http://journals.cambridge.org Downloaded: 02 Oct 2013 IP address: 92.99.63.126 Mechanisms of Ti nanocluster formation by inert gas condensation Ahmad I. Ayesh a) Department of Physics, United Arab Emirates University, Al Ain, United Arab Emirates Haya A. Ahmed Department of Chemical and Petroleum Engineering, United Arab Emirates University, Al Ain, United Arab Emirates Falah Awwad Department of Electrical Engineering, United Arab Emirates University, Al Ain, United Arab Emirates Samir I. Abu-Eishah Department of Chemical and Petroleum Engineering, United Arab Emirates University, Al Ain, United Arab Emirates Saleh T. Mahmood Department of Physics, United Arab Emirates University, Al Ain, United Arab Emirates (Received 14 May 2013; accepted 13 August 2013) The mechanisms involved in the formation of titanium (Ti) nanoclusters produced by sputtering and inert gas condensation were investigated experimentally and numerically. Ti nanoclusters were generated inside an ultrahigh vacuum compatible system under different source parameters, i.e., inert gas flow rate (f Ar ), length of the aggregation region (L), and sputtering discharge power (P). Nanocluster size and yield were measured using a quadrupole mass filter (QMF). The variation of the above source parameters enabled fine-tuning of the nanocluster size and yield. Herein, Ti nanoclusters were produced within the size range 3.0–10.0 nm. The combination between the nanocluster size and yield as a function of source parameters enabled understanding Ti nanocluster formation mechanisms, i.e., three-body and two-body collisions. The results show that two-body collisions dominate nanocluster production at low f Ar while the three-body collisions dominate at high f Ar . In addition, nanocluster size increases as L increases due to the increase in nanocluster nucleation and growth times. The maximum nanocluster yield was obtained at f Ar that maximize the probability of three-body and two-body collisions. Nanoclusters could be produced within an optimum range of the sputtering discharge power wherein the nanocluster size and yield increase with increasing the discharge power as a result of increasing the amount of sputtered material. The experimental results were compared with a theoretical model of nanocluster formation via three-body collision. Detailed understanding of the evolution of size and yield of Ti (and Ti-oxide) nanoclusters is essential for producing nanoclusters that can be utilized for environmental applications such as conversion of carbon dioxide and water vapor into hydrocarbons. I. INTRODUCTION Nanocluster production using inert gas condensation technique has become progressively popular for both basic and applied fields of material science. This technique re- presents a novel method for nanocluster production since it allows fine-tuning of nanocluster size, composition, as well as structure. 1 In addition, size selection of nanoclusters is possible since they are produced unsupported and mostly ionized. 2 Therefore, nanoclusters with particular sizes can be used as building blocks to generate self-assembled nanocluster devices. 3 Herein, a material is vaporized and the vapor meets a cooling inert gas that decreases the tem- perature of the vapor rapidly leading to a high supersatu- ration causing vapor nucleation and nanocluster formation. The source material can be vaporized by various methods including Joule heating, thermal plasma, laser ablation, and sputtering. Since the introduction of a nanocluster source that com- bines sputtering and inert gas condensation by Haberland et al., 4 various modifications were introduced as nanocluster sources to optimize the size and yield of the produced nanoclusters. This technique is highly versatile for material production since most metals and semiconductors can be sputtered using direct current (DC) or radio frequency (RF) sputtering regardless of their melting temper- atures. In addition, many unique advantages are associ- ated with this technique such as generation of nanoclusters without passivation layers, since they are produced inside a vacuum chamber, and production of nanoclusters with designated compositions by controlling the composition of the sputtering target. a) Address all correspondence to this author. e-mail: ayesh@uaeu.ac.ae DOI: 10.1557/jmr.2013.246 J. Mater. Res., Vol. 28, No. 18, Sep 28, 2013 Ó Materials Research Society 2013 2622