Pulsed laser de wetting of Au films: Experiments and modeling of nanoscale behavior

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Mikhail Khenner Department of Mathematics, Western Kentucky University, Bowling Green, Kentucky 42101

Ramki Kalyanaramana) Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996; Department of Material Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996; and Sustainable Energy Education and Research Center, University of Tennessee, Knoxville, Tennessee 37996 (Received 26 November 2012; accepted 27 March 2013)

Ultrathin metal film dewetting continues to grow in interest as a simple means to make nanostructures with well-defined properties. Here, we explored the quantitative thickness-dependent dewetting behavior of Au films under nanosecond (ns) pulsed laser melting on glass substrates. The trend in particle spacing and diameter in the thickness range of 3–16 nm was consistent with predictions of the classical spinodal dewetting theory. The early stage dewetting morphology of Au changed from bicontinuous-type to hole-like at a thickness between 8.5 and 10 nm, and computational modeling of nonlinear dewetting dynamics also captured the bicontinuous morphology and its evolution quite well. The thermal gradient forces were found to be significantly weaker than dispersive forces in Au due to its large effective Hamaker coefficient. This also resulted in Au dewetting length scales being significantly smaller than those of other metals such as Ag and Co.

I. INTRODUCTION

Self-assembling and self-organizing techniques are now well-established routes to manufacture nanoscale materials in one or more dimensions. In the context of making metallic nanostructures, the spontaneous dewetting of thin films under thermal treatment has been widely investigated.1–6 Among the possible heating sources, nanosecond (ns) laser pulses have proved to be important because of their ability to rapidly raise the metal temperature to beyond its melting point and also being able to capture intermediate stages of dewetting due to high quenching rates (;1010 K/s for ns pulses). As a result, several important fundamental principles of dewetting, first seen in polymeric systems,7–10 have been observed and confirmed in metallic systems. For example, in single layer metal films of nanometer thickness, dewetting of metal films via the classical spinodal instability has been quantitatively verified by a number of investigations. In spinodal dewetting, an initially smooth film is destabilized because attractive intermolecular forces, such as the Hamaker forces, overcome the stabilizing effect of surface tension.11 In such systems, the ambient/ film/substrate free energy behavior is reminiscent of that in immiscible binary systems with spinodal behavior, i.e., the free energy curvature has negative values over a a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.90 J. Mater. Res., Vol. 28, No. 13, Jul 14, 2013

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range of concentration, and in this region, the microstructure can spontaneously evolve int