Low temperature annealing method for fabricating alloy nanostructures and metasurfaces: Unlocking a novel degree of freedom

TL;DR

This paper introduces a low temperature annealing method at 300°C for fabricating alloy nanostructures and metasurfaces, enabling shape preservation and spectral tuning for optical applications.

Contribution

It presents a novel low temperature alloying process for creating diverse Au-Ag nanostructures with controlled optical properties and complex metasurfaces.

Findings

Homogeneous alloying confirmed by EDX and XPS analyses.

Tunable optical responses through alloy stoichiometry.

Demonstrated optical functions like holograms at 532 nm.

Abstract

The material and exact shape of a nanostructure determine its optical response, which is especially strong for plasmonic metals. Unfortunately, only a very 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, we develop a low temperature alloying process at only 300{\deg}C and fabricate Au-Ag nanostructures with a broad diversity of shapes, aspect ratios and stoichiometries. EDX and XPS analyses confirm the homogeneous alloying through the entire sample. Varying the alloy stoichiometry tunes the optical response of the nanostructure 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 hologram or Fresnel zone plate is demonstrated at the visible wavelength of 532 nm. This low temperature annealing technique provides a versatile and cost-effective way of fabricating complex Au-Ag nanostructures with arbitrary stoichiometry.