Plasmon management in index engineered 2.5D hybrid nanostructures for surface-enhanced Raman scattering

Abstract

Leaky mode resonance (LMR) and optical standing waves (SWs) generated in suitably illuminated vertically aligned supported nanowires (NWs) were demonstrated to enhance the plasmonic electric (E) field behavior of silver nanoparticles (AgNPs) dispersed on the wires. The combination of LMR and SW can significantly enhance the local plasmonic E field around highdensity AgNPs. The results of the finite difference time domain (FDTD) calculations were experimentally verified by comparing the surface-enhanced Raman scattering (SERS) sensitivity, which is directly dependent on the E field, in AgNP-coated silicon, germanium and silver NWs. As LMR and SW are functions of the substrate optical index (n, k), an engineering of the indices predicted a (3.88, 0) theoretical value to maximize the plasmonic E-field on the hybrid scaffold at a given AgNP density. These SERS substrates were utilized for the detection of marine toxins, L-BMAA (2-amino-3-(methylamino) propionic acid hydrochloride) and Malachite green, which are used extensively in seafood. AgNP-decorated silicon NWs, whose index (3.88, 0.02) lies close to the theoretically predicted value, exhibit at least pico-molar sensitivity toward those marine toxins. A plasmon management strategy is developed that could assist in lowering the detection level of environmental toxins by SERS. NPG Asia Materials (2014) 6, e123; doi:10.1038/am.2014.67; published online 12 September 2014 INTRODUCTIONThe involvement of plasmonics 1 in applications such as biosensing 2 through surface-enhanced Raman scattering (SERS), energy, 3 photodetectors 4 and lasers, 5 among others, is extensive because surface plasmons can concentrate and manipulate light energy on scales lower than their diffraction limit. 6 The common goal has generally been to be able to intensify and control the plasmonic electric (E) fields in preferred locations within the material. For example, the so-called 'hot-spots' in SERS, for molecular sensing, are reportedly the regions of extraordinarily intense E field at the junctions of critically close gold (Au) or silver (Ag) nanoparticles (NPs). The interparticle spacing 7 and fractal-like aggregations 8 within the metal NPs were observed to be more critical for the E enhancement than the basic material properties or the size of the NPs.The recent observation of enhanced optical absorption in suitably illuminated semiconductor nanowires (NWs) via leaky mode resonances (LMRs) 9 opens up the possibility of using LMR for the E-field enhancement of metal NPs. In addition, researchers have observed optical standing waves (SWs) along the length of the NWs when studying optically illuminated vertically aligned NW structures grown on a substrate by finite difference time domain (FDTD) calculations. However, the effect of the SWs or the LMR in modulating the total effective E field has been ignored or addressed inadequately. The fact that Ag-or AuNP-dispersed vertically aligned 1D NW/nanorods 10

Polarization response of nanowires à la carte

“…Absorption tends to decrease for longer wavelengths, with two absorption peaks at 500 and 680 nm . These peaks are related to photonic resonances in the NW and have been studied extensively in the literature 24 25 . Figure 2b depicts the intensity of light within the nanowire cross-section in these resonant cases.…”

Unravelling the formation of carbyne nanocrystals from graphene nanoconstrictions through the hydrothermal treatment of agro-industrial waste molasses

“…This manifests highly reactive graphene defect sites such as edges, single vacancies, and grain boundaries. 97 The broadening phenomenon of sp 3 C atoms (FWHM = 2.39 eV) can be directly correlated to the increase in the concentration of defects, 98 plausibly causing a zone of nanoconstriction, and ultimately nanogaps (Fig. 2a).…”

“…Inspired by monolayer thick graphene, 1 the controlled synthesis of sp–sp 2 hybridized monolayer nanostructures acknowledging certain extra degree of freedom fetch paradigm shift towards new age 2D nanomaterials 2,3 that has attracted great attention among scientists. The ‘flat-and-twist’ nature of graphene moiré layers has several significant characteristic properties, including sizeable band gaps, 4 giant electron delocalization, 5 superconductivity, 6,7 interfacial charge transfer and surface reactivity.…”

Non-enzymatic glucose sensing by enhanced Raman spectroscopy on flexible ‘as-grown’ CVD graphene

“…7 SERS makes use of the internal field of the surface plasmons to electromagnetically enhance the Raman signals from analyte molecules in the vicinity of the metal nanoparticles. 8 For example, Van Duyne and his group did detailed studies of glucose detection using SERS with thiol to adsorb glucose on a silver film coated nanosphere substrate. [9][10][11][12] Other SERS studies for glucose sensing also exist but the fabrication of a sensitive biocompatible substrate is a major limiting factor.…”