Towards stable and sustainable Raman imaging of large samples at the nanoscale

Raman spectroscopy, an optical microscopy technique, is a non-destructive chemical analysis technique that provides rich molecular fingerprint information about chemical structure, phase, crystallinity and molecular interactions. This technique relies on the interaction of light with chemical bonds in a material. However, because light is a wave, optical microscopy cannot resolve distances of less than half the wavelength from the light incident on the sample. This is known as the “diffraction limit,” which prevents Raman spectroscopy and other optical microscopy techniques from achieving nanoscale resolution.

To increase the spatial resolution, another technique called “tip-enhanced Raman spectroscopy” (TERS) was invented, which can achieve a spatial resolution below the diffraction limit. In TERS, a nano-sized metal tip confines light in a nano-sized volume just above the sample. The light interacts with the sample molecules on the surface and imaging is done by analyzing the scattered light.

TERS has been used successfully to analyze chemical composition and surface defects in samples at nanoscale resolution. However, during imaging, the nanotip tends to drift due to unavoidable thermal fluctuations and vibrations in ambient conditions, causing the sample to become out of focus or misaligned between the nanotip and the focus point, or both. This causes considerable distortion of the scattered signal. To avoid this, TERS imaging must be completed within a 30-minute timeframe, a limitation which prevents imaging of any samples larger than 1 m.² with nano resolution.

In a new study published in Science Advances, a team of researchers from Japan, led by Dr. Ryo Kato, Assistant Professor appointed at the Institute of Post-LED Photonics at Tokushima University, and Associate Professor Takayuki Umakoshi and Professor Prabhat Verma from Osaka University, have now developed, for the first time, a stable TERS system that is not confined to an imaging time window. short one. The team demonstrated its capabilities by successfully imaging nanoscale defects for 6 hours in two-dimensional micrometer (2D) tungsten disulfide (WS).₂) films – materials commonly used in optoelectronic devices. “Our new optical nano-imaging system enables the characterization of flaw analysis in large-sized WS₂ coating at high pixel resolutions up to 10 nm without significant optical signal loss,” said Dr. Kato.

To compensate for long-term drift, the team developed a feedback system that tracks the displacement of the focused light source and readjusts the position of the focal plane. The focal position of the light source is tracked by measuring the displacement of the reflected laser guide beam aimed at the microscope. Focus is then stabilized with a piezo-controlled objective scanner whenever the system senses deviations or changes in the focus position of the light source.

To stabilize the nanotips, the team designed a laser scan-assisted tip drift compensation system. In this case, the galvanic scanner takes an image of the laser dot around the metal nanotip as it approaches the sample surface. This image appears as a bright dot and shows the position of the nano tip. After measurements at specific pixels are made, the image of the laser dot around the nanotip is recaptured. The laser spot is then moved to match the new position of the nanotip in this image. The process continues throughout the imaging process, ensuring the nanotip remains in a constant position.

By applying this correction, the team was able to create a 2D WS . sheet image₂ (Figure 1) with a scanning area of ​​1 × 4 m². With an imaging time window 12 times longer than conventional imaging, they can detect unique defects missed in conventional TER imaging. They also show that the density of defects in the larger WS₂ sample (comparable to device scale) is higher than reported for smaller samples.

This study could open the door for precise high-resolution imaging of not only optoelectronic devices but of biological samples as well. “Our new drift-compensated TERS microscopy can not only better evaluate the surface properties of device materials, but also allow us to study biological processes such as the mechanisms underlying disease progression. This, in turn, could help develop new clinical methods and therapies,” speculates Dr. Umakoshi. These are certainly some interesting possibilities to ponder!

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