Invited
Detection of surface modes and symmetry-prohibited modes in strontium titanate by tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) occupies a unique position for the investigation of nonmetallic nanomaterials: it provides a wealth of information of Raman spectroscopy even under ambient conditions with the opportunity for spatial resolution below the diffraction limit. The high sensitivity of the optical near field to surfaces has been exploited on self-assembled monolayers on multiple occasions, and yet, the potential for the investigation of crystalline surfaces remains to be unfolded. Using strontium titanate (SrTiO₃) as a model system, we demonstrate that TERS does not only provide insight into surface modes but also activates otherwise symmetry-forbidden modes. The bulk phase of strontium titanate is Raman-inactive, and the optical far field therefore does not provide any first-order Raman signature: as a consequence, any peak in TERS configuration originates from the optical near field, confined to a few nanometers at the apex of the tip. We observe first-order Raman peaks interpreted as TO2, TO4, and LO4 phonon modes and the strong field enhancement of both infrared-active LO3 and Raman surface modes in agreement with density functional theory (DFT) calculations.