Tip-enhanced Raman spectroscopy (TERS) is a powerful tool to investigate chemical composition, for obtaining molecular information and for recording images with a spatial resolution on the nanometer scale. However, it typically has been limited to a fixed excitation wavelength. We demonstrate excitation-dependent hyperspectral imaging by implementing a wavelength-tunable laser to our TERS setup. Varying the excitation wavelength during the TERS experiments is a key to perform spatially resolved resonant Raman scattering with nanometer resolution, which enables mapping of transition centers and to study, for example, the quantum properties of electrons and phonons. To present the application potential and to verify the setup, we recorded excitation-dependent hyperspectral nanoimages of a densely packed film of carbon nanotubes (CNTs) on an Au surface and use the spectral position and intensity of the radial breathing modes for a unique assignment of the CNTs. We succeeded in identifying and imaging at least nine different tube species. The nanoimages revealed the exact position and the distribution of certain CNTs inside the film. e-TERS will have manifold application in nanoimaging, for chemical analysis, and electronic studies on the nanometer scale, making it highly interesting in fields ranging from biomedicine and chemistry to material science.
