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O: Fachverband Oberflächenphysik
O 91: Scanning Probe Microscopy: Light Matter Interaction at Atomic Scales III
O 91.2: Vortrag
Donnerstag, 21. März 2024, 15:30–15:45, MA 041
Triplet emitter electroluminescence from Pd-octaethylporphyrin (PdOEP) in a scanning tunneling microscope (STM) — Li-Qing Zheng1, Fábio Costa1,2, Abhishek Grewal1, Anna Rosławska1, •Klaus Kuhnke1, and Klaus Kern1,3 — 1Max-Planck Institut für Festkörperforschung, 70569 Stuttgart, Germany — 2Gleb Wataghin Institute of Physics, UNICAMP, Campinas 13083-859, Brazil — 3Institut de Physique, École Polytechnique Fédéral de Lausanne, 1015 Lausanne, Switzerland
Triplet emitters produce significant emission (phosphorescence) even below their lowest singlet transition (fluorescence) due to spin-orbit coupling by a heavy metal center. While this property makes them interesting for OLEDs, the study of their luminescence at the single molecule level in high resolution STM is challenging.
We investigate isolated PdOEP molecules decoupled from Ag(100) and Ag(111) by an ultrathin NaCl layer. Singlet and triplet emission lines are observed in the STM at visible wavelengths, only about 100 nm apart from each other. The singlet S1 state of PdOEP emits photons even when the energy of one tunneling electron is lower than the emitted S1 photon energy. The mechanism requires a relay (or shelving) state in which energy is stored in the molecule for the interval between two tunneling electrons. Emission is then expected to follow a quadratic current dependence. The S1 emission, in contrast, exhibits a linear dependence, suggesting a relay state with nanoseconds life time. We discuss possible scenarios for the S1 overbias emission.
Keywords: scanning tunneling microscopy; electroluminescence; energy upconversion; phosphorescence; porphyrin