Manufacturing technologies
Material science, Advanced electronics devices, Life science

Ultrastable long-duration super-resolution
Raman microscopy
Institute for Advanced Co-Creation Studies/Department of Applied Physics, Graduate School of Engineering
Associate Professor Takayuki Umakoshi

https://researchmap.jp/TakayukiUmakoshi?lang=en

Department of Applied Physics, Graduate School of Engineering
Professor Prabhat Verma						https://researchmap.jp/Verma_Prabhat?lang=en

Abstract
Near-field Raman microscopy utilizes nanoscale near-field light
generated at the apex of a metallic probe to perform Raman imaging of chemical properties of samples with nanoscale spatial
resolution (~10 nm). In this study, we developed an ultrastablenear-field Raman microscope capable of extended measurement
durations. In previous works, mechanical drift limited measurement
times to around 30 minutes. By developing an automated drift compensation system, we realized a near-field Raman microscope ideally capable of imaging without any limitation to the measurement
time. We demonstrated near-field Raman imaging of two-dimensional material MoS2 (molybdenum disulfide) over a 6-hour span.

analyze, for instance, micro-sized semiconductor transistors at thedevice scale. Furthermore, it holds the potential to observe biological samples that could not be easily imaged due to weak Raman
signals. Importantly, the high stability and reproducibility significantly enhance its reliability and practicality. As a truly practicalsuper-resolution Raman microscope, it offers contributions ranging
from advanced materials to life sciences.

Manufacturing technologies▍ Background & Results
Near-field Raman microscopy, a powerful super-resolution microscopy technique, detects Raman scattered light from samples
with about 10 nm spatial resolution, using near-field light generated
at the apex of a metallic probe tip by illuminating it with incident
light. Raman scattered light possesses rich optical and chemical
information, such as molecular vibrations and chemical bonding.
Super-resolution Raman imaging is possible by raster-scanning thesample. However, even a slight drift from the focal spot of the incident light can cause the near-field light to vanish. Imaging time istherefore limited to roughly 30 minutes until this drift occurs, which
also restricts the observation area to only about 1 µm2. This not
only decreases measurement stability and reproducibility but also
limits the samples that can be observed.
In this research, we developed a unique compensation systemthat three-dimensionally holds the metallic probe tip at the centerof the focal spot with nanoscale precision, achieving an ultrastablenear-field Raman microscope. To hold the tip within the focal spot,drifts in both the lateral and vertical directions need to be corrected. We introduced a galvanometer scanner for detecting and correcting tip drift in the lateral direction by obtaining scattering images from the probe tip within a certain period of time. For the verticaldirection, we introduced a guide laser to detect vertical drift of thefocus spot in terms of a lateral displacement of the guide laser on
a position sensor, correcting it with an objective lens positioner.
Using the developed compensation system, we confirmed that theRaman signal did not degrade even after much longer than 30 minutes. In addition, we successfully demonstrated super-resolution
Raman imaging of the two-dimensional material MoS2 on a microscale area over 6 hours. This comprehensive large-area observationeven enabled us to detect rare Raman signals (distributed in 0.2%of the entire area) originating from defect structures, which could
be overlooked with precious methods.

Overview of ultrastable near-field Raman Spectroscopy

Large-area super-resolution Raman image obtained of MoS2 through 6-hour imaging▍ Significance of the research and Future perspective
Previously, due to time constraints, only a very narrow area of thesample within a few hundred nanometers could be observed with
the near-field Raman microscope. With high stability and an extended imaging period, we can now comprehensively image andP a t e n tKato, Ryo; Moriyama, Toki; Umakoshi, Takayuki et al. Ultrastable tip-enhanced heperspectral optical nanoimaging for defect analysis of large-sized WS2layers. Science Advances 2022, 8, eabo4021. doi: 10.1126/sciadv.abo4021
Umakoshi, Takayuki; Kowashima, Koji; Moriyama, Toki et al. Tip-enhanced Raman spectroscopy with amplitude-controlled tapping-mode AFM. Scientific
Reports 2022, 12, 12776. doi: 10.1038/s41598-022-17170-7
URL http://naspec.ap.eng.osaka-u.ac.jp
K e y w o r d near-field optical microscopy, Raman spectroscopy, super-resolution Raman microscopy
Tr e a t i s e