Materials Science and Engineering B 177 (2012) 1452–1459 Contents lists available at SciVerse ScienceDirect Materials Science and Engineering B j o ur nal homep age: www.elsevier.com/locate/mseb Structural and optical characterization of mechanochemically synthesized copper doped CdS nanopowders P. Reyes ∗ , S. Velumani Departamento de Ingeniería Eléctrica-SEES, CINVESTAV-IPN, Zacatenco, D.F., C.P.07360, México, Mexico a r t i c l e i n f o Article history: Received 1 October 2011 Received in revised form 18 February 2012 Accepted 3 March 2012 Available online 28 March 2012 Keywords: Cu doped CdS Ball milling Nanopowder HRTEM a b s t r a c t Incorporation of copper into CdS crystals has been successfully prepared by mechanical alloying using a planetary ball mill. The powders are prepared with different milling times at 300 rpm with various Cu/Cd ratios from 0.1 to 25 at%. X-ray diffraction (XRD) analysis of milled powders showed peaks corre- sponding to hexagonal structure with a detection of phase transition to a cubic structure with increasing milling time. Grain sizes varied from 21 to 30 nm corresponding to different Cu/Cd ratios. Field emis- sion scanning electron microscopy (FESEM) images reveal agglomerated materials with particle size of approximately 28 nm (5 Cu at%) and layered structures caused due to the milling process. Powder com- position by energy dispersive analysis of X-rays (EDAX) reveals the incorporation of copper into the CdS. Micro Raman spectroscopy showed peaks approximately at 301 and 585 cm -1 corresponding to first and second order scatterings of longitudinal optical phonon mode. The LO mode at 301 cm -1 shifted towards lower wave number due to decrease of grain size by increase in milling time. From high resolution trans- mission electron microscope (HRTEM), the dominant phase of individual CdS nanocrystals was found to be hexagonal structure along with cubic structure. © 2012 Elsevier B.V. All rights reserved. 1. Introduction CdS is a II–VI n-type semiconductor with a direct band-gap of 2.42 eV and a hexagonal (stable) or cubic (metastable) structure. It has applications in laser materials, photo resistance, light emitting diode, nonlinear optical devices, etc. It is widely used as window material for solar cells [1]. Two basic requirements for the window material are the low electrical resistivity and high optical transmit- tance. An effective way to obtain CdS with these requirements can be achieved by the creation of Cd excess or S vacancies [2] with the incorporations of different impurities such as Cu [3], Fe [4] Ag [5], Al [6] and Ga [7]. Copper impurities behave as an acceptor in CdS, changing the resistivity, bandgap energy, photoelectrical proper- ties [3] and also changing the type of semiconductor from n to p (useful for CdS:Cu/CdS PV devices) [3]. Many methods have been used to incorporate dopants into CdS, some of these methods are: ion-exchange reaction [8], dry process [9], spray pyrolysis [10], and vacuum deposition [11]. Mechani- cal alloying (MA) can be considered as a simple, inexpensive and novel alternate process to obtain CdS with various Cu concen- trations. MA is a solid-state processing technique that involves welding, fracturing and rewelding of powder particles at atomic ∗ Corresponding author. Tel.: +52 55 5747 4001; fax: +52 55 5747 4003. E-mail addresses: pireyes@cinvestav.mx, itzam29@gmail.com (P. Reyes). level [12], capable of synthesizing a variety of stable and metastable alloy phases [13]. Some of the important parameters that have an effect on the final constitution of the powder are the milling speed, milling time and ball-to-powder weight ratio. The milling time is an important parameter and normally it is chosen to achieve a steady state between the fracturing and cold welding of the powder particles. The samples obtained from mechanical alloying are poly- crystalline with aggregates of crystallites forming grains with grain boundaries. The structure of the powders influences all its optoelec- tronic and optical properties. As an example, the photoconductivity response in polycrystalline materials composed of several micro- crystallites has an additional adsorption band beyond the band edge. This band is related with the decrease in crystalline size [6]. Different systems have been obtained by mechanochemical synthesis reactions. Experiments reported by Suryanarayana [12] shows successfully achieved reactions on binary and ternary com- pounds related with copper and aluminum (CuO, CuO + Al) using 2 h milling time. Taking this into consideration, we decided to use milling times equal or higher than 2 h. The prepared CdS nanopar- ticle powder can be deposited in films by spraying, curtain coating, roll coating, doctor blading, or screen-printing [13] for applications in low-cost thin-film solar cells or thin photoconductive layers. Although there are some reports on Cu doped CdS [3,11,14], so far to our knowledge this is the first attempt to make Cu doped CdS through mechanochemical alloying. Hence this paper focuses on the preparation and structural characterization of Cu doped CdS nanoparticles obtained by a simple mechanochemical route 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2012.03.002