Au@Ag Dendritic Nanoforests for Surface-Enhanced Raman Scattering Sensing

Abstract

The effects of Au cores in Ag shells in enhancing surface-enhanced Raman scattering (SERS) were evaluated with samples of various Au/Ag ratios. High-density Ag shell/Au core dendritic nanoforests (Au@Ag-DNFs) on silicon (Au@Ag-DNFs/Si) were synthesized using the fluoride-assisted Galvanic replacement reaction method. The synthesized Au@Ag-DNFs/Si samples were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, reflection spectroscopy, X-ray diffraction, and Raman spectroscopy. The ultraviolet-visible extinction spectrum exhibited increased extinction induced by the addition of Ag when creating the metal DNFs layer. The pure Ag DNFs exhibited high optical extinction of visible light, but low SERS response compared with Au@Ag DNFs. The Au core (with high refractive index real part) in Au@Ag DNFs maintained a long-leaf structure that focused the illumination light, resulting in the apparent SERS enhancement of the Ag coverage.

Enhanced Photoluminescence of R6G Dyes from Metal Decorated Silicon Nanowires Fabricated through Metal Assisted Chemical Etching

“…The increased SEF for Au NPs enriched nanostructured Ag is interesting, and similar effects for bimetallic Ag-Au alloys and core-shell nanostructures have been observed in other works [ 27 , 43 ]. However, the effects are reproduced in the present study, even by simple drop-casting Au NPs on an Ag nanostructured surface.…”

“…A smaller drop with a minimum around 370 nm exists for the Ag-aggregates ( Figure 4 b), while the addition of the Au-NP results in a lower reflectivity-minimum that shifts towards slightly higher wavelengths, 380–390 nm. This can be attributed to light absorption due to particle-plasmon excitation [ 16 ] since the plasmon resonance in bimetallic Ag-Au nanostructures shifts to longer wavelengths as the Au concentration increases [ 42 ], and this was also observed in Ag-Au core-shell dendritic nanoforests [ 27 ]. The plasmon resonance shift by adding Au on the Ag nanostructures can be more clearly seen in the case of dendrites ( Figure 4 c) where the minimum observed around 360 nm shifts to 390 nm when Au is added.…”

“…These effects were used in various surface-enhanced phenomena such as Surface Enhanced Raman Spectroscopy (SERS) [ 15 , 16 ] or Surface Enhanced Photoluminescence Spectroscopy [ 17 ] (SEPS), and Surface Enhanced Fluorescence (SEF) [ 18 , 19 ]. In particular, structures such as silver nanoshells [ 20 ], nanocubes [ 21 , 22 ], nanoplates [ 23 ] nanoclusters [ 24 , 25 ], dendrites [ 26 , 27 , 28 ], nanorods, nanowires [ 29 , 30 ] and aggregates [ 31 , 32 ], can offer high Electromagnetic field (EM) Enhancement Factors (EF). However, it is important to note that the size, shape, and morphology of the structures have a significant effect on the optical properties and influence the wavelength and width of the SPP resonance band of the metal nanostructures [ 13 , 15 , 33 ].…”

“…Bimetallic Au-Ag core-shell nanostructures and dendrites [ 27 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 ] have also attracted some attention since they have shown improved SEF and SERS compared to single metal Au or Ag structures [ 40 , 41 , 46 ]. The position of the plasmon resonance was controlled through the alloy composition to custom-design substrates that match the Raman excitation or the dye emission band.…”

Identifying Surface-Enhanced Raman Spectra with a Raman Library Using Machine Learning

“…With vast Raman spectral libraries already available, it is worthwhile to examine whether one can circumvent the issues of SERS substrate specificity that arise when using a variety of different SERS-based substrates for chemical identification (i.e., NP aggregates (hot spots) and complex 3D nanostructures with sharp corners or junctions). ,,− These nanostructured geometries each offer their own specific near-field properties. Here, we report a machine learning (ML) approach for identifying unknown chemicals from their SERS spectrum using a standard Raman library.…”