Design of Active Defects in Semiconductors: 3D Electron Diffraction Revealed Novel Organometallic Lead Bromide Phases Containing Ferrocene as Redox Switches

Once the optical, electronic, or photocatalytic properties of a semiconductor are set by adjusting composition, crystal phase, and morphology, one cannot change them anymore, respectively, on demand. Materials enabling postsynthetic and reversible switching of features such as absorption coefficient, bandgap, or charge carrier dynamics are highly desired. Hybrid perovskites facilitate exceptional possibilities for progress in the field of smart semiconductors because active organic molecules become an integral constituent of the crystalline structure. This paper reports the integration of ferrocene ligands into semiconducting 2D phases based on lead bromide. The complex crystal structures of the resulting, novel ferrovskite (≃ ferrocene perovskite) phases are determined by 3D electron diffraction. The ferrocene ligands exhibit strong structure-directing effects on the 2D hybrid phases, which is why the formation of exotic types of face- and edge-sharing lead bromide octahedra is observed. The bandgap of the materials ranges from 3.06 up to 3.51 eV, depending on the connectivity of the octahedra. By deploying the redox features of ferrocene, one can create defect states or even a defect band leading to control over the direction of exciton migration and energy transport in the semiconductor, enabling fluorescence via indirect to direct gap transition. © 2022 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH

On the shape-selected, ligand-free preparation of hybrid perovskite (Ch3nh3pbbr3) microcrystals and their suitability as model-system for single-crystal studies of optoelectronic properties

Hybrid perovskite materials are one of the most promising candidates for optoelectronic applications, e.g., solar cells and LEDs, which can be produced at low cost compared to established materials. Although this field of research has seen a huge upsurge in the past decade, there is a major lack in understanding the underlying processes, such as shape-property relationships and the role of defects. Our aerosol-assisted synthesis pathway offers the possibility to obtain methylammonium lead bromide (MAPbBr3 ) microcrystals from a liquid single source precursor. The differently shaped particles are aligned on several substrates, without using a directing agent or other additives. The obtained particles show good stability under dry conditions. This allows us to characterize these materials and their pure surfaces at the single-crystal level using time-and spatially resolved methods, without any influences of size-dependent effects. By optimizing the precursor for the aerosol process, we were able to eliminate any purification steps and use the materials as processed. In addition, we performed theoretical simulations to deepen the understanding of the underlying processes in the formation of the different crystal facets and their specific properties. The model system presented provides insights into the shape-related properties of MAPbBr3 single crystals and their directed but ligand-free synthesis. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

From 'One Namibia, One Nation' towards 'Unity in Diversity? Shifting representations of culture and nationhood in Namibian Independence Day celebrations, 1990-2010

In 2010 Namibia celebrated its twentieth anniversary of independence from South African rule. The main celebrations in the country’s capital Windhoek became the stage for an impressively orchestrated demonstration of maturing nationhood, symbolically embracing postcolonial policy concepts such as ‘national reconciliation’, ‘unity’ and ‘diversity’. At the same time, nation building in post-apartheid Namibia is characterised by a high degree of social and political fragmentation that manifests itself in cultural and/or ethnic discourses of belonging. Taking the highly significant independence jubilee as our vantage point, we map out a shift of cultural representations of the nation in Independence Day celebrations since 1990, embodied by the two prominent slogans of ‘One Namibia, one Nation’ and ‘Unity in Diversity’. As we will argue, the difficult and at times highly fragile postcolonial disposition made it necessary for the SWAPO government, as primary nation builder, to accommodate the demands of regions and local communities in its policy frameworks. This negotiation of local identifications and national belonging in turn shaped, and continues to shape, the performative dimension of Independence Day celebrations in Namibia.Web of Scienc

Determination of test weight during threshing by analysing air-filled pore volume in grain fills

Die Bestimmung des Hektolitergewichts ist eine weitverbreitete Messmethode, um schnell Informationen über die Qualitätseigenschaften von Getreide zu erhalten. Es gibt jedoch keine weltweit einheitliche Vorgehensweise bei der Bestimmung; dies führt – je nach Messmethode – zu unterschiedlichen Ergebnissen. Des Weiteren wird das Messergebnis mit den aktuell gängigen Methoden auch von Umgebungsbedingungen wie z. B. Vibrationen, Entmischungsvorgängen oder Guteigenschaften wie der Korngeometrie beeinflusst. Nicht zuletzt beeinflusst auch der Faktor Mensch das Messergebnis. Aufgrund von Vibrationen oder Luftströmen auf dem Mähdrescher ist es zurzeit nur schwer möglich, das Hektolitergewicht online während der Ernte zu bestimmen. Das hierfür notwendige Wiegen der einzelnen Getreideproben erfordert konstante, definierte und vor allem vibrationsfreie Umgebungsbedingungen. Gemeinsam mit der BlueMethano GmbH wurde an der Humboldt-Universität zu Berlin deshalb ein neuartiges System zur Bestimmung des Hektolitergewichts entwickelt. Das Gewicht wird in diesem Verfahren über das luftgefüllte Porenvolumen einer Getreideprobe bestimmt. Hierfür ist keine Waage notwendig, sodass das System auch online auf der Erntemaschine eingesetzt werden kann

Design of Active Defects in Semiconductors: 3D Electron Diffraction Revealed Novel Organometallic Lead Bromide Phases Containing Ferrocene as Redox Switches

Once the optical, electronic or photocatalytic properties of a semiconductor are set by adjusting composition, crystal phase and morphology, one cannot change them anymore, respectively on demand. Materials enabling a post-synthetic and reversible switching of features such as absorption coefficient, band-gap or charge carrier dynamics represent are highly desired. Hybrid perovskites facilitate exceptional possibilities for progress in the field of smart semiconductors because active organic molecules become an integral constituent of the crystalline structure. We report the integration of ferrocene ligands into semiconducting 2D phases based on lead bromide. The complex crystal structures of the resulting, novel ferrovskite phases were determined by 3D electron diffraction. The ferrocene ligands exhibit strong structure directing effects on the 2D hybrid phases, which is why the formation of exotic types of face and edge sharing lead bromide octahedra is observed. The band gap of the materials ranges from 3.06 eV up to 3.51 eV, depending on the connectivity of the octahedra. Deploying the redox features of ferrocene, one can create defect states or even a defect band leading to the control over the direction of exciton migration and energy transport in the semiconductor

Optical properties of NIR photoluminescent PbS nanocrystal-based three-dimensional networks

The assembly of nanocrystals (NCs) into three-dimensional network structures is a recently established strategy to produce macroscopic materials with nanoscopic properties. These networks can be formed by the controlled destabilization of NC colloids and subsequent supercritical drying to obtain NC-based aerogels. Even though this strategy has been used for many different semiconductor NCs, the emission of NC-based aerogels is limited to the ultraviolet and visible and no near-infrared (NIR) emitting NC-based aerogels have been investigated in literature until now. In the present work we have optimized a gelation route of NIR emitting PbS and PbS/CdS quantum dots (QDs) by means of a recently established gel formation method using trivalent ions to induce the network formation. Thereby, depending on the surface ligands and QDs used the resulting network structure is different. We propose, that the ligand affinity to the nanocrystal surface plays an essential role during network formation, which is supported by theoretical calculations. The optical properties were investigated with a focus on their steady-state and time resolved photoluminescence (PL). Unlike in PbS/CdS aerogels, the absorption of PbS aerogels and their PL shift strongly. For all aerogels the PL lifetimes are reduced in comparison to those of the building blocks with this reduction being especially pronounced in the PbS aerogels

Ligand-Programmed Consecutive Symmetry Break(s) in Nanoparticle Based Materials Showing Emergent Phenomena: Transitioning from Six-Fold to Three-Fold Symmetry in Anisotropic ZnO Colloids

The central promise of nanoparticle-based materials is that cooperative properties may emerge, when individual quantum dots are positioned on a periodic lattice. Yet, there are only few papers in literature reporting about such effects. Nevertheless, it is clear that the symmetry of the superlattice is decisive for the desired emergent phenomena. An interesting question is, how the symmetry of the initial monodisperse nanoparticles affects the structure of the colloidal crystal during self-assembly processes. For instance, particles with hexagonal cross-section show self-organization which is very similar to spherical colloids. Like-wise one would also expect that trigonal nanoparticles behave similar. Unfortunately, it is very hard to obtain monodisperse semiconductur colloids with trigonal shape, because this requires a symmetry break during morphogenesis of the nanocrystal. While such a symmetry-break is known in literature for structures attached to a solid substrate, it is shown here, colloidal synthesis of trigonal ZnO nanorods is successful, and the mechanism is elucidated by experimental and theoretical methods. 2D-superlattices formed by such particles with trigonal cross-section were compared to hexagonal analogues. It was found, there are distinct differences, which result in important differences in properties such as the formation of voids and also in optical properties

Ligand‐Programmed Consecutive Symmetry Break(s) in Nanoparticle Based Materials Showing Emergent Phenomena : Transitioning from Sixfold to Threefold Symmetry in Anisotropic ZnO Colloids

The central promise of nanoparticle‐based materials is that cooperative properties may emerge, when individual quantum dots are positioned on a periodic lattice. Yet, there are only a few papers in the literature reporting such effects. Nevertheless, it is clear that the symmetry of the superlattice is decisive for the desired emergent phenomena. An interesting question is, how the symmetry of the initial monodisperse nanoparticles affects the structure of the colloidal crystal during self‐assembly processes. For instance, particles with a hexagonal cross‐section demonstrate self‐organization, which is very similar to spherical colloids. Likewise, one would also expect that trigonal nanoparticles behave similarly. Unfortunately, it is very hard to obtain monodisperse semiconductor colloids with a trigonal shape, because this requires a symmetry break during morphogenesis of the nanocrystal. While such a symmetry break is known in the literature for structures attached to a solid substrate, herein, colloidal synthesis of trigonal ZnO nanorods is successfully demonstrated, and the mechanism is elucidated via experimental and theoretical methods. 2D‐superlattices formed by such particles with trigonal cross‐section are compared to hexagonal analogues. It is found that there are distinct differences, which result in important differences in properties such as the formation of voids and also in optical properties.publishe

On the Shape-Selected, Ligand-Free Preparation of Hybrid Perovskite (CH₃NH₃PbBr₃) Microcrystals and Their Suitability as Model-System for Single-Crystal Studies of Optoelectronic Properties

Hybrid perovskite materials are one of the most promising candidates for optoelectronic applications, e.g., solar cells and LEDs, which can be produced at low cost compared to established materials. Although this field of research has seen a huge upsurge in the past decade, there is a major lack in understanding the underlying processes, such as shape-property relationships and the role of defects. Our aerosol-assisted synthesis pathway offers the possibility to obtain methylammonium lead bromide (MAPbBr3) microcrystals from a liquid single source precursor. The differently shaped particles are aligned on several substrates, without using a directing agent or other additives. The obtained particles show good stability under dry conditions. This allows us to characterize these materials and their pure surfaces at the single-crystal level using time- and spatially resolved methods, without any influences of size-dependent effects. By optimizing the precursor for the aerosol process, we were able to eliminate any purification steps and use the materials as processed. In addition, we performed theoretical simulations to deepen the understanding of the underlying processes in the formation of the different crystal facets and their specific properties. The model system presented provides insights into the shape-related properties of MAPbBr3 single crystals and their directed but ligand-free synthesis.publishe