Tip-Enhanced Raman Spectroscopy
231
Being based on a completely different mechanism to generate near-field light,
plasmon nanofocusing provides distinct properties of the background suppression
and the broadband, which holds a great promise for the next generation of TERS
techniques.
4.5 Single Molecular Resolution
Last but not least, a dramatic progress that has recently been seen in TERS is the
spatial resolution. Spatial resolution in TERS had typically been 10–20 nm, and only
in some particular cases, such as using tip-pressure, the limitation could be pushed
down to a few nanometers of spatial resolution as described earlier. It has been a
common understanding in TERS for a long time that the typical spatial resolution
in TERS is limited by the size of the tip apex, which is typically about several
nanometers. However, single molecular spatial resolution has suddenly been achieved
in TERS, which was first reported in 2013 [16]. In this work, STM-based TERS was
utilized under the conditions of ultrahigh vacuum and low temperature. Even though
a solid physical reason to achieve such ultrahigh spatial resolution was not provided
at that time, it has had an impact high enough to stimulate the TERS community
or even other research fields outside the nanophotonics field. Since then, several
research papers have been published on the ultrahigh-spatial-resolution TERS [86,
87]. Moreover, sub-molecular spatial resolution was recently reported in 2019 [88],
in which surprisingly the authors reported a spatial resolution of around 1Å. The
mechanism of such high spatial resolution is still under debate, but one of the ideas
that have started to be somehow accepted in the community is that an extremely tight
confinement of light field may be achieved between a few atoms protruded at the
tip apex and the substrate in the gap-mode regime, so-called the “pico-cavity”. In
this sense, only if a single or a few atoms protrude from the tip apex, atomic level
confinement of light field can be possible when the tip is brought to an angstrom
distance from the substrate. Although it has a limitation in the gap-distance and it
may work only for studying atomically thin molecules, there is no exaggeration in
saying that TERS has now become a powerful tool to study states of a single molecule
at a sub-molecule spatial resolution.
5 Conclusion
In this chapter, we described from basic instrumentations to recent developments and
trends in TERS. It has been around 20 years since the invention of TERS, and during
this journey TERS has been recognized and well-accepted as a unique and powerful
tool of Raman spectroscopy and imaging with the nanoscale spatial resolution. At
the same time, we have witnessed tremendous efforts made by many researchers that
has kept TERS evolving in different aspects, which further encourages researchers
231
Being based on a completely different mechanism to generate near-field light,
plasmon nanofocusing provides distinct properties of the background suppression
and the broadband, which holds a great promise for the next generation of TERS
techniques.
4.5 Single Molecular Resolution
Last but not least, a dramatic progress that has recently been seen in TERS is the
spatial resolution. Spatial resolution in TERS had typically been 10–20 nm, and only
in some particular cases, such as using tip-pressure, the limitation could be pushed
down to a few nanometers of spatial resolution as described earlier. It has been a
common understanding in TERS for a long time that the typical spatial resolution
in TERS is limited by the size of the tip apex, which is typically about several
nanometers. However, single molecular spatial resolution has suddenly been achieved
in TERS, which was first reported in 2013 [16]. In this work, STM-based TERS was
utilized under the conditions of ultrahigh vacuum and low temperature. Even though
a solid physical reason to achieve such ultrahigh spatial resolution was not provided
at that time, it has had an impact high enough to stimulate the TERS community
or even other research fields outside the nanophotonics field. Since then, several
research papers have been published on the ultrahigh-spatial-resolution TERS [86,
87]. Moreover, sub-molecular spatial resolution was recently reported in 2019 [88],
in which surprisingly the authors reported a spatial resolution of around 1Å. The
mechanism of such high spatial resolution is still under debate, but one of the ideas
that have started to be somehow accepted in the community is that an extremely tight
confinement of light field may be achieved between a few atoms protruded at the
tip apex and the substrate in the gap-mode regime, so-called the “pico-cavity”. In
this sense, only if a single or a few atoms protrude from the tip apex, atomic level
confinement of light field can be possible when the tip is brought to an angstrom
distance from the substrate. Although it has a limitation in the gap-distance and it
may work only for studying atomically thin molecules, there is no exaggeration in
saying that TERS has now become a powerful tool to study states of a single molecule
at a sub-molecule spatial resolution.
5 Conclusion
In this chapter, we described from basic instrumentations to recent developments and
trends in TERS. It has been around 20 years since the invention of TERS, and during
this journey TERS has been recognized and well-accepted as a unique and powerful
tool of Raman spectroscopy and imaging with the nanoscale spatial resolution. At
the same time, we have witnessed tremendous efforts made by many researchers that
has kept TERS evolving in different aspects, which further encourages researchers