Nanoscale chemical imaging of custom patterns of organic monolayers by gap-mode tunneling tip-enhanced Raman spectroscopy

In the present work, nanostructured thiols monolayers have been imaged by tip-enhanced Raman spectroscopy (TERS). TERS is a novel technique that allows to obtain simultaneous chemical and morphological imaging of Raman active molecules with nanometric resolution. TERS imaging is achievable in different configurations in terms of illumination (top down, side or bottom up) and techniques and tips (Atomic Force Microscope or Scanning Tunneling Microscope probes). Nowadays no widespread reference samples have been established in order to test, evaluate and compare the performances of TERS setups and techniques. To this aim, nanopatterned samples of TERS active molecules play a fundamental role in building a reference material for the metrological characterization of the imaging capabilities of a STM- controlled TERS apparatus. In this respect, self-assembled monolayers (SAMs) of thiols on gold surfaces are ideal candidates, due the high TERS activity of thiol molecules, the good homogeneity and reproducibility of the monolayer and the high conductivity of the substrate. Here, nanopatterned thiol monolayers have been fabricated by an electron beam lithography (EBL) based approach, on highly conductive gold thin films deposited on Si/SiO2 substrates. TERS images (Fig. 1) of the resulting structures have been taken with a top-visual apparatus in STM feedback using electrochemically etched silver tips, with an excitation wavelength of 633 nm. Chemical maps of stripes of width less than 25 nm have been measured, featuring high enhancement factors and contrast, since no thiol signal has been detected in the areas previously covered by the masks, indicating the viability of the production process.