Surface Enhanced Raman Spectroscopy SERS

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Surface Enhanced Raman Spectroscopy (SERS)

• BMEN 489/689

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Raman Spectroscopy Review

• Allows indirect probing of vibrational states
  in molecules.
• Unique spectral signatures provide means of
  molecular identification
• Used extensively for identification of chemical
  constituents in liquid mixtures (i.e. in
  petroleum industry)
• Can be used in biomedicine as well

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Optical Activity in Molecules

• Rotational Spectra Molecular system rotates
  about axis perpendicular to internuclear axis
  (low energy)
• Vibrational Spectra Nuclei vibrate about
  equilibrium positions (medium energy)
• Electronic Spectra Change in electronic
  configuration (high energy)

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Vibrational Spectra

• Quantum mechanics tells us we get discrete states
  within the potential well
• For parabolic well En (n½)h?0

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Electronic Spectra

• Optical absorption emission arises from change
  in electronic configuration

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Vibrational Spectroscopy

• Vibrational states can be accessed utilizing two
  methods
• Infrared Spectroscopy Direct absorption of low
  energy light
• Raman Spectroscopy Inelastic scattering with
  energy transfer to molecule (Stokes Line) or
  energy transfer from the molecule (anti-Stokes
  Line)

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Vibrational Spectroscopy

• CO2 Example
• Infrared active is permanent dipole moment
  changes as in bending
• Raman active if polarizability changes as in
  symmetric stretch

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Vibrational Spectroscopy

• Raman and Infrared spectroscopy are complementary
  due to selection rules
• Highly symmetric polyatomic molecules with center
  of inversion are IR active
• Homonuclear diatomic molecules are Raman active
• Non-symmetric molecules demonstrate both
  modalities

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Raman Spectroscopy

• Briefly descend into the math
• Induced dipole
• µind aE0cos(?t)
• Polarizability
• a a0 a1cos(?1t)
• Internal motion modulates the induced dipole
  moment
• µind a0 E0cos(?t) Rayleigh
• ½ a1E0cos(? - ?1)t Stokes
• ½ a1E0cos(? ?1)t anti-Stokes

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Raman Spectroscopy

• Both rotational and vibrational states can be
  observed but rotational states are difficult to
  spectrally separate
• Uses inelastic scattering

?
?
Stokes band transfers energy to the molecule
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Raman Spectrum (Shift)

• Cycloxenane excited at 1064nm
• Spectrum independent of excitation
• Exact peak values depend on molecular conformation

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Raman Spectroscopy

• Generally, the Raman intensity, IR, can be
  written as
• However, sR tends to be very small (i.e.
  10-30cm2/molecule)
• So, we have to use lots of laser power and wait a
  long time

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Surface Enhanced Raman

• In the 1970s, it was observed that rough metal
  surfaces produced enhanced Raman signals (up to
  106)
• Observations
• Required roughened (10-100nm) surface of highly
  reflective metals
• Raman intensity generally falls off with
  increasing vibrational frequency
• Raman shifts are sometimes shifted from standard
  Raman and new peaks appear

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Surface Enhanced Raman

• The effect was also observed in metal
  nanoparticles (NP) we will use this as the
  basis for our explaination

• Recall the dielectric function for a metal
• At resonance for NP
• em -2 eH

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Surface Field Enhancement

• At resonance, we also observe a field enhancement
  at the surface of the NP when in resonance

Kelly et al., J. Chem. Phys. B 107, 668 (2003).
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Surface Field Enhancement

• So, the particle acts as an antenna and
  concentrates the incident field at the
  nanoparticle surface can be 10-100x
• Can plot the Poynting vector of the incident
  radiation and absorption around the sphere

Below resonance, particle polarizes in phase and
thus reduces the field inside and slightly
increases field outside
As resonance, particle polarizes resonantly and
greatly enhances the field at the particle
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Surface Field Enhancement

• Additionally, the nanoparticle enhances the Raman
  scattered field as well can be 10-100x as well
• Overall, the Raman signal actually experiences a
  surface enhancement proportional to

Can be 108!
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Field Decay

• If we move away from the surface, the dipole
  decay has to be considered
• Single molecule
• Monolayer

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Surface Enhanced Raman

• So what does this tell us?
• Want em to be small (low loss) near the
  resonance metals
• Because signal depends on both E2laser(?) and
  E2Raman(?), as ?-? gets larger, G will get
  smaller since the resonance condition is NOT met
  at both ? and ?

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Chemical Enhancement

• When molecules are adsorbed to the surface, their
  electronic states can interact with the states in
  the metal and produce new transitions
• True nature of this still not fully understood