Nonlinear response of 2D plasmonic gold patches for SERS applications

In this paper we describe the fabrication of a periodic, two-dimensional arrangement of gold square patches on a Silicon substrate, and highlight technological limitations due to the roughness of the metal layer. Scanning Electron Microscope (SEM) and Atomic Force Microscope analyses are also reported showing that the geometrical parameters obtained are almost identical to the nominal parameters of the simulated structure. The device is functionalized by means of a conjugated rigid thiol forming a very dense, closely packed, reproducible 18 angstrom-thick, self-assembled monolayer. The nonlinear response of the 2D array is characterized by means of a micro-Raman spectrometer and it is compared with a conventional plasmonic platform consisting of a gold nano-particles ensemble on Silicon substrate, revealing a dramatic improvement in the Raman signal. The SERS response is empirically investigated using a laser source operating in the visible range at 633 nm. SERS mapping and estimation of the provided SERS enhancement factor (EF) are carried out to evaluate their effectiveness, stability and reproducibility as SERS substrate. Moreover, we take advantage of the simple geometry of this 2D array to investigate the dependence of the SERS response on the number of total illuminated nano-patches.