Toward High-Contrast Atomic Force Microscopy-Tip-Enhanced Raman Spectroscopy Imaging: Nanoantenna-Mediated Remote-Excitation on Sharp-Tip Silver Nanowire Probes.

The tip-enhanced Raman spectroscopy (TERS) imaging technique is designed to provide correlated morphological and chemical information with a nanoscale spatial resolution by utilizing the plasmonic resonance supported by metallic nanostructures at the tip apex of a scanning probe. However, limited by the scattering cross sections of these nanostructures, only a small fraction of the incident light can be coupled to the plasmonic resonance to generate Raman signals. The uncoupled light then directly excites background spectra with a diffraction-limited resolution, which becomes the background noise that often blurs the TERS image. Here, we demonstrate how this problem can be solved by physically separating the light excitation region from the Raman signal generation region on the scanning probe. The remote-excitation TERS (RE-TERS) probe, which can be fabricated with a facile, robust and reproducible method, utilizes silver nanoparticles as nanoantennas to mediate the coupling of free-space excitation light to propagating surface plasmon polaritons (SPPs) in a sharp-tip silver nanowire to excite Raman signals remotely. With this RE-TERS probe, a 10 nm spatial resolution was demonstrated on a single-walled carbon nanotube sample, and the strain distribution in a monolayer molybdenum disulfide (MoS2) was mapped.