Near-field chemical mapping of gold nanostructures using a functionalized scanning probe.

We report on photochemical and photophysical properties produced by Surface Plasmon Resonance (SPR) on metallic nanograins by means of high resolution Functionalized Tip-Enhanced Raman Spectroscopy (F-TERS). This technique relies on a sharp gold tip functionalized with Raman-active molecules to be scanned relatively to plasmonic hot-spots on a surface. We describe the local variation of plasmon-induced Raman enhancement on the surface of nanostructures that also affects the photochemistry while the quantitative interpretation of peak intensities requires the consideration of surface topography near the tip apex. Our F-TERS maps show Raman modes of hot electron reduction of 4-nitrothiophenol (4-NTP) molecules on the tip and indicate at least partial photochemical dimerization. An apparent photo-induced reversibility of this dimerization can be conservatively explained by a local topography feature that we simulate in a finite element environment. Our experimental results reveal a spatial resolution of approximately 10 nm, corresponding to a few hundred 4-NTP molecules exposed to the near-field.

High resolution scanning near field mapping of enhancement on SERS substrates: comparison with photoemission electron microscopy.

The need for a dedicated spectroscopic technique with nanoscale resolution to characterize SERS substrates pushed us to develop a proof of concept of a functionalized tip-surface enhanced Raman scattering (FTERS) technique. We have been able to map hot spots on semi-continuous gold films; in order to validate our approach we compare our results with photoemission electron microscopy (PEEM) data, the complementary electron microscopy tool to map hot spots on random metallic surfaces. Enhanced Raman intensity maps at high spatial resolution reveal the localisation of hotspots at gaps for many neighboring nanostructures. Finally, we compare our findings with theoretical simulations of the enhancement factor distribution, which confirms a dimer effect as the dominant origin of hot spots.

High optical enhancement in Au/Ag alloys and porous Au using Surface-Enhanced Raman spectroscopy technique.

We report high optical enhancement in Ag/Au alloys and porous gold nanostructures using Surface Enhanced Raman Spectroscopy (SERS) technique. Scanning electron microscopy investigation shows the formation of Ag/Au alloys particles during irradiation of Ag-Au bilayer deposited on FTO (SnO2:F) substrate by laser fluency equal to 0.5 J/cm2 or 1.0 J/cm2 with 12 ns laser pulse duration. The dealloying process of these Au-Ag alloy particles leads to the formation of Au nanoporous particles. The obtained nanostructures were studied with SERS and revealed a promising enhancement factor in porous Au nanostructure and tunability of localized surface plasmon resonance. The highly dense strong hot spots and large specific area in porous structure of gold nanostructures is the origin of the highly enhancement factor observed experimentally and theoretically. A very good agreement between simulation and experimental results was found confirming the potential of Au/Ag alloys and particularly porous gold nanostructure in SERS application.