Abstract
The development of efficient plasmonic nanostructures with controlled and reproducible surface-enhanced Raman spectroscopy (SERS) signals is an important task for the evolution of ultrasensitive sensor-related methods. One of the methods to improving the characteristics of nanostructures is the development of hybrid structures that include several types of materials. Here, we experimentally investigate ultrathin gold films (3–9 nm) near the percolation threshold on Si/Au/SiO2 and Si/Au/SiO2/graphene multilayer structures. The occurring field enhanced (FE) effects were characterized by a recording of SERS signal from Crystal Violet dye. In this geometry, the overall FE principally benefits from the combination of two mechanisms. The first one is associated with plasmon excitation in Au clusters located closest to each other. The second is due to the gap plasmons’ excitation in a thin dielectric layer between the mirror and corrugated gold layers. Experimentally obtained SERS signals from sandwiched structures fabricated with Au film of 100 nm as a reflector, dielectric SiO2 spacer of 50 nm and ultrathin gold atop could reach SERS enhancements of up to around seven times relative to gold films near the percolation threshold deposited on a standard glass substrate. The close contiguity of the analyte to graphene and nanostructured Au efficiently quenches the fluorescent background of the model compound. The obtained result shows that the strategy of combining ultrathin nano-island gold films near the percolation threshold with gap plasmon resonances is promising for the design of highly efficient SERS substrates for potential applications in ultrasensitive Raman detection.
Highlights
We study the efficiency of hybrid SERS structures consisting of ultrathin gold films (3–9 nm) near the percolation threshold deposited over a gold mirror
This investigation is combined with a detailed investigation of structural and optical properties of gold films near the percolation threshold, deposited as a topmost part of three types of substrates-glass, Au/SiO2, and
The percolation threshold was defined by two methods as a four-probe sheet resistance measurement and spectroscopic ellipsometry
Summary
Surface-enhanced Raman spectroscopy (SERS) is an analytical tool that allows detecting chemical compounds at extremely low concentrations (down to single molecules) and provides unique spectra based on the specific vibration bonds of molecules [1,2,3]. SERS is achieved due to strong electromagnetic field enhancement (FE), which occurs due to the excitation of plasmons in resonant metal nanostructures [4,5,6,7]. The requirements for SERS structures can be listed as follows: simplicity, reliability, and low manufacturing cost, as well as high reproducibility and uniformity over a large area. Note that highly sensitive substrates, for instance, for single-molecule level detection are not always required. Reproducible and satisfactory signal amplification is Nanomaterials 2021, 11, 3205.
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