Excitons and Trions in One‐Photon‐ and Two‐Photon‐Excited MoS2: A Study in Dispersions

Herein, various dispersions of MoS₂ obtained by means of liquid phase exfoliation are spectroscopically, (spectro‐) electrochemically, and microscopically characterized. At the core of these studies are transient absorption assays. Importantly, small‐angle X‐ray scattering measurements are employed to corroborate the exfoliated character of the MoS₂ flakes in dispersion, on the one hand, and to correlate the results with TEM, AFM, and Raman characterization in the solid state, on the other. It is, then, demonstrated that transient absorption spectroscopy responds sensitively not only to changes in the sample preparation but also to instrumental and environmental parameters. It is documented that the spectroscopic features and their underlying lifetimes are tuneable on the femto‐, pico‐, and nanosecond scales by changing, for example, the centrifugation speed, the pump fluence, or the temperature. In other words, transient absorption spectroscopy provides an in situ method to quantitatively characterize liquid dispersions of MoS₂ without facing the problems of reaggregated samples due to their drying for microscopic assays. The most far reaching results stem from resonantly and nonresonantly changing the pump fluence to characterize either single‐ or multiple‐excited‐state species such as excitons, trions, and bi‐/multiexcitons and to follow their formation and deactivation pattern.     

Process Control in Aluminum Foam Production Using Real‐Time X‐ray Radioscopy

Aluminum alloy foams were created by expanding foamable precursors containing a gas‐releasing blowing agent in a dense metallic matrix. The precursors were prepared in two different ways: either by hot‐compaction of powder mixtures or by thixocasting of billets obtained by cold compaction of powder blends. Foam evolution was visualized by means of real‐time X‐ray radioscopy with image frequencies ranging up to 18 Hz and spatial resolutions down to 10 μm. The difference in pore formation between the two processing routes could be studied. Rupture of cell walls during foam expansion could be visualized, critical rupture thickness measured, and the time‐scale of the rupture process estimated. By manufacturing foam precursors in which defects were incorporated deliberately, the question of the origin of very large pores in solid metal foams could be examined. By forced cooling of liquid metal foams while recording their structure, the importance of solidification‐induced changes of foam morphology was illustrated.     

Following the Kinetics of Barium Titanate Nanocrystal Formation in Benzyl Alcohol Under Near‐Ambient Conditions

In complex chemical syntheses (e.g., coprecipitation reactions), nucleation, growth, and coarsening often occur concurrently, obscuring the individual processes. Improved knowledge of these processes will help to better understand and optimize the reaction protocol. Here, a form‐free and model independent approach, based on a combination of time‐resolved small/wide‐angle X‐ray scattering, is employed to elucidate the effect of reaction parameters (such as precursor concentration, reactant stoichiometry, and temperature) on the nucleation, crystallization, and growth phenomena during the formation of nanocrystalline barium titanate. The strength of this approach is that it relies solely on the total scattered intensity (i.e., scattering invariant) of the investigated system, and no prior knowledge is required. As such, it can be widely applied to other synthesis protocols and material's systems. Through the scattering invariant, it is found that the amorphous‐to‐crystalline transformation of barium titanate is predominantly determined by the total amount of water released from the gel‐like barium hydroxide octahydrate precursor, and three rate‐limiting regimes are established. As a result of this improved understanding of the effect of varying reaction conditions, elementary boundary conditions can be set up for a better control of the barium titanate nanocrystal synthesis.     

In Situ Real‐Time Study of the Dynamic Formation and Conversion Processes of Metal Halide Perovskite Films

Metal halide perovskite solar cells (PSCs) have advanced to the forefront of solution‐processed photovoltaic techniques and made stunning progress in power conversion efficiency (PCE). Further improvements in device performances rely on perfecting the structure and morphology of perovskite films. However, undesirable defects such as pinholes and grain boundaries are often created in film preparations due to lack of knowledge of the precise reaction mechanism. Here, in situ grazing‐incidence X‐ray diffraction (GI‐XRD) investigations are performed, facilitated by other techniques, on the formation of the widely adopted MAPbI₃ (MA = methylammonium) perovskite films from their intermediate adduct (IA) phases. The influences of solvent vapor atmospheres on MAPbI₃ films are also systematically investigated, where the dynamic conversion processes between different phases are visualized in real time. Further in situ GI‐XRD and infrared spectroscopy measurements reveal that the IA phases contain both N,N‐dimethylformamide and dimethyl sulfoxide (DMSO) as coordinating molecules. By tuning the DMSO concentration in perovskite precursors, the ideal perovskite film is formed and the best PCE is achieved for the planar MAPbI₃‐based PSCs. These findings highlight the role of IA phases and the effect of solvent atmospheres on the quality of perovskite films, providing direct insights into their growth mechanism.     

Saturated Lightweight Aggregate for Internal Curing in Low w/c Mixtures: Monitoring Water Movement Using X‐ray Absorption

Abstract: The susceptibility for early‐age shrinkage cracking in low w/c concrete has led to the development of new technologies that can reduce the risk of cracking. Internal curing, one of these new technologies, uses saturated lightweight aggregate to supply ‘curing water’ to low w/c pastes as they hydrate. While significant research has been performed to determine the effects of internal curing on shrinkage and stress development, relatively little detailed information exists on movement of water from the LWA to the paste. The details of when water moves and how far it travels has implications on the volume of aggregate and the aggregate gradation that should be used in mixture proportioning with internal curing. This study examines the timing of the release of water from saturated lightweight aggregate (LWA) and the distance that water can travel. X‐ray absorption is used to determine the time at which water is drawn out of the lightweight aggregate and the loss of water is traced over time. Further, it examines the travel distance of the water into the paste. Experimental details associated with the X‐ray technique are provided to describe the importance of specimen alignment. Further, in sealed specimens, the results indicate little water movement prior to set. After set however, the distance of water movement was as much as 2 mm. The importance of water movement is discussed in terms of mixture proportioning and its influence on shrinkage and stress development.