In situ electrical modulation and monitoring of nanoporous gold morphology

The ability to fine-tune feature size in nanostructured thin films is critical, as many desirable properties of these materials are dictated by their nanostructure. Accordingly, there is a need for techniques that allow for modifying nanostructure while monitoring the morphological changes in situ. Here, we demonstrate a closed-loop electro-annealing system which enables in situ monitoring of morphology evolution in sub-micron nanoporous gold (np-Au) thin films. Np-Au is produced by a microfabrication-compatible self-assembly process that produces a network of interconnected ligaments with tunable diameter (10 s to 100 s of nanometers), making it a desirable material for numerous applications and fundamental studies alike. We specifically investigate the relationship between np-Au morphology (i.e., ligament diameter) and electrical resistance of the thin film. A strong correlation emerges between ligament size and electrical resistance, which puts forward resistance as an effective parameter for monitoring morphology evolution. Surprisingly, np-Au films with thicker ligaments lead to an increase in electrical resistance, which is unexpected since the extent of charge carrier scattering at the ligament surface should decrease with increasing ligament size. Further examination of np-Au morphology with high-resolution electron microscopy revealed grain growth on the ligaments in highly-annealed np-Au thin films. This suggests that grains act as scattering centers for charge carriers and this becomes the dominant mechanism in dictating electrical resistance in a percolated network of thin conductive ligaments.