Development and characterization of a novel reference sample for tip-enhanced Raman spectroscopy

The tip-enhanced Raman spectroscopy (TERS) is a modern technique for nano-scale analyses, which combines the excellent spatial resolution of scanning probe microscopy and the chemical sensitivity of surface-enhanced Raman scattering spectroscopy. These unique properties of TERS are achieved via the use of a plasmonic nano-tip positioned only a few nanometers away from the studied sample. However, the tips are prone to mechanical and chemical degradation, which may hinder the achieved spatial resolution and signal enhancement. Therefore, periodic checks of the state of the tip are crucial to maintaining the high quality of TERS experiments. Reference samples are commercially available for these purposes, but they are costly and have a quite short expiration date of several months. For this reason, we have developed a simple procedure for the preparation of reference probe samples for testing TERS tips using copper(II) phthalocyanine on a Au nanolayer, which is prepared by thermal vacuum evaporation of Au on a Si wafer. The analysis of the prepared system by atomic force microscopy and scanning electron microscopy confirmed a relatively smooth surface morphology, which is an important parameter for minimizing the risk of mechanical damage of the tip. TERS experiments proved that the developed system is suitable as a reference sample for TERS at the 785 nm excitation wavelength, enabling a repeated detection of well-resolved TERS spectra. The use of the reference sample for 633 nm excitation is limited because of possible photo-induced processes, which decrease the reproducibility and stability of the collected TERS spectra.