Cryst. Res. Technol. 46, No. 9, 991 – 996 (2011) / DOI 10.1002/crat.201100141 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Structure-controlled growth of ZnO nanonails by thermal evaporation technique M. Senthil Kumar* 1 , D. Chhikara 2 , and K. M. K. Srivatsa 2 1 Physics of Energy Harvesting Division, National Physical Laboratory, Council of Scientific and Industrial Research (CSIR), New Delhi - 110 012, India 2 Materials Physics and Engineering Division, National Physical Laboratory, Council of Scientific and Industrial Research (CSIR), New Delhi - 110 012, India Received 5 April 2011, revised 30 June 2011, accepted 4 July 2011 Published online 15 July 2011 Key words nanostructures, thermal evaporation, electron microscopy, photoluminescence spectroscopy. Wurtzite ZnO nanonail structures have been grown on sapphire substrate by simple thermal evaporation of Zn powder in oxygen ambient. Growth parameters such as growth temperature and oxygen gas flow have been examined for the growth of nanonail structure. It is found that the nanonail structures repeatedly grow under a certain relation between the growth temperature and the oxygen flow. Also, at higher growth temperature, the nanonails grow in the form of branched-structures. The grown ZnO nanonails have hexagonally well-faceted cap and grow mostly perpendicular to the sapphire substrate. Excellent luminescence properties of a strong UV emission peak with negligible green band have been obtained at room temperature. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction ZnO is one of the most important semiconducting materials with excellent physical properties, such as wide direct band gap (3.37 eV) with large exciton binding energy (60 meV) and good piezoelectricity, that are useful for a broad range of applications in optoelectronics, electronics, sensors, photovoltaic, actuators, biomedical sciences and spintronics [1]. ZnO nanostructures have superior properties compared to bulk counterpart due to their small size. ZnO nanomaterials have been grown in a large number of interesting structures, like nanowires, nanorods, nanobelts, nanonail, nanocomb, nanocables, nanoribbons, nanosprings, nanorings, nanobridge, etc. by using various growth techniques such as metal organic chemical vapor deposition (MOCVD), thermal evaporation, pulse laser deposition , hydrothermal method and chemical vapor deposition on various kinds of substrates [2]. Consequently, various ZnO nanoscale-devices like transistor, LED, lasing device, electrochemical and biosensors, etc. have been developed based on these nanostructures [2-4]. Two major growth mechanisms such as vapor-liquid-solid (VLS) and vapor-solid (VS) have been proposed to understand the formation of different nanostructures. However, the factual relation among the formation of various kinds of ZnO nanostructures, growth parameters and ZnO inherent properties is still unclear. In the present work, we report catalyst-free growth of individual and branched ZnO nanonail structures by thermal evaporation of Zn powder in the presence of oxygen under various experimental conditions. Nanonail structure consists of a hexagonal cap at the top and a stem at the bottom. ZnO nanonails with their hexagonal cap have been studied for their optical cavity effects on whispering gallery modes and for field emission property [5,6]. Recently, Zang et al. have demonstrated the lasing action from p-type ZnO nanonail array/n-Si p-n heterojunction [7]. ZnO nanonails of various dimensions have been grown by thermal evaporation of Zn powder [6], carbothermal reduction of ZnO powder [8], and nanoparticle assisted pulsed laser deposition [9] under a wide range of experimental conditions. Lao et al. and Shen et al. have achieved growth of ZnO nanonails with additional indium vapor in the growth zone and reported that indium atom plays a curial role in determining the nanonail structure [8,10], whereas, many other researchers have grown ZnO nanonails without using any additional indium atoms [6,11]. Thus, the inter-relation of the experimental parameters that produces ____________________ * Corresponding author: e-mail: senthilmk@nplindia.org