The Influence of the Mechanical Compliance of a Substrate on the Morphology of Nanoporous Gold Thin Films

Author:

Shahriar Sadi1ORCID,Somayajula Kavya2,Winkeljohn Conner1,Mason Jeremy K.1,Seker Erkin3ORCID

Affiliation:

1. Department of Materials Science and Engineering, University of California—Davis, Davis, CA 95616, USA

2. Department of Mechanical and Aerospace Engineering, University of California—Davis, Davis, CA 95616, USA

3. Department of Electrical and Computer Engineering, University of California—Davis, Davis, CA 95616, USA

Abstract

Nanoporous gold (np-Au) has found its use in applications ranging from catalysis to biosensing, where pore morphology plays a critical role in performance. While the morphology evolution of bulk np-Au has been widely studied, knowledge about its thin-film form is limited. This work hypothesizes that the mechanical compliance of the thin film substrate can play a critical role in the morphology evolution. Via experimental and finite-element-analysis approaches, we investigate the morphological variation in np-Au thin films deposited on compliant silicone (PDMS) substrates of a range of thicknesses anchored on rigid glass supports and compare those to the morphology of np-Au deposited on glass. More macroscopic (10 s to 100 s of microns) cracks and discrete islands form in the np-Au films on PDMS compared to on glass. Conversely, uniformly distributed microscopic (100 s of nanometers) cracks form in greater numbers in the np-Au films on glass than those on PDMS, with the cracks located within the discrete islands. The np-Au films on glass also show larger ligament and pore sizes, possibly due to higher residual stresses compared to the np-Au/PDMS films. The effective elastic modulus of the substrate layers decreases with increasing PDMS thickness, resulting in secondary np-Au morphology effects, including a reduction in macroscopic crack-to-crack distance, an increase in microscopic crack coverage, and a widening of the microscopic cracks. However, changes in the ligament/pore widths with PDMS thickness are negligible, allowing for independent optimization for cracking. We expect these results to inform the integration of functional np-Au films on compliant substrates into emerging applications, including flexible electronics.

Funder

National Science Foundation

Publisher

MDPI AG

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