Near-Field Response on the Far-Field Wavelength-Scanned Surface-Enhanced Raman Spectroscopic Study of Methylene Blue Adsorbed on Gold Nanocolloidal Particles (original) (raw)
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The Journal of Physical Chemistry Letters, 2010
Raman signals from molecules adsorbed on a noble metal surface are enhanced by many orders of magnitude due to the plasmon resonances of the substrate. Additionally, the enhanced spectra are modified compared to the spectra of neat molecules: many vibrational frequencies are shifted and relative intensities undergo significant changes upon attachment to the metal. With the goal of devising an effective scheme for separating the electromagnetic and chemical effects, we explore the origin of the Raman spectra modification of benzenethiol adsorbed on nanostructured gold surfaces. The spectral modifications are attributed to the frequency dependence of the electromagnetic enhancement and to the effect of chemical binding. The latter contribution can be reproduced computationally using moleculemetal cluster models. We present evidence that the effect of chemical binding is mostly due to changes in the electronic structure of the molecule rather than to the fixed orientation of molecules relative to the substrate.
Journal of Raman Spectroscopy, 2012
Surface enhanced Raman scattering (SERS) of adsorbed molecule on colloidal gold nanoparticles of different shapes, namely nanospheres (NSs), nanorods (NRs), and nanoprisms (NPs) as well as the three NPs arrays of different interstice prepared by NS lithography, are studied with incident wavenumbers in the near-dipole and near-quadrpole regions of the nanoparticles. In the colloidal gold nanoparticles, the SERS enhancement is the largest for the sharp tip followed by the truncated tip NPs, then the NRs and least enhancement for the NSs. This decreasing order of enhancement occurs although the incident wavenumber was near the dipole resonance of NSs and the quadrupole resonance for the NPs. These varied enhancements are explained in part as due to the binding energies of the nanocrystal facets, but the larger contribution results from the plasmon electromagnetic fields. A parallel finite difference time domain (FDTD) calculations were carried out, which corporate the experimental results and show agreement with ratios of the SERS enhancement for the different shapes. The normalized SERS intensity for NPs of different interstice distances show a sharp rise with the decrease of the interstice distances because of interparticle dipolar and quadrupolar coupling as evidenced also by FDTD calculations. Furthermore, these calculations show that the enhancement is polarization independent for an incident wavelength near quadrupole resonance but polarization dependent for an incident wavelength near the plasmon dipole transition. In the last case, the enhancement is larger by an order of magnitude for a polarization parallel to the NPs bisector than for polarization normal to the bisector with no hot spots for the relatively large interstice dimensions used.
Nanomaterials, 2020
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 ...
OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), 2007
A general many-body formulation of surface-enhanced Raman scattering from admolecules is presented. Both local-field as well as chemisorption effects are taken into account. The theory can be applied to an arbitrary nonstatistical surface. It is shown that the calculation of the cross section can essentially be reduced to (a) the calculation of the classical Green's function for the electromagnetic field in the presence of the bounded metal but without the admolecule and (1) the inversion of "small" matrices. In order to illustrate the general theory, the cross section is worked out for a Newns-Anderson model of chemisorption using a plane metal surface with and without a weak grating and with the plasmon-pole approximation for the dielectric function of the metal. The enhancements of the cross section due to local-field effects, chemisorption, and grating are calculated using available or estimated values for the electronic matrix elements.
Near-Infrared Surface-Enhanced Raman Scattering (NIR SERS) on Colloidal Silver and Gold
Applied Spectroscopy, 1994
Near-infrared surface-enhanced Raman scattering (NIR SERS) was detected with high sensitivity for two model compounds, crystal violet and the DNA base adenine, adsorbed on silver and gold colloidal particles in aqueous solutions. The NIR SERS spectra were measured with a fiber-optic probe with the use of a Ti:sapphire laser at 850 nm as the excitation source and a charge-coupled device (CCD) as the detection system. In a step to achieve the optimal electromagnetic SERS-enhancement conditions, silver and gold sols were modified to shift their extinction spectra more to the NIR region. Surface enhancement factors for both crystal violet and adenine were estimated to be on the order of 106 and 108 for gold and silver, respectively. A larger enhancement factor was observed for adenine with NIR excitation than with visible excitation. The benefits of NIR SERS and its potential applications are discussed.
2005
We present a theoretical analysis of surface-enhanced Raman scattering and fluorescence emission from chromophoric molecules located at electromagnetic 'hot spots' in nanoparticle aggregates. The model combines classical electrodynamic enhancement effects with molecular quantum dynamics and allows us to quantify various molecular cross-sections and spectral properties. For a model molecule that simulates rhodamine 6G, we find that an electromagnetic Raman enhancement of the order of 10 10 results in an effective Raman cross-section of the order of 10 −14 cm 2 , in agreement with single-molecule Raman measurements.
Spectral dependence of the amplification factor in surface enhanced Raman scattering
Multi-wavelength SERS measurements on dyes molecules adsorbed on gold evaporated nanoparticles substrates are presented. The spectra are correlated with the Localized Surface Plasmon Resonance (LSPR) profile of the nanostructured substrate before and after the adsorption process. The enhancement of the typical vibrational modes of the methylene blue is investigated for the different excitation wavelengths and we show that its behavior follows properly the LSPR profile of nanoparticles "dressed" with the dye molecules.
Journal of Nanotechnology, 2012
Surface-enhanced Raman spectroscopy (SERS) of thiols and dye molecules adsorbed on triangular silver nanostructures was investigated. The SERS hot-spots are localized at the edges and corners of the silver triangular particles. AFM and SEM measurements permit to observe many small clusters formed at the edges of triangular particles fabricated by nanosphere lithography. Finite-element calculations show that near-field enhancements can reach values of more than 200 at visible wavelengths, in the gaps between small spherical particles and large triangular particles, although for the later no plasmon resonance was found at the wavelengths investigated. The regions near the particles showing strong near-field enhancement are well correlated with spatial localization of SERS hot-spots done by confocal microscopy. Silver nanostructures fabricated by thermal evaporation present strong and fast fluctuating SERS activity, due to amorphous carbon contamination. Thiols and dye molecules seem to be able to passivate the undesired SERS activity on fresh evaporated silver.
Applied Spectroscopy, 1998
In agreem ent with previous resu lts reported for colloidal silver clusters, effective surface-enhanced Raman cross sections of about 10 2 16 cm 2 per molecule, corresp onding to enhancem ent factors on the order of 10 14 , have also been obtained for molecules attached to colloidal gold clusters. Spatially isolated nearly spherical colloidal gold particles of about 60 nm size show maximum enhancem ent factors on the order of 10 3 at 514 nm excitation, close to the single plasmon resonance. The enhancement factor increases by eleven orders of m agnitude when colloidal gold clusters are formed by aggregation of the gold colloids and when near-infrared excitation is applied. The large effective surface-en hanced Raman cross section has been estim ated by a straightforward method based on steady-state population redistribution due to the pumping of m olecules to the ® rst excited vibrational state via the strongly enhanced Raman process. Our experim ental ® nding con® rms the im portant role of colloidal clusters for extrem ely large surface-en hanced Raman scattering (SERS) enhancement factors. Simultaneously, it suggests colloidal gold clusters as a substrate for high-sensitivity surface-en hanced Raman scattering, which can provide an enhancement level suf®cient for Raman single m olecule detection. Due to its chemical inactivity, gold m ight have some advantages compared to silver, particularly in biomedical spectroscop y.