Tip-Enhanced Raman Spectroscopy
225
TERS can provide such a unique way, with which one can understand at nanoscale
how chemical bonds change by an external force and/or how strong the chemical
bonds are. The shift of Mode 2 from the original G
+ -band at Mode 1 is large enough
to experimentally measure. For example, it is about 10 cm
−1 for a tip-applied force of
2.4 nN. Therefore, the shift can be easily observed in TERS measurements. Note that
the contact area between the tip apex and CNT at this value of tip-applied pressure is
ideally about 1 nm, even when the size of the tip apex was about 35 nm. This means
Mode 2 can only be observed from a tiny part of a few nanometers of the CNT, while
the enhanced Raman scattering in TERS experiment will have a spatial resolution
comparable to the size of the tip apex. By taking this advantage, the authors scanned
the CNT while measuring the shift in Mode 2 at a tip-applied force of 2.4 nN. As the
tip passed over the CNT during a one-dimensional scan, Mode 2 was observed only
within a small rage of about 4 nm, as shown in Fig. 8d. Therefore, with the inclusion
of tip-applied force, an extremely high spatial resolution of 4 nm was demonstrated.
This was the world record of spatial resolution in any kind of optical microscopy at
the time of publication of this work in 2009.
In another interesting work, an effect of local pressure applied to one CNT by
another CNT was investigated by TERS (Fig. 8e) [61]. Here, two bundles of 2–3
semiconducting CNTs were placed in the shape of the letter “X”, where one CNT
bundle lay on the other, so that a local pressure was induced at the crossing point of
the two bundles. Through the detailed analysis of nano-Raman spectra, the authors
revealed that a small part of the semiconducting CNTs turned into metallic CNTs only
within an extremely localized area around the crossing point due to an alternation
of the electric properties caused by a localized pressure-induced bond deformation
inside CNTs.
Although we do not discuss in details here, TERS is of course not limited only to
CNTs or nano carbon materials, but a variety of samples from many different fields
have been investigated by TERS. TERS is quite effective for advanced semiconducting materials as they are Raman active materials [62]. Recently, TERS investigation of 2D materials, such as graphene, MoS 2 or WSe 2 , has received great interests
because of the attractive electric properties of these materials [33, 63–65]. The suitable structure of a 2D material with atomically thin thickness has also facilitated the
use of TERS for nano-Raman analysis of 2D materials.
Biomolecules such as DNA, nucleic acids, lipids, and polypeptides are also important targets for TERS. A number of papers have been published on bio-related TERS
research, where nanoscale details of chemical bonds were well investigated and
analyzed [60, 66, 67]. The critical next step will be TERS investigation of biological
samples in their physiological environment. This usually means measuring a biosample, preferably live, when it is kept in a watery environment. An issue preventing
from achieving this goal is contamination of tips during the measurement as TERS
tips can be easily contaminated by substances floating in the liquid surrounding. The
contamination may also emit background Raman signals that would deteriorate TERS
measurements. Although some methods to protect the tip from contamination have
225
TERS can provide such a unique way, with which one can understand at nanoscale
how chemical bonds change by an external force and/or how strong the chemical
bonds are. The shift of Mode 2 from the original G
+ -band at Mode 1 is large enough
to experimentally measure. For example, it is about 10 cm
−1 for a tip-applied force of
2.4 nN. Therefore, the shift can be easily observed in TERS measurements. Note that
the contact area between the tip apex and CNT at this value of tip-applied pressure is
ideally about 1 nm, even when the size of the tip apex was about 35 nm. This means
Mode 2 can only be observed from a tiny part of a few nanometers of the CNT, while
the enhanced Raman scattering in TERS experiment will have a spatial resolution
comparable to the size of the tip apex. By taking this advantage, the authors scanned
the CNT while measuring the shift in Mode 2 at a tip-applied force of 2.4 nN. As the
tip passed over the CNT during a one-dimensional scan, Mode 2 was observed only
within a small rage of about 4 nm, as shown in Fig. 8d. Therefore, with the inclusion
of tip-applied force, an extremely high spatial resolution of 4 nm was demonstrated.
This was the world record of spatial resolution in any kind of optical microscopy at
the time of publication of this work in 2009.
In another interesting work, an effect of local pressure applied to one CNT by
another CNT was investigated by TERS (Fig. 8e) [61]. Here, two bundles of 2–3
semiconducting CNTs were placed in the shape of the letter “X”, where one CNT
bundle lay on the other, so that a local pressure was induced at the crossing point of
the two bundles. Through the detailed analysis of nano-Raman spectra, the authors
revealed that a small part of the semiconducting CNTs turned into metallic CNTs only
within an extremely localized area around the crossing point due to an alternation
of the electric properties caused by a localized pressure-induced bond deformation
inside CNTs.
Although we do not discuss in details here, TERS is of course not limited only to
CNTs or nano carbon materials, but a variety of samples from many different fields
have been investigated by TERS. TERS is quite effective for advanced semiconducting materials as they are Raman active materials [62]. Recently, TERS investigation of 2D materials, such as graphene, MoS 2 or WSe 2 , has received great interests
because of the attractive electric properties of these materials [33, 63–65]. The suitable structure of a 2D material with atomically thin thickness has also facilitated the
use of TERS for nano-Raman analysis of 2D materials.
Biomolecules such as DNA, nucleic acids, lipids, and polypeptides are also important targets for TERS. A number of papers have been published on bio-related TERS
research, where nanoscale details of chemical bonds were well investigated and
analyzed [60, 66, 67]. The critical next step will be TERS investigation of biological
samples in their physiological environment. This usually means measuring a biosample, preferably live, when it is kept in a watery environment. An issue preventing
from achieving this goal is contamination of tips during the measurement as TERS
tips can be easily contaminated by substances floating in the liquid surrounding. The
contamination may also emit background Raman signals that would deteriorate TERS
measurements. Although some methods to protect the tip from contamination have