3 Surface-enhanced Raman Scattering ...
31
vibrational Raman effect is classically described by Placzek’s theory, which is
a simplified depiction of interaction between a molecule and electromagnetic wave
that induces dipole moment in the molecule. As far as oscillation Raman effect is
concerned, one needs to realize that molecular vibrations influence properties of the
charge distribution in the molecule. therefore, dipole oscillates with frequencies
ν 0 , ν
ν
0
- osc , and ν
ν
0
+ osc , emitting light of frequency equal to its own ( ν 0 being
frequency of incident light beam and ν osc : frequency of molecular vibrations).
As it follows from the classical description, two types of Raman scattering are
distinguished: when the energy of the scattered photons is lower or higher than the
energy of the incident ones. It is explained as losing or gaining energy to vibrational
or rotational modes of the molecule.
unfortunately, Raman effect concerns only a small number of photons; vast majority of incident photons is scattered elastically without the energy change. Raman scattering involves only 1 of about 10
7
photons. Such obstacle prevented this
technique from effectively developing in application fields for years. It was only
in 1970s when Fleischmann et al. [1] observed a sudden rise in the Raman scattering intensity of the molecules adsorbed on a roughened electrode surface. Even
though the contribution of the metal surface to the enhancement was not initially
recognized, this technique is fully referred to as surface-enhanced Raman scattering
(SERS) nowadays. Intrinsically weak Raman scattering can be enhanced thanks to
the interaction between all three components of the system: light, adsorbed molecule and the metal surface. Nature of the effect, which is a borderline case between
chemistry and physics, makes it necessary to combine languages of both fields in
order to fully describe its mechanism. theories explaining SERS belong to one
of the two categories: chemical and electromagnetic ones. It is now well known
that the dominant contribution to the enhancement arises from the electromagnetic
effect. however, some observations, such as stronger enhancement of the Raman
scattering of chemically adsorbed molecules as compared to physically adsorbed
ones cause that chemical mechanism should be also taken into account.
Chemical explanation of the surface-enhanced Raman scattering mechanism
makes use of charge-transfer theory. It focuses on the influence of the metal surface on energy levels of adsorbed molecules [3]. According to the chemical theory,
Raman intensity gain in SERS is obtained by matching the energy of the incident
beam with the energy of electron transition between Fermi level of the metal and a
molecular level of the adsorbate. Situation when intensity of Raman scattering rises
due to laser-induced intermolecular transitions is analogous to resonance Raman
mechanism. In that case, enhancement of the Raman scattering is obtained when a
laser beam of energy equal to electronic transition of the molecule is applied.
Some features of SERS prove that chemical mechanism plays a role in the enhancement; potential-dependent experiments show that the phenomenon might be
tuned to a certain beam energy through applying appropriate bias to the metal adsorbent. however, there are features that cannot be understood on the ground of
charge-transfer theory. It turns out that every SERS-active metal has its own energy
work range. It is widely known that silver nanostructures yield the best enhancement
when exposed to blue/green laser beam; on the other hand, red light supports SERS
31
vibrational Raman effect is classically described by Placzek’s theory, which is
a simplified depiction of interaction between a molecule and electromagnetic wave
that induces dipole moment in the molecule. As far as oscillation Raman effect is
concerned, one needs to realize that molecular vibrations influence properties of the
charge distribution in the molecule. therefore, dipole oscillates with frequencies
ν 0 , ν
ν
0
- osc , and ν
ν
0
+ osc , emitting light of frequency equal to its own ( ν 0 being
frequency of incident light beam and ν osc : frequency of molecular vibrations).
As it follows from the classical description, two types of Raman scattering are
distinguished: when the energy of the scattered photons is lower or higher than the
energy of the incident ones. It is explained as losing or gaining energy to vibrational
or rotational modes of the molecule.
unfortunately, Raman effect concerns only a small number of photons; vast majority of incident photons is scattered elastically without the energy change. Raman scattering involves only 1 of about 10
7
photons. Such obstacle prevented this
technique from effectively developing in application fields for years. It was only
in 1970s when Fleischmann et al. [1] observed a sudden rise in the Raman scattering intensity of the molecules adsorbed on a roughened electrode surface. Even
though the contribution of the metal surface to the enhancement was not initially
recognized, this technique is fully referred to as surface-enhanced Raman scattering
(SERS) nowadays. Intrinsically weak Raman scattering can be enhanced thanks to
the interaction between all three components of the system: light, adsorbed molecule and the metal surface. Nature of the effect, which is a borderline case between
chemistry and physics, makes it necessary to combine languages of both fields in
order to fully describe its mechanism. theories explaining SERS belong to one
of the two categories: chemical and electromagnetic ones. It is now well known
that the dominant contribution to the enhancement arises from the electromagnetic
effect. however, some observations, such as stronger enhancement of the Raman
scattering of chemically adsorbed molecules as compared to physically adsorbed
ones cause that chemical mechanism should be also taken into account.
Chemical explanation of the surface-enhanced Raman scattering mechanism
makes use of charge-transfer theory. It focuses on the influence of the metal surface on energy levels of adsorbed molecules [3]. According to the chemical theory,
Raman intensity gain in SERS is obtained by matching the energy of the incident
beam with the energy of electron transition between Fermi level of the metal and a
molecular level of the adsorbate. Situation when intensity of Raman scattering rises
due to laser-induced intermolecular transitions is analogous to resonance Raman
mechanism. In that case, enhancement of the Raman scattering is obtained when a
laser beam of energy equal to electronic transition of the molecule is applied.
Some features of SERS prove that chemical mechanism plays a role in the enhancement; potential-dependent experiments show that the phenomenon might be
tuned to a certain beam energy through applying appropriate bias to the metal adsorbent. however, there are features that cannot be understood on the ground of
charge-transfer theory. It turns out that every SERS-active metal has its own energy
work range. It is widely known that silver nanostructures yield the best enhancement
when exposed to blue/green laser beam; on the other hand, red light supports SERS