Modulated-Illumination Intermittent-Contact Tip-Enhanced Raman Spectroscopy

This article presents a proof-of-concept for a new imaging method that combines tip-enhanced Raman spectroscopy with intermittent-contact atomic force microscopy to provide simultaneous nanometer-scale mechanical imaging with chemical contrast. The foremost difference from a standard tip-enhanced Raman microscope is the Raman illumination, which is modulated by the cantilever drive signal so that illumination is only active when the tip is close to the surface. This approach significantly reduces contact forces and thermal damage due to constant illumination while simultaneously reducing background Raman signals. Near-field optical and dynamic cantilever simulations highlight the effect of the imaging parameters on the tip-sample force and the evanescent field enhancement. The experimental images obtained with this new imaging method demonstrate a lateral resolution sufficient to identify single-walled carbon nanotube bundles with a full width at half-maximum of 20 nm.