been synthesized. However, one dimensional (1-D) nanostructures like nanowires and nanorods have received special attention due their applications as efficient gas sensors, 11–13 photocatalysts, 14 and intracellular nanosensors. 15 On the other hand, ZnO nano- structures of different morphologies have been tried to fabri- cate hybrid (organic-inorganic) and dye-sensitized solar cells (DSSCs). For example, Beek et al. 16 have utilized ZnO nanoparti- cles of 5.0 nm average diameter to blend with poly[2-methoxy-5- (3 ′ ,7 ′ -dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PVP) to produce hybrid photovoltaic cells with high fill factor and open circuit voltage. Jiang et al. 17 have utilized ZnO nanoflowers as ∗ Author to whom correspondence should be addressed. photoanode in DSSCs and revealed their higher solar conversion efficiency than ZnO nanowires. On the other hand, Cheng et al. 18 have utilized ZnO microflowers of hierarchical morphologies to fabricate quasi-solid state DSSCs of efficiency as high as 4.12%. Guillén et al. 19 could obtain DSSCs of 2.9% efficiency utilizing commercial ZnO nanoparticles and non-volatile ionic electrolytes of low-viscosities. Though the fabrication of those hybrid and dye-sensitized solar cells have been performed with inorganic ZnO nanostructures of different sizes and morphologies, using different organic dyes and solvents, 20 it is expected that the solar conversion efficiency of the fabricated devices would depend strongly on the specific surface area and hence the morphology of the nanostructures. Therefore, a general method for fabricating ZnO nanostructures of different morphologies and dimensions is essential for their applications in solar cells. On the other hand, before applying those nanostructures in devices like DSSC it is essential to evaluate their crystallinity, and defect structures. A large number of physical or chemical methods have been adopted for the synthesis of ZnO nanostructures. Most of the exotic morphologies of ZnO have been grown using physical techniques like chemical vapor deposition (CVD), 21 molecu- lar beam epitaxy (MBE), 22 and sputtering. 23 Nevertheless, wet- chemical routs 24 25 are preferred for the production of ZnO as Adv. Sci. Lett. Vol. 4, No. xx, 2011 1936-6612/2011/4/001/008 doi:10.1166/asl.2011.1998 1