Monitoring Nanocrystal Self‐Assembly in Real Time Using In Situ Small‐Angle X‐Ray Scattering

Self‐assembled nanocrystal superlattices have attracted large scientific attention due to their potential technological applications. However, the nucleation and growth mechanisms of superlattice assemblies remain largely unresolved due to experimental difficulties to monitor intermediate states. Here, the self‐assembly of colloidal PbS nanocrystals is studied in real time by a combination of controlled solvent evaporation from the bulk solution and in situ small‐angle X‐ray scattering (SAXS) in transmission geometry. For the first time for the investigated system a hexagonal closed‐packed (hcp) superlattice formed in a solvent vapor saturated atmosphere is observed during slow solvent evaporation from a colloidal suspension. The highly ordered hcp superlattice is followed by a transition into the final body‐centered cubic superlattice upon complete drying. Additionally, X‐ray cross‐correlation analysis of Bragg reflections is applied to access information on precursor structures in the assembly process, which is not evident from conventional SAXS analysis. The detailed evolution of the crystal structure with time provides key results for understanding the assembly mechanism and the role of ligand–solvent interactions, which is important both for fundamental research and for fabrication of superlattices with desired properties.     

Real‐Time Tracking of Superparamagnetic Nanoparticle Self‐Assembly

The spontaneous self‐assembly process of superparamagnetic nanoparticles in a fast‐drying colloidal drop is observed in real time. The grazing‐incidence small‐angle X‐ray scattering (GISAXS) technique is employed for an in situ tracking of the reciprocal space, with a 3 ms delay time between subsequent frames delivered by a new generation of X‐ray cameras. A focused synchrotron beam and sophisticated sample oscillations make it possible to relate the dynamic reciprocal to direct space features and to localize the self‐assembly. In particular, no nanoparticle ordering is found inside the evaporating drop and near‐surface region down to a drop thickness of 90 µm. Scanning through the shrinking drop‐contact line indicates the start of self‐assembly near the drop three‐phase interface, in accord with theoretical predictions. The results obtained have direct implications for establishing the self‐assembly process as a routine technological step in the preparation of new nanostructures.     

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.     

Urchin‐Like Fe3Se4 Hierarchitectures: A Novel Pseudocapacitive Sodium‐Ion Storage Anode with Prominent Rate and Cycling Properties

Transition metal chalcogenides have received great attention as promising anode candidates for sodium‐ion batteries (SIBs). However, the undesirable cyclic life and inferior rate capability still restrict their practical applications. The design of micro–nano hierarchitectures is considered as a possible strategy to facilitate the electrochemical reaction kinetics and strengthen the electrode structure stability upon repeated Na⁺ insertion/extraction. Herein, urchin‐like Fe₃Se₄ hierarchitectures are successfully prepared and developed as a novel anode material for SIBs. Impressively, the as‐prepared urchin‐like Fe₃Se₄ can present an ultrahigh rate capacity of 200.2 mAh g^‐1 at 30 A g^‐1 and a prominent capacity retention of 99.9% over 1000 cycles at 1 A g^‐1, meanwhile, a respectable initial coulombic efficiency of ≈100% is achieved. Through the conjunct study of in situ X‐ray diffraction, ex situ X‐ray absorption near‐edge structure spectroscopy, as well as cyclic voltammetry curves, it is intriguing to reveal that the phase transformation from monoclinic to amorphous structure accompanied by the pseudocapacitive Na⁺ storage behavior accounts for the superior electrochemical performance. When paired with the Na₃V₂(PO₄)₃ cathode materials, the assembled full cell enables high energy density and decent cyclic stability, demonstrating potential practical feasibility of the present urchin‐like Fe₃Se₄ anode.     

Analysis of Carbon Materials by X‐ray Photoelectron Spectroscopy and X‐ray Absorption Spectroscopy

Since hard coating such as ta:C films are of growing interest for protecting surfaces of industrial machines and high capacity tools against wear and friction or for biomedical applications, more information about the structure‐to‐property relation is required. Therefore, core level X‐ray photoelectron sprectroscopy and X‐ray absorption spectroscopy and spectromicroscopy are used to derive chemical information about selected carbon species.     

Real‐time X‐ray Radioscopy on Metallic Foams Using a Compact Micro‐Focus Source

A measurement apparatus for observing the foaming of metals is described. It consists of a microfocus X‐ray source, a small heater and a panel detector. The foaming of samples can be recorded in real‐time with frequencies up to 9 Hz and resolutions down to 5 μm due to the small spot size. Magnifications up to 10x are obtained by simply changing the distance between source, sample and detector. Foaming of Al alloys and sandwiches was investigated applying different final foaming temperatures, heating rates and blowing agent pre‐treatments. An image analysis program was developed to automatically recognise the foam expansion.