Two-dimensional metal halide perovskites of Ruddlesden-Popper type have recently moved into the centre of attention of perovskite research due to their potential for light generation and for ...stabilisation of their 3D counterparts. It has become widespread in the field to attribute broad luminescence with a large Stokes shift to self-trapped excitons, forming due to strong carrier-phonon interactions in these compounds. Contrarily, by investigating the behaviour of two types of lead-iodide based single crystals, we here highlight the extrinsic origin of their broad band emission. As shown by below-gap excitation, in-gap states in the crystal bulk are responsible for the broad emission. With this insight, we further the understanding of the emission properties of low-dimensional perovskites and question the generality of the attribution of broad band emission in metal halide perovskite and related compounds to self-trapped excitons.
Tin‐based perovskites have long remained a side topic in current perovskite optoelectronic research. With the recent efficiency improvement in thin film solar cells and the observation of a long hot ...carrier cooling time in formamidinium tin iodide (FASnI3), a thorough understanding of the material's photophysics becomes a pressing matter. Since pronounced background doping can easily obscure the actual material properties, it is of paramount importance to understand how different processing conditions affect the observed behavior. Using photoluminescence spectroscopy, thin films of FASnI3 fabricated through different protocols are therefore investigated. It is shown that hot carrier relaxation occurs much faster in highly p‐doped films due to carrier–carrier scattering. From high quality thin films, the longitudinal optical phonon energy and the electron–phonon coupling constant are extracted, which are fundamental to understanding carrier cooling. Importantly, high quality films allow for the observation of a previously unreported state of microsecond lifetime at lower energy in FASnI3, that has important consequences for the discussion of long lived emission in the field of metal halide perovskites.
Formamidinium tin iodide (FASnI3) thin films of different quality exhibit drastically different photophysics. Strongly increased carrier scattering and recombination causes hot carriers to cool down quickly in samples of poor quality. The low temperature crystal phase of FASnI3 possesses a long‐lived emitting state with an energy smaller than the main emission.
Crises trigger both learning and unlearning at both intra-organizational and inter-organizational levels. This article stresses the need to facilitate unlearning for effective crisis management and ...shows how we could use mindfulness practice to enhance unlearning and transformative learning in a crisis. This study proposes the conceptualization of mindful unlearning in crisis with different mechanisms to foster unlearning in three stages of crisis (pre-crisis, during-crisis, and post-crisis). These mechanisms include mindful awareness of impermanence and sensual processing (pre-crisis stage), mindful awareness of interdependence and right intention (crisis management stage), and mindful awareness of transiency and past experiences (post-crisis stage).
Knowledge of the mechanism of formation, orientation, and location of phases inside thin perovskite films is essential to optimize their optoelectronic properties. Among the most promising, low ...toxicity, lead‐free perovskites, the tin‐based ones are receiving much attention. Here, an extensive in situ and ex situ structural study is performed on the mechanism of crystallization from solution of 3D formamidinium tin iodide (FASnI3), 2D phenylethylammonium tin iodide (PEA2SnI4), and hybrid PEA2FAn−1SnnI3n+1 Ruddlesden–Popper perovskites. Addition of small amounts of low‐dimensional component promotes oriented 3D‐like crystallite growth in the top part of the film, together with an aligned quasi‐2D bottom‐rich phase. The sporadic bulk nucleation occurring in the pure 3D system is negligible in the pure 2D and in the hybrid systems with sufficiently high PEA content, where only surface crystallization occurs. Moreover, tin‐based perovskites form through a direct conversion of a disordered precursor phase without forming ordered solvated intermediates and thus without the need of thermal annealing steps. The findings are used to explain the device performances over a wide range of composition and shed light onto the mechanism of the formation of one of the most promising Sn‐based perovskites, providing opportunities to further improve the performances of these interesting Pb‐free materials.
The structure and formation mechanism of spin‐coated films of lead‐free Sn‐based Ruddlesden–Popper (Sn‐RDP) perovskites are unveiled by combining results from in situ grazing incidence wide‐angle X‐ray scattering measurements and other extensive ex situ characterization methods. The formation of films with oriented Sn‐RDP crystallites is the result of bulk crystallization suppression induced by the presence of the 2D component (PEA+) during the drying process.
Replacing a carbon atom with silicon (see figure) on the main chain of a conjugated polythiophene gives a polysilole with higher crystallinity, improved charge transport, reduced bimolecular ...recombination, and reduced formation of charge transfer complexes when blended with a fullerene derivative. Optimized bulk heterojunction solar cells using this blend give certified efficiencies of 5.2% under AM1.5 illumination.
Nanostructured molecular semiconductor films are promising Surface-Enhanced Raman Spectroscopy (SERS) platforms for both fundamental and technological research. Here, we report that a nanostructured ...film of the small molecule DFP-4T, consisting of a fully π-conjugated diperfluorophenyl-substituted quaterthiophene structure, demonstrates a very large Raman enhancement factor (>10
) and a low limit of detection (10
M) for the methylene blue probe molecule. This data is comparable to those reported for the best inorganic semiconductor- and even intrinsic plasmonic metal-based SERS platforms. Photoluminescence spectroscopy and computational analysis suggest that both charge-transfer energy and effective molecular interactions, leading to a small but non-zero oscillator strength in the charge-transfer state between the organic semiconductor film and the analyte molecule, are required to achieve large SERS enhancement factors and high molecular sensitivities in these systems. Our results provide not only a considerable experimental advancement in organic SERS figure-of-merits but also a guidance for the molecular design of more sensitive SERS systems.
2D perovskites offers a rich playing field to explore exciton physics and they possess a great potential for a variety of opto‐electronic applications. Whilst their photophysics shows intricate ...interactions of excitons with the lattice, most reports have so far relied on single compound studies. With the exception of variations of the organic spacer cations, the effect of constituent substitution on the photophysics and the nature of emitting species, in particular, have remained largely under‐explored. Here PEA2PbBr4, PEA2PbI4, and PEA2SnI4 (where PEA stands for phenylethylammonoium) are studied through a variety of optical spectroscopy techniques to reveal a complex set of excitonic transitions at low temperature. Weak high‐energy features are attributed to vibronic transitions breaking Kasha's, for which the responsible phonons cannot be accessed through simple Raman spectroscopy. Bright peaks at lower energy are due to two distinct electronic states, of which the upper is a convolution of the free exciton and a localized dark state and the lower is attributed to recombination involving shallow defects. This study offers deeper insights into the photophysics of 2D perovskites through compositional substitution and highlights critical limits to the communities’ current understanding of processes in these compounds.
The photophysics of 2D perovskites is studied upon variation of the constituent metal and halide anions. All compounds exhibit persistent luminescence of hot excitons that violate Kasha's rule and a complex set of low‐energy transitions involving defects.
Immersions into Sasakian space forms Loi, A.; Placini, G.; Zedda, M.
Mathematische Zeitschrift,
07/2024, Letnik:
307, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We study immersions of Sasakian manifolds into finite and infinite dimensional Sasakian space forms. After proving Calabi’s rigidity results in the Sasakian setting, we characterise all homogeneous ...Sasakian manifolds which admit a (local) Sasakian immersion into a nonelliptic Sasakian space form. Moreover, we give a characterisation of homogeneous Sasakian manifolds which can be embedded into the standard sphere both in the compact and noncompact case.
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