Light and Thermal Activation: Decoupled Responses in CsPbBr3 Perovskites
Giulia Fulvia Mancini a
a Laboratory for Ultrafast X-ray and Electron Microscopy (LUXEM), Department of Physics, University of Pavia, Strada Nuova, 65, Pavia, Italy
nanoGe Fall Meeting
Proceedings of Materials for Sustainable Development Conference (MAT-SUS) (NFM22)
#NANOMAT - Advances on the Understanding and Synthesis of Nanomaterials for Photocatalysis and Optoelectronics
Barcelona, Spain, 2022 October 24th - 28th
Invited Speaker, Giulia Fulvia Mancini, presentation 241
Publication date: 11th July 2022

The flexibility of lead-halide perovskites plays a crucial role in their functional response, but the exact structural distortions and their effects on physical properties are still unclear [1]. Here, we provide for the first time the description of the photoinduced and thermally-activated structural distortions in CsPbBr3 perovskite nanocrystals with atomic-level precision. We combine time-resolved X-ray absorption spectroscopy (TR-XAS) and ab initio simulations to show that the photoexcitation of CsPbBr3 nanocrystals (NCs) leads to well-defined polaronic lattice changes, rather than photoinduced structural phase transitions. Additionally, our analysis rules out thermal effects in the photoactivation of the system [2]. We also investigated the purely thermal response of CsPbBr3 with temperature-dependent XAS and first principles molecular dynamics (MD) simulations across its phase diagram. We show that the thermally-activated lattice cannot be reduced to a cubic average structure, because of the presence of dynamically distorted local configurations and lattice anharmonicity [3].

Our comprehensive investigation demonstrates that the structural changes of CsPbBr3 NCs induced by light and thermal functional triggers have fundamentally different physical origins. Even though both effects are related to the flexibility of the perovskite lattice, the photoexcitation is selectively driven by electron-phonon coupling, while the thermal activation drives lattice phonon anharmonicity. The latter leads to significant distortions from the CsPbBr3 space- and time- average lattice symmetry, and it is well rationalized by the soft-mode model in the framework of displacive thermal phase transitions.

These finding clarify the underlying mechanisms of the lattice response under functional activation and offers strategies to control the perovskite nuclear degrees of freedom with different external stimuli. Understanding the thermal processes acting at the atomic-level represents the first step toward a rational design of perovskite-based devices with improved stability.

G.F.M. acknowledges 
- all co-authors from (i) Journal of the American Chemical Society 143.24 (2021): 9048-9059, and (ii) The Journal of Physical Chemistry Letters 13.15 (2022): 3382-3391.
And, on behalf of all authors:
- European Union’s Horizon 2020 research and innovation program: grant agreement no. 851154 (ULTRAIMAGE), no. 695197 755 (DYNAMOX) FET Open research and innovation action under the grant
agreement no. 899141 (PoLLoC). 
- Fondazione Cariplo (NanoFast 2020.2544). 
- Swedish Research Council (2019-03993)and the Chalmers Gender Initiative for Excellence (Genie).
- DE-AC02-06CH11357
- DE-AC02-06CH11357

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