UC Davis

Nanostructured materials offer tremendous opportunities for engineering advanced device components for diagnostic and therapeutic applications. One such material, nanoporous gold (np-Au), has found use in applications ranging from catalysis to biosensing, where pore morphology plays a critical role in performance. Np-Au is typically produced by a process known as dealloying, where immersion in nitric acid selectively removes silver from a gold-silver alloy and gold surface atoms diffuse at the metal-electrolyte interface arranging into a bicontinuous ligament network. While morphology evolution of bulk np-Au has been widely studied, knowledge about its thin film form, which is influenced by the underlying substrate, is limited. This thesis delves into the mechanical behavior of nanoporous gold (np-Au) thin films on substrates of varying mechanical compliance, focusing on the role of substrate stiffness controlled by the thickness of polydimethylsiloxane (PDMS) layers anchored onto a rigid glass slide. Using a finite element analysis (FEA) framework, the study simulates the deformation and strain energy characteristics of np-Au films, revealing a nuanced interplay between substrate compliance and film morphology. Simulation results indicate that the effective elastic modulus of PDMS, modulated by its thickness, critically affects the deformation patterns in np-Au thin films. At the film-substrate interface, simulations show that the np-Au/PDMS system undergoes significantly greater deformation than np-Au/glass, characterized by both in-plane compressive and out-of-plane vertical displacements. The study further presents a detailed analysis of strain energies, with the simulations uncovering that the total strain energy of the film-PDMS system decreases as PDMS thickness increases. Corresponding experiments performed in our group show that the decrease in strain energy is associated with the diminished presence of macroscopic cracks in the np-Au films on thicker PDMS substrates from experiments, as opposed to those on glass. The simulations also highlight that the distribution and intensity of microscopic cracks are contingent on the PDMS thickness, confirming experimental observations of the hierarchical crack formation in the np-Au/PDMS system and offering predictive insights into the mechanical stability of the films. In conclusion, the simulations provide compelling evidence that the mechanical characteristics of np-Au films can be finely tuned by adjusting the thickness of the anchored compliant substrate. This paves the way for engineering advanced materials with tailored morphological properties, optimizing np-Au thin films for applications in flexible electronics and wearable sensors.