Long-range manifestation of surface-enhanced Raman scattering

Abstract

The long-range action of surface-enhanced Raman scattering (SERS) is probed via distancedependent measurements of molecular Raman spectra. To this end, identical SERS substrates composed of irregular silver nanoisland arrays were covered by dielectric spacer layers with variable thickness, and the strength of the SERS signal produced from analyte molecules deposited on top of the structure was analyzed. The obtained distance dependence of the signal strength exhibited a shelf-like behavior up to 30 nm away from the enhancing surface and then rapidly decreased further away. Thus, the observed behavior of the electromagnetic mechanism of SERS enhancement in metal island films contradicts the widely accepted picture of extremely rapid (2-3 nm) decay of SERS-enhancement of 2D nanoparticle ensembles. Because of the observed steady enhancement factors at distances of ∼30 nm from the surface, SERS can be used for probing the spectra of macromolecules or other objects relatively distant from the metal surface. PACS numbers:Since its first observation by Fleischmann et al.(1974) 1 surface-enhanced Raman scattering (SERS) has been thoroughly investigated as an amazing physical phenomenon itself and one of the most promising tools for analytical applications. The first perception of SERS as a giant enhancement over conventional Raman scattering in experiments of Jeanmaire, Van Duyne 2 and Albrecht, Creighton 3 was followed by an extensive theoretical and experimental analysis, searching for the most general explanation of the enhancement mechanism. The enhancement of Raman scattering signals from organic molecules absorbed on nanostructured metal surfaces and photoexcited in a certain spectral range has been shown to reach 6-10 orders of magnitude 2,4 . Further enhancement of up to ∼14 orders of magnitude was observed on molecules residing in silver colloidal aggregates, enabling single molecule detection 5 . It is now generally agreed that more than one effect contributes to the total enhancement of Raman signals. The enhancement mechanisms are roughly divided into so-called electromagnetic (EM) field enhancement 6-8 and chemical first-layer effects 9-11 . The electromagnetic enhancement is caused by the enhanced local optical fields at the place of the molecule nearby the metal surface due to excitation of electromagnetic resonances, called surface plasmon polaritons. For isolated silver or gold spheroidal nanoparticles typical values for electromagnetic enhancement of SERS are on the order of 12,13 10 6 − 10 7 . For the more sophisticated case of closely spaced interacting particles (or clusters of particles), the individual dipole oscillators of the small particles couple, thereby generating normal modes of plasmon excitation that embrace the cluster. According to theoretical evaluations, the excitation is not distributed uniformly over the cluster but tends to be spatially localized in so-called "hot" areas [14][15][16] . Effects of chemical enhancement arise from the electronic coupling between the adsorbate molecule...

Suppression of short‐range scattering via hydrophobic substrates and the fractional quantum Hall effect in graphene

“…We emphasize that in previous FQHE work on graphene 5, 10, 11, 21, the product of $\mu n$ > 10 16 V –1 s –1 (conductivity > 40$e^2/h),$ is exceeded by the one in our work of $\mu n$ > 2 × 10 16 V –1 s –1 at n > 1 × 10 12 cm –2 . Also, the factor $\mu n$ > 10 16 V –1 s –1 is similarly found for 2D electron gases in III–V 25, 26 VI 27–29 and transition metal–VI 30, 31 systems, as well as for two‐dimensional hole gases 32 in the FQHE regime. In all these studies $\ell/d$ is approximately the same, $\ell/d$ ≥ 10, indicating a similar EEI contribution to the overall scattering.…”

Out-diffused silver island films for surface-enhanced Raman scattering protected with TiO2 films using atomic layer deposition

“…However, only the recent use of a very precise atomic layer deposition (ALD) technique has allowed obtaining quantitative results related to the SERS influence by alumina spacers deposited on metal microspheres [3], MIFs [12], and metal nanowires [13]. However, due to the difference in metal nanoislands and nanoparticles used in the experiment, these results can hardly be compared, and they contradict the data obtained in SERS experiments using MIFs covered with non-ALD spacers [14]. It is worth to note that the key issues in this comparison are the SPR shift and the local electric field decay vs the spacer thickness.…”

Impact of magnetic field inhomogeneity on electron cyclotron radiative loss in tokamak reactors

“…In the model of the EC radiative transport [4,5,11], we use the following averaging over a magnetic surface: (1) where g(ρ, θ) is the plasma volume lying within the intervals {ρ, ρ + dρ} and {θ, θ + dθ}. Accordingly, the volume averaging and the total plasma volume, V tot , are given by the formulas…”

“…In the models [4,11], EC radiative transport depends only on the angle averaged spectral coeffi cients of absorption, , and emission, ,…”

“…This model was implemented in the earlier version of the CYNEQ code [5,6]. The model is based on the approach [11], which enables semianalytic solution of the transport problem for R w ≥ 0.5 by extending the escape probability method developed in the theory of nonlocal radiative transport in atomic spectral lines. This model improves the semianalytic approach used in the CYTRAN code [4] for typical tokamak reactor conditions:…”