J. Bukowska and P. Piotrowski
32
on gold structures. In addition, enhancement factor depends on electric permittivity
of the medium. this inconsistency led to the second theory of SERS: electromagnetic one. It explores the influence of electromagnetic wave on a roughened surface. First attempt to describe SERS electromagnetically was dipole image theory.
It presented a molecule as an oscillating electric dipole. vibrations of the induced
dipole are caused by oscillating electric compound of the electromagnetic wave.
As the molecule is situated near the surface, it polarizes the metal. thus, image
dipole in the metal is created. It, in turn, causes extra polarization of the molecule
which completes the feedback loop of interaction leading to the enhancement of the
electric field near the metal surface. one of the main flaws of this theory is that it
does not answer the question why metal nanostructures on the surface are essential
to Raman enhancement. more refined theory includes the idea of surface plasmon
resonance (SPR). Physicists define plasmons as collective oscillations of electron
plasma (collection of valence electrons in the solid) [4]. Plasmons of different energies describe different excitation modes of such oscillations. Special properties of
longitudinal charge density waves propagating at a metal/dielectric interface led to
distinction of a subgroup of plasmons: surface plasmons [5]. these quasi-particles
can be excited only by a transverse electromagnetic wave. What is more, optical
properties of the system change drastically when energies of surface plasmons and
incident light beam match. In that case, giant rise in electric field intensity near the
interface is observed. this phenomenon is called surface plasmon resonance. For
light from near uv/vIS/near IR range (usually used to excite Raman scattering),
SPR is possible to obtain for plasmons confined in small metallic nanostructures;
then we talk about localized surface plasmons (more accurately localized surface
plasmon-polaritons, in order to refer to coupling between plasmons and light).
When SPR occurs, molecules situated in the vicinity of such a metal nanostructure
are exposed to electric field of intensity magnified several orders of magnitude (in
comparison to the incident beam). As a result, electromagnetic enhancement factors
reach up to 10
11
(see critical paper on this topic by Ru et al. [6]). Exceptionally high
enhancement factors are seen on the edges of the nanostructures. Such places might
be called hot-spots (Fig. 3.1). Needless to say, only selected metals enhance Raman
scattering effectively in the visible and near infrared range. Coinage metals, silver,
gold and copper, face all the extra requirements needed to provide SERS signal.
Extensive outlook on the problem of SERS substrates is presented in sect. 3.
one more thing needs to be highlighted. If one is already acquainted with Raman spectroscopy and knows Raman spectra of investigated molecules, it might be
tempting to compare them to SERS spectra. however, they may—and usually they
do—differ. due to adsorption onto the metal surface, energies of molecular vibrations change which leads to modification of the Raman shift of certain bands. Furthermore, selection rules that apply to electromagnetic and charge transfer mechanisms of surface enhancement make some vibrational modes selectively enhanced,
dependent on the relative contribution of each mechanism to total enhancement
[8]. thus, relative intensities of the bands in the SERS spectra are also altered in
comparison to the normal Raman ones. As normal Raman selection rules do not
apply in SERS, some bands ascribed to forbidden transitions may appear. Signifi-
32
on gold structures. In addition, enhancement factor depends on electric permittivity
of the medium. this inconsistency led to the second theory of SERS: electromagnetic one. It explores the influence of electromagnetic wave on a roughened surface. First attempt to describe SERS electromagnetically was dipole image theory.
It presented a molecule as an oscillating electric dipole. vibrations of the induced
dipole are caused by oscillating electric compound of the electromagnetic wave.
As the molecule is situated near the surface, it polarizes the metal. thus, image
dipole in the metal is created. It, in turn, causes extra polarization of the molecule
which completes the feedback loop of interaction leading to the enhancement of the
electric field near the metal surface. one of the main flaws of this theory is that it
does not answer the question why metal nanostructures on the surface are essential
to Raman enhancement. more refined theory includes the idea of surface plasmon
resonance (SPR). Physicists define plasmons as collective oscillations of electron
plasma (collection of valence electrons in the solid) [4]. Plasmons of different energies describe different excitation modes of such oscillations. Special properties of
longitudinal charge density waves propagating at a metal/dielectric interface led to
distinction of a subgroup of plasmons: surface plasmons [5]. these quasi-particles
can be excited only by a transverse electromagnetic wave. What is more, optical
properties of the system change drastically when energies of surface plasmons and
incident light beam match. In that case, giant rise in electric field intensity near the
interface is observed. this phenomenon is called surface plasmon resonance. For
light from near uv/vIS/near IR range (usually used to excite Raman scattering),
SPR is possible to obtain for plasmons confined in small metallic nanostructures;
then we talk about localized surface plasmons (more accurately localized surface
plasmon-polaritons, in order to refer to coupling between plasmons and light).
When SPR occurs, molecules situated in the vicinity of such a metal nanostructure
are exposed to electric field of intensity magnified several orders of magnitude (in
comparison to the incident beam). As a result, electromagnetic enhancement factors
reach up to 10
11
(see critical paper on this topic by Ru et al. [6]). Exceptionally high
enhancement factors are seen on the edges of the nanostructures. Such places might
be called hot-spots (Fig. 3.1). Needless to say, only selected metals enhance Raman
scattering effectively in the visible and near infrared range. Coinage metals, silver,
gold and copper, face all the extra requirements needed to provide SERS signal.
Extensive outlook on the problem of SERS substrates is presented in sect. 3.
one more thing needs to be highlighted. If one is already acquainted with Raman spectroscopy and knows Raman spectra of investigated molecules, it might be
tempting to compare them to SERS spectra. however, they may—and usually they
do—differ. due to adsorption onto the metal surface, energies of molecular vibrations change which leads to modification of the Raman shift of certain bands. Furthermore, selection rules that apply to electromagnetic and charge transfer mechanisms of surface enhancement make some vibrational modes selectively enhanced,
dependent on the relative contribution of each mechanism to total enhancement
[8]. thus, relative intensities of the bands in the SERS spectra are also altered in
comparison to the normal Raman ones. As normal Raman selection rules do not
apply in SERS, some bands ascribed to forbidden transitions may appear. Signifi-