ABSTRACT
The realization of large-scale and high-density gaps with sizes as small as possible is crucial for designing ultra-sensitive surface-enhanced Raman scattering (SERS) substrates. As known, the ultrathin alumina mask (UTAM) surface nanopatterning technique allows the fabrication of periodic nanoparticle (NP) arrays with 5 nm gaps among the NPs, however, it still faces a significant challenge in realizing the reliable distribution of nanogaps over a large area, because of the unavoidable collapse of the UTAM pore wall during the traditional one-step homothermal pore-widening process. Herein, an efficient two-step poikilothermal pore-widening process was developed to precisely control the pore wall etching of a UTAM, enabling effectively avoiding the fragmentation of the UTAM and finally obtaining a large-scale UTAM with a pore wall thickness of about 5 nm. As a result, large-scale NP arrays with high-density sub-5 nm and even smaller gaps between the neighboring NPs have been realized through applying the as-prepared UTAM as the nanopatterning template. These NP arrays with sub-5 nm gaps show ultrahigh SERS sensitivity (signal enhancement improved by an order of magnitude compared with NP arrays with 5 nm gaps) and good reproducibility, which demonstrates the practical feasibility of this promising two-step pore-widening UTAM technique for the fabrication of high-performance active SERS substrates with large-scale ultra-small nanogaps.
ABSTRACT
The spatial distribution of localized surface plasmon resonance (LSPR) plays a key role in many plasmonic applications. Based on the thermal stability of alumina templates, this work reports a novel approach to manipulate the distribution of LSPR and exhibits its significance for an important plasmonic application, the surface-enhanced Raman spectroscopy (SERS). A suitable thermal annealing sharpens the edges in top surfaces (far from the substrates) of Ag nanoparticles, which significantly strengthens the distal mode (DM) with the LSPR excited on the top surfaces. Because the top surface is the major place to adsorb probe molecules, this manipulation greatly improves the detection sensitivity of SERS. Our research provides a new way to improve the sensitivity of SERS, which also indicates that great care has to be taken on special LSPR mode which is largely responsible for a certain plasmonic application (e.g., the DM for SERS although it is not the major mode).
ABSTRACT
Applicable surface enhanced Raman scattering (SERS) active substrates require high enhancement factor (EF), excellent spatial reproducibility, and low-cost fabrication method on a large area. Although several SERS substrates with high EF and relative standard deviation (RSD) of signal less than 5% were reported, reliable fabrication for large area SERS substrates with both high sensitivity and high reproducibility via low-cost routes remains a challenge. Here, we report a facile and cost-effective fabrication process for large-scale SERS substrate with Ag inter-nanoparticle (NP) gaps of 5 nm based on ultrathin alumina mask (UTAM) surface pattern technique. Such closely packed Ag NP arrays with high density of electromagnetic field enhancement ("hot spots") on large area exhibit high SERS activity and excellent reproducibility, simultaneously. Rhodamine 6G molecules with concentration of 1 × 10(-7) M are used to determine the SERS performance, and an EF of â¼10(9) is obtained. It should be noted that we obtain RSDs about 2% from 10 random spots on an area of 1 cm(2), which implies the highly reproducible signals. Finite-difference time-domain simulations further suggest that the enhanced electric field originates from the narrow gap, which agrees well with the experimental results. The low value of RSD and the high EF of SERS signals indicate that the as-prepared substrate may be promising for highly sensitive and uniform SERS detection.
