Raman signal enhancement factor

Vibrational hyper-Raman scattering [40-43]. Another nonlinear phenomenon observed for metal/molecule system is the surface-enhanced hyper-Raman scattering (SEHRS), consisting in the emission from a sample irradiated with iight at a frequency radiation with frequency 2 > com, where com is a frequency t3q)ical of the molecule (such as a vibrational frequency). For h3q)er-Raman signals enhancement factors of 10 have been measured [41]. 

The molecular orbital molecule-surface interaction generates new electronic states that can be resonant with light laser at lower frequencies than the states of either the molecule or the surface. This is the so called chemical enhancement, the other SERS important mechanism (12). The results presented here demonstrate that the signals of the Raman spectrum of the target nitroaromatic HE TNT and DNT are enhanced strongly by contact with the metal oxide particles. Since Raman signal enhancement is clear in this experiment, other mechanisms could be responsible for the enhancement factors found. 

As with RRS, but with less fluorescence problems, the signal-enhancement factors (up to 10 ) afforded by surfaced-enhanced Raman scattering has proven to be useful in studies that required trace-level detection limits and high selectivity. Detection limits in the pi-cogram level and dynamic ranges over two to three orders of magnitude are often achieved (see Table 1). Note that, however, because the analyte signal for trace levels from NRS cannot be detected, actual enhancement factors are difficult to determine for most compounds. 

Local electromagnetic field effects on certain roughened materials (e.g., Ag, Au, and Cu) enhance the Raman signal by factors of up to 10 and make detection of monolayers straightforward [35], Enhanced Raman spectra that are 10 -10 as strong as normal Raman scattering were reported for pyridine [36]. 

Ren et al. reported a method to prepare a gold tip with a tip apex radius of 30 nm reproducibly [27]. They observed the TERS of a Malachite Green isothiocyanate (MGITC) monolayer on an Au(lll) surface and obtained an enhancement factor of about 1.6 X 10, by using the relation, q= /TERs/lRRs=g /l focus where q is the net increase in the signal. Iters snd rrs the signal intensities for TERS and RRS (resonance Raman scattering), respectively is the TERS enhancement (gis the field enhancement), a denotes the radius of the enhanced field, and Rfocus the radius of the laser focus. 

Since most biomolecules normally exhibit medium or low Raman cross sections, an enhancement of the signal intensity for the ability to characterize even low concentrations would be preferable. Besides the application of resonance Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS) is a promising alternative. In doing so the vicinity of molecules to rough noble metal surfaces leads to Raman enhancement factors of 106-108 and even up to 1014 leading to a single molecule detection limit [9]. 

The size of the enhancement factor or the effective SERS cross section is a key question for the application of SERS as a tool for ultrasensitive detection. The effective cross section must be high enough to provide a detectable Raman signal from a few molecules. In the early SERS experiments, Van... 

Duyne and co-workers estimated enhancement factors on the order of 105 to 106 for pyridine on rough silver electrodes. The value was obtained from a comparison between surface-enhanced and normal bulk Raman signals from pyridine by taking into account the different number of molecules on the electrode and in solution. The size of the enhancement was found to correlate with the electrode roughness, indicating that enhancement occurs via a strong electromagnetic field. On the other hand, the dependence of the... 

In order to determine the efficacy of a SERS substrate for a given adsorbed molecule, it is necessary to quantify the degree to which the substrate increases the Raman scattered signal with respect to the non-SERS Raman scattered signal of the same molecule. This enhancement factor (EF) is most usually defined as ... 

Since the HRS scattering cross section is so small, enhancement of the signal intensity is indispensable for a practical use of the HRS technique in the field of molecular science. As already mentioned, one of the methods for signal enhancement is to use plasmonic resonances of metal nanostructures [20, 21, 25]. This enhancement technique is called surface-enhanced hyper-Raman scattering (SEHRS) because it is similar to that of surface-enhanced Raman scattering (SERS). In SERS or SEHRS, both excitation absorption and scattered emission benefit from plasmonic field enhancement. Therefore, the plasmonic enhancement factor M is described as... 

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. 

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