**4. Enhancement mechanism observed in the SERS effect**

Even though, several theoretical and experimental research studies based on the SERS phenomena have been carried out and a great deal of research publications based on these works are well cited in the literature, the correct nature of the gigantic enhancement in Raman intensity found in the SERS effect is still controversial. However, it is generally accepted that two enhancement mechanisms, a long-range electromagnetic (EM) effect and a short-range chemical (CM) effect, are functioning simultaneously. The EM mechanism is based on the amplification of the electromagnetic field generated by coupling of the radiation field with the localized surface plasmons (LSP) of the metal nanoparticles. The localized surface plasmon resonanace (LSPR) arises as a result of resonance condition between the incident wavelength of light and the electrons in the nanoparticles. This facilitates a combined oscillation of the conduction electrons and it will give rise to two main consequences. The first consequence is the absorption of the wavelengths of light selectively by nanoparticles, which is responsible for this collective oscillation. The second consequence is the enhancement of electromagnetic fields that extend from the nanoparticle surfaces. These fields are mainly responsible for the large enhancement observed in SERS. The enhancement is approximately proportional to |*E*<sup>4</sup> | and generally in the order of 108 or more, where *E* is the intensity of the electromagnetic field.

Localized surface plasmon resonance (LSPR), the lightning rod effect, and the image field effect, all these effects are responsible for the enhancement in SERS. Among them, LSPR contributes mainly to the electromagnetic field enhancement and SERS effect. Anisotropic metallic nanostructures have all of the characteristics of excellent SERS active substrates with good stability and high reproducibility. It has been demonstrated in the literature that anisotropic nanostructures such as nanorods, nanodisks, and nanoprisms will exhibit interesting size and shape-dependent properties. Anisotropic metal nanoparticles will exhibit "lightningrod effect" [24], which is a new kind of field enhancement refers to enhanced charge density localization at a tip or vertex of a nanoparticle. The theory based on "lightning-rod effect" was developed by Liao and Wokaun in the year 1982. The excitation of the free electrons of a metallic tip by an electromagnetic field (laser light) will generate an extremely localized, sturdy electric field at these sharp tips or vertex with large curvatures leading to large field enhancement in those regions. This effect gives rise to high SERS activity of the anisotropic nanostructures. Anisotropic metallic nanostructures have been extensively utilized as an effective SERS active substrate with high SERS activity [25, 26] as found in the literature.

The chemical enhancement (CM) mechanism corresponds to the enhancement effect arises as a result of the chemical interaction between the adsorbates and the metal surface. The CM mechanism can also be referred as the charge transfer (CT) mechanism, involving the photoinduced transfer of an electron from the Fermi level of the metal to an unoccupied molecular orbital of the adsorbate (LUMO) or vice versa. The enhancement factor of CM is generally in the order of 10–102 . EM has a long-range effect, for which rough metallic surfaces can be used as an SERS active substrate, while CM has a short-range effect taking place on the molecular scale. The two mechanisms of EM and CM are not reciprocally restricted, but these two effects work simultaneously to generate the overall SERS effect. However, it is very hard to differentiate CM from the EM effect. Several research groups all over the world have tried to solve this problem, but the problem is unsolved so far.
