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Nowadays Raman spectroscopy is a routine optical technique for the study of the chemical bonding and the structural properties of a great variety of materials. However, its spatial resolution is limited by the excitation spot size, i.e. by the diffraction of light. When combined either with atomic force microscopy (AFM) or with scanning tunnelling microscopy (STM), Raman spectroscopy becomes capable of probing the sample with a resolution below the diffraction limit and is commonly designed as tip-enhanced Raman spectroscopy (TERS). In the case of both AFM and STM operation modes, a solid tip (or probe) with an apex size of several tens of nm, is introduced inside the Raman laser spot. The tip, covered with a noble metal coating for plasmon resonance excitation, enhances the near field Raman signal in its immediate proximity, thus increasing the lateral resolution of the spectroscopic signal down to nm scale.
In this work, we present a comparative study of AFM and STM mode TERS performed on thin dye layer as well as on bulk crystalline silicon samples. Gold-coated silicon and silicon nitride tips were used as probes in the AFM mode, while gold wire tips were used in the STM. The near field enhancement was estimated from the experimental data for both operation modes under various experimental conditions. The field enhancement factor was found to be higher in STM than in AFM mode TERS which can be readily attributed to the geometrical and structural differences existing between the probes used. Factors influencing the near field enhancement like tip material, geometry and coating nature (in the AFM case) have also been studied and discussed in a comparative manner. In particular, the importance of the refractive index of the tip material as a field enhancement factor through its direct influence on the plasmon resonance frequency is pointed out.
The near field enhancement obtained in the two operation modes with different tips was likewise modelled as a function of the polarization by using a recently proposed phenomenological model*. The results were found to be in a very good agreement with theoretical and numerical simulations, as well as with previous experimental evidence.
*R. Ossikovski, Q. Nguyen, G. Picardi, Phys. Rev. B 75, 045412 (2007)
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