Investigations on the use of thin adhesion layers in top-illumination AFM tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) is known to work in different configurations, modes and combinations of tip materials and excitation laser wavelengths. Although very promising, atomic force microscopy (AFM)-based TERS is not yet at the level of other competitive techniques like scanning tunneling microscopy or shear force based TERS, for instance, in terms of imaging resolution or tips control and usability. However, AFM TERS has several structural advantages over others, such as the versatility in the characterization of both electrical conducting and non-conducting samples or the abundance of collateral information provided, which make it ideal for future routine analyses and market applications. One of the main issues in AFM TERS concerns the fabrication processes of tips, which generally make use of commercial silicon cantilevers as starting supports for the plasmonic materials, like silver or gold, required in TERS. These can be chemically deposited or vacuum evaporated directly on the pristine silicon tips, but they can easily peel off due to dynamics of measurements and mechanical interaction with the sample surface. In particular, gold has a bad adhesion with silicon oxide surfaces, and commercial tips have a native oxide layer which is difficult to remove, without dulling the tip apex, with other than wet chemical etching methods.In the present contribution, we explore with TERS, in the top-illumination geometry, the possibility to use thin adhesion layers to avoid metal detachment without severely affecting, like in other plasmonic applications, the sensing performances.