rus
Issue #6/2015
T.Dieing, U.Schmidt, S.Breuninger
Nearfield-Raman – Imaging beyond the diffraction limit
Nearfield-Raman – Imaging beyond the diffraction limit
Microscope systems which combine several microscopy techniques within one microscope setup are ideally suited for applications, which require Raman information
with resolutions below the diffraction limit.
with resolutions below the diffraction limit.
Теги: diffraction limit nearfield microscopy raman spectroscopy ближнепольная микроскопия дифракционный предел рамановская спектроскопия
Nearfield-Raman Imaging is an exceptional combined microscopy technique which links chemical Raman information to high resolution Scanning Nearfield Optical Microscopy (SNOM). It allows for the acquisition of complete high-resolution confocal Raman images. Typically, lateral resolutions of below 100 nm can be achieved. Through the unique combination of a high-throughput spectroscopic system with the cantilever-based SNOM technique, an unrivaled sensitivity and imaging quality can be provided by a single microscope setup.
The Nearfield-Raman Imaging principle
The excitation laser light is focused through the SNOM-tip resulting in a "nearfield" (evanescent field) on the far side of the aperture (fig.2). While the sample is moved on a piezo-driven scan stage, the transmitted light is spectroscopically detected point by point and line by line in order to generate a hyperspectral image. The optical resolution of the transmitted light is thereby only limited by the diameter of the aperture (< 100 nm). Using a beam deflection setup as in AFM contact mode, it is ensured that the cantilever is always in contact with the sample. In this way the topography is recorded simultaneously to the Nearfield-Raman measurement.
Experimental Results
A sample of an exfoliated graphite flake was investigated by Near-field-Raman (fig.3). An exemplary Raman spectrum was acquired through the SNOM-tip with 5 seconds total integration time per spectrum (fig.4). Along a line trace indicated in Red in fig.5a the Raman signal of the graphite flake was analyzed through the integrated intensity of the G-Band (near 1600 rel. cm-1). The dimensions of the graphite flake can be displayed by plotting the increase in the intensity relatively to the distance. The complete length of the line trace was 20 μm with 400 measurement points and 1 second integration time per spectrum (fig.5b). Through a line trace zoom-in on the edge region of the graphene flake the single measurement points can be distinguished. Thereby a lateral resolution of < 100 nm of the Nearfield-Raman technique can be determined (fig.5c).
Fig.6a shows the topography image of the exfoliated graphite simultaneously recorded during the Nearfield- Raman measurement. The corresponding topography curve along the blue line in fig.6a shows a height of ~10 nm of the graphite flake and displays the small size of the graphite sample (fig.6b).
With the same image scan a Nearfield-Raman image of the G-Band intensity (same sample area as shown in fig.5a) can be generated. Due to the extremely sensitive measurement technique a total laser power of ~5 μW at the sample and an illuminated area < 100 nm was sufficient for image generation (fig.7a). At each image pixel a complete Raman spectrum was acquired with only 0.53 seconds total integration time per spectrum. The scan range was 5 Ч 1.7 μm and the image size 100 Ч 35 pixels. The G-band intensity (fig.7b) along the red line reveals the measurable signal variations between the small sample and the substrate.
Summary and Conclusion
WITec’s Nearfield-Raman imaging enables the generation of Raman images with an optical resolution beyond the diffraction limit. Thus the distribution of the chemical and molecular components can be imaged with the highest resolution. The technique pairs unrivaled sensitivity with the ease-of-use and reliability of the established cantilever SNOM-probe technique within a single microscope system and is suitable for all fields of application, where a detailed sample characterization is required.
The Nearfield-Raman Imaging principle
The excitation laser light is focused through the SNOM-tip resulting in a "nearfield" (evanescent field) on the far side of the aperture (fig.2). While the sample is moved on a piezo-driven scan stage, the transmitted light is spectroscopically detected point by point and line by line in order to generate a hyperspectral image. The optical resolution of the transmitted light is thereby only limited by the diameter of the aperture (< 100 nm). Using a beam deflection setup as in AFM contact mode, it is ensured that the cantilever is always in contact with the sample. In this way the topography is recorded simultaneously to the Nearfield-Raman measurement.
Experimental Results
A sample of an exfoliated graphite flake was investigated by Near-field-Raman (fig.3). An exemplary Raman spectrum was acquired through the SNOM-tip with 5 seconds total integration time per spectrum (fig.4). Along a line trace indicated in Red in fig.5a the Raman signal of the graphite flake was analyzed through the integrated intensity of the G-Band (near 1600 rel. cm-1). The dimensions of the graphite flake can be displayed by plotting the increase in the intensity relatively to the distance. The complete length of the line trace was 20 μm with 400 measurement points and 1 second integration time per spectrum (fig.5b). Through a line trace zoom-in on the edge region of the graphene flake the single measurement points can be distinguished. Thereby a lateral resolution of < 100 nm of the Nearfield-Raman technique can be determined (fig.5c).
Fig.6a shows the topography image of the exfoliated graphite simultaneously recorded during the Nearfield- Raman measurement. The corresponding topography curve along the blue line in fig.6a shows a height of ~10 nm of the graphite flake and displays the small size of the graphite sample (fig.6b).
With the same image scan a Nearfield-Raman image of the G-Band intensity (same sample area as shown in fig.5a) can be generated. Due to the extremely sensitive measurement technique a total laser power of ~5 μW at the sample and an illuminated area < 100 nm was sufficient for image generation (fig.7a). At each image pixel a complete Raman spectrum was acquired with only 0.53 seconds total integration time per spectrum. The scan range was 5 Ч 1.7 μm and the image size 100 Ч 35 pixels. The G-band intensity (fig.7b) along the red line reveals the measurable signal variations between the small sample and the substrate.
Summary and Conclusion
WITec’s Nearfield-Raman imaging enables the generation of Raman images with an optical resolution beyond the diffraction limit. Thus the distribution of the chemical and molecular components can be imaged with the highest resolution. The technique pairs unrivaled sensitivity with the ease-of-use and reliability of the established cantilever SNOM-probe technique within a single microscope system and is suitable for all fields of application, where a detailed sample characterization is required.
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