Engineering a Hierarchical Al Nanocap Array with Tunable Plasmonic Properties for Sensing Applications

In this study, we propose an easy and cost-effective method to improve the refractive index (RI) sensitivity of hierarchical aluminum (Al) nanocap arrays (HAlNAs) that are suitable for RI sensing applications. By adjusting the deposited Al thickness, deposition rate, and period, HAlNAs with tunable plasmonic and sensing properties could be achieved. The optimal sensitivity could reach 631 nm/RIU, and the detection limit of adenine ethanol solution with different concentration was as low as 1 × 10–6 M. The experimental results confirmed that the RI sensitivity of the proposed sensor was primarily related to the period and deposited Al thickness. Moreover, a higher deposition rate could narrow the full width at half-maximum (FWHM) of the reflection spectra, resulting in a higher figure of merit (FOM). The simulated electric field (EF) distribution revealed that the high RI sensitivity could be attributed to the strong coupling between propagating surface plasmon polaritons (PSPPs) and localized surface plasmon resonance (LSPR). In addition, HAlNAs with larger periods could generate a greater hot spot density, resulting in higher sensing sensitivity. The results of this study provide a method to easily fabricate Al-based RI sensors that exhibit great promise for the development of highly sensitive and selective biosensors.