EBL-Based Fabrication and Different Modeling Approaches for Nanoporous Gold Nanodisks

We report electron beam lithography (EBL) based fabrication and different modeling techniques for disk-shaped nanoporous gold nanoparticles (NPG disk). The EBL technique can provide large area 2D patterns of regularly or randomly distributed nanodisks with narrow size distribution and flexible interdisk (center to center) distance. Such flexibility is essential to obtain quasi-single NPG disk response, which typically peaks in the near-infrared (NIR) spectrum beyond 1 pm, from ensemble measurements by common UV/vis/NIR spectrometers instead of a specialized NIR spectroscopic microscope. NPG disks of 200 to 500 nm diameter and 50 nm thickness have been fabricated and characterized. To model the NPG disk and calculate its plasmonic properties, two different modeling approaches have been developed. A model based on the Bruggeman effective medium theory (B-EMT model) requires little information about the nanoporous structure. In contrast, the nanoporous model (NP model) retains the essential nanoporous structural features of NPG disk. To evaluate the performance of these models, simulated extinction spectra have been compared to the experimental data. Both the B-EMT and NP models perform well to estimate the far-field plasmon resonance peak position. However, to obtain the accurate information about the plasmon peak width/plasmon lifetime and near-field plasmonic hot-spots formation within the nanopores, the NP model is essential since the B-EMT model lacks the nanoporous network.