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A reproducible route for tuning localized surface plasmon resonance in scattering type near-field optical microscopy probes is presented. The method is based on the production of a focused-ion-beam milled single groove near the apex of electrochemically etched gold tips. Electron energy-loss spectroscopy and scanning transmission electron microscopy are employed to obtain highly spatially and spectroscopically resolved maps of the milled probes, revealing localized surface plasmon resonance at visible and near-infrared wavelengths. By changing the distance L between the groove and the probe apex, the localized surface plasmon resonance energy can be fine-tuned at a desired absorption channel. Tip-enhanced Raman spectroscopy is applied as a test platform, and the results prove the reliability of the method to produce efficient scattering type near-field optical microscopy probes.

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The knowledge of the phonon coherence length is of great importance for two-dimensional-based materials since phonons can limit the lifetime of charge carriers and heat dissipation. Here we use tip-enhanced Raman spectroscopy (TERS) to measure the spatial correlation length L c of the A1g 1 and A1g 2 phonons of monolayer and few-layer gallium sulfide (GaS). The differences in L c values are responsible for different enhancements of the A1g modes, with A1g 1 always enhancing more than the A1g 2, independently of the number of GaS layers. For five layers, the results show an L c of 64 and 47 nm for A1g 1 and A1g 2, respectively, and the coherence lengths decrease when decreasing the number of layers, indicating that scattering with the surface roughness plays an important role.