http://journals.cambridge.org Downloaded: 04 Jul 2012 IP address: 178.250.250.21 ARTICLES Fabrication of size-selected bimetallic nanoclusters using magnetron sputtering Ahmad I. Ayesh, a) Naser Qamhieh, and Saleh T. Mahmoud Department of Physics, United Arab Emirates University, Al Ain, United Arab Emirates Hussain Alawadhi Department of Applied Physics, University of Sharjah, United Arab Emirates (Received 11 March 2012; accepted 31 May 2012) Copper–tin (Cu x Sn 1Àx ) nanocluster is a promising system for gas sensing applications, mainly because of its sensitivity and selectivity for H 2 S. In this work, pure Sn and Cu as well as composite Cu x Sn 1Àx nanoclusters were synthesized using the dc magnetron sputtering gas condensation technique. Nanoclusters with different Sn to Cu ratios were produced by changing the ratio of Sn and Cu in the target. The dependence of Sn, Cu, and Cu x Sn 1Àx nanoclusters’ size distribution on various source parameters, such as the inert gas flow rate and aggregation length, has been investigated in detail. The results show that as the inert gas flow rate increases, the mean nanocluster size increases for Sn, decreases for Cu, while increases and then decreases for Cu x Sn 1Àx . The results could be understood in terms of the contribution percentage of the nanocluster formation mechanism. Furthermore, this work demonstrates the ability of tuning the Cu x Sn 1Àx nanoclusters’ size and composition by a proper optimization of the source operation conditions. I. INTRODUCTION Metal nanoclusters are the subject of numerous studies for applications in several fields, and the attention dedicated to this topic is witnessed by the increasing number of related contributions within the material science community. These nanoclusters exhibit striking optical properties 1–3 and super- paramagnetism with enhanced coercivity and a shift of their hysteresis loop. 4,5 Tin (Sn) has been used since early 1960s as a solid-state gas sensor, and it became the main material commercially available in sensors for detecting fuel gas, carbon monoxide, general purpose combustible gases, ammonia, water vapor, etc. A considerable enhancement in the sensitivity was reported for Sn alloyed with noble metals. 6,7 Furthermore, alloying Sn thin films with copper (Cu) nanoclusters was found to enhance their sensing characteristics for gases such as H 2 S considerably. 8–14 This makes such material particu- larly useful in industries where, e.g., leak detection and process control are significant issues. Therefore, in addition to the large surface area of Sn nanoclusters, alloying them with Cu is expected to further improve their sensing properties. However, to use those nanoclusters for applications, precise control of their size, yield, alloying concentration, and of size distribution is needed. The present investigation focuses on the synthesis of pure Cu, pure Sn, and composite Cu–Sn nanoclusters using the magnetron sputtering technique inside an ultra-high vacuum (UHV) compatible system. This method of prepar- ing composite Cu–Sn nanoclusters has many advantages: (i) nanoclusters are produced with a narrow size distribu- tion; (ii) the average nanocluster size can be tuned by controlling the source conditions; (iii) no passivation layer is formed on the prepared nanoclusters, which provides direct access to their surface properties; (iv) a large portion of the produced nanoclusters are ionized, which allows size selection; (v) nanoclusters are self-assembled directly on a substrate without additional experimental stages; and (vi) the composition of nanoclusters can be controlled easily by varying the percentage of the metals within the sputtering target. In this work, we investigate the composition of the pro- duced nanoclusters and how to control their size and number by changing the source parameters. To the best of our knowl- edge, this is the first work that investigates the production parameters of composite Cu–Sn nanoclusters using the dc magnetron sputtering gas-condensation technique. II. EXPERIMENTAL Cu–Sn nanoclusters were synthesized using a magnetron sputtering plasma aggregation source inside the UHV com- patible system. 15,16 The system consisted of two chambers: the source and main chambers, which were evacuated to a base pressure of ;10 À8 mbar. The dc magnetron-type dis- charge was utilized to generate nanoclusters from a target using a discharge power (P) of 15 W unless it is stated otherwise. For pure nanoclusters, Sn and Cu targets with a) Address all correspondence to this author. e-mail: ayesh@uaeu.ac.ae DOI: 10.1557/jmr.2012.205 J. Mater. Res., 2012 Ó Materials Research Society 2012 1