ABSTRACT
The control of magnetic properties at the nanoscale is a current topic of intense research. It was shown that combining both magnetic and plasmonic nanoparticles (NPs) led to the improvement of their magneto-optical signal. In this context, common strategies consist of the design of bimetallic NPs. However, the understanding of the physics leading to the coupling between magnetic and plasmonic NPs is lacking, preventing any significant progress for the development of future photonic devices. In this article, we propose to focus our attention on an efficient and commonly used magnetic metal, cobalt, and evaluate its plasmonic properties at the nanoscale through the use of NP regular arrays, as a potential candidate combining both optical and magnetic functionalities within the same metal. We show that such NPs display plasmonic properties within a large spectral range from the UV to the NIR spectral range, with efficient quality factors, when the inter-particle distance is properly selected. These as-fabricated simple materials could find applications in integrated photonic devices for telecommunications.
ABSTRACT
Long-range interaction in regular metallic nanostructure arrays can provide the possibility to manipulate their optical properties, governed by the excitation of localized surface plasmon (LSP) resonances. When assembling the nanoparticles in an array, interactions between nanoparticles can result in a strong electromagnetic coupling for specific grating constants. Such a grating effect leads to narrow LSP peaks due to the emergence of new radiative orders in the plane of the substrate, and thus, an important improvement of the intensity of the local electric field. In this work, we report on the optical study of LSP modes supported by square arrays of gold nanodiscs deposited on an indium tin oxyde (ITO) coated glass substrate, and its impact on the surface enhanced Raman scattering (SERS) of a molecular adsorbate, the mercapto benzoic acid (4-MBA). We estimated the Raman gain of these molecules, by varying the grating constant and the refractive index of the surrounding medium of the superstrate, from an asymmetric medium (air) to a symmetric one (oil). We show that the Raman gain can be improved with one order of magnitude in a symmetric medium compared to SERS experiments in air, by considering the appropriate grating constant. Our experimental results are supported by FDTD calculations, and confirm the importance of the grating effect in the design of SERS substrates.
ABSTRACT
The design of surface-enhanced Raman spectroscopy (SERS) platforms based on the coupling between plasmonic nanostructures and stimuli-responsive polymers has attracted considerable interest over the past decades for the detection of a wide range of analytes, including pollutants and biological molecules. However, the SERS intensity of analytes trapped inside smart hybrid nanoplatforms is subject to important fluctuations because of the spatial and spectral variation of the plasmonic near-field enhancement (i.e., its dependence with the distance to the nanoparticle surface and with the localized surface plasmon resonance). Such fluctuations may impair interpretation and quantification in sensing devices. In this paper, we investigate the influence of the plasmonic near-field profile upon the Raman signal intensity of analytes trapped inside thermoresponsive polymer-coated gold nanoarrays. For this, well-defined plasmonic arrays (nanosquares and nanocylinders) were modified by poly(N-isopropylacrylamide) (PNIPAM) brushes using surface-initiated atom-transfer radical polymerization. Molecular probes were trapped inside these Au@PNIPAM nanostructures by simple physisorption or by covalent grafting at the end of PNIPAM brushes, using click chemistry. The SERS spectra of molecular probes were studied along various heating/cooling cycles, demonstrating a strong correlation between SERS intensities and near-field spectral profile of underlying nanoparticles, as confirmed by simulations based on the finite difference time domain method. Thermoresponsive plasmonic devices thus provide an ideal dynamic SERS platform to investigate the influence of the near-field plasmonic profile upon the SERS response of analytes.
