Proceedings of MATSUS Spring 2025 Conference (MATSUSSpring25)
DOI: https://doi.org/10.29363/nanoge.matsusspring.2025.194
Publication date: 16th December 2024
Colloidal lead halide perovskite nanocrystals (NCs) are very promising materials for next-generation light-emitting devices due to their large gap tunability and high emission quantum yield. At ambient conditions, metal halide perovskites, whether in bulk or nanocrystalline form, exhibit a fairly linear increase in fundamental gap with increasing temperature, when crystallized in a tetragonal or cubic phase [1]. This also holds for CsPbBr3 NCs, although they presumably crystallize in an orthorhombic phase. Strikingly, the chlorine counterpart exhibits a sign reversal in the temperature slope of the fundamental gap [2]. We combined temperature and pressure-dependent photoluminescence (PL) measurements of colloidal CsPb(Br1-xClx)3 mixed-anion NCs to unravel the origin of such a reversal. The NC composition was varied continuously by ionic exchange in the primal colloidal solution. Careful analysis of the PL data allowed us to disentangle the effects of thermal expansion and electron-phonon interaction on the variation of the gap with temperature. We show that, concomitant with a transition into an orthorhombic phase, occurring for Cl contents exceeding ca. 40%, the electron-phonon interaction undergoes a sudden and radical change in sign and magnitude. In contrast, thermal expansion effects remain the same. Based on recent observations in mixed-cation CsxMA1-xPbI3 single crystals with low Cs content [3], we interpret such behavior as due to the activation of an anomalous electron-phonon coupling mechanism linked to lattice anharmonicities induced by coupling with Cs rattling modes. This takes place in the shrunken cage voids of the orthorhombic NC lattice for high Cl concentrations. In this way, we have clarified a puzzling result about metal halide perovskite NCs, providing valuable insights into the role of A-site cation dynamics in their optoelectronic properties.
The Spanish "Ministerio de Ciencia, Innovación y Universidades" (MICIU) through the Agencia Estatal de Investigación (AEI) is gratefully acknowledged for its support through grant CEX2023-001263-S (MATRANS42) in the framework of the Spanish Severo Ochoa Centre of Excellence program and the AEI/FEDER(UE) grants PID2020-117371RA-I00 (CHIRALPERO), PID2021-128924OB-I00 (ISOSCELLES), PID2022-141956NB-I00 (OUTLIGHT) and TED2021-131628A-I00 (MACLEDS). The authors also thank the Catalan agency AGAUR for grant 2021-SGR-00444 and the National Network "Red Perovskitas" (MICIU funded). S.F. acknowledges a FPI grant PRE2021-100097 from MICIU and the PhD programme in Materials Science from Universitat Autònoma de Barcelona in which she is enrolled. B.S. acknowledges the support of the Erasmus+ programme of the European Union through the internship project 2022-1-DE01-KA131-HED-000055364 (STREAM 2022). L.P. acknowledges support from the Spanish MICIU through Ramón y Cajal grant (RYC2018-026103-I) and a grant from the Xunta de Galicia (ED431F2021/05). S.G.G. acknowledges support from project CNS2022-135531 (HARDTOP) funded by 14 MCIN/AEI/10.13039/501100011033.
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