Rational side-by-side self-assembly of gold nanorods with short and medium aspect ratios via the self-evaporation method to boost their potential as a surface-enhanced Raman scattering (SERS) substrate†

Abstract

Surface-enhanced Raman scattering (SERS) represents a compelling detection methodology centered on the electromagnetic fields, commonly termed “hot spots”, generated around noble nanoparticles. Nonetheless, the efficacy of electromagnetic field (EMF) amplification is constrained when utilizing individual nanoparticles. There has been a notable lack of experimental and theoretically simulated studies regarding the increase of the electromagnetic field when gold nanorods with different aspect ratios undergo self-assembly in either perpendicular or parallel orientations to substrates. This research presents a novel and facile methodology for fabricating SERS nanosubstrates. This method entails self-assembling gold nanorods (AuNRs) with short and medium aspect ratios (ARs) through natural evaporation. By manipulating the water-to-ethanol ratios, we ascertain the appropriate conditions for the rational alignment of the nanorods in both perpendicular and parallel orientations relative to the silicon substrate. These nanosubstrates have been experimentally evaluated for their ability to improve the Surface-Enhanced Raman Scattering (SERS) performance, presenting a novel perspective in this field. In addition, a computational analysis employing the finite-difference time-domain (FDTD) method was conducted to elucidate the electromagnetic field generated by nanoarrays when subjected to incident light of varying wavelengths, including 532 nm, 638 nm, and 785 nm. Notably, the FDTD simulation outcomes indicated that gold nanorods (AuNRs) possessing an aspect ratio of 3.0 and nanogaps of 2.0 nm exhibited exceptional electromagnetic field characteristics when aligned parallel to the substrate under 532 nm laser illumination. Conversely, when the AuNRs were oriented perpendicular to the substrates, they produced lower EMFs upon interaction with excitation laser light. These findings can potentially contribute to the advancement of SERS nanosubstrate design.