Ligand-Dependent Coalescence Behaviors of Gold Nanoparticles Studied by Multichamber Graphene Liquid Cell Transmission Electron Microscopy.

The formation mechanism of colloidal nanoparticles is complex because significant nonclassical pathways coexist with the conventional nucleation and growth processes. Particularly, the coalescence of the growing clusters determines the final morphology and crystallinity of the synthesized nanoparticles. However, the experimental investigation of the coalescence mechanism is a challenge because the process is highly kinetic and correlates with surface ligands that dynamically modify the surface energy and the interparticle interactions of nanoparticles. Here, we employ quantitative in situ TEM with multichamber graphene liquid cell to observe the coalescence processes occurring in the synthesis of gold nanoparticles in different ligand systems, thus affording us an insight into their ligand-dependent coalescence kinetics. The analyses of numerous liquid-phase TEM trajectories of the coalescence and MD simulations of the ligand shells demonstrate that enhanced ligand mobility, employing a heterogeneous ligand mixture, results in the rapid nanoparticle pairing approach and a fast post-merging structural relaxation.

Amorphous-Phase-Mediated Crystallization of Ni Nanocrystals Revealed by High-Resolution Liquid-Phase Electron Microscopy.

Nonclassical features of crystallization in solution have been recently identified both experimentally and theoretically. In particular, an amorphous-phase-mediated pathway is found in various crystallization systems as an important route, different from the classical nucleation and growth model. Here, we utilize high-resolution in situ transmission electron microscopy with graphene liquid cells to study amorphous-phase-mediated formation of Ni nanocrystals. An amorphous phase is precipitated in the initial stage of the reaction. Within the amorphous particles, crystalline domains nucleate and eventually form nanocrystals. In addition, unique crystallization behaviors, such as formation of multiple domains and dislocation relaxation, are observed in amorphous-phase-mediated crystallization. Theoretical calculations confirm that surface interactions can induce amorphous precipitation of metal precursors, which is analogous to the surface-induced amorphous-to-crystalline transformation occurring in biomineralization. Our results imply that an unexplored nonclassical growth mechanism is important for the formation of nanocrystals.

Two-dimensional demixing within multilayered nanoemulsion films.

Benefiting from the demixing of substances in the two-phase region, a smart polymer laminate film system that exhibits direction-controlled phase separation behavior was developed in this study. Here, nanoemulsion films (NEFs) in which liquid nanodrops were uniformly confined in a polymer laminate film through the layer-by-layer deposition of oppositely charged emulsion nanodrops and polyelectrolytes were fabricated. Upon reaching a critical temperature, the NEFs exhibited a micropore-guided demixing phenomenon. A simulation study based on coarse-grained molecular dynamics revealed that the perpendicular diffusion of oil droplets through the micropores generated in the polyelectrolyte layer is crucial for determining the coarsening kinetics and phase separation level, which is consistent with the experimental results. Considering the substantial advantages of this unique and tunable two-dimensional demixing behavior, the viability of using the as-proposed NEF system for providing an efficient route for the development of smart drug delivery patches was demonstrated.

Motion-Programmed Bar-Coating Method with Controlled Gap for High-Speed Scalable Preparation of Highly Crystalline Organic Semiconductor Thin Films.

Solution-processed organic semiconductor thin films with high charge carrier mobility are necessary for development of next-generation electronic applications, but the rapid processing speed demanded for the industrial-scale production of these thin films poses a challenge to control of their thin-film properties, such as crystallinity, morphology, and film-to-film uniformity. Here, we show a new solution coating method that is compatible with a roll-to-roll printing process at a rate of 2 mm s-1 by using a gap-controllable wire bar, motion-programming strategy, and blended active inks. We demonstrate that the coating bar, the horizontal motion of which is repeatedly brought to an intermittent standstill, results in an improved vertically self-stratified structure and a high crystallinity for organic active inks comprising a semiconducting small molecule and a semiconducting polymer. Furthermore, organic transistors prepared by the developed method show superior hole mobility with high operational stability (hysteresis and kink-free transfer characteristics), high uniformity over a large area of a 4 in. wafer, good reproducibility, and superior electromechanical stabilities on a flexible plastic substrate. The bar-coating approach demonstrated here will be a step toward developing industrial-scale practical organic electronics applications.