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T. Umakoshi and P. Verma
1 Introduction
Tip-enhanced Raman spectroscopy (TERS) is an interesting combination of Raman
spectroscopy and plasmonic near-field optical technique that takes Raman spectroscopy and microscopy to the next level beyond its conventional limits to establish
it as an inevitable optical tool to investigate various materials at the nanoscale [1–3].
Raman spectroscopy has already been well-recognized as a powerful tool to investigate molecular vibrations by spectroscopically analyzing Raman scattering from
molecules. Due to the inherent spectral and chemical specificities, it tells us which
chemical bonds the molecules have and how the bonds vibrate [4]. As it is also an
invasive and non-contact method, it has been widely used in a variety of research
fields from material science to biological science as a versatile tool [5–7]. It has
contributed in analyzing the physical properties of novel materials and in revealing
molecular activities of biological systems.
However, just like most of the classical optical techniques, Raman spectroscopy
also suffers from the issue of limited spatial resolution due to the wave nature of light.
This is because an incident light cannot be focused into a size smaller than approximately half of its wavelength due to the diffraction limit of light. For the visible light
that is often used in Raman spectroscopy, the spatial resolution is limited to a few
hundred nanometers. Improvement of the spatial resolution in Raman spectroscopy
has been in high demand, especially since nanotechnology and nanoscience have
become of great importance and interests in various scientific fields.
TERS is one of the distinctive solutions to meet this demand. It is the only technique that measures scattered light from a nanometric volume of the sample and
thus enables one to obtain Raman image of samples with a true nanoscale spatial
resolution (typically 10–20 nm). Although there are several super-resolution optical
microscopy techniques such as STED, PALM or STORM [8, 9], these techniques
do not include a direct optical observation at nanoscale. TERS is the only technique
where light is spatially confined to a true nanometric volume, which is done through
a plasmonic approach known as the near-field scanning optical microscopy (NSOM).
In NSOM, an aperture probe was initially used to generate near-field light at the tiny
aperture, which is so-called aperture-type NSOM [10]. Later in 1994, Kawata et al.
reported a new type of NSOM that uses an apertureless metallic tip instead of an
aperture tip, which is known as scattering-type (or apertureless-type) NSOM [11]. In
the scattering-type NSOM, a metallic tip that works as a plasmonic antenna is used
to generate near-field light that is highly localized at the end of the tip apex [12].
Because the spatial extent of the near-field light is almost comparable to the size of
nanometrically small tip apex, the tip apex practically behaves as a nano-light-source
that invokes Raman scattering from a tiny volume of the sample directly beneath the
tip apex. One can therefore detect Raman scattering signal from a nanoscale area
of the sample with the near-field light. Therefore, by detecting Raman signal with
the near-field light at the tip apex, nano-Raman spectroscopic analysis is possible.
Actually, this is the basic idea of TERS. It was reported for the first time in 2000
by three different groups at almost the same time [13–15]. Since then, TERS has
T. Umakoshi and P. Verma
1 Introduction
Tip-enhanced Raman spectroscopy (TERS) is an interesting combination of Raman
spectroscopy and plasmonic near-field optical technique that takes Raman spectroscopy and microscopy to the next level beyond its conventional limits to establish
it as an inevitable optical tool to investigate various materials at the nanoscale [1–3].
Raman spectroscopy has already been well-recognized as a powerful tool to investigate molecular vibrations by spectroscopically analyzing Raman scattering from
molecules. Due to the inherent spectral and chemical specificities, it tells us which
chemical bonds the molecules have and how the bonds vibrate [4]. As it is also an
invasive and non-contact method, it has been widely used in a variety of research
fields from material science to biological science as a versatile tool [5–7]. It has
contributed in analyzing the physical properties of novel materials and in revealing
molecular activities of biological systems.
However, just like most of the classical optical techniques, Raman spectroscopy
also suffers from the issue of limited spatial resolution due to the wave nature of light.
This is because an incident light cannot be focused into a size smaller than approximately half of its wavelength due to the diffraction limit of light. For the visible light
that is often used in Raman spectroscopy, the spatial resolution is limited to a few
hundred nanometers. Improvement of the spatial resolution in Raman spectroscopy
has been in high demand, especially since nanotechnology and nanoscience have
become of great importance and interests in various scientific fields.
TERS is one of the distinctive solutions to meet this demand. It is the only technique that measures scattered light from a nanometric volume of the sample and
thus enables one to obtain Raman image of samples with a true nanoscale spatial
resolution (typically 10–20 nm). Although there are several super-resolution optical
microscopy techniques such as STED, PALM or STORM [8, 9], these techniques
do not include a direct optical observation at nanoscale. TERS is the only technique
where light is spatially confined to a true nanometric volume, which is done through
a plasmonic approach known as the near-field scanning optical microscopy (NSOM).
In NSOM, an aperture probe was initially used to generate near-field light at the tiny
aperture, which is so-called aperture-type NSOM [10]. Later in 1994, Kawata et al.
reported a new type of NSOM that uses an apertureless metallic tip instead of an
aperture tip, which is known as scattering-type (or apertureless-type) NSOM [11]. In
the scattering-type NSOM, a metallic tip that works as a plasmonic antenna is used
to generate near-field light that is highly localized at the end of the tip apex [12].
Because the spatial extent of the near-field light is almost comparable to the size of
nanometrically small tip apex, the tip apex practically behaves as a nano-light-source
that invokes Raman scattering from a tiny volume of the sample directly beneath the
tip apex. One can therefore detect Raman scattering signal from a nanoscale area
of the sample with the near-field light. Therefore, by detecting Raman signal with
the near-field light at the tip apex, nano-Raman spectroscopic analysis is possible.
Actually, this is the basic idea of TERS. It was reported for the first time in 2000
by three different groups at almost the same time [13–15]. Since then, TERS has