Fabrication and Characterization of Percolating Nanocluster Devices

Abstract

IntroductionInvestigating nanocluster properties is of strategic importance, since the properties of nanoclusters can be tuned by controlling their size which creates innovative materials and devices. Recently, research has been focused on palladium (Pd) nanoclusters because of possible applications in the fields of hydrogen storage [1,2], hydrogen detection [3], and catalysts [4,5]. Therefore, intensive research is required to develop nanocluster sources to produce more intensive nanocluster beam, and to provide further control of nanocluster size distributions [6,7]. Sputtering and inert gas-condensation is a standard candidate that combines both plasma sputtering and inert gas condensation without the complications of target heating [8]. In addition, plasma gas condensation is a predominant technology that can be used for large scale production by the industries to produce high quality metallic nanoclusters [9,10]. Nevertheless, only few systematic investigations were devoted to improve this technique to reach an optimal large scale production of nanoclusters [8,10,11].The growth of nanoclusters may involve either or both of the following processes: i) embryo formation via three-body collision (two sputtered atoms and an inert gas atom to remove their excess kinetic energy); and ii) nucleation of large nanoclusters by two-body collision through nanocluster-nanocluster collision and accepting atoms that have arrived to the nanocluster surface (atomic condensation) [12,13]. Hence, number of produced nanoclusters and nanoclusters' size are determined by the probability of the two-body and/or three-body collisions which are mainly dependent on the time nanoclusters spend within the growth region.Nanoclusters deposited for a device fabrication are arranged randomly to form a network between electrical electrodes. The electrical conduction between each pair of nanoclusters is determined by the nature of connection at the interface between nanoclusters. Rigorous research effort is needed to address the electronic transport in Pd nanocluster devices.In this work, the production mechanisms of Pd nanoclusters are investigated. In addition, the produced nanoclusters were deposited between electrical electrodes to study the charge carrier transport within the device. ExperimentalDC discharge magnetron sputtering technique was used to fabricate Pd nanoclusters inside an Ultra-High Vacuum compatible system (UHV) shown in Figure 1(a) [12]. UHV system consists of: i) dc magnetron sputtering source, ii) Quadrupole Mass Filter (QMF), and iii) deposition chamber. The plasma is ignited in a mixture of Ar and He inert gases by applying a dc voltage to create a dense vapor of atomic ions and small nanoclusters from the target. The nanoclusters are formed from the vapor by inert gas condensation within an aggregation region of length L. The aggregation length can be varied during the experiment to optimize nanocluster production. The size of the produced nanoclusters depends on the aggregation length, the type of inert gases, their...