Fundamentals and Applications of Surface Enhanced Raman …
191
the incident electromagnetic wave and a Hertzian dipole on the nanoscale is generated. This Hertzian dipole can emit frequency at the same frequency as the incident
wave. Another dipole moment is created in the analyte molecule due to the enhanced
localized electric field around the metal nanostructure.
3.1.2 Chemical Enhancement Theory of SERS
In different SERS experiments, it is observed that the plasmon theory could not
explain alone the overall enhancement. In a system, where both the mechanisms are
simultaneously working, then the effects are multiplicative. Chemical enhancement
is mainly depending on the local electronic structures of both the probe molecule
and the SERS substrate [27, 28]. The chemical enhancement is the combination of
non-resonant charge in the molecular polarization, charge transfer between analyte
molecule and SERS substrate and enhancements from molecular excitation resonances. The non-resonant chemical enhancement is independent of the excitation
wavelength as Raman scattering process is explained on the basis of virtual energy
levels (shown in Fig. 1). However, the molecular excitation resonances arise when
Raman scattering happens via electronic levels of the analyte instead of virtual states
[29].
3.1.3 Relative Magnitude of Enhancement of Different Mechanisms
A unified explanation of SERS was explained by Herzberg–Teller coupling, which
includes the surface plasmon resonance (SPR), charge-transfer resonance and molecular resonance [23, 30]. Figure 4 shows a hypothetical illustration of the spectral
behavior and relative magnitude of the enhancement of different mechanisms. Nonresonant chemical enhancement is independent of excitation frequency and the effect
is moderate, which produces Raman signal enhancements between 10
0 and 10
2 [29].
The excitation wavelength (resonance condition) plays an important role to enhance
Fig. 4 Hypothetical
example of the spectral
dependence of SERS [29]
Electromagnetic
Charge transfer
Static chemical
Resonant
Raman
Total enhancement
191
the incident electromagnetic wave and a Hertzian dipole on the nanoscale is generated. This Hertzian dipole can emit frequency at the same frequency as the incident
wave. Another dipole moment is created in the analyte molecule due to the enhanced
localized electric field around the metal nanostructure.
3.1.2 Chemical Enhancement Theory of SERS
In different SERS experiments, it is observed that the plasmon theory could not
explain alone the overall enhancement. In a system, where both the mechanisms are
simultaneously working, then the effects are multiplicative. Chemical enhancement
is mainly depending on the local electronic structures of both the probe molecule
and the SERS substrate [27, 28]. The chemical enhancement is the combination of
non-resonant charge in the molecular polarization, charge transfer between analyte
molecule and SERS substrate and enhancements from molecular excitation resonances. The non-resonant chemical enhancement is independent of the excitation
wavelength as Raman scattering process is explained on the basis of virtual energy
levels (shown in Fig. 1). However, the molecular excitation resonances arise when
Raman scattering happens via electronic levels of the analyte instead of virtual states
[29].
3.1.3 Relative Magnitude of Enhancement of Different Mechanisms
A unified explanation of SERS was explained by Herzberg–Teller coupling, which
includes the surface plasmon resonance (SPR), charge-transfer resonance and molecular resonance [23, 30]. Figure 4 shows a hypothetical illustration of the spectral
behavior and relative magnitude of the enhancement of different mechanisms. Nonresonant chemical enhancement is independent of excitation frequency and the effect
is moderate, which produces Raman signal enhancements between 10
0 and 10
2 [29].
The excitation wavelength (resonance condition) plays an important role to enhance
Fig. 4 Hypothetical
example of the spectral
dependence of SERS [29]
Electromagnetic
Charge transfer
Static chemical
Resonant
Raman
Total enhancement