Charge Transfer Within Multilayered Films of Gold Nanorods

Abstract

Controlling the charge transport across hybrid nanostructures composed of metal nanoparticles, carbon nanonostructures or quantum dots is very important for developing functional sensing, optoelectronic, and photovoltaic devices (Kamat, 2008; Katz & Willner, 2004). The unique electronic, optical, and catalytic properties of metal nanoparticles (1–100 nm), together with the large variety of the synthetic methods available for their synthesis with precise control over the shape and size, provide exciting possibilities for the fabrication of nanoscale assemblies, structures, and devices (Feldheim & Foss Jr, 2004). The selfassembly of nanoparticles into hybrid nanostructures is achieved by using functional molecular linkers which exhibit specific interactions with both the nanoparticles and the substrate. The most stable hybrid nanostructures can be built based on covalent interactions. For example, self-assembled monolayers (SAMs) of aliphatic chain thiols form well defined films on gold surfaces based on spontaneous covalent bonding to the substrate (Love et al., 2005). The charge transfer through these types of molecular linkers is characterized by a decay of the tunnelling current with the length of the linker, the tunnelling coefficient being around β≈0.8A-1 (Adams et al., 2003; Finklea, 1987, 1996). However, recent studies demonstrate that the tunnelling decay is not the dominant property of the films when the nanomaterials are tethered at the end of the molecular bridges determining a fast electron transfer process and moreover, this fast electron transfer process is dependent on the size and the shape of the nanomaterials (Bradbury et al., 2008; Chirea et al, 2009, 2010; Kissling et al., 2010). A more detailed research has to be developed in order to fully understand the mechanism behind this fast electron transfer processes taking place at electrodes modified with hydrid nanostructures. Various electrochemical techniques have been use for the investigation of the electron transfer processes taking place at hybrid nanostructured modified electrodes. Cyclic voltammetry, electrochemical impedance spectroscopy, square wave voltammetry, differential pulse voltammetry, potential step chronoamperometry and rotating disk electrodes are among the most used techniques for these electrochemical studies (Chen P Chirea et al, 2005, 2007; Hicks et al., 2002, Horswell et al.,2003). Electrochemical impedance spectroscopy (EIS) has been proven as one of the most powerful tools for the investigation of interfacial electrode reaction mechanisms (Katz & Willner, 2003, Yan & Sadik, 2001, Lasia, A. 1999). EIS measures the response (current and its phase) of an electrochemical system to an applied oscillating potential as a function of the frequency. The electrochemical behaviour of redox probes at hybrid nanostructures