Abstract
Low-temperature tip-enhanced Raman spectroscopy (TERS) enables chemical identification with single-molecule sensitivity and extremely high spatial resolution even down to the atomic scale. The large enhancement of Raman scattering obtained in TERS can originate from physical and/or chemical enhancement mechanisms. Whereas physical enhancement requires a strong near-field through excitation of localized surface plasmons, chemical enhancement is governed by resonance in the electronic structure of the sample, which is also known as resonance Raman spectroscopy. Here we report on tip-enhanced resonance Raman spectroscopy (TERRS) of ultrathin ZnO layers epitaxially grown on a Ag(111) surface, where both enhancement mechanisms are operative. In combination with scanning tunneling spectroscopy (STS), it is demonstrated that the TERRS intensity strongly depends on the local electronic resonance of the ZnO/Ag(111) interface. We also reveal that the spatial resolution of TERRS is dependent on the tip-surface distance and reaches nearly 1 nm in the tunneling regime, which can be rationalized by strong-field confinement resulting from an atomic-scale protrusion on the tip apex. Comparison of STS and TERRS mapping clearly shows a correlation between resonantly enhanced Raman scattering and the local electronic states at near-atomic resolution. Our results suggest that TERRS is a new approach for the atomic-scale optical characterization of local electronic states.