Enhancement electromagnetic

The excitation of the dipole particle plasmon resonance provides the theoretical basis for the development of the electromagnetic enhancement mechanism (EM).  

For a recent review on the EM mechanism of enhancement see Schatz and Van Duyne. ° The theoretical approach allows the calculation of enhancement factors for metal particles of different sizes and shapes. It is, in fact, electromagnetic enhancements that define surface-enhanced spectroscopies. 

For a sphere of charge q=0, with dielectric function (assumed equal to that of the bulk metal), imbedded in a medium with dielectric constant and with an incident electric field Eq, the most significant part of the local field outside the sphere. 

The Hrst part of the total external field is identical to that of a field created by a dipole p at the center of a sphere  

Within the dipolar model, the enhancement of the local excitation rate varies as (ala+df, where d is the distance from a metal sphere of radius a to the molecule. When both the excitation and the fluorescence are enhanced by the nanostructure, the total enhancement varies approximately as (a/a+d) It can also be seen that larger metal particles should provide fields that extend further out. For simplicity, let s consider an incident field ,(r,ffib) in vacuum, where in=l then 

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As a result of the above considerations and the observed experimental findings, the best estimate of the electromagnetic enhancement expected from the DSP-cxcitation EM model is c, 10z-104, Again, this clearly shows the requirement to invoke other enhancement mechanisms. 

Haller K.L., Bumm L.A., Altkom R.I., Zeman E.J., Schatz G.C., Vanduyne R.P., Spatially resolved surface enhanced 2nd harmonic-generation - theoretical and experimental-evidence for electromagnetic enhancement in the near-Infrared on a laser microfabricated Pt surface, J. Chem. Phys. 1989 90 1237-1252. 

It is likely that the decreases observed can be rationalized in terms of two contributions. Changes in surface optical properties resulting from modification by the foreign metal have been shown to decrease the electromagnetic enhancement contribution to SERS. However, for the case of Pb UPD on Ag, this effect has been shown to account for only ca. 40% of the decrease in going from zero coverage to one monolayer.(14) Moreover, this model does not account for the relatively rapid decrease in intensity observed with the deposition of small (i.e., less than 20% of a monolayer) amounts of Pb on the Ag surface. 

In section IID, we introduced the utilization of chemical enhancement effect for higher sensitivity in TERS. Here, it should be pointed out that in addition to electromagnetic enhancement and chemical enhancement effects, physical deformation induced by tip-applied force showed extra enhancement effect in TERS on carbon materials such as SWNTs and fullerene molecules (Yano et al. 2005, 2006 Verma et al. 2006). This tip-pressurized effect is a unique feature of TERS and not observable in SERS. Since the spatial resolution of TERS with tip-pressurized effect is determined by the size of the very end of the metallic tip that has direct contact with the molecules, this is a very promising approach to improve the spatial resolution of the near-field microscope. 

Figure 10.5. Schematic model for electromagnetic enhancement in SEIRA.

The plasma frequency which is the important property for obtaining electromagnetic enhancement can be easily characterized by the absorption spectrum of the colloidal solution. 

In the case of a bull s eye structure, the relative electromagnetic enhancement is about 100 (computed for an incoming plane wave and consistent with experiment (72)), but the structure radius and thus the laser beam focus spot are about 4 times the diffraction limit (minimum focus size). If we compare the relative intensity enhancement to the case of a diffraction-limited spot, the intensity with the bull s eye is increased by 100x(l/4) = 6.25, which is about the enhancement with a single (bare) aperture with a tightly focused beam. Of course, this argument has to be supplemented with the influence the emission rate and the directivity of the fluorescence beam, which contribute to the overall enhanced fluorescence. However,... 

Itoh T, Yoshikawa H, Yoshida K, Biju Y, Ishikawa M (2009) Evaluation of electromagnetic enhancement of surface enhanced hyper Raman scattering using plasmonic properties of binary active sites in single Ag nanoaggregates. J Chem Phys 130 214706... 

Lithographic methods are able to generate very uniform nanoparticle arrays and can be used to monitor the preparation process. In combination with additional techniques, even various three-dimensionally shaped structures can be formed (see, e.g., [16]). However, lithographic methods suffer from the limitation that the small interparticle spacings required for huge electromagnetic enhancement are not technically feasible. 

Computational simulations have provided a theoretical backdrop for many experimental measurements focusing on the electromagnetic enhancement... 

Since the discovery of the surface enhancement effect, it has been the subject of much debate as to what the origins of the effect are. It is generally understood that there are several mechanisms which are responsible for the observed enhancement. There are two main theories electromagnetic enhancement and charge-transfer or chemical enhancement which will be mentioned only briefly here. 

Although electromagnetic enhancement can explain important features of the SERS effect, it does not take into account in any way the chemical structure of the analyte species. Therefore, chemical enhancement models take into account the structural properties of the analyte and it is thought to operate independently from electromagnetic enhancement. Chemical enhancement can result from charge-transfer or bond formation between the metal and the analyte molecule which can result in an increase in the polarizability, ot, of the molecule. Therefore, a surface complex between the analyte and the metal must form before chemical enhancement can occur. 

The SERS electromagnetic enhancement originates from the resonance between incident radiation and electronic excitation wave on the metal surface, called surface plasmon band, as explained below. The resonance condition depends on the dielectric constant of the metal s (co) = Sj + i 2, which is a complex function of the frequency co. The enhancement factor can be expressed as ... 

The condition concerning the dielectric constant is necessary to have electromagnetic enhancement, but for observing a sizeable SERS effect, it is also... 

In contrast electromagnetic enhancement [6] relies on the intensification of the local electromagnetic field at the metal surface and the interaction of this localized electromagnetic field with the molecules close to, but not necessarily directly in contact with or chemisorbed at, the metal surface. The electromagnetic enhancement is of longer range than the chemical enhancement decaying over a distance of the order of 50 to 100 nm. 

For electromagnetic enhancement the exciting laser light must first couple to the metal surface to produce a surface plasmon. A surface plasmon is an interaction between free surface charges and the electromagnetic field [26-28] (Figure... 

One of the key developments in SE(R)RS over the last 10 or so years has been the development of structured surfaces designed to achieve strong SERS enhancements by controlling the plasmonics of the surface and thus the electromagnetic enhancement. This is a very active area of research which overlaps with work in the wider fields of nanophotonics and optical metamaterials [104, 105]. A number of excellent reviews of the area have been published in the last few years [11,15-17, 26-28, 106]. 

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