ABSTRACT
We investigate ex situ the kinetics of a thin, thermally evaporated gold layer on glass below and at the percolation thickness. Using grazing incidence ultra-small-angle X-ray scattering (GIUSAXS), we follow the domain formation during annealing well below the bulk melting point of gold. We observe a strong dewetting of the gold layer for thicknesses near the critical percolation thickness. We compare our findings with optical spectroscopy, which corroborates the GIUSAXS data. Synchrotron Radiation and Structural Proteomics Edited by Eugenia Pechkova and Christian Riekel Copyright © 2012 Pan Stanford Publishing Pte. Ltd. www.panstanford.com
8.1 INTRODUCTIONControlling the morphology of metal nanoparticle layers is of high technological and scientific interest. The broad field of applications of such layers includes catalysts (Valden et al., 1998; Renaud et al., 2003), solar cells (Westphalen et al., 2000), biorecognition (Elghanian et al., 1997), magnetic data storage (Lin et al., 2005), and optoelectronic devices (Bauer et al., 2003). The nanoparticle layer morphology is tuned to the specific application. This means either a fully established island geometry or a percolated or smooth system (Bauer et al., 2003; Roth et al., 2003, 2006b, 2007; Walter et al., 2006). Thus, one can create full-scale homogenous contacts (Kaune et al., 2009), color effects for anti-forging devices (Bauer et al., 2003), or increase the efficiency of organic solar cells (Catchpole and Polman, 2008). The latter is done by incorporating such noble metal nanoparticles into the photoactive organic film (Metwalli et al., 2008).Especially the two latter applications make use of the plasmon resonance in noble metal nanoparticles. This resonance stems from collective oscillations of the free electron gas in external electromagnetic fields (Sun and Xia, 2003) in the confined geometry of the nanoparticle layer. The oscillation leads to pronounced absorption bands in the visible light (Hulteen et al., 1997). Being a resonant phenomenon, it depends strongly on the nanoparticle layer structure and morphology. This influences the width and position of the plasmon resonance in metal layers. The size of the metal particles ranges from 1 nm to 100 nm.Essential for contact application are the questions of percolation and thermal stability of the metal layer. This means that an electrically conducting path exists in the metal layer. Moreover, for annealing well below Tm ripening and diffusion must be taken into account (Revenant et al., 2004). 8.2 ROUTES TO NANOSTRUCTURING
To produce a nanostructured metal layer, a multitude of preparation methods exists. Favored methods are creation from liquid colloidal solution or vacuum deposition. In solution casting, the solvent
evaporates and the colloidal nanoparticles are deposited on the surface (Bigioni et al., 2006; Roth et al., 2007, 2009; Siffalovic et al., 2008). The colloidal particles can be, e.g., spheres or triangles (Roth et al., 2009). Large-scale arrangements over several millimeters can be achieved (Siffalovic et al., 2008; Roth et al., 2010). To create regular arrangements, the self-assembly processes are exploited (Xia et al., 2000) during evaporation. The ordering takes place at the triple phase boundary contact line (Roth et al., 2007) and is strongly governed by thermodynamic fields and capillary effects (Haw et al., 2002; Müller-Buschbaum et al., 2006b).Vacuum deposition is another route employed in roll-to-roll applications (Bauer et al., 2003; Walter et al., 2006). Vacuum deposition includes thermal evaporation, sputter deposition, or pulsed laser deposition (Roth et al., 2003, 2006b; Biswas et al., 2006; Röder et al., 2008) to create large-scale nanostructured composite films for the above-mentioned applications. The methods strongly differ in the thermal energy of the metal atoms being deposited. In the case of sputter deposition, this energy is around 1 eV, while for thermal evaporation the kinetic energy is around 25 meV. The installed structures are different, as is seen in the case of polymer-metal films (Roth et al., 2003, 2006b), e.g., during thermal evaporation larger structures are installed.Further steps of nanostructuring include thermal annealing (Lin et al., 2005; Kashem et al., 2009), leading to hierarchically organized structures on multiple length scales. The structures installed strongly depend on the initial layer thickness. Processes like dewetting, Vollmer-Weber growth, and ripening may occur (Lazzari et al., 1999; Revenant et al., 2004). 8.3 SAMPLE PREPARATIONIn the approach described here, we investigated a basic two-layer system, namely gold (Au) on an amorphous substrate (glass) (Parrill et al., 1993; Naudon et al., 2000). We used thermal evaporation in a vacuum chamber (tectra GmbH). The glass slides had a dimension of 25 mm × 30 mm × 1 mm and were coated successively using a rotation stage in the vacuum chamber. The vacuum pressure was 4 × 10-5 mbar. The gold target was heated
using a current of I = 3.5 A and a voltage of U = 78 V, applied for a certain period tevap. The target-to-sample distance was about 350 mm. Different gold layer thicknesses d were achieved using different evaporation times tevap. The following gold layer thicknesses d were prepared: d = 3, 5, and 8 nm (d1, d2, d3). The critical thickness for percolation is around dc = 7 ± 1 nm for noble metals such as Ag and Au (Charton et al., 2001, 2004; Walter et al., 2002; Roth et al., 2003, 2004, 2006b). Thus, d covers the range well below to just above the percolation threshold. During thermal evaporation, the gold atoms self-assemble and form islands (Levine et al., 1989; Parrill et al., 1993; Naudon et al., 2000). At the critical thickness, percolation is observed and the optical and electrical properties change significantly.
