Synthesis of ZnO Nanoneedles on Flexible Polymer Substrates at Room Temperature by Activated Reactive Evaporation

Abstract

Abstract: ZnO nanoneedles were successfully deposited on flexible polymer substrates at room temperature by activated reactive evapo-ration. Neither a catalyst nor a template was employed in this synthesis. These synthesized needles measured 500 - 600 nm in length and its diameter varied from 30 - 15 nm from the base to the tip. The single-crystalline nature of the nanoneedle was observed by high-resolution transmission electron microscopy studies. The Raman studies on these nanoneedles had shown that they are oxygen deficient in nature. A possible growth mechanism has been proposed here, in which the nanoneedles nucleate and grow in the gas phase by vapor-solid mechanism. Keywords: Nanoneedle, Activated reactive evaporation, ZnO, Gas phase, Nanostructures on flexible substrates. 1. INTRODUCTION Low dimensional nanostructured materials have attracted great interest because of their unique physical and chemical properties. ZnO, an important semiconductor of the II-VI group has many in-teresting properties such as wide band gap (3.37eV), large exciton binding energy, good piezoelectric characteristics, chemical stabil-ity and bio-compatibility [1, 2]. Nanostructures of ZnO have been widely investigated for potential application in UV laser, light emit-ting diode, sensors and solar cells [3-6]. Several techniques such as thermal evaporation, pulsed laser deposition (PLD), chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) are widely employed to synthesis one dimensional (1D) ZnO nanostructures [7-11]. In general, all the above mentioned methods require high processing temperature and crystalline substrates for nanostructure growth. Hence they cannot be employed for synthesizing nanostruc-tures on flexible polymer substrates. Recently there has been increasing interest on the growth of 1D nanostructures over non-wafer based substrates such as flexible polymer and glass using inexpensive processing techniques. The assembly of semiconductor nanostructures on flexible polymer substrates is important and might provide merits in terms of per-formance, flexibility, shock resistance, reliability and cost over the analogous devices built on Si wafers. Combining the high flexibil-ity of the polymer substrates with the structural and electrical prop-erties of the semiconductor nanostructures may open up new areas of application such as flexible electronic devices and solar cells. Currently ZnO nanostructures on flexible polymer substrates for different applications are synthesized by wet-chemical techniques using different precursors and expensive top to bottom pattern transfer methods [4, 12-15]. To the best of our knowledge there is no report on the synthesis of 1D ZnO nanostructures on polymer substrates using an evaporation process. ZnO nanoneedles have been synthesized on highly flexible polymer and indium tin oxide (ITO) coated glass substrates at room temperature by activated reactive evaporation (ARE). The advan-tage of this process over the conventional process is that it does not require a catalyst or high substrate temperatures for the growth of the nanostructures [7, 8]. In this process nanostructures were syn-thesized at room temperature, independent of the substrate and relatively at high vacuum (~10