Tip-Enhanced Raman Spectroscopy
213
metallic particle, respectively. The dielectric constant is a function of the wavelength.
At a certain incident wavelength that satisfies the condition of ε 1 = −2ε 2 , the polarizability α diverges to infinity, which indicates the resonant excitation of the localized
surface plasmons in the metallic particle [25]. In reality, the polarizability α cannot
be infinity as we have to consider damping factor of the metallic particles. However,
one can still obtain enormous enhancement of Raman signals as mentioned above.
2.2 LSPR at the Tip for TERS
While SERS provides huge enhancement in Raman scattering to an extent that even
an extremely weak Raman scatterer such as an isolated single molecule can be
detected, it does not solve the issue of poor spatial resolution of conventional Raman
spectroscopy. This is because the light field is confined only in the direction normal to
the metallic substrate, and is not confined in the lateral direction along the substrate.
In order to achieve the lateral confinement of the light field, one must reduce the
size of the substrate to the range of a few nanometers. This is realized in TERS,
where the nanometric apex of the metallic tip used in TERS behaves as the tiny
substrate of SERS. In this sense, TERS can be considered as a special case of SERS
where a single metallic nanoparticle sitting right at the apex of the nano tip invokes
SERS [1]. Figure 1c illustrates how localized surface plasmons can be resonantly
excited in a single metallic nanoparticle by an incident light that results in the creation
of enhanced near-field evanescent light in the close vicinity of the nanoparticle. This
enhanced light field is confined in all three directions within a volume compared
to or smaller than that of the nanoparticle. If a sample is placed in this enhanced
and confined light, Raman scattering can be invoked only from a tiny part of the
sample that is immersed in this confined light. In this way, Raman scattering can be
measured from a nanometric volume of the sample. If one would like to measure
Raman scattering from different areas of the sample, but at a spatial resolution of
a few nanometers, then it would be necessary to move this metallic nanoparticle
along the sample surface while measuring Raman scattering at every position of the
nanoparticle. An easy way of doing this is by replacing the metallic nanoparticle with
a metallic nanotip that can be conveniently scanned over the sample surface by means
of a scanning probe microscopy (SPM) control, such as an atomic force microcopy
(AFM) or a scanning tunneling microscopy (STM). The apex of a metallic nanotip
can be approximated with a single metallic nanoparticle, as illustrated in Fig. 2,
which would also generate a highly confined and strongly enhanced light field at the
very end of the tip apex. When Raman scattering from a sample placed beneath the
apex of a metallic nanotip is enhanced in this way, it is called the tip-enhanced Raman
scattering. An important difference between SERS and TERS is the confinement of
light in the lateral direction that provides high spatial resolution to TERS. Here one
can position the tip apex at an arbitrary location and scan it over the sample surface
under the precise control of a SPM. This makes it possible not only to obtain nanoRaman signal from any arbitrary location of the sample, but also to construct Raman
213
metallic particle, respectively. The dielectric constant is a function of the wavelength.
At a certain incident wavelength that satisfies the condition of ε 1 = −2ε 2 , the polarizability α diverges to infinity, which indicates the resonant excitation of the localized
surface plasmons in the metallic particle [25]. In reality, the polarizability α cannot
be infinity as we have to consider damping factor of the metallic particles. However,
one can still obtain enormous enhancement of Raman signals as mentioned above.
2.2 LSPR at the Tip for TERS
While SERS provides huge enhancement in Raman scattering to an extent that even
an extremely weak Raman scatterer such as an isolated single molecule can be
detected, it does not solve the issue of poor spatial resolution of conventional Raman
spectroscopy. This is because the light field is confined only in the direction normal to
the metallic substrate, and is not confined in the lateral direction along the substrate.
In order to achieve the lateral confinement of the light field, one must reduce the
size of the substrate to the range of a few nanometers. This is realized in TERS,
where the nanometric apex of the metallic tip used in TERS behaves as the tiny
substrate of SERS. In this sense, TERS can be considered as a special case of SERS
where a single metallic nanoparticle sitting right at the apex of the nano tip invokes
SERS [1]. Figure 1c illustrates how localized surface plasmons can be resonantly
excited in a single metallic nanoparticle by an incident light that results in the creation
of enhanced near-field evanescent light in the close vicinity of the nanoparticle. This
enhanced light field is confined in all three directions within a volume compared
to or smaller than that of the nanoparticle. If a sample is placed in this enhanced
and confined light, Raman scattering can be invoked only from a tiny part of the
sample that is immersed in this confined light. In this way, Raman scattering can be
measured from a nanometric volume of the sample. If one would like to measure
Raman scattering from different areas of the sample, but at a spatial resolution of
a few nanometers, then it would be necessary to move this metallic nanoparticle
along the sample surface while measuring Raman scattering at every position of the
nanoparticle. An easy way of doing this is by replacing the metallic nanoparticle with
a metallic nanotip that can be conveniently scanned over the sample surface by means
of a scanning probe microscopy (SPM) control, such as an atomic force microcopy
(AFM) or a scanning tunneling microscopy (STM). The apex of a metallic nanotip
can be approximated with a single metallic nanoparticle, as illustrated in Fig. 2,
which would also generate a highly confined and strongly enhanced light field at the
very end of the tip apex. When Raman scattering from a sample placed beneath the
apex of a metallic nanotip is enhanced in this way, it is called the tip-enhanced Raman
scattering. An important difference between SERS and TERS is the confinement of
light in the lateral direction that provides high spatial resolution to TERS. Here one
can position the tip apex at an arbitrary location and scan it over the sample surface
under the precise control of a SPM. This makes it possible not only to obtain nanoRaman signal from any arbitrary location of the sample, but also to construct Raman