Question

 

Explain (a) the surface plasmon polariton, (b) localized surface plasmon (LSP), and (c)
how to apply the LSP to realize the surface‐enhanced Raman scattering spectroscopy
(SERS). Please describe the working principle of SERS and the argument whether SERS
can be applied to detect single molecule.

Answer 1

Plasmon: free electrons at the metal surface i.e. the electron liquid of high-density n ≈10²³ cm^-3

Surface Plasmon Polariton: The light-matter interaction is called the polariton i.e. the coupled state between the photon and elementary excitation. The plasmon-polariton is the coupled state between the photon and plasmon  

• Surface Plasmons (SPs) are collective oscillations of free electrons at metal/dielectric interface see in figure 1
• Surface Plasmon Polariton (SPPs) are highly localized to interfaces is called LSPR and propagate easily for microns. The energy in SPPs modes enhances absorption

  

Figure 1: The shows the oscillations of free electrons at the metal/dielectric interface

SERS: SERS is a phenomenon associated with the enhancement of the electromagnetic field surrounding small objects optically excited near an intense and sharp plasmon resonance. The enhanced fields excite the adsorbate (probe) and the scattered radiation will again be enhanced.

or

Surface-enhanced Raman scattering (SERS) consists in using the large local field enhancements that can exist at metallic surfaces (under the right conditions, typically by profiting from localized surface plasmon resonances) to boost the Raman scattering signal of molecules at (or close to) the surface

SERS conditions: – specific metals (e.g. Au, Pt, Ag, Cu, ...) – surfaces with roughness on the nanometer scale – proper wavelength of excitation

The Raman scattering intensity coming from molecules located in the vicinity of nanostructure metallic surface excited by visible light can strongly be enhanced; these phenomena are called surface-enhanced Raman scattering shown in figure 2(a,b)

Figure 2: The Raman scattering show in figure 2(a, b) and υ is independent of υ₀

The majority of vibrational bands will correspond to the vibration of the closet bonds to the metallic surface shown in figure 2(b)

Advantages –

• This is Non-destructive, non-invasive techniques
• Works in-situ and in-vitro for biological samples
• Molecules with very low efficiency
• No vacuum needed –
• Very small analyzing volume
• till single molecule (SERS) –
• Extremely high spatial resolution (SERS)