Applied Surface Science 270 (2013) 578–583 Contents lists available at SciVerse ScienceDirect Applied Surface Science jou rn al h om epa g e: www.elsevier.com/locate/apsusc Concentration dependent structural and optical properties of electrochemically grown ZnO thin films and nanostructures Trilok Singh a,b,∗ , D.K. Pandya a , R. Singh a a Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Haus Khas, New Delhi 110016, India b Institute of Inorganic and Materials Chemistry, University of Cologne, Greinstrasse 6, D-50939 Cologne, Germany a r t i c l e i n f o Article history: Received 2 December 2012 Received in revised form 11 January 2013 Accepted 11 January 2013 Available online 26 January 2013 Keywords: ZnO Electrodeposition Molar concentration Photoluminescence a b s t r a c t ZnO thin films and nanostructures have been grown on ITO and Si (100) substrates with varying con- centration of electrolyte by electrodeposition at low temperatures. The structural analysis showed an evolution of directional growth along (002) crystallographic plane with the change of electrolyte con- centrations. The peak position of defect-related bands depends upon the electrolyte concentration, and at higher molar concentration the dominant luminescent center corresponds to green emission. Fur- thermore, the nature of substrate also contributes to the origin of the different luminescent center. These results demonstrate that the electrolyte concentration and choice of substrate can tune the optical properties of ZnO thin films and nanostructures. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Zinc oxide (ZnO), an II–VI semiconductor, which is a direct wide bandgap (3.4 eV at room temperature) material with a large exci- ton binding energy (60 meV), has attracted interest, especially in the nanoscale science and nanotechnology [1–3]. It crystallizes preferentially in the hexagonal wurtzite crystal structure [4]. A lot of experimental investigations have been performed over the recent years owing to its unique optical, electrical and optoelec- tronic properties. Using bottom-up approach, ZnO nanostructures (nanorods, nanowires etc.) can be assembled into larger func- tional blocks and nanoscale electronics [5], electro-optic devices [6], nanogenerators [7], sensors [8], field-effect transistors [9] and solar cells [10]. ZnO nanostructures have been prepared by various process such as MOCVD [11], thermal evaporation [12], hydro- thermal [13], solution chemical [14] and electrodeposition [15–18]. Electrochemical deposition has many advantages over the high temperature process for its simplicity, low cost and easy control. It has been shown that the growth behavior and properties of pre- pared ZnO nanorods significantly depend on the substrate [19,20], electrochemical parameters [21,22], electrolyte [23,24] and tem- perature [25]. However, a detailed study of the effect of electrolyte concentration on the structural and optical properties of ZnO is missing in the literature. Therefore, in this article an attempt has ∗ Corresponding author at: Institute of Inorganic and Materials Chemistry, Uni- versity of Cologne, Greinstrasse 6, D-50939 Cologne, Germany. E-mail address: trilok.singh@uni-koeln.de (T. Singh). been made to synthesize the ZnO thin films and nanostructures with varying zinc nitrate concentration at low temperatures, using electrodeposition. A correlation between electrolyte concentra- tions and morphological, structural and optical properties of ZnO thin films and nanostructures has been proposed. 2. Experimental details Zinc nitrate [Zn(NO 3 ) 2 6H 2 O] was used as the precursor for ZnO and the zinc nitrate concentration was varied from 1 M to 1 mM. The electrolyte (bath) temperature was maintained 80 ◦ C. Electrochem- ical depositions of ZnO thin films and nanostructures were carried out at the deposition potential of -1.0 V (vs. SCE) for 5–300 min. After the deposition, the samples were removed from electrolyte and rinsed in deionized water. The morphology of thin films and nanostructures were obtained using Scanning electron microscopy (SEM, Zeiss, EVO-50) with operating voltage 20 kV. For the struc- tural studies, X-ray diffractometer (XRD, Philips Xpert Pro) using Cu K ( = 1.5405 ˚ A) radiation in 2 range 20–80 ◦ and the X-rays were generated by applying 45 kV voltage on anode material Cu at a cur- rent of 40 mA. The Photoluminescence (PL) spectra were obtained using 266 nm laser source (RPM 2000, Accent Optics, USA). 3. Results and discussion The reduction of nitrate ions into the solution generates OH - , which is subsequently captured by the Zn 2+ to produce its hydrox- ides or oxides upon dehydration. Therefore, the metals having high stability constants for its hydroxide species would favor the 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.088