Comparative study of atomic force mode and tunneling mode tip-enhanced Raman spectroscopy

In Tip-Enhanced Raman Spectroscopy (TERS) a metal (or metallized) sharp tip is used to enhance the electromagnetic field by a localized surface-plasmon excitation. Two different modes - atomic force mode (AFM) and scanning tunneling mode (STM) - together with their respective types of probe tips are used in TERS experiments. We have compared the efficiency in enhancing the Raman signal on a thin dye layer for metal-coated AFM tips as well as for electrochemically etched metal STM tips. A much higher enhancement factor and better reproducibility were found when using STM tips. The very different performance is mainly attributed to the more efficient plasmonic excitation when using bulk-metal tips and possibly to the morphological differences in the tip and apex shapes existing between the two tip types.