Polarons on Oxides Detected by Infrared-Reflection Absorption Spectroscopy (IRRAS): The Case of TiO2 and ZnO

Christof Wöll (KIT, Karlsruhe)

Dec 09. 2019, 14:00 — 14:40

Christof Wöll Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), 76021 Karlsruhe, Germany, christof.woell@kit.edu Despite their pronounced importance for oxide-based photochemistry, optoelectronics and photovoltaics, only fairly little is known about the polaron lifetimes and binding energies. Polarons represent a crucial intermediate step populated immediately after dissociation of the excitons formed in the primary photoabsorption process. Here, we present a novel approach to studying photoexcited polarons in oxides, namely infrared (IR) reflection-absorption spectroscopy (IRRAS) measured in an UHV-compatible apparatus. Advanced instrumentation not only allows studying weak changes in the reflection intensity down to 1x10-6, but also makes it possible to investigate temporal variation with a time resolution of 100 ms. In this presentation, we will discuss data for well-defined single crystals of rutile TiO2(110) [1] and ZnO(10-10) [2]. In both cases, we observe intense IR absorption bands after photoexcitation and/or doping with H-atoms. On the basis of first-principles-based electronic structure calculations, in case of ZnO the peak at around 200 meV is assigned to hole polarons of intermediate coupling strength. From the pronounced temperature dependence, the binding energy of the hole polarons can be estimated, and an excellent agreement with the theoretical results is obtained. In case of TiO2, the situation is less clear. Although from the fact that similar excitations can also be seen after exposure to H-atoms it can be safely concluded that in this case the features in the IR-data must be due to electron polarons, the previous assignment to excitations of polarons [1] has to be reconsidered. In the final part of the presentation, we will discuss recent data on a photoactive titanate, KTaO3where polaron-induced features could also be detected. References: [1] H. Sezen, M. Buchholz, A. Nefedov, C. Natzeck, S. Heißler, C. Di Valentin, Ch. Wöll Nature Scientific Reports, 4, 3808, (2014) [2] H. Sezen, HH., Shang, F. Bebensee, CW. Yang, M. Buchholz, A. Nefedov, S. Heissler, C. Carbogno, M. Scheffler, P. Rinke, Ch. Wöll Nature Comm., 6, 6901 (2015)

Further Information
ESI Boltzmann Lecture Hall
Associated Event:
Polarons in the 21st Century (Workshop)
Jozef Devreese (U Antwerpen)
Cesare Franchini (U of Vienna)
Georg Kresse (U of Vienna)
Jacques Tempere (U Antwerpen)