Delivered by Ingenta to: Pohang Gongkwa Daehakkyo (Pohang University of Science & Technology) IP : 141.223.167.71 Sun, 25 Jul 2010 10:19:14 Copyright © 2010 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 10, 3562–3565, 2010 Local Structural and Optical Properties of ZnO Nanoparticles Eun-Suk Jeong 1 , Hyo-Jong Yu 1 , Yong-Jin Kim 2 , Gyu-Chul Yi 3 , Yong-Dae Choi 4 , and Sang-Wook Han 1 ∗ 1 Division of Science Education, Institute of Fusion Science, and Institute of Science Education, Chonbuk National University, Jeonju 561-756, Korea 2 Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea 3 National CRI Center for Semiconductor Nanorods and Department of Physics, Seoul National University, Seoul 151-742, Korea 4 Department of Techno-Marketing, Mokwon University, Daejeon 302-718, Korea This study examined the local structural and optical properties of ZnO nanoparticles (NPs) with mean diameters of 4.5 and 70 nm using extended X-ray absorption fine structure (EXAFS) mea- surements at the Zn K edge and photoluminescence (PL) measurements. EXAFS revealed that the average bond length of atomic pairs in the NPs was shorter than that of the powder. Furthermore, a substantial amount of structural disorder existed in the NPs. From the PL measurements, we observed the direct band gap peak of 3.41 eV from the 70 nm ZnO NPs at low temperatures. This blue shift was related to the structural property changes. Keywords: EXAFS, Nanoparticle, ZnO, Structure, Photoluminescence, Quantum Confinement. 1. INTRODUCTION ZnO nanostructures have been studied extensively for pos- sible practical applications to nanometer-scale electronics and photonics including transistors, gas sensors, light emit- ting diodes (LEDs), ultra-violet (UV) sensors, piezoelec- tric applications, biosensors, and field emission devices. Recently, researchers have paid considerable attention to ZnO nanoparticles for their quantum confinement effects. Many studies reported observations of a quantum confine- ment effect of ZnO nanoparticles (NPs) 1–4 and nanowires 5 using photoluminescence (PL) and Raman scattering mea- surements. The previous studies reported that the ZnO free exciton recombination peak was shifted from 3.3 eV to ∼3.4 eV at low temperatures, as the ZnO NPs reduced to a few nanometers. This blue shift was attributed to the quantum confinement effect. However, the particle size was still much larger than the Bohr radius of ZnO, which is approximately 1.5 nm. The blue shift was even observed from the ZnO nanoparticles (NPs) with a mean diameter of more than 10 nm. 3 4 The blue shift could be attributed to a structural change because the energy band gap can be engi- neered by controlling the lattice constants of crystals. 6–8 ∗ Author to whom correspondence should be addressed. This study compared the local structural and the opti- cal properties of NPs. Field-emission tunneling electron microscopy (FE-TEM) is used to observe the atomic arrangement in certain areas. However, it has limited res- olution and cannot detect a small amount of lattice dis- tortion. X-ray diffraction (XRD) is a powerful tool for investigating the structures of crystalline materials but has limitations when examining nanomaterials due to a small number of scattering sources. Extended X-ray absorption fine structure (EXAFS) can reveal the bond lengths, the disorder of the bond lengths from a probe atom, the coordi- nation numbers, and the species of the atoms. 9 Therefore, EXAFS was used to quantify the local structural proper- ties of ZnO NPs with an average particle size of 4.5 and 70 nm. 2. EXPERIMENTAL DETAILS ZnO NPs, with two different sizes as shown in Figure 1, were fabricated with a solution process. 10 Zinc acetate in ethanol was boiled at approximately 80  C in air while vigorously stirring. In an ultrasonic bath, ZnO NPs were synthesized by supplying OH ions of lithium hydroxide into the zinc-ethanol solution at approximately 0  C. The 3562 J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 5 1533-4880/2010/10/3562/004 doi:10.1166/jnn.2010.2334