Calculations of the direct electromagnetic enhancement in surface enhanced Raman scattering on random self-affine fractal metal surfaces

Plasmonic Cu/CuCl/Cu2S/Ag and Cu/CuCl/Cu2S/Au Supports with Peroxidase-Like Activity: Insights from Surface Enhanced Raman Spectroscopy

In the present study, we present nanostructured bimetallic Cu/CuCl/Cu2S/Au(Ag) supports that exhibit plasmonic electromagnetic field enhancement and peroxidase-like catalytic activity. The Cu2S component acts as the peroxidase-like catalyst, while the Au or Ag component provides the necessary light enhancement for surface enhanced Raman spectroscopic (SERS) studies of surface bound molecular reactants. As a test reaction the catalytic oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in presence of H2O2 was investigated. The comparison of product evolution in solution measured by UV-Vis spectroscopy and on the surface measured via SERS is able to give more insight into the different steps involved in the overall catalysis.

Spontaneous Formation of Fractal Aggregates of Au Nanoparticles in Epoxy-Siloxane Films and Their Application as Substrates for NIR Surface Enhanced Raman Spectroscopy

We present a facile, inexpensive route to free-standing, thermo-mechanically robust and flexible epoxy-siloxane substrates embedded with fractal aggregates of Au nanoparticles, and demonstrate their efficiency as substrates for surface enhanced Raman spectroscopy (SERS) at NIR wavelengths. The metallodielectric films are prepared by generating Au nanoparticles through the in-situ reduction of gold (III) chloride trihydrate in epoxypropoxypropyl terminated polydimethyl siloxane (EDMS). The metal nanoparticles spontaneously aggregate into fractal structures in the colloid, which could then be drop-cast onto a substrate. Subsequent UV-initiated cationic polymerization of epoxide moieties in EDMS transforms the fluid colloid into a thin, free-standing film, which contains a dense distribution of fractal aggregates of Au nanoparticles. We used electron and optical microscopy as well as UV⁻Vis⁻NIR spectrometry to monitor the evolution of nanoparticles and to optically and structurally characterize the resulting films. Raman spectroscopy of the chromophore Eosin Y adsorbed onto the metallodielectric films showed that they are excellent SERS substrates at NIR excitation with an enhancement factor of ~9.3 × 10³.

FDTD simulations of localization and enhancements on fractal plasmonics nanostructures

A parallelized 3D FDTD (Finite-Difference Time-Domain) solver has been used to study the near-field electromagnetic intensity upon plasmonics nanostructures. The studied structures are obtained from AFM (Atomic Force Microscopy) topography measured on real disordered gold layers deposited by thermal evaporation under ultra-high vacuum. The simulation results obtained with these 3D metallic nanostructures are in good agreement with previous experimental results: the localization of the electromagnetic intensity in subwavelength areas ("hot spots") is demonstrated; the spectral and polarization dependences of the position of these "hot spots" are also satisfactory; the enhancement factors obtained are realistic compared to the experimental ones. These results could be useful to further our understanding of the electromagnetic behavior of random metal layers.

Induced surface enhancement in coral Pt island films attached to nanostructured Ag electrodes

Coral Pt islands films are deposited via electrochemical reduction on silica-coated nanostructured Ag electrodes. From these devices surface-enhanced (resonance) Raman [SE(R)R] signals of molecules exclusively attached to Pt are obtained with intensity up to 50% of the value determined for Ag. SE(R)R spectroscopic investigations are carried out with different probe molecules, silica-coating thicknesses, and excitation lines. Additionally, field enhancement calculations on Ag-SiO(2)-Pt support geometries are performed to elucidate the influence of the Pt island film nanostructure on the observed Raman intensities. It is concluded that the nonperfect coating of the Pt island film promotes the efficiency of the induced Pt SER activity. Comparison with similar measurements on Ag-SiO(2)-Au electrodes further suggests that the chemical nature of the deposited metal island film plays a minor role for the SE(R)R intensity.

Characterization of Defects and Stress in Polycrystalline Silicon Thin Films on Glass Substrates by Raman Microscopy

Raman microscopy was applied to characterize polycrystalline silicon (poly-Si) on glass substrates for application as thin-film transistors (TFTs) integrated on electronic display panels. This study examines the crystallographic defects and stress in poly-Si films grown by industrial techniques: solid phase crystallization and excimer laser crystallization (ELC). To distinguish the effects of defects and stress on the optical-phonon mode of the Si–Si bond, a semiempirical analysis was performed. The analysis was compared with defect images obtained through electron microscopy and atomic force microscopy. It was found that the Raman intensity for the ELC film is remarkably enhanced by the hillocks and ridges located around grain boundaries, which indicates that Raman spectra mainly reflect the situation around grain boundaries. A combination of the hydrogenation of films and the observation of the Si-hydrogen local-vibration mode is useful to support the analysis on the defects. Raman microscopy is also effective for detecting the plasma-induced damage suffered during device processing and characterizing the performance of Si layer in TFTs.

In situ Raman monitoring of competitive adsorption of Ag and Au nanoparticles onto a poly(4-vinyl pyridine) surface

The competitive adsorption of citrate-capped Ag and Au nanoparticles (~25 nm in diameter) onto a poly(4-vinyl pyridine) (P4VP) surface has been investigated by means of Raman scattering spectroscopy. The P4VP film prepared on a glass slide was too thin for its normal Raman spectrum to be observed, but the Raman peaks of P4VP could be detected upon the adsorption of Ag and/or Au nanoparticles onto the film, due to the surface-enhanced Raman scattering (SERS) effect associated with the localized surface plasmon of Ag and/or Au nanoparticles. Neither quartz crystal microbalance nor atomic force microscopy (AFM) nor scanning electron microscopy (SEM) methodologies can distinguish between Ag and Au nanoparticles during their adsorption onto P4VP, but it is possible through Raman scattering spectroscopy because Ag (though not Au) nanoaggregates are SERS active at 514.5 nm excitation, while both Ag and Au nanoaggregates are SERS active at 632.8 nm excitation. Coupled with the AFM data, we were thus able to infer that about 120 Ag nanoparticles per 1 μm(2) were adsorbed, along with 60 Au nanoparticles per 1 μm(2), onto the P4VP film over a period of 1.5 h from a 1 : 1 mixture of Ag and Au sols at 1.6 nM each.

Electromagnetic model and calculations of the surface-enhanced Raman-shifted emission from Langmuir-Blodgett films on metal nanostructures

We present a theoretical study of the electromagnetic contribution to surface-enhanced Raman scattering (SERS) from a Langmuir-Blodgett film close to a metal surface. This macroscopic dipolar model fully accounts for the Raman-shifted emission so that meaningful SERS (electromagnetic) enhancement factors that do not depend only on the local electromagnetic field enhancement at the pump frequency are defined. For a plane metal surface, analytical SERS enhancement factors that are consistent for all pump beam polarization and molecular orientation are obtained. In order to investigate SERS on complex nanostructured metal surfaces, we introduce this model into the formally exact, Green's theorem surface integral equation formulation of the scattered electromagnetic field. This formulation is thus employed to calculate numerically the near-field and far-field emissions at the Raman-shifted frequency for very rough, random nanostructured surfaces, with emphasis on the impact of collective processes for varying pump frequency and Raman shift. Our results reveal that the widely used |E|4 approximation tends to overestimate average SERS enhancement factors.

Electromagnetic mechanism in surface-enhanced Raman scattering from Gaussian-correlated randomly rough metal substrates

We investigate the electromagnetic mechanism in surfaceenhanced Raman scattering (SERS) from randomly rough metal surfaces with Gaussian statistics and Gaussian correlation function. By means of rigorous numerical calculations, large average SERS enhancement factors (above 104) are encountered when the correlation length is of the order of (or lower than) a hundred nanometers, with excitation in the visible and near infrared. These Gaussian-correlated metal surfaces can be used as SERS substrates. Furthermore, local SERS enhancement factors are obtained of up to 108 that make them appropriate for resonant SERS single molecule detection.

Light scattering from self-affine fractal silver surfaces with nanoscale cutoff: far-field and near-field calculations

We study the light scattered from randomly rough, one-dimensional, self-affine fractal silver surfaces with nanoscale lower cutoff illuminated by s- or p-polarized Gaussian beams a few micrometers wide. By means of rigorous numerical calculations based on the Green's theorem integral equation formulation (GTIEF), we obtain both the far- and near-field scattered intensities. The influence of diminishing the size of the fractal lower-scale irregularities (from approximately 50 nm to a few nanometers) is analyzed in the case of both single realization and ensemble-average magnitudes. For s polarization, variations are small in the far field, being significant only in the higher-spatial-frequency components of evanescent character in the near field. In the case of p polarization, however, the nanoscale cutoff has remarkable effects stemming from the roughness-induced excitation of surface-plasmon polaritons. In the far field, the effect is noticed both in the speckle pattern variation and in the decrease of the total reflected energy upon ensemble averaging, as a result of increased absorption. In the near field, more efficient excitation of localized optical modes is achieved with smaller cutoff, which in turn leads to huge surface electric field enhancements.

Influence of nanoscale cutoff in random self-affine fractal silver surfaces on the excitation of localized optical modes

We report rigorous numerical calculations of the near field scattered from rough, one-dimensional self-affine fractal silver surfaces. We show that fractal lower-scale cutoff (decreased to the order of tens of nanometers) has a strong effect on excitation and strength of localized optical modes, leading to very large enhancements of the intensity (larger than 10(4)) and fluctuations of the electric field.

Electromagnetic wave scattering from conducting self-affine surfaces: an analytic and numerical study

We derive an analytical expression for the scattering of an s-polarized plane wave from a perfectly conducting self-affine one-dimensional surface in the framework of the Kirchhoff approximation. We show that most of the results can be recovered by means of a scaling analysis. We identify the typical slope taken over one wavelength as the relevant parameter controlling the scattering process. We compare our predictions with direct numerical simulations performed on surfaces of varying roughness parameters and confirm the broad range of applicability of our description up to very large roughness. Finally we verify that a nonzero electrical resistivity, provided that it is small, does not invalidate our results.