8 research outputs found
2D metal halide perovskites: a new fascinating playground for exciton fine structure investigations
Two-dimensional (2D) metal halide perovskites are natural quantum wells which consist of low bandgap metal-halide slabs, surrounded by organic spacers barriers. The quantum and dielectric confinements provided by the organic part lead to the extreme exciton binding energy which results in a huge enhancement of exciton fine structure in this material system. This makes 2D perovskites a fascinating playground for fundamental excitonic physics studies. In this review, we summarize the current understanding and quantification of the exciton fine structure in 2D perovskites. We discuss what is the role of exciton fine structure in the optical response of 2D perovskites and how it challenges our understanding of this fundamental excitation. Finally, we highlight some controversy related to particularly large bright-dark exciton states splitting and high efficiency of light emission from these materials. This can result from the unique synergy of excitonic and mechanical properties of 2D perovskites crystals
Quantification of Exciton Fine Structure Splitting in a Two-Dimensional Perovskite Compound
International audienceApplications of two-dimensional (2D) perovskites have significantly outpaced the understanding of many fundamental aspects of their photophysics. The optical response of 2D lead halide perovskites is dominated by strongly bound excitonic states. However, a comprehensive experimental verification of the exciton fine structure splitting and associated transition symmetries remains elusive. Here we employ low temperature magneto-optical spectroscopy to reveal the exciton fine structure of (PEA)2PbI4 (here PEA is phenylethylammonium) single crystals. We observe two orthogonally polarized bright in-plane free exciton (FX) states, both accompanied by a manifold of phonon-dressed states that preserve the polarization of the corresponding FX state. Introducing a magnetic field perpendicular to the 2D plane, we resolve the lowest energy dark exciton state, which although theoretically predicted, has systematically escaped experimental observation (in Faraday configuration) until now. These results corroborate standard multiband, effective-mass theories for the exciton fine structure in 2D perovskites and provide valuable quantification of the fine structure splitting in (PEA)2PbI4
Polaronic Mass Enhancement and Polaronic Excitons in Metal Halide Perovskites
International audienceIn metal halide perovskites, the complex dielectric screening together with low energy of phonon modes leads to non-negligible Fröhlich coupling. While this feature of perovskites has already been used to explain some of the puzzling aspects of carrier transport in these materials, the possible impact of polaronic effects on the optical response, especially excitonic properties, is much less explored. Here, with the use of magneto-optical spectroscopy, we revealed the non-hydrogenic character of the excitons in metal halide perovskites, resulting from the pronounced Fröhlich coupling. Our results can be well described by the polaronic-exciton picture where electron and hole interactions are no longer described by a Coulomb potential. Furthermore, we show experimental evidence that the carrier-phonon interaction leads to the enhancement of the carrier’s effective mass. Notably, our measurements reveal a pronounced temperature dependence of the carrier’s effective mass, which we attribute to a band structure renormalization induced by the population of low-energy phonon modes. This interpretation finds support in our first-principles calculations
Quantification of Exciton Fine Structure Splitting in a Two-Dimensional Perovskite Compound
Applications of two-dimensional (2D) perovskites have significantly outpacedthe understanding of many fundamental aspects of their photophysics. The optical response of2D lead halide perovskites is dominated by strongly bound excitonic states. However, acomprehensive experimental verification of the excitonfine structure splitting and associatedtransition symmetries remains elusive. Here we employ low temperature magneto-opticalspectroscopy to reveal the excitonfine structure of (PEA)2PbI4(here PEA is phenyl-ethylammonium) single crystals. We observe two orthogonally polarized bright in-plane freeexciton (FX) states, both accompanied by a manifold of phonon-dressed states that preservethe polarization of the corresponding FX state. Introducing a magneticfield perpendicular tothe 2D plane, we resolve the lowest energy dark exciton state, which although theoreticallypredicted, has systematically escaped experimental observation (in Faraday configuration)until now. These results corroborate standard multiband, effective-mass theories for theexcitonfine structure in 2D perovskites and provide valuable quantification of the finestructure splitting in (PEA)2PbI
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Layered BiOI single crystals capable of detecting low dose rates of X-rays
Acknowledgements: We would like to thank Prof. Richard Phillips (University of Cambridge) for useful feedback on the manuscript and help with optical measurements. The authors also thank Zhuotong (Thomas) Sun (University of Cambridge) for assistance on the powder X-ray diffraction measurements, and Prof. James Marrow and Marcus Williamson (University of Oxford) for assistance in taking radiographs. R.A.J. acknowledges funding from an EPSRC Department Training Partnership studentship (no. EP/N509620/1), as well as Bill Welland and the Winton Programme for the Physics of Sustainability. L.E. and T.V.D.G. acknowledge support from the EPSRC Cambridge NanoDTC (no. EP/L015978/1). L.E. acknowledges funding by the DFG (project no. 387651688). T.V.D.G. also acknowledges financial support from the Schiff Foundation. K.G. and S.D.S. acknowledge the EPSRC (no. EP/R023980/1) for funding. S.D.S. acknowledges the Royal Society and Tata Group (no. UF150033) and EPSRC (no. EP/W004445/1) for funding. The work has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (HYPERION - grant agreement no. 756962; PEROVSCI - 957513). The work was supported by a Royal Society International Exchanges Cost Share award (no. IEC\R2\170108) and the Alliance Hubert Curien Programme of the British Council (no. 608412749). K.R.D. thanks the Department of Chemistry at the University of Oxford for a studentship. P.P. appreciates support from National Science Centre Poland within the OPUS program (no. 2019/33/B/ST3/01915). This work was partially supported by OPEP project, which received funding from the ANR-10-LABX-0037-NEXT. The Polish participation in European Magnetic Field Laboratory is supported by the DIR/WK/2018/07 grant from Ministry of Science and Higher Education, Poland. F.D. acknowledges support from the DFG Emmy Noether Programme (project no. 387651688) and the Winton Programme for the Physics of Sustainability. J.L.M.-D. acknowledges funding from the Royal Academy of Engineering under the Chair in Emerging Technologies Scheme (no. CIET1819_24). R.L.Z.H. acknowledges support from the Royal Academy of Engineering under the Research Fellowship scheme (no. RF\201718\1701), the Isaac Newton Trust (Minute 19.07(d)), Downing College Cambridge through the Kim and Juliana Silverman Research Fellowship, and an EPSRC grant (no. EP/V014498/2). I.B. and B.M. acknowledge support from the Winton Programme for the Physics of Sustainability. B.M. also acknowledges support from a UKRI Future Leaders Fellowship (no. MR/V023926/1) and from the Gianna Angelopoulos Programme for Science, Innovation and Technology. The calculations are conducted using resources provided by the Cambridge Tier-2 system, operated by the University of Cambridge Research Computing Service (www.hpc.cam.ac.uk) and funded by EPSRC Tier-2 capital grant (no. EP/P020259/1).AbstractDetecting low dose rates of X-rays is critical for making safer radiology instruments, but is limited by the absorber materials available. Here, we develop bismuth oxyiodide (BiOI) single crystals into effective X-ray detectors. BiOI features complex lattice dynamics, owing to the ionic character of the lattice and weak van der Waals interactions between layers. Through use of ultrafast spectroscopy, first-principles computations and detailed optical and structural characterisation, we show that photoexcited charge-carriers in BiOI couple to intralayer breathing phonon modes, forming large polarons, thus enabling longer drift lengths for the photoexcited carriers than would be expected if self-trapping occurred. This, combined with the low and stable dark currents and high linear X-ray attenuation coefficients, leads to strong detector performance. High sensitivities reaching 1.1 × 103 μC Gyair−1 cm−2 are achieved, and the lowest dose rate directly measured by the detectors was 22 nGyair s−1. The photophysical principles discussed herein offer new design avenues for novel materials with heavy elements and low-dimensional electronic structures for (opto)electronic applications.</jats:p
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Layered BiOI single crystals capable of detecting low dose rates of X-rays.
Detecting low dose rates of X-rays is critical for making safer radiology instruments, but is limited by the absorber materials available. Here, we develop bismuth oxyiodide (BiOI) single crystals into effective X-ray detectors. BiOI features complex lattice dynamics, owing to the ionic character of the lattice and weak van der Waals interactions between layers. Through use of ultrafast spectroscopy, first-principles computations and detailed optical and structural characterisation, we show that photoexcited charge-carriers in BiOI couple to intralayer breathing phonon modes, forming large polarons, thus enabling longer drift lengths for the photoexcited carriers than would be expected if self-trapping occurred. This, combined with the low and stable dark currents and high linear X-ray attenuation coefficients, leads to strong detector performance. High sensitivities reaching 1.1 × 103 μC Gyair-1 cm-2 are achieved, and the lowest dose rate directly measured by the detectors was 22 nGyair s-1. The photophysical principles discussed herein offer new design avenues for novel materials with heavy elements and low-dimensional electronic structures for (opto)electronic applications
Recommended from our members
Layered BiOI single crystals capable of detecting low dose rates of X-rays.
Detecting low dose rates of X-rays is critical for making safer radiology instruments, but is limited by the absorber materials available. Here, we develop bismuth oxyiodide (BiOI) single crystals into effective X-ray detectors. BiOI features complex lattice dynamics, owing to the ionic character of the lattice and weak van der Waals interactions between layers. Through use of ultrafast spectroscopy, first-principles computations and detailed optical and structural characterisation, we show that photoexcited charge-carriers in BiOI couple to intralayer breathing phonon modes, forming large polarons, thus enabling longer drift lengths for the photoexcited carriers than would be expected if self-trapping occurred. This, combined with the low and stable dark currents and high linear X-ray attenuation coefficients, leads to strong detector performance. High sensitivities reaching 1.1 × 103 μC Gyair-1 cm-2 are achieved, and the lowest dose rate directly measured by the detectors was 22 nGyair s-1. The photophysical principles discussed herein offer new design avenues for novel materials with heavy elements and low-dimensional electronic structures for (opto)electronic applications