Regensburg 2019 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 97: Ultrafast Electron Dynamics at Surfaces and Interfaces III

O 97.4: Vortrag

Freitag, 5. April 2019, 11:15–11:30, H16

Exciton dynamics and energy transfer at PTCDA/noble-metal interfaces — •Klaus Stallberg and Ulrich Höfer — Fachbereich Physik, Philipps-Universität, D-35032 Marburg

Combining time-resolved photoluminescence (TRPL) and two-photon photoemission (2PPE) we investigate thin layered films of PTCDA on the (111) surfaces of silver and gold. Our combined experimental approach enables us to study the exciton dynamics from optical excitation in the organic layers to annihilation at the organic/metal interface. More specifically, we identify three different excitonic states – a monomer-related Frenkel exciton, a charge-transfer (CT) exciton, and an excimer – based on their spectral signatures and their radiative lifetimes in TRPL. 2PPE spectra of the same samples exhibit distinct low-energy features which we attribute to hot conduction electrons at the metal surfaces. The exciton dynamics, as probed with TRPL, strongly depends on the PTCDA film thickness which is varied from several nanometers down to a single monolayer. The decrease of the CT exciton radiative lifetime with decreasing film thickness is explained in terms of exciton diffusion along the stacking direction of the PTCDA molecules. Interestingly, exactly the same dependence on the PTCDA film thickness is observed for the hot-electron dynamics, as probed with 2PPE. This strongly points to the CT excitons as source of hot electrons after diffusion to the organic/metal interface. This energy transfer is found much more efficient on Ag(111) than on Au(111) indicating involvement of the metal–organic interface state of PTCDA/Ag(111) which is absent at the PTCDA/Au(111) interface.

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