Morphology dependent electrical transport behavior in gold nanostructures
Introduction
Usage of conductive thin-film substrates and electrodes has grown substantially in the last decade. These conductive thin films are used in recent research for applications such as conductive electrodes and catalytic surfaces [1], [2], surface plasmon resonance substrates [3], [4], solar cells [5], and patterning lithography in nanoscale structures [6]. Other applications include DNA mediated mono and multilayer [7], disposable biosensors [8], and DNA sensors [9]. As the films decrease in thickness, structural and electrical properties of such films changes significantly. Structural investigation of sputtered and evaporated gold (Au) thin films has shown to be discontinuous, and randomly distributed as nanoparticles along the surface before a certain threshold at a thickness of few hundred angstroms. Only when the film thickness is greater than 30 nm do the films become completely continuous [10], [11], [12], [13], [14], [15], [16]. Electrical transport in such metallic films; both in-plane and cross-plane, differs markedly from that of their bulk counterparts depending on the film thickness. When approaching the electron mean free path (EMFP) an abrupt increase in resistivity is noted and often attributed to different scattering mechanisms [17], [18], [19]. As the thickness decreases, surface scattering, grain boundary scattering and roughness play an important role [20]. Dominating scattering mechanisms are tightly related to the film structure [21].
Non-ohmic behavior of discontinuous thin Au films was studied in the past. Tunneling models of conduction were suggested to interpret the electron transport in such films [22], [23], [24], [25]. In such studies the experiments used are usually macroscopic in nature, which makes it difficult to correlate the electrical transport behavior to the local structure of the thin film [10], [26], [27]. In the vast majority of recent experimental studies using Atomic Force Microscopy (AFM), where non-ohmic behavior is observed, results are usually interpreted as either tip/sample mechanical interaction effects, or oxide or tip bio-contamination effects [28], [29], [30], [31]. The correlation between non-ohmic behavior observed at the nanoscale (i.e. using AFM) for thinner Au films and their discontinuous nature is not stressed in literature. In a previous study, the authors employed Conductive AFM (C-AFM) to show a non-ohmic behavior of Au ultra-thin films deposited on glass [32].
In this work, in-plane electrical conduction behavior in Au ultra-thin films of dispersed nano Au particles is studied in relation to the film morphology, showing the evolution of film structure and its effect on the electrical transport behavior. A threshold between ohmic and non-ohmic behavior transitions is recorded at film thicknesses below 39 nm, indicating different film structure during the film evolution. We show the effect of dispersed Au nanoparticles in lower film thicknesses and comment on its effect on the change of ultra-thin Au films. Simmons's model is used to describe the observed non-ohmic behavior, attributed to tunneling in the nano-scale. Using the proposed model we describe two different tunneling mechanisms in Au nano-particle structure thin films. We also derive and explain the effect of grain size, and average distance between grains on the transport behavior, especially as the film becomes discontinuous with random nanoparticles distributed along the film sheet in early film growth stages.
Section snippets
Experimental setup
Au films of thicknesses 5–140 nm are deposited using physical vapor deposition on soda-lime glass substrates. The sputtering targets are 99.999% Au. The samples are prepared under 0.133 Pa residual Argon pressure and 50 W RF power during deposition. The deposition rate of sputtered material is ~ 0.5 Å/s for all samples.
Quanta-FEG scanning electron microscope (SEM) is employed for high resolution imaging of different Au films. Both Secondary Electron (SE) detector for structural analysis and
Modeling
I–V curves shown are typical curves obtained for all samples with thicknesses below 34 nm. The curves show symmetric hyperbolic sine curves for 23 and 34 nm thicknesses, see Fig. 4 (a), and asymmetric curves with zero current region for thicknesses below 23 nm, see Fig. 4 (b). I–V curves in Fig. 4 (a) are characteristic of the direct tunneling regime. In fact, according to Stratton [37], the direct tunneling current iswhere h is Planck's constant and τ is the tunnel traversal time
Summary
In the process of growing gold films, the structure starts as discontinuous, with spread metallic islands, and as the film thickness increases, the film starts to become continuous. This structural progression in the film – from discontinuity to continuity – is observed to affect the electrical transport behavior significantly. Local I–V curves revealed an ohmic behavior in higher thicknesses and a non-ohmic behavior in lower thicknesses with the transition happening between 34 and 39 nm. In the
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