Positron annihilation characterization of nanocrystalline ZnO Mahuya Chakrabarti a, * , D. Jana a , D. Sanyal b a Department of Physics, University of Calcutta, 92 Acharya Prafulla Chandra Road, Kolkata, West Bengal 700009, India b Variable Energy Cyclotron Centre, 1/AF, Bidhannagar, Kolkata 700064, India article info Article history: Received 7 May 2012 Received in revised form 18 June 2012 Accepted 18 June 2012 Keywords: Positron annihilation technique Nanoparticles ZnO abstract Nanocrystalline (average crystal size w 3e4 nm) ZnO have been prepared by chemical route. Employing positron annihilation techniques these nanocrystalline ZnO have been characterized. Positron annihilation lifetime spectroscopy indicates the presence of large number of vacancy defects at the surface of these nanocrystaline ZnO. A large percentage of positronium formation has been observed in this nanocrystalline wide band gap semiconductor. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Semiconducting nanocrystals nowadays become very important due to their optical and electronics properties which can be modified by changing the size of the nanocrystals [1]. Among these nano-sized semiconductors, ZnO is one of the interesting candi- dates to produce light emitting devices in the ultraviolet region and is considered as an important material for the next generation LED [2]. ZnO also has a potential application as a Dye sensitized solar cell (DSSC) material [3]. The other important feature of ZnO is that it is a wide band gap semiconductor with strong excitonic emission in the ultraviolet regime with large exciton binding energy (w60 meV) [1]. Even at room temperature ZnO can be used as ultraviolet luminescent material. It is now possible to grow high quality large size ZnO crystals and also high quality nanocrystalline (crystal size w 2e5 nm) ZnO powder, which are essential for large scale industrial applications. Probing such high quality ZnO materials by suitable characterization technique is very important to understand the carrier driven processes, e.g., optical, transport, magnetic, etc., and can be a guide for further tuning of material properties for technological aspect. Defects play an important role in determining the optical, transport, magnetic properties of ZnO [4,5]. Characterization of such defects by positron annihilation spectroscopy (PAS) provides useful information regarding the nature and abundance of defects in a system like ZnO [6e13]. In literature there are number of reports [6e13] on positron annihilation characterizations in single crystalline ZnO and undoped polycrystalline ZnO with different crystal sizes. In the present work we have prepared nanocrystalline ZnO with average crystal size w3e4 nm by chemical route. We have employed positron annihilation technique to characterize the defect states of this nanocrystalline ZnO and the results have been compared with a high quality single crystal ZnO and “as received” powder ZnO. 2. Experimental outline The nanocrystalline ZnO of average size w3e4 nm has been prepared by chemical route. 1.1 g of Zinc Acetate [Zn(Ac) 2 .2H 2 O] has been dissolved in 50 ml boiling ethanol (Solution 1) at atmospheric pressure. Solution 1 is then directly cooled to 0  C. At room temperature this solution produces white powder of anhydrous zinc acetate. In a parallel process 0.29 g of Lithium Hydroxide [LiOH.H 2 O] has been dissolved in 50 ml ethanol in ultrasonic bath at room temperature to produce Solution 2. Solution 2 is then directly cooled to 0  C. This solution 2 is then added drop wise to solution 1 under vigorous stirring maintaining the temperature at 0  C. pH of the mixed solution has been noticed during addition of each drop. The solution becomes transparent after addition of approximately 0.1 g of LiOH. This solution is then kept at temperature <4  C for preventing rapid particle growth. To remove the reaction products (LiOH, H 2 O) from the required ZnO solution Heptane has been added to this ZnO solution with 2:1 (Heptane: solution) ratio. The supernatant of the solution has been removed by centrifugation. The collected ZnO precipitated has been re-dispersed in ethanol. This process has been repeated several times. The final product was then heated at 150  C for 15 min to remove the solvent. * Corresponding author. Tel.: þ91 33 23184462; fax: þ91 33 23346871. E-mail address: mahuyacuphy@gmail.com (M. Chakrabarti). Contents lists available at SciVerse ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2012.06.013 Vacuum 87 (2013) 16e20