Influence of mechanical compliance of the substrate on the morphology of nanoporous gold thin films

Nanoporous gold (np-Au) has found use in applications ranging from catalysis to biosensing where pore morphology plays a critical role in performance. While morphology evolution of bulk np-Au has been widely studied, knowledge about its thin film form is limited. This work hypothesizes that 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 (10s to 100s of microns) cracks and discrete islands form in the np-Au films on PDMS compared to glass. Conversely, uniformly-distributed microscopic (100s of nanometers) cracks form in greater numbers in the np-Au films on glass than 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.