Giant Oscillator Strength and Excitonic Molecule Transitions in Bulk CsPbBr3 Nanocrystals

Chia-Kai Lin^a^,^b, Pieter Geiregat^a^,^b, Zeger Hens^a^,^b, Böhme Simon Christian^c^,^d, Maryna Bodnarchuk^c^,^d, Kseniia Shcherbak^c^,^d, Maksym Viktorovych Kovalenko^c^,^d

^aPhysics and Chemistry of Nanostructures, Ghent University, Gent 9000, Belgium

^bNOLIMITS Center for Microscopy and Spectroscopy, Ghent University, Gent 9000, Belgium

^cInstitute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich, Switzerland

^dLaboratory for Thin Films and Photovoltaics, Empa—Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland

Proceedings of MATSUS Spring 2026 Conference (MATSUSSpring26)

D7 Low-Dimenisonal Halide Perovskites – Exploring Unique Challenges and Opportunities in 0D, 1D and 2D Materials

Barcelona, Spain, 2026 March 23rd - 27th

Organizers: Kunal Datta, Silvia Motti and Ajay Ram Srimath Kandada

Poster, Chia-Kai Lin, 926
Publication date: 15th December 2025

Bulk nanocrystals, the nanocrystals with a size in the regime of weak quantum confinement, have emerged recently as optical gain materials. Notably, the strongly correlated excitons in perovskite nanocrystals demonstrate enhanced transition oscillator strength as volume increases. This giant oscillator strength (GOS) facilitates phenomena such as single-photon superradiance and superabsorption; however, its influence on excitonic molecule dynamics and overall optical gain remains poorly understood. In this work, we employ cryogenic transient absorption spectroscopy on 18 nm bulk CsPbBr3 nanocrystals to investigate the GOS effect, exciton/molecule dynamics, and optical gain performance. We observe an acceleration in exciton decay upon cooling, revealing an expansion of the exciton coherent volume as exciton-phonon coupling is suppressed. Furthermore, optical gain increases significantly at low temperatures, correlating with enhanced bleaching from the molecule state transition. Our results indicate that the equilibrium and transitions between excitons and excitonic molecules play a decisive role in the stimulated emission process of bulk nanocrystals. Moreover, cryogenic polarized transient absorption (TA) spectroscopy, which is utilized to discuss selection rules and thermal fluctuations, serves as a critical diagnostic for isolating specific transitions. By leveraging these rules, we resolve the transition from exciton to excitonic molecule states; additionally, an anomalous shift of the exciton resonance, originating from the optical Stark effect, was observed. This approach ultimately provides a high-resolution probe into the dynamics of carrier transitions and spin relaxation, significantly deepening our understanding of many-body quantum correlations in the system.

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