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Advanced materials 35(5), 2208061 (2023). doi:10.1002/adma.202208061, Incorporating large organic cations to form 2D and mixed 2D/3D structures significantly increases the stability of perovskite solar cells. However, due to their low electron mobility, aligning the organic sheets to ensure unimpeded charge transport is critical to rival the high performances of pure 3D systems. While additives such as methylammonium chloride (MACl) can enable this preferential orientation, so far, no complete description exists explaining how they influence the nucleation process to grow highly aligned crystals. Here, by investigating the initial stages of the crystallization, as well as partially and fully formed perovskites grown using MACl, the origins underlying this favorable alignment are inferred. This mechanism is studied by employing 3-fluorobenzylammonium in quasi-2D perovskite solar cells. Upon assisting the crystallization with MACl, films with a degree of preferential orientation of 94%, capable of withstanding moisture levels of 97% relative humidity for 10 h without significant changes in the crystal structure are achieved. Finally, by combining macroscopic, microscopic, and spectroscopic studies, the nucleation process leading to highly oriented perovskite films is elucidated. Understanding this mechanism will aid in the rational design of future additives to achieve more defect tolerant and stable perovskite optoelectronics., Published by Wiley-VCH, Weinheim

In this paper, we study the mutual solid-state solubility in Au Ni nanoparticles with a mean size of ≈6 nm. Solid solution formation was probed via electron microdiffraction, EDX, and high-resolution STEM during sample heating in an electron microscope. As a result, a miscibility gap curve was constructed on the Au Ni phase diagram for ≈6 nm particles. It was shown that a quasi-homogeneous solid solution is already formed at 300–325 °C. The coarsening of particles induces decomposition of the solid solution and segregation of gold to the surface of nickel nanoparticles above 325 °C.

Melting temperature is one of the principal parameters of phase equilibria thermodynamics and its value affects the majority of materials properties. Despite previous studies, the fundamental mechanisms of interphase interactions at melting in two-component nanomaterials are under consideration. We address these issues in a comprehensive experimental study of phase transformations in nanosized Ag-Ge eutectic system by means of in situ TEM techniques. The Ge/Ag/Ge and Ag/Ge layered films with thin Ag layer (1–50 nm) on top or in-between amorphous Ge films have been used as a model of a binary system of variable size. Morphology, crystalline structure and composition of Ag-Ge films have been examined in situ and ex situ by advanced (S)TEM techniques, the eutectic temperature (TE) and the Ge crystallization temperature have been measured as a function of Ag film thickness. A few hundred degrees lowering of TE with the film thickness reduction has been registered. Complex changes of the Ge/Ag/Ge films morphology below the eutectic temperature driven by Ag-mediated crystallization of amorphous Ge have been observed and related to the formation of metastable (liquid and/or Ag-Ge hcp) phases at the metal-semiconductor interface at 250 °C already.

We present a novel technique for investigating the melting and crystallization of nanomaterials in confinement. According to this method, the melting-crystallization phase transitions are traced by sharp changes of viscoelastic properties of matter using quartz resonator Q-factor analysis during heating-cooling cycles. Ge/Bi/Ge layered films, formed by successive condensation of components in high vacuum, were used as the model of nanoscale Bi confined in Ge matrix. Size dependences of the melting and crystallization temperatures have been systematically determined for the samples with Bi film thickness of 5-50 nm. The obtained data are in good agreement with the data obtained previously using in situ transmission electron microscopy techniques.

The group IV Ge–Sn system is a promising candidate for a variety of electronic and photonic applications due to its tunable bandgap and compatibility with Si-based technology. However, the peculiarities of the components interaction, phases evolution and, especially, their thermal stability are still far from a brimming comprehension. In the course of in situ TEM heating experiments, we demonstrate the high potential of using nanosized, layered Sn/amorphous-Ge films for the synthesis of Ge–Sn solid solutions with an enhanced Sn content. Particularly, we observe the formation of highly uniform, diamond structured Ge1-xSnx solid solution with Sn content of 33 at% by metal-induced crystallization (MIC) at 90 °C. The Ge0.67Sn0.33 alloy is stable in the range of 20–150 °C, whereat additional heating leads to partial Sn segregation and subsequent melting of the Sn-rich phase. The formation of the metastable and stable Sn-rich liquid phases, which was observed at 90 and 150–190 °C, respectively, has a key role in the system thermal stability. The temperatures and the observed complex interphase interactions are discussed in the frame of the size effects in nanoscaled structures.

We investigate the melting behavior of Au/Ge bilayer (Ge: 5 or 2 nm; Au: 0.1–1.1 nm) thin films using transmission electron microscopy and high energy electron diffraction. It was found that the liquid-phase formation in the system at eutectic temperature takes place only if the gold film mass thickness values are greater than the critical one (0.2 nm). This effect is revealed to be independent of the contact-heating sequence of the components.