Nano-gapped Al/Al2O3 nanorods: a novel paradigm for efficient SERS-based sensing in the deep UV region

Abstract

Deep-UV surface-enhanced Raman spectroscopy (DUV-SERS) has emerged as a promising method for sensing molecular structure and dynamics in biological systems, projecting potential applications in disease diagnosis. In this context, this work presents an original theoretical study demonstrating the potential benefits of a hypothetical and novel plasmonic nanoantenna system based on nano-gapped Al nanorods (NRs) for improving DUV-SERS performance. The FDTD method was used to study the electric near-field distribution for wavelengths between 200 and 300 [nm] and the corresponding electric near-field enhancement (ENFE) factors. The analysis was focused on studying the performance of typically-used laser sources in deep UV Raman, such as 224 (NeAg), 248.6 (NeCu) and 266 [nm] (Nd:YAG). Pure Al and oxidized (Al/Al₂O₃) NRs were studied, considering native oxide (Al₂O₃) layers from 2 to 6 [nm], to assess the potential impact of surface oxidation on the ENFE and future feasibility of these nanostructures for DUV-SERS-based sensing applications. Results show that the plasmon resonance wavelength, a condition of the maximum ENFE, can be tuned between 200 and 300 [nm] by varying the nanorod length between 180 and 340 [nm]. High ENFE factors in the order of 10³ could be achieved for pure Al by varying the nanorod length, unlike other metals (Au, Ag, and Cu), which showed very poor enhancements (<50) in this UV region. Surface oxidation in Al NRs induced a considerable redshift of the plasmon resonance with appreciable changes in the ENFE factors as the oxide layer thickness increased. Nevertheless, the oxide-influenced ENFE factors could be kept at reasonably good values (∼10³) for wavelengths of interest (for example, 266 [nm], the most common one) by choosing the adequate nanorod length. This fact highlights the relevance of considering the formation of inherent native oxides when designing plasmonic nanoantennas for real applications, suggesting potential tunability and tolerance to oxidation of the proposed systems as practical advantages over traditional Al nanostructures. Our research proposes a novel and interesting paradigm for analyzing the future perspectives of Al-based plasmonic nanoantennas toward a smart structural design for fabricating efficient DUV-SERS-based sensors.