Speakers
Description
Ultrathin ZnO on Ag(111) has emerged as an interesting material platform for the atomic-scale investigation of light-matter interaction in plasmonic tunnel junctions. As ultrathin layers, ZnO forms a two-dimensional hexagonal lattice with a layer-thickness dependent electronic structure. In addition, an interface state (IS) is formed at the ZnO/Ag(111) interface, whose coupling to the ZnO-CBE plays an important role for the light-matter interaction in this system. Here, we use scanning tunneling microscopy (STM) combined with optical laser excitation and local light detection to study light-matter interaction of ZnO/Ag(111) inside a plasmonic nanocavity.
Part I: We use STM-induced luminescence (STML) to study the influence of ultrathin ZnO/Ag(111) on plasmonic light emission from a plasmonic nanocavity. At positive sample bias, the plasmonic luminescence - resulting from the radiative decay of localized surface plasmons (LSP) excited by inelastic tunneling - is spectrally modified by the ZnO layers. In particular, we observe a low-pass filtering effect which is absent at negative bias and depends on the local electronic structure of the ZnO, as confirmed by spatial STML mapping. Our findings demonstrate that the conduction band of ZnO serves as initial state for plasmonic luminescence driven by inelastic electron transport across the ZnO/Ag(111) interface.
Part II: While tip-enhanced Raman spectroscopy (TERS) has been implemented in STM with even sub-molecular resolution, time domain vibrational spectroscopy approaches are still under development. We recently showed that optical pump-probe STM on ultrathin ZnO/Ag(111) allows to realize coherent phonon (CP) spectroscopy with few nanometer spatial resolution. Beyond this proof-of-concept study, the mechanisms for CP excitation and detection and the role of the IS-CBE optical resonance are not fully understood. To gain further insights, we implement two-color pump-probe STM for controlled on- and off-resonant ultrafast optical STM and to selectively excite the IS-CBE resonance on 2ML and 3ML. With a new laser setup, we could realize resonant CP spectroscopy on 2ML ZnO, and show first results which indicate that the CP excitation and detection is also possible under non-resonant conditions.
S. Liu, Sci. Adv. 8, 42, eabq5682 (2022)
Liu et al., Nano Lett. 19, 8, 5725 (2019)
Wiedenhaupt et al., in preparation
