First observation of dual electrical-optical percolation in metallic nanowire networks

Metallic nanowire (MNW) networks have attracted sustained interest due to their remarkable optical transparency, electrical conductivity, and mechanical flexibility, making them promising candidates for transparent electrodes in applications such as photovoltaics, touchscreens, electrochromic devices, transparent heaters, and low-emissivity coatings. Among MNWs, silver nanowires (AgNWs) have been the most extensively studied due to their well-controlled chemical synthesis and good stability. The strong link between the physical properties of MNW networks and their structural parameters-nanowire dimensions and network density-has been widely investigated, particularly regarding electrical percolation. We demonstrate in this article for the first time that in addition to the well-known electrical percolation, MNW networks also exhibit an optical percolation at a network density approximately six times higher. This optical percolation is revealed through the angular dependence of the infrared emissivity of AgNW networks with varying densities. When the network density exceeds an optical critical threshold, the AgNW networks display a distinctly metallic optical response. We further show, also for the first time, that AgNW networks above a critical density follow the Hagen-Rubens relation which was originally developed for metallic films. The ratio between critical network density for electrical and optical percolation was found to be independent of the diameter of the silver nanowire in the range of 58-112 nm. These findings provide new insights into the optical behaviour of MNW networks and offer valuable guidelines for optimizing their integration into industrial devices.