Proceedings of MATSUS Spring 2024 Conference (MATSUS24)
DOI: https://doi.org/10.29363/nanoge.matsus.2024.049
Publication date: 18th December 2023
Macroscopic Bi2Se3 is a layered material consisting of a high number (>10) Se/Bi/Se/Bi/Se quintuple layers, separated by van der Waals gaps. The material has been characterized as a topological quantum spin Hall insulator, with an inverted bandgap of 300 meV, and protected helical surface states. Now we consider the high-energy electron-hole excitations involving bands below - and further above theFermi-level: In bulk Bi2Se3, the absorption spectrum shows intriguing optical transitions in the 0.5 – 3.5 eV energy region: a strong optical transition at 2.8 eV located at the surface, followed by luminescence in the region of 2.3 eV, preserving chirality (1,2).
We used colloidal two-dimensional Bi2Se3 nanoplatelets as a model system to monitor the transition from bulk Bi2Se3 to two-dimensional Bi2Se3, to finally a single quintuple layer. We studied the optical transitions in the 1-3 eV region with absorption and transient absorption quenching spectroscopy (3). We observe more than 10 strong optical transitions. Comparison with DFT-GW theory allowed us to identify these transitions, group them as surface and interior transitions, and locate them in the two-dimensional Brillouin zone. In the time region below 10 ps, the electron and hole cooling and recombination dynamics are very specific for the bands involved, with in some cases signatures of electron-hole separation in momentum space by cooling.
1. Observation of chiral surface excitons in a topological insulator Bi2Se3. Proceedings of the National Academy of Sciences of the United States of America 116, 4006-4011 (2021)
2. Complex optical conductivity of Bi2Se3 thin film: Approaching two-dimensional limit Applied Physics Letters 118, DOI: 10.1063/5.0049170 (2021)
3. Identification of high-energy excitations in two-dimensional topological Bi2Se3 platelets. Jara Vliem, Ricardo Reho, Servet Ataberk Cayan*, Pieter Geiregat*, Zeila Zanolli, and D. Vanmaekelbergh
Daniel Vanmaekelbergh and Jara Vliem wish to thank the Duch NWO Physics project 680.PR.030 and the ERC Advanced Grant 692691, and the Dutch Gravity Program "Materials for the quantum age".
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