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What is SERS?
Surface Enhanced Raman Spectroscopy
(SERS)
A molecule adsorbed on a corrugated metal produces a Raman signal
which is 6 to 12 orders of magnitude more intense than the signal
it would emit if deposited on a flat substrate. The enhancement
comes from the increase of the local optical field intensity in
the proximity of sharp points of textured metals [Au, Ag, Cu], or
in the nano-scale gaps between colloidal particles.
It was observed for the first time
in 1974 [1] on pyridine molecules absorbed unto an electrochemically
roughen silver surface. To date, however, the theoretical understanding
of SERS is not clear, but it is accepted that it has two linked
components.
The electromagnetic contribution is due to the increase of the optical
field intensity in the proximity of sharp points, whereas the chemical
effect is due to the mixing of the orbital of the adsorbed molecule
and the metal atoms.
How does SERS work?
The phenomena mediating the enhanced Raman scattering interaction
between the laser light and the molecule is called a “surface
plasmon”, and can be viewed as collective charge oscillation
at the metal air interface.
SERS process steps: (1) laser light incident on the metal substrate
(2) plasmons excitation (3) light scattered by the molecule (4)
Raman scattered light transferred back to plasmons and scattered
in air (5)
Plasmons at the metal act as antennas, which assist in coupling
light into molecules close to the surface and couple out photons
into specific directions. It is this enhanced coupling both into
and out of the molecule that enhances the Raman signal.
The plasmon properties – such a wavelength and width of its
resonance – depend on the nature of the metal surface and on
its geometry. Many early SERS substrates used a random roughening
of the surface so only small uncontrolled areas of the total metal
surface would have the correct geometry for Raman enhancement. Other
techniques have relied on aggregating gold colloids and with only
some colloids in solution being SERS active. Most traditional techniques
of preparing SERS surfaces have therefore been plagued by 100% variations
in the raman signal across the surface and by hot spots where only
small areas of the total devices had the right metal geometry for
sers enhancement.
Photonic Crystal SERS substrates
Photonic Crystal SERS substrates that are used in Klarite are a
new class of highly engineered surfaces with sub-micron metal cavities.
Instead of depending on random roughening or nanoparticle separation
and sharp metallic features - as used in previous techniques - they
exploit voids architecture. Such continuous flat-voids metal film
can support two types of plasmons. The delocalised plasmons are
distributed on the metal surface, whereas localised plasmons are
trapped in the void features. Delocalised and localised plasmons
interact strongly, both mutually and with the incident light. By
modifying the size, separation and geometry of texture features,
the properties of photonic crystal SERS substrates can be tuned,
making them extremely versatile.
Furthermore by exploiting semiconductor lithographic fabrication
technology the photonic crystal pattern of the Klarite surface can
be reproducibly fabricated with high precision over large areas.
By providing a uniform patterned surface, Klarite slides provide
control of the Raman process giving consistent SERS signals from
anywhere on the active surface.
Example of Photonic Crystal SERS substrates. The cross section shows
the details of nano-structured metal. The top view diagram shows
the uniform distribution of the localised and delocalised plasmons
on the textured metal.
[1] Fleischman M, Hendra PJ, McQuillan AJ. Chem. Phys. Lett. 26,
123 (1974)
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