Tip-enhanced Raman scattering for atomic-scale spectroscopy and imaging
Atomic scale spectroscopy provides an exceptional ability to define electronic, optical, thermal, mechanical, and chemical properties of materials at the nanoscale. At these scales, dimensional confinement can lead to new and unusual properties, where the intrinsic natures and interfaces of individual atoms and molecules can result in quantum phenomena that are not exhibited in bulk materials. Realizing the full potential of these unique properties hinges upon the ability to probe them with suitable spatial resolution. Scanning tunneling microscopy (STM) uses tunneling current between a sharp tip and the sample to yield real space images of individual atoms and molecules. But it lacks chemical sensitivity and therefore it can become difficult to define unknown species or structures. By contrast, complementary approaches, such as spectroscopy can provide chemical identification, as well as define physical and electronic structures. However, there is a mismatch between the spatial resolution of the two techniques, as the diffraction limit of light leads to ensemble-based measurements where average properties are observed, obscuring local inhomogeneities. Instead, scanning near-field optical microscopy methods, such as tip-enhanced Raman spectroscopy (TERS), can be used to probe individual atoms and molecules with atomically confined light.