Organic nanopatterned samples by electron beam for gap-mode STM TERS and chemical imaging

In the present work, Tip Enhanced Raman Scattering (TERS) samples with nanometers line patters of thiols have been fabricated using electron beam lithography. Nanopattern samples of TERS active molecules are interesting to build a reference material for metrology characterization of TERS apparatus imaging. A highly conductive, chemically stable and with low roughness thin film of gold with a titanium adhesion layer was evaporated by electron gun deposition method, on Si/SiO2 substrate. Then, a nanopatterned self-assembled monolayer of thiol molecules was obtained on the gold substrate by electron beam lithography and subsequent soaking in a thiol solution. The resulting surface was tested not to be Raman enhancing so to exclude any signal contribution from the substrate. The obtained sample was suitable for measurements by both Scanning Tunneling Microscope (STM) and Atomic Force Microscope (AFM) TERS setups. This fabrication method has been applied to manufacture organic molecule patterns of stripes with dimensions below 100nm in width. By STM TERS measurements the enhancement factors and the spatial resolutions of the Raman signals have been calculated. The STM TERS signals were obtained using a 633nm laser excitation wavelength in a top visual configuration with a 100× microscope objective (0.7 NA) and with a 3 cm-1 resolution. The STM silver tips were electrochemically etched using an electronic control circuit. Chemical TERS images of periodic lines narrower than 25 nm were acquired (Fig.1). A remarkable spatial contrast was achieved, since no molecule signal could be observed in the areas previously covered by the mask. This configuration leads to no observable contamination of either the tip or the pristine gold areas, thus validating the aforementioned procedure for the production of a viable, custom nanopatterned sample of organic molecules for STM TERS imaging.