Thickness dependent growth of needle-like and flower-like ZnO nanostructures S. K. Panda Æ N. Singh Æ S. Pal Æ C. Jacob Received: 20 May 2008 / Accepted: 17 September 2008 / Published online: 14 October 2008 Ó Springer Science+Business Media, LLC 2008 Abstract Needle-like nanorods and micron-scale flower- like structures of ZnO were synthesized by thermal evap- oration of metallic zinc films with different thicknesses, followed by thermal annealing. Needle-like nanorods of ZnO were found through out the sample surface after annealing of the 1.3 lm thick Zn film. Three-dimensional crystalline nanorod-based flower-like structures of ZnO were also observed after annealing of the relatively thick (3.3 lm) Zn film. Thermal annealing of the Zn films was done at 800 °C in air for different time durations (30, 45, and 90 min). The flower size and number increase with increase in film thickness for the same annealing temper- ature and time. The X-ray diffraction results show that both the needle-like nanorods and flower-like structures are hexagonal wurtzite structure of ZnO. The room tempera- ture PL spectrum shows a strong defect related violet emission peak centered at 441 nm for both the structures. The possible growth process based on root growth tech- nique is proposed. 1 Introduction One-dimensional nanomaterials have attracted great inter- est because of their unique and fascinating optical, electrical, mechanical, and thermoelectrical properties together with their wide uses in fundamental scientific research and potential technical applications. Among these, ZnO is one of the most promising materials for short- wavelength light-emitting application since it has a high mechanical and thermal stability [1], a wide direct band gap (3.37 eV), low lasing threshold, high free-exciton binding energy (60 meV), friendliness to the environment, and cheapness. ZnO nanostructures can be used as gas sensors [2–6], ultraviolet (UV) and blue light emitting devices (LED) [7], laser diodes, etc. One-dimensional ZnO nanostructures have proved to be valuable candidates for field emission devices [8]. Furthermore, its piezoelectric property enables it to be used in nanoscale mechanical devices [9]. A variety of techniques have been used to synthesize ZnO nanostructures. Thermal evaporation, solid–vapor and high temperature vapor–liquid–solid (VLS) are often used to synthesize them. Some catalysts such as Au, Ag, Ni, Co nanoparticles influence growth [10] and structural/elec- tronic properties of ZnO nanowires. The use of catalysts may introduce some impurities which are undesirable for device applications. Recently, highly vertically aligned ZnO nanorods have been developed without using metal catalyst by metalorganic chemical vapor deposition (MOCVD) technique [11]. This process is very expensive. Annealing of the thermally evaporated Zn thin film in open atmosphere is a very inexpensive and simple method for the synthesis of ZnO nanostructures. Here, we describe how nanostructure of ZnO depends on the thickness of the Zn film and annealing time. 2 Experimental details ZnO nanostructures were synthesized on p-type Si(111) wafers by two-step processing. At first, the deposition of Zn thin films was performed on silicon substrates with various thicknesses, then annealing of the as-grown Zn films S. K. Panda Á N. Singh Á S. Pal Á C. Jacob (&) Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India e-mail: cxj14_holiday@yahoo.com 123 J Mater Sci: Mater Electron (2009) 20:771–775 DOI 10.1007/s10854-008-9800-4