Berlin 2024 – scientific programme
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O: Fachverband Oberflächenphysik
O 72: Poster: Scanning Probe Microscopy: Light Matter Interaction at Atomic Scales
O 72.8: Poster
Wednesday, March 20, 2024, 18:00–20:00, Poster D
STM Luminescence of Ultrathin ZnO Films on Ag(111) — •Henrik Wiedenhaupt1, Fabian Schulz1, Luis Parra López1, S. Liu1, Adnan Hammud1, Martin Wolf1, Takashi Kumagai1,2, and Melanie Müller1 — 1Fritz-Haber Institute of the Max-Planck Society, Berlin, Germany — 2Institute for Molecular Science, Okazaki, Japan
ZnO films on Ag(111) exhibit a layer-dependent electronic structure and conduction band edge (CBE) whose strong coupling to an interface state (IS) dominates the optical properties of the system. Previous studies combining optical excitation with scanning tunneling microscopy (STM) have shown that resonant optical excitation at the IS-CBE transition gives rise to chemical enhancement in tip-enhanced Raman spectroscopy (TERS) [1] and to enhanced photoinduced tunneling [2]. The optical enhancement thereby exhibits pronounced dependence on the local electronic structure which can vary on the nanoscale. In this work, we perform STM-luminescence (STML) spectroscopy to gain deeper insights into the local optical and electronic properties of ultrathin ZnO/Ag(111). We find that tunneling into the CB at positive sample bias gives rise to strong plasmonic luminescence at photon energies smaller than CBE onset, while higher photon energies are quenched. In contrast, STML at negative sample bias results in plasmonic luminescence as observed on bare Ag(111), excluding that dipole-coupling (re-absorption) is responsible for luminescence quenching at high photon energies. Our results show that efficient light-emitting states reside near the CBE, which serve as a bias-independent initial state for radiative plasmonic decay after elastic charge injection into the CB. The layer-dependent energetic position of the CBE thus allows the plasmonic luminescence spectra to be controlled by the ZnO film thickness. [1] Liu et al., Nano Lett. 19, 8, 5725 (2019) [2] Liu et al., Sci. Adv. 8, 42 (2022)
Keywords: photocurrent; STM; plasmonics; optical rectification; photon-assisted tunneling