Nanoparticles of ZnO obtained by mechanical milling L.C. Damonte a , L.A. Mendoza Ze ´lis a , B. Marı ´ Soucase b , M.A. Herna ´ndez Fenollosa b, * a Department of Physics, Universidad Nacional de La Plata, IFLP, CONICET, C.C.67 (1900) La Plata, Argentina b Dpto. de Fı ´sica Aplicada, Universitat Polite `cnica de Vale `ncia, Camı ´ de Vera s/n, 46071-Valencia, Spain Abstract Structural and optical characterization of mechanically milled ZnO powders are presented in this paper. It is shown that the application of mechanical milling is a very effective and simple technique to produce nanocrystalline powders, with the possibility of obtaining large quantities of materials. The milled powders are analyzed by X-ray diffraction, positron annihilation spectroscopy, scanning electron microscopy (SEM) and photoluminescence spectroscopy (PL). As milling proceeds, a clear reduction of grain size and homogenization are observed. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline semiconductors; Mechanical milling; Free volume; Solar cells 1. Introduction Zinc oxide and zinc-oxide-based powders have been widely studied recently because of their unique optical and electrical/electronic properties and thus they can be used in many demanding technological applications [1], such as low-voltage varistors [2,3]. In order to obtain high-quality zinc oxide powders with fine particle size, narrow size distribution and special morphology, various preparation techniques have been used to synthesize ultrafine zinc oxide powders [4,5], including precipitation, sol–gel, microemul- sion, etc. In general, nanocrystalline semiconductors with dimen- sion less than 100 nm represent a relatively new class of materials. Their short-range structures are essentially the same as bulk semiconductors but their optical and electronic properties are dramatically different. In partic- ular, they have been intensively studied because of their quantum size effects, photo catalysis, nonlinear optical properties and photoconductivity. They can be prepared in the form of dispersed colloids or trapped or stabilized within micelles, polymers, zeolites or glasses. Mechanical milling has proved to be an effective and simple technique to produce nanocrystalline powders and the possibility of obtained large quantities of materials. In this work, we present preliminary studies on mechanically milled ZnO powders together with its structural and optical character- ization. The milled powders were analyzed by X-ray diffraction, positron annihilation spectroscopy and optical measurements (photoluminescence). In addition, the evo- lution of annihilation parameters with milling time and annealing temperature is analyzed and related with the kind of mechanically induced defect involved. The effect of grain size on the optical properties of ZnO is discussed in terms of the deformation and defects generated during the milling. 2. Experimental procedure Commercially obtained ZnO powder (sized i500 nm, purity 99%) was milled in a steel cylinder (8 cm 3 ) with one steel ball (diameter 12 mm) with a ball to powder weight ratio of 14. The mechanical milling was performed in a horizontal oscillatory mill (Retsch MM2) operating at 25 Hz, during different milling times. As milling proceeds, a clearly reduction of grain size was observed. The milled powder was compacted under uniaxial pressure of 1.7 GPa into disk-shaped pellets (diameter 8 mm). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.09.014 * Corresponding author. E-mail address: mhernan@fis.upv.es (M.A. Herna ´ndez Fenollosa). Powder Technology 148 (2004) 15– 19 www.elsevier.com/locate/powtec