Synthesis and optical properties of ZnO nanostructures with different morphologies U. Pal a, * , J. Garcia Serrano a , P. Santiago b , Gang Xiong c,d , K.B. Ucer d , R.T. Williams d a Instituto de Fı ´sica, Universidad Auto ´ noma de Puebla, Apdo. Postal J-48, Puebla, Pue. 72570, Mexico b Instituto de Fı ´sica, Universidad Nacional Auto ´noma de Me ´xico, A.P. 20-365, C.P. 01000, Me ´xico D.F., Mexico c Pacific Northwest National Laboratory, P.O. Box 999, K8-88, Richland, WA 99352, USA d Department of Physics, Wake Forest University, Winston Salem, NC 27109, USA Available online 18 April 2006 Abstract ZnO nanostructures with different morphologies were grown by a low-temperature hydrothermal technique. The morphology, crys- tallinity and defect content in the nanostructures could be controlled by adjusting the synthesis conditions. Nanostructures prepared with optimum growth conditions were of good structural and optical qualities. Effects of growth conditions and thermal annealing on the optical properties of the nanostructures were studied by Raman and photoluminescence spectroscopy techniques. It is found that the nanostructures grown with particular initial and final pH values of the reaction mixture and air-annealed at about 250 °C are of best crystalline and optical quality. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Nanostructures; Optical properties 1. Introduction Semiconductor nanostructures of desired dimension and morphology are critical for the fabrication of optoelectronic devices. Zinc oxide is a direct band gap semiconductor (3.37 eV at room temperature) and has large exciton bind- ing energy (60 meV) with applications in the fields such as near-ultraviolet emission [1], gas and piezoelectric sensing [2,3], and as transparent electrode material for solar cells [4]. The optoelectronic properties of ZnO nanostructures depend strongly on their morphology, crystalline structure, defect and impurity contents. While several physical meth- ods like vapor-phase epitaxy [5], vapor–liquid–solid growth [6,7], vapor transport [8] and chemical methods like tem- plate-assisted growth [9] and solution growth [10,11] have been used to prepare ZnO nanostructures of different mor- phologies and of good structural and/or optical qualities, there are only a few reports on the synthesis of ZnO nano- structures of good structural and optical qualities through low-temperature synthesis process. Here we report the synthesis of ZnO nanostructures of different morphologies via low-temperature hydrothermal process with excellent reproducibility. The structure and composition of the nanostructures were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission elec- tron microscopy (TEM) and high resolution electron microscopy (HREM) techniques. The optical properties were studied by Raman and photoluminescence spectros- copy. Our study show that the nanostructures are of good crystalline quality with low structural and electronic defect contents. Air annealing of as-grown nanostructures at 250 °C for 2 h further improves their structural and opto- electronic properties. 0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.03.015 * Corresponding author. Tel.: +52 222 295500; fax: +52 222 295611. E-mail address: upal@sirio.ifuap.buap.mx (U. Pal). www.elsevier.com/locate/optmat Optical Materials 29 (2006) 65–69