Journal of Alloys and Compounds 479 (2009) L11–L14 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Letter Dependence of photoluminescence peaks and ZnO nanowires diameter grown on silicon substrates at different temperatures and orientations Ramin Yousefi ∗ , Burhanuddin Kamaluddin Solid State Laboratory, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia article info Article history: Received 31 July 2008 Received in revised form 9 December 2008 Accepted 15 December 2008 Available online 20 January 2009 PACS: 78.67.Lt 78.55.Cr Keywords: ZnO nanowires Photoluminescence Blue-shift Orientation Aspect ratio Surface effect Lattice mismatch abstract ZnO nanowires were grown on Si(1 0 0) and Si(1 1 1) substrates by a simple physical deposition method in a conventional tube furnace without the use of any catalyst. The substrates were placed at different temperature zones. ZnO nanowires with different diameters were obtained at different substrate tem- peratures. Photoluminescence (PL) spectroscopy has been employed to study the optical properties of ZnO nanowires with average diameters ranging from 29 to 75nm. The shapes of the photoluminescence curves are dependent on the temperature and orientation of substrates. The ultraviolet photolumines- cence peak exhibited a blue-shift in position with a decrease in the diameter of nanowires. Large diameter nanowires tend to have more intense UV emission. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The ZnO is an important wide-band gap semiconductor and has a direct band gap (3.37eV) with high exiton binding energy (60 meV) larger than the thermal energy at room temperature. It presents itself as a promising material for the ultraviolet nano- optoelectronic devices and lasers operating at room temperature [1]. Understanding heteroepitaxial growth of nanometer size ZnO structures such as wires, rods, nanostructures and quantum dots on various substrates is important for assembling ordered arrays of these nanostructures into functional ultraviolet optoelectronic, nanoscale electronic devices and chemical sensor [2–15]. The study of the size effect on the properties of nanostructures is of great importance. The nanostructure materials show a high sur- face/volume ratio, which critically affects the electronic and optical properties, especially near the band gap of such materials [16]. The effect of size on the optical properties of ZnO nanorods [17], nanobelts [18] and nanoclusters [19] has been reported. Also Shal- ish et al. [20] has reported size-dependent surface luminescence in ZnO nanowires by a simple mathematical model and obtained ∗ Corresponding author. Tel.: +60 172909546; fax: +60 379674146. E-mail address: yousefi.ramin@gmail.com (R. Yousefi). a relationship between nanowire radius and luminescence peak intensity ratio. The size effect is expected to weaken as the sizes of the nanomaterial exceed that of the exciton Bohr radius, which is about 2 nm for ZnO [21]. One of the most important parameters to control the morphological and optical properties of ZnO nanostruc- tures obtained using the physical vapor deposition method is the substrate temperature [22–28]. Thus, various amounts of substrate temperatures can lead to ZnO nanowires with different sizes. In this work, we report the synthesis and characterization of high-aspect-ratio ZnO nanowires on silicon substrates with differ- ent orientations by a simple physical vapor deposition method at different substrate temperatures in a conventional tube furnace. Nitrogen was used as carrier gas, while no catalyst was applied on the substrates. We investigated the effect of silicon substrate orientations at different temperatures on the morphological and optical properties of ZnO nanowires. As a consequence, we obtained nanowires with different diameters, and investigated the size effect (aspect ratio (L/D)) of those on optical properties by photolumines- cence (PL) spectroscopy. 2. Experimental The ZnO nanowires were deposited in a conventional horizontal furnace. Silicon substrates were ultrasonically cleaned with ethanol and de-ionized water and then lightly etched using mixture of HF (33%) and water (1:10) for about 10 min to remove 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.147