Post-annealing effects on the structural and optical properties of vertically aligned undoped ZnO nanorods grown by radio frequency magnetron sputtering P. Sundara Venkatesh, a S. Balakumar b and K. Jeganathan * a We report the nature of point defects associated with the visible transitions and X-ray photoelectron emissions of post-growth annealed ZnO nanorods under vacuum and air atmospheres. The ZnO nanorods are vertically aligned along the c-axis with a hexagonal cross section. The compressive strain in the as-grown ZnO nanorods has been completely relaxed by the post-growth annealing under vacuum. The relative quantity of oxygen deficiencies in the as-grown and post-annealed ZnO nanorods is calculated from the X-ray photoelectron spectra. Despite high oxygen deficiencies, the intense bi- donor bound exciton emission with narrow full width at half maximum reflects good optical quality of the vacuum annealed ZnO nanorods. The additional green and red emissions are attributed to electron transitions owing to the oxygen mediated defects in the nanorods. 1. Introduction Zinc oxide (ZnO) is one of the most morphologically rich materials and crystallizes in manifold structures such as nanoribbons, 1 nanotetrapods, 2 nanobres, 3 nanosheets, 4 nanowires, 5 nanobelts 6 and nanoowers. 7 Various deposition techniques such as thermal evaporation, 4 chemical bath depo- sition, 8 metal–organic chemical vapor deposition, 9 spray pyrol- ysis 10 and sputtering 11 have been employed for the fabrication of various types of ZnO nanostructures. However, sputtering is one of the least investigated techniques for the fabrication of ZnO nanostructures since it is typically engaged for the deposition of thin lms. However, one can easily grow nanostructures by ne tuning of the growth parameters, particularly pressure and temperature. Recently, one-dimensional (1D) nanostructures of ZnO have received considerable attention as fundamental building blocks for optoelectronic devices such as light emitting and laser diodes in the short wavelength region 12 due to their wide and direct band gap of 3.37 eV with a large exciton binding energy (60 meV) at room temperature. 13 Their large exciton binding energy in comparison with the thermal energy at room temperature (26 meV) allows an ultraviolet (UV) lasing action to occur even at room temperature. 14,15 The lasing efficiency of ZnO depends on the quality of the material. In general, ZnO commonly exhibits different defect mediated emissions in the visible region in addition to a dominant UV emission owing to its band edge. However, the correlation of visible emissions with the point defects is unclear, especially the origin of green luminescence in ZnO has been attributed to several intrinsic point defects such as oxygen vacancies (V O ), 16–18 oxygen inter- stitial sites (O i ), 19,20 zinc vacancies (V Zn ), 21,22 zinc interstitial sites (Zn i ) 23 and antisites of zinc and oxygen. 24 Furthermore, an earlier report suggested that the visible emissions in the pho- toluminescence (PL) spectrum of ZnO may be attributed to the various types of point defects in the same peak positions. 25 Hence, a detailed investigation of the point defects is essential to identify the nature of the visible emissions. It is obvious that temperature dependent photoluminescence (TDPL) spectros- copy can be used to analyze the point defects and exciton recombinations in ZnO along with a quantitative analysis using X-ray photoelectron spectroscopy (XPS). In the present work, we have investigated the origin of the visible emissions and correlated these with the point defects in the as-grown (AG), vacuum annealed (VA) and air annealed (AA) ZnO nanorods. The relative oxygen deciency for the VA sample is very high compared with the AG and AA ZnO nanorods. The neutral donor bound exciton transition is found to be inde- pendent of the oxygen mediated point defects in ZnO nanorods. 2. Experimental section Vertically aligned ZnO nanorods were fabricated on n-type silicon (111) substrates by a radio frequency (rf) magnetron sputtering technique under a pure argon atmosphere using a a Centre for Nanoscience and Nanotechnology, School of Physics, Bharathidasan University, Tiruchirappalli 620 024, Tamilnadu, India b National Centre for Nanoscience and Nanotechnology, University of Madras, Chennai 600 025, Tamilnadu, India. E-mail: kjeganathan@yahoo.com Cite this: RSC Adv. , 2014, 4, 5030 Received 15th July 2013 Accepted 7th November 2013 DOI: 10.1039/c3ra43639j www.rsc.org/advances 5030 | RSC Adv. , 2014, 4, 5030–5035 This journal is © The Royal Society of Chemistry 2014 RSC Advances PAPER Published on 08 November 2013. Downloaded by University of Rhode Island on 02/01/2014 15:35:24. View Article Online View Journal | View Issue