International Journal of Mass Spectrometry 309 (2012) 176–181 Contents lists available at SciVerse ScienceDirect International Journal of Mass Spectrometry jou rn al h om epa ge: www.elsevier.com/locate/ijms Growth kinetics of Al clusters in the gas phase produced by a magnetron-sputtering source Zhixun Luo a , W. Hunter Woodward a , Jordan C. Smith a , A.W. Castleman Jr. a,b,∗ a Department of Chemistry, Pennsylvania State University, University Park, PA 16802, United States b Department of Physics, Pennsylvania State University, University Park, PA 16802, United States a r t i c l e i n f o Article history: Received 1 May 2011 Received in revised form 22 September 2011 Accepted 22 September 2011 Available online 1 October 2011 Keywords: Metallic clusters Size-controllability Magnetron-sputtering Cluster growth a b s t r a c t A magnetron-sputtering (MagS) cluster source was used to produce metal clusters of different size distri- butions by varying individual source parameters. Selectivity of the size of the Al clusters was observed and the mass distribution presents wide-range controllability by the MagS-source, which enables experimen- tal determination of the growth process of Al clusters. By implementing the extended Smoluchowski rate equation, here we propose an interpretation of the cluster-growth kinetics in the gas phase environment. A collision-dependent growing time domain is demonstrated. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Cluster formation processes are typically very fast; hence it is difficult to directly observe the initial real-time growing pro- cess [1–7]. In this regard, despite tremendous advancements in cluster science, the underlying mechanistic processes leading to micro-scale assembly, crystal formation, and cluster growth have not been fully explored [8–11]. Recent advances in cluster science have inspired considerable interest in the fundamental principles of cluster growth [12–16]. A better understanding of the growth kinet- ics of all-metal clusters, which have demonstrated advantageous properties for use in catalysts and cluster-assembled materials, would greatly expedite the eventual large-scale use of these sub- nanoscale materials. In the case of clusters deposited onto a solid substrate [17–20], the growth processes can be investigated via high-resolution microscopy to monitor the early stages of the phenomenon [21,22]; additionally, the growth mechanism of liquid clusters can be identi- fied using the Wolde–Frenkenl cluster definition [23,24]. However, although growing processes may be observed in these examples, the size-regime of clusters makes it difficult to monitor the ini- tial stage. A number of subtle questions regarding the growth of ∗ Corresponding author at: Department of Chemistry, Pennsylvania State Univer- sity, 309 Chemistry Building, University Park, PA 16802, United States. Tel.: +1 814 865 7242; fax: +1 814 865 5235. E-mail address: awc@psu.edu (A.W. Castleman Jr.). clusters remain undetermined and are still debated due to a lack of evidence on how burgeoning clusters initially begin to grow. Indirectly observing the growth kinetics of gas-phase clusters may provide useful information in the fields of surface, liquid, and gas-phase nanomaterials. Because of the difficulty in a direct approach, a controllable source that provides uniform distributions of the target atoms or small clusters is an important precondition [25]. Among the several available sources at present, most utilize the vaporization of a metal in a relatively high pressure volume and a subsequent expansion into a low pressure volume [26–37]. However, the ability to produce different cluster distributions is limited in almost all of these techniques due to a source that is non-modifiable without interrupting the experiment. This creates a cluster distribution that is almost entirely fixed. Alternatively, a cluster distribution can be obtained by employing a magnetron sputtering technique which has been widely utilized to perform atomic layer deposition and produce thin film materials [38–44]. The appeal of this source in the current study is that it operates under several variable parameters that affect the cluster distri- bution in some way, and thus allows one to indirectly observe the influences that affect cluster growth. Herein, we employ a magnetron-sputtering (MagS) cluster source to investigate the growth kinetics of gas-phase clusters. Variable size distributions were observed and several parameters of the MagS-source system affecting the mass distributions were identified. These parame- ters produce distributions which mimic the theoretical modeling results based on the extended Smoluchowski rate equation [45,46]. Such experimental and theoretical investigations provide detailed 1387-3806/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ijms.2011.09.016