SERS enhancement factor, values

At this point, it is important to clarify what is meant by the enhancement factor before discussing hot spots further. Typically, the enhancement factor is calculated as the ratio of the detected Raman signal under SERS conditions compared to the signal obtained under normal conditions, for equal numbers of active molecules and surface area exposed to the laser beam. This value is proportional to the intensity of the local electromagnetic field ( ) to the fourth power, i.e., jEj which results from the enhancement of both the incident and emitted photons [15]. Throughout the rest of the chapter, the enhancement factor that is quoted refers to this the enhancement of SERS intensity, rather than that associated with the local electromagnetic field unless otherwise stated. 

To examine the role of the LDOS modification near a metal nanobody and to look for a rationale for single molecule detection by means of SERS, Raman scattering cross-sections have been calculated for a hypothetical molecule with polarizability 10 placed in a close vicinity near a silver prolate spheroid with the length of 80 nm and diameter of 50 nm and near a silver spherical particle with the same volume. Polarization of incident light has been chosen so as the electric field vector is parallel to the axis connecting a molecule and the center of the silver particle. Maximal enhancement has been found to occur for molecule dipole moment oriented along electric field vector of Incident light. The position of maximal values of Raman cross-section is approximately by the position of maximal absolute value of nanoparticle s polarizability. For selected silver nanoparticles it corresponds to 83.5 nm and 347.8 nm for spheroid, and 354.9 nm for sphere. To account for local incident field enhancement factor the approach described by M. Stockman in [4] has been applied. To account for the local density of states enhancement factor, the approach used for calculation of a radiative decay rate of an excited atom near a metal body [9] was used. We... 

A word of caution in addition to the uncertainties and problems discussed above, one should take note that all the enhancement factors quoted in the literature are merely average values. The average is over the total amount of adsorbed material, be it at submonolayer, monolayer, or multilayer coverages. If there are special SERS-active sites which are a small fraction of the surface, then the true intrinsic enhancement factor per molecule may be much larger than the estimated one. We should be aware of this fact when interpretation of results are considered in Section III. 

Blondeau et used radiochemical measurements in conjunction with Raman scattering. They find that the amount of adsorbed pyridine is the equivalent of several layers (based on the assumption of a coverage of 1.5 x 10 mol cm, i.e., 9 x 10 " molecules cm ). Even without almost any electrochemical treatment they see about two to three layers, while at typical conditions for SERS (25 mC cm ) the equivalent of eight layers is seen. This, of course, decreases the values calculated for the enhancement factor based on monolayer coverage. For cyanide only a monolayer of Ag(CN) ion is seen, emphasizing the large enhancement factors evaluated for this ion. 

Surface-enhanced Raman spectroscopy (SERS) provides a means of obtaining the Raman spectmm of a monolayer. Although this method requires careful preparation of a roughened surface (necessary for intensity enhancement), and the absorbance may vary from sample to sample, it is very sensitive to the functionalities located close to the metal surface. The enhancement factor depends inter alia on the distance between the functional group and the metal surface on one hand and the surface coverage on the other. A typical distance at which the enhancement factor decreases to half of its initial value in a well-packed gold-thiol monolayer is about 3.5-7. This technique provides useful... 

Furthermore, when the plasmon band and the excitation wavelength overlap, SERS enhancement can increase by a factor of 10-10, in comparison with the systems whose plasmon bands do not feature such overlap. Since the plasmon band of individual metal NPs changes when they form chain-like structures, the system may show strong enhancement when the aggregation number of the self-assembled structure reaches a threshold value. 

The ratios k k are observed to be 2.2 0.39 1. Thus, if At2 is taken to be normal , the rate constant for the reaction of ser with [Cu(en)] + is also consistent with normal substitution whereas the presence of the bulky histamine molecule leads to a relatively low rate constant. Furthermore, k- the rate constant for the dissociation of ser from [Cu(hm)(ser)]+ is also substantially lower than the rate constant for the dissociation of this ion from the other serinato-complexes (Ar i, A 2, At-ij), which all have the same value to within a factor of two. This rate reduction may indicate steric hindrance between the co-ordinated histamine molecule and an entering water molecule. In a dissociative mechanism for ligand substitution bulky non-leaving ligands appear to enhance the rate. The opposite effect shown here by histamine suggests that an associative mechanism holds for Cu substitution. Further evidence for this conclusion has been obtained from the copper(n)-ethylenediamine-histamine system. 

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