Molecular Sensing and Color Manipulation Based on Dimension-Controlled Plasmon-Enhanced Silicon Nanotube SERS Substrates

The system of suspended gold nanoring on silicon nanotube substrate with enhanced light harvesting and electromagnetic field enhancements was proposed in the present study. The effects of outer/inner diameter (<i>D</i>/<i>d</i>) ratio of the ring and tube on the plasmonic behavior were studied by systemic simulations and experiments. In simulations, the high order quadrupole–dipole mode was also excited in addition to the typical dipole–dipole mode, and the resonant configurations were characterized by both electric field profile and resonant surface charge distribution. Experimentally, both dark-field and Raman microscopies were conducted to examine the plasmonic behavior. The plasmon-enhanced scattering could be controlled by tailoring the <i>D</i>/<i>d</i> ratio, and the dark-field image colors could be manipulated covering the visible range. Raman spectra using two excitation wavelengths were also recorded and showed good agreement with calculated enhancement factor which, in turn, provided the evidence of the evolution of resonance mode and denoted our designed structure as a potential candidate for surface-enhanced Raman scattering applications

Surface Plasmon Excited on Imprintable Thin-Film Metallic Glasses for Surface-Enhanced Raman Scattering Applications

Metallic glasses (MGs) are a class of amorphous alloys in contrast with crystalline metals and provide a challenge of engineering applications for unique structure and properties. However, plasmonic applications remain a virgin area for MGs. In this work, we discovered that certain compositions of gold-based MGs possessed negative dielectric constants and could be used as plasmonic materials. Furthermore, with a low glass-transition temperature of gold-based thin-film MGs (TFMGs), we were able to fabricate large dimensions of nanostructures using an inexpensive thermal imprint method in air instead of other costly lithography methods. We performed both measurements and simulations to demonstrate that our designed nanostructures were suitable for surface-enhanced Raman scattering (SERS) applications. In addition, in the absence of grain boundaries in amorphous TFMGs, damping due to increased scattering at grain boundaries does not occur, and SERS could be improved. Also, compared to regular SERS substrates of textured Si with deposited Au films, imprinted Au-based TFMGs provided complete coverage of Si underneath, and the vibrational signal of Si lattice would not show in Raman spectra to possibly overlap signals of analyte and decrease the accuracy of sensing. Our results suggested new avenues for applying a low-cost and high-throughput method on TFMGs to fabricate large dimensions of substrates for plasmonic applications

3D Nanostructures of Silver Nanoparticle-Decorated Suspended Graphene for SERS Detection

The silver nanoparticle-decorated suspended graphene was proposed and fabricated to increase the efficiency of surface-enhanced Raman scattering (SERS) mainly by the enhanced electric field resulting from exciting the localized surface plasmon resonance. The morphology of cavity under the graphene was controlled by the thickness of catalyst and the etching time in the metal-assisted chemical etching process (MacEtch). The reflectance and ellipsometric spectra were examined to understand the optical behaviors of silver nanoparticle-decorated suspended graphene as functions of the etching time. For the samples treated with MacEtch, the Raman signals of graphene and <i>p</i>-mercaptoaniline were greatly enhanced due to the plasmonic cavity effect. Moreover, the graphene could increase the Raman intensity of the probed molecules by chemical enhancement. With the optimal etching time of 15 s, the SERS signals reached the maximum that was 13–15 times larger than those without etching. The electric field enhancement profiles and the SERS enhancement factor were simulated by finite-difference time-domain method to characterize the field distribution around the silver nanoparticles and to verify the enhanced SERS phenomenon observed in measurements

White Light Emission from Black Germanium

We demonstrate the nearly perfect absorption and quantum dots-mediated enhanced visible light emissions from defect engineered Ge nanopyramids or black germanium. High-resolution 3D photoluminescence (PL) imaging of the pyramid structure elucidated the position dependency of defects and their emission: Stronger photoluminescence yield was observed at the nanopyramid tips, which is correlated to the efficient light nanofocusing at the tips where increased defect density and roughness at the interface between Ge and surface oxide (GeO₂) also takes place. Furthermore, the visible light emissions from this GeO₂/Ge interface can be enhanced ∼15-fold when CdTe quantum dots (QDs) are adsorbed on the GeO₂/Ge system. The enhanced luminescence of our structure can be attributed to the extraordinary light harvesting property of pyramid structure; strong antireflection effect, pronounced defect formation at the nanopyramid tips, and anomalous resonant energy transfer between GeO₂ defects and CdTe QDs. The proposed methodology can be applied to other nanostructured wide bandgap materials to turn them into solar light harvesters and bright white light-emitting phosphors

Giant Electric Field Enhancement and Localized Surface Plasmon Resonance by Optimizing Contour Bowtie Nanoantennas

The surface plasmon resonances of gold contour bowtie nanostructures were simulated in the present study. The local electromagnetic field enhancement and the resonance wavelength for different dimensions of contour bowtie antennas with various contour thicknesses were investigated to find the critical conditions to induce additional enhancement compared to the solid bowtie antenna. Both the phase of the electric field and the bound surface charge distribution on the surface of the contour bowtie were studied to characterize the coupled plasmon configurations of the contour bowtie antenna. Also, a model was proposed to explain the resonance and hybridization behavior in the contour bowtie nanoantenna, and it was verified by examining the phase of the electric field in the polarization direction