Tip-Enhanced Raman Spectroscopy
211
been a key technology in nanoscience, and many researchers have been involved in
developing and improving TERS. In the past two decades after it was first reported
in 2000, various new and exciting technological developments have been established
to improve detection sensitivity, spatial resolution, imaging speed, and so forth. In
particular, a spatial resolution of the single molecular level or even the atomic level
has been recently achieved [16]. Measurement capability of TERS is still growing
owing to the tremendous ongoing efforts from researchers.
As a comparison, in terms of optical microscopy with the high spatial resolution,
one may first come up with the super-resolution fluorescent microscopy, which was
awarded the Noble prize in Chemistry in 2014 for its prominent contribution to the
life science by imaging nanoscale details of biological samples [8, 9]. However, it
is literally a technique to observe fluorescent label, which is tagged with a target
molecule and therefore contains no information about the intrinsic properties of
the target molecules. It is therefore not possible to observe an optical signal originating from the sample itself with this technique. In this sense, as TERS achieves
the super-resolution imaging using a physically small light source at the tip apex,
nanoscale Raman analysis is possible. Although both TERS and the super-resolution
fluorescent microscopies are categorized in the super-resolution optical microscopy,
TERS has such a clearly different property. Also, some other far-field techniques to
improve the spatial resolution in Raman microscopy have been recently proposed and
demonstrated [17, 18]. However, as mentioned above, only TERS can reach down
to 10–20 nm or even single molecular level of the spatial resolution. In this chapter,
we describe this powerful technique from the basic principle to recent important
developments.
2 Basic Principle of TERS
2.1 Field Enhancement by Localized Surface Plasmon
Resonance (LSPR)
In Raman scattering process, when molecules are irradiated with light, they scatter
light that has a wavelength shifted from that of the incident light [4]. As the wavelength shift occurs due to the interaction between the incident light and the vibration
of molecular bonds, it contains a signature of the physical and chemical properties of
the sample, and thus one can obtain information about molecular bonds by analyzing
Raman scattering. Raman scattered signals are detected through a spectroscope in
the form of Raman spectrum, an example of which is shown in Fig. 1a.
Compared to fluorescence spectroscopy, a drawback in Raman spectroscopy is
the low detection sensitivity due to the intrinsically weak Raman signal. This is why
surface-enhanced Raman spectroscopy (SERS) technique has received much attention in wide research fields [19]. SERS can dramatically improve the sensitivity by
strongly enhancing both the incident and the scattered light in Raman spectroscopy.
211
been a key technology in nanoscience, and many researchers have been involved in
developing and improving TERS. In the past two decades after it was first reported
in 2000, various new and exciting technological developments have been established
to improve detection sensitivity, spatial resolution, imaging speed, and so forth. In
particular, a spatial resolution of the single molecular level or even the atomic level
has been recently achieved [16]. Measurement capability of TERS is still growing
owing to the tremendous ongoing efforts from researchers.
As a comparison, in terms of optical microscopy with the high spatial resolution,
one may first come up with the super-resolution fluorescent microscopy, which was
awarded the Noble prize in Chemistry in 2014 for its prominent contribution to the
life science by imaging nanoscale details of biological samples [8, 9]. However, it
is literally a technique to observe fluorescent label, which is tagged with a target
molecule and therefore contains no information about the intrinsic properties of
the target molecules. It is therefore not possible to observe an optical signal originating from the sample itself with this technique. In this sense, as TERS achieves
the super-resolution imaging using a physically small light source at the tip apex,
nanoscale Raman analysis is possible. Although both TERS and the super-resolution
fluorescent microscopies are categorized in the super-resolution optical microscopy,
TERS has such a clearly different property. Also, some other far-field techniques to
improve the spatial resolution in Raman microscopy have been recently proposed and
demonstrated [17, 18]. However, as mentioned above, only TERS can reach down
to 10–20 nm or even single molecular level of the spatial resolution. In this chapter,
we describe this powerful technique from the basic principle to recent important
developments.
2 Basic Principle of TERS
2.1 Field Enhancement by Localized Surface Plasmon
Resonance (LSPR)
In Raman scattering process, when molecules are irradiated with light, they scatter
light that has a wavelength shifted from that of the incident light [4]. As the wavelength shift occurs due to the interaction between the incident light and the vibration
of molecular bonds, it contains a signature of the physical and chemical properties of
the sample, and thus one can obtain information about molecular bonds by analyzing
Raman scattering. Raman scattered signals are detected through a spectroscope in
the form of Raman spectrum, an example of which is shown in Fig. 1a.
Compared to fluorescence spectroscopy, a drawback in Raman spectroscopy is
the low detection sensitivity due to the intrinsically weak Raman signal. This is why
surface-enhanced Raman spectroscopy (SERS) technique has received much attention in wide research fields [19]. SERS can dramatically improve the sensitivity by
strongly enhancing both the incident and the scattered light in Raman spectroscopy.