A novel method for synthesis of well-aligned hexagonal cone-shaped ZnO nanostructures in field emission applications J. Rouhi a,b,n , M. Alimanesh c , S. Mahmud c , R.A. Dalvand c , C.H. Raymond Ooi d , M. Rusop a,b a Centre of Nanoscience and Nanotechnology (NANO-SciTech Centre), Institute of Science, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia b NANO-ElecTronic Centre, Faculty of Electrical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia c Nano-Optoelectronic Research (NOR) Lab, School of Physics, Universiti Sains Malaysia, 11800 Pulau Pinang, Malaysia d Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia article info Article history: Received 13 February 2014 Accepted 29 March 2014 Available online 5 April 2014 Keywords: Semiconductors ZnO nanocone arrays Electric field-assisted chemical bath deposition Electrical properties Field emission properties abstract Well-aligned and high-density ZnO nanocone arrays were prepared on Si substrates using an electric field-assisted chemical bath deposition (FECBD) method. The photoluminescence (PL) spectra of ZnO nanostructures indicate that near-band-edge (NBE) emission shifted slightly toward lower wavelengths with the increase in procedure temperature, and the intensity increased with improved ZnO crystal- lization. The cone-shaped ZnO nanorods had the lowest turn-on electric field, largest field-enhancement factor, and good stability, which were attributed to the small emitter radius at the tip, uniform density distribution, and high aspect ratio. These results indicate that the cone-shaped ZnO nanostructures fabricated by FECBD are promising for application as field emission (FE) electron sources. & 2014 Elsevier B.V. All rights reserved. 1. Introduction FE is a quantum tunneling phenomenon in which electrons are emitted toward an anode through a barrier (vacuum) by a very high electric field [1]. ZnO nanostructures with various morphol- ogies, such as nanotubes, nanowires, nanopyramids, and nanopins, have been widely studied for FE applications. ZnO is a wide band gap semiconductor with excellent chemical stability and high exciton binding energy. ZnO nanostructures of different morphol- ogies were recently synthesized by various techniques, including pulsed laser deposition, hydrothermal method, electrochemical method, sonicated sol–gel immersion, and so on [2,3]. To our knowledge, for the first time, well-aligned ZnO nano- cone arrays were synthesized using a simple and inexpensive EFCBD method and the FE properties of the as-grown ZnO nanostructure were studied. 2. Material and methods A ZnO seed layer was deposited on P-type Si substrates using a radio frequency magnetron sputtering system. Sputtering was performed at an incident power of 100 W for 1 h at 100 1C using a ceramic ZnO target with 99.99% purity. A two-electrode setup was used in the electrochemical deposition of ZnO nanostructures on Si substrates. Zinc nitrate hexahydrate (Zn(NO 3 ) 2  6H 2 O) and hexamethylenetetramine (C 6 H 12 N 4 ) were individually dissolved in DI water at equal molar concentrations at room temperature. The seed layer-coated ZnO/Si film and platinum electrode were immersed into the aqueous solution as the cathode and anode, respectively. During the growth procedure, the growth time and current density were maintained at 1 h and 0.5 mA/cm 2 , respectively. After the synthesis of ZnO nanocone arrays, the sample was rinsed with DI water and dried by nitrogen gas flow. The samples were characterized by FE scanning electron microscopy (FE-SEM, FEI Sirion 200) analysis. The optical proper- ties were studied at room temperature by PL spectroscopy (Jobin Yvon HR 800 UV, Edison, NJ, USA) which used He-Cd Laser 325 as a source. X-ray diffraction (XRD, Rigaku Ultima IV) was employed to investigate the crystallinity of the ZnO nanostructures. The FE measurements were performed in an ultrahigh vacuum system under a base pressure of 10 7 Torr at room temperature. The I–V behavior was automatically recorded using a self-developed signal acquisition system. 3. Results and discussion The FE-SEM images of the electrodeposited ZnO nanostructure arrays at various temperatures are shown in Fig. 1. During the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2014.03.183 0167-577X/& 2014 Elsevier B.V. All rights reserved. n Corresponding author at: Centre of Nanoscience and Nanotechnology (NANO- SciTech Centre), Institute of Science, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia. Tel.: þ603 55444412. E-mail address: jalal.rouhi@gmail.com (J. Rouhi). Materials Letters 125 (2014) 147–150