Structure and Electrical Properties of an Assembly of Au Nanoclusters
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Structure and Electrical Properties of an Assembly of Au Nanoclusters G. Muralidharan, L. Maya and T. Thundat Oak Ridge National Laboratory Oak Ridge, TN-37831-6123. ABSTRACT Conduction through an assembly of nanosized clusters coupled by tunneling barriers is of significant interest both for understanding the fundamental physics involved and for potential applications. In this study, we describe a technique for preparing relatively large (dimensions of a few 100 µm to a few mm in size) monolayer films consisting of 3 nm diameter Au clusters coated with mercaptododecanoic acid, using low molecular weight-polymers as coupling agents. Electrical measurements of the assembly show non-linear characteristics. Below a certain threshold voltage, the current does not vary with an increase in voltage. Above this threshold voltage, current increases with voltage and can be described by a power-law relationship with an exponent close to unity. These characteristics of the I-V curve are discussed with specific reference to theoretical studies on conduction through an array of capacitance-coupled metallic islands and previous experimental results in similar systems. INTRODUCTION Recently, there has been significant interest in observing Coulomb blockade effects at room temperature in systems containing multiple tunnel junctions comprising of small, metallic clusters separated by tunnel barriers due to potential applications in nanoscale electronics [1-5]. Two criteria have to be satisfied for observing Coulomb blockade in such systems. Firstly, the Coulomb charging energy of the cluster or island (Ec = e2/2C) where e is the electron charge and C is the capacitance of the island, has to be larger than the ambient thermal energy kT. The second criterion to be satisfied is that the tunnel barrier between islands must have a tunnel resistance greater than the quantum resistance h/e2, where h is the Planck’s constant. Ligandstabilized metal particles offer an attractive method for synthesizing two-dimensional arrays of multiple tunnel junctions, since the ligand shells not only facilitate coupling amongst the metallic clusters but they also serve as tunnel barriers [6]. Various methods have been used to synthesize two-dimensional assemblies consisting of nm sized metallic clusters separated by ligand shells. Spin casting has been used to prepare ordered assemblies of size 450 nm consisting of 3.7 nm Au clusters [2, 7]. Biopolymer templates have been used to prepare two-dimensional assemblies consisting of 0.7 nm Au nanoparticles between electrodes separated by distances of 15 µm [3-5]. Room temperature electrical conductance of such an assembly showed evidence of Coulomb gap. In the current study, we present a methodology to prepare relatively large assemblies (100 µm – 2 mm in size) consisting of Au clusters coated with mercaptododecanoic acid. We also present room temperature electrical properties of such assemblies, which show evidence of a Coulomb gap. EXPERIMENTAL METHOD Au clusters coated with mercaptododecanoic acid were obtai
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