Surface Enhanced Infrared Absorption SEIRA Spectroscopy

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Surface Enhanced Infrared Absorption (SEIRA)
Spectroscopy
07/15/08 Benjamin Dach
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The Theory

• Molecules adsorbed on metal island films or
  particles exhibit 101 103 times more intense
  infrared absorption than without the metal.
• The electromagnetic (EM) interactions of the
  incident photon field with the metal and
  molecules predominantly cause the enhancement.
• Surface Enhanced Raman Scattering (SERS) is a
  very similar phenomenon to SEIRA, yet its
  enhancement is on the order of lt106.

The metal islands are polarized by the incident
photon field through the excitation of collective
electron resonance, or localized plasmon modes,
and the dipole p induced in an island generates a
local EM field stronger than the incident photon
field around the island
Topics Appl. Phys. 2001, 81, 163187
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Attenuated Total Reflection (ATR) Spectroscopy

• This reflection forms the evanescent wave which
  extends into the sample.
• The observed band intensities depend on the
  polarization and angle of incidence of the
  infrared radiation.
• Coupling ATR with SEIRA can give signal
  enhancement at least on the order of 104.

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First Finding of SEIRA in 1980
The IR absorption from molecular monolayers of
4-nitro-benzoic acid, benzoic acid, and
4-pyridine-COOH is enhanced by a factor of 20 by
thin metal overlayers or underlayers with the use
of ATR. The total enhancement, including
contributions from the ATR geometry, is almost
104.
Phys. Rev. Lett. 1980, 45(3), 201-204
of 4-nitro-benzoic acid.
on metal overlayer thickness.
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On Low Reflective Surfaces
- BaF2, Ge, microscope slide glass, and
poly(acrylonitrile-butadiene-styrene) resin.

• The sensitivity of the reflection IR spectroscopy
  at these surfaces is remarkably improved by
  evaporating very thin layer films on the sample
  surfaces.

Anal. Chem. 1993, 65, 556-562
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The Importance of Metal Morphology

• Enhanced IR absorption of C60 on thin evaporated
  Pd island films onto surface-oxidized (Si_Ox)
  Si(100) and hydrogen-terminated (Si_Hterm)
  Si(111) substrates

Isr. J. Chem. 2006, 46, 257-263
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H2_at_C60?

• Can surfaced enhanced IR techniques be used to
  detect the H2 inside the C60?
• Off center translational motion creates a dipole
  on the H2_at_C60 complex detectable by mid-infrared
  spectroscopy.
• Will the surface enhancement give a more
  powerful signal?

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Post-annealing temperature dependence of infrared
absorption enhancement of polymer on evaporated
silver films Thin Solid Films 2007, 515,
3073-3078
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Conclusions

• SEIRA is a relatively recent innovation in
  detecting vibrational modes on surfaces.
• Ultrathin films adsorbed onto metal island films
  can be detected most readily.
• Metal layer morphology, thin film thickness,
  temperature effects, and angle of incident beam
  are important factors in maximizing the enhanced
  signal
• Future Research
• Bilayers and monolayers at a liquidliquid
  interface
• Adsorption monolayers and Langmuir
  (water-insoluble) monolayers at an airwater
  interface
• Adsorption films and self-assembled monolayers
  (SAMs) at a liquidsolid interface
• LangmuirBlodgett films, cast (deposit) films,
  and spin-coat films at an airsolid interface.