Laser-assisted dealloying for direct-write patterning of plasmonic nanostructures

Abstract

Recently, nanoporous gold (NPG) has attracted significant interest due to its unique properties such as large specific surface area, bi-continuous nanostructure, high electrical conductivity and the applicability of thiol-gold surface chemistry. Patterned NPG disks showcase tunable pore and ligament sizes ranging from nanometers to microns. The nanoporous structure and sub-wavelength nanoparticle shape contribute to its unique LSPR properties. NPG disk not only features large specific surface area, but high-density plasmonic field enhancement known as “hot-spots”. Hence, NPG disks have found many applications in nanoplasmonic sensor development. In our recent studies, we have shown that NPG disks array chip can be utilized for high-sensitivity detection by various enhanced spectroscopic modalities, as photothermal agents, and for disease biomarker detection. To date, patterned NPG disks have been exclusively fabricated by colloidal nanosphere lithography. Starting with pattern transfer into alloy disks, dealloying subsequently turns the alloy disks into NPG disks. In this paper, we present another NPG patterning method by localized laser heating, during which dealloying occurs at the laser focal spots due to elevated temperature. This approach has enabled us to pattern NPG entity with various sizes and shapes. We have investigated fabrication parameters such as laser power, irradiation duration, and solution environment. We have also characterized the plasmonic resonance of the patterned NPG disks by extinction spectroscopy. The noncontact nature of this technique is well suited for the processing of substrates immersed in an aqueous environment. Further, this technique shares the same advantages as maskless laser direct writing.