Abstract
Full spectroscopic imaging by means of tip-enhanced Raman spectroscopy (TERS) was used to measure the distribution of two isomeric thiols (2-mercaptopyridine (2-PySH) and 4-mercaptopyridine (4-PySH)) in a self-assembled monolayer (SAM) on a gold surface. From a patterned sample created by microcontact printing, an image with full spectral information in every pixel was acquired. The spectroscopic data is in good agreement with the expected molecular distribution on the sample surface due to the microcontact printing process. Using specific marker bands at 1000 cm−1 for 2-PySH and 1100 cm−1 for 4-PySH, both isomers could be localized on the surface and semi-quantitative information was deduced from the band intensities. Even though nanometer size resolution information was not required, the large signal enhancement of TERS was employed here to detect a monolayer coverage of weakly scattering analytes that were not detectable with normal Raman spectroscopy, emphasizing the usefulness of TERS.
Highlights
The chemical characterization of surface adsorbates is of great interest in several areas of research
We show that tip-enhanced Raman spectroscopy (TERS) can be used to image chemically heterogeneous surfaces without the need for labeling, even when the different adsorbates have very similar properties, i.e., a monolayer consisting of both 2-PySH and 4-PySH
The height signal shows the elevation of the thiols a little more clearly, suggesting a step height
Summary
The chemical characterization of surface adsorbates is of great interest in several areas of research. Due to the increased signal, TERS can detect small amounts of analyte in a short time, allowing acquisition of Raman images of a surface area covered with weakly scattering molecules. The information from such a Raman image was used here to chemically identify and localize two different thiol isomers in an inhomogeneous self-assembled monolayer (SAM).
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