Lead-free, luminescent perovskite nanocrystals obtained through ambient condition synthesis

Heterovalent substitution of toxic lead is an increasingly popular design strategy to obtain environmentally sustainable variants of the exciting material class of halide perovskites. Perovskite nanocrystals (NCs) obtained through solution-based methods exhibit exceedingly high optical quality. Unfortunately, most of these synthesis routes still require reaction under inert gas and at very high temperatures. Herein we present a novel synthesis routine for lead-free double perovskite NCs. We combine hot injection and ligand-assisted reprecipitation (LARP) methods to achieve a low-temperature and ambient atmosphere-based synthesis for manganese-doped Cs_{2}NaBiCl_{6} NCs. Mn incorporation is critical for the otherwise non-emissive material, with a 9:1 Bi:Mn precursor ratio maximizing the bright orange photoluminescence (PL) and quantum yield (QY). Higher temperatures slightly increased the material's performance, yet NCs synthesized at room temperature were still emissive, highlighting the versatility of the synthetic approach. Furthermore, the NCs show excellent long-term stability in ambient conditions, facilitating additional investigations and energy-related applications

Fine‐Tuning Blue‐Emitting Halide Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) with narrow, bright emission in the visible range are promising candidates for light-emitting applications. Near-unity quantum yields have been realized for green and red-emitting perovskites, but efficient, stable blue-emitting perovskite materials are scarce. Current methods to synthesize quantum-confined CsPbBr3 NCs with blue emission are limited to specific wavelength ranges and still suffer from inhomogeneously broadened emission profiles. Herein, anisotropic blue-green emitting CsPbBr3 NCs are synthesized in ambient atmosphere using a spontaneous crystallization method. Optical spectroscopy reveals a gradual, asymptotic photoluminescence (PL) redshift of pristine colloidal NCs after synthesis. During this process, the emission quality improves notably as the PL spectra become narrower and more symmetric, accompanied by a PL intensity increase. Electron microscopy indicates that the gradual redshift stems from an isotropic growth of the CsPbBr3 NCs in at least two dimensions, likely due to residual precursor ions in the dispersion. Most importantly, the growth process can be halted at any point by injecting an enhancement solution containing PbBr2 and organic capping ligands. Thus, excellent control over NC size is achieved, allowing for nanometer-precise tunability of the respective emission wavelength in the range between 475 and 500 nm, enhancing the functionality of these already impressive NCs

Dark-Bright Exciton Splitting Dominates Low-Temperature Diffusion in Halide Perovskite Nanocrystal Assemblies

Semiconductor nanocrystals could replace conventional bulk materials completely in displays and light-emitting diodes. Exciton transport dominates over charge carrier transport for materials with high exciton binding energies and long ligands, such as halide perovskite nanocrystal films. Here, we investigate how beneficial superlattices - nearly perfect 3D nanocrystal assemblies of nanocrystals are to exciton transport. Surprisingly, the high degree of order is not as crucial as the individual nanocrystal size, which strongly influences the splitting of the excitonic manifold into bright and dark states. At very low temperatures, the energetic splitting is large for the smallest nanocrystals, and dark levels with low oscillator strength effectively trap excitons inside individual nanocrystals, suppressing diffusion. The effect is reversed at elevated temperatures, and the larger NC size becomes detrimental to exciton transport due to enhanced exciton trapping and dissociation. Our results reveal that the nanocrystal size must be strongly accounted for in design strategies of future optoelectronic applications

Doubly stabilized perovskite nanocrystal luminescence downconverters

Halide perovskite nanocrystals (NCs) have emerged as a promising material for applications ranging from light-emitting diodes (LEDs) to solar cells and photodetectors. Still, several issues impede the realization of the nanocrystals' full potential, most notably their susceptibility to degradation from environmental stress. This work demonstrates highly stable perovskite nanocrystals (NCs) with quantum yields as high as 95 % by exploiting a ligand-assisted copolymer nanoreactor-based synthesis. The organic ligands thereby serve a dual function by enhancing the uptake of precursors and passivating the NCs. The polymer micelles and ligands thus form a double protection system, shielding the encapsulated NCs from water-, heat- and UV-light-induced degradation. We demonstrate the optoelectronic integrability by incorporating the perovskite NCs as spectrally pure downconverters on top of a deep-blue-emitting organic LED. These results establish a way of stabilizing perovskite NCs for optoelectronics while retaining their excellent optical properties

Praxisbericht aus dem mediendidaktischen Projekt FEEDBACK in der Lehre

„Was ist gute Lehre?“ Dieser Frage wollen wir auf den Grund gehen. Erneut haben wir zur Auseinandersetzung damit ein Vorhaben auf den Weg gebracht, das insbesondere auch nach dem Beitrag die digitalen Medien zur Entwicklung didaktischer Qualität fragt und die Rahmenbedingungen, unter denen sich ihr Potenzial für die Hochschullehre erschließen lässt, zu beschreiben versucht. Mit dem Projekt „FEEDBACK in der Lehre“ fokussieren wir einen wesentlichen Aspekt unseres Lehr-Lern-Handelns und machen ihn explizit zum Gegenstand der Reflexion von Unterrichtsmethoden und Mediengebrauch. Ein weiteres Mal versuchen wir Antworten auf diese Frage aus der Praxis unserer Lehre an der Hochschule Bremen zu erzeugen – nicht zuletzt als Konsequenz aus dem vorhergehenden Projekt „WELLDONE“, in dessen Evaluation einer angemessenen und konstruktiven Kommunikation über Lernerwartungen und Lernergebnissen für den Studienerfolg von der Mehrheit der Teilvorhaben eine besondere Bedeutung beigemessen wurde. Wie gute Lehre gestaltet werden kann oder als solche erfahren wird erleben wir alltäglich im Studienbetrieb – auch ohne besondere Vorhaben. Sie äußern sich in den kleinen und großen Beispielen positiver Rückmeldungen, nach einer als gelungen empfundenen Diskussion im Seminar, in der Freude eines Studierenden-Teams über einen erfolgreichen Laborversuch, in der von einer Kollegin ausgesprochenen Ermutigung, den Einsatz eines neuen Mediums weiterhin zu verfolgen, auch wenn es beim ersten Mal noch nicht optimal geklappt hat. Die in der Alltagspraxis vorgefundenen Antworten sind so unterschiedlich und mannigfaltig wie das Spektrum der Studienangebote an unserer Hochschule. Sie sind einem ständigen Wandel unterworfen, einerseits bedingt durch neue Ziele und Rahmenbedingungen, andererseits auch immer wieder gründend in der didaktischen Kreativität von Lehrenden, die mit der Freiheit der Lehre auch methodische Spielräume verbinden und sie zur Bereicherung ihres Lehrangebots nutzen. All das ist es wert, anderen zugänglich gemacht und – gern auch kontrovers – diskutiert zu werden. Dafür brauchen wir den besonderen Rahmen, den wir mittels unserer Vorhaben herstellen wollen: Eine Umgebung, in der wir aus Beispielen lernen können, in der wir im kollegialen Gespräch erfragen können, wie sich theoretisch fundierte und didaktisch sorgfältig ausgewählte Methoden und Medien in der konkreten Unterrichtspraxis bewährt haben, in der wir uns die Zeit nehmen können, anderen Lehrenden zuzuhören, auch unsere Neugier zu stillen oder den kritischen Blick auf unsere eigene Routine zu schärfen.8

Does Religion Matter on Financial Decisions? The Case of SMEs Capital Structure

Different finance theories have tried to explain the capital structure of companies considering it as a rational choice with limited results. In contrast, upper echelons theory highlights the influence of top managers’ values and beliefs in the decisions of firms. This research aims to fill this gap and looks into the linkage of religious belonging to explain why SMEs differ in their preferences between equity and liabilities. Therefore, the present study uses the two main Christian faiths to analyze the impact on the capital structure. We hypothesize that SMEs in countries with a mainly Catholic faith preferably carry higher amounts of debt, and contrary, firms in predominantly Protestant nations are more willing to issue equity.134672022

Finanzcontrolling in Start-ups

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Dark-Bright Exciton Splitting Dominates Low-Temperature Diffusion in Halide Perovskite Nanocrystal Assemblies

Semiconductor nanocrystals could replace conventional bulk materials completely in displays and light-emitting diodes. However, the organic ligands enabling their unique optical properties, prevent current flow in nanocrystal films, leaving energy transfer as the only means of injecting or extracting carriers. Here, we investigate exciton diffusion in halide perovskite superlattices - nearly perfect 3D nanocrystal assemblies. This high degree of order is not as crucial as the individual nanocrystal size, which affects transport differently depending on temperature. Up to 70 K, a confinement-induced splitting of excitonic energies, especially for the smallest nanocrystals, traps excitons into dark levels, suppressing diffusion. At intermediate temperatures, the distance of individual FRET steps corresponding to nanocrystal size enhances diffusion in the larger nanocrystals. This trend is reversed, as exciton dissociation and carrier trapping in less strongly confined nanocrystals become dominant up to room temperature. Our results reveal that transport must be factored strongly into nanocrystal design strategies for future optoelectronic applications.ISSN:2693-501