A Low-Temperature Annealing Method for Alloy Nanostructures and Metasurfaces: Unlocking a Novel Degree of Freedom.

The material and exact shape of a nanostructure determine its optical response, which is especially strong for plasmonic metals. Unfortunately, only a few plasmonic metals are available, which limits the spectral range where these strong optical effects can be utilized. Alloying different plasmonic metals can overcome this limitation, at the expense of using a high-temperature alloying process, which adversely destroys the nanostructure shape. Here, a low-temperature alloying process is developed where the sample is heated at only 300 °C for 8 h followed by 30 min at 450 °C and Au-Ag nanostructures with a broad diversity of shapes, aspect ratios, and stoichiometries are fabricated. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses confirm the homogeneous alloying through the entire sample. Varying the alloy stoichiometry tunes the optical response and controls spectral features, such as Fano resonances. Binary metasurfaces that combine nanostructures with different stoichiometries are fabricated using multiple-step electron-beam lithography, and their optical function as a hologram or a Fresnel zone plate is demonstrated at the visible wavelength of λ = 532 nm. This low-temperature annealing technique provides a versatile and cost-effective way of fabricating complex Au-Ag nanostructures with arbitrary stoichiometry.